LOSS OF RETINOIC ACID RECEPTORS IN MOUSE SKIN AND SKIN TUMORS IS ASSOCIATED WITH ACTIVATION OF THE RAS(HA) ONCOGENE AND HIGH-RISK FOR PREMALIGNANT PROGRESSION

Retinoic acid receptor transcripts (RAR alpha and RAR gamma) are decre ased in benign mouse epidermal tumors relative to normal skin and are almost absent in carcinomas, In this report, the expression of RAR alp ha and RAR gamma proteins was analyzed by immunoblotting in benign ski n tumors induced by two different promotion protocols designed to yiel d tumors at low; or high risk for malignant conversion, RAR alpha mas slightly reduced in papillomas promoted with 12-O-tetradecanoylphorbol -13-acetate (low risk) and markedly decreased or absent in papillomas promoted by mezerein (high risk). However, mezerein also caused substa ntial reduction of RAR alpha in nontumorous skin, RAR gamma was not de tected in tumors from either protocol and was greatly reduced in skin treated by either promoter. Both RAR alpha and RAR gamma proteins were decreased in keratinocytes overexpressing an oncogenic v-ras(H alpha) gene, and RAR alpha was underexpressed in a benign keratinocyte cell line carrying a mutated c-ras(H alpha) gene, Introduction of a recombi nant RAR alpha expression vector into benign keratinocyte tumor cells reduced the S-phase population and inhibited [(3H)]thymidine incorpora tion in response to retinoic acid, Furthermore, transactivation of B-R ARE-tk-LUC by retinoic acid was markedly decreased in keratinocytes tr ansduced with the v-ras(H alpha) oncogene (v-ras(H alpha)-keratinocyte s). Blocking protein kinase C function in v-ras(H alpha)-keratinocytes with bryostatin restored RAR alpha protein to near normal levels, ref lecting the involvement of protein kinase C in RAR alpha regulation. B oth RAR alpha and RAR gamma are downregulated in cultured keratinocyte s by 12-O-tetradecanoylphorbol-13-acetate, further implicating PKC in the regulation of retinoid receptors, Our data suggest that modulation of RARs could contribute to the neoplastic phenotype in mouse skin ca rcinogenesis and map be involved in the differential promoting activit y of mezerein and 12-O-tetradecanoylphorbol-13-acetate, particularly f or selecting tumors at high risk for malignant conversion.