Mechanisms controlling competence gene expression in murine fibroblasts stimulated with minimally modified LDL

Abstract

Mildly oxidized low density lipoprotein (minimally modified low density lipoprotein [MM-LDL]) is capable of inducing gene expression in cells of the artery wall. In this study, we investigated the mechanisms that control the mRNA expression of JE, KC, c-myc, and c-fos in quiescent mouse L-cell fibroblasts stimulated with MM-LDL. The data demonstrate that MM-LDL induces increases ≥20-fold in the levels of transcripts of these genes within 15-60 minutes. Of the four genes examined, JE and KC mRNA showed the greatest response to MM-LDL. The pattern of induction by MM-LDL is distinct from that observed in fibroblasts stimulated with serum, a known inducer of these genes. Treatment with cycloheximide (10 μg/ml) did not block the MM-LDL-induced increase in the mRNA levels of these genes. The increase of JE and KC mRNA levels in response to MM-LDL could be blocked by treatment with actinomycin D (5 μg/ml). In nuclear runoff studies, MM-LDL increased the transcription rate of JE and KC at 4 hours by 13-fold and fivefold, respectively. Small but reproducible stimulations of c-fos and c-myc transcription by MM-LDL were also observed. In addition, the half-life of JE mRNA was increased after addition of MM-LDL to fibroblasts, suggesting that the MM-LDL-induced accumulation of these mRNAs might be accomplished by both transcriptional and posttranscriptional mechanisms. In protein kinase C-depleted fibroblasts, MM-LDL increased JE and KC mRNA levels, whereas the stimulatory effect of 12-O-tetradecanoyl phorbol-13-acetate on JE and KC expression was blocked under these conditions. Taken together, these data demonstrate that MM-LDL increases transcription of the JE and KC genes by a mechanism that does not require de novo protein synthesis and is not mediated via a 12-O-tetradecanoyl phorbol-13-acetate-responsive protein kinase C pathway. These results provide a basis for further investigations of the molecular mechanism of action of MM-LDL. (Arteriosclerosis and Thrombosis 1992;12:800-806).