DEVELOPMENT OF CELLULAR-RESISTANCE TO PP60(V-SRC) KINASE-INDUCED CELL-DEATH

The v-src gene of Rous sarcoma virus (RSV) encodes pp60(v-src) a tyros ine kinase that can initiate cellular transformation. High levels of v -src gene expression can either be cytotoxic or the cause of altered e xpression of cellular genes, Examination of cytotoxic thresholds is di fficult because cells expressing high levels of a cytotoxic oncogene w ill die. To evaluate quantitatively the cytotoxicity of pp60(v-src) on growth, we amplified two different v-src genes, under the control of the human hsp70B heat shock promoter to establish cell clones with var ying copy numbers of the heat-inducible v-src gene. The viability of c ell lines over a prolonged period of time depended on the particular s rc gene, the expression of v-src mRNA, synthesis of the pp60(v-src) pr otein and, most importantly, the tyrosine kinase activity of the pp60( v-src) protein. We found a relatively sharp threshold in v-src-encoded tyrosine kinase activity above which cell viability rapidly declines. However, over time, tyrosine kinase activity was exponentially suppre ssed at about a 10-fold higher rate than pp60(v-src) protein during pa ssage. Our results indicate that homeostasis: of tyrosine phosphorylat ion is important for cell viability, that perturbation of this balance results in cell mortality, and that cells can evolve to accommodate o verexpression of oncogene by downregulating the level of tyrosine kina se activity.