Effect of IL-6 on Osteoclast Generation and Regulation of Its Production in Osteoblastic Cells

다국어 초록 (Multilingual Abstract)

Interleukin-6 （IL-6） is a multifunctional cytokine that known to be synthesized by osteoblasts and suggested to be an important regulator of bone resorption in some physiologic and／or pathologic conditions. However, the mechanism of cation of IL-6 responsible for bone resorption in vitro and role as a possible mediator of bone resorptive action of systemic or local osteotropic agents are not clear yet. Therefore, to further understand the mechanism of IL-6 action on bone resorption, we examined the effects of IL-6 on the generation of osteoclast-like cells using mouse bone marrow culture system. We also observed the regulation of IL-6 secretion in human osteoblastic cells by several cytokines and systemic osteotropic hormones, known to stimulate or inhibit bone resorption.
To observe the effects of IL-6 on the generation of osteoclast-like cells, mouse bone marrow cells were isolated from femurs and tibiae of 4 to 6 week-old mice, plated at 3.0×10^6 cells／well in 24-well plates and cultured for 8 days. In order to examine the role of IL-6 in osteoclast-like multinucleated cell （MNC） generation, bone marrow cells were treated with IL-6 alone or in combination with prostaglandin E_2（PGE_2）. After culture, cells were stained for tartrate-resistant acid phosphatase （TRAP）, a marker enzyme of osteoclasts, according to the modified method of Burstone. The TRAP-positive MNCs, which have 3 or more nuclei, were counted. Also we observed the regulation of IL-6 secretion in human osteoblastic cells （MG-63） by several osteotropic agents using the quantitative enzyme-linked immunosorbent assay.
The present study showed that IL-6 alone did not generate TRAP-positive MNCs, but significantly enhanced the generation of TRAP-positive MNCs induced by PGE_2（10^－6 M） in a dose-dependent manner. Though the augmenting effect of IL-6 was significant through all the culture period, it was greater at later period of culture. And augmenting effect of IL-6 on generation of TRAP-positive MNCs significantly appeared from the 6th-day of culture. These results suggest that IL-6 enhances the effect of PGE_2 on TRAP-positive MNC generation by stimulating both differentiation and fusion of osteoclast precursors. On the otherhand, bone resorbing cytokines such as interleukin-1β（IL-1β）and tumor necrosis factor-α（TNF-α） stimulated the secretion of IL-6 from MG-63 cells, but interferon-γ（IFN-γ）, known to be an inhibitor of bone resorption, significantly inhibited the TNF-α-stimulated secretion of IL-6. However, parathyroid hormone, 1,25（OH）_2-vitamin D_3, and PGE_2, well known bone resorbing agents, did not induce the secretion of IL-6 from MG-63 cells.
Taken together, these results suggest that IL-6 may play an important role in bone resorption by regulating the generation of osteoclasts and at least partly, mediate the bone modulating effects of several cytokines.