Adaptation of chondrocytes to low oxygen tension: Relationship between hypoxia and cellular metabolism

Abstract

In endochondral bone, the growth cartilage is the site of rapid growth. Since the vascular supply to the cartilage is limited, it is widely assumed that cells of the cartilage are hypoxic and that limitations in the oxygen supply regulate the energetic state of the maturing cells. In this report, we evaluate the effects of oxygen tension on chondrocyte energy metabolism, thiol status, and expression of transcription elements, HIF and AP-1. Imposition of an hypoxic environment on cultured chondrocytes caused a proportional increase in glucose utilization and elevated levels of lactate synthesis. Although we observed a statistical increase in the activities of phosphofructokinase, pyruvate kinase, lactate dehydrogenase, and creatine kinase after exposure to lowered oxygen concentrations, the effect was small. The cultured cells exhibited a decreased utilization of glutamine, possibly due to down regulation of mitochondrial function and inhibition of oxidative deamination. With respect to total energy generation, we noted that these cells are quite capable of maintaining the energy charge of the cell at low oxygen tensions. Indeed, no changes in the absolute quantity of adenine nucleotides or the energy charge ratio was observed. Hypoxia caused a decrease in the glutathione content of cultured chondrocytes and a concomitant rise in cell and medium cysteine levels. It is likely that the fall in cell glutathione level is due to decreased synthesis of the tripeptide under reduced oxygen stress and the limited supply of glutamate. The observed rise in cellular and medium cysteine levels probably reflects an increase in the rate of degradation of glutathione and a decrease in synthesis of the peptide. To explore how cells transduce these metabolic effects, gel retardation assays were used to study chondrocyte HIF and AP-1 binding activities. Chondrocyte nuclear preparations bound an HIF-oligonucleotide; however, at low oxygen tensions, no increase in HIF binding was observed. In addition, we found that AP-1 binding activities in chondrocytes exposed to low oxygen tensions was elevated, although the response was lower than that exhibited by fibroblasts exposed to the same range of oxygen concentrations. We compared these results to HIF and AP-1 binding by cells in the growth plate. There was strong HIF and AP-1 binding throughout the plate, but no evidence of selective binding to any one zone. The results of the study lend strong support to the view that chondrocytes are very well adapted to low oxygen tensions; thus, under hypoxic conditions, there is a high level of expression of both HIF and AP-1, and energy conservation appears to be near-maximum.

Disruption of thrombospondin‐2 accelerates ischemic fracture healing

“…19 Studies have also shown that chondrocyte differentiation can be induced with hypoxia, although results can be conflicting. 27,[30][31][32] On the other hand, the anoxic environment caused by ischemia delays both chondrogenesis and osteoblast differentiation and limits bone repair. 27 It was our intent therefore to characterize fracture healing of TSP2-null mice in an ischemic fracture environment.…”

Computational Study of Culture Conditions and Nutrient Supply in Cartilage Tissue Engineering

“…Under conditions of nutrient limitations cellular regulation mechanisms become more important. Articular chondrocytes have been shown to display either a decrease in glycolysis as a function of decreasing oxygen (28,30,39) or by contrast, an increase (Pasteur effect) (27,29,32,59), modulated by differences in culture conditions and associated dedifferentiation. As a first order estimation, the model assumed that glucose and oxygen uptake were uncoupled, although such regulation mechanisms can be subsequently included.…”

Voltage-Dependent Calcium Channels in Chondrocytes: Roles in Health and Disease

“…Chondrocytes are surrounded by a pericellular matrix and are situated in cavities called lacunae within the ECM, where they exist in a hypoxic micro-environment [3,4] and control ECM turnover and homeostasis in response to mechanical loading [5] despite having a rather slow (mainly anaerobic) metabolism. Due to the fact that chondrocytes reside within the peculiar milieu of the ECM (which is acidic, hypertonic and hyperosmotic [6][7][8]), and since their function and differentiation is strongly dependent on intracellular calcium homeostasis [9,10], they deploy a whole range of ion channels to enable ion transport across the plasma membrane; the collection of ion channels is referred to as the chondrocyte 'channelome' [11].…”