Synergistic Enhancement by 12-O-Tetradecanoylphorbol-13- acetate and Dibutyryl cAMP of 1α, 25-Dihydroxyvitamin D₃ Action in Human Promyelocytic Leukemic HL-60 Cells

Abstract

We have reported that dibutyryl cAMP (dbcAMP), an activator of cAMP-dependent protein kinase (PKA), potentiated the effects of 1α, 25-dihydroxyvitamin D₃(1, 25-(OH)₂D₃)-induced 24-hydroxylation activity inHL-60 cells by increasing 1, 25-(OH)₂D₃ receptor (VDR). The present study demonstrated that 12-O-tetradecanoylphorbol- 13-acetate (TPA), a potent phorbol ester, also potentiated the effect of 1, 25-(OH)₂D₃ on HL-60 cells and that TPA and dbcAMP acted in a synergistic manner to enhance the effect of 1, 25-(OH)₂D₃. It is interesting that TPA induced 24-hydroxylation activity far more efficiently than dbcAMP, in addition to their effects in increasing VDR. TPA increased basal levels of c-fos mRNA to the maximum by 1h after the treatment, whereas dbcAMP failed to affect c-fos gene expression. Together with the previous data indicating the presence of AP-1-like sequence in the promoter of 24-hydroxylase gene, it was suggested that TPA potentiated the effect of 1, 25-(OH)₂D₃ through an activation of c-fos gene expression. This notion was further supported by the data showing that TPA and dbcAMP also acted in a synergistic manner to activate c-fos gene expression. Neither TPA nor dbcAMP affected c-jun early response gene in the HL-60 clone used in the present study. The present study suggested that the activation of early c-fos response gene by TPA might be another mechanism to enhance the effect of 1, 25-(OH)₂D₃, besides up-regulation of VDR.