Mechanisms of cyclophosphamide action on hepatic P-450 expression.
Cyclophosphamide was administered to adult male rats (130 mg/kg, single i.p. injection) and its effects on the P-450 enzymes that contribute to the activation of this drug in rat liver were then assessed. P-450-mediated cyclophosphamide 4-hydroxylase activity in isolated rat liver microsomes decreased by approximately 70% over a 9-day period following drug treatment. This decrease was due to the loss of cytochrome P-450 form 2c (IIC11), a major contributor to cyclophosphamide 4-hydroxylation in untreated male rat liver, while the other major hepatic cyclophosphamide 4-hydroxylase, P-450 PB-1 (IIC6), was largely unaffected. The loss of P-450 2c activity did not result from a decrease in P-450 reductase or from direct inactivation of the P-450 protein by cyclophosphamide or its metabolites, but rather was due to a reduction in hepatic P-450 2c protein and mRNA levels. Hepatic P-450 2a (IIIA2) and P-450 RLM2 (IIA2) were also suppressed by cyclophosphamide treatment. Serum testosterone, which contributes to the expression of P-450s 2c, 2a, and RLM2, was severely depleted in the cyclophosphamide-treated rats; however, this loss was not the direct cause of the decreases in these hepatic P-450s, since the decreases were not reversed upon restoration of normal testosterone levels by human chorionic gonadotropin stimulation of testicular androgen production. In contrast to the suppression of these testosterone-dependent P-450s, P-450 3 (IIA1), P-450j (IIE1), and the P-450-independent microsomal enzyme steroid 5 alpha-reductase were each elevated in rat liver following cyclophosphamide administration. In contrast to P-450 3 and steroid 5 alpha-reductase, however, the elevation of P-450j protein was transient and was not accompanied by an increase in P-450j-associated hepatic microsomal aniline hydroxylase activity. In vitro experiments revealed that P-450j was severalfold more susceptible to inactivation by the cyclophosphamide metabolite acrolein as compared with P-450 3. These observations suggest that P-450j protein is induced by cyclophosphamide treatment but that the protein is inactivated by the cyclophosphamide metabolite acrolein. These findings establish that cyclophosphamide treatment can modulate hepatic P-450 activities through multiple mechanisms and in a manner that may alter P-450 metabolism of cyclophosphamide and perhaps other anticancer drugs that undergo bioactivation in the liver.
LeBlanc
G A
GA
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115.
Waxman
D J
DJ
D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase / Q000032:analysis; D000818:Animals; D003520:Cyclophosphamide / Q000378:metabolism / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000032:analysis* / Q000235:genetics; D005260:Female; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008297:Male; D009251:NADPH-Ferrihemoprotein Reductase / Q000032:analysis; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D011916:Rats, Inbred F344; D013739:Testosterone / Q000097:blood
D012333:RNA, Messenger; D013739:Testosterone; D003520:Cyclophosphamide; D003577:Cytochrome P-450 Enzyme System; D013741:3-Oxo-5-alpha-Steroid 4-Dehydrogenase; D009251:NADPH-Ferrihemoprotein Reductase