EXPRESSION OF THE V-SRC ONCOPROTEIN IN FIBROBLASTS DISRUPTS NORMAL REGULATION OF THE CDK INHIBITOR P27 AND INHIBITS QUIESCENCE

To determine how an oncogenic tyrosine kinase disturbs cell cycle cont rol we examined expression of cell cycle proteins and growth of fibrob lasts reversibly transformed by a temperature sensitive mutant of v-Sr c (ts LA 29). ts LA 29 Rat-1 cells and normal Rat-1 cells had similar growth rates but the transformed cells traversed the G(1) phase of the cell cycle more rapidly and failed to exit cycle efficiently in respo nse to serum starvation and cell confluence. Cyclin D1 and cyclin E le vels were not elevated in growing ts LA 29 Rat-1 cells and the abbrevi ated GI was further accelerated by overexpression of cyclin E. A fall in cyclin E and cyclin A dependent kinase activities in Rat-1 cells in response to inhibitory growth conditions was abrogated in ts LA 29 Ra t-1 cells and correlated with lack of p27 accumulation or cyclin A dow n regulation, the latter due to sustained cyclin A promoter activity. The expression of p27 mRNA was lower in ts LA 29 Rat-1 cells than Rat- 1 cells and was elevated following v-Src inactivation concurrent with an increase in p27 promoter activity and temporary cell cycle exit. Th e suppression of mRNA or transcription is a novel way an oncoprotein c an induce down regulation of p27 and contributes to the G(1) shortenin g and perturbed cell cycle regulation of the v-Src transformed cells.