The viral tyrosine protein kinases

Phosphoproteomics in cancer

Reversible protein phosphorylation serves as a basis for regulating a number of cellular processes. Aberrant activation of kinase signaling pathways is commonly associated with several cancers. Recent developments in phosphoprotein/phosphopeptide enrichment strategies and quantitative mass spectrometry have resulted in robust pipelines for high-throughput characterization of phosphorylation in a global fashion. Today, it is possible to profile site-specific phosphorylation events on thousands of proteins in a single experiment. The potential of this approach is already being realized to characterize signaling pathways that govern oncogenesis. In addition, chemical proteomic strategies have been used to unravel targets of kinase inhibitors, which are otherwise difficult to characterize. This review summarizes various approaches used for analysis of the phosphoproteome in general, and protein kinases in particular, highlighting key cancer phosphoproteomic studies.

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.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Activation of phosphatidylinositol 3-kinase in cells expressing abl oncogene variants

A phosphoinositide kinase specific for the D-3 position of the inositol ring, phosphatidylinositol (PI) 3-kinase, associates with activated receptors for platelet-derived growth factor, insulin, and colony-stimulating factor 1, with products of the oncogenes src, fms, yes, crk, and with polyomavirus middle T antigen. Efficient fibroblast transformation by proteins of the abl and src oncogene families requires activation of their protein-tyrosine kinase activity and membrane association via an amino-terminal myristoylation. We have demonstrated that the PI 3-kinase directly associates with autophosphorylated, activated protein-tyrosine kinase variants of the abl protein. In vivo, this association leads to accumulation of the highly phosphorylated products of PI 3-kinase, PI-3,4-bisphosphate and PI-3,4,5-trisphosphate, only in myristoylated, transforming abl protein variants. Myristoylation thus appears to be required to recruit PI 3-kinase activity to the plasma membrane for in vivo activation and correlates with the mitogenicity of the abl protein variants.

Activation of phosphatidylinositol 3-kinase in cells expressing abl oncogene variants

A phosphoinositide kinase specific for the D-3 position of the inositol ring, phosphatidylinositol (PI) 3-kinase, associates with activated receptors for platelet-derived growth factor, insulin, and colony-stimulating factor 1, with products of the oncogenes src, fms, yes, crk, and with polyomavirus middle T antigen. Efficient fibroblast transformation by proteins of the abl and src oncogene families requires activation of their protein-tyrosine kinase activity and membrane association via an amino-terminal myristoylation. We have demonstrated that the PI 3-kinase directly associates with autophosphorylated, activated protein-tyrosine kinase variants of the abl protein. In vivo, this association leads to accumulation of the highly phosphorylated products of PI 3-kinase, PI-3,4-bisphosphate and PI-3,4,5-trisphosphate, only in myristoylated, transforming abl protein variants. Myristoylation thus appears to be required to recruit PI 3-kinase activity to the plasma membrane for in vivo activation and correlates with the mitogenicity of the abl protein variants.

Monoclonal antibodies to individual tyrosine-phosphorylated protein substrates of oncogene-encoded tyrosine kinases

Cellular transformation by oncogenic retroviruses encoding protein tyrosine kinases coincides with the tyrosine-specific phosphorylation of multiple protein substrates. Previous studies have shown that tyrosine phosphorylation of a protein of 120 kDa, p120, correlated with src transformation in chicken embryo fibroblasts. Additionally, we previously identified two phosphotyrosine-containing cellular proteins, p130 and p110, that formed stable complexes with activated variants of pp60src, the src-encoded tyrosine kinase. To study transformation-relevant tyrosine kinase substrates, we have generated monoclonal antibodies to individual tyrosine phosphoproteins, including p130, p120, p110, and five additional phosphoproteins (p210, p125, p118, p85, and p185/p64). These antibodies detected several of the same tyrosine phosphoproteins in chicken embryo fibroblasts transformed by avian retroviruses Y73 and CT10, encoding the yes and crk oncogenes, respectively. Protein substrates in mouse, rat, hamster, and human cells overexpressing activated variants of chicken pp60src were also detected by several of the monoclonal antibodies.

Vimentin phosphorylation by p37mos protein kinase in vitro and generation of a 50-kDa cleavage product in v-mos-transformed cells

Previous studies have shown that vimentin, an intermediate filament protein, is reduced in amount in cells acutely infected with Moloney mouse sarcoma virus (Mo-MuSV). In this report, we provide evidence for specific alteration of vimentin in Mo-MuSV-transformed cells and demonstrate specific phosphorylation of vimentin by the p37mos protein kinase in vitro. Specificity of the phosphorylation reaction was demonstrated by using viral mos proteins encoded by various isolates of Mo-MuSV and p37mos produced in yeast. A phosphotransfer domain mutant lacking the ability to autophosphorylate p37mos failed to phosphorylate vimentin. Similarly, vimentin was not phosphorylated by the temperature-sensitive P85gag-mos kinase derived from infected cells maintained at the restrictive temperature. In ts110 MuSV-transformed NRK cells, vimentin was phosphorylated at both the permissive and nonpermissive temperatures for transformation. However, at the permissive temperature, an altered form of vimentin (about 50 kDa) with a more basic isoelectric point and lower apparent molecular weight was detected. This 50-kDa product was highly phosphorylated as compared to the bulk of the normal 55-kDa form of vimentin. On the basis of its mobility in two-dimensional gels, the 50-kDa form of vimentin should lack the carboxy terminus. This type of alteration could conceivably modulate the function of vimentin filaments in the transformed cell.

Transformation-specific tyrosine phosphorylation of a novel cellular protein in chicken cells expressing oncogenic variants of the avian cellular src gene

We used myristylated and nonmyristylated c-src-based variants and phosphotyrosine-specific antibodies to reevaluate the role of tyrosine phosphorylation in cellular transformation by pp60src. Prior methods used to detect tyrosine-phosphorylated proteins failed to discriminate predicted differences in tyrosine phosphorylation which are clearly observed with phosphotyrosine-specific antibodies and Western blotting (immunoblotting). Here we report the observation of a 120,000-Mr protein whose phosphorylation on tyrosine correlates with the induction of morphological transformation. p120 was not observed in cells overexpressing the regulated, nononcogenic pp60c-src, whereas phosphorylation of p120 was greatly enhanced in cells expressing activated, oncogenic pp60527F. Furthermore, phosphorylation of p120 was not induced by expression of the activated but nonmyristylated src variant pp602A/527F, which is transformation defective. p120 partitioned preferentially with cellular membranes, consistent with the observation that transforming src proteins are membrane associated. Although a number of additional putative substrates were identified and partially characterized with respect to intracellular localization, tyrosine phosphorylation of these proteins was not tightly linked to transformation.

The epidermal growth factor receptor from prostate cells is dephosphorylated by a prostate-specific phosphotyrosyl phosphatase

Human prostatic acid phosphatase (PAcP) has been found to have phosphotyrosyl-protein phosphatase activity (H. C. Li, J. Chernoff, L. B. Chen, and A. Kirschonbaun, Eur. J. Biochem. 138:45-51, 1984; M.-F. Lin and G. M. Clinton, Biochem. J. 235:351-357, 1986) and has been suggested to negatively regulate phosphotyrosine levels, at least in part, by inhibition of tyrosine protein kinase activity (M.-F. Lin and G. M. Clinton, Adv. Protein Phosphatases 4:199-228, 1987; M.-F. Lin, C. L. Lee, and G. M. Clinton, Mol. Cell. Biol. 6:4753-4757, 1986). We investigated the molecular interaction of PAcP with a specific tyrosine kinase, the epidermal growth factor (EGF) receptor, from prostate carcinoma cells. Of several proteins phosphorylated in membrane vesicles from prostate carcinoma cells, PAcP selectively dephosphorylated the EGF receptor. The prostate EGF receptor was more efficiently dephosphorylated by PAcP than by another phosphotyrosyl phosphatase, potato acid phosphatase. Further characterization of the interaction of PAcP with the EGF receptor revealed that the optimal rate of dephosphorylation occurred at neutral rather than at acid pH. Thus, the enzyme that we formerly referred to as PAcP we now call prostatic phosphotyrosyl-protein phosphatase. Hydrolysis of phosphate from tyrosine residues in the immunoprecipitated EGF receptor catalyzed by purified prostatic phosphotyrosyl-protein phosphatase caused a 40 to 50% decrease in the receptor tyrosine kinase activity with angiotensin as the substrate. In contrast, autophosphorylation of the receptor was associated with an increase in tyrosine kinase activity.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Transformation-specific tyrosine phosphorylation of a novel cellular protein in chicken cells expressing oncogenic variants of the avian cellular src gene

We used myristylated and nonmyristylated c-src-based variants and phosphotyrosine-specific antibodies to reevaluate the role of tyrosine phosphorylation in cellular transformation by pp60src. Prior methods used to detect tyrosine-phosphorylated proteins failed to discriminate predicted differences in tyrosine phosphorylation which are clearly observed with phosphotyrosine-specific antibodies and Western blotting (immunoblotting). Here we report the observation of a 120,000-Mr protein whose phosphorylation on tyrosine correlates with the induction of morphological transformation. p120 was not observed in cells overexpressing the regulated, nononcogenic pp60c-src, whereas phosphorylation of p120 was greatly enhanced in cells expressing activated, oncogenic pp60527F. Furthermore, phosphorylation of p120 was not induced by expression of the activated but nonmyristylated src variant pp602A/527F, which is transformation defective. p120 partitioned preferentially with cellular membranes, consistent with the observation that transforming src proteins are membrane associated. Although a number of additional putative substrates were identified and partially characterized with respect to intracellular localization, tyrosine phosphorylation of these proteins was not tightly linked to transformation.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

The epidermal growth factor receptor from prostate cells is dephosphorylated by a prostate-specific phosphotyrosyl phosphatase

Human prostatic acid phosphatase (PAcP) has been found to have phosphotyrosyl-protein phosphatase activity (H. C. Li, J. Chernoff, L. B. Chen, and A. Kirschonbaun, Eur. J. Biochem. 138:45-51, 1984; M.-F. Lin and G. M. Clinton, Biochem. J. 235:351-357, 1986) and has been suggested to negatively regulate phosphotyrosine levels, at least in part, by inhibition of tyrosine protein kinase activity (M.-F. Lin and G. M. Clinton, Adv. Protein Phosphatases 4:199-228, 1987; M.-F. Lin, C. L. Lee, and G. M. Clinton, Mol. Cell. Biol. 6:4753-4757, 1986). We investigated the molecular interaction of PAcP with a specific tyrosine kinase, the epidermal growth factor (EGF) receptor, from prostate carcinoma cells. Of several proteins phosphorylated in membrane vesicles from prostate carcinoma cells, PAcP selectively dephosphorylated the EGF receptor. The prostate EGF receptor was more efficiently dephosphorylated by PAcP than by another phosphotyrosyl phosphatase, potato acid phosphatase. Further characterization of the interaction of PAcP with the EGF receptor revealed that the optimal rate of dephosphorylation occurred at neutral rather than at acid pH. Thus, the enzyme that we formerly referred to as PAcP we now call prostatic phosphotyrosyl-protein phosphatase. Hydrolysis of phosphate from tyrosine residues in the immunoprecipitated EGF receptor catalyzed by purified prostatic phosphotyrosyl-protein phosphatase caused a 40 to 50% decrease in the receptor tyrosine kinase activity with angiotensin as the substrate. In contrast, autophosphorylation of the receptor was associated with an increase in tyrosine kinase activity.

A single point mutation has pleiotropic effects on pp60v-src function

The Rous sarcoma virus mutant tsLA29 encodes a pp60v-src molecule that is temperature sensitive for both tyrosine kinase activity and its ability to locate at the cell periphery. The defect in localization appears to be due to a perturbation in events following complex dissociation, since the mutant enzyme shows a rapidly reversible association with the cytoskeleton when shifted between permissive and restrictive temperatures. Although tsLA29 pp60v-src differs from the wild type at three amino acid residues, studies with chimeric proteins show that only one of the mutations, an alanine-for-proline substitution at residue 507, accounts for all the temperature-sensitive characteristics. Moreover, a single second site mutation, at residue 427, can restore the wild phenotype. Cells infected with a chimeric virus encoding only the alanine substitution at position 507 have a conspicuously fusiform morphology, suggesting that this mutation also has subtle effects on pp60v-src function that are apparently compensated for by the other mutations in native tsLA29.

pp60c-src expression in the developing rat brain

We have studied pp60c-src expression in the striatum, hippocampus, and cerebellum of the developing rat brain. In the striatum, pp60c-src protein kinase activity peaks during embryonic development and then declines in the adult. The peak activity occurs in the striatum on embryonic day 20 (E20) when it is 18- to 20-fold higher than the activity in fibroblasts and 4- to 5-fold higher than the activity in the striatum at E15 or in the adult striatum. In the hippocampal region, pp60c-src activity reaches a maximum shortly after birth but remains high throughout life. On postnatal day 2 (P2) the activity in the hippocampus is 9- to 13-fold higher than the activity in fibroblasts and twice as high as the activity in the hippocampus at E18. In the cerebellum, the kinase activity remains constant from E20 onward and is 6- to 10-fold higher than that observed in fibroblasts. The increase in pp60c-src kinase activity observed during the development of the striatum and hippocampus is due to an increase in the amount of pp60c-src protein and to an increase in the specific activity of the kinase. The increase in specific activity in these regions coincides with the peak periods of neurogenesis and neuronal growth. In the striatum, we have found that the increase in pp60c-src activity also parallels the increase observed in culture as embryonic striatal neurons differentiate. Taken together, our results are consonant with the idea that pp60c-src is the product of a developmentally regulated gene that is important for the differentiation and/or the continuing function of neurons.

Tyrosine phosphorylations in vivo associated with v-fms transformation

The role of tyrosine-specific phosphorylation in v-fms-mediated transformation was examined by immunoblotting techniques together with a high-affinity antibody that is specific for phosphotyrosine. This antiphosphotyrosine antibody detected phosphorylated tyrosine residues on the gp140v-fms molecule, but not gP180v-fms or gp120v-fms, in v-fms-transformed cells. This antibody also identified a number of cellular proteins that were either newly phosphorylated on tyrosine residues or showed enhanced phosphorylation on tyrosine residues as a result of v-fms transformation. However, the substrates of the v-fms-induced tyrosine kinase activity were not the characterized pp60v-src substrates. The phosphorylation of some of these cellular proteins and of the gp140fms molecule was found to correlate with the ability of v-fms/c-fms hybrids to transform cells. In addition, immunoblotting with the phosphotyrosine antibody allowed a comparison to be made of the substrates phosphorylated on tyrosine residues in various transformed cell lines. This study indicates that the pattern of tyrosine phosphorylation in v-fms-transformed cells is strikingly similar to that in v-sis-transformed cells.

Tyrosine phosphorylations in vivo associated with v-fms transformation

The role of tyrosine-specific phosphorylation in v-fms-mediated transformation was examined by immunoblotting techniques together with a high-affinity antibody that is specific for phosphotyrosine. This antiphosphotyrosine antibody detected phosphorylated tyrosine residues on the gp140v-fms molecule, but not gP180v-fms or gp120v-fms, in v-fms-transformed cells. This antibody also identified a number of cellular proteins that were either newly phosphorylated on tyrosine residues or showed enhanced phosphorylation on tyrosine residues as a result of v-fms transformation. However, the substrates of the v-fms-induced tyrosine kinase activity were not the characterized pp60v-src substrates. The phosphorylation of some of these cellular proteins and of the gp140fms molecule was found to correlate with the ability of v-fms/c-fms hybrids to transform cells. In addition, immunoblotting with the phosphotyrosine antibody allowed a comparison to be made of the substrates phosphorylated on tyrosine residues in various transformed cell lines. This study indicates that the pattern of tyrosine phosphorylation in v-fms-transformed cells is strikingly similar to that in v-sis-transformed cells.

Expression of Rous sarcoma virus transforming protein pp60v-src in Saccharomyces cerevisiae cells

The Rous sarcoma virus (RSV) pp60v-src protein was expressed in Saccharomyces cerevisiae cells either from a plasmid vector carrying the v-src gene or in yeast cells containing a single-copy v-src gene chromosomally integrated. In both yeast strains, v-src gene transcription is regulated by the galactose-inducible GAL10 promoter. Growth in galactose-containing medium resulted in constitutive expression of pp60v-src in the integrated strain and transient expression of higher levels of pp60v-src in the plasmid-bearing strain. The concentration of pp60v-src in the plasmid-bearing strain at its peak of expression was approximately threefold lower than that found in RSV-transformed mammalian cells. pp60v-src synthesized in yeast cells was phosphorylated in vivo on sites within the amino and carboxyl halves of the molecule. In immune complex kinase assays, the yeast pp60v-src was autophosphorylated on tyrosine and was able to phosphorylate exogenous substrates such as casein and enolase. The specific activity of pp60v-src synthesized in yeast cells was approximately 5- to 10-fold higher than that made in mammalian cells. Induction of pp60v-src caused the death of the plasmid-bearing yeast strain and transient inhibition of growth of the single-copy strain. Concomitantly, this induction resulted in high levels of tyrosine phosphorylation of yeast cell proteins. This indicates that pp60v-src functions as a tyrosine-specific phosphotransferase in yeast cells and suggests that hyperphosphorylation of yeast proteins is inimical to cell growth.

Expression of Rous sarcoma virus transforming protein pp60v-src in Saccharomyces cerevisiae cells

The Rous sarcoma virus (RSV) pp60v-src protein was expressed in Saccharomyces cerevisiae cells either from a plasmid vector carrying the v-src gene or in yeast cells containing a single-copy v-src gene chromosomally integrated. In both yeast strains, v-src gene transcription is regulated by the galactose-inducible GAL10 promoter. Growth in galactose-containing medium resulted in constitutive expression of pp60v-src in the integrated strain and transient expression of higher levels of pp60v-src in the plasmid-bearing strain. The concentration of pp60v-src in the plasmid-bearing strain at its peak of expression was approximately threefold lower than that found in RSV-transformed mammalian cells. pp60v-src synthesized in yeast cells was phosphorylated in vivo on sites within the amino and carboxyl halves of the molecule. In immune complex kinase assays, the yeast pp60v-src was autophosphorylated on tyrosine and was able to phosphorylate exogenous substrates such as casein and enolase. The specific activity of pp60v-src synthesized in yeast cells was approximately 5- to 10-fold higher than that made in mammalian cells. Induction of pp60v-src caused the death of the plasmid-bearing yeast strain and transient inhibition of growth of the single-copy strain. Concomitantly, this induction resulted in high levels of tyrosine phosphorylation of yeast cell proteins. This indicates that pp60v-src functions as a tyrosine-specific phosphotransferase in yeast cells and suggests that hyperphosphorylation of yeast proteins is inimical to cell growth.