HIF-1α metabolically controls collagen synthesis and modification in chondrocytes

Abstract

Endochondral ossification, an important bone formation process in vertebrates, highly depends on proper functioning of growth plate chondrocytes 1 . Their proliferation determines longitudinal bone growth and the matrix deposited provides a scaffold for future bone formation. However, these two energy-dependent anabolic processes occur in an avascular environment 1,2 . In addition, the centre of the expanding growth plate becomes hypoxic and local activation of the hypoxiainducible transcription factor HIF-1α is necessary for chondrocyte survival by still unknown cellintrinsic mechanisms [3][4][5][6] . Whether HIF-1α signalling has to be contained in the other regions of the growth plate and whether chondrocyte metabolism controls cell function remains undefined. We here show that prolonged HIF-1α signalling in chondrocytes leads to skeletal dysplasia by interfering with cellular bioenergetics and biosynthesis. Decreased glucose oxidation results in an energy deficit, which limits proliferation, activates the unfolded protein response (UPR) and reduces collagen synthesis. However, enhanced glutamine flux increases α-ketoglutarate (αKG) levels, which in turn increases collagen proline and lysine hydroxylation. This metabolically regulated collagen modification renders the cartilaginous matrix more resistant to proteasemediated degradation and thereby increases bone mass. Thus, inappropriate HIF-1α signalling results in skeletal dysplasia caused by collagen overmodification, an effect that may also contribute to other extracellular matrix-related diseases such as cancer and fibrosis.To investigate whether HIF signalling needs to be controlled in growth plate chondrocytes, we conditionally inactivated HIF prolyl hydroxylase 2 (PHD2; Phd2 chonmice), its main negative regulator 7 , resulting in HIF-1α accumulation (Extended Data Fig. 1a-d).This approach caused skeletal dysplasia, characterized by impaired longitudinal bone growth and increased trabecular bone mass (Fig. 1a,b, Extended Data Fig. 1e,f). The growth plate was shorter, but normally organized and, interestingly, the high bone mass was not due to altered bone resorption or formation (Extended Data Fig. 1g-l). Instead, we observed more cartilage remnants in the bony trabeculae, evidenced by more type II collagen (COL2)positive and proteoglycan-rich matrix (Fig. 1c, Extended Data Fig. 1m). The decreased serum CTx-II levels, measuring COL2 degradation, indicated that the cartilage matrix was incompletely resorbed, and the unaltered chondrocyte-to-matrix ratio pointed to a qualitative, rather than quantitative, change in matrix properties (Extended Data Fig. 1j,n). Thus, inactive oxygen sensing in chondrocytes increases trabecular bone mass, caused by abundant cartilage remnants, likely resulting from modifications in the cartilage matrix itself.HIF-1α stabilization in PHD2-deficient chondrocytes resulted, as expected 7,8 , in metabolic reprogramming. Mitochondrial content was reduced, likely because of decreased biogenesis without changing autophagy (Extended D...

Molecular Insights Into Lysyl Oxidases in Cartilage Regeneration and Rejuvenation

“…Articular cartilage is residing in a hypoxic microenvironment of avascular hypoxic zone in vivo under normal physiological conditions, ranging from 7∼10% oxygen tensions in the superficial zone, and 1% oxygen in the deep zones (Ferrell and Najafipour, 1992). Hypoxia-inducible factor 1α (HIF-1α) have been demonstrated essential for cartilage maturation (Duval et al, 2009;Stegen et al, 2019). Also, local activation of HIF-1α is necessary for survival and homeostatic function of chondrocytes, as well as normal joint development (Aro et al, 2012;Long, 2019).…”

Conditional ablation of MAPK7 expression in chondrocytes impairs endochondral bone formation in limbs and adaptation of chondrocytes to hypoxia

“…First, in the present study, we have not obtained direct evidence that the abnormal growth plate development in the absence of MAPK7 is caused by an inhibition of HIF1α signaling. Some studies have found that knockout of von Hippel-Lindau tumor suppressor (Vhl) and HIF prolyl hydroxylase 2 (Phd2), two key genes for HIF1α degradation, can increase HIF1α protein levels [46,47]. In our future research, we will investigate whether the observed phenotypes can be rescued by deleting Vhl and/ or Phd2 in chondrocytes.…”

UCP2-induced hypoxia promotes lipid accumulation and tubulointerstitial fibrosis during ischemic kidney injury

“…The critical mediator of cellular adaptation to hypoxia is HIF. Clinical and genetic evidence show that activation of HIF signaling in renal epithelial cells is associated with the development of chronic renal fibrosis and may promote expression and accumulation of ECM 23,42,43 . Recent studies demonstrated that HIF-1α regulates lipid metabolism and the involved key regulator and enzymes [24][25][26] .…”