Synergistic enhancement by 12-O-tetradecanoylphorbol-13-acetate and dibutyryl cAMP of 1 alpha,25-dihydroxyvitamin D-3 action in human promyelocytic leukemic HL-60 cells

We have reported that dibutyryl cAMP (dbcAMP), an activator of cAMP-depende nt protein kinase (PKA), potentiated the effects of 1 alpha,25-dihydroxyvit amin D-3(1,25-(OH)(2)D-3)-induced 24-hydroxylation activity in HL-60 cells by increasing 1,25-(OH)(2)D-3 receptor (VDR). The present study demonstrate d that 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent phorbol ester, also potentiated the effect of 1,25-(OH)(2)D-3 on HL-60 cells and that TPA and dbcAMP acted in a synergistic manner to enhance the effect of 1,25-(OH) (2)D-3. It is interesting that TPA induced 24-hydroxylation activity far mo re efficiently than dbcAMP, in addition to their effects in increasing VDR. TPA increased basal levels of c-fos mRNA to the maximum by 1 h after the t reatment, whereas dbcAMP failed to affect c-fos gene expression. Together w ith 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)(2)D-3 through an activation of c-fos gene expression. T his notion was further supported by the data showing that TPA and dbcAMP al so 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 gen e by TPA might be another mechanism to enhance the effect of 1,25-(OH)(2)D- 3, besides up-regulation of VDR.