We have previously shown that the abundance of vitamin D receptors (VDR) in cultured cells is increased by mitogens such as serum and growth factors, whereas activation of protein kinase-C (PK-C) causes inhibition of VDR gene expression. This study examines the effect of the cAMP-activated protein kinase-A (PK-A) second messenger system on VDR abundance and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] action. Elevation of intracellular cAMP levels in NIH-3T3 mouse fibroblasts by forskolin or (Bu)2cAMP caused a substantial (8- to 12-fold) increase in VDR abundance, as measured by ligand binding and Western blot analysis. The time course of the forskolin effect on VDR expression was complex. An early rise in VDR abundance occurred at 4 h, followed by a decrease and then a broad secondary rise at 18 h. At the mRNA level, forskolin caused a rapid rise in VDR transcripts after 1 h of exposure, a peak at 2 h, followed by a decline and a subsequent increase at 15 h. Activation of PK-C with the phorbol ester phorbol myristate acetate abolished the forskolin-induced increase in VDR protein and mRNA abundance. NIH-3T3 cells were stably transfected with phOC-CAT, a plasmid carrying a human osteocalcin promoter fragment containing the vitamin D response element fused to the reporter gene chloramphenicol acetyl transferase (CAT). 1,25-(OH)2D3 treatment of transfected cells induced a dose-dependent increase in CAT activity. Up- or down-regulation of VDR in these transfected cells by forskolin or phorbol myristate acetate pretreatment, respectively, resulted in corresponding enhancement or attenuation of 1,25-(OH)2D3-inducible CAT activity.(ABSTRACT TRUNCATED AT 250 WORDS)