Suppression of constitutive cytochrome P-450 gene expression in livers of rats undergoing an acute phase response to endotoxin

Administration of purified bacterial lipopolysaccharide (LPS) to male rats suppressed the constitutive hepatic expression of the male-specific cytochrome P-450 AH, reduced flavoprotein:oxygen oxidoreductase (RH hydroxylating), E.C.1.14.14.1] isozyme P-450h (P450IIC11) to about 35% of control levels within 24 hr. The mRNA for P-450h was more rapidly and more profoundly suppressed than was the protein, indicating (a) that the decrease in the mRNA was responsible for the suppression of the protein and (b) that other mechanisms work to maintain expression of P-450h apoprotein in the face of repression of its mRNA. Suppression of P-450h expression was maximal at an endotoxin dose of 30-100 micrograms/kg, indicating that P-450 suppression is concomitant with the acute-phase response of hepatic secretory proteins. The female-specific cytochrome P-450 isozyme, P-450i (P450IIC12), was suppressed to 17% of control levels by LPS administration in female rats. Suppression of the P-450i apoprotein by LPS, and recovery of its expression, was more rapid than was suppression of P-450h in males. P-450i protein and mRNA levels were concomitantly suppressed by LPS, indicating that although there is a pretranslational component to the suppression, other mechanisms may also contribute. Calculations based on estimations of the microsomal contents of P-450h and P-450i relative to the total cytochrome P-450 in untreated rat livers indicate that suppression of these forms contributes significantly to the decreases in total microsomal P-450 after LPS treatment. In these studies, hepatic microsomal NADPH-cytochrome c reductase (TPNH2-cytochrome c reductase, E.C.1.6.2.4) activities and content of cytochrome b5 were decreased by LPS administration in both male and female rats. Like its effects on cytochrome P-450 expression, endotoxin suppression of NADPH-cytochrome c reductase activities and cytochrome b5 levels was more rapid in female rats than in males. The production of a local inflammatory response in male rats by subcutaneous injection of turpentine caused effects on cytochrome P-450, P-450h expression, and cytochrome b5 that were similar to those of endotoxin but were less rapidly achieved.