[Regulation of tumor promotion by 1 alpha, 25-dihydroxyvitamin D3, an active form of vitamin D3].

1 alpha, 25-Dihydroxyvitamin D3 (1 alpha, 25(OH)2D3) is a hormonally active from of vitamin D3 and has been known to regulate plasma calcium level and cell differentiation. In addition to these functions, we found that 1 alpha, 25 (OH)2D3 is involved in tumor promotion in vitro and in vivo. It enhances chemically-induced transformation of BALB/3T3 cells but paradoxically inhibits tumor promotion in mouse skin carcinogenesis and induction of ornithine decarboxylase by tumor promoters in skin, stomach, liver and colon. Cellular and biochemical mechanisms of actions of 1 alpha, 25(OH)2D3 have been studies in comparison with phorbol ester tumor promoters.