IMPAIRED EXPRESSION OF GENES REGULATING CHOLESTEROL EFFLUX IN HUMAN OSTEOARTHRITIC CHONDROCYTES

Abstract

Osteoarthritis is a complex joint disease in which all involved tissues play an important role in its onset and progression. It has been suggested that osteoarthritis is likely to be a systemic disease involving stromal cell differentiation and lipid metabolism while altered lipid metabolism has been implicated as a critical player in its pathogenesis.

As excessive accumulation of free cholesterol is toxic for the cells, the accumulation of lipids in chondrocytes may signify a causal relationship to development and/or progression of osteoarthritis; therefore we investigated the expression of genes regulating reverse cholesterol transport, as ABCA1, ApoA1, LXRa, LXR_, in human osteoarthritic chondrocytes. We also investigated the effect of an LXR agonist on ABCA1 and ApoA1 expression and, for the first time, on cholesterol effiux and lipid accumulation in osteoarthritic chondrocytes.

Articular cartilage samples were obtained from femoral condyles and tibial plateaus of patients with primary OA undergoing knee replacement surgery while normal cartilage was obtained from eight individuals undergoing fracture repair surgery, with no history of joint disease. Total cellular RNA was extracted from all samples and ABCA1, ApoA1, and LXRα and LXRβ mRNA and protein expression levels were evaluated using real-time PCR and Western blot analysis respectively.

The effect of the synthetic LXR agonist TO-901317 was studied after treatment of osteoarthritic chondrocytes and subsequent investigation of ABCA1 and ApoA1 mRNA expression levels. Cholesterol effiux was evaluated in osteoarthritic chondrocytes radiolabeled with [1,2(n)-3H] cholesterol after LXR treatment, while intracellular lipid accumulation was studied after Oil-red-O staining.

ApoA1 and ABCA1 mRNA levels were significantly lower in osteoarthritic cartilage compared to normal (p< 0.01 and p< 0.001 respectively). In addition, the two subtypes of the LXR, namely LXRα and LXRβ, mRNA levels were also found to be significantly lower in osteoarthritic cartilage (p< 0.05 and p< 0.01 respectively). The differential expression pattern of the cholesterol effiux genes between normal and osteoarthritic cartilage remained the same at the protein level as well. Treatment of osteoarthritic chondrocytes with the LXR agonist TO-901317 significantly increased ApoA1 and ABCA1 expression levels, as well as cholesterol effiux. Additionally, osteoarthritic chondrocytes presented intracellular lipids deposits, while no deposits were found after treatment with TO-901317.

Our findings suggest that impaired expression of genes regulating cholesterol effiux may be a critical player in osteoarthritis, while the ability of the LXR agonist to facilitate cholesterol effiux suggests that it may be a target for therapeutic intervention in osteoarthritis.