(The) bone anabolic effects of irisin are through preferential stimulation of aerobic glycolysis

Abstract

Irisin, a recently identified hormone secreted by skeletal muscle in response to exercise, exhibits anabolic effects on the skeleton primarily through the stimulation of bone formation. However, the mechanism underlying the irisin-stimulated anabolic response remains largely unknown. Emerging evidence indicated that anabolic agents for osteoporosis enhanced the work of osteoblast by stimulating osteoblast glucose metabolism. Furthermore, an increase in aerobic glycolysis is considered to be a common feature activated by bone anabolic signals. To uncover the underlying mechanism of irisin-mediated anabolic actions in bone, we biosynthesized recombinant irisin (r-irisin) using an Escherichia coli expression system and used it to treat several osteoblast cell types. Our biosynthesized r-irisin was successfully purified and characterized. The synthesized r-irisin could promote proliferation and differentiation of osteoblasts as evidenced by enhanced expression of osteoblast-specific transcriptional factors, including Runt-related transcription factor-2 (Runx2), Oster (Osx), as well as early osteoblastic differentiation markers such as alkaline phosphatase (Alp) and collagen type I alpha 1 (Col1a1), and suppression of the expression of sclerostin (Sost), the main function of which is to inhibit osteoblastic bone formation, in MLO-Y4 osteocyte-like cells. Furthermore, we showed that the promotion of r-irisin on the proliferation and differentiation of osteoblast lineage cells are preferentially through aerobic glycolysis, as indicated by the enhanced abundance of representative enzymes such as lactate dehydrogenase A (LDHA) and pyruvate dehydrogenase kinase 1 (PDK1), together with increased lactate levels, but not oxidative phosphorylation (OXPHOS). The expression of OXPHOS related proteins decreased after r-irisin treatment in primary cultured calvarial cells. Suppression of r-irisin-mediated aerobic glycolysis with Dichloroacetate blunted its anabolic effects. The favorite of the aerobic glycolysis after r-irisin treatment was then confirmed in primary calvarial cells by metabolic analysis using gas chromatography–mass spectrometry. Thus, our results suggest that the anabolic actions of r-irisin on the regulation of osteoblast lineage cells are preferentially through aerobic glycolysis, which may help to develop new irisin based bone anabolic agents.