Hypoxia Drives Material‐Induced Heterotopic Bone Formation by Enhancing Osteoclastogenesis via M2/Lipid‐Loaded Macrophage Axis

Abstract

Heterotopic ossification (HO) is a double‐edged sword. Pathological HO presents as an undesired clinical complication, whereas controlled heterotopic bone formation by synthetic osteoinductive materials shows promising therapeutic potentials for bone regeneration. However, the mechanism of material‐induced heterotopic bone formation remains largely unknown. Early acquired HO being usually accompanied by severe tissue hypoxia prompts the hypothesis that hypoxia caused by the implantation coordinates serial cellular events and ultimately induces heterotopic bone formation in osteoinductive materials. The data presented herein shows a link between hypoxia, macrophage polarization to M2, osteoclastogenesis, and material‐induced bone formation. Hypoxia inducible factor‐1α (HIF‐1α), a crucial mediator of cellular responses to hypoxia, is highly expressed in an osteoinductive calcium phosphate ceramic (CaP) during the early phase of implantation, while pharmacological inhibition of HIF‐1α significantly inhibits M2 macrophage, subsequent osteoclast, and material‐induced bone formation. Similarly, in vitro, hypoxia enhances M2 macrophage and osteoclast formation. Osteoclast‐conditioned medium enhances osteogenic differentiation of mesenchymal stem cells, such enhancement disappears with the presence of HIF‐1α inhibitor. Furthermore, metabolomics analysis reveals that hypoxia enhances osteoclastogenesis via the axis of M2/lipid‐loaded macrophages. The current findings shed new light on the mechanism of HO and favor the design of more potent osteoinductive materials for bone regeneration.

Intersections of Fibrodysplasia Ossificans Progressiva and Traumatic Heterotopic Ossification

“…Recent studies have shown that vascular endothelial growth factor A (VEGFA), an angiogenic protein whose expression is modulated by HIF-1α, is upregulated in MSCs as well as macrophages following injury, suggesting that hypoxia may modulate tHO formation through cell populations other than MSCs [ 36 , 113 ]. In another recent study, HIF-1α was shown to influence ectopic bone formation by promoting M2 macrophage phenotypes and osteoclast formation following intramuscular implantation of osteoinductive material [ 114 ]. Given that inflammation and hypoxia tend to exist concurrently, more studies are warranted to better understand MSC-specific mechanisms and other immune cell population contributions in FOP and tHO.…”

Inhibition of PI3K/AKT signaling pathway prevents blood-induced heterotopic ossification of the injured tendon

“…Besides, cellular components play a significant role in tendon HO. Macrophages are key mediators that link inflammation and HO pathogenesis, which can promote MSCs osteochondrogenic differentiation and significantly contribute to the hypoxic microenvironment [ [13] , [14] , [15] ]. So far, various types of progenitors have been detected as the origin of HO, including TSPCs, circulating MSCs, and peripheral neural progenitor cells [ [16] , [17] , [18] , [19] , [20] ].…”