Expression of the v-Src oncoprotein in fibroblasts disrupts normal regulation of the CDK inhibitor p27 and inhibits quiescence

Abstract

To determine how an oncogenic tyrosine kinase disturbs cell cycle control we examined expression of cell cycle proteins and growth of ®broblasts reversibly transformed by a temperature sensitive mutant of v-Src (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 e ciently in response to serum starvation and cell con¯uence. Cyclin D1 and cyclin E levels were not elevated in growing ts LA 29 Rat-1 cells and the abbreviated G 1 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 Rat-1 cells and correlated with lack of p27 accumulation or cyclin A down 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. The suppression of mRNA or transcription is a novel way an oncoprotein can induce down regulation of p27 and contributes to the G 1 shortening and perturbed cell cycle regulation of the v-Src transformed cells.

Induction of p21WAF1/CIP1 and cyclin D1 expression by the Src oncoprotein in mouse fibroblasts: role of activated STAT3 signaling

“…Consistent with this possibility, earlier studies have shown that ectopic overexpression of cyclin D1 resulted in increased levels of p21 WAF1/CIP1 in rodent ®broblasts and human gliomas (Hiyama et al, 1997). Second, other studies have indicated that overexpression of cyclin D1 and E causes a shortening of G1 phase (Liu et al, 1995;Johnson et al, 1998a). Thus, the increased p21 WAF1/CIP1 levels may be part of a regulatory mechanism for restricting the CDK-associated kinase activities within a limited range, which in turn may ensure viability of the cells by preventing premature initiation of DNA synthesis.…”

“…There is evidence that v-Src transformation results in altered expression of cyclins and CKIs, suggesting that Src may induce oncogenesis at least in part by modulating the expression of critical components of cell cycle regulation (Johnson et al, 1998a;Lee et al, 1999). Furthermore, other studies indicate that STATs regulate expression of the p21 WAF1/CIP1 and cyclin D genes (Chin et al, 1996;Matsumura et al, 1997;Bellido et al, 1998;Johnson et al, 1998b;Florenes et al, 1999;Wen et al, 1999).…”

Activated Src abrogates the Myc requirement for the G 0 /G 1 transition but not for the G 1 /S transition

“…2A). Both Gas1 and p27 Kip1 have been shown to be repressed by both Myc and Src at the transcriptional level (14)(15)(16). Consistent with these observations, loss of c-myc led to induction of Gas1 and p27 mRNAs.…”

Limiting amounts of p27Kip1 correlates with constitutive activation of cyclin E-CDK2 complex in HTLV-I-transformed T-cells

“…Low levels of p27 Kip1 have been correlated with cell transformation (Johnson et al, 1998). The level of p27 Kip1 mRNA and protein expression was analysed in ten HTLV-I-infected T-cell lines, four IL-2 dependent (N1186, N1185, LAF, E55) and six IL-2 independent (C91/PL, MJ.…”