Endocrine Unit, VA Medical Center, University of California, San Francisco, California, U.S.A.

Correspondence: Dr Daniel D. Bikle, Endocrine Unit, VA Medical Center, 4150 Clement Street (111N), San Francisco, CA 94121

Received 3 April 1997; Revised 14 December 1997; Accepted 14 January 1998.

Abstract

Squamous cell carcinomas (SCC) derived from human epidermis fail to differentiate normally under the influence of 1,25-dihydroxyvitamin D₃[1,25(OH)₂D₃[ despite the presence of the vitamin D receptor. Previous studies from our laboratory showed that phospholipase C-1 (PLC-1) was upregulated transcriptionally by 1,25(OH)₂D₃ in normal human keratinocytes, and a vitamin D responsive element (VDRE) in its promoter region has been identified. To examine the inducibility of human PLC-1 transcription by 1,25(OH)₂D₃ and/or retinoic acid in SCC cell lines, we transiently transfected SCC4 and SCC12B2 cells with human PLC-1 promoter-luciferase constructs containing the VDRE and tested the response of these constructs to 1,25(OH)₂D₃ and/or all-trans retinoic acid. The induction of the human PLC-1 VDRE by 1,25(OH)₂D₃ was synergistic with all-trans retinoic acid in normal human keratinocytes, but none of the constructs was induced by 1,25(OH)₂D₃ and/or all-trans retinoic acid in SCC4 and SCC12B2 cells. In contrast, the construct containing the VDRE of the human 24-hydroxylase gene was induced several fold by 1,25(OH)₂D₃ in normal human keratinocytes and by both 1,25(OH)₂D₃ 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-1 VDRE similarly to that seen in normal keratinocytes. The data indicate that the VDRE in the human PLC-1 gene is not functional in SCC4 and SCC12B2 cells, unlike normal human keratinocytes, even though vitamin D receptors bind normally to it. Failure of transcriptional control of the PLC-1 gene by 1,25(OH)₂D₃ suggests the lack of a cofactor(s) linking the VDRE to the transcriptional machinery.