Effect of β-carotene on the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase in rat liver

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase), is a rate-limiting enzyme in the biosynthesis of not only cholesterol but also a variety of non-sterol isoprenoids. It is subjected to multivalent feedback suppression by transcriptional and post-transcriptional control mechanisms mediated by sterols and non-sterol substances. In the present study, the effect of a plant isoprenoid, β-carotene, on the expression of HMG-CoA reductase in rat liver was investigated. In control rats the hepatic levels of mRNA transcripts of HMG-CoA reductase increased following 23 partial hepatectomy with two peaks, one at 8 h and the other at 24 h. Administration of the carotenoid (70 mg/kg, given every alternate day for 3 consecutive weeks) partially inhibited the increase in the transcript level with a 50% reduction at 8 h and 30% reduction at 24 h post partial hepatectomy. Nuclear run-off assays with nuclei isolated from the resting liver and from livers of control rats and rats exposed to β-carotene for 3 consecutive weeks and killed 8 h after partial hepatectomy indicated that β-carotene did not inhibit the rate of transcription of HMG-CoA reductase gene. These observations suggest that β-carotene regulates the expression of HMG-CoA reductase by some post-transcriptional mechanisms.