VEGF-independent cell-autonomous functions of HIF-1α regulating oxygen consumption in fetal cartilage are critical for chondrocyte survival

Abstract

Fetal growth plate cartilage is nonvascularized, and chondrocytes largely develop in hypoxic conditions. We previously found that mice lacking the hypoxia-inducible transcription factor HIF-1a in cartilage show massive death of centrally located, hypoxic chondrocytes. A similar phenotype was observed in mice with genetic ablation of either all or specifically the diffusible isoforms of vascular endothelial growth factor (VEGF), a prime angiogenic target of HIF-1a. Here, we assessed whether VEGF is a critical downstream component of the HIF-1a-dependent survival pathway in chondrocytes. We used a genetic approach to conditionally overexpress VEGF164 in chondrocytes lacking HIF-1a, evaluating potential rescuing effects. The effectiveness of the strategy was validated by showing that transgenic expression of VEGF164 in Col2-Cre;VEGF f / f mice stimulated angiogenesis in the perichondrium, fully corrected the excessive hypoxia of VEGF-deficient chondrocytes, and completely prevented chondrocyte death. Yet, similarly crossed double-mutant embryos lacking HIF-1a and overexpressing VEGF164 in the growth plate cartilage still displayed a central cell death phenotype, albeit slightly delayed and less severe compared with mice exclusively lacking HIF-1a. Transgenic VEGF164 induced massive angiogenesis in the perichondrium, yet this only partially relieved the aberrant hypoxia present in HIF-1a-deficient cartilage and thereby likely inflicted only a partial rescue effect. In fact, excessive hypoxia and failure to upregulate phosphoglycerate-kinase 1 (PGK1), a key enzyme of anaerobic glycolytic metabolism, were among the earliest manifestations of HIF-1a deficiency in cartilaginous bone templates, and reduced PGK1 expression was irrespective of transgenic VEGF164. These findings suggest that HIF-1a activates VEGF-independent cell-autonomous mechanisms to sustain oxygen levels in the challenged avascular cartilage by reducing oxygen consumption. Hence, regulation of the metabolic pathways by HIF-1a and VEGF-dependent regulation of angiogenesis coordinately act to maintain physiological cartilage oxygenation. We conclude that VEGF and HIF-1a are critical preservers of chondrocyte survival by ensuring an adequate balance between availability and handling of oxygen in developing growth cartilage. ß

Hypoxia-driven pathways in bone development, regeneration and disease

“…36 The mechanism underlying this role of HIF-1α in cell survival is likely to involve a number of factors, but some evidence suggests that HIF-1α turns on genes that enable chondrocytes to switch to oxygen-sparing metabolic pathways. 1, 43 As such, by activating anaerobic glycolysis in cartilage, HIF-1α prevents overconsumption of scarce oxygen in this challenged avascular tissue. 43 The oxygen tension, therefore, is held within a likely narrow though optimum range for hypoxic chondrocytes to survive and differentiate normally.…”

Interaction of HIF1α and β-catenin inhibits matrix metalloproteinase 13 expression and prevents cartilage damage in mice

“…hypoxia-inducible factor 1α | chondrocyte | osteoarthritis | Wnt signaling | matrix metalloprotease 13 L ow oxygen tension (hypoxia) orchestrates several cell functions and is critical in health and disease (1)(2)(3)(4). Hypoxiainducible factor 1α (HIF1α) is an essential factor to maintain chondrocyte homeostasis and allow cell differentiation (5,6).…”

Up-regulation of glycolytic metabolism is required for HIF1α-driven bone formation

“…However, when HIF1α and HIF2α were deleted separately in osteoblasts, although both affected vascular density in long bones, only HIF1α deletion had an appreciable effect on bone formation (15). More recently, a VEGF-independent effect of HIF1α was demonstrated in mouse fetal cartilage (16). Thus, HIF1α may play important direct roles in osteoblasts, independent of effects on angiogenesis caused by up-regulation of VEGF.…”