CHOLESTEROL 7-ALPHA-HYDROXYLASE IS UP-REGULATED BY THE COMPETITIVE INHIBITOR 7-OXOCHOLESTEROL IN RAT-LIVER

Rats of the Sprague-Dawley strain were infused intravenously with a fa t emulsion (Intralipid, trademark of Kabi Pharmacia, Uppsala, Sweden) containing 7-oxocholesterol. This resulted in an increased cholesterol 7alpha-hydroxylase activity in liver microsomes as compared to contro ls and was accompanied by increased levels of cholesterol 7alpha-hydro xylase mRNA and microsomal cholesterol 7alpha-hydroxylase protein. Rat s were also fed a cholestyramine-supplemented diet and infused with 7- oxocholesterol. These animals excreted about half as much bile acids i n faeces as cholestyramine-fed controls. Addition of 7-oxocholesterol to liver microsomes from normal rats in amounts corresponding to those present in microsomes from 7-oxocholesterol-treated rats inhibited th e cholesterol 7alpha-hydroxylase activity by about 75%. Cholesterol in duced a type-I binding spectrum when added to a purified bacterial-exp ressed cholesterol 7alpha-hydroxylase (P-450c7DELTA2-24). 7-Oxocholest erol competitively inhibited the cholesterol binding spectrum, while 7 beta-hydroxycholesterol did not interfere with binding of cholesterol to the enzyme. It is concluded that treatment with the competitive inh ibitor 7-oxocholesterol leads to a reduced bile acid biosynthesis and, as a consequence of reduced bile acid inhibition, a compensatory incr ease in cholesterol 7alpha-hydroxylase synthesis. The high enzyme acti vity measured in microsomal preparations from 7-oxocholesterol-treated rats may be due to a continuous conversion of 7-oxocholesterol into l ess inhibitory metabolites, e.g. 7beta-hydroxycholesterol. The latter compound was found in high concentrations in liver microsomes from rat s treated with 7-oxocholesterol. The physiological importance of these results is discussed in relation to the previous findings that 7-oxoc holesterol is accumulated in liver after cholesterol feeding and that 7-oxocholesterol is formed from cholesterol during lipid peroxidation.