Oxygen-sensing PHDs regulate bone homeostasis through the modulation of osteoprotegerin

Abstract

The bone microenvironment is composed of niches that house cells across variable oxygen tensions. However, the contribution of oxygen gradients in regulating bone and blood homeostasis remains unknown. Here, we generated mice with either single or combined genetic inactivation of the critical oxygen-sensing prolyl hydroxylase (PHD) enzymes (PHD1-3) in osteoprogenitors. Hypoxia-inducible factor (HIF) activation associated with Phd2 and Phd3 inactivation drove bone accumulation by modulating osteoblastic/osteoclastic cross-talk through the direct regulation of osteoprotegerin (OPG). In contrast, combined inactivation of Phd1, Phd2, and Phd3 resulted in extreme HIF signaling, leading to polycythemia and excessive bone accumulation by overstimulating angiogenic-osteogenic coupling. We also demonstrate that genetic ablation of Phd2 and Phd3 was sufficient to protect ovariectomized mice against bone loss without disrupting hematopoietic homeostasis. Importantly, we identify OPG as a HIF target gene capable of directing osteoblast-mediated osteoclastogenesis to regulate bone homeostasis. Here, we show that coordinated activation of specific PHD isoforms fine-tunes the osteoblastic response to hypoxia, thereby directing two important aspects of bone physiology: cross-talk between osteoblasts and osteoclasts and angiogenic-osteogenic coupling.

Prolyl hydroxylase domain proteins regulate bone mass through their expression in osteoblasts

“…In a more recent study, Wu et al reported that combined inactivation of Phd1-3 genes in Osx-expressing cells using Osx-Cre-mediated recombination increased bone mass as a result of impaired osteoclast formation, though with no change on osteoblast function. Collectively, results from these studies suggest the complex function of PHD proteins in bone, which deserves further investigation (Wu et al, 2015b).…”

Vhl deletion in osteoblasts boosts cellular glycolysis and improves global glucose metabolism

“…The repercussions of enhanced signaling by HIF-1α versus HIF-2α were recently studied in models of Osx-Cre:GFP-driven overexpression of the respective HIFs (38), confirming and extending earlier studies indicating that the genes regulated by HIF-1α and HIF-2α in osteoblasts are overlapping but nonidentical. While HIF-1α appears to be primarily responsible for meditating the metabolic switch to glycolysis, VEGF upregulation in osteogenic cells is controlled by both HIF-1α and HIF-2α (10,21,38). Additionally, HIF-2α has been shown to be the main regulator for EPO production by osteoblasts (12) and to regulate OPG, the factor that inhibits osteoclastogenesis by counteracting RANKL (38).…”

“…Second, besides regulating erythropoiesis, EPO has also been shown to stimulate bone formation and repair (42). Third, OPG has been recognized as a direct transcriptional target of HIF-2α, and at least part of the net bone anabolic effect of HIF-2α has been ascribed to reduced bone resorption (38).…”

“…While HIF-1α appears to be primarily responsible for meditating the metabolic switch to glycolysis, VEGF upregulation in osteogenic cells is controlled by both HIF-1α and HIF-2α (10,21,38). Additionally, HIF-2α has been shown to be the main regulator for EPO production by osteoblasts (12) and to regulate OPG, the factor that inhibits osteoclastogenesis by counteracting RANKL (38). All of these downstream effectors of the VHL/HIF axis -whose increased expression fully correlated with the presence of the local bone phenotype in our Vhl-deficient models, including in being unaffected by DCA treatment -could have contributed to or caused the high bone mass and BM alterations.…”

Targeting the hypoxic response in bone tissue engineering: A balance between supply and consumption to improve bone regeneration

“…Recent evidence highlights that metabolic adaptations in osteoblasts, resulting in activation of glycolysis, are essential for the increased bone formation when HIF-1a is stabilized in osteoprogenitors postnatally (Regan et al, 2014). Furthermore, HIF activation by deleting PHD2 and PHD3 in osteoprogenitors enhanced the expression of osteoprotegerin, an osteoclast-inhibiting factor, and the associated decrease in osteoclast formation resulted in increased accumulation of bone mass (Wu et al, 2015). However, one has to be cautious to extrapolate these findings to bone tissue engineering, as these studies were performed by activating HIF signalling in cells experiencing a wellvascularized environment.…”