DIFFERENTIAL REGULATION OF VITAMIN-D RESPONSIVE ELEMENTS IN NORMAL AND TRANSFORMED KERATINOCYTES

Squamous cell carcinomas (SCC) derived from human epidermis fail to di fferentiate normally under the influence of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] despite the presence of the vitamin D receptor. Previ ous studies from our laboratory showed that phospholipase C-gamma 1 (P LC-gamma 1) was upregulated transcriptionally by 1,25(OH)(2)D-3 in nor mal human keratinocytes, and a vitamin D responsive element (VDRE) in its promoter region has been identified. To examine the inducibility o f human PLC-gamma 1 transcription by 1,25(OH)(2)D-3 and/or retinoic ac id in SCC cell lines, we transiently transfected SCC4 and SCC12B2 cell s with human PLC-gamma 1 promoter-luciferase constructs containing the VDRE and tested the response of these constructs to 1,25(OH)2D, and/o r all-trans retinoic acid. The induction of the human PLC-gamma 1 VDRE by 1,25(OH)(2)D-3 was synergistic with ah-trans retinoic acid in norm al human keratinocytes, but none of the constructs was induced by 1,25 (OH)(2)D-3 and/or all-trans retinoic acid in SCC4 and SCC12B2 cells. I n contrast, the construct containing the VDRE of the human 24-hydroxyl ase gene was induced several fold by 1,25(OH)(2)D-3 in normal human ke ratinocytes and by both 1,25(OH)(2)D-3 and all-trans retinoic acid in SCC4 and SCC12B2 cells. DNA mobility shift assays showed that both the vitamin D receptor and the retinoic acid receptor in SCC4 and SCC12B2 cells bound the human PLC-gamma 1 VDRE similarly to that seen in norm al keratinocytes. The data indicate that the VDRE in the human PLC-gam ma 1 gene is not functional in SCC4 and SCC12B2 cells, unlike normal h uman keratinocytes, even though vitamin D receptors bind normally to i t. Failure of transcriptional control of the PLC-gamma 1 gene by 1,25( OH)(2)D-3 suggests the lack of a cofactor(s) linking the VDRE to the t ranscriptional machinery.