Interrelationship between signal transduction pathways and 1,25(OH)(2)D-3 in UMR106 osteoblastic cells

In this study, the interrelationship between signal transduction pathways a nd 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] action was examined in UMR106 osteoblastic cells. Treatment of these cells with 8-bromo-cAMP (1 mM) resu lted in an upregulation of the vitamin D receptor (VDR) and an augmentation in the induction by 1,25(OH)(2)D-3 of 25(OH)D-3 24-hydroxylase [24(OH) ase ] and osteopontin (OPN) mRNAs as well as gene transcription. Transfection w ith constructs containing the vitamin D response element devoid of other pr omoter regulatory elements did not alter the cAMP-mediated potentiation, su ggesting that cAMP-enhanced transcription is due, at least in part, to upre gulation of VDR. Treatment with phorbol ester [12-O-tetradecanoyl-phorbol-1 3-acetate (TPA) 100 nM], an activator of protein kinase C, significantly en hanced 1,25(OH)(2)D-3-induced OPN mRNA and transcription but had no effect on VDR or on 24( OH) ase mRNA or transcription. Studies using OPN promoter constructs indicate that TPA-enhanced OPN transcription is mediated by an e ffect on the OPN promoter separate from an effect on VDR. Thus interactions with signal transduction pathways can enhance 1,25(OH)(2)D-3 induction of 24(OH) ase and OPN gene expression, and, through different mechanisms, chan ges in cellular phosphorylation may play a significant role in determining the effectiveness of 1,25(OH)(2)D-3 on transcriptional control in cells exp ressing skeletal phenotypic properties.