Abstract
IntroductionBone remodelling and increased subchondral densification are important in osteoarthritis (OA). Modifications of bone vascularization parameters, which lead to ischemic episodes associated with hypoxic conditions, have been suspected in OA. Among several factors potentially involved, leptin and dickkopf-related protein 2 (DKK2) are good candidates because they are upregulated in OA osteoblasts (Obs). Therefore, in the present study, we investigated the hypothesis that hypoxia may drive the expression of leptin and DKK2 in OA Obs.MethodsObs from the sclerotic portion of OA tibial plateaus were cultured under either 20% or 2% oxygen tension in the presence or not of 50 nM 1,25-dihydroxyvitamin D3 (VitD3). The expression of leptin, osteocalcin, DKK2, hypoxia-inducible factor 1α (Hif-1α) and Hif-2α was measured by real-time polymerase chain reaction and leptin production was measured by enzyme-linked immunosorbent assay (ELISA). The expression of Hif-1α, Hif-2α, leptin and DKK2 was reduced using silencing RNAs (siRNAs). The signalling pathway of hypoxia-induced leptin was investigated by Western blot analysis and with mitogen-activated protein kinase (MAPK) inhibitors.ResultsThe expression of leptin and DKK2 in Obs was stimulated 7-fold and 1.8-fold, respectively (P <0.05) under hypoxia. Interestingly, whereas VitD3 stimulated leptin and DKK2 expression 2- and 4.2-fold, respectively, under normoxia, it stimulated their expression by 28- and 6.2-fold, respectively, under hypoxia (P <0.05). The hypoxia-induced leptin production was confirmed by ELISA, particularly in the presence of VitD3 (P <0.02). Compared to Obs incubated in the presence of scramble siRNAs, siHif-2α inhibited VitD3-stimulated leptin mRNA and protein levels by 70% (P =0.004) and 60% (P <0.02), respectively, whereas it failed to significantly alter the expression of DKK2. siHif-1α has no effect on these genes. Immunoblot analysis showed that VitD3 greatly stabilized Hif-2α under hypoxic conditions. The increase in leptin expression under hypoxia was also regulated, by p38 MAPK (P <0.03) and phosphoinositide 3-kinase (P <0.05). We found that the expression of leptin and DKK2 were not related to each other under hypoxia.ConclusionsHypoxic conditions via Hif-2 regulation trigger Obs to produce leptin, particularly under VitD3 stimulation, whereas DKK2 is regulated mainly by VitD3 rather than hypoxia.
Highlights
Bone remodelling and increased subchondral densification are important in osteoarthritis (OA)
The expression of leptin, osteocalcin, dickkopf-related protein 2 (DKK2), hypoxia-inducible factor 1α (Hif-1α) and Hypoxia-inducible transcription factor (Hif)-2α was measured by real-time polymerase chain reaction and leptin production was measured by enzyme-linked immunosorbent assay (ELISA)
Compared to Obs incubated in the presence of scramble Silencing RNA (siRNA), siHif-2α inhibited VitD3-stimulated leptin mRNA and protein levels by 70% (P =0.004) and 60% (P
Summary
Bone remodelling and increased subchondral densification are important in osteoarthritis (OA). Similar blood flow disturbances were demonstrated in the subchondral bone medial plateau of guinea pigs, which develop spontaneous knee OA [14] In this model, the observation of outflow obstruction preceding bone and cartilage OA lesions lends support to a key role of abnormal vascularization of OA bone tissues. Chang et al demonstrated that hypoxia induced vascular endothelial growth factor (VEGF) expression as well as other genes involved in bone vasculature in OA osteoblasts (Obs) [21]. They suggested that hypoxia modifies the OA Ob phenotype. An accumulated Hif-α protein translocates to the nucleus and forms a dimer with Hif-β
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