Background Autophagy is an essential, homeostatic process by which cells digest unnecessary cellular organelles. Beclin1 and Atg7 are markers of autophagy that are required for the initiation of the autophagasomeand fusion of peroxisomal and vaculuolar membranes, respectively. Accumulating evidence demonstrates that autophagy is involved in the pathology of varies diseases including infections, cancer, neurodegeneration, aging and heart disease.
Objectives Rheumatoid arthritis (RA) is a chronic, systemic inflammatory disorder characterized by increased activation of osteoclasts and progressive joint destruction. The underlying mechanisms are incompletely understood. Here, we investigated the role of autophagy in bone destruction in rheumatoid arthritis.
Methods The expression of Beclin1 and Atg7 in osteoclasts was investigated by immunofluorescence staining in RA and OA patients. To specifically inhibit autophagy in the monocyte linage, Atg7 fl/fl mice were crossbred with LysMCre+ mice. We also applied electron microscopy and acridine orange fluorescent stain to quantify autophagy in osteoclasts. Bone marrow cells from Atg7 fl/fl × LysMCre+ or Atg7 fl/fl × LysMCre– BMCs were transplanted into radiated hTNFα tg mice to generate Atg7 fl/fl × LysMCre+ × hTNFα tg mice. Microcomputed tomography and histomorphometry were used to analyze bone ersoion in vivo. Lentiviral constructs encoding for Beclin1 were used to enhance autophagy in osteoclasts in vitro and in vivo.
Results We demonstrate that autophagy is activated in RA with overexpression of Atg7 and Beclin1 in osteoclasts of RA patients. We identified TNFα as a major inducer of autophagy in osteoclasts in vitro and in vivo. Stimulation of autophagy in monocytes by lentiviral overexpression of Beclin-1 induced osteoclastogenesis and significantly increased the number and the average size of multinucleated, TRAP positive cells. Overexpression of Beclin-1 upregulated the expression of osteoclast-associated genes such as NFATc1, OSCAR, TRAP and cathepsin K. Moreover, overexpression of Beclin1 significantly enhanced the resorptive capacity of osteoclasts. On the other hand, silencing autophagy by knockdown of Atg7 or incubation with Bafilomycin A1 prevented osteoclast differentiation and reduced bone resorption in vitro. Microcomputed tomography and histomorphometry analyses showed that radiated hTNFα tg mice transplanted with Atg7 fl/fl x LysMCre+BMCs significantly protected from TNFα induced bone erosion and reduced 20% of proteoglycan loss, 28% of chondrocyte death and also reduced 67% of the total number of osteoclasts and 50% of erosion area.
Conclusions We provide first evidence for a central role of autophagy in joint destruction. Autophagy is activated in RA by TNFα and stimulates osteoclast differentiation and activity. Inhibition of autophagy selectively in monocytic cells strongly reduces osteoclast differentiation and joint destruction in hTNFα tg mice.
Disclosure of Interest None Declared
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