Constitutive expression of c-fos antisense RNA blocks c-fos gene induction by interferon and by phorbol ester and reduces c-myc expression in F9 embryonal carcinoma cells.

To address the role of c-fos proto-oncogene we constructed a plasmid that allows constitutive expression of RNA complementary to c-fos mRNA, and stably introduced this plasmid into F9 embryonal carcinoma cells. Some F9 clones expressing c-fos antisense RNA had a reduced basal level of c-fos mRNA, and were unable to induce a c-fos mRNA as well as its protein when stimulated with phorbol ester or with interferon (IFN). Nevertheless, the ability to induce major histocompatibility class I genes following IFN treatment was not impaired in these clones. Clones expressing c-fos antisense RNA grew as rapidly as control F9 cells, and underwent differentiation after retinoic acid treatment. Unexpectedly, constitutive expression of c-myc mRNA was reduced on average by 10-fold in clones expressing c-fos antisense RNA. However, expression of the p53 gene and heat shock gene hsp 70 was not affected in these clones, indicating the existence of a specific regulatory linkage between c-fos and c-myc genes. Cycloheximide treatment led to induction of a large amount of c-fos mRNA in clones expressing c-fos antisense RNA as well as in control F9 clones. The amount of c-fos antisense RNA was also increased by cycloheximide treatment. We postulate that c-fos antisense RNA blocks expression of the endogenous c-fos gene by accelerating the degradation of c-fos mRNA and that cycloheximide treatment interferes with this degradation.