The Pathways Connecting G Protein-coupled Receptors to the Nucleus through Divergent Mitogen-activated Protein Kinase Cascades (1998)

Receptors coupled to heterotrimeric GTP-binding proteins (G proteins) are integral membrane proteins involved in the transmission of signals from the extracellular environment to the cytoplasm. The best known family of G protein-coupled receptors (GPCRs), 1The abbreviations used are: GPCR, G protein-coupled receptor; PIP2, phosphatidylinositol bisphosphate; MAP, mitogen-activated protein; ERK, extracellular signal-regulated kinase; MAPK, mitogen-activated protein kinase; MEK, MAP or ERK kinase; PLC, phospholipase C; PKC, protein kinase C; PDGF, platelet-derived growth factor; PI3K, phosphatidylinositol 3-kinase; JNK, Jun kinase; SAPK, stress-activated protein kinase. currently comprising more than 1000 members, exhibits a common structural motif consisting of seven membrane-spanning regions (1Dohlman H.G. Caron M.G. Lefkowitz R.J. Biochemistry. 1987; 26: 2657-2664Crossref PubMed Scopus (419) Google Scholar) (Fig. 1). A diverse array of external stimuli including neurotransmitters, hormones, phospholipids, photons, odorants, certain taste ligands, and growth factors can activate specific members of this receptor family and promote interaction between the receptor and the G protein on the intracellular side of the membrane. This causes the exchange of GDP for GTP bound to the G protein α subunit and apparently the dissociation of the βγ heterodimers. In turn, GTP-bound G protein α subunits or βγ complexes initiate intracellular signaling responses by acting on effector molecules such as adenylate cyclases or phospholipases or directly regulating ion channel or kinase function (Fig. 1, and see below). Sixteen distinct mammalian G protein α subunits have been molecularly cloned and are divided into four families based upon sequence similarity: αs, αi, αq, and α12. Similarly, eleven G protein γ subunits and five G protein β subunits have been identified. Thus, GPCRs are likely to represent the most diverse signal transduction systems in eukaryotic cells. The biochemical and biological consequences of such diversity in subunit composition and coupling specificity for each receptor have just begun to be elucidated. In this review, we will briefly describe the role of G proteins and their coupled receptors in normal growth control and tumorigenesis and then focus on current efforts to elucidate the signaling pathways connecting this class of cell surface receptors to nuclear events regulating gene expression. Proliferative signaling has generally been associated with polypeptide growth factor receptors that possess an intrinsic protein tyrosine kinase activity (2Yarden Y. Escobedo J.A. Kuang W.J. Yang-Feng T.L. Daniel T.O. Tremble P.M. Chen E.Y. Ando M.E. Harkins R.N. Francke U. Fried V.A. Ullrich A. Williams L.T. Nature. 1986; 323: 226-232Crossref PubMed Scopus (769) Google Scholar). A variety of oncogenes have been found to code for mutated forms of these receptors (3Sherr C.J. Rettenmier C.W. Sacca R. Roussel M.F. Look A.T. Stanley E.R. Cell. 1985; 41: 665-676Abstract Full Text PDF PubMed Scopus (982) Google Scholar) and their ligands (4Doolittle R.F. Hunkapiller M.W. Hood L.E. Devare S.G. Robbins K.C. Aaronson S.A. Antoniades H.N. Science. 1983; 221: 275-277Crossref PubMed Scopus (983) Google Scholar) or for molecules that participate in their growth-promoting pathways (5Ullrich A. Schlessinger J. Cell. 1990; 61: 203-212Abstract Full Text PDF PubMed Scopus (4611) Google Scholar). On the other hand, GPCRs have been traditionally linked to tissue-specific, fully differentiated cell functions (1Dohlman H.G. Caron M.G. Lefkowitz R.J. Biochemistry. 1987; 26: 2657-2664Crossref PubMed Scopus (419) Google Scholar). However, GPCRs are also expressed in proliferating cells, and they have been implicated in embryogenesis, tissue regeneration, and growth stimulation (reviewed in Ref. 6Rozengurt E. Science. 1986; 234: 161-166Crossref PubMed Scopus (852) Google Scholar). In this regard, many ligands acting via GPCRs, including thrombin, bombesin, bradykinin, substance P, endothelin, serotonin, acetylcholine, gastrin, prostaglandin F2α, and lysophosphatidic acid, are known to elicit a mitogenic response in a variety of cell types (reviewed in Refs. 6Rozengurt E. Science. 1986; 234: 161-166Crossref PubMed Scopus (852) Google Scholar and 7van Biesen T. Luttrell L.M. Hawes B.E. Lefkowitz R.J. Endocr. Rev. 1996; 17: 698-714Crossref PubMed Scopus (390) Google Scholar), and recent gene knock-out studies indicate that certain GPCRs are essential for cell growth under physiological conditions (8Nagata A. Ito M. Iwata N. Kuno J. Takano H. Minowa O. Chihara K. Matsui T. Noda T. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 11825-11830Crossref PubMed Scopus (219) Google Scholar). Furthermore, accumulating evidence indicates that GPCRs and their signaling molecules can harbor oncogenic potential. For example, the mas oncogene, which encodes a putative GPCR, was initially cloned using standard transfection assays by virtue of its ability to induce tumors in mice (9Young D. Waitches G. Birchmeier C. Fasano O. Wigler M. Cell. 1986; 45: 711-719Abstract Full Text PDF PubMed Scopus (328) Google Scholar). Subsequently, serotonin 1C (10Julius D. Livelli T.J. Jessell T.M. Axel R. Science. 1989; 244: 1057-1062Crossref PubMed Scopus (287) Google Scholar), muscarinic m1, m3, and m5 (11Gutkind J.S. Novotny E.A. Brann M.R. Robbins K.C. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 4703-4707Crossref PubMed Scopus (262) Google Scholar), and adrenergic α1 (12Allen L.F. Lefkowitz R.J. Caron M.G. Cotecchia S. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 11354-11358Crossref PubMed Scopus (292) Google Scholar) receptors were shown effectively to transform contact-inhibited cultures of rodent fibroblasts when persistently activated. Together these studies demonstrated that GPCRs can behave as agonist-dependent oncogenes and prompted several groups to explore the transforming potential of G protein α subunits. In recent studies, constitutively active mutants of Gαi, Gαq, Gα0, Gα12, and Gα13 were shown to behave as transforming genes in a variety of cell types (reviewed in Ref. 13Dhanasekaran N. Heasley L.E. Johnson G.L. Endocr. Rev. 1995; 16: 259-270Crossref PubMed Scopus (178) Google Scholar). The recent discovery of activating mutations in GPCRs and G proteins in several disease states, including cancer, further supports a role for GPCRs in normal and aberrant growth control. For example, mutationally activated Gαs results in hyperplasia of endocrine cells and has been found in human thyroid and pituitary tumors (reviewed in Ref. 13Dhanasekaran N. Heasley L.E. Johnson G.L. Endocr. Rev. 1995; 16: 259-270Crossref PubMed Scopus (178) Google Scholar) and in the McCune-Albright syndrome, a disease in which multiple endocrine glands exhibit autonomous hyperproliferation (14Weinstein L.S. Shenker A. Gejman P.V. Merino M.J. Friedman E. Spiegel A.M. N. Engl. J. Med. 1991; 325: 1688-1695Crossref PubMed Scopus (1399) Google Scholar). Interestingly, activated Gαs contributes significantly to hyperplasia only in tissues where cAMP stimulates proliferation, thus acting as an oncogene referred to as the gsp oncogene (15Landis C.A. Masters S.B. Spada A. Pace A.M. Bourne H.R. Vallar L. Nature. 1989; 340: 692-696Crossref PubMed Scopus (1225) Google Scholar). Activating mutations have also been identified for Gαi2, referred to as the gip2 oncogene, in a subset of ovarian sex cord stromal tumors and adrenal cortical tumors (16Lyons J. Landis C.A. Harsh G. Vallar L. Grunewald K. Feichtinger H. Duh Q.Y. Clark O.H. Kawasaki E. Bourne H.R. McCormick F. Science. 1990; 249: 655-659Crossref PubMed Scopus (927) Google Scholar). On the other hand, Gα12, referred as the gep oncogene (17Xu N. Voyno-Yasenetskaya T. Gutkind J.S. Biochem. Biophys. Res. Commun. 1994; 201: 603-609Crossref PubMed Scopus (88) Google Scholar,18Xu N. Bradley L. Ambdukar I. Gutkind J.S. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 6741-6745Crossref PubMed Scopus (174) Google Scholar), was isolated as a transforming gene from a soft tissue sarcoma-derived cell (19Chan A.M. Fleming T.P. McGovern E.S. Chedid M. Miki T. Aaronson S.A. Mol. Cell. Biol. 1993; 13: 762-768Crossref PubMed Scopus (145) Google Scholar), although its role in tumorigenesis remains unclear. Naturally occurring activated mutations in members of the Gαq family have not yet been described. At the receptor level, the identification of constitutively active thyroid-stimulating hormone receptor mutations in 30% of thyroid adenomas (20Parma J. Duprez L. Van Sande J. Cochaux P. Gervy C. Mockel J. Dumont J. Vassart G. Nature. 1993; 365: 649-651Crossref PubMed Scopus (851) Google Scholar) provided a direct link between this class of receptors and human cancer. Similarly, mutationally activated luteinizing hormone receptors have been identified in a form of familial male precocious puberty, which results from hyperplastic growth of Leydig cells (21Shenker A. Laue L. Kosugi S. Merendino Jr., J.J. Minegishi T. Cutler Jr., G.B. Nature. 1993; 365: 652-654Crossref PubMed Scopus (653) Google Scholar). Perhaps more frequently than activating mutations, paracrine and autocrine stimulation of multiple GPCRs for neuropeptides and prostaglandins has been implicated in a number of human neoplasias, including small cell lung carcinoma (22Cuttitta F. Carney D.N. Mulshine J. Moody T.W. Fedorko J. Fischler A. Minna J.D. Nature. 1985; 316: 823-826Crossref PubMed Scopus (1149) Google Scholar), colon adenomas and carcinomas (23Hoosein N.M. Kiener P.A. Curry R.C. Rovati L.C. McGilbra D.K. Brattain M.G. Cancer Res. 1988; 48: 7179-7183PubMed Google Scholar), and gastric hyperplasia and cancer (24Tahara E. J. Cancer Res. Clin. Oncol. 1990; 116: 121-131Crossref PubMed Scopus (185) Google Scholar). Sequences encoding functional GPCRs have also been found in the genome of transforming DNA viruses, including herpesvirus saimiri (25Nicholas J. Cameron K.R. Honess R.W. Nature. 1992; 355: 362-365Crossref PubMed Scopus (155) Google Scholar) and Kaposi's sarcoma-associated herpesvirus (26Arvanitakis L. Geras-Raaka E. Varma A. Gershengorn M.C. Cesarman E. Nature. 1997; 385: 347-350Crossref PubMed Scopus (575) Google Scholar). Currently available evidence suggests that, at least for Kaposi's sarcoma-associated herpesvirus, these viral GPCRs are sufficient to subvert normal growth control. The mechanism(s) whereby GPCRs regulate cell proliferation remain poorly understood. Although inhibition of adenylyl cyclase has been observed in cells responding to growth-promoting agents acting on Gi-coupled receptors, there is no formal proof that induction of DNA synthesis results from decreasing intracellular levels of cAMP. Conversely, several lines of investigation have implicated phosphatidylinositol bisphosphate (PIP2) hydrolysis as a critical component of mitogenesis (6Rozengurt E. Science. 1986; 234: 161-166Crossref PubMed Scopus (852) Google Scholar). However, recent studies using mutant tyrosine kinase receptors suggested that PIP2hydrolysis is neither necessary nor sufficient for mitogenesis (27Mohammadi M. Dionne C.A. Li W. Li N. Spivak T. Honegger A.M. Jaye M. Schlessinger J. Nature. 1992; 358: 681-684Crossref PubMed Scopus (336) Google Scholar,28Coughlin S.R. Escobedo J.A. Williams L.T. Science. 1989; 243: 1191-1194Crossref PubMed Scopus (288) Google Scholar). Furthermore, a number of GPCR agonists induce the PIP2turnover pathway but fail to stimulate growth when added alone to quiescent cells (29Moolenaar W.H. Cell Growth Differ. 1991; 2: 359-364PubMed Google Scholar). 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