Osteoarthritis: the role and mechanisms of the chondroprotective peptide Urocortin I
Osteoarthritis (OA), a chronic degenerative condition of the diarthrodial joints is most predominant amongst the middle aged and elderly population, where it is associated with progressive disability and deformity of the joints. Whilst the exact mechanism of OA pathogenesis is currently unknown, chondrocyte death by apoptosis is seen as one of the main contributors to the disease process. It is increasingly recognised that the overproduction of mediators such as Nitric Oxide (NO) in Osteoarthritic cartilage contributes to the disease pathology partly by inducing chondrocyte apoptosis. Therefore, agents, which protect against NOinduced mitochondrial injury, may have therapeutic potential in Osteoarthritis. The CRF related peptide, Urocortin I (UCNI), has been shown to protect the human chondrocyte cell line, C-20/A4, from SNAP/NO induced apoptotic death, but little is yet known about its mechanism of action. Therefore the aim of this project is to investigate the potential mechanism through which UCNI exerts its observed protective effects. The studies documented here, have demonstrated the endogenous expression of this cytoprotective peptide, UCNI, by C-20/A4 chondrocytes and have shown that treatment of chondrocytes with SNAP (a NO donor) resulted in a significant increase in apoptosis which was abrogated by the addition of exogenous UCNI. Therefore, an investigation into the potential mechanism for UCNI mediated protection against apoptosis in chondrocytes (i.e. chondroprotection) detected the mRNA expression of both CRFR1 and CRFR2 receptors specifically the CRFR1α and CRFR2β splice variants by the C-20/A4 chondrocytes. However the CRFR antagonist, α−helical CRH failed to induce apoptosis or abrogate the protective effects of UCNI against SNAP/NO induced apoptosis. RT-PCR studies have also demonstrated C-20/A4 mRNA expression of the Kir6.1, Kir6.2, SUR1 and SUR2B subunits of the KATP channels suggesting that functional KATP channels are also present in these cells, which represent a further putative mechanism of action for UCNI. Quantitative PCR studies were performed to determine regulation of CRFR and Kir6.1 expression by UCNI. These studies showed that treatment of chondrocytes with UCNI and SNAP alone and concurrently resulted in no significant change in mRNA expression of either CRF receptors or kir6.1 (the pore forming subunit of the mitochondrial KATP channel). Treatment of chondrocytes with Diazoxide, a mitochondrial KATP channel opener, resulted in a reduction in SNAP induced apoptosis, mimicking that seen for UCNI, suggesting a similar protective role for these channels in chondrocytes, to that proposed in other cell types and supporting these channels as potential targets for UCNI in chondroprotection.
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