Protein kinase C involvement in interleukin-6 production by parathyroid hormone and tumor necrosis factor-α in UMR-106 osteoblastic cells

The cytokine interleukin-6 (IL-6) is increased in bone and bone cells by several resorptive stimuli, including parathyroid hormone (PTH), IL-1β, and tumor necrosis factor-α (TNF-α). The current studies were designed to determine the contribution of the protein kinase C (PKC) signaling pathway to the effects of these three agents to increase IL-6 in UMR-106 rat osteoblastic cells. Cells were pretreated with vehicle (dimethylsulfoxide [DMSO]) or the phorbol ester, phorbol 12,13-dibutyrate (PDB; 300 nM) for 48 h to down-regulate phorbol-sensitive PKC isozymes. Either PTH (0.1-10 nM), IL- 1β (0.1-10 nM), or TNF-α (5 nM and 10 nM) was then added for 24 h in the continued presence of vehicle or PDB. PKC isozymes were visualized by Western immunoblotting and IL-6 was determined by bioassay. PDB pretreatment caused a partial downregulation of the conventional α-PKC and β₁-PKC isozymes and complete down-regulation of the novel δ-isoenzyme and ε-isozymes but it had no effect on the atypical ζ-PKC isozyme. PDB pretreatment reduced IL-6 responses to 5 nM and 10 nM PTH by 61% and 33%, respectively, reduced IL-6 responses to 5 nM and 10 nM TNF-α by 54% and 42%, respectively, and failed to inhibit the IL-6 responses to 0.1-10 nM IL-1β. The PDB pretreatment protocol significantly enhanced PTH-stimulated cyclic adenosine monophosphate (cAMP) production. The PKC inhibitor calphostin C also decreased IL-6 responses to PTH. Thus, in this osteoblast cell line, the PKC pathway is an important component of the signaling pathway for the IL-6 production stimulated by PTH and TNF-α but not that from IL-1β.