Hypoxia‐inducible factor 1‐alpha does not regulate osteoclastogenesis but enhances bone resorption activity via prolyl‐4‐hydroxylase 2

Abstract

Osteogenic–angiogenic coupling is promoted by the hypoxia‐inducible factor 1‐alpha (HIF‐1α) transcription factor, provoking interest in HIF activation as a therapeutic strategy to improve osteoblast mineralization and treat pathological osteolysis. However, HIF also enhances the bone‐resorbing activity of mature osteoclasts. It is therefore essential to determine the full effect(s) of HIF on both the formation and the bone‐resorbing function of osteoclasts in order to understand how they might respond to such a strategy. Expression of HIF‐1α mRNA and protein increased during osteoclast differentiation from CD14+ monocytic precursors, additionally inducing expression of the HIF‐regulated glycolytic enzymes. However, HIF‐1α siRNA only moderately affected osteoclast differentiation, accelerating fusion of precursor cells. HIF induction by inhibition of the regulatory prolyl‐4‐hydroxylase (PHD) enzymes reduced osteoclastogenesis, but was confirmed to enhance bone resorption by mature osteoclasts. Phd2 +/− murine osteoclasts also exhibited enhanced bone resorption, associated with increased expression of resorption‐associated Acp5, in comparison with wild‐type cells from littermate controls. Phd3 −/− bone marrow precursors displayed accelerated early fusion, mirroring results with HIF‐1α siRNA. In vivo, Phd2 +/− and Phd3 −/− mice exhibited reduced trabecular bone mass, associated with reduced mineralization by Phd2 +/− osteoblasts. These data indicate that HIF predominantly functions as a regulator of osteoclast‐mediated bone resorption, with little effect on osteoclast differentiation. Inhibition of HIF might therefore represent an alternative strategy to treat diseases characterized by pathological levels of osteolysis. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Osteoblast-osteoclast co-culture amplifies inhibitory effects of FG-4592 on osteoclast formation and reduces bone resorption activity

“…This is driven predominantly by direct transcriptional effects of HIF-2on Opg expression, which does not affect osteoblast proliferation or mineralisation [5]. We have observed elevated serum concentrations of OPG in Phd3 −/− mice associated with reduced serum CTXI, indicative of reduced osteoclast activity in vivo [19].…”

“…Osteoclasts form by fusion of CD14+ monocytic precursors, induced by the cytokines macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa B ligand (RANKL), to produce multi-nucleated bone-resorbing cells [17,18]. By directly comparing HIF knockdown, HIF induction and PHD enzyme depletion in in vitro cultured murine and human osteoclasts we showed a striking role for HIF-1α and PHD2 in driving bone resorption by mature osteoclasts [19][20][21][22]. Osteoclast-specific inactivation of HIF-1α antagonises osteoporotic bone loss in mice, suggesting that HIF-1α also promotes osteoclast activation and bone loss in vivo [23].…”

“…Increased trabecular bone mass in mice with an osteoblast-specific mutation in Phd1-3 was described as the result of enhanced osteoclast-mediated bone resorption exceeded by increased bone formation [6]. This dampening of HIF-mediated osteoclast activation could encompass a moderate effect of HIF to delay cell fusion during osteoclast differentiation [19].…”

ERK1/2 and Akt phosphorylation were essential for MGF E peptide regulating cell morphology and mobility but not proangiogenic capacity of BMSCs under severe hypoxia

“…After bone defect, the initial repair process is always exposed to a much lower oxygen tension than the physiological condition in the bone marrow (1‐7%) . Hypoxic condition is intricately involved in angiogenesis, osteogenesis, and bone regeneration and remodelling, and hypoxia‐inducible factor 1α (HIF‐1α) is the master transcriptional regulator of osteogenic‐angiogenic coupling under hypoxia . Physiological hypoxic stress has a major role in bone defect regeneration in regulating cell mobility and angiogenesis .…”

“…7 Hypoxic condition is intricately involved in angiogenesis, osteogenesis, and bone regeneration and remodelling, and hypoxia-inducible factor 1α (HIF-1α) is the master transcriptional regulator of osteogenic-angiogenic coupling under hypoxia. 8,9 Physiological hypoxic stress has a major role in bone defect regeneration in regulating cell mobility 10,11 and angiogenesis. 12 Unfortunately, acute severe hypoxia significantly endangers the cell mobility of BMSCs, 13 but little is known about the effect of severe hypoxia on cell adhesion capacity and proangiogenic capacity of BMSCs.…”