CIS-UROCANIC ACID DOWN-REGULATES HISTAMINE-MEDIATED ACTIVATION OF ADENYLATE-CYCLASE IN THE PIG EPIDERMIS

Urocanic acid (UCA), one of the skin's major components for absorbing UV radiation, is present naturally in the stratum corneum as a trans-i somer. On absorption of UVB radiation either in vitro or in the skin, UCA undergoes trans- to cis-isomerization in a dose-dependent manner. Although the mechanism by which cis-UCA suppresses immunity remains un elucidated, recent studies have indicated that cis-UCA appears to inhi bit the induction of cyclic AMP in fibroblasts, which suggests that th is molecule plays an active role in modifications to the skin, Here, w e report that although neither trans-UCA nor cis-UCA increases cyclic AMP in the pig epidermis, cis-UCA actively down-regulates the increase of cyclic AMP induced by histamine, The effects of cis-UCA on the pig epidermis are revealed through the modulation of the effects caused b y histamine, These findings suggest that in the pig epidermis, the ini tial biochemical and cellular event for UVB-induced immune suppression - that is, the step immediately following the isomerization of trans- UCA to cis-UCA is down-regulation of cyclic AMP brought about by the a ctivity of cis-UCA.