Stimulation of pp60c-src tyrosyl kinase activity in polyoma virus-infected mouse cells is closely associated with polyoma middle tumor antigen synthesis

Activation of human pp60c-src

pp60c-src and the structurally related members of the Src family are non-receptor tyrosine kinases that reside within the cell associated with cell membranes and appear to transduce signals from transmembrane receptors to the cell interior. Many intracellular pathways can be stimulated upon Src activation, and a variety of cellular consequences can result, including morphological changes and cell proliferation. pp60c-src activity is normally suppressed by phosphorylation on its carboxy-terminal tail by an enzyme known as CSK. Various cellular stimuli or mutations within pp60c-src can activate its endogenous kinase activity. In this paper, we review aspects of pp60c-src activation and regulation and discuss results obtained in our laboratory in two experimental systems: (i) in melanoma cell lines and primary pigmented normal human melanocytes and (ii) using activated mutant forms of purified human pp60c-src protein.

pp60c-src kinase expression in brain of adult rats in relation to age

Although there is evidence of alterations in brain protein phosphorylation patterns with age, it is not known if the protein kinases that phosphorylate only at tyrosine residues are involved in these changes. For this reason, we examined the age-related expression of pp60c-src, a tyrosine protein kinase enriched in neural tissues, in whole brain of adult Fischer-344 rats. The pp60c-src kinase activity was immunoprecipitated using a monoclonal antibody and the incorporation of [32P] from radiolabeled ATP into an exogenous substrate (casein) measured. The results showed that there was a substantial amount of pp60c-src kinase activity in brain of the adult animals ranging in age from 4 to 23 months and that it was not significantly different among these groups. Also, immunoprecipitates obtained under conditions of monoclonal antibody excess and utilized for immunoblot analysis indicated that the relative levels of the pp60c-src protein were unchanged in the same animals. These results suggest that, at the whole brain level, the pp60c-src kinase has a stable turnover and that a high amount of activity is biologically important in brain of adult rats through early senescence.

Mutants of polyomavirus middle-T antigen

Polyomavirus middle-T antigen induces the transformation of established cell lines in culture and is known to interact with and/or modulate the activity of several enzymes (pp60c.src, protein kinase C and phosphatidylinositol kinase) in vitro. This review is a compilation of the reported mutants of middle-T antigen and their biochemical and biological properties as they relate to the transformation event. The mutants of polyomavirus middle-T antigen have been previously classified phenotypically. Given the now large number of mutants, the classification presented here is based upon the position within the molecule. A model of middle-T is presented in which the protein is considered as consisting of three domains: a hydrophobic domain (the putative membrane-binding domain), the amino-terminal half of the molecule (the putative pp60c.src-binding domain) and the intervening amino acids (the putative modulatory domain). A current model for the induction of transformation by polyomavirus middle-T is presented.

Analysis of middle tumor antigen and pp60c-src interactions in polyomavirus-transformed rat cells

The relative abundance of pp60c-src molecules associated with polyomavirus (Py) middle tumor antigen (MTAg) and the relative abundance of MTAg associated with pp60c-src in a variety of Py-transformed rat cells was determined by quantitative immunoblot analyses which detect pp60c-src or Py MTAg. The results demonstrate that approximately 5 to 10% of the total immunoprecipitable pp60c-src molecules in Py-transformed rat cells are stably associated with MTAg and have elevated protein kinase activities. In these same cells, it was found that approximately 10 to 15% of the detectable MTAg molecules are stably associated with pp60c-src. Other results presented in this report demonstrate that approximately 50 to 75% of the total MTAg-associated cellular tyrosine kinase activity potentially represents the enzymatic activity of pp60c-src, while the remaining 25 to 50% represents the activity of other cellular tyrosine kinases. Our results also show that most pp60c-src molecules associated with Py MTAg do not possess electrophoretic mobilities that are altered from those of pp60c-src molecules not associated with MTAg or pp60c-src molecules obtained from normal rodent cells.

Analysis of polyomavirus middle-T-antigen-transformed rat cell variants expressing different levels of pp60c-src

We characterize two independent variant cellular clones which arose following in vitro passage of polyomavirus middle-T-antigen (MTAg)-transformed FR3T3 cells expressing RNA complementary to c-src mRNA. These clones were initially flat and underwent morphologic transformation at a high frequency to a phenotype indistinguishable from that of parental MTAg-transformed FR3T3 cells. Biochemical analysis of the flat clones prior to phenotypic conversion revealed that these cells synthesized little detectable pp60c-src and had correspondingly low levels of pp60c-src protein kinase activity and MTAg-associated protein kinase activity. The flat cell clones did not possess detectable focus-forming activity, were not capable of detectable anchorage-independent growth, and had saturation densities and doubling times below those normally observed for FR3T3 cells. Following conversion of the flat clones to a shape resembling that of typical MTAg-transformed cells, the abundance of pp60c-src, pp60c-src kinase activity, and MTAg-associated in vitro protein kinase activity were all restored to the levels found in the parental MTAg transformants. These cells had growth rates, focus-forming activities, anchorage-independent growth rates, and saturation densities similar to those of the parental MTAg-transformed rat cells. These data provide additional evidence that maintenance of a transformed phenotype by polyomavirus MTAg in established rat cell lines depends, at least in part, on a minimal threshold level of pp60c-src.

Activation of pp60c-src protein kinase activity in human colon carcinoma

The tyrosine-specific protein kinase activity of pp60c-src molecules obtained from human colon carcinoma tissues and tumor-derived cell lines was found to be elevated over that from normal colon tissues or cultures of normal colon mucosal cells. The elevated pp60c-src protein kinase activity in tumor tissues and in cultured colon carcinoma cells does not appear to result solely from an increase in the abundance of the c-src-encoded protein, suggesting that the specific activity of the pp60c-src tyrosine phosphotransferase is enhanced. These results raise the possibility that activation of the pp60c-src protein kinase may contribute to the genesis of human colon tumors.

Regulation of pp60c-src synthesis by inducible RNA complementary to c-src mRNA in polyomavirus-transformed rat cells

To determine the potential role of pp60c-src in polyomavirus-transformed cells, we constructed a recombinant plasmid with the mouse metallothionein-I promoter upstream of a src gene in an anti-sense orientation. We cotransfected this plasmid into middle tumor antigen-transformed FR3T3 cells with a plasmid containing the neomycin resistance gene, and G418 resistant colonies were selected. Analysis of these cells for pp60c-src expression revealed that 50 of the 200 cellular clones screened were found to have decreased levels of c-src expression when compared with the parental middle tumor antigen-transformed cells. Three independent clones which transcribed the expected 3.6-kilobase src complementary RNA and had levels of pp60c-src kinase activity comparable to that of normal FR3T3 cells were further analyzed. In the presence of Cd2+, these clones grew significantly slower in monolayer cultures than either the parental transformed cells (FR18-1) or FR18-1 cells transfected with the neomycin resistance gene alone. The morphology of these clones in the presence of Cd2+ was distinct from that of either the parental FR18-1 cells or normal FR3T3 cells. The clones expressing the complementary src RNA were found to form fewer colonies in soft agar, form fewer foci on monolayers of normal rat cells, and form tumors more slowly following injection into syngenic rats when compared with parental FR18-1 cells. The results of these studies suggest that the level of pp60c-src kinase activity affects the growth characteristics and transformation properties of polyoma virus-transformed rat cells.

Regulation of pp60c-src synthesis by inducible RNA complementary to c-src mRNA in polyomavirus-transformed rat cells

To determine the potential role of pp60c-src in polyomavirus-transformed cells, we constructed a recombinant plasmid with the mouse metallothionein-I promoter upstream of a src gene in an anti-sense orientation. We cotransfected this plasmid into middle tumor antigen-transformed FR3T3 cells with a plasmid containing the neomycin resistance gene, and G418 resistant colonies were selected. Analysis of these cells for pp60c-src expression revealed that 50 of the 200 cellular clones screened were found to have decreased levels of c-src expression when compared with the parental middle tumor antigen-transformed cells. Three independent clones which transcribed the expected 3.6-kilobase src complementary RNA and had levels of pp60c-src kinase activity comparable to that of normal FR3T3 cells were further analyzed. In the presence of Cd2+, these clones grew significantly slower in monolayer cultures than either the parental transformed cells (FR18-1) or FR18-1 cells transfected with the neomycin resistance gene alone. The morphology of these clones in the presence of Cd2+ was distinct from that of either the parental FR18-1 cells or normal FR3T3 cells. The clones expressing the complementary src RNA were found to form fewer colonies in soft agar, form fewer foci on monolayers of normal rat cells, and form tumors more slowly following injection into syngenic rats when compared with parental FR18-1 cells. The results of these studies suggest that the level of pp60c-src kinase activity affects the growth characteristics and transformation properties of polyoma virus-transformed rat cells.

A subclass of polyomavirus middle tumor antigen binds to DNA cellulose

We examined the binding of polyomavirus large (L-T)-, middle (M-T)-, and small-tumor antigens to DNA cellulose. At pH 6.0, the majority of L-T bound to calf thymus DNA cellulose, while little or no small tumor antigen was retained under these conditions. Unexpectedly, a small but reproducible proportion of M-T bound to both native and denatured DNA cellulose. M-T encoded by polyomavirus mutant dl 8, which expressed shortened L-T and M-T, bound to DNA, indicating that the deleted sequences are not required for DNA binding. Also, M-T from transformed BMT-1 rat cells, which synthesize exclusively this polyomavirus tumor antigen, bound to DNA, indicating that its binding is not due to association with other polyomavirus-encoded proteins. Using the DNA fragment immunoassay, we found that, under conditions in which L-T bound specifically to DNA fragments containing viral regulatory sequences, no viral DNA fragments were bound by M-T. The existence of distinct subpopulations of M-T that differ in their DNA-binding properties was indicated by rebinding experiments in which M-T that had bound to DNA cellulose rebound very efficiently, while that which had not been originally retained by DNA cellulose rebound poorly. Furthermore, the M-T-pp60 c-src complex did not bind to DNA cellulose. These data suggest that polyomavirus M-T is heterogeneous, consisting of populations of molecules that differ in their interactions with DNA cellulose.

Analysis of pp60c-src protein kinase activity in hamster embryo cells transformed by simian virus 40, human adenoviruses, and bovine papillomavirus 1

We have examined the effect of DNA tumor virus transformation of primary hamster embryo cells on the tyrosyl kinase activity of pp60c-src. Our present study demonstrates that some clones of hamster embryo cells transformed by simian virus 40, adenovirus type 2, adenovirus type 12, or bovine papillomavirus 1 can possess elevated pp60c-src kinase activity when compared with normal hamster embryo cells. However, other clones of hamster embryo cells transformed by these same viruses were found to have normal levels of pp60c-src kinase activity. In those clones of transformed cells where pp60c-src kinase activity was elevated, the increased levels of kinase activity were the result of an apparent increase in the specific activity of the pp60c-src phosphotransferase rather than an increase in the amount of the src gene product. Additionally, pp60c-src was not found to be physically associated with tumor antigens known to be encoded by these viruses. These results indicate that elevated levels of pp60c-src kinase activity can be found in hamster embryo cells transformed by several different DNA tumor viruses and suggest that the molecular mechanism by which pp60c-src kinase activity is elevated may differ from that previously observed in polyomavirus-transformed cells. These results also imply that elevation of pp60c-src kinase activity is not required for the transformation of hamster cells by these viruses.

Increased pp60c-src tyrosyl kinase activity in human neuroblastomas is associated with amino-terminal tyrosine phosphorylation of the src gene product

We have observed a 20- to 40-fold increase in pp60c-src tyrosyl kinase activity in human neuroblastoma cell lines over that found in either human glioblastoma cells or human fibroblasts. The level of c-src gene transcripts and pp60c-src protein synthesis in the neuroblastoma cells was not significantly increased when compared to the levels found in glioblastoma cells. Approximately one-half of the pp60c-src molecules synthesized during a 4-hr [35S]methionine or [32P]orthophosphate labeling period in neuroblastoma cells were found to migrate more slowly on NaDodSO4/polyacrylamide gels than pp60c-src molecules labeled in glioblastoma cells. Peptide and phosphoamino acid analysis of the in vivo phosphorylated c-src molecules from these two cell types revealed that pp60c-src molecules from the neuroblastoma cells possess in the amino-terminal portion of the protein at least one unique tyrosine phosphorylation site not found in pp60c-src derived from glioblastoma cells.