Transcriptional regulation of target genes by c-Myc proteins

The c-myc proto-oncogene plays a critical role in the regulation of cell proliferation. Several translational products of the c-myc gene have been identified including the major translational product c-Myc2, and the downstream initiated c-MycS. Similar to c-Myc2, c-MycS has been shown to stimulate proliferation, induce transformation and apoptosis. We show here that c-MycS induces transcription of known c-Myc targets such as CAD and eIF4e and autorepresses the c-myc gene. To further investigate transcriptional regulation by c-MycS, we screened for genes that are regulated by the inducible fusion protein c-MycS-ER.; Using Differential Display we identified GAP4, an unknown gene, and α-tropomyosin as a targets of activation by c-MycS. We also identified GABA_A-α1 (alpha1 subunit of GABA receptor, the major inhibitory neurotransmitter receptor in the CNS) as a target of repression by c-MycS. GABA_A-α1 expression is repressed 2.5 fold by c-MycS and c-Myc2 within one hour. In addition, GABA_A-α1 is repressed in the presence of cycloheximide, suggesting it is repressed directly by c-Myc. The expression of GABA _A-α1 is elevated in c-myc null fibroblasts whereas it is undetectable in their parental cells. We also found that GABA_A -α1 expression is markedly induced during growth at high cell density.; Attempts to overexpress GABA_A-α1 in fibroblasts results in massive cell death. Using immunofluorescence microscopy we show that GABA _A-α1 overexpression in NIH-3T3 and P19 cells results in activation of caspase-3 and induction of cytochrome c release, which occur during apoptosis. In addition, bcl-2 or bcl-x expression block GABA_A-α1 induced caspase-3 activation and cytochrome-c release.; The expression of both GABA_A-α1 and c-Myc has been shown to be inversely regulated during development of the brain. We further show here that during differentiation of P19 cells into neuronal cells, repression of c-Myc is accompanied by marked elevation in GABA_A-α1 expression. Our data suggest that GABA_A-α1 may play a role in regulating cell death in the developing brain.