Protective effects of the synthetic flavonoid α-naphthoflavone against transcriptional and post-transcriptional changes induced by the carcinogen benzo[a]pyrene

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The exposure to polycyclic aromatic hydrocarbons (PAHs) has been regarded as a causative factor in the etiology of breast cancer. Acute exposure to PAHs are hallmarked by an induction of S-phase arrest. Flavonoids are a large family of dietary phenolic phytochemicals found in fruits and vegetables. Key features of flavonoids include the ability to compete with PAHs for binding to the aromatic hydrocarbon receptor (AhR) and suppress cell cycle progression. Therefore, dietary and synthetic flavonoids have been considered as chemotherapeutic agents against PAHs. 7,8-naphthoflavone (α-naphthoflavone:ANF) is a prototype synthetic flavonoid, which shares structural similarities with other natural flavonoids including genestein and quercitin. In this study, we examined in breast cancer MCF-7 cells the effects of ANF on B[a]P-induced changes in cDNA microarray profiles and protein levels of factors that contribute to DNA repair and cell cycle progression in S-phase. The treatment with B[a]P induced mRNA levels for members of the cytochrome P450 family (CYP1A1, CYP1B1, CYP2J2), cell cycle regulators (cdc25a, cdc25c), and DNA damage repair (ATM, BRCA1). In contrast, B[a]P down regulated transcript levels for various cyclins including A2, D1, and E1. The co-treatment with ANF tended to counteract the effects of B[a]P. Treatment with the B[a]P metabolite 7r,8r-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydro-benzo[a]pyrene (BPDE) also triggered a reduction in mRNA levels for cyclins A2, D1 and E, but at lower doses and earlier time points. Changes in the cDNA microarray profiles of cyclins A2, D1, and E following exposure to B[a]P and BPDE were validated by RT-PCR. In contrast, western blot analysis indicated that B[a]P did not induce changes in expression of phosphorylated checkpoint kinase1 (Chk1-pSer345) and cyclins A2, D1 and E1. Conversely, B[a]P did induce p53, BRCA1, phosphorylated checkpoint kinase2 (Chk2-pThr68), while reducing CDC25a/CDC25c protein levels. The co-treatment of ANF restored the normal levels of these proteins. We conclude that the potential chemotherapeutic properties of dietary flavonoids may stem from their ability to modulate transcriptional and post-transcriptional changes induced by PAHs.