Analysis of the fibroblast transformation potential of GTPase-deficient gip2 oncogenes

Clinical potential of the Hippo-YAP pathway in bladder cancer

Bladder cancer (BC) is one of the world’s most frequent cancers. Surgery coupled with adjuvant platinum-based chemotherapy is the current standard of therapy for BC. However, a high proportion of patients progressed to chemotherapy-resistant or even neoplasm recurrence. Hence, identifying novel treatment targets is critical for clinical treatment. Current studies indicated that the Hippo-YAP pathway plays a crucial in regulating the survival of cancer stem cells (CSCs), which is related to the progression and reoccurrence of a variety of cancers. In this review, we summarize the evidence that Hippo-YAP mediates the occurrence, progression and chemotherapy resistance in BC, as well as the role of the Hippo-YAP pathway in regulating bladder cancer stem-like cells (BCSCs). Finally, the clinical potential of Hippo-YAP in the treatment of BC was prospected.

Naturally occurring hotspot cancer mutations in Gα13 promote oncogenic signaling

Heterotrimeric G-proteins are signaling switches broadly divided into four families based on the sequence and functional similarity of their Gα subunits: Gs, Gi/o, Gq/11, and G12/13 Artificial mutations that activate Gα subunits of each of these families have long been known to induce oncogenic transformation in experimental systems. With the advent of next-generation sequencing, activating hotspot mutations in Gs, Gi/o, or Gq/11 proteins have also been identified in patient tumor samples. In contrast, patient tumor-associated G12/13 mutations characterized to date lead to inactivation rather than activation. By using bioinformatic pathway analysis and signaling assays, here we identified cancer-associated hotspot mutations in Arg-200 of Gα13 (encoded by GNA13) as potent activators of oncogenic signaling. First, we found that components of a G12/13-dependent signaling cascade that culminates in activation of the Hippo pathway effectors YAP and TAZ is frequently altered in bladder cancer. Up-regulation of this signaling cascade correlates with increased YAP/TAZ activation transcriptional signatures in this cancer type. Among the G12/13 pathway alterations were mutations in Arg-200 of Gα13, which we validated to promote YAP/TAZ-dependent (TEAD) and MRTF-A/B-dependent (SRE.L) transcriptional activity. We further showed that this mechanism relies on the same RhoGEF-RhoGTPase cascade components that are up-regulated in bladder cancers. Moreover, Gα13 Arg-200 mutants induced oncogenic transformation in vitro as determined by focus formation assays. In summary, our findings on Gα13 mutants establish that naturally occurring hotspot mutations in Gα subunits of any of the four families of heterotrimeric G-proteins are putative cancer drivers.

Probing the mutational landscape of regulators of G protein signaling proteins in cancer

The advent of deep-sequencing techniques has revealed that mutations in G protein-coupled receptor (GPCR) signaling pathways in cancer are more prominent than was previously appreciated. An emergent theme is that cancer-associated mutations tend to cause enhanced GPCR pathway activation to favor oncogenicity. Regulators of G protein signaling (RGS) proteins are critical modulators of GPCR signaling that dampen the activity of heterotrimeric G proteins through their GTPase-accelerating protein (GAP) activity, which is conferred by a conserved domain dubbed the "RGS-box." Here, we developed an experimental pipeline to systematically assess the mutational landscape of RGS GAPs in cancer. A pan-cancer bioinformatics analysis of the 20 RGS domains with GAP activity revealed hundreds of low-frequency mutations spread throughout the conserved RGS domain structure with a slight enrichment at positions that interface with G proteins. We empirically tested multiple mutations representing all RGS GAP subfamilies and sampling both G protein interface and noninterface positions with a scalable, yeast-based assay. Last, a subset of mutants was validated using G protein activity biosensors in mammalian cells. Our findings reveal that a sizable fraction of RGS protein mutations leads to a loss of function through various mechanisms, including disruption of the G protein-binding interface, loss of protein stability, or allosteric effects on G protein coupling. Moreover, our results also validate a scalable pipeline for the rapid characterization of cancer-associated mutations in RGS proteins.

Mutations in G protein β subunits promote transformation and kinase inhibitor resistance

Activating mutations in genes encoding G protein α (Gα) subunits occur in 4-5% of all human cancers, but oncogenic alterations in Gβ subunits have not been defined. Here we demonstrate that recurrent mutations in the Gβ proteins GNB1 and GNB2 confer cytokine-independent growth and activate canonical G protein signaling. Multiple mutations in GNB1 affect the protein interface that binds Gα subunits as well as downstream effectors and disrupt Gα interactions with the Gβγ dimer. Different mutations in Gβ proteins clustered partly on the basis of lineage; for example, all 11 GNB1 K57 mutations were in myeloid neoplasms, and seven of eight GNB1 I80 mutations were in B cell neoplasms. Expression of patient-derived GNB1 variants in Cdkn2a-deficient mouse bone marrow followed by transplantation resulted in either myeloid or B cell malignancies. In vivo treatment with the dual PI3K-mTOR inhibitor BEZ235 suppressed GNB1-induced signaling and markedly increased survival. In several human tumors, mutations in the gene encoding GNB1 co-occurred with oncogenic kinase alterations, including the BCR-ABL fusion protein, the V617F substitution in JAK2 and the V600K substitution in BRAF. Coexpression of patient-derived GNB1 variants with these mutant kinases resulted in inhibitor resistance in each context. Thus, GNB1 and GNB2 alterations confer transformed and resistance phenotypes across a range of human tumors and may be targetable with inhibitors of G protein signaling.

Suppression of GNAI2 message in ovarian cancer

Background

Understanding the integration of hormone signaling and how it impacts oncogenesis is critical for improved cancer treatments. Here we elucidate GNAI2 message alterations in ovarian cancer (OvCa). GNAI2 is a heterotrimeric G protein which couples cell surface hormone receptors to intracellular enzymes, and is best characterized for its direct role in regulating cAMP response element-binding protein (CREB) function by decreasing intracellular cAMP through inhibiting adenylyl cyclase.

Methods

We probed the Origene human OvCa array for the presence of polymorphisms and gene expression alterations of GNAI2 using directing sequencing and qPCR. These data were supported by database mining of the [NCBI NIH GSE:6008, GSE:14764, GSE:29450, GDS:4066, GDS:3297, GSE:32474, and GSE:2003] datasets.

Results

No significant polymorphisms were found, including an absence of the gip2 oncogene. However, 85.9% of (506 of 589) OvCa patients had decreased GNAI2 message. Further characterization demonstrated that the GNAI2 message was on average decreased 54% and maximally decreased by 2.8 fold in clear cell carcinoma. GNAI2 message decreased in early stage cancer while message was increased compared to normal in advanced cancers. The changes in GNAI2 also correlated to deregulation of CREB, Fos, Myc, cyclins, Arf, the transition from estrogen dependence to independence, and metastatic potential.

Conclusion

These data strongly implicate GNAI2 as a critical regulator of oncogenesis and an upstream driver of cancer progression in OvCa.

Transformation by a nucleotide-activated P2Y receptor is mediated by activation of Galphai, Galphaq and Rho-dependent signaling pathways

Background

Nucleotide-actived P2Y receptors play critical roles in the growth of tumor cells by regulating cellular proliferation, differentiation and survival.

Results

Here we demonstrate that an avian P2Y purinoceptor (tP2YR) with unique pharmacological and signal transduction properties induces morphologic and growth transformation of rodent fibroblasts. tP2YR induced a transformed phenotype similar to the mas oncogene, a G protein-coupled receptor which causes transformation by activation of Rac-dependent pathways. tP2YR-transformed cells exhibited increased steady-state activation of Rac1 and RhoA. Like activated Rho GTPases, tP2YR cooperated with activated Raf and caused synergistic transformation of NIH3T3 cells. Our data indicate that the ability of tP2YR to cause transformation is due to its unique ability among purinergic receptors to simultaneously activate Gα_q and Gα_i. Co-expression of constitutively activated mutants of these two Gα subunits caused the same transformed phenotype as tP2YR and Mas. Furthermore, transformation by both tP2YR and Mas was blocked by pharmacological inhibition of Gα_I by pertussis toxin (PTX) indicating an essential role for Gα_i in transformation by these G-protein coupled receptors.

Conclusions

Our data suggest that coordinated activation of Gα_q and Gα_i may link the tP2YR and possibility the Mas oncogene with signaling pathways resulting in activation of Rho family proteins to promote cellular transformation.

Activation of endogenous thrombin receptors causes clustering and sensitization of epidermal growth factor receptors of swiss 3T3 cells without transactivation

The G protein-coupled thrombin receptor can induce cellular responses in some systems by transactivating the epidermal growth factor (EGF) receptor. This is in part due to the stimulation of ectoproteases that generate EGF receptor ligands. We show here that this cannot account for the stimulation of proliferation or migration by thrombin of Swiss 3T3 cells. Thrombin has no direct effect on the activation state of the EGF receptor or of its downstream effectors. However, thrombin induces the subcellular clustering of the EGF receptor at filamentous actin-containing structures at the leading edge and actin arcs of migrating cells in association with other signaling molecules, including Shc and phospholipase Cgamma1. In these thrombin-primed cells, the subsequent migratory response to EGF is potentiated. Thrombin did not potentiate the EGF-stimulated EGF receptor phosphorylation. Thus, in Swiss 3T3 cells the G protein-coupled thrombin receptor can potentiate the EGF tyrosine kinase receptor response when activated by EGF, and this appears to be due to the subcellular concentration of the receptor with downstream effectors and not to the overall ability of EGF to induce receptor transphosphorylation. Thus, the EGF receptor subcellular localization which is altered by thrombin appears to be an important determinant of the efficacy of downstream EGF receptor signaling in cell migration.

Activation of B-Raf and regulation of the mitogen-activated protein kinase pathway by the G(o) alpha chain

Many receptors coupled to the pertussis toxin-sensitive G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) pathway. The role of the alpha chains of these G proteins in MAPK activation is poorly understood. We investigated the ability of Galpha(o) to regulate MAPK activity by transient expression of the activated mutant Galpha(o)-Q205L in Chinese hamster ovary cells. Galpha(o)-Q205L was not sufficient to activate MAPK but greatly enhanced the response to the epidermal growth factor (EGF) receptor. This effect was not associated with changes in the state of tyrosine phosphorylation of the EGF receptor. Galpha(o)-Q205L also potentiated MAPK stimulation by activated Ras. In Chinese hamster ovary cells, EGF receptors activate B-Raf but not Raf-1 or A-Raf. We found that expression of activated Galpha(o) stimulated B-Raf activity independently of the activation of the EGF receptor or Ras. Inactivation of protein kinase C and inhibition of phosphatidylinositol-3 kinase abolished both B-Raf activation and EGF receptor-dependent MAPK stimulation by Galpha(o). Moreover, Galpha(o)-Q205L failed to affect MAPK activation by fibroblast growth factor receptors, which stimulate Raf-1 and A-Raf but not B-Raf activity. These results suggest that Galpha(o) can regulate the MAPK pathway by activating B-Raf through a mechanism that requires a concomitant signal from tyrosine kinase receptors or Ras to efficiently stimulate MAPK activity. Further experiments showed that receptor-mediated activation of Galpha(o) caused a B-Raf response similar to that observed after expression of the mutant subunit. The finding that Galpha(o) induces Ras-independent and protein kinase C- and phosphatidylinositol-3 kinase-dependent activation of B-Raf and conditionally stimulates MAPK activity provides direct evidence for intracellular signals connecting this G protein subunit to the MAPK pathway.

Distinct roles for Galpha(i)2 and Gbetagamma in signaling to DNA synthesis and Galpha(i)3 in cellular transformation by dopamine D2S receptor activation in BALB/c 3T3 cells

Control of cell proliferation depends on intracellular mediators that determine the cellular response to external cues. In neuroendocrine cells, the dopamine D2 receptor short form (D2S receptor) inhibits cell proliferation, whereas in mesenchymal cells the same receptor enhances cell proliferation. Nontransformed BALB/c 3T3 fibroblast cells were stably transfected with the D2S receptor cDNA to study the G proteins that direct D2S signaling to stimulate cell proliferation. Pertussis toxin inactivates G(i) and G(o) proteins and blocks signaling of the D2S receptor in these cells. D2S receptor signaling was reconstituted by individually transfecting pertussis toxin-resistant Galpha(i/o) subunit mutants and measuring D2-induced responses in pertussis toxin-treated cells. This approach identified Galpha(i)2 and Galpha(i)3 as mediators of the D2S receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity; Galpha(i)2-mediated D2S-induced stimulation of p42 and p44 mitogen-activated kinase (MAPK) and DNA synthesis, whereas Galpha(i)3 was required for formation of transformed foci. Transfection of toxin-resistant Galpha(i)1 cDNA induced abnormal cell growth independent of D2S receptor activation, while Galpha(o) inhibited dopamine-induced transformation. The role of Gbetagamma subunits was assessed by ectopic expression of the carboxyl-terminal domain of G protein receptor kinase to selectively antagonize Gbetagamma activity. Mobilization of Gbetagamma subunits was required for D2S-induced calcium mobilization, MAPK activation, and DNA synthesis. These findings reveal a remarkable and distinct G protein specificity for D2S receptor-mediated signaling to initiate DNA synthesis (Galpha(i)2 and Gbetagamma) and oncogenic transformation (Galpha(i)3), and they indicate that acute activation of MAPK correlates with enhanced DNA synthesis but not with transformation.

Rheb interacts with Raf-1 kinase and may function to integrate growth factor- and protein kinase A-dependent signals

Rheb is a recently described member of the Ras family that was originally identified as an immediate-early gene in brain but is also widely expressed in other tissues. Here we demonstrate that Rheb interacts with and appears to regulate Raf-1 kinase, an essential component of the H-Ras signaling pathway. In direct contrast to H-Ras, however, the interaction of Rheb with Raf-1 is potentiated by growth factors in combination with agents that increase cyclic AMP (cAMP) levels. Protein kinase A-dependent phosphorylation of serine 43 within the regulatory domain of Raf-1 reciprocally potentiates its interaction with Rheb and decreases its interaction with H-Ras. A single amino acid in the G2 effector domain is critical for the differential properties of Rheb. Since Rheb is an immediate-early gene, our studies suggest that Rheb functions in concert with H-Ras to dynamically integrate cAMP and growth factor signaling.

Coupling of the thrombin receptor to G12 may account for selective effects of thrombin on gene expression and DNA synthesis in 1321N1 astrocytoma cells

In 1321N1 astrocytoma cells, thrombin, but not carbachol, induces AP-1-mediated gene expression and DNA synthesis. To understand the divergent effects of these G protein-coupled receptor agonists on cellular responses, we examined Gq-dependent signaling events induced by thrombin receptor and muscarinic acetylcholine receptor stimulation. Thrombin and carbachol induce comparable changes in phosphoinositide and phosphatidylcholine hydrolysis, mobilization of intracellular Ca2+, diglyceride generation, and redistribution of protein kinase C; thus, activation of these Gq-signaling pathways appears to be insufficient for gene expression and mitogenesis. Thrombin increases Ras and mitogen-activated protein kinase activation to a greater extent than carbachol in 1321N1 cells. The effects of thrombin are not mediated through Gi, since ribosylation of Gi/Go proteins by pertussis toxin does not prevent thrombin-induced gene expression or thrombin-stimulated DNA synthesis. We recently reported that the pertussis toxin-insensitive G12 protein is required for thrombin-induced DNA synthesis. We demonstrate here, using transfection of receptors and G proteins in COS-7 cells, that G alpha 12 selectively couples the thrombin receptor to AP-1-mediated gene expression. This does not appear to result from increased mitogen-activated protein kinase activity but may reflect activation of a tyrosine kinase pathway. We suggest that preferential coupling of the thrombin receptor to G12 accounts for the selective ability of thrombin to stimulate Ras, mitogen-activated protein kinase, gene expression, and mitogenesis in 1321N1 cells.

The G alpha i homologue gna-1 controls multiple differentiation pathways in Neurospora crassa

Heterotrimeric G proteins are components of principal signaling pathways in eukaryotes. In higher organisms, alpha subunits of G proteins have been divided into four families, Gi, Gs, Gq, and G12. We previously identified a G alpha i homologue gna-1 in the filamentous fungus Neurospora crassa. Now we report that deletion of gna-1 leads to multiple phenotypes during the vegetative and sexual cycles in N. crassa. On solid medium, delta gna-1 strains have a slower rate of hyphal apical extension than wild type, a rate that is more pronounced under hyperosmotic conditions or in the presence of a cellophane overlay. delta gna-1 mutants accumulate less mass than wild-type strains, and their mass accumulation is not affected in the same way by exposure to light. delta gna-1 strains are defective in macroconidiation, possessing aerial hyphae that are shorter, contain abnormal swellings, and differentiate adherent macroconidia. During the sexual cycle, delta gna-1 strains are fertile as males. However, the mutants are female-sterile, producing small, aberrant female reproductive structures. After fertilization, delta gna-1 female structures do not enlarge and develop normally, and no sexual spores are produced. Thus, mutation of gna-1 results in sex-specific loss of fertility.

Gi2-mediated activation of the MAP kinase cascade

The Gi class of heterotrimeric G proteins has been implicated in transmitting mitogenic signals from a variety of seven-transmembrane domain receptors. In addition, the alpha subunit of Gi2 (alpha i2) is oncogenic when mutated to a constitutively active form (gip2). The mechanism by which Gi2 stimulates cellular proliferation is unknown, but is believed to involve activation of the mitogen-activated protein kinase (MAPK) signaling cascade. To study Gi2 activation of the cascade, we transiently expressed a mutant, pertussis toxin (PTX)-resistant alpha i2 in Chinese hamster ovary cells. After PTX treatment of these cells, Gi-coupled receptors specifically activated PTX-resistant Gi2 without activating other Gi proteins. Receptor-mediated activation of Gi2 led to activation of MAPK and its upstream activator, MAPK/ERK-activating kinase (MEK). Activation of MAPK and MEK by Gi2 was blocked by expression of a dominant-negative mutant of Ras. Gi2 activation did not, however, detectably increase the proportion of Ras protein in the GTP-bound form. Additional experiments suggest that Gi2 stimulates the MAPK pathway, at least in part, by mechanisms that involve release of its beta gamma subunit, as well as activation of phosphatidylinositol-3 kinase.

Gi2 alpha protein deficiency: a model of inflammatory bowel disease

Mice deficient for the G protein subunit Gi2 alpha were obtained by gene targeting. They displayed a growth retardation that was apparent at 6 weeks of age. They subsequently developed diffuse colitis with clinical and histopathological features closely resembling those of ulcerative colitis in humans. Seven of 20 Gi2 alpha-deficient mice with colitis also developed adenocarcinomas of the colon. Gi2 alpha-deficient thymocytes displayed two- to fourfold increases in mature CD4+8- and CD4-8+ phenotypes, an approximately threefold increase in high-intensity CD3 staining and enhanced proliferative responses to T-cell receptor stimuli. Stimulation of Gi 2 alpha-deficient peripheral T cells induced a hyperresponsive profile of interleukin-2, tumour necrosis factor, and interferon-gamma production, which may reflect a heightened response of primed cells or a defective negative regulation. We suggest that Gi 2 alpha-deficient mice may represent a useful animal model for dissecting the pathomechanisms of inflammatory bowel disease and also for the development of novel therapeutic strategies.

Point mutations of ras genes in human adrenal cortical tumors: absence in adrenocortical hyperplasia

Point mutations of ras genes (K-, H-, and N-ras) at codons 12, 13, and 61 and of the Gi2 alpha gene at codons 179 and 205, were studied in 56 primary adrenal cortical tumors and 6 adrenal cortical hyperplasias. Of 56 tumors, 24 were carcinomas and 32 were benign. The 24 carcinomas and 20 of the benign tumors were from American patients; the 12 remaining adenomas were from Japanese patients. Of the benign tumors 12 were cortisol-producing adenomas, 15 were aldosterone-producing adenomas, 3 were nonfunctioning adenomas, and 2 were adenomas that produced a virilizing syndrome. Tumor DNA obtained from archival formalin-fixed, paraffin-embedded tissue or fresh frozen tissue was amplified by polymerase chain reaction; and point mutations were detected by sequence-specific oligonucleotide hybridization. Activating ras mutations were found in 7 of 56 (12.5%) of all tumors: 3 of 24 (12.5%) carcinomas and 4 of 32 (12.5%) adenomas. Of adenomas from an American population, 4 of 20 (20%) exhibited positive ras mutations, whereas none was present in the Japanese tumors. All mutations detected were adenine to guanine transitions at the second position of N-ras codon 61, resulting in a conversion from glutamine to arginine. No mutations were found in K-ras or H-ras genes. Furthermore, no mutations of the Gi2 alpha gene were identified. These findings demonstrate that N-ras mutations at codon 61 may contribute to the genesis of both benign and malignant human adrenal cortical tumors. Finally, no mutations of the ras or Gi2 alpha genes were identified in hyperplastic adrenocortical tissues.

Aberrant function of the Ras-related protein TC21/R-Ras2 triggers malignant transformation

Although the human Ras proteins are members of a large superfamily of Ras-related proteins, to date, only the proteins encoded by the three mammalian ras genes have been found to possess oncogenic potential. Among the known Ras-related proteins, TC21/R-Ras2 exhibits the most significant amino acid identity (55%) to Ras proteins. We have generated mutant forms of TC21 that possess amino acid substitutions analogous to those that activate Ras oncogenic potential [designated TC21(22V) and TC21(71L)] and compared the biological properties of TC21 with those of Ras proteins in NIH 3T3 and Rat-1 transformation assays. Whereas wild-type TC21 did not show any transforming potential in vitro, both TC21(22V) and TC21(71L) displayed surprisingly potent transforming activities that were comparable to the strong transforming activity of oncogenic Ras proteins. Like Ras-transformed cells, NIH 3T3 cells expressing mutant TC21 proteins formed foci of morphologically transformed cells in monolayer cultures, proliferated in low serum, formed colonies in soft agar, and developed progressive tumors in nude mice. Thus, TC21 is the first Ras-related protein to exhibit potent transforming activity equivalent to that of Ras. Furthermore, mutant TC21 proteins also stimulated constitutive activation of mitogen-activated protein kinases as well as transcriptional activation from Ras-responsive promoter elements (Ets/AP-1 and NF-kappa B). We conclude that aberrant TC21 function may trigger cellular transformation via a signal transduction pathway similar to that of oncogenic Ras and suggest that deregulated TC21 activity may contribute significantly to human oncogenesis.

Aberrant function of the Ras-related protein TC21/R-Ras2 triggers malignant transformation

Although the human Ras proteins are members of a large superfamily of Ras-related proteins, to date, only the proteins encoded by the three mammalian ras genes have been found to possess oncogenic potential. Among the known Ras-related proteins, TC21/R-Ras2 exhibits the most significant amino acid identity (55%) to Ras proteins. We have generated mutant forms of TC21 that possess amino acid substitutions analogous to those that activate Ras oncogenic potential [designated TC21(22V) and TC21(71L)] and compared the biological properties of TC21 with those of Ras proteins in NIH 3T3 and Rat-1 transformation assays. Whereas wild-type TC21 did not show any transforming potential in vitro, both TC21(22V) and TC21(71L) displayed surprisingly potent transforming activities that were comparable to the strong transforming activity of oncogenic Ras proteins. Like Ras-transformed cells, NIH 3T3 cells expressing mutant TC21 proteins formed foci of morphologically transformed cells in monolayer cultures, proliferated in low serum, formed colonies in soft agar, and developed progressive tumors in nude mice. Thus, TC21 is the first Ras-related protein to exhibit potent transforming activity equivalent to that of Ras. Furthermore, mutant TC21 proteins also stimulated constitutive activation of mitogen-activated protein kinases as well as transcriptional activation from Ras-responsive promoter elements (Ets/AP-1 and NF-kappa B). We conclude that aberrant TC21 function may trigger cellular transformation via a signal transduction pathway similar to that of oncogenic Ras and suggest that deregulated TC21 activity may contribute significantly to human oncogenesis.

Epitope-tagged Gq alpha subunits: expression of GTPase-deficient alpha subunits persistently stimulates phosphatidylinositol-specific phospholipase C but not mitogen-activated protein kinase activity regulated by the M1 muscarinic acetylcholine receptor

Gq is the heterotrimeric guanine nucleotide-binding protein that activates the beta isoforms of phosphatidyl-inositol-specific phospholipase C (PI-PLC). The Gq alpha-subunit polypeptide (alpha qa) was N-terminally modified by addition of a 9-aa sequence, YPYDVPDYA. Placement of the 9-aa epitope tag at the N terminus allowed expression of functional alpha q polypeptides and selective identification of plasmid-expressed wild-type and mutant G-protein alpha subunits. Mutation of glutamine-209 to leucine in the N-terminally epitope-tagged alpha q (N(epi) alpha qQ209L) inhibited GTPase activity and persistently activated PI-PLC, resulting in high steady-state levels of inositol phosphates. The elevated levels of inositol phosphates resulting from N(epi) alpha qQ209L expression were similar to those obtained with carbachol activation of the M1 muscarinic acetylcholine receptor. The Gq-coupled M1 receptor, which stimulates PI-PLC activity, and phorbol esters, acting via protein kinase C, activate the cytoplasmic mitogen-activated protein kinase in COS cells. However, the constitutive activation of PI-PLC enzymatic activity resulting from expression of GTPase-deficient alpha q was unable to persistently activate this kinase. The results indicate that persistent PI-PLC activation is insufficient to sustain the stimulation of a cytoplasmic serine/threonine protein kinase regulated by Gq-coupled receptor signal-transduction pathways.

Expression cDNA cloning of a transforming gene encoding the wild-type G alpha 12 gene product

Using an expression cDNA cloning approach, we examined human tumor cell lines for novel oncogenes that might evade detection by conventional techniques. We isolated a transforming sequence that was highly efficient in transforming NIH 3T3 mouse fibroblasts. DNA sequence analysis identified the gene as the human homolog of a recently cloned alpha subunit of mouse GTP-binding protein G alpha 12. NIH 3T3 cells transfected with G alpha 12 cDNA grew in soft agar and were tumorigenic in nude mice. There were no apparent mutations in the cloned cDNA in comparison with a G alpha 12 cDNA clone isolated from a normal human epithelial cell library, implying that overexpression alone was sufficient to cause NIH 3T3 cell transformation. The observed altered growth properties mediated by G alpha 12 showed a certain degree of dependency on serum factors, and its mitogenic potential was also potently inhibited by suramin treatment.

Expression cDNA cloning of a transforming gene encoding the wild-type G alpha 12 gene product

Using an expression cDNA cloning approach, we examined human tumor cell lines for novel oncogenes that might evade detection by conventional techniques. We isolated a transforming sequence that was highly efficient in transforming NIH 3T3 mouse fibroblasts. DNA sequence analysis identified the gene as the human homolog of a recently cloned alpha subunit of mouse GTP-binding protein G alpha 12. NIH 3T3 cells transfected with G alpha 12 cDNA grew in soft agar and were tumorigenic in nude mice. There were no apparent mutations in the cloned cDNA in comparison with a G alpha 12 cDNA clone isolated from a normal human epithelial cell library, implying that overexpression alone was sufficient to cause NIH 3T3 cell transformation. The observed altered growth properties mediated by G alpha 12 showed a certain degree of dependency on serum factors, and its mitogenic potential was also potently inhibited by suramin treatment.

Myristoylation of the G alpha i2 polypeptide, a G protein alpha subunit, is required for its signaling and transformation functions

GTPase-inhibiting mutations of the alpha subunit (alpha i2) of the G protein, Gi2, result in constitutive activation of alpha i2 signal transduction functions. GTPase-inhibited alpha i2 mutant polypeptides, referred to as gip2 oncoproteins, have glutamine-205 mutated to leucine (alpha i2Q205L). Expression of the alpha i2Q205L polypeptide inhibits adenylyl cyclase stimulation, constitutively activates p42 mitogen-activated protein kinase, and transforms Rat 1a fibroblasts. The alpha i2 polypeptides are N-terminal-myristoylated, but the function of myristoylation is unclear in alpha i2 signal transduction. We have tested the requirement for myristoylation on the ability of the alpha i2Q205L mutant polypeptide to constitutively regulate signal pathways and cell transformation. When expressed in Rat 1a cells, the nonmyristoylated alpha i2Q205L polypeptide is membrane associated but is unable to regulate adenylyl cyclase or p42 mitogen-activated protein kinase and does not induce cellular transformation. We conclude that myristoylation is absolutely necessary for alpha i2Q205L signal transduction and regulation of effector enzymes in the cell.

Activation of 5-HT1A receptors expressed in NIH-3T3 cells induces focus formation and potentiates EGF effect on DNA synthesis

NIH-3T3 fibroblasts have been transfected with human serotonin 5-HT1A receptors. Clonal cell lines expressed between 40 and 500 fmol receptor/mg. 5-HT1A agonists strongly inhibited nonstimulated- as well as forskolin- or isoproterenol-stimulated adenylyl cyclase. The effects of 5-HT1A receptor activation on cell growth were investigated. 5-HT1A agonists accelerated cell division, generated foci, and increased DNA synthesis. The stimulation of [3H]thymidine incorporation was much stronger when tyrosine kinase receptors were activated concomitantly. Cyclic AMP (cAMP) elevating agents inhibited DNA synthesis induced by all mitogens tested. The mitogenic activity of 5-HT1A agonists did not seem to be linked to adenylyl cyclase inhibition because 1) we were not able to measure any decrease in intracellular cAMP levels under the conditions of DNA synthesis assay and 2) 2',5'-dideoxyadenosine, which strongly inhibited adenylyl cyclase, was not mitogenic and did not modify the mitogenic effects of 5-HT1A agonists. Pertussis toxin completely blocked potentiation of epidermal growth factor effect induced by 8-hydroxy-di-(n-propyl)aminotetralin, a 5-HT1A agonist, but only partially blocked the one induced by insulin. In conclusion, in transfected NIH-3T3 cells, transforming and mitogenic effects of 5-HT1A agonists involve a pertussis toxin-sensitive G protein but do not seem to be linked to adenylyl cyclase inhibition.

Mitogen-activated protein kinase activation resulting from selective oncogene expression in NIH 3T3 and rat 1a cells

Mitogen-activated protein kinases (MAPKs) are serine/threonine kinases that are rapidly activated in response to a variety of growth factors in many cell types. MAPKs are activated by phosphorylation of both tyrosine and threonine residues. They are proposed to be key integrators of growth factor receptor transduction systems involving conversion of tyrosine kinase signals to serine/threonine kinase activation. We have studied the influence of specific oncogenes on the regulation of MAPK activity in NIH 3T3 and Rat 1a fibroblasts. In NIH 3T3 cells, ras or raf oncogene expression, but not gip2 oncogene expression, induces a significant constitutive MAPK activation. In contrast, in Rat 1a cells, gip2, but not ras or raf oncogene expression, induces a strong constitutive MAPK activation. The findings indicate that, in a cell type-selective manner, different oncoproteins are capable of causing the constitutive activation of MAPK. However, the magnitude of oncogene-induced MAPK activation is not directly correlated with cellular transformation in either cell type. It appears that expression of only a subset of transforming oncogenes in a specific cell type is able to alter the regulation of the MAPK activation pathway. Thus, the network of cytoplasmic serine/threonine kinases will be differentially regulated when the same oncogene is expressed in different cell types.