Cytochrome P450 CYP1B1 protein expression:

Abstract

The overexpression of human cytochrome P450 CYP1B1 has been observed in a wide variety of malignant tumours, but the protein is undetectable in normal tissues. A number of cytochrome P450 enzymes are known to metabolise a variety of anticancer drugs, and the consequence of cytochrome P450 metabolism is usually detoxification of the drug, although bioactivation occurs in some cases. In this study, a Chinese hamster ovary cell line expressing human cytochrome P450 CYP1B1 was used to evaluate the cytotoxic profile of several anticancer drugs (docetaxel, paclitaxel, cyclophosphamide, doxorubicin, 5-fluorouracil, cisplatin, and carboplatin) commonly used clinically in the treatment of cancer. The MTT (3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide) assay was used to determine the levels of cytotoxicity. The key finding of this study was that on exposure to docetaxel, a significant decrease in sensitivity towards the cytotoxic effects of docetaxel was observed in the cell line expressing CYP1B1 compared to the parental cell line (P = 0.03). Moreover, this difference in cytotoxicity was reversed by co-incubation of the cells with both docetaxel and the cytochrome P450 CYP1 inhibitor alpha-naphthoflavone. This study is the first to indicate that the presence of CYP1B1 in cells decreases their sensitivity to the cytotoxic effects of a specific anticancer drug.

Introduction

We have demonstrated that human cytochrome P450 CYP1B1 is overexpressed in a wide variety of cancers [1], [2], [3], [4]. This finding raises the question as to what the functional role of CYP1B1 is in tumours. CYP1B1 has been shown to be capable of the 4-hydroxylation of oestradiol [5], [6], [7], and a significant increase in the 4-hydroxylation of oestradiol has been demonstrated in breast cancer [8]. Several cytochrome P450 enzymes are involved in the metabolism of a range of anticancer drugs, including cyclophosphamide, paclitaxel, and docetaxel [9], [10], [11], [12], [13], [14]. Cytochrome P450-mediated metabolism usually results in reduced activity or inactivation of the anticancer drugs, but in some cases bioactivation to a more cytotoxic metabolite occurs. One example of detoxification of anticancer drugs is shown by the taxanes. The major pathway of metabolism of paclitaxel, an anticancer drug used in the treatment of breast, ovarian, and non-small cell lung cancer, is catalysed by CYP2C8 and involves the hydroxylation of position 6 on the taxane ring [15]. The metabolite 6-hydroxytaxol is 30-fold less cytotoxic than the parent compound paclitaxel [16], and this metabolite is further metabolised by CYP3A4 [13]. Docetaxel, a semisynthetic taxane currently under going phase II and phase III trials for use in first-line therapy of ovarian cancer, is metabolised by CYP3A to apparently less cytotoxic metabolites [12], [13].

Several anticancer drugs are metabolically activated by P450s. In particular, cyclophosphamide, an inactive prodrug used in the treatment of a number of different cancers, including ovarian, must first undergo a 4-hydroxylation reaction to 4-hydroxycyclophosphamide by cytochrome P450 enzymes (CYP2B6 and CYP3A4) [17] before becoming cytotoxic [18]. CYP3A4 has also been shown to be involved in the metabolic activation of doxorubicin to the more cytotoxic morpholino doxorubicin [19], [20].

In this study, the influence of CYP1B1 on the cytotoxic profile of several anticancer drugs commonly in clinical practice was evaluated in a Chinese hamster ovary cell line expressing human CYP1B1.