Heat shock interferes with steroidogenesis by reducing transcription of the steroidogenic acute regulatory protein gene

A key regulatory point in fine tuning of steroidogenesis is the synthesis o f steroidogenic acute regulatory protein, which transfers cholesterol into mitochondria. Heat shock and toxic insults reduce steroidogenic acute regul atory protein, severely compromising steroid synthesis. As the molecular me chanisms for this reduction remain elusive, we tested the hypothesis that h eat shock directly interferes with transcription of the steroidogenic acute regulatory protein gene. We show that, in mouse MA-10 Leydig tumor cells, heat shock causes drastic declines in (Bu)(2)cAMP-induced progesterone accu mulation and steroidogenic acute regulatory protein transcript abundance. A proximal steroidogenic acute regulatory protein promoter fragment (-85 to +39) is sufficient to direct both cAMP inducibility and heat shock inhibiti on. Nuclear extracts from MA-10 cells displayed binding to this proximal pr omoter fragment as a low mobility complex in gel shift experiments. This co mplex disappeared in nuclear extracts taken at 5 and 10 min after initiatio n of heat shock and reappeared in extracts taken at 2 and 8 h. Similar low- mobility complexes formed on oligonucleotides representing the overlapping subfragments of the minimal steroidogenic acute regulatory protein promoter fragment sensitive to the heat shock effect. Extracts from heat-shocked MA -10 cells displayed reduced complex formation to each of the subfragments. We conclude that heat shock reduces progesterone synthesis, steroidogenic a cute regulatory protein mRNA abundance, and activity and disrupts binding o f nuclear proteins to the proximal region of the steroidogenic acute regula tory protein promoter. Together these observations provide strong evidence for a mechanism of transcriptional inhibition in the down-regulation of ste roidogenic acute regulatory protein expression by heat shock.