Transformation by pp60src or stimulation of cells with epidermal growth factor induces the stable association of tyrosine-phosphorylated cellular proteins with GTPase-activating protein.

GTPase-activating protein (GAP) is a cytosolic protein that stimulates the rate of hydrolysis of GTP (GTP to GDP) bound to normal p21ras, but does not catalyze the hydrolysis of GTP bound to oncogenic, activated forms of the ras protein. Transformation of cells with v-src or activated transforming variants of c-src or stimulation of cells with epidermal growth factor resulted in the stable association of GAP with two tyrosine-phosphorylated cellular proteins of 64 kDa (p64) and 190 kDa (p190). Analysis of GAP immune complexes isolated from extracts of metabolically labeled src-transformed cells and epidermal growth factor-stimulated cells indicated that tyrosine phosphorylation of p64 and p190 appeared to be coincident with the stable association of these proteins with GAP. Quantitation of the amount of p64 associated with GAP in v-src-transformed cells, however, indicated that only 15 to 25% of tyrosine-phosphorylated p64 was found in complex with GAP. Mutations within the SH2 region of pp60src that render activated pp60src defective for transformation inhibited the efficient formation of complexes between GAP and the tyrosine-phosphorylated forms of p64 and p190. From these data, we suggest that tyrosine phosphorylation and stable association of p64 with GAP is an important step in mediating cellular signaling through the p21ras-GAP pathway.     

Inhibition of v-src-induced transformation by a GTPase-activating protein.

Previous work has shown that microinjection into cells of antibodies against p21ras blocks transformation by src, suggesting that oncogenic transformation by pp60v-src is dependent on p21ras. The activity of p21ras itself is regulated by its cyclic association with GDP-GTP, where p21ras-GTP is the active form and p21ras-GDP is the inactive form. A GTPase-activating protein (GAP) mediates the inactivation of p21ras by facilitating the conversion of the active p21ras-GTP to the inactive p21ras-GDP. This predicts that overexpression of GAP would inactivate p21ras and block transformation of cells by src. In this paper, we confirm this prediction. We report that overexpression of GAP in NIH 3T3 cells blocks transformation by pp60v-src but not by v-ras. Susceptibility to transformation by v-src is restored when GAP expression is lowered to levels comparable to that in control cells. These results support the suggestion that p21ras plays a central role in the signalling pathway used by pp60v-src.     

Activation of the oncogenic potential of the avian cellular src protein by specific structural alteration of the carboxy terminus.

The role of tyrosine phosphorylation in the regulation of tyrosine protein kinase activity was investigated using site-directed mutagenesis to alter the structure and environment of the three tyrosine residues present in the C terminus of avian pp60c-src. Mutations that change Tyr 527 to Phe or Ser activate in vivo tyrosine protein kinase activity and induce cellular transformation of chicken cells in culture. In contrast, alterations of tyrosine residues present at positions 511 or 519 in c-src do not induce transformation or in vivo tyrosine protein kinase activity. Amber mutations, which alter the structure of the pp60c-src C terminus by inducing premature termination of the c-src protein at either residue 518 or 523 also induce morphological transformation and increase in vivo tyrosine phosphorylation, whereas removal of the last four residues of c-src by chain termination at residue 530 does not alter the kinase activity or the biological activity of the resultant c-src protein. We conclude from these studies that C-terminal alterations which either remove or replace Tyr 527 serve to activate the c-src protein resulting in cellular transformation and increased in vivo tyrosine protein kinase activity.     

Cell transformation by pp60c-src mutated in the carboxy-terminal regulatory domain

We introduced two mutations into the carboxy-terminal regulatory region of chicken pp60c-src. One, F527, replaces tyrosine 527 with phenylalanine. The other, Am517, produces a truncated pp60c-src protein lacking the 17 carboxy-terminal amino acids. Both mutant proteins were phosphorylated at tyrosine 416 in vivo. The specific activity of the Am517 mutant protein kinase was similar to that of wild-type pp60c-src whereas that of the F527 mutant was 5- to 10-fold higher. Both mutant c-src genes induced focus formation on NIH 3T3 cells, but the foci appeared at lower frequency, and were smaller than foci induced by polyoma middle tumor antigen (mT). The wild-type or F527 pp60c-src formed a complex with mT, whereas the Am517 pp60c-src did not. The results suggest that one, inability to phosphorylate tyrosine 527 increases pp60c-src protein kinase activity and transforming ability; two, transformation by mT involves other events besides lack of phosphorylation at tyrosine 527 of pp60c-src; three, activation of the pp60c-src protein kinase may not be required for transformation by the Am517 mutant; and four, the carboxyl terminus of pp60c-src appears to be required for association with mT.     

Altered sites of tyrosine phosphorylation in pp60c-src associated with polyomavirus middle tumor antigen.

We characterized the tyrosine phosphorylation sites of free pp60c-src and of pp60c-src associated with the polyomavirus middle tumor antigen (mT) in transformed avian and rodent cells. The sites of tyrosine phosphorylation in the two populations of pp60c-src were different, both in vitro and in vivo. Free pp60c-src was phosphorylated in vitro at a single site, tyrosine 416. pp60c-src associated with mT was phosphorylated in vitro on tyrosine 416 and on one or more additional tyrosine residues located in the amino-terminal region of the molecule. Free pp60c-src in polyomavirus mT-transformed cells was phosphorylated in vivo on tyrosine 527. In contrast, pp60c-src associated with mT was phosphorylated in vivo on tyrosine 416 and not detectably on tyrosine 527. Thus, the in vivo phosphorylation sites of pp60c-src associated with mT in transformed cells are identical to those of pp60v-src, the Rous sarcoma virus transforming protein. The results suggest that altered phosphorylation of pp60c-src associated with mT may play a role in the enhancement of the pp60c-src protein kinase activity and in cell transformation by polyomavirus.     

Regulation of the oncogenic activity of the cellular src protein requires the correct spacing between the kinase domain and the C-terminal phosphorylated tyrosine (Tyr-527).

Repression of the tyrosine kinase activity of the cellular src protein (pp60c-src) depends on the phosphorylation of a tyrosine residue (Tyr-527) near the carboxy terminus. Tyr-527 is located 11 residues C terminal from the genetically defined end of the kinase domain (Leu-516) and is therefore in a negative regulatory region. Because the precise sequence of amino acids surrounding Tyr-527 appears to be unimportant for regulation, we hypothesized that the conformational constraints induced by phosphorylated Tyr-527 may require the correct spacing between the kinase domain (Leu-516) and Tyr-527. In this report, we show that deletions at residue 518 of two, four, or seven amino acids or insertions at this residue of two or four amino acids activated the kinase activity and thus the transforming potential of pp60c-src. As is the case for the prototype transforming variant, pp60527F, activation caused by these deletions or insertions was abolished when Tyr-416 (the autophosphorylation site) was changed to phenylalanine. In comparison with wild-type pp60c-src, the src proteins containing the alterations at residue 518 showed a lower phosphorylation state at Tyr-527 regardless of whether residue 416 was a tyrosine or a phenylalanine. Mechanisms dealing with the importance of spacing between the kinase domain and Tyr-527 are discussed.     

Activation and suppression of pp60c-src transforming ability by mutation of its primary sites of tyrosine phosphorylation

pp60c-src is phosphorylated in vivo at tyrosine 527, a residue not present in pp60v-src (its transforming homolog), and not at tyrosine 416, its site of in vitro autophosphorylation. To test the hypothesis that tyrosine phosphorylation regulates pp60c-src biological activity, we constructed and studied pp60c-src mutants in which Tyr 527 and Tyr 416 were separately or coordinately altered to phenylalanine. Tyr----Phe 527 mutation strongly activated pp60c-src transforming and kinase activities, whereas the additional introduction of a Tyr----Phe 416 mutation suppressed these activities. Tyr----Phe 416 mutation of normal pp60c-src eliminated its partial transforming activity, which suggests that transient or otherwise restricted phosphorylation of Tyr 416 is important for pp60c-src function even though stable phosphorylation is not observed in vivo.     

Functional role of GTPase-activating protein in cell transformation by pp60v-src.

Morphological transformation of NIH 3T3 cells was observed following coexpression of a portion of the ras GTPase-activating protein (GAP) comprising the amino terminus (GAP-N) and a mutant of v-src (MDSRC) lacking the membrane-localizing sequence. Cells expressing either of these genes alone remained nontransformed. Coexpression of GAP-N with MDSRC did not alter the subcellular localization, kinase activity, or pattern of cellular substrates phosphorylated by the MDSRC product. In contrast to SHC, phospholipase C-gamma 1, and the p85 alpha phosphatidylinositol 3'-kinase subunit, the endogenous GAP product (p120GAP) was highly tyrosine-phosphorylated only in cells transformed by wild-type v-src. Furthermore, for transformation induced by wild-type v-src as well as by coexpression of MDSRC and GAP-N, a strict correlation was observed between cell transformation, elevated tyrosine phosphorylation of p62, p190, and a novel protein of 150 kDa, and complex formation between these proteins and p120GAP. As with cells transformed by wild-type v-src, the MDSRC plus GAP-N transformants remained dependent on endogenous Ras. The results suggest that tyrosine phosphorylation and complex formation involving p120GAP represent critical elements of cell transformation by v-src and that complementation of the cytosolic v-src mutant by GAP-N results, at least in part, from the formation of these complexes.     

Polyamines are essential for cell transformation by pp60v-src: delineation of molecular events relevant for the transformed phenotype

Ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, becomes upregulated during cell proliferation and transformation. Here we show that intact ODC activity is needed for the acquisition of a transformed phenotype in rat 2R cells infected with a temperature-sensitive mutant of Rous sarcoma virus. Addition of the ODC inhibitor alpha-difluoromethyl ornithine (DFMO) to the cells (in polyamine-free medium) before shift to permissive temperature prevented the depolymerization of filamentous actin and morphological transformation. Polyamine supplementation restored the transforming potential of pp60v-src. DFMO did not interfere with the expression of pp60v-src or its in vitro tyrosine kinase activity. The tyrosine phosphorylation of most cellular proteins, including ras GAP, did not either display clear temperature- or DFMO-sensitive changes. A marked increase was, however, observed in the tyrosine phosphorylation of phosphatidylinositol 3-kinase and proteins of 33 and 36 kD upon the temperature shift, and these hyperphosphorylations were partially inhibited by DFMO. A DFMO-sensitive increase was also found in the total phosphorylation of calpactins I and II. The well-documented association of GAP with the phosphotyrosine-containing proteins p190 and p62 did not correlate with transformation, but a novel 42-kD tyrosine phosphorylated protein was complexed with GAP in a polyamine- and transformation-dependent manner. Further, tyrosine phosphorylated proteins of 130, 80/85, and 36 kD were found to coimmunoprecipitate with pp60v-src in a transformation-related manner. Altogether, this model offers a tool for sorting out the protein phosphorylations and associations critical for the transformed phenotype triggered by pp60v- src, and implicates a pivotal role for polyamines in cell transformation.     

v-src mutations outside the carboxyl-coding region are not sufficient to fully activate transformation by pp60c-src in NIH 3T3 cells.

Previous studies have shown that carboxyl-terminal mutation of pp60c-src can activate its transforming ability. Conflicting results have been reported for the transforming ability of pp60c-src mutants having only mutations outside its carboxyl-terminal region. To clarify the effects of such mutations, we tested the activities of chimeric v(amino)- and c(carboxyl)-src (v/c-src) proteins at different dosages in NIH 3T3 cells. The focus-forming activity of Rous sarcoma virus long terminal repeat (LTR)-src expression plasmids was significantly reduced when the v-src 3' coding region was replaced with the corresponding c-src region. This difference was masked when the Rous sarcoma virus LTR was replaced with the Moloney murine leukemia virus LTR, which induced approximately 20-fold more protein expression, but even focus-selected lines expressing v/c-src proteins were unable to form large colonies in soft agarose or tumors in NFS mice. This suggests that pp60c-src is not equally sensitive to mutations in its different domains and that there are at least two distinguishable levels of regulation, the dominant one being associated with its carboxyl terminus. v/c-src chimeric proteins expressed with either LTR had high in vitro specific kinase activity equal to that of pp60v-src but, in contrast, were phosphorylated at both Tyr-527 and Tyr-416. Total cell protein phosphotyrosine was enhanced in cells incompletely transformed by v/c-src proteins to the same extent as in v-src-transformed cells, suggesting that the carboxyl-terminal region may affect substrate specificity in a manner that is important for transformation.     

Relationship between c-src tyrosine kinase activity and the control of glucose transporter gene expression

It has been shown previously that the rate of glucose transport in fibroblasts is accelerated by oncoproteins such as v-src, ras, and the transforming protein of feline sarcoma virus. This induction of glucose transport is associated with, and presumably caused by, induction of Hep-G2/rat brain glucose transporter gene expression. To determine the mechanism underlying the induction of glucose transporter gene expression by the v-src oncogene we studied cell lines that overexpress the normal counterpart of the v-src protein (c-src), or various mutants of the c-src protein. In these mutants, the tyrosines at positions 416, 527, or 519, or various combinations of these, have been replaced by phenylalanine by site directed mutagenesis, resulting in mutated c-src proteins that possess varying tyrosine kinase activity and transforming potential. Cells that overexpress the c-src protein show no changes in glucose transporter gene expression. However, when Tyr 527 in the COOH terminus of the c-src protein is replaced with Phe, the tyrosine kinase activity and transforming potential of the protein are increased and the protein acquires a potent ability to increase levels of glucose transporter mRNA and protein, as well as the rate of 2-deoxy-D-glucose uptake. This ability is abolished by the double mutation of Tyr to Phe in positions 416 and 527, which reduces the tyrosine kinase activity of the 527 single mutant. Thus, the ability of src proteins to induce expression of the glucose transport system is linked to the tyrosine kinase activity of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin-like growth factor I receptor signaling in transformation by src oncogenes.

R- cells, a line of mouse embryo fibroblasts with a targeted disruption of the insulin-like growth factor I (IGF-I) receptor genes, are refractory to transformation by several viral and cellular oncogenes. Using colony formation in soft agar as a measure of full transformation, we report here that R- cells can be transformed by v-src, although they still cannot be transformed by the activated c-src527 (mutation at tyrosine 527 to phenylalanine), which readily transforms mouse embryo cells with a wild-type number of IGF-I receptors (W cells). Although v-src is a more potent inducer of tyrosine phosphorylation than c-src527, the extent of phosphorylation of either insulin receptor substrate 1 or Shc, two of the major substrates of the IGF-I receptor, does not seem sufficiently different to explain the qualitative difference in soft agar growth. v-src, however, is considerably more efficient than c-src527 in its ability to tyrosyl phosphorylate, in R- cells, the focal adhesion kinase, Stat1, and p130cas. These results indicate that v-src, but not c-src527, can bypass the requirement for a functional IGF-I receptor in the full transformation of mouse embryo fibroblasts and suggest that qualitative and quantitative differences between the two oncogenes can be used to identify some of the signals relevant to the mechanism(s) of transformation.     

Dephosphorylation or antibody binding to the carboxy terminus stimulates pp60c-src.

Phosphorylation of pp60c-src at Tyr-527, six residues from the carboxy terminus, has been implicated in regulation of the protein-tyrosine kinase activity of pp60c-src. Here we show that dephosphorylation of pp60c-src by phosphatase treatment in vitro caused a 10- to 20-fold increase in pp60c-src protein-tyrosine kinase activity. Binding of specific antibody to the region of pp60c-src which contains phosphotyrosine-527 also increased kinase activity. Each treatment increased phosphorylation of added substrates and of Tyr-416 within pp60c-src by a similar mechanism that involved altered interactions with ATP and increased catalytic rate. We suggest that the phosphorylated carboxy terminus acts as an inhibitor of the protein kinase domain of pp60c-src, unless its conformation is altered by either dephosphorylation or antibody binding. The antibody additionally stimulated the phosphorylation of forms of pp60c-src that had reduced gel mobility, much like those phosphorylated in kinase reactions containing pp60c-src activated by polyomavirus medium tumor antigen. These in vitro experiments provide models for the activation of pp60c-src in cells transformed by polyomavirus. We also show that autophosphorylation of pp60c-src at Tyr-527 occurs only to a very limited extent in vitro, even when Tyr-527 is made available for phosphorylation by treatment with phosphatase. This suggests that other protein-tyrosine kinases may normally phosphorylate Tyr-527 and regulate pp60c-src in the cell.     

Regulation of tetradecanoyl phorbol acetate-induced responses in NIH 3T3 cells by GAP, the GTPase-activating protein associated with p21c-ras.

Proteins of the ras family of oncogenes have been implicated in signal transduction pathways initiated by protein kinase C (PKC) and by tyrosine kinase oncogenes and receptors, but the role that ras plays in these diverse signalling systems is poorly defined. The activity of ras proteins has been shown to be controlled in part by a cellular protein, GAP (GTPase-activating protein), that negatively regulates p21c-ras by enhancing its intrinsic GTPase activity. Thus, overexpression of GAP provides a tool for determining the step(s) in signal transduction dependent on p21c-ras activity. In this paper, we report that overexpression of GAP blocks the phorbol ester (tetradecanoyl phorbol acetate [TPA])-induced activation of p42 mitogen-activated protein kinase (p42mapk), c-fos expression, and DNA synthesis. GAP overexpression did not block responses to serum or fluoroaluminate. Moreover, not all biochemical events elicited by TPA were affected by GAP overexpression, as increased glucose uptake and phosphorylation of MARCKS, a major PKC substrate, occurred normally. Reduction of GAP expression to near normal levels restored the ability of the cells to activate p42mapk in response to TPA. These findings suggest that ras and GAP together play a key role in a PKC-dependent signal transduction pathway which leads to p42mapk activation and cell proliferation.     

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.     

Mutation of amino acids in pp60c-src that are phosphorylated by protein kinases C and A.

The product of the c-src proto-oncogene, pp60c-src, is phosphorylated at Ser-17 by cyclic AMP-dependent protein kinase A and at Ser-12 by calcium-phospholipid-dependent protein kinase C (when stimulated by 12-O-tetradecanoyl phorbol acetate). We tested the effects of Ser----Ala and Ser----Glu mutations at these sites in pp60c-src and in pp60c-src(F527) (a mutant whose transforming activities are enhanced by Tyr-527----Phe mutation) by transfecting single-, double-, and triple-mutant src expression plasmids into NIH 3T3 cells. Tryptic phosphopeptide analyses of the mutant proteins confirmed prior biochemical identifications of the phosphorylation sites and showed that neither separate nor coordinate mutations at Ser-12 and Ser-17 affected Tyr-416, Tyr-527, or Ser-48 phosphorylation or prevented mitosis-specific phosphorylations of either pp60c-src or pp60c-src(F527). Ser-12 mutation did not affect phosphorylation of the Ser-17-containing peptide, but mutation of Ser-17 significantly increased phosphorylation at Ser-12. Specific kinase activities (both with and without in vivo 12-O-tetradecanoyl phorbol acetate treatment) and the abilities of pp60c-src and pp60c-src(F527) to induce foci, transformed morphologies, and anchorage-independent growth were unaffected by any of the serine mutations. Thus, pp60c-src transforming activity in NIH 3T3 cells is relatively insensitive to phosphorylation at these sites, but there is a suggestion that Ser-17 phosphorylation may have a subtle regulatory effect.     

Altered phosphorylation and activation of pp60c-src during fibroblast mitosis

At least half the pp60c-src in NIH 3T3-derived c-src overexpresser cells in modified by novel threonine and, possibly, serine phosphorylation within its amino 16 kd region during mitosis. At the same time, the tryptic phosphopeptide containing Ser 17, the site of cyclic AMP-dependent phosphorylation, is either modified or dephosphorylated. While the amount of pp60c-src is not significantly altered, the in vitro-specific kinase activity of modified pp60c-src is enhanced 4- to 7-fold. Modified pp60c-src has the same tyrosine-containing tryptic phosphopeptides as pp60c-src from unsynchronized cells, indicating that activation is independent of Tyr 416/Tyr 527 phosphorylation. Electrophoretic mobility retardations indicated that endogenous pp60c-src and pp60v-src are similarly modified during mitosis. The modifications and enhanced activity disappear near the time of cell division. These results suggest that pp60c-src is regulated by and, in turn, may regulate mitosis-specific events in fibroblasts.     

Rous sarcoma virus variants that encode src proteins with an altered carboxy terminus are defective for cellular transformation.

The src gene of Rous sarcoma virus (v-src) and its cellular homolog, the c-src gene, share extensive sequence homology. The most notable differences between these genes reside in the region encoding the carboxy terminus of the src proteins. We constructed mutations within the 3' end of the v-src gene to determine the significance of this region to the transforming potential of the v-src protein, pp60v-src. The mutants CHdl300 and CHis1511 contain mutations that alter the last 23 amino acids of pp60v-src, whereas the mutant CHis1545-C contains a linker insertion that alters the last 11 amino acids of pp60v-src, and the mutant CHis1545-H contains a linker insertion that results in a 9-amino-acid insertion at position 415. Plasmids bearing each of these mutations were unable to transform chicken cells when introduced into these cells by DNA transfection. In addition, the structurally altered src proteins encoded by the mutants had much-reduced levels of tyrosine protein kinase activity in vivo, as measured by autophosphorylation and phosphorylation of the 34,000-Mr cellular protein, and in vitro, as determined by measuring the level of pp60src autophosphorylation. These data indicate that the carboxy-terminal amino acid sequences play an important role in maintaining the structure of the catalytic domain of pp60v-src. In contrast, the transfection of chicken cells with plasmid DNA containing a chimeric v-c-src gene resulted in morphological cell transformation and the synthesis of an enzymatically active hybrid protein. Therefore, the carboxy-terminal sequence alterations observed in the c-src protein do not alone serve to alter the functional activity of a hybrid v-c-src protein appreciably.     

Stable association of activated pp60src with two tyrosine-phosphorylated cellular proteins.

We have identified two phosphotyrosine-containing cellular proteins with relative molecular masses of 130,000 (pp130) and 110,000 (pp110) daltons in chicken embryo cells that coimmunoprecipitated with pp60v-src and activated forms of chicken pp60c-src (pp60(527)F). Most if not all of the tyrosine-phosphorylated forms of pp130 and pp110 could be immunoprecipitated from lysates with any of several src protein-specific monoclonal antibodies directed against at least three spatially distinct epitopes. Consequently, of the more than 15 prominent phosphoproteins detected on immunoblots with phosphotyrosine-specific antibodies, pp130 and pp110 were selectively removed by src protein-specific immunoprecipitation, and their presence in the immunoprecipitates appears to have been due to a direct interaction with activated src proteins. src protein variants that induce different morphological phenotypes were altered in their ability to form detergent-stable complexes with pp130 and pp110 or with pp110 alone. Mutant src proteins, defective for myristylation, showed increased tyrosine phosphorylation of and association with pp110. Expression of src variants with mutations in the A box (pp60dl92/527F) or B box (pp60dl155/527F) of the src homology region induced differences in phosphorylation of pp130 and pp110, as well as changes in their association with variant src proteins. Sequences within the B-box region appeared to be necessary for stable complex formation with pp130 and pp110 and may be involved in the interaction of activated src proteins with cellular substrates.