Bd. Murphy et al., Heat shock interferes with steroidogenesis by reducing transcription of the steroidogenic acute regulatory protein gene, MOL ENDOCR, 15(8), 2001, pp. 1255-1263
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.