RACK1 mediates the advanced glycation end product‐induced degradation of HIF‐1α in nucleus pulposus cells via competing with HSP90 for HIF‐1α binding

Abstract Hyperglycemia can drive advanced glycation end product (AGE) accumulation and associated nucleus pulposus cell (NPC) dysfunction, but the basis for this activity has not been elucidated. Hypoxia‐inducible factor‐1α (HIF‐1α) is subject to cell‐type‐specific AGE‐mediated regulation. In the current study, we assessed the mechanistic relationship between AGE accumulation and HIF‐1α degradation in NPCs. Immunohistochemical staining of degenerated nucleus pulposus (NP) samples was used to assess AGE levels. AGE impact on NPC survival and glycolysis‐related gene expression was assessed via 3‐(4,5)‐dimethylthiazol(‐z‐y1)‐3,5‐di‐phenyltetrazolium bromide assay and quantitative reverse‐transcription polymerase chain reaction (qRT‐PCR), while HIF‐1α expression in NPCs following AGE treatment was monitored via Western blot analysis and qRT‐PCR. Additionally, a luciferase reporter assay was used to monitor HIF‐1α transcriptional activity. The importance of the receptor for activated C‐kinase 1 (RACK1) as a mediator of HIF‐1α degradation was evaluated through gain‐ and loss‐of‐function experiments. Competitive binding of RACK1 and HSP90 to HIF‐1α was evaluated via immunoprecipitation. Increased AGE accumulation was evident in NP samples from diabetic patients, and AGE treatment resulted in reduced HIF‐1α protein levels in NPCs that coincided with reduced HIF‐1α transcriptional activity. AGE treatment impaired the stability of HIF‐1α, leading to its RACK1‐mediated proteasomal degradation in a manner independent of the canonical PHD‐mediated degradation pathway. Additionally, RACK1 competed with HSP90 for HIF‐1α binding following AGE treatment. AGE treatment of NPCs leads to HIF‐1α protein degradation. RACK1 competes with HSP90 for HIF‐1α binding following AGE treatment, resulting in posttranslational HIF‐1α degradation. These results suggest that AGE is an intervertebral disc degeneration risk factor, and highlight potential avenues for the treatment or prevention of this disease.