Biological functions of cytochrome P450 1A1 in the proliferation and survival of breast cancer cells

Abstract

Cytochrome P450 1A1 (CYP1A1) is an extrahepatic phase I metabolizing enzyme whose expression is suppressed under physiologic conditions, but can be induced by substrates via the aryl hydrocarbon receptor (AhR). Nonetheless, recent studies show that the majority of breast tumors constitutively express CYP1A1. These findings led us to test the hypothesis that CYP1A1 promotes breast cancer progression by evaluating the effects of CYP1A1 knockdown on the proliferation and survival of the MCF7 and MDA-MB-231 lines. Independent of estrogen receptor status, CYP1A1 knockdown decreases cell proliferation, decreases colony formation, blocks the cell cycle at G0/G1 associated with reduction of cyclin D1, and increases apoptosis associated with reduction of survivin. CYP1A1knockdown markedly increases phosphorylation of AMP-activated protein kinase (AMPK) and decreases phosphorylation of AKT, extracellular signal-regulated kinases (ERK)-1 and 2, and 70 kDa ribosomal protein S6 kinase (P70S6K). AMPK inhibition by compound C partially abrogates the proapoptotic effects of CYP1A1siRNA, suggesting that CYP1A1siRNA effects are mediated, in part, through AMPK signaling. Consistent with CYP1A1 knockdown results, pharmacologic reduction of CYP1A1 levels by the phytopolyphenol carnosol also correlates with impaired proliferation and induced AMPK phosphorylation. These results indicate that reduction of basal CYP1A1 expression is critical for inhibition of proliferation, which is neither affected by alpha-naphthoflavone-mediated inhibition of CYP1A1 activity nor modulated by AhR silencing. Growth complementation experiments were performed to identify the exact mechanism by which CYP1A1 regulates these signaling pathways. Our results show that CYP1A1 regulates cell signal transduction through mechanisms other than synthesis of 8,9- epoxyeicosatrienoic acid, 11,12- epoxyeicosatrienoic acid, 14,15- epoxyeicosatrienoic acid, 20-hydroxyeicosatetraenoic acid, and 17,18-epoxyeicosatetraenoic acid or metabolism of all-trans-retinoic acid, 9-cis retinoic acid, 13-cis retinoic acid and pregnenolone. These studies support that CYP1A1 may promote breast cancer proliferation and survival, at least in part, through AMPK signaling and that reduction of CYP1A1 levels is a potential strategy for breast cancer therapeutics.