Human osteoarthritic chondrocytes up-regulate the expression of osteoprotegerin in osteoblasts via the Indian hedgehog signaling pathway
Li Jiale1, 2, Luo Dasheng1, Zheng Liujie1, Liu Wei1, Yao Yunfeng1
1The Second Hospital of Anhui Medical University, Hefei 230601, Anhui Province, China; 2Fuyang Hospital of Anhui Medical University, Fuyang 236000, Anhui Province, China
Abstract: BACKGROUND: Upregulation of hedgehog protein signaling can increase the expression of osteoarthritis markers, Runx2, a disintegrin and metalloproteinase with thrombospondin motifs, collagen type X alpha 1, and matrix metalloproteinase 13, while inhibition of hedgehog proteins attenuates the severity of osteoarthritis. It is speculated that osteoarthritic chondrocytes can influence bone formation by affecting osteoblasts through the Indian hedgehog protein (IHH) signaling pathway.
OBJECTIVE: To investigate the effect of human osteoarthritic chondrocytes on subchondral osteoblasts.
METHODS: Tibial plateau specimens from patients with osteoarthritis were collected. Chondrocytes were extracted using enzymatic digestion, and osteoblasts were extracted using enzymatic pre-digestion + bone block method. Chondrocytes were identified by toluidine blue staining and immunofluorescence and osteoblasts were identified by alkaline phosphatase staining and immunofluorescence. Chondrocytes were cultured in sodium alginate beads to maintain chondrocyte phenotype and co-cultured with osteoblasts. The co-culture system was added with IHH signaling pathway inhibitor (cyclopamine, 10 nmol/L) and activator (purmorphamine, 10 nmol/L) separately. After 48 hours of co-culture, osteoblasts from each group were collected, mRNA expressions of Gli1, osteoprotegerin, Runx2, parathyroid hormone-related peptide, alkaline phosphatase, receptor activator of nuclear factor-kB ligand (RANKL) and osteocalcin were detected by qRT-PCR, and protein expressions of GLi1, oseoprotegerin and RANKL in osteoblasts were detected by western blot.
RESULTS AND CONCLUSION: The mRNA expression levels of GLi1, osteoprotegerin and RUNX2 in osteoblasts were significantly increased, while the mRNA expression levels of parathyroid hormone-related peptide were decreased (P < 0.05) when co-cultured with human osteoarthritic chondrocytes. The mRNA and protein levels of Gli1 were significantly decreased after the addition of IHH signaling pathway inhibitor (cyclopamine) (P < 0.05), and the mRNA and protein levels of Gli1 were significantly increased after the addition of IHH signaling pathway activator (purmorphamine) (P < 0.05). Osteoprotegerin showed the same trend as Gli1 in the experiment. The osteoprotegerin/RANKL ratio followed the same trend as osteoprotegerin. To conclude, human osteoarthritic chondrocytes can promote the expression of Gli1, osteoprotegerin, Runx2 and other proteins in osteoblasts. The upregulation of osteoprotegerin is related to the IHH signaling pathway. Osteoarthritic chondrocytes can up-regulate the expression of osteoprotegerin in osteoblasts through the IHH signaling pathway and thus up-regulate the osteoprotegerin/RANKL ratio, which will contribute to bone formation in subchondral bone.
Key words: osteoarthritis, osteoblast, chondrocyte, Indian hedgehog signaling pathway, osteoprotegerin, receptor activator of nuclear factor-kB ligand