p21 and bax: Two novel targets of c -Myc

The molecular mechanism that c-Myc uses to initiate cellular proliferation and/or programmed cell death is not completely understood. Much attention has focused on the pathways involved in each of these biological functions. The present study identifies two novel targets of c-Myc, p21 and bax, involved in cell cycle regulation and the induction of apoptosis, respectively. p21 is a well characterized inhibitor of cell cycle progression, while bax is a proapoptotic member of the bcl-2 family of proteins. To study the effects of c-Myc on cell cycle progression in human cells a c-Myc-expressing adenovirus (Ad-cMyc) was generated. We show here that overexpression of c-Myc in cells growth arrested by serum-deprivation lead to the abrogation of cell cycle arrest. We see that exogenous c-Myc inhibits p21 expression in SkBr3 and LNCaP cells induced to enter into S-phase. A time course after infection of TPA-arrested cells with the Ad-cMyc revealed that the inhibition of p21 expression preceded entry into S-phase. Overexpression of c-Myc reduced the levels of endogenous p21 mRNA, and transfection of c-Myc repressed p21-promoter Iuciferase-reporter gene expression. The results suggest that the downregulation of p21 expression may contribute to c-Myc-dependent entry into S-phase, possibly in situations in which growth arrest is associated with increased p21 expression. Using the aforementioned c-Myc adenovirus, we also show that overexpressed c-Myc upregulates endogenous pro-apoptotic bax gene expression in human cells. Furthermore, c-Myc transcriptionally regulates bax through binding to E-box elements located in the bax promoter region. Overexpression of c-Myc also leads to apoptosis which is significantly reduced in bax −/− as compared to bax +/+ mouse embryonic fibroblasts. The results suggest that the cell death-promoting gene bax is a potential mediator of c-Myc-induced apoptosis through transcriptional control