Negative Regulation of Osteoclastogenesis by Inflammatory Signals

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Negative Regulation of Osteoclastogenesis by Inflammatory signals Rheumatoid arthritis (RA) is a chronic inflammatory disease in which immune cells and synovial fibroblasts produce pro-inflammatory cytokines, in particular TNF and IL-1, and drive an inflammatory state leading to destruction of affected joints. One important consequence of inflammation is the generation of osteoclasts, myeloid lineage cells that effectively resorb bone and thus are directly responsible for bone erosion and morbidity in RA. This application will focus on mechanisms of inhibition of the generation of osteoclasts. TLRs (Toll-like Receptors) are potent activators of inflammation and have been implicated in driving inflammatory bone resorption. However, at the same time that they activate inflammation, TLRs also induce potent homeostatic mechanisms to limit the intensity of inflammation and thus limit associated tissue damage. Disease progression is evidence of relatively ineffective feedback inhibition that is unable to restrain inflammation and bone resorption. Thus, we have initiated studies to understand the effective homeostatic regulation that occurs during physiological resolution of inflammation and quiescent phases of disease. Our overall hypothesis is that augmentation of physiological homeostatic mechanisms represents an effective approach to limiting bone resorption associated with inflammation and thus can form the basis for novel therapeutic approaches. We have found that TLR stimulation strongly suppresses osteoclastogenesis by inhibiting RANK, a key receptor required for osteoclastogenesis. The mechanism of TLR-mediated inhibition involves induction of a M-CSF receptor (c-Fms) shedding and proteolytic processing by activated TNF-alpha converting enzyme (TACE, also known as ADAM-17), potentially leading to cellular unresponsiveness to M-CSF. Cleaved c-Fms subsequently undergoes a series of proteolytic reactions that results in generation of the 50-kDa intracellular domain cleavage fragments (referred to as MICD). Interestingly, our results reveal that MICD is able to translocate into the nucleus, and expression of MICD enhances osteoclastogenesis. MICD generation is also diminished by inflammatory signals. We will use human systems that are directly relevant for RA pathogenesis as well as mouse system to test the in vivo role of MICD and TACE in inflammatory diseases. The long-term goals of this project are to: 1) understand molecular mechanisms by which c-Fms shedding and processing into MICD regulate osteoclastogenesis and the association of TACE with this process, and 2) identify the role of MICD in M-CSF signaling and differentiation of osteoclasts and macrophages. We anticipate that our studies will yield insights into homeostatic regulation that can not only be exploited for therapeutic interventions to suppress bone resorption associated with joint destruction but will also broaden understanding of the actions of M-CSF in the field of osteoimmunology.