The relationship between treadmill running and HIF-2α on rat knee articular cartilage

Purpose: Appropriate mechanical stress is needed to maintain normal articular cartilage metabolism. Therefore, exercise therapy is recommended as an effective treatment for osteoarthritis (OA). On the contrary, excessive exercise is detrimental for articular cartilage causing OA progression. In our previous study, hydrostatic pressure stimulation acts as anabolic or catabolic effect dependent on its intensity for chondrocytes and its mechanism was related on hypoxia-inducible factor (HIF)-2α in vitro. The purpose of this study was to investigate the metabolism of articular cartilage in knees of rats running on a treadmill, and to elucidate the relationship between treadmill exercise and HIF-2α expression. Methods: Twelve-week-old male Wistar rats ran on a treadmill at 12 m/min or 20 m/min for 45-min at a single time (n = 16 in each group). Rats in control group were kept sedentarily in cages (n = 16). All were sacrificed after running. Articular cartilage in right knee was removed from patellar, distal femur, and proximal tibia immediately after sacrifice. The cartilage was homogenized, and total RNA was extracted. We analyzed gene expression of NF-κB as a transcription factor, and ADAM-TS5, MMP-13, and Col10a1 as catabolic factors with quantitative reverse transcriptase polymerase chain reaction (RT-PCR). Left knee of each group was stained with immunostaining for HIF-2α (n = 4). Results: Gene expression of NF-κB was significantly higher in the 12 m/min group than control group. Gene expression of ADAM-TS5 in the 20 m/min group was significantly higher than the others. There was no significant difference among all groups in gene expression of MMP13 or Col10a1. On immunological staining, HIF-2α was more strongly stained in 20 m/min group than in the control group and less strongly in 12 m/min group. Conclusions: Homeostasis in articular cartilage is constantly maintained by the balance of anabolic and catabolic factors. OA is caused by collapse of this balance due to excessive mechanical stress, aging or something. HIF-2α plays a major role in the progression of OA, and hydrostatic pressure, which is a representative mechanical stress, influences HIF-2α expression in chondrocytes. According to previous reports, running on treadmill at around 12 m/min prevented OA in the destabilization of the medial meniscus model. In the 12 m/min group of our study, almost all catabolic factors didn’t change compared with control group, and HIF-2α gene expression was significantly lower than control group. These results show that 12 m/min running on treadmill may reduce the catabolism of articular cartilage via down-regulating HIF-2α. On the other hand, normal rats became OA by 20 m/min constant running in previous study. Our study revealed that the 20 m/min running increased the expression of ADAM-TS5, and HIF-2α was more strongly stained than control group. These results show that cartilage catabolism in the joint may be activated with even a single excessive treadmill running. In other words, excessive mechanical stress up-regulated HIF-2α expression, causing the activation of the catabolic factors. Our results indicate that HIF-2α may be involved in the balance of cartilage metabolism on exercise. In conclusion, even a single treadmill running affects the metabolism of articular cartilage and HIF-2α may be related to its mechanism.