ucOCN Inhibits Chondrocyte Hypertrophy and Osteoarthritis Development through GPRC6A/HIF-1α Cascade

Abstract

Abstract Osteocalcin (OCN), as a characteristic protein expressed at the mature stage of osteoblasts, plays an important role in crosstalk within the skeletal muscle system. Mature OCN is secreted out of the cell after carboxylation in the endoplasmic reticulum. According to the degree of carboxylation, it can be divided into carboxylated OCN (cOCN) and undercarboxylated OCN (ucOCN). The two forms of proteins exhibit different biological activities, which make their functions more complex than those currently recognized. Many literatures reported that more osteocalcin was expressed in the hypertrophic stage of chondrocytes, even as a marker protein of osteoarthritis (OA) chondrocytes. However, these studies paid few attention to the potential different roles of different carboxylated forms of OCN in chondrocytes. Here, we discovered that ucOCN played a significant protecting role in OA development. In OCN knockout (OCN-/-) mice, articular chondrocytes showed a hypertrophic tendency. But in mice OA model, OCN-/- mice didn’t develop more severe OA than WT mice. Meanwhile, OCN in chondrocytes increased significantly and ucOCN in synovial fluid decreased dramatically in WT mice. We hypothesized that intracellular OCN and extracellular ucOCN may play different roles in the development of OA. By manipulating the amount of ucOCN in the synovial fluid in wild type mice and a series of experiments in vitro, we confirmed the protective effect of ucOCN in OA. We further explored the downstream pathway of ucOCN in chondrocytes and verified that ucOCN activates HIF-1α pathway through G protein coupled receptor 6A (GPRC6A) to inhibit chondrocyte hypertrophy.