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

The Journal of Pathology

Bishop

Vernet

et al. 2017

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.

Section: Discussionmentioning

confidence: 99%

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

The Journal of Pathology

Bishop

Vernet

et al. 2017

“…Genetic studies initially defined the HIF‐driven link between osteogenesis and angiogenesis. Mice with osteoblast‐specific overexpression of HIFα caused by deletion of Vhl ( 2 ) or the Phd1‐3 enzymes ( 5,6 ) overexpress VEGF and develop dense, heavily vascularized long bones, whereas osteoblast‐specific deletion of Hif1a or Hif2a produces the reverse phenotype. ( 2,7 ) Similarly, HIF stabilization with PHD enzyme inhibitors increases vascularity and stimulates new bone formation, improving BMD and bone strength in murine models of bone fracture, ( 8–11 ) distraction osteogenesis, (12 ) and osteoporosis.…”

Section: Introductionmentioning

confidence: 99%

“…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 ) Any dampening of HIF‐mediated osteoclast activation could encompass a moderate effect of HIF to delay cell fusion during osteoclast differentiation. ( 19 )…”

Section: Introductionmentioning

confidence: 99%

Osteoblast–Osteoclast Coculture Amplifies Inhibitory Effects of FG‐4592 on Human Osteoclastogenesis and Reduces Bone Resorption

Papadimitriou-Olivgeri

Knowles

2020

JBMR Plus

The link between bone and blood vessels is regulated by hypoxia and the hypoxia-inducible transcription factor, HIF, which drives both osteogenesis and angiogenesis. The recent clinical approval of PHD enzyme inhibitors, which stabilize HIF protein, introduces the potential for a new clinical strategy to treat osteolytic conditions such as osteoporosis, osteonecrosis, and skeletal fracture and nonunion. However, bone-resorbing osteoclasts also play a central role in bone remodeling and pathological osteolysis, and HIF promotes osteoclast activation and bone loss in vitro. It is therefore likely that the result of PHD enzyme inhibition in vivo would be mediated by a balance between increased bone formation and increased bone resorption. It is essential that we improve our understanding of the effects of HIF on osteoclast formation and function and consider the potential contribution of inhibitory interactions with other musculoskeletal cells. The PHD enzyme inhibitor FG-4592 stabilized HIF protein and stimulated osteoclast-mediated bone resorption, but inhibited differentiation of human CD14+ monocytes into osteoclasts. Formation of osteoclasts in a more physiologically relevant 3D collagen gel did not affect the sensitivity of osteoclastogenesis to FG-4592, but increased sensitivity to reduced concentrations of RANKL. Coculture with osteoblasts amplified inhibition of osteoclastogenesis by FG-4592, whether the osteoblasts were proliferating, differentiating, or in the presence of exogenous M-CSF and RANKL. Osteoblast coculture dampened the ability of high concentrations of FG-4592 to increase bone resorption. These data provide support for the therapeutic use of PHD enzyme inhibitors to improve bone formation and/or reduce bone loss for the treatment of osteolytic pathologies and indicate that FG-4592 might act in vivo to inhibit the formation and activity of the osteoclasts that drive osteolysis.

“…Genetic studies initially defined the HIF-driven link between osteogenesis and angiogenesis. Mice with osteoblast-specific overexpression of HIFα due to deletion of Vhl [2] or the Phd1-3 enzymes [5,6] over-express VEGF and develop dense, heavily vascularised long bones, whereas osteoblastspecific deletion of Hif1a or Hif2a produces the reverse phenotype [2,7]. Similarly HIF stabilisation with PHD enzyme inhibitors increases vascularity and stimulates new bone formation, improving bone mineral density and bone strength in murine models of bone fracture [8][9][10][11], distraction osteogenesis [12] and osteoporosis [13,14].…”

Section: Introductionmentioning

confidence: 99%

“…It is likely that the final outcome of PHD enzyme inhibition in vivo would be mediated by a balance between increased bone formation and increased bone resorption. 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].…”

Section: Introductionmentioning

confidence: 99%

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

Papadimitriou-Olivgeri

Knowles

2019

Preprint

The link between bone and blood vessels is regulated by hypoxia and the hypoxia-inducible transcription factor, HIF, which drives both osteogenesis and angiogenesis. The recent clinical approval of PHD enzyme inhibitors, which stabilise HIF protein, introduces the potential for a new clinical strategy to treat osteolytic conditions such as osteoporosis, osteonecrosis and skeletal fracture and non-union. However, bone-resorbing osteoclasts also play a central role in bone remodelling and pathological osteolysis and HIF promotes osteoclast activation and bone loss in vitro. It is therefore likely that the final outcome of PHD enzyme inhibition in vivo would be mediated by a balance between increased bone formation and increased bone resorption. It is essential that we improve our understanding of the effects of HIF on osteoclast formation and function, and consider the potential contribution of inhibitory interactions with other musculoskeletal cells.The PHD enzyme inhibitor FG-4592 stabilised HIF protein and stimulated osteoclast-mediated bone resorption, but inhibited differentiation of human CD14+ monocytes into osteoclasts. Formation of osteoclasts in a more physiologically relevant 3D collagen gel did not affect the sensitivity of osteoclastogenesis to FG-4592, but increased sensitivity to reduced concentrations of RANKL. Coculture with osteoblasts amplified inhibition of osteoclastogenesis by FG-4592, whether the osteoblasts were proliferating, differentiating or in the presence of exogenous M-CSF and RANKL. Osteoblast co-culture dampened the ability of high concentrations of FG-4592 to increase bone resorption.This data provides support for the therapeutic use of PHD enzyme inhibitors to improve bone formation and/or reduce bone loss for treatment of osteolytic pathologies, and indicates that FG-4592 might also act to inhibit the formation and activity of the osteoclasts that drive osteolysis.