Differential expression of prostaglandin H synthase isozymes during multistage carcinogenesis in mouse epidermis

An anti-tumor-promoting effect of indomethacin and related nonsteroidal anti-inflammatory drugs (NSAIDs) as well as the ability of the tumor promoter 12-O-tetradecanoylphorbol-13- acetate (TPA) to increase the level of prostaglandins in murine keratinocytes and mouse epidermis in vivo has been repeatedly documented. Here, the expression of prostaglandin H synthase (PGHS) isozymes, which are major targets of NSAIDs, was investigated in different stages of tumor development in mouse skin. Mouse epidermis in vivo constitutively expressed PGHS-1. PGHS-1 steady-state levels remained unchanged upon induction of acute or chronic epidermal hyperplasia by TPA and in papillomas and carcinomas generated by the initiation-promotion procedure, with 7,12-dimethylbenz[a]anthracene as initiator and TPA as promoter. Thus, the elevated prostaglandin level in the acute hyperplastic epidermis was very likely due to PGHS-2 induction. Repeated applications of TPA resulted in stationary hyperplasia and downregulation of PGHS-2 expression and prostaglandin levels, suggesting that the epidermis had adapted to the TPA stimulus. In papillomas and carcinomas, however, constitutive overexpression of PGHS-2 was found, with a large amount of prostaglandin E2 and prostaglandin F2 alpha. Keratinocyte cell lines corresponding to different stages of tumor development also constitutively over-expressed PGHS-2. Considered with inhibitor studies, these data suggest that PGHS-2 has a critical role in skin carcinogenesis. The anti-tumor-promoting effect of the PGHS inhibitor indomethacin is specifically reversed by prostaglandin F2 alpha, indicating that this prostaglandin type has a significant role in tumor development.