Objective: Autologous chondrocyte implantation requires expansion of cells ex vivo, leading to dedifferentiation of chondrocytes (loss of aggrecan and type II collagen to the profit of type I and type III collagens). Several approaches have been described for redifferentiation of these cells. Among them, low oxygen tension has been exploited to restore the differentiated chondrocyte phenotype, but molecular mechanisms of this process remain unclear. However, under conditions of hypoxia, one of the major factors involved is hypoxia‐inducible factor 1α (HIF‐1α). The purpose of this study was to investigate the role of HIF‐1α during human chondrocyte redifferentiation.Methods: We used complementary approaches to achieving HIF‐1α loss (inhibition by cadmium ions and dominant‐negative expression) or gain (ectopic expression and cobalt ion treatment) of function. Expression of chondrocyte, as well as fibroblast‐like, phenotype markers was determined using real‐time reverse transcription–polymerase chain reaction and Western blot analyses. Binding activities of HIF‐1α and SOX9, a pivotal transcription factor of chondrogenesis, were evaluated by electrophoretic mobility shift assays and by chromatin immunoprecipitation assay.Results: We found that hypoxia and HIF‐1α not only induced the expression of SOX9, COL2A1, and aggrecan, but they simultaneously inhibited the expression of COL1A1, COL1A2, and COL3A1. In addition, we identified the binding of HIF‐1α to the aggrecan promoter, the first such reported demonstration of this binding.Conclusion: This study is the first to show a bimodal role of HIF‐1α in cartilage homeostasis, since HIF‐1α was shown to favor specific markers and to impair dedifferentiation. This suggests that manipulation of HIF‐1α could represent a promising approach to the treatment of osteoarthritis.

Type II collagen is composed of α1(II) chains encoded by the COL2A1 gene. Alteration of this cartilage marker is a common feature of osteoarthritis. Interleukin-6 (IL-6) is a pro-inflammatory cytokine that needs a soluble form of receptor called sIL-6R to exert its effects in some cellular models. In that case, sIL-6R exerts agonistic action. This mechanism can make up for the partial or total absence of membrane-anchored IL-6 receptors in some cell types, such as chondrocytes. Our study shows that IL-6, sIL-6R, or both inhibit type II collagen production by rabbit articular chondrocytes through a transcriptional control. The cytokine and/or sIL-6R repress COL2A1 transcription by a -63/-35 sequence that binds Sp1·Sp3. Indeed, IL-6 and/or sIL-6R inhibit Sp1 and Sp3 expression and their binding activity to the 63-bp promoter. In chromatin immunoprecipitation experiments, IL-6·sIL-6R induced an increase in Sp3 recruitment to the detriment of Sp1. Knockdown of Sp1·Sp3 by small interference RNA and decoy strategies were found to prevent the IL-6- and/or sIL-6R-induced inhibition of COL2A1 transcription, indicating that each of these Sp proteins is required for down-regulation of the target gene and that a heterotypic Sp1·Sp3 complex is involved. Additionally, Sp1 was shown to interact with Sp3 and HDAC1. Indeed, overexpression of a full-length Sp3 cDNA blocked the Sp1 up-regulation of the 63-bp COL2A1 promoter activity, and by itself, inhibits COL2A1 transcription. We can conclude that IL-6, sIL-6R, or both in combination decrease both the Sp1·Sp3 ratio and DNA-binding activities, thus inhibiting COL2A1 transcription.

In osteoarthritis (OA), interleukin-1 (IL-1) stimulates the expression of metalloproteinases and aggrecanases, which induce cartilage degradation. IL-1 is also capable of reducing the production of cartilage-specific macromolecules, including type II collagen, through modulation of the transcription factors Sp1 and Sp3. Conversely, Transforming growth factor-ß (TGF-β) counteracts with most of the IL-1 deleterious effects and contributes to cartilage homeostasis. However, OA chondrocytes progressively loose the expression of TGF-β type II receptor and become insensitive to the factor. This downregulation is also driven by IL-1. This review provides insights into the molecular mechanisms that underly the interplay between IL-1 and TGF-β in OA cartilage metabolism and enlightens the central role of Sp1 and Sp3 transcription factors in the matrix pathological alterations.