We have investigated the role of protein kinase C (PKC) signal transduction pathways in parathyroid hormone (PTH) regulation of insulin-like growth factor-binding protein-5 (IGFBP-5) gene expression in the rat osteoblast-like cell line UMR-106-01. Involvement of the PKC pathway was determined by the findings that bisindolylmaleimide I inhibited 40% of the PTH effect, and 1 µM bovine PTH-(3-34) stimulated a 10-fold induction of IGFBP-5 mRNA. PTH-(1-34) and PTH-(3-34) (100 nM) both stimulated PKC- translocation from the membrane to the nuclear fraction. Rottlerin, a PKC--specific inhibitor, and a dominant negative mutant of PKC- were both able to significantly inhibit PTH-(1-34) and PTH-(3-34) induction of IGFBP-5 mRNA, suggesting a stimulatory role for PKC- in the effects of PTH. Phorbol 12-myristate 13-acetate (PMA) stimulated PKC- translocation from the cytosol to the membrane and inhibited ~50% of the PTH-(1-34), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate-stimulated IGFBP-5 mRNA levels, suggesting that PKC- negatively regulates protein kinase A (PKA)-mediated induction of IGFBP-5 mRNA. These results suggest that the induction of IGFBP-5 by PTH is both PKA and PKC dependent and PKC- is the primary mediator of the effects of PTH via the PKC pathway.