title: 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-1α 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-1α. Here, we assessed whether VEGF is a critical downstream component of the HIF-1α-dependent survival pathway in chondrocytes. We used a genetic approach to conditionally overexpress VEGF164 in chondrocytes lacking HIF-1α, 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-1α 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-1α. Transgenic VEGF164 induced massive angiogenesis in the perichondrium, yet this only partially relieved the aberrant hypoxia present in HIF-1α-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-1α deficiency in cartilaginous bone templates, and reduced PGK1 expression was irrespective of transgenic VEGF164. These findings suggest that HIF-1α 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-1α and VEGF-dependent regulation of angiogenesis coordinately act to maintain physiological cartilage oxygenation. We conclude that VEGF and HIF-1α are critical preservers of chondrocyte survival by ensuring an adequate balance between availability and handling of oxygen in developing growth cartilage.
authors.affiliation: Laboratory of Experimental Medicine and Endocrinology, KULeuven, Leuven, Belgium.
authors.forename: Christa
authors.initials: C
authors.lastname: Maes
authors.forename: Elisa
authors.initials: E
authors.lastname: Araldi
authors.forename: Katharina
authors.initials: K
authors.lastname: Haigh
authors.forename: Richa
authors.initials: R
authors.lastname: Khatri
authors.forename: Riet
authors.initials: R
authors.lastname: Van Looveren
authors.forename: Amato J
authors.initials: AJ
authors.lastname: Giaccia
authors.forename: Jody J
authors.initials: JJ
authors.lastname: Haigh
authors.forename: Geert
authors.initials: G
authors.lastname: Carmeliet
authors.forename: Ernestina
authors.initials: E
authors.lastname: Schipani
mesh_terms: D000818:Animals; D017209:Apoptosis; D002356:Cartilage / Q000166:cytology / Q000196:embryology / Q000502:physiology*; D002470:Cell Survival / Q000502:physiology*; D019902:Chondrocytes / Q000166:cytology*; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000502:physiology*; D051379:Mice; D008822:Mice, Transgenic; D010101:Oxygen Consumption / Q000502:physiology*; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000502:physiology*
chemical_list: D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D042461:Vascular Endothelial Growth Factor A