TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics

Parathyroid hormone (PTH) and tumor necrosis factoralpha (TNFalpha) are bone resorptive agents that upregulate interleukin-6 (IL-6) and RANKL production by osteoblasts. IL-6 mRNA expression induced by PTH is rapid and transient in osteoblasts both in vitro and in vivo. This study found that IL-6 secretion induced by PTH is also rapid and transient. The induction of RANKL mRNA by PTH is also rapid and transient although with an extended time course compared to that of IL-6 mRNA. In contrast, the effects of TNFalpha are biphasic. During the first 2 h of stimulation with TNFalpha, the responses are similar to those induced by PTH. This is followed by a period of relatively low IL-6 and RANKL mRNA levels and little IL-6 secretion. A late phase of increased IL-6 and RANKL mRNA expression occurs 12-24 h after stimulation with TNFalpha leading to a significant increase in IL-6 secretion. A similar biphasic pattern of activation of p38 MAP kinase is induced by TNFalpha. p38alpha/beta activation is required for the increased RANKL mRNA during the early phase of stimulation by TNFalpha but not in the late phase. In contrast, p38alpha/beta activation is not required for increased IL-6 mRNA or IL-6 protein secretion in either the early or late phases of stimulation by TNFalpha. Blocking the increases in IL-6 transcription completely eliminates IL-6 secretion induced during the early phases of stimulation by either PTH or TNFalpha. Consistent with the dependence on transcription, IL-6 mRNA is rapidly degraded with half-lives of 10-14 min following stimulation with either PTH or TNFalpha. In contrast to IL-6, RANKL mRNA is substantially more stable with half-lives of 40-60 min. Taken together, our results show that TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics. The more extensive effect of TNFalpha likely reflects that TNFalpha stimulates IL-6 production and bone resorption in pathological situations. In contrast, the less extensive effect of PTH likely reflects that it acts in physiological situations where it is important to minimize the potential adverse effects of high levels of IL-6 on bone and/or surrounding tissues.