We have previously demonstrated that long-term heparin treatment causes cancellous bone loss in rats due in part to an increase in the number of osteoclasts lining the trabecular bone surface. In the present study, we investigated this phenomenon by examining the ability of heparin to synergistically enhance interleukin-11 (IL-11)-induced osteoclast formation. Treatment of murine calvaria and bone marrow cells with IL-11 was found to induce the formation of tartrate-resistant acid phosphatase-positive (TRAP ؉ ) multinucleated cells (MNCs) in a dose-dependent fashion. No effect was seen when cocultures were treated with heparin alone. However, when cocultures were treated with both IL-11 and heparin, IL-11's ability to induce TRAP ؉ MNC formation was enhanced 6-fold. In an attempt to resolve the mechanism responsible for this effect, we examined the ability of heparin to influence IL-11 signaling using murine calvaria cells. Heparin was found to enhance both IL-11-induced STAT3-DNA complex formation and transactivation without altering either STAT3 (signal transducer and activator of transcription-3) tyrosine or serine phosphorylation. Heparin was also found to enhance IL-11's ability to induce the expression of both receptor activator of nuclear factor-B ligand (RANKL) and glycoprotein (gp) 130. When taken together, these findings suggest a plausible mechanism by which heparin may cause increased osteoclastogenesis and therefore bone loss when administered longterm. (Blood. 2002;100:2530-2536)

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Using an animal model of heparin-induced osteoporosis we previously demonstrated that heparin causes bone loss, in part, by increasing osteoclast number and activity. In an effort to account for these findings, we examined the effect of heparin on osteoclast formation in vitro. We found that while heparin alone had no effect, it was able to synergistically enhance Interleukin-11 (IL-11) - induced STAT3 (Signal Transducer and Activator of Transcription) activation and thus, increase both gp130 expression and osteoclast formation. In the present study, we examine the effect of various serine kinase inhibitors on heparin’s ability to synergistically enhance both STAT3 activation and gp130 expression. As assessed by electrophoretic mobility shift assays (EMSAs), treatment of murine calvaria cells with heparin was found to increase IL-11-induced STAT3-DNA complex formation by 2 fold. Inhibition of the c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein (MAP) kinase, or the phosphatidylinositol 3-kinase (PI3-K) pathway had no effect on heparin’s ability to promote STAT3-DNA complex formation. In contrast, the mitogen-activated protein kinase kinase (MEK) inhibitor, PD098059, completely abolished the ability of heparin to act synergistically with IL-11. Similar results were obtained when northern blot analysis was used to assess heparin’s ability to enhance IL-11-induced gp130 expression. In an attempt to resolve the mechanism by which this was occurring, we examined the ability of heparin to influence STAT3 Ser727 phosphorylation in the presence or absence of IL-11. Heparin alone was found to have no effect on Ser727 phosphorylation, nor did heparin alter the phosphorylation status of Ser727 in the presence of IL-11. Heparin was, however, found to increase ERK1/2 (extracellular signal regulated kinase) activation in both a time and dose-dependent manner. When taken together, these findings suggest that heparin enhances IL-11-induced STAT3 activation and thus, gp130 expression, by a mechanism that is independent of STAT3 Ser727 phosphorylation, but which involves upregulation of the MAP Kinase pathway.