DIFFERENTIAL INDUCTION OF CYP1A1, CYP1B1, AHD4, AND NMO1 IN MURINE SKIN TUMORS AND ADJACENT NORMAL EPIDERMIS BY LIGANDS OF THE ARYL-HYDROCARBON RECEPTOR

Products of several phase I and II genes transcriptionally activated b y ligands of the aryl hydrocarbon receptor (AHR) were quantitated in c utaneous samples isolated from non-tumor-bearing SENCAR or SSIN mice, and animals bearing skin tumors generated in initiation-promotion prot ocols. The constitutive 7-ethoxyresorufin O-deethylase (EROD) activiti es in papillomas and squamous cell carcinomas were less than or equal to 37% of the values measured in the adjacent normal cutaneous tissue. Dermal and epidermal EROD specific activities in microsomal samples p repared from both tumor-bearing and non-tumor-bearing mice were elevat ed 9- to 14- and 43- to 77-fold, respectively, above constitutive leve ls 16-20 h after a single topical application of 100 nmol of dibenz[a, c]anthracene (DB [a,c]A). EROD specific activities in tumors were maxi mally elevated two-fold after topical application of DB[a,c]A. Western blot, northern blot, and reverse transcription (RT)-polymerase chain reaction (PCR) analyses confirmed that the EROD measurements reflected cutaneous cytochrome P450 (CYP) 1A1 protein, mature mRNA, and heterog eneous nuclear RNA contents, respectively. Analyses of CYP1A1, CYP1B1, cytosolic aldehyde dehydrogenase class 3, and NAD(P)H:menadione oxido reductase (NMO1) mRNA content by RT-PCR revealed significant increases in all four mRNAs in the normal tissue adjacent to papillomas after e xposure to 4 nmol of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) but no increases in the tumors. NMO1 mRNA content in acetone-treated papillo mas approached the levels detected in TCDD-treated normal skin. RT-PCR analyses also demonstrated elevated constitutive aryl hydrocarbon rec eptor nuclear translocator mRNA content (an approximately two-fold inc rease) in skin tumors. In contrast, AHR mRNA content in the tumors was about 20% of that measured in adjacent normal tissue. Collectively, t hese studies demonstrated that ligand-induced, AHR-mediated processes are absent in murine skin tumors that develop in initiation-promotion protocols. (C) 1998 Wiley-Liss, Inc.