Retinoic acid receptor transcripts (RARalpha and RARgamma) are decreased in benign mouse epidermal
tumors relative to normal skin and are almost absent in
carcinomas. In this report, the expression of RARalpha and RARgamma
proteins was analyzed by immunoblotting in benign skin
tumors induced by two different promotion protocols designed to yield
tumors at low or high risk for malignant conversion. RARalpha was 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 substantial reduction of RARalpha in nontumorous skin. RARgamma was not detected in
tumors from either protocol and was greatly reduced in skin treated by either promoter. Both RARalpha and RARgamma
proteins were decreased in keratinocytes overexpressing an oncogenic v-ras(Ha) gene, and RARalpha was underexpressed in a benign keratinocyte cell line carrying a mutated c-ras(Ha) gene. Introduction of a recombinant RARalpha expression vector into benign keratinocyte
tumor cells reduced the S-phase population and inhibited [3H]
thymidine incorporation in response to
retinoic acid. Furthermore, transactivation of B-RARE-tk-LUC by
retinoic acid was markedly decreased in keratinocytes transduced with the v-ras(Ha) oncogene (v-ras(Ha)-keratinocytes). Blocking
protein kinase C function in v-ras(Ha)-keratinocytes with
bryostatin restored RARalpha
protein to near normal levels, reflecting the involvement of
protein kinase C in RARalpha regulation. Both RARalpha and RARgamma are down-regulated in cultured keratinocytes 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
carcinogenesis and may be involved in the differential promoting activity of
mezerein and 12-O-tetradecanoylphorbol-13-acetate, particularly for selecting
tumors at high risk for malignant conversion.