ALL-TRANS-RETINOIC ACID BLOCKS THE ANTIPROLIFERATIVE PRODIFFERENTIATING ACTIONS OF 1,25-DIHYDROXYVITAMIN D-3 IN NORMAL HUMAN KERATINOCYTES

1,25-Dihydroxyvitamin D-3 [1,25(OH)(2)D-3] and all-trans retinoic acid (RA), the active metabolites of vitamins D and A respectively, regula te the proliferation and differentiation of keratinocytes. Both the vi tamin D receptor (VDR) and the retinoic acid receptor family (RAR) bin d to DNA response elements as heterodimers with the retinoic X recepto r (RXR), suggesting that there are pathways of action that are shared by both compounds. Therefore, we examined the interactions of 1,25(OH) (2)D-3 and RA upon the proliferation and differentiation of normal hum an keratinocytes (NHK) and of a squamous cell carcinoma cell line, SCC 4. Although both 1,25(OH)(2)D-3 and RA were each able to inhibit NHK p roliferation in a dose-dependent manner, when they were administered i n combination, proliferation was stimulated, suggesting mutual antagon ism. In contrast, SCC4 cells proved insensitive in terms of proliferat ion to 1,25(OH)(2)D-3 and to all but the highest concentration (10(-6) M) of RA. 1,25(OH)(2)D-3 exerted a biphasic effect on transglutaminas e (TGase) and involucrin (INV) mRNA levels, with maximal stimulation a t 10(-9) M. RA inhibited TGase and INV mRNA levels and antagonized the stimulation by 1,25(OH)(2)D-3. A similar pattern was observed for TGa se protein, but, RA, which, by itself, reduced INV, markedly enhanced the ability of 1,25(OH)(2)D-3 to raise INV levels, possibly by inhibit ing 1,25(OH)(2)D-3-stimulated TGase activity and cross-linking cif sol uble INV into the insoluble cornified envelope (CE). Thus, in NHK cell s, RA antagonizes the antiproliferative prodifferentiating actions of 1,25(OH)(2)D-3, but assessment of a single marker, such as INV protein , may be misleading. (C) 1998 Wiley-Liss, Inc.