ROLE OF C-MYC IN SIMIAN-VIRUS-40 LARGE TUMOR ANTIGEN-INDUCED DNA-SYNTHESIS IN QUIESCENT 3T3-L1 MOUSE FIBROBLASTS

Stably transfected NIH 3T3-L1 mouse fibroblasts (L1 cells) expressing the simian virus 40 large tumor antigen (LTAg) maintain c-myc expressi on and proliferation in low serum, whereas cells expressing the mutant form LTAg-K1, defective in binding of the retinoblastoma suppressor g ene product pRb, showed reduced levels of c-myc RNA and only backgroun d levels of DNA synthesis in low serum. The role of the c-Myc protein in LTAg-induced DNA synthesis was studied in microinjection experiment s. Expression of LTAg induced cellular DNA synthesis in >95% of microi njected serum-starved L1 cells, whereas the mutant LTAg-K1 could not i nduce DNA synthesis. Coexpression of dominant negative c-Myc or Max mu tants with LTAg inhibited DNA synthesis, indicating that functional c- Myc is necessary for induction of DNA synthesis by LTAg. Expression of c-Myc induced programmed cell death (apoptosis) in serum-starved L1 c ells. Coexpression of c-Myc with LTAg-K1 restored induction of DNA syn thesis without apoptosis. Expression of a truncated LTAg, LTAg-(1-259) , defective in binding of the tumor suppresser gene product p53, faile d to prevent c-Myc-induced apoptosis. The data indicate that c-Myc can restore the ability of LTAg-K1 to induce DNA synthesis and that LTAg- K1 prevents c-Myc-induced apoptosis in serum-starved L1 cells by its i nteraction with p53.