PARTICIPATION OF CYCLIN-A IN MYC-INDUCED APOPTOSIS

The involvement of c-Myc in cellular proliferation or apoptosis has be en linked to differential cyclin gene expression. We observed that in both proliferating cells and cells undergoing apoptosis, cyclin A (but not B, C, D1, and E) mRNA level was elevated in unsynchronized Myc-ov erexpressing cells when compared with parental Rat1a fibroblasts. We f urther demonstrated that Zn2+ inducible cyclin A expression was suffic ient to cause apoptosis. When Myc-induced apoptosis was blocked by coe xpression of Bcl-2, the levels of cyclin C, D1, and E mRNAs were also elevated. Thus, while apoptosis induced by c-Myc is associated with an elevated cyclin A mRNA level, protection from apoptosis by coexpresse d Bcl-2 is associated with a complementary increase in cyclin C, D1, a nd E mRNAs.