The fine balance between proliferation and apoptosis plays a primary role in carcinogenesis. Proto-oncogenes that induce both proliferation and apoptosis provide a powerful inbuilt system to inhibit clonal expansion of cells with high proliferation rates. This provides a restraint to the development of neoplasms. C-myc expressing cells undergo apoptosis in low serum by an unknown mechanism. Several lines of evidence suggested that c-myc induces apoptosis by a transcriptional mechanism. However, the target genes of this program have not been fully defined. Protein synthesis inhibitors induce apoptosis in c-myc over-expressing cells at high serum levels suggesting that inhibition of synthesis of a survival factor may induce apoptosis. We show that the expression of c-myc directly correlates with an increase in the level of a survival protein, bcl-\rm x\sb{L}, and a decrease in the pro-apoptotic protein, bax, at both the protein and mRNA level. Furthermore, a significant decrease of the bcl-\rm x\sb{L} protein levels is observed under low serum conditions. In order to investigate the mechanism of regulation of bcl-\rm x\sb{L} and bax by c-myc, the bcl-x and bax promoters were cloned, sequenced and shown to contain c-myc binding sites. The chloramephenicol acetyl transferase (CAT) reporter assay was used to demonstrate activation of the bcl-x promoter by increasing levels of c-myc when co-transfected in COS cells. The bax promoter was also shown to be transrepressed in c-myc expressing cells. The role of bcl-\rm x\sb{L} in apoptosis regulation in c-myc cell lines in normal and low serum was then investigated. Cells lines expressing c-myc and bcl-\rm x\sb{L} were generated and were shown to be resistant to apoptosis induction in low serum. Furthermore, cell lines expressing c-myc, anti-sense bcl-\rm x\sb{L} and β-galactosidase demonstrated significantly enhanced rates of apoptosis in high serum compared to c-myc Rat 1a cells. These findings suggest that c-myc activates a survival program involving bcl-\rm x\sb{L} upregulation and bax downregulation. However, this survival signal is reduced under low serum conditions by the relative downregulation of bcl-\rm x\sb{L} allowing for apoptosis to proceed. These data also directly demonstrates that downregulation in the level of bcl-\rm x\sb{L} associated with low serum conditions is a critical determinant of c-myc induced apoptosis
