Toxicological and endocrinolgical aspects of cytochrome P450 in breast and lung

Cytochrome P450 (CYP) enzymes have well characterized functions in disposition and pharmacokinetics of xenobiotics as well as in biosynthesis and/or metabolism of endogenous compounds, such as steroid hormones, bile acids, arachidonic acid and its derivatives. Cytochrome P450 mediated metabolic activation is known to be a prerequisite for the cytotoxic and mutagenic activity of many environmental toxins and procarcinogens. Xenobiotic and steroid-metabolizing P450 enzymes are found in many tissues in the body, although at lower levels than in the liver and adrenal glands. It is the major hypothesis of this thesis that cell-specific expression of P450 can influence hormone metabolism and xenobiotic toxicity in target cells. In order to test this theory we have investigated P450 mediated activation of procarcinogens in the lung and characterized the P450 content of rat and human breast. Environmental factors have been implicated in the etiology of both lung and breast cancer. Heterocyclic amines are a group of dietary procarcinogens, that produce tumors at multiple sites in experimental animals and that have been proposed to play a role in human cancer. The capacity of rat and mouse lung microsomes to activate the heterocyclicamines IQ, MelQx and PhlP was demonstrated using the Ames mutagenicity test. With inhibitory antibodies it was shown that CYP2A3 mediated a large part of the activation of IQ in rat lung microsomes. CYP2A3 has been detected in rat lung and nasal mucosa, but not in the liver and thus these studies underscore the importance of studying extrahepatic reactions. The mammary gland is characterized by its hormonal regulation of growth and differentiation. In studies on rat breast, we could show that the P450 profile also varies with age and endocrine status of the animals. Quantitation of enzyme levels was achieved after partial purification of P450 and expression of P450 enzymes was analysed by Western blot of the protein in combination with RT PCR of mRNA. The studied P450 enzymes were distinctly expressed in virgin (CYP2A3, 2D6, 3A), pregnant (CYPIA1), lactating (CYP IAI ) and post lactating (CYP2B and 3A) rat breast, whereas other forms (CYP2EI,4A, aromatase) were found in all samples. Immunohistochemical localization revealed that CYP2A3 was specifically expressed in a limited number of epithelial cells in the ducts of 6-week-old rat breast. This finding permits future studies to evaluate whether these cells are at risk for genotoxic effects of procarcinogens that are activated by CYP2A3. Analysis of P450 profiles in human breast samples from reduction mammaplasties clearly showed the usefulness of this tissue source for characterization of human breast P450. A large interindividual variation was found in the expression of CYPIAI,2A6, 2B6 and 3A. These P450s are known to metabolize many xenobiotics and drugs, such as tamoxifen as well as steroid hormones. As in the rat CYP2E I, 4A and aromatase were detected in most human samples. In addition, CYP I B I and 2C were expressed in all samples. These studies clearly reveal cellular and developmental (in rat) as well as interindividual (in human) differences in the P450 profile and indicate that P450 forms in the mammary gland may play a role in in situ activation of procarcinogens and metabolism of hormones as well as antihormones.