Hypoxia Inducible Factor-1 alpha Is a Regulator of Autophagy in Osteoarthritic Chondrocytes

Objective To investigate the relationship between hypoxia inducible factor-1 alpha (HIF-1 alpha) and the autophagic response in osteoarthritic chondrocytes (OA), under inflammatory insult as represented by in vitro OA model. Methods Human chondrocyte cell line C28/I2 was cultured in both normoxic and hypoxic conditions and treated with interleukin-1 beta (IL1 beta) to emulate OA inflammatory insult in vitro. Cellular HIF-1 alpha expression was silenced using siRNA transfection and cellular autophagic (P62/LC3II) response and OA chondrocyte damage (COL2A1/MMP13) related proteins were examined using western blotting. Cellular mitophagic (BNIP3/PINK1/Parkin) and apoptotic (Caspase/Cleaved Caspase 3) were also evaluated to assess mitophagy-mediated cell death due to HIF-1 alpha silencing. Results Chondrocyte basal autophagy levels were higher in a HIF-1 alpha elevated environment and was more resistant to IL1 beta-induced inflammatory insult. Increase in autophagic proteins showed better chondrocyte repair, which resulted a lower level of reactive oxygen species production, and lesser damage to chondrocyte integrity. Silencing HIF-1 alpha activates cellular PINK1/Parkin and BNIP3 mitophagic proteins, which leads to the activation of Caspase/Cleaved Caspase 3 apoptotic cascade. Conclusion Our results show that chondrocyte autophagy is dependent on HIF-1 alpha expression, showing the importance of HIF-1 alpha in hypoxic chondrocyte function in OA. Dysregulation of HIF-1 alpha expression results in the activation of mitophagy-mediated apoptosis.