Parathyroid Hormone-Related Protein (107-139) Stimulates Interleukin-6 Expression in Human Osteoblastic Cells

The N-terminal region of both parathyroid hormone (PTH) and PTH-related protein (PTHrP) binds to the same PTH/PTHrP receptor in osteoblasts. However, C-terminal PTHrP (107-139) inhibits growth and various functions of osteoblasts and osteoclasts apparently through PTHrP-specific receptors. PTH (1-34) and PTHrP (1-34) rapidly induce interleukin-6 (IL-6) expression by osteoblasts. The aim of the present study was to assess the effects of PTHrP (107-139) on IL-6 gene expression and secretion by osteoblastic cells from human trabecular bone (hOB). Using reverse transcription followed by PCR, it was found that IL-6 mRNA was twofold maximally increased by either PTHrP (1-34) or PTHrP (107-139), at 10 nM, over basal within 1 to 2 h in hOB cells. This effect of PTHrP (107-139), and that of PTHrP (1-34), were abolished by the transcription inhibitor actinomycin D. Meanwhile, puromycin, a protein synthesis inhibitor, superinduced IL-6 expression in the presence or absence of each PTHrP peptide. Both PTHrP (1-34) and PTHrP (107-139), but not PTHrP (38-64), stimulated IL-6 secretion to the hOB cell-conditioned medium at 24 h, dose dependently. In addition, this maximal stimulatory effect (twofold over basal) was similar with each PTHrP peptide alone, and not additive when added together. PTHrP (107-139) stimulation of mRNA and protein in hOB cells was abolished by bisindolylmaleimide I, a protein kinase C inhibitor, but not by either adenosine 3′,5′-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS), or N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H89), two protein kinase A inhibitors. These results indicate that C-terminal PTHrP, like its N-terminal domain, induces IL-6 production by human osteoblastic cells. This effect of both PTHrP regions could provide a mechanism to modulate bone turnover.