DEVELOPMENTALLY REGULATED CYTOCHROME-P-450IA1 EXPRESSION IN CULTURED RAT VASCULAR SMOOTH-MUSCLE CELLS

We have examined the expression of cytochrome P-450IA1 (CypIa1) in cultured rat arterial smooth muscle cells (SMC). We report here that newborn (pup) aortic and adult carotid neointimal SMC express 10- 20-fold higher steady-state levels of CypIa1 mRNA compared with identically treated SMC derived from adult aorta or uninjured carotid media, respectively. Nuclear transcription experiments suggest that the difference in basal expression of CypIa1 reflects differences in transcription rates of the gene in these two cell types. Treatment of adult aortic SMC with cycloheximide dramatically elevated CypIa1 mRNA levels and had little effect on the amount of this mRNA in pup SMC. That the cycloheximide effect in adult SMC was due to enhanced transcription of the CypIa1 gene was demonstrated by 1) the ability of actinomycin D to completely block the cycloheximide-induced increase in steady-state mRNA levels, and 2) the increased transcription rate of the CypIa1 gene in cycloheximide-treated adult nuclei determined via nuclear transcription assays. Treatment of either cell type with beta-naphthoflavone increased CypIa1 mRNA levels to a similar extent, suggesting that the differential response to cycloheximide was specific and not a general lack of pup SMC to respond to transcriptional inducers of the CypIa1 gene. Our data suggest that a labile repressor of the CypIa1 gene is present in cultured adult aortic SMC and is absent, inactive, or at a greatly reduced level in cultured pup SMC.