ACTIVATION OF RAF-MITOGEN-ACTIVATED PROTEIN-KINASE SIGNALING PATHWAY BY 1,25-DIHYDROXYVITAMIN-D-3 IN NORMAL HUMAN KERATINOCYTES

The biologic effects of the vitamin D hormone 1,25-dihydroxyvitamin D- 3 (1,25(OH)(2)D-3) are believed to be mediated by an intracellular vit amin D receptor, which after ligand binding acts as a transcription fa ctor modulating expression of a variety of genes, Besides having a wel l-known role in calcium metabolism, this hormone is an important regul ator of proliferation in a majority of normal and neoplastic cells, Ke ratinocytes provide a convenient model for investigating the growth-re lated effects of vitamin D in normal cells, Growth of keratinocytes ma y be either stimulated or inhibited by 1,25(OH)(2)D-3, depending on th e degree of cell differentiation. We show here that 1,25(OH)(2)D-3 sti mulates DNA synthesis via sequential activation of Raf and the mitogen -activated protein kinase, Activation of these kinases is independent on protein and mRNA synthesis and is preceded by rapid tyrosine phosph orylation of an adaptor protein p66(Shc) and formation of a complex be tween p66(Shc), a bridging molecule Grb2, and a Ras activator, mSos. V itamin D receptor protein associates with She, indicating that this st eroid hormone is able to signal through the transcription-independent pathways similar to those used by peptide hormones and cytokines.