Hormonal perturbation as a possible mechanism for the alteration of cytochrome P450 by cyclophosphamide

Abstract

The mechanism by which the cytotoxic agent cyclophosphamide (CP) alters cytochrome P450 and some associated enzymes in the male rat has been investigated. CP, administered as a high single dose, decreases the activity of the enzymes CYP2C6, CYP2C11, CYP3A2 and CYP2E1 and NADPH P450 reductase and increases the activity of steroid 5α-reductase. CP appears to exert its effect via an indirect mechanism that reaches its maximal effect 7 days after administration. The decreases in the activity of the enzymes CYP2C11, CYP2E1 and CYP3A2 are accompanied via a corresponding change in the amount of enzyme protein indicating that the alteration in expression of these enzymes occurs via changes in transcription and/or translation. Michaelis-Menten analysis confirmed this conclusion for the enzymes CYP2C11 and CYP3A2. The change in enzyme profile is accompanied by a reduction in the hormones, testosterone, TSH, T 4 and T 3. The reduction in hormone levels is also maximal 7 days after CP administration. To determine whether CP alters enzyme expression in the male rat via perturbation of hormonal regulation, daily replacement doses of hCG and/or T 3 were administered for 7 days after a single dose of CP and hepatic CYP and associated enzyme activities assessed. Results indicated that daily hormone replacement with either hCG and/or T 3 prevented the changes in expression of the hormone dependent enzymes NADPH P450 reductase, steroid 5α-reductase and CYP3A2 following a single dose of CP. In contrast for other enzymes including the male sex-specific enzyme CYP2C11, the female predominant enzyme CYP2E1 and the sex-independent enzyme CYP2C6, daily replacement of hCG and/or T 3 did not prevent the changes that occur 7 days following CP administration. As rats appeared anorexic and dehydrated and significant weight losses were recorded following CP treatment, blood was collected at the time of killing and subjected to biochemical analysis and a complete blood picture to identify any changes in such parameters that may have contributed to the changes in hormones and/or enzyme expression that occurred. However, significant variation in the treatment groups compared with controls for all parameters was not observed to occur except for an anticipated leukopoenia. We have concluded that CP alters the enzymes NADPH P450 reductase, steroid 5α-reductase and CYP3A2 via perturbation of the regulation of these enzymes by testosterone and/or thyroid hormones. However, while interference with regulation by testosterone and/or thyroid hormones may be part of the mechanism by which CP alters CYP2C11 and CYP2E1, other factors are contributing. Furthermore, the CP-mediated decrease in activity of CYP2C6, an enzyme shown to be independent of hormonal regulation, cannot be attributed to changes in hormone levels as expected. In addition we were able to eliminate the possibility that CP alters enzyme expression and/or hormone levels via alteration of biochemical parameters.