MOlecular regulaTiOn Of bOne reSOrpTiOn by hypOXia

In pathological lesions of osseous tissues, including inflammation, fracture, and tumors, pO2 is evidently lower. 3, 4 Low O2 can alter bone homeostasis, leading to osteolysis. Patients exposed to long-term hypoxic states are at risk for accelerated bone loss. Decreased vascular flow to the lower extremities correlates with an increased rate of bone loss at the hip and calcaneus.5 Chronic respiratory failure patients with lower blood pO2 show significant time-dependent loss of bone mass density.6 This phenomenon can also be seen in experimental animals. Bone mass is significantly reduced in Wistar rats exposed to hypobaric atmosphere.6 Bone is continuously resorbed and reformed by osteoclasts and osteoblasts, respectively. Formation and function of these cells depends on a ‘normal’ bone microenvironment. Reduced oxygen tension causes a variety of biological responses. Low oxygen conditions are potent inducers or suppressors of differentiation in numerous cell types including osteoblasts, chondrocytes, and osteoclasts. In this article we review the hypoxic regulation of osteoblasts, chondrocytes, and osteoclasts, and mechanisms relevant to these responses. Furthermore, we include our recent measurements of the effects of hypoxia on osteoclasts and related actions of nickel and cobalt ions. HYPOXIA AND OSTEOBLASTS/STROMAL CELLS Osteoblasts are bone-forming cells originating from mesenchymal stem cells (MSC). In addition to osteoblasts, these progenitor cells can differentiate into chondrocytes, adipocytes, myoblasts, and fibroblasts by different stimulating factors.7 Mild hypoxic condition (8% O2, vs the 20% O2 v/v in a typical tissue culture incubator) accelerates MSC differentiation toward both osteogenic and adipogenic phenotypes.8 Differentiation of MIAMI cells is inhibited by 3% O2, thus favoring stemness over osteogenic differentiation.9 Rat osteoblasts cultured under 2-5% O2 form fewer mineralized bone nodules, and express less alkaline phosphatase (ALP), while anoxia (0.2% O2) almost abolishes nodule formation.10 The inhibitory effects of hypoxia are due to decreased osteoblast proliferation and differentiation.10, 11 Cultures exposed to 2% O2 decrease osteoblast formation by reducing expression of Runx2, a transcription factor essential for osteoblast differentiation. Expression of type I collagen, osteocalcin, and ALP in human MG3 cells is reduced by low oxygen tension in a time-dependent manner.11 Salim et al report that brief exposure to anoxia, but not hypoxia (2% O2) inhibits osteoblast differentiation by down-regulating BMP2 and Runx2 levels.12 Collagen fibrils are less organized in osteoblasts cultured in 2% O2 and much less abundant than in 20% O2. 10 In mice, haploinsufficiency of hypoxia inducible factor-1α (HIF-1α), a transcriptional activator that regulates O2 homeostasis, results in decreased chondrocytic and osteoblastic apoptosis, and fracture callus exhibits greater mineralization and is larger, stronger, and stiffer in HIF-1α+/than in wild-type mice.13 HYPOXIA AND CHONDROCYTES Endochondral bone is formed in a low oxygen environment. The cartilaginous growth plate at the end of long bone is avascular, thus hypoxic.14 Low oxygen tension affects chondrocyte energy metabolism. Culture of chondrocytes under hypoxia causes a proportional increase in glucose utilization and elevated level of lactate synthesis.15 The limitations in the oxygen supply regulate chondrocyte growth arrest and survival in the growth cartilage.14 Under hypoxia, MSC preferentially differentiate into chondrocytes and synthesize cartilage matrix.16, 17 Real-time PCR of RNA isolated from pluripotent C3H10T1/2 cells reveals hypoxia-induced changes in the expression of Sox9 and its downstream targets aggrecan and col2α. 16 Although the growth plate is avascular, it requires blood vessel invasion and resorption of calcified cartilage by osteoclasts to be replaced by bone. An important mediator of angiogeneis, vascular endothelial growth factor A (VEGFA), is upregulated by hypoxia. VEGFA plays a significant role in both early and late stages of cartilage vascularization. Knock-out of VEGFA in mice shows delayed invasion of blood vessels into primary ossification centers and delayed removal of terminal hypertrophic chondrocytes.18 Somying Patntirapong, DDS is a DMSc candidate in Oral Biology, Harvard School of Dental Medicine, Boston, MA and a member of the Department of Orthopaedic Surgery, Children’s Hospital, Boston, MA.