High glucose upregulates BMAL1 in chondrocytes from macroscopically normal and osteoarthritic human cartilage but this only results in increased collagen synthesis in chondrocytes from macroscopically normal cartilage regions

Objective Osteoarthritic (OA) chondrocytes exhibit limited capacity for new collagen synthesis. High glucose promotes collagen I and III production by fibroblasts. The purpose of this study was to determine if it also promotes collagen II (COL2) synthesis by chondrocytes. Design Chondrocytes isolated from osteoarthritic (OA) and macroscopically normal (MN) cartilage from patients with knee OA were maintained in 1 % oxygen in 4 mM or 10 mM glucose for up to 24 h. Results Secreted COL2 levels were higher in MN but not OA chondrocytes maintained in 10 mM versus 4 mM glucose with increased COL2 production specifically evident in the first 6 h of high glucose exposure. COL2A1 and proly-4-hydroxylase α2 (P4HA2) RNA and protein were upregulated by high glucose in MN and OA chondrocytes at this timepoint. While P4HA2 levels were similar in MN and OA chondrocytes, COL2A1 RNA and SOX9 protein levels were lower in OA chondrocytes. BMAL1 protein levels were elevated by high glucose in both MN and OA chondrocytes at 6 h and BMAL1 knockdown reduced P4HA2 and COL2A1 in both MN and OA chondrocytes at this timepoint. Conclusions These data demonstrate a novel mechanism by which high glucose-mediated upregulation of BMAL1 promotes collagen synthesis in chondrocytes. Although high glucose leads to similar upregulation of BMAL1 and P4HA2 in both MN and OA chondrocytes, overall lower COL2A1 RNAin OA chondrocytes likely limits their ability to upregulate secreted procollagen II levels. Hence, MN but not OA chondrocytes show increased collagen II production in high glucose.