Suppression of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated CYP1A1 and CYP1B1 induction by 12-O-tetradecanoylphorbol-13-acetate: role of transforming growth factor beta and mitogen-activated protein kinases

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances or su ppresses the transcriptional activation of CYP1A1 by 2,3,7,8-tetrachlorodib enzo-p-dioxin (TCDD) in a cell/tissue-specific manner. The basis for these effects is not known. Exposure of the immortalized human breast epithelial cell line MCF10A-Neo to TPA at the time of, or up to 12 hr prior to, the ad dition of TCDD strongly suppressed the transcriptional activation of CYP1A1 and CYP1B1 (IC50 similar to0.5 nM). A recent study (Carcinogenesis 2000;21 :1303-12) demonstrated that TPA-treated MCF10A-Neo cells rapidly activate t he latent transforming growth factor beta (TGF beta) in the serum used to s upplement the culture medium. The suppressive effects of TPA on CYP1A1 indu ction by TCDD in MCF10A-Neo cultures could be partially suppressed by: (a) co-incubation of TCDD + TPA-treated cultures with a neutralizing TGF beta p an antibody; (b) prior removal of latent TGF beta from the culture medium; or (c) switching cultures to serum- and growth factor-free medium immediate ly before the addition of TPA and TCDD. Exposure of cultures to TPA 24-48 h r prior to subsequent TPA + TCDD treatment not only inhibited the suppressi ve effects of TPA, but markedly enhanced CYP1A1 mRNA accumulation. TPA caus ed a rapid and protracted activation of extracellular signal-regulated kina ses (ERKs). Pretreatment of cultures with the mitogen-activated protein kin ase kinase (MEK) inhibitor PD184352 [2-(2-chloro-4-iodo-phenylamino)-N-cycl opropyl-methoxy-3,4-difluoro-benzamide] completely inhibited ERK activation by TPA. However, PD184352 did not prevent the suppressive effects of TPA o n CYP1A1 activation by TCDD. These studies demonstrate that TPA initiates p rotein kinase C-dependent, ERK-independent processes that suppress CYP1A1 a ctivation by TCDD in MCF10A-Neo cells. Furthermore, TGF beta mediates a sma ll portion of this suppressive activity. (C) 2001 Elsevier Science Inc. All rights reserved.