Objective: Findings of recent in vivo and in vitro studies suggest that oxidized low-density lipoprotein (ox-LDL) plays a role in the degeneration of cartilage. The purpose of this study was to determine whether ox-LDL induces chondrocyte senescence through binding to lectin-like ox-LDL receptor 1 (LOX-1).
Methods: The effects of ox-LDL on senescence of cultured bovine articular chondrocytes (BACs) were investigated by observing senescence-associated (SA) beta-galactosidase (beta-gal) activity, cell proliferation activity, and telomerase activity. Telomerase activity was measured after adding LY294002 (a specific inhibitor of phosphatidylinositol 3-kinase [PI3K]) or after adding insulin-like growth factor 1 (IGF-1; an activator of PI3K) plus ox-LDL to the culture medium to elucidate the involvement of the PI3K/Akt pathway. Immunoblot analysis was used to investigate whether ox-LDL affects the phosphorylation of Akt. To ascertain whether these effects were attributable to ox-LDL binding to LOX-1, BACs were preincubated with TS-20, an anti-bovine LOX-1 blocking antibody.
Results: The activity of SA beta-gal was increased and the incorporation of bromodeoxyuridine into BACs was decreased by ox-LDL in a dose-dependent manner. The telomerase activity of BACs was suppressed by the addition of ox-LDL in a time- and dose-dependent manner. LY294002 suppressed the telomerase activity of BACs, and IGF-1 reversed the ox-LDL-induced suppression of telomerase activity. In addition, ox-LDL rapidly decreased the amount of phosphorylated Akt in BACs. Pretreatment of cultured BACs with TS-20 recovered these effects.
Conclusion: These data show that ox-LDL binding to LOX-1 induces stress-induced premature senescence of chondrocytes and results in suppression of telomerase activity by inactivating the PI3K/Akt pathway. Oxidized LDL may play an important role in the pathogenesis of osteoarthritis by inducing chondrocyte senescence.