REVIEW ARTICLE G protein-coupled receptor-induced Akt activity in cellular proliferation and apoptosis David C New, Kelvin Wu, Alice W S Kwok and Yung H Wong Department of Biochemistry, the Molecular Neuroscience Center, and the Biotechnology Research Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Keywords Akt; protein kinase B; G protein; G proteincoupled receptor; cell cycle; apoptosis Correspondence Y H Wong, Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Fax: +852 2358 1552 Tel: +852 2358 7328 E-mail: boyung@ust.hk (Received 17 August 2007, revised 17 September 2007, accepted 24 September 2007) Akt (also known as protein kinase B) plays an integral role in many intracellular signaling pathways activated by a diverse array of extracellular signals that target several different classes of membrane-bound receptors Akt plays a particularly prominent part in signaling networks that result in the modulation of cellular proliferation, apoptosis and survival Thus, the overexpression of Akt subtypes has been measured in a number of cancer types, and dominant-negative forms of Akt can trigger apoptosis and reduce the survival of cancer cells G protein-coupled receptors act as cell-surface detectors for a diverse spectrum of biological signals and are able to activate or inhibit Akt via several direct and indirect means In this review, we shall document how G protein-coupled receptors are able to control Akt activity and examine the resulting biochemical and physiological changes, with particular emphasis on cellular proliferation, apoptosis and survival doi:10.1111/j.1742-4658.2007.06116.x Akt, also known as protein kinase B (PKB), is a serine ⁄ threonine protein kinase that plays a pivotal role in many physiological processes, including metabolism, development, cell cycle progression, migration and survival [1–4] The Akt subfamily of protein kinases consists of three isoforms – Akt1, Akt2 and Akt3 (also termed PKBa, PKBb and PKBc) – which are the products of distinct genes All three proteins share a conserved tertiary structure of an N-terminal pleckstrin homology domain, a kinase domain and a C-terminal regulatory domain containing the hydrophobic motif phosphorylation site [5] While the homology between the three isoforms allows for a degree of functional redundancy [1], there also seems to be considerable scope for isoform-specific activation and substrate specificity [3,6] Akt plays an integral role in the phosphoinositide 3-kinase (PI3K) signaling pathways PI3K pathways are activated in response to extracellular signals mediated by cell-surface receptors of the G protein-coupled receptor (GPCR), integrin and growth factor ⁄ receptor tyrosine kinase (RTK) superfamilies Receptor-mediated activation of PI3K results in the generation of phosphatidylinositol (3,4,5)-trisphosphate from phosphatidylinositol (4,5)-bisphosphate, a reaction that is reversed by the enzymes phosphatase and tensin homologue (PTEN) and SH2-domain-containing inositol polyphosphate 5-phosphatase (SHIP) Both Akt and Abbreviations CDK, cyclin-dependent kinase; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; FH, forkhead; GPCR, G protein-coupled receptor; LPA, lysophosphatidic acid; MAPK, mitogen-activated protein kinase; mTOR, mammalian target of rapamycin; NF-jB, nuclear factor jB; p70S6K, p70 ribosomal protein S6 kinase; PAR-2, protease-activated receptor-2; PDGFRa, platelet-derived growth factor receptor a; PI3K, phosphoinositide 3-kinase; PKB, protein kinase B; PTEN, phosphatase and tensin homologue; RTK, receptor tyrosine kinase; SHIP, SH2-domain-containing inositol polyphosphate 5-phosphatase; TNF, tumour necrosis factor; TSC, tuberous sclerosis complex; TSH, thyroid-stimulating hormone receptor FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS 6025 Regulation of Akt signaling pathways by GPCRs D C New et al phosphoinositide-dependent kinase are recruited to the plasma membrane by phosphatidylinositol (3,4,5)-trisphosphate through their pleckstrin homology domains, where phosphoinositide-dependent kinase phosphorylates Akt1 on residue Thr308 in its kinase domain [7] A second phosphorylation takes place at Ser473 in the hydrophobic motif region of Akt1 This phosphorylation event seems to be catalyzed by a number of different kinases, which are probably stimulus- and ⁄ or cell type-specific This stabilizes the active conformation of Akt and allows it to translocate to the cytoplasm or nucleus to search for its many target proteins [8] Akt’s role in physiology suggests that aberrant Akt signaling may be a factor in disease states Most notably, amplification of Akt isoform genes and Akt mRNA overexpression has been observed in many human cancers [9] Akt activity in cancer cells may also be enhanced by the amplification of genes encoding PI3K or by a reduction in the activity of PTEN or SHIP [9] It is therefore to be expected that the inhibition of PI3K, Akt and their downstream effectors has been targeted in the development of cancer therapies [10] The involvement of Akt in cancer is not surprising given the ability of Akt to promote cellular proliferation through the direct and indirect phosphorylation of a number of cell cycle regulatory proteins [11], and its ability to inactivate pro-apoptotic factors, such as Bad, caspase-9 and forkhead (FH) transcription factors [12] In contrast, it is thought that a reduction in Akt signaling may contribute to diabetes by reducing the survival of pancreatic b cells [13] GPCRs act as cell-surface detectors for a diverse spectrum of biological signals To date, over 200 GPCRs have been matched with a ligand that activates the receptor to promote a wide variety of intracellular biochemical changes [14], even though it is estimated that the human genome encodes between 800 and 1000 GPCR subtypes [15,16] Their pervasive influence, coupled with their cell-surface accessibility, has resulted in GPCRs becoming the targets of as many as 45% of modern medicines [17], which are used to treat conditions as diverse as inflammation, incontinence, hypertension, depression and pain [18] GPCRs preferentially couple to heterotrimeric G proteins (consisting of a, b and c subunits) that are grouped into four classes, known as Gaq ⁄ 11, Gai ⁄ o, Gas and Ga12 ⁄ 13, based on the effector with which the a-subunit primarily interacts The activated G proteins in turn promote the activation or inhibition of a variety of intracellular events, including the activation of phos6026 pholipases, mitogen-activated protein kinases (MAPKs), activation ⁄ inhibition of adenylyl and guanylyl cyclases, and the opening and closing of ion channels In this review, we shall investigate the ability of GPCRs to activate Akt signaling pathways both directly, through the interaction of Gbc subunits with PI3K, and indirectly, through the GPCR transactivation of RTKs and integrins We shall also examine the downstream signaling and physiological consequences of GPCR-induced Akt activation, paying particular attention to the consequences for cellular proliferation, survival and apoptosis GPCR activation of PI3K ⁄ Akt signaling pathways GPCRs promote intracellular signaling through both Ga and Gbc subunits, which can activate distinct, complementary or antagonistic pathways As we will demonstrate, a large number of GPCRs, coupled to all four classes of G protein, activate PI3K ⁄ Akt pathways through either Ga or Gbc subunits (see below, Table and Fig 1) Gbc subunits are able to bind directly to and activate PI3K heterodimeric proteins containing either the p110b or the p110c subunits [19] Furthermore, it has been demonstrated that muscarinic and lysophosphatidic acid (LPA) GPCRs are only able to activate Akt in cells expressing the p110b or p110c subunits, and that this activation is mediated by Gbc subunits, but not by Ga subunits [20] Direct activation of PI3K by Ga subunits has not been specifically measured but they can activate PI3K ⁄ Akt pathways by transactivating integrins, RTKs and other growth factor receptors Numerous RTKs and integrins can independently activate PI3K ⁄ Akt pathways [21] but it has also been reported that they can be transactivated by GPCRs through Ga- or Gbc-dependent pathways [22,23] For example, ligands for the LPA, endothelin-1 and thrombin receptors all promote DNA synthesis in Rat1 fibroblasts by transactivating the epidermal growth factor receptor (EGFR, an RTK) Such transactivation requires the activation of matrix metalloproteases to release EGF from its membrane-bound form, which then stimulates the EGFR and downstream extracellular signal-regulated kinase (ERK) pathways [24] PI3K ⁄ Akt pathways are also activated by a similar method of transactivation [25] In Swiss 3T3 cells, bradykinin and bombesin promote cellular proliferation by an EGFR-dependent formation of a signaling complex that activates PI3K ⁄ Akt cascades [26] A number of other RTKs are also transactivated by GPCRs [27–29], and it has been demonstrated that FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS D C New et al Regulation of Akt signaling pathways by GPCRs Table GPCR-induced Akt activity and the consequences for cellular proliferation and apoptosis A selection of examples is presented here demonstrating the signaling components employed in connecting GPCR-initiated signals to downstream events regulated by Akt ›, indicates an increase in protein levels or activity; fl, indicates a decrease in protein levels or activity AC, adenylyl cyclase; AFX, FOX04; ALXR, lipoxin A4 receptor; CDK, cyclin-dependent kinase; CREB, cAMP-response element binding; cyt c, cytochrome c; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; FKHR, forkhead in rhabdomyosarcoma; FSH, follicle stimulating hormone; IGF-1, insulin growth factor-1; IGF-IRb, insulin-like growth factor receptor; IRS-1, insulin receptor substrate 1; GnRH, gonadotropin-releasing hormone; GSK3b, glycogen synthase kinase 3b; LHRH, luteinizing hormone-releasing hormone; LPA, lysophophatidic acid; MMP, matrix metalloproteinase; mTOR, mammalian target of rapamycin; NF-jB, nuclear factor-jB; p70S6k, p70 ribosomal protein S6 kinase; PAR-2, protease-activated receptor-2; PDGF, platelet-derived growth factor; PDGFR, platelet-derived growth factor receptor; PDK1, phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3-kinase; PKA, protein kinase A; PKC, protein kinase C; PP2A, protein phosphatase 2A; PTP, protein tyrosine phosphatase, ROCK, Rho-associated kinase; ROS, reactive oxygen species; SHP, Src homology 2-containing phosphatase; TSC2, tuberous sclerosis complex 2; TSH, thyroid stimulating hormone; VPAC, vasoactive intestinal peptide receptor GPCR Intracellular pathway Functional consequences Dopamine D2 ›PI3K ⁄ ›Akt ⁄ flTSC2 ›PLC ⁄ ›PI3K ⁄ ›Akt ⁄ flERK flPDGF ⁄ ›EGF ⁄ ›PI3K ⁄ ›Akt ⁄ ›p27Kip1 ⁄ ›p21Cip1 ⁄ flCDK2 ⁄ flcyclin E ›Ca2+ ⁄ ›Pyk2 ⁄ ›PI3K ⁄ ›ERK ⁄ ›Akt ›PI3K ⁄ ›Akt ⁄ flTSC2 ›PI3K ⁄ ›Akt ⁄ flGSK3b ⁄ flAFX ⁄ flFKHR ›PP2A ⁄ flAkt ⁄ ›GSK-3 j-opioid LPA ›PI3K ⁄ ›Akt ⁄ flTSC2 ›MMP ⁄ ›EGFR ⁄ ›PI3K ⁄ ›Akt l-opioid Muscarinic M2 Muscarinic M4 Pheromone V2R Somatostatin SST2 ›PI3K ⁄ ›Akt ⁄ ›p70S6k ›PI3K ⁄ ›Akt ⁄ ›NF-jB ›PI3K ⁄ ›Akt ⁄ flTSC2 ›PI3K ⁄ ›Akt ›PI3K ⁄ ›Akt ⁄ flTSC2 ›ERK ⁄ ›Akt ⁄ ›CREB flPI3K ⁄ flAkt ⁄ flNF-jB Somatostatin SST2b ›Src ⁄ ›SHP-1 ⁄ ›SHP-2 ⁄ ›PI3K ⁄ ›Ras ⁄ ›ERK ⁄ ›p27Kip1 ›PI3K ⁄ ›Akt ⁄ ›p70S6K Survival Anti-proliferation Anti-inflammatory effects, antiproliferation ›DNA synthesis, proliferation Survival Survival, proliferation Dopamine-induced neurological responses Survival ›Cyclin D1,›DNA synthesis, proliferation, survival Survival Survival Survival Survival Survival Survival Anti-proliferative, apoptotic, antiangiogenic and anti-invasive G1 cell cycle arrest Gi ⁄ o pathways Adenosine A2 Adenosine A3 ALXR CXCR4 d-opioid Gq ⁄ 11 pathways a-thrombin Angiotensin II type Apelin APJ Bradykinin Bombesin Endothelin-1 GnRH LPA LHRH Muscarinic M1 ›PI3Kb ⁄ ›Akt ⁄ ›cyclin D ⁄ ›CDK4 ›MMP ⁄ ›EGFR ⁄ ›PI3K ⁄ ›Akt ›EGFR ⁄ ›PI3K ⁄ ›Akt ⁄ ›mTOR ⁄ ›p70S6K ›PI3K ⁄ ›Akt ⁄ flcaspase ⁄ flcyt c ⁄ flcaspase ⁄ flcaspase ›EGFR ⁄ ›PI3K ⁄ ›Akt ›EGFR ⁄ ›PI3K ⁄ ›Akt ›IGF-IRb ⁄ ›Src ⁄ ›PI3K ⁄ ›Akt ›IGF-IRb ⁄ ›Src ⁄ ›PI3K ⁄ ›Akt ›Src ⁄ ›PI3K ⁄ ›Akt ›PKC ⁄ ›Src ⁄ ›Pyk2 ⁄ ›MMP ⁄ ›EGFR ⁄ ›PI3K ⁄ ›Akt ›PI3K ⁄ ›Akt ⁄ ›NF-jB ›PKCa ⁄ flAkt ⁄ ›GSK3 ⁄ ›Bad ⁄ ›caspase ›PI3K ⁄ ›Akt ›PTP ⁄ flphosphorylated-IRS-1 ⁄ flPI3K ⁄ flAkt ⁄ ›RhoA ⁄ flROCK-I FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS References [41] [56] [47] [34] [50] [81] [37] [50] [24,25] [70] [80] [50] [63] [40] [64] [36] [57] ›DNA synthesis, survival, proliferation [45] ›G1-S phase transition Survival, proliferation ›G1 cyclins, flp27Kip1, flp21Cip1, proliferation Survival [46] [25] [52] ›Cyclin D1, ›Cyclin E, ›DNA synthesis, proliferation ›DNA synthesis, proliferation Survival Survival Glut4 translocation Survival, proliferation Survival Apoptosis Survival Apoptosis [26] [68] [26] [29] [29] [38] [25] [80] [75] [63] [73] 6027 Regulation of Akt signaling pathways by GPCRs D C New et al Table (Continued) GPCR Intracellular pathway Functional consequences Muscarinic subtypes ›Src ⁄ ›PI3K ⁄ ›Akt ⁄ ›ERK ⁄ flGSK3b ⁄ flcaspase ⁄ ›CREB ›Ca2+ ⁄ ›PKC ⁄ ›Pyk2 ⁄ ›Src ⁄ ›PI3K ⁄ ›Akt Survival, proliferation [67] Actin reorganization and cell migration Proliferation Cell growth, proliferation [33] Survival Mucin secretion Follicular survival and development Proliferation, thyroid follicle activity DNA synthesis, mitogenesis Survival, development, differentiation [65] [31] [69] flTranscription of apoptotic and antiproliferative genes flTranscription of apoptotic and antiproliferative genes [30] PAR-2 Serotonin Vasopressin V1 ›PI3K ⁄ ›ROS ⁄ ›Akt ⁄ ›mTOR ⁄ ›p70S6K ›Ca2+ ⁄ ›PKC ⁄ ›Pyk2 ⁄ ›Src ⁄ ›EGFR ⁄ ›PI3K ⁄ ›Akt ⁄ ›mTOR ⁄ ›p70S6K Gs pathways Adenosine A2A b-adrenergic FSH ›Ca2+ ⁄ ›Src ⁄ ›Trk ⁄ ›Akt ›AC ⁄ ›PKA ⁄ ›Src ⁄ ›EGFR ⁄ ›PI3K ⁄ ›Akt ›PI3K ⁄ ›Akt ⁄ flFOXO1a TSH ›cAMP ⁄ ›PKA ⁄ ›PI3K ⁄ ›PDK1 ⁄ ›mTOR ⁄ ›p70S6K VPAC-1 ›cAMP ⁄ ›PKA ⁄ ›PI3K-Ras complex ⁄ flERK ›TrkA ⁄ ›PI3K ⁄ ›Akt G12 ⁄ 13 pathways LPA ›Rho ⁄ ›p160ROCK ⁄ ›PDGFRa ⁄ ›PI3K ⁄ ›Akt ⁄ flFKHR Thrombin ›Rho ⁄ ›p160ROCK ⁄ ›PDGFRa ⁄ ›PI3K ⁄ ›Akt ⁄ flFKHR References [53] [51] [54] [55] [28] [30] Fig Routes to Akt activation G12 ⁄ 13-, Gi ⁄ o-, Gq-, and Gs-coupled receptors are all known to activate the phosphoinositide 3-kinase (PI3K) ⁄ Akt pathway through either Ga or Gbc subunits Gaq and Gas subunits utilize secondary messenger systems [Ca2+, cAMP, reactive oxygen species (ROS)] to promote PI3K ⁄ Akt activation Whilst Ca2+-induced PI3K activation is a feature of Gi ⁄ o-coupled GPCRs, Gbc subunits released from these receptors are also able to directly bind to and activate PI3K In addition to these mechanisms, all four classes of GPCRs are able to activate the PI3K ⁄ Akt pathway by transactivating RTK at the plasma membrane either through matrix metalloproteinases (Gi ⁄ o-, Gq-, and Gs-coupled receptors) or through Rho ⁄ Rho-associated kinase (Rock)-mediated expression of RTK ligands (G12 ⁄ 13-coupled receptors) The Rho ⁄ Rock pathway can also indirectly inhibit PI3K activity, although the signaling components involved have not yet been elucidated (indicated by a dashed line) 6028 FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS D C New et al constitutively active Ga12 subunits activate PI3K ⁄ Akt signaling via the transactivation of the platelet-derived growth factor receptor a (PDGFRa) [30] However, it is not clear whether transactivation of RTKs by GPCRs can also occur through the induced expression of RTK ligands Alternatively, the RTK ligand requirement may be bypassed by the GPCR-induced Src family tyrosine kinase activation of RTKs [27], as evidenced by the Src kinase-dependent EGFR transactivation promoted by the b-adrenergic receptor in gastric mucosal cells [31] Src-family kinases are firmly embedded in signal transduction pathways activated by diverse extracellular stimuli playing a significant role in the crosstalk between many pathways, including those that facilitate the GPCR activation of Akt [32] The proteaseactivated receptor-2 (PAR-2) utilizes Gaq subunits to promote the activation of protein kinase C and the mobilization of intracellular Ca2+, leading to the formation of a complex containing Src-family kinases, the focal adhesion kinase, Pyk2, and PI3K [33] Similar findings have been made for the Gi ⁄ o-coupled CXCR4 receptor, which promotes DNA synthesis via a Pyk2 ⁄ PI3K ⁄ ERK pathway [34] These complexes may form as part of larger integrin ⁄ paxillin signaling platforms that promote the phosphorylation and activation of PI3K subunits [35] GPCR activation may also lead to the inactivation of Akt It has been reported that somatostatin SST2 receptors directly form a complex with the p85 regulatory subunit of PI3K Agonist activation induced the dissociation of this complex, preventing PI3K activation [36] Following agonist-induced activation, dopamine D2 receptors are internalized and form a multiprotein complex that includes b-arrestin, protein phosphatase 2A and Akt Protein phosphatase 2A inactivates Akt, thereby relieving Akt’s inhibition of glycogen synthase kinase 3b and allowing it to mediate dopamine-induced neurological responses [37] An alternative mode of b-arrestin-mediated PI3K ⁄ Akt inhibition is proposed to occur upon activation of the Gaq-coupled PAR-2 receptor Upon recruitment to the PAR-2 receptor, b-arrestin forms a complex with PI3K and spatially restricts its enzyme activity, thereby modulating the PAR-2 receptor activation of PI3K ⁄ Akt mediated by Pyk2 and Src-family kinases (see the preceding discussion) [33] In contrast, another study has indicated that b-arrestins mediate the endothelin A receptor activation of Akt by recruiting Src-family kinases that phosphorylate and activate Gaq, ultimately leading to PI3K ⁄ Akt pathway activation [38] Nevertheless, the vital role of b-arrestins in modulating the apoptotic events following the activation of some Regulation of Akt signaling pathways by GPCRs GPCRs was highlighted by a study which showed that in mouse embryonic fibroblasts devoid of b-arrestins the N-formyl peptide receptor, vasopressin V2, chemokine CXCR2 and the angiotensin II AT1A receptors all promote apoptosis through the activation of PI3K, MAPKs and Src kinases, leading to the activation of caspase pathways [39] Reconstituting the b-arrestins prevented the GPCR-induced apoptosis, suggesting that for some GPCRs b-arrestins constrain their apoptotic abilities The same study also demonstrated the GPCR selectivity of these events because in the absence of b-arrestins the CXCR4 and b2-adrenergic receptors were unable to activate apoptosis Recent studies have indicated that constitutively active Ga subunits of the Gaq ⁄ 11 and Ga12 ⁄ 13 subfamilies may actually inhibit the EGFR-mediated activation of Akt in transfected HEK-293 cells [40] This contradicts the previously noted ability of constitutively active Ga12 subunits to potentiate PDGFRa-mediated PI3K ⁄ Akt signaling [30] It is not immediately apparent whether these studies relate to GPCR signaling because RTKs are able to utilize heterotrimeric G protein pathways independently of GPCR activation [41] Akt mediation of GPCR-induced cell cycle control GPCRs have been widely reported to mediate mitogenic signals leading to cellular proliferation [2,42], and the overexpression or mutation of many GPCR subtypes in numerous cell types is thought to contribute to deregulated growth and tumour development [43,44] The transmembrane and intracellular pathways mediating the GPCR control of cell cycle progression are extensive [2], with all pathways converging in the nucleus to regulate the expression, localization, activity or stability of a small number of cell cycle proteins that are critical for the orderly progression from the G1 to S phases of the cell cycle Akt, in response to GPCR activation, directly interacts with some of these cell cycle proteins or exerts its effects through its downstream partners (Fig 2) Evidence suggests that the GPCR activation of Akt pathways can be either proliferative or antiproliferative, depending on the nature of the stimulus and the cell type observed Competing effects on cell cycle progression generated simultaneously by the same extracellular signal have been observed, suggesting that the final outcome of a signaling event relies on the balance of several competing mechanisms For example, activation of the SST2a receptor in Chinese hamster ovary cells promotes the sustained activation of the MAPK FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS 6029 Regulation of Akt signaling pathways by GPCRs D C New et al Fig Targets of Akt phosphorylation Activated (phosphorylated) Akt isoforms are able to regulate key cellular and physiological processes by phosphorylating a wide range of substrates involved in cellular survival (blue), glucose metabolism (orange), cell cycle progression (green), and protein synthesis (pink) Dashed lines indicate a translocation event family member p38 and the up-regulation of the cell cycle inhibitory protein p21Cip1 Conversely, activation of the SST2b receptor resulted in the activation by PI3K of both Akt and the p70 ribosomal protein S6 kinase (p70S6K), which led to cell cycle progression [45], probably through induction of the expression of cyclins (key proteins for the G1 to S phase transition) [2] Both somatostatin receptor subtypes were shown to be activating the same Gai subtypes but it was postulated that the Gbc subunit pairings may have been receptor subtype selective [45] Although we now know that GPCR interactions with b-arrestins may also control PI3K ⁄ Akt activation (as discussed above), a study on a-thrombin receptor signaling demonstrated that this GPCR activated Akt in b-arrestin-dependent and -independent ways b-arrestin-independent activation of Akt was more prolonged than b-arrestin-dependent activation and led to cyclin D1 accumulation, cyclin D1-cyclin-dependent kinase (CDK) activity and cell cycle progression [46] The intermediaries between Akt and cyclin D1 accumulation were not determined but it is known that the cyclin D1 protein is stabilized by the Akt-mediated inactivation of glycogen synthase kinase 3b, which normally phosphorylates and promotes the degradation of cyclin D1 In addition, Akt also phosphorylates and inactivates FH transcription factors, which bind to and activate the p27Kip1 promoter (another cell cycle inhibitory protein) Akt may also reduce the stability of p27Kip1, and Akt phosphorylation of p27Kip1 adversely affects its nuclear localization [11] Indeed, the anti-inflammatory lipoxins act 6030 through GPCRs to inhibit the PDGFR-mediated activation of Akt and the subsequent decrease in the levels of p21Cip1 and p27Kip1, as well as inhibiting the PDGFR-mediated cyclin E–CDK2 complex formation and cell cycle progression [47] Akt-induced phosphorylation of the tumour suppressor tuberous sclerosis complex (TSC)2 (also known as tuberin) causes the dissociation of TSC2 and TSC1 (also known as hamartin), relieving their inhibition of the mammalian target of rapamycin (mTOR) kinase [48] Increased mTOR activity reduces the stability of p27Kip1, releasing its restrictions on cell cycle progression In addition, mTOR activates the proliferative kinase p70S6K [11] Some GPCRs have now been shown to couple to this PI3K ⁄ Akt ⁄ tuberin ⁄ mTOR system In PC-12 and other neuronal cells, the Gi ⁄ o-coupled a2-adrenergic receptors, muscarinic M4 receptors, as well as the d-, j- and l-opioid receptors, all promote TSC2 phosphorylation via a PI3K ⁄ Akt-dependent pathway [41,49,50] Despite such evidence, a direct role for a Gi-coupled GPCR ⁄ Akt ⁄ mTOR signaling axis in cellular proliferation has not been demonstrated, as it has been for anti-apoptotic, pro-survival pathways (see below) However, activation of the Gqcoupled vasopressin V1 receptor in mesangial cells potently stimulates cell growth and proliferation by a Pyk2 ⁄ Src-dependent transactivation of EGFR followed by an mTOR-dependent activation of p70S6K and cell cycle progression [51] A very similar proliferative EGFR ⁄ PI3K ⁄ Akt ⁄ mTOR ⁄ p70S6K pathway is activated by Gq-coupled angiotensin II type receptors in FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS D C New et al mouse embryonic stem cells, leading to increases in the expression levels of G1 cyclins and their CDK partners, along with decreases in the levels of p21Cip1 and p27Kip1 [52] Mitogenic responses through these pathways have been reported for a number of other Gqcoupled receptors, including those for serotonin [53] The proliferative actions of Gs-coupled GPCRs mediated by these pathways have not been reported Nevertheless, activation of the Gs-coupled thyroidstimulating hormone receptor (TSH) in thyrocytes results in proliferation via an Akt-independent pathway activated by the TSH receptor interaction with PI3K, leading to the activation of p70S6K and mTOR [54] A separate study has indicated that the TSH receptor promotes PI3K pathway activation and DNA synthesis by stimulating the association of PI3K with Ras [55] Ras is known to bind to and activate several PI3K subtypes [19], and itself is a major target of GPCR activity [2] In relation to GPCR control of proliferation, Akt control of ERK has also been recorded For example, agonist stimulation of the Gi-coupled adenosine A3 receptor expressed in human melanoma cells triggers PI3K phosphorylation of Akt, leading to a reduction in the levels of active, phosphorylated ERK1 ⁄ and an inhibition of cellular proliferation [56] ERKs are known to regulate the transcriptional activity of several transcription factors that control the expression of G1 cyclins and CDK inhibitors [2] A seemingly similar PI3K ⁄ ERK-dependent pathway is activated by SST2 receptors, leading to the induction of p27Kip1 [57] Akt mediation of GPCR-induced survival and anti-apoptotic pathways A key role of Akt is to facilitate cell survival and to prevent apoptotic cell death In fact, dominant negative alleles of Akt reduce the ability of growth factors, extracellular matrix and other stimuli to support cell survival Conversely, the overexpression of Akt can rescue cells from apoptosis [9] This is achieved by the phosphorylation and inactivation of pro-apoptotic factors such as Bad, caspase-9 and FH transcription factors Bad belongs to the Bcl2 family of apoptotic proteins In some cell types, unphosphorylated Bad forms a complex with pro-survival members of the Bcl2 family at the mitochondrial membrane, inducing the release of cytochrome c from the mitochondria and triggering caspase-mediated apoptosis Akt phosphorylation of Bad leads to its sequestration in the cytosol by 14-3-3 proteins, preventing it from binding to its partners at the mitochondrial membrane [9] Likewise, Regulation of Akt signaling pathways by GPCRs Akt also phosphorylates and inactivates caspase-9, thereby inhibiting the terminal execution phase of apoptosis [12,58] In the absence of Akt activity, FH family members are found in the nucleus where they initiate apoptosis through the enhanced expression of specific pro-apoptotic Bcl2 family members Additionally, FH transcription factors promote the expression of the tumour necrosis factor (TNF) receptorassociated death domain and of the TNF-related apoptosis-inducing ligand, leading to the activation of death-receptor signaling and caspase-mediated apoptosis [59] Activated Akt phosphorylates FH family members, which are then exported from the nucleus and sequestered in the cytoplasm by their interaction with 14-3-3 proteins [12] Akt-dependent cell survival may also be achieved by the activation of the nuclear factor-jB (NF-jB) transcription factor and the direct phosphorylation and activation of the cAMP-response element binding protein These two transcription factors have been implicated in the promotion of the expression of genes encoding survival proteins, such as c-myc, inhibitor-of-apoptosis proteins ⁄ and Bcl2 [9,60] GPCR-mediated inhibition of apoptosis was observed many years ago when, for example, the activation of muscarinic M3 receptors endogenously expressed in rat cerebellar granule neurons protected the cells against K+-induced apoptosis [61] In neuronal PC12 cells, agonist activation of exogenously expressed muscarinic M1 receptors protected against apoptosis induced by growth factor withdrawal [62] The intracellular pathways responsible for mediating these effects are gradually being revealed and it is now clear that Akt-dependent signaling is a vital avenue for the transmission of pro-survival, anti-apoptotic signals emanating from GPCRs For example, in transfected COS-7 cells both Gq-coupled M1 and Gi-coupled M2 muscarinic GPCRs are able to activate Akt and prevent UV-induced apoptosis [63], while the Go-coupled V2R pheromone receptor promotes the survival of vomeronasal stem cells via a pathway dependent on Akt and cAMP-response element binding protein activation [64] Gs-coupled receptors have also been noted to utilize Akt-dependent mechanisms to promote cell survival Adenosine acting through the A2A receptor transactivates the Trk neurotrophin RTKs, which in turn activate Akt and cell survival [65], while an uncharacterized Gs-coupled receptor for the peptide hormone ghrelin protects pancreatic b-cells against induced apoptosis via both Akt and MAPK pathways [66] The GPCR-mediated signaling events downstream of Akt have also begun to be characterized In oligo- FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS 6031 Regulation of Akt signaling pathways by GPCRs D C New et al dendrocytes, carbachol (a nonselective muscarinic receptor agonist) significantly reduces caspase-mediated apoptosis by stimulating PI3K ⁄ Akt pathways [67] The role of caspases in GPCR-induced cell survival is further confirmed by the ability of the peptide hormone apelin to decrease the activation of caspase 9, as well as caspases and In mouse osteoblasts, this inhibition of caspase activity and the apoptotic activity induced by serum deprivation, steroids or TNF-a were blocked by inhibitors of PI3K and Akt [68] It is apparent that GPCRs also modify the expression and activity of members of the Bcl2 family of proteins in order to regulate cell survival and apoptosis As discussed above, the Akt-mediated phosphorylation of FH transcription factors removes them from the nucleus, preventing them from promoting the expression of pro-apoptotic Bcl2 proteins The initiation of these pro-survival responses by GPCRs is little studied, but there are indications that GPCR ⁄ Akt ⁄ FH transcription factor ⁄ Bcl2 protein pathways are relevant to cell survival For example, follicle stimulating hormone is thought to play a role in follicular survival and development in the ovary When expressed in HEK293 cells, the follicle stimulating hormone receptor rapidly promoted the phosphorylation and inactivation of the FOXO1a FH transcription factor, probably by Akt [69] The consequences of FOXO1a inactivation were not examined in this study but other work has described the ability of GPCRs to inhibit the expression of Bcl2 proteins In multiple cell types, the Gicoupled LPA receptors activate PI3K ⁄ Akt ⁄ p70S6K pathways as part of their cell survival mechanism [70] It is suspected that the activation of these pathways by LPA and sphingosine 1-phosphate receptors results in the suppression of the cellular levels of the pro-apoptotic Bcl2 family member Bax [71] In PC12 cells, it was determined that M4 receptors induced a Gbc subunitdependent activation of Akt and were able to augment nerve growth factor (NGF)-mediated cell survival [40] This Akt activation was accompanied by the degradation of TSC2 While not directly measured in this study, removal of TSC2 would be expected to promote mTOR activity mTOR is thought to affect apoptosis and cell survival in several different ways, including by regulating the expression levels of the anti-apoptotic Bcl2 family member Bcl-XL [72] It seems that given the correct conditions, GPCRs can actually promote apoptosis In HeLa cells, this can be achieved by an M1 receptor-mediated inhibition of insulin receptor-stimulated Akt activation or by a direct activation of caspase ⁄ RhoA ⁄ Rho-associated kinase pathways [73], possibly by up-regulating the expression of the pro-apoptotic Bax [74], in contrast to 6032 the previously noted suppression of the cellular levels of Bax by the LPA and sphingosine 1-phosphate receptors [71] An alternative approach to the induction of apoptosis has been adopted by the luteinizing hormone-releasing hormone, which inhibits the insulin growth factor-1 receptor-mediated activation of Akt Inhibition by the LHRH receptor in pituitary cells results in a reduction in Bad phosphorylation and a reduction in the ability of insulin growth factor-1 to rescue cells from apoptosis [75] It is clear that Akt-dependent survival pathways represent an attractive target for the development of anticancer agents In fact, inhibitors of mTOR not only cause cell cycle arrest but also promote apoptosis directly by sensitizing cells to the effects of DNA-damaging agents [72,76] The contribution that the regulation of GPCR activity may make to the modulation of these potentially therapeutic pathways is being intensively investigated [77,78] One approach to cancer therapy has been to target nonselectively the activity of heterotrimeric G proteins using compound BIM46174 A variety of biochemical assays indicate that BIM-46174 inhibits the formation and ⁄ or dissociation of the Ga ⁄ Gbc heterotrimeric complex Exposure of a variety of human cancer cells to BIM-46174 inhibits their growth by inducing caspase-dependent apoptosis In mice, this drug seems to complement established chemotherapeutic regimes [79] Individual GPCRs have also been targeted For example, LPA receptors couple to several different G protein subfamilies to activate Akt and the transcriptional activity of NF-jB In androgen-insensitive prostate cancer PC3 cells, this activation of Akt and NF-jB is required to escape cell death Therefore, it has been suggested that as NF-jB is constitutively activated in prostate cancer, a strategy of targeted disruption of the LPA ⁄ Akt ⁄ NF-jB pathway may benefit androgen-insensitive prostate cancer treatment [80] In small cell lung cancer cells with constitutively active Akt signaling pathways, the application of the d-opioid receptor antagonist naltrindole promotes apoptosis This correlated with reduced levels of phosphorylation and activity of PI3K, Akt, glycogen synthase kinase 3b and FH transcription factors, as well as the up-regulation of several proapoptotic gene products [81] Conclusions There is little doubt that Akt is a crucial intermediary in many intracellular signaling pathways initiated by diverse extracellular stimuli acting at several classes of membrane-bound receptors Recent years have produced a growing body of evidence that clearly FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS D C New et al establishes GPCRs as key initiators of the modulation of Akt-dependent signaling events Furthermore, the regulation of Akt-dependent cellular proliferation and cellular survival in human cancer cells by numerous GPCRs has opened up the possibility of controlling cellular events through the use of ligands for a variety of receptors The investigation of the applicability of such an approach for therapeutic benefit is in its infancy but, as we have described, progress has been made with efforts to control growth and trigger apoptosis in human cancer cells by targeting heterotrimeric G proteins and individual GPCRs [79,80] However, experimental data obtained from transgenic and knockout mice dictates that a cautious approach to targeting Akt activity will be necessary Mice deficient in Akt2 display insulin resistance and type-II diabetes-like syndrome, while both Akt1 and Akt2 are required for platelet activation [8] Encouragingly, expression of a dominant negative, kinase dead mutant of Akt using an adenoviral vector selectively induced apoptosis in tumor cells with elevated levels of Akt activity but not in normal cells [82] This suggests that unlike normal cells, tumor cells are dependent on increased Akt activity for survival, indicating that short-term inhibition of Akt signaling may not be toxic to normal cells As with many aspects of cellular signaling, much remains to be uncovered in order to deepen our understanding of how individual GPCRs functionally interact with different G proteins to initiate a cascade of events leading to the activation of different Akt subtypes, which in turn trigger a multitude of downstream pathways Many of these GPCR-initiated events are likely to be cell-type specific and modulated by the actions of a host of other extracellular and intracellular cues that must ultimately be integrated to achieve the required biochemical ⁄ physiological outcomes Acknowledgements This work was supported, in part, by grants from the Research Grants Council of Hong Kong (HKUST ⁄ 03C, HKUST 6443 ⁄ 06 m), the University Grants Committee (AoE ⁄ B-15 ⁄ 01), and the Hong Kong Jockey Club YHW was a recipient of the Croucher Senior Research Fellowship References Dummler B & Hemmings BA (2007) Physiological roles of PKB ⁄ Akt isoforms in development and disease Biochem Soc Trans 35, 231–235 Regulation of Akt signaling pathways by GPCRs New DC & Wong YH (2007) Molecular mechanisms mediating the G protein-coupled regulation of cell cycle progression J Mol Signal 2, Stambolic V & Woodgett JR (2006) Functional distinctions of protein kinase B ⁄ Akt isoforms defined by their influence on cell migration Trends Cell Biol 16, 461– 466 Downward J (2004) PI 3-kinase, Akt and cell survival Semin Cell Dev Biol 15, 177–182 Hanada M, Feng J & Hemmings BA (2004) Structure, regulation and function of PKB ⁄ AKT - a major therapeutic target Biochim Biophys Acta 1697, 3–16 Okano J, Gaslightwala I, Birnbaum MJ, Rustgi AK & Nakagawa H (2000) Akt ⁄ protein kinase B isoforms are differentially regulated by epidermal growth factor stimulation J Biol Chem 275, 30934–30942 Alessi DR, James SR, Downes CP, Holmes AB, Gaffney PR, Reese CB & Cohen P (1997) Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Ba Curr Biol 7, 261–269 Fayard E, Tintignac LA, Baudry A & Hemmings BA (2005) Protein kinase B ⁄ Akt at a glance J Cell Sci 118, 5675–5678 Nicholson KM & Anderson NG (2002) The protein kinase B ⁄ Akt signalling pathway in human malignancy Cell Signal 14, 381–395 10 Chen YL, Law PY & Loh HH (2005) Inhibition of PI3K ⁄ Akt signaling: an emerging paradigm for targeted cancer therapy Curr Med Chem Anti Canc Agents 5, 575–589 11 Liang J & Slingerland JM (2003) Multiple roles of the PI3K ⁄ PKB (Akt) pathway in cell cycle progression Cell Cycle 2, 339–345 12 Vanhaesebroeck B & Alessi DR (2000) The PI3KPDK1 connection: more than just a road to PKB Biochem J 346, 561–576 13 Zdychova J & Komers R (2005) Emerging role of Akt kinase ⁄ protein kinase B signaling in pathophysiology of diabetes and its complications Physiol Res 54, 1–16 14 Pierce KL, Premont RT & Lefkowitz RJ (2002) Seventransmembrane receptors Nat Rev Mol Cell Biol 3, 639–650 15 Marchese A, George SR, Kolakowski LF Jr, Lynch KR & O’Dowd BF (1999) Novel GPCRs and their endogenous ligands: expanding the boundaries of physiology and pharmacology Trends Pharmacol Sci 20, 370–375 16 Venter JC, Adams MD, Myers EW, Li PW, Mural RJ, Sulton GG, Smith HO, Yandell M, Evans CA, Holt RA et al (2001) The sequence of the human genome Science 291, 1304–1351 17 Drews J (2000) Drug discovery: a historical perspective Science 287, 1960–1964 18 Nambi P & Aiyar N (2003) G protein-coupled receptors in drug discovery Assay Drug Dev Technol 1, 305–310 FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS 6033 Regulation of Akt signaling pathways by GPCRs D C New et al 19 Schwindinger WF & Robishaw JD (2001) Heterotrimeric G-protein bc-dimers in growth and differentiation Oncogene 20, 1653–1660 20 Murga C, Fukuhara S & Gutkind JS (2000) A novel role for phosphatidylinositol 3-kinase b in signaling from G protein-coupled receptors to Akt J Biol Chem 275, 12069–12073 21 Wymann MP, Zvelebil M & Laffargue M (2003) Phosphoinositide 3-kinase signalling – which way to target? Trends Pharmacol Sci 24, 366–376 22 Schafer B, Gschwind A & Ullrich A (2004) Multiple Gprotein-coupled receptor signals converge on the epidermal growth factor receptor to promote migration and invasion Oncogene 23, 991–999 23 Ohtsu H, Dempsey PJ & Eguchi S (2006) ADAMs as mediators of EGF receptor transactivation by G protein-coupled receptors Am J Physiol Cell Physiol 291, C1–C10 24 Schafer B, Marg B, Gschwind A & Ullrich A (2004b) Distinct ADAM metalloproteinases regulate G proteincoupled receptor-induced cell proliferation and survival J Biol Chem 279, 47929–47938 25 Shah BH, Neithardt A, Chu DB, Shah FB & Catt KJ (2006) Role of EGF receptor transactivation in phosphoinositide 3-kinase-dependent activation of MAP kinase by GPCRs J Cell Physiol 206, 47–57 26 Santiskulvong C, Sinnett-Smith J & Rozengurt E (2001) EGF receptor function is required in late G1 for cell cycle progression induced by bombesin and bradykinin Am J Physiol Cell Physiol 281, C886– C898 27 Piiper A & Zeuzem S (2004) Receptor tyrosine kinases are signaling intermediates of G protein-coupled receptors Curr Pharm Des 10, 3539–3545 28 El Zein N, Badran BM & Sariban E (2007) The neuropeptide pituitary adenylate cyclase activating protein stimulates human monocytes by transactivation of the Trk ⁄ NGF pathway Cell Signal 19, 152–162 29 Sumitomo M, Milowsky MI, Shen R, Navarro D, Dai J, Asano T, Hayakawa M & Nanus DM (2001) Neutral endopeptidase inhibits neuropeptide-mediated transactivation of the insulin-like growth factor receptor-Akt cell survival pathway Cancer Res 61, 3294–3298 30 Kumar RN, Ha JH, Radhakrishnan R & Dhanasekaran DN (2006) Transactivation of platelet-derived growth factor receptor a by the GTPase-deficient activated mutant of Ga12 Mol Cell Biol 26, 50–62 31 Slomiany BL & Slomiany A (2005) Gastric mucin secretion in response to b-adrenergic G protein-coupled receptor activation is mediated by Src kinase-dependent epidermal growth factor receptor transactivation J Physiol Pharmacol 56, 247–258 32 Parsons SJ & Parsons JT (2004) Src family kinases, key regulators of signal transduction Oncogene 23, 7906– 7909 6034 33 Wang P & DeFea KA (2006) Protease-activated receptor-2 simultaneously directs b-arrestin-1-dependent inhibition and Gaq-dependent activation of phosphatidylinositol 3-kinase Biochemistry 45, 9374–9385 34 Bajetto A, Barbero S, Bonavia R, Piccioli P, Pirani P, Florio T & Schettini G (2001) Stromal cell-derived factor-1a induces astrocyte proliferation through the activation of extracellular signal-regulated kinases ⁄ pathway J Neurochem 77, 1226–1236 35 Luttrell DK & Luttrell LM (2004) Not so strange bedfellows: G-protein-coupled receptors and Src family kinases Oncogene 23, 7969–7978 36 Bousquet C, Guillermet-Guibert J, Saint-Laurent N, Archer-Lahlou E, Lopez F, Fanjul M, Ferrand A, Fourmy D, Pichereaux C, Monsarrat B et al (2006) Direct binding of p85 to sst2 somatostatin receptor reveals a novel mechanism for inhibiting PI3K pathway EMBO J 25, 3943–3954 37 Beaulieu JM, Gainetdinov RR & Caron MG (2007) The Akt-GSK-3 signaling cascade in the actions of dopamine Trends Pharmacol Sci 28, 166–172 38 Imamura T, Huang J, Dalle S, Ugi S, Usui I, Luttrell LM, Miller WE, Lefkowitz RJ & Olefsky JM (2001) bArrestin-mediated recruitment of the Src family kinase Yes mediates endothelin-1-stimulated glucose transport J Biol Chem 276, 43663–43667 39 Revankar CM, Vines CM, Cimino DF & Prossnitz ER (2004) Arrestins block G protein-coupled receptor-mediated apoptosis J Biol Chem 279, 24578–24584 40 Wu EH, Tam BH & Wong YH (2006) Constitutively active a subunits of Gq ⁄ 11 and G12 ⁄ 13 families inhibit activation of the pro-survival Akt signaling cascade FEBS J 273, 2388–2398 41 Wu EH & Wong YH (2005) Involvement of Gi ⁄ o proteins in nerve growth factor-stimulated phosphorylation and degradation of tuberin in PC-12 cells and cortical neurons Mol Pharmacol 67, 1195–1205 42 Moolenaar WH (1991) G protein-coupled receptors, phosphoinositide hydrolysis, and cell proliferation Cell Growth Differ 2, 359–364 43 Li S, Huang S & Peng SB (2005) Overexpression of G protein-coupled receptors in cancer cells: involvement in tumor progression Int J Oncol 27, 1329–1339 44 Schoneberg T, Schulz A, Biebermann H, Hermsdorf T, Rompler H & Sangkuhl K (2004) Mutant G proteincoupled receptors as a cause of human diseases Pharmacol Ther 104, 173–206 45 Sellers LA, Alderton F, Carruthers AM, Schindler M & Humphrey PP (2000) Receptor isoforms mediate opposing proliferative effects through Gbc-activated p38 or Akt pathways Mol Cell Biol 20, 5974–5985 46 Goel R, Phillips-Mason PJ, Raben DM & Baldassare JJ (2002) a-Thrombin induces rapid and sustained Akt phosphorylation by b-arrestin1-dependent and -indepen- FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS D C New et al 47 48 49 50 51 52 53 54 55 56 57 dent mechanisms, and only the sustained Akt phosphorylation is essential for G1 phase progression J Biol Chem 277, 18640–18648 Mitchell D, Rodgers K, Hanly J, McMahon B, Brady HR, Martin F & Godson C (2004) Lipoxins inhibit Akt ⁄ PKB activation and cell cycle progression in human mesangial cells Am J Pathol 164, 937–946 Wu EH, Wu KK & Wong YH (2007) Tuberin: a stimulus-regulated tumor suppressor protein controlled by a diverse array of receptor tyrosine kinases and G protein-coupled receptors Neurosignals 15, 217–227 Wu EH & Wong YH (2006) Activation of muscarinic M4 receptor augments NGF-induced pro-survival Akt signaling in PC12 cells Cell Signal 18, 285–293 Wu EH & Wong YH (2005) Activation of d-, j-, and lopioid receptors induces phosphorylation of tuberin in transfected HEK 293 cells and native cells Biochem Biophys Res Commun 334, 838–844 Ghosh PM, Mikhailova M, Bedolla R & Kreisberg JI (2001) Arginine vasopressin stimulates mesangial cell proliferation by activating the epidermal growth factor receptor Am J Physiol Renal Physiol 280, F972–F979 Han HJ, Han JY, Heo JS, Lee SH, Lee MY & Kim YH (2007) ANG II-stimulated DNA synthesis is mediated by ANG II receptor-dependent Ca2+ ⁄ PKC as well as EGF receptor-dependent PI3K ⁄ Akt ⁄ mTOR ⁄ p70S6K1 signal pathways in mouse embryonic stem cells J Cell Physiol 211, 618–629 Liu Y & Fanburg BL (2006) Serotonin-induced growth of pulmonary artery smooth muscle requires activation of phosphatidylinositol 3-kinase ⁄ serine-threonine protein kinase B ⁄ mammalian target of rapamycin ⁄ p70 ribosomal S6 kinase Am J Respir Cell Mol Biol 34, 182– 191 Suh JM, Song JH, Kim DW, Kim H, Chung HK, Hwang JH, Kim JM, Hwang ES, Chung J, Han JH et al (2003) Regulation of the phosphatidylinositol 3kinase, Akt ⁄ protein kinase B, FRAP ⁄ mammalian target of rapamycin, and ribosomal S6 kinase signaling pathways by thyroid-stimulating hormone (TSH) and stimulating type TSH receptor antibodies in the thyroid gland J Biol Chem 278, 21960–21971 Ciullo I, Diez-Roux G, Di Domenico M, Migliaccio A & Avvedimento EV (2001) cAMP signaling selectively influences Ras effectors pathways Oncogene 20, 1186– 1192 Merighi S, Benini A, Mirandola P, Gessi S, Varani K, Leung E, Maclennan S & Borea PA (2005) A3 adenosine receptor activation inhibits cell proliferation via phosphatidylinositol 3-kinase ⁄ Akt-dependent inhibition of the extracellular signal-regulated kinase ⁄ phosphorylation in A375 human melanoma cells J Biol Chem 280, 19516–19526 Lahlou H, Saint-Laurent N, Esteve JP, Eychene A, Pradayrol L, Pyronnet S & Susini C (2003) sst2 Regulation of Akt signaling pathways by GPCRs 58 59 60 61 62 63 64 65 66 67 68 69 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation J Biol Chem 278, 39356–39371 Kumar S (2007) Caspase function in programmed cell death Cell Death Differ 14, 32–43 Burgering BM & Medema RH (2003) Decisions on life and death: FOXO Forkhead transcription factors are in command when PKB ⁄ Akt is off duty J Leukoc Biol 73, 689–701 Li X & Stark GR (2002) NF-jB-dependent signaling pathways Exp Hematol 30, 285–296 Yan GM, Lin SZ, Irwin RP & Paul SM (1995) Activation of muscarinic cholinergic receptors blocks apoptosis of cultured cerebellar granule neurons Mol Pharmacol 47, 248–257 Lindenboim L, Pinkas-Kramarski R, Sokolovsky M & Stein R (1995) Activation of muscarinic receptors inhibits apoptosis in PC12M1 cells J Neurochem 64, 2491– 2499 Murga C, Laguinge L, Wetzker R, Cuadrado A & Gutkind JS (1998) Activation of Akt ⁄ protein kinase B by G protein-coupled receptors A role for a and bc subunits of heterotrimeric G proteins acting through phosphatidylinositol-3-OH kinase c J Biol Chem 273, 19080– 19085 Xia J, Sellers LA, Oxley D, Smith T, Emson P & Keverne EB (2006) Urinary pheromones promote ERK ⁄ Akt phosphorylation, regeneration and survival of vomeronasal (V2R) neurons Eur J Neurosci 24, 3333–3342 Lee FS, Rajagopal R & Chao MV (2002) Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors Cytokine Growth Factor Rev 13, 11–17 Granata R, Settanni F, Biancone L, Trovato L, Nano R, Bertuzzi F, Destefanis S, Annunziata M, Martinetti M, Catapano F et al (2007) Acylated and unacylated ghrelin promote proliferation and inhibit apoptosis of pancreatic b-cells and human islets: involvement of 3¢,5¢-cyclic adenosine monophosphate ⁄ protein kinase A, extracellular signal-regulated kinase ⁄ 2, and phosphatidyl inositol 3-kinase ⁄ Akt signaling Endocrinology 148, 512–529 Cui QL, Fogle E & Almazan G (2006) Muscarinic acetylcholine receptors mediate oligodendrocyte progenitor survival through Src-like tyrosine kinases and PI3K ⁄ Akt pathways Neurochem Int 48, 383–393 Tang SY, Xie H, Yuan LQ, Luo XH, Huang J, Cui RR, Zhou HD, Wu XP & Liao EY (2007) Apelin stimulates proliferation and suppresses apoptosis of mouse osteoblastic cell line MC3T3–E1 via JNK and PI3-K ⁄ Akt signaling pathways Peptides 28, 708–718 Nechamen CA, Thomas RM, Cohen BD, Acevedo G, Poulikakos PI, Testa JR & Dias JA (2004) Human follicle-stimulating hormone (FSH) receptor interacts FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS 6035 Regulation of Akt signaling pathways by GPCRs 70 71 72 73 74 75 D C New et al with the adaptor protein APPL1 in HEK 293 cells: potential involvement of the PI3K pathway in FSH signaling Bio Reprod 71, 629–636 Ye X, Ishii I, Kingsbury MA & Chun J (2002) Lysophosphatidic acid as a novel cell survival ⁄ apoptotic factor Biochim Biophys Acta 1585, 108–113 Goetzl EJ, Kong Y & Mei B (1999) Lysophosphatidic acid and sphingosine 1-phosphate protection of T cells from apoptosis in association with suppression of Bax J Immunol 162, 2049–2056 Tirado OM, Mateo-Lozano S & Notario V (2005) Rapamycin induces apoptosis of JN-DSRCT-1 cells by increasing the Bax: Bcl-xL ratio through concurrent mechanisms dependent and independent of its mTOR inhibitory activity Oncogene 24, 3348–3057 Ueda H, Morishita R, Narumiya S, Kato K & Asano T (2004) Gaq ⁄ 11 signaling induces apoptosis through two pathways involving reduction of Akt phosphorylation and activation of RhoA in HeLa cells Exp Cell Res 298, 207–217 Del Re DP, Miyamoto S & Brown JH (2007) RhoA ⁄ Rho kinase up-regulate Bax to activate a mitochondrial death pathway and induce cardiomyocyte apoptosis J Biol Chem 282, 8069–8078 Rose A, Froment P, Perrot V, Quon MJ, LeRoith D & Dupont J (2004) The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary aT3 cells by protein kinase Ca-mediated negative regulation of Akt J Biol Chem 279, 52500–52516 6036 76 Beuvink I, Boulay A, Fumagalli S, Zilbermann F, Ruetz S, O’Reilly T, Natt F, Hall J, Lane HA & Thomas G (2005) The mTOR inhibitor RAD001 sensitizes tumor cells to DNA-damaged induced apoptosis through inhibition of p21 translation Cell 120, 747–759 77 Youn BS, Yu KY, Oh J, Lee J, Lee TH & Broxmeyer HE (2002) Role of the CC chemokine receptor ⁄ TECK interaction in apoptosis Apoptosis 7, 271–276 78 Fischer OM, Hart S, Gschwind A & Ullrich A (2003) EGFR signal transactivation in cancer cells Biochem Soc Trans 31, 1203–1208 79 Prevost GP, Lonchampt MO, Holbeck S, Attoub S, Zaharevitz D, Alley M, Wright J, Brezak MC, Coulomb H, Savola A et al (2006) Anticancer activity of BIM46174, a new inhibitor of the heterotrimeric Ga ⁄ Gbc protein complex Cancer Res 66, 9227–9234 80 Raj GV, Sekula JA, Guo R, Madden JF & Daaka Y (2004) Lysophosphatidic acid promotes survival of androgen-insensitive prostate cancer PC3 cells via activation of NF-kappaB Prostate 61, 105– 113 81 Chen YL, Law PY & Loh HH (2004) Inhibition of Akt ⁄ protein kinase B signaling by naltrindole in small cell lung cancer cells Cancer Res 64, 8723–8730 82 Jetzt A, Howe JA, Horn MT, Maxwell E, Yin Z, Johnson D & Kumar CC (2003) Adenoviral-mediated expression of a kinase-dead mutant of Akt induces apoptosis selectively in tumor cells and suppresses tumor growth in mice Cancer Res 63, 6697– 6706 FEBS Journal 274 (2007) 6025–6036 ª 2007 The Authors Journal compilation ª 2007 FEBS ... Anderson NG (2002) The protein kinase B ⁄ Akt signalling pathway in human malignancy Cell Signal 14, 381–395 10 Chen YL, Law PY & Loh HH (2005) Inhibition of PI3K ⁄ Akt signaling: an emerging... protein kinase A, extracellular signal-regulated kinase ⁄ 2, and phosphatidyl inositol 3-kinase ⁄ Akt signaling Endocrinology 148, 512–529 Cui QL, Fogle E & Almazan G (2006) Muscarinic acetylcholine... demonstrating the signaling components employed in connecting GPCR-initiated signals to downstream events regulated by Akt ›, indicates an increase in protein levels or activity; fl, indicates a