Interleukin-4: a molecular switch from osteoclasts to multinucleated giant cells

Abstract

Interleukin 4 (IL-4) inhibited Receptor Activator of NF-κB Ligand (RANKL)-induced osteoclast (OC) differentiation, while at the same time promoted macrophage fusion to form multinucleated giant cells (MNG). This was dependent on the expression of signal transducer and activator of transcription-6 (STAT6). The precise mechanism of this is not understood. The regulation of one typical osteoclastogenic gene, tartrate-resistant acid phosphatase (TRAP), by IL-4 was studied in detail. RANKL significantly upregulated TRAP expression and its expression was inhibited in the presence of IL-4. We found that IL-4 activated-STAT6 directly bound to the TRAP promoter. However, the effect of STAT6 binding was to modestly enhance TRAP transcription. The ability of IL-4 to inhibit RANKL- induced TRAP was through an indirect mechanism: IL-4 significantly inhibited the expression of the transcription factor NFATc1. The IL-4-induced downregulation of NFATc1 was responsible for the downregulation of TRAP. Several groups have proposed that IL-4 inhibits osteoclastogenesis by suppressing the RANKL-induced activation of NF-κB. However, we found that IL-4 did not inhibit the ability of RANKL to activate the canonical NF-κB pathway. Rather, IL-4 inhibited activation of the alternative pathway. IL-4 also upregulated p105/50 expression in the presence of RANKL. IL-4 did not inhibit osteoclastogenesis in p105/50 deficient bone marrow-derived macrophages (BMM). However, in p105/50 deficient BMM, the formation of both multinucleated OC and MNG induced by RANKL or IL-4 respectively was impaired. This suggests that NF-κB also played an important role in IL-4-induced macrophage fusion. In addition, an inhibitor of NF-κB activation effectively blocked both OC and MNG formation. Furthermore, transduction of BMM with p50, p65, p52 or RelB enhanced both OC and MNG formation. These results suggest that NF-κB molecules participates in the IL-4-induced MNG formation and that alteration of the NF-κB pathway may induce a switch in cell fate from OC to MNG.