Chondrocyte apoptosis is a central pathological feature of cartilage in osteoarthritis (OA). Accumulating evidence suggests that calcium ions (Ca2+) are an important regulator of apoptosis. Previously, we reported that the transient receptor potential channel vanilloid (TRPV5) is upregulated in monoiodoacetic acid (MIA)-induced OA articular cartilage.
The protein levels of TRPV5, phosphorylated Ca2+/calmodulin-dependent kinase II (p-CaMKII), and total CaMKII were detected in vivo using western blotting techniques. Primary chondrocytes were isolated and cultured in vitro. Then, p-CAMKII was immunolocalized by immunofluorescence in chondrocytes. Fluo-4AM staining was used to assess intracellular Ca2+. Annexin V-fluorescein isothiocyanate / propidium iodide flow cytometric analysis was performed to determine chondrocyte apoptosis. Western blotting techniques were used to measure the expression of apoptosis-related proteins.
We found that ruthenium red (aTRPV5inhibitor)or(1-[N,O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperaze (KN-62) (an inhibitor of Ca2+/calmodulin-dependent kinase II (CaMKII) phosphorylation) can relieve or even reverse OA in vivo. We found that TRPV5 has a specific role in mediating extracellular Ca2+ influx leading to chondrocyte apoptosis in vitro. The apoptotic effect in chondrocytes was inhibited by KN-62. We found that activated p-CaMKII could elicit the phosphorylation of extracellular signal-regulated protein kinase 1/2, c-Jun N-terminal kinase, and p38, three important regulators of the mitogen-activated protein kinase (MAPK) cascade. Moreover, we also showed that activated p-CaMKII could elicit the phosphorylation of protein kinase B (Akt) and two important downstream regulators of mammalian target of rapamycin (mTOR): 4E-binding protein, and S61 kinase.
Our results demonstrate that upregulated TRPV5 may be an important initiating factor that activates CaMKII phosphorylation via the mediation of Ca2+ influx. In turn, activated p-CaMKII plays a critical role in chondrocyte apoptosis via MAPK and Akt/mTOR pathways.