The effect of phosphate availability on chondrocyte metabolism
Dietary phosphate is essential for normal fracture healing and bone growth. Previous studies have established that mice given a phosphate deficient diet after a fracture demonstrate delayed cartilage maturation and callus mineralization, as well as changes in gene expression consistent with oxidative phosphorylation dysfunction. This study was undertaken in order to examine the role of inorganic and organic phosphate availability on chondrocyte differentiation and mineralization, and to define the relationship between these processes and changes in chondrocyte metabolic function. ATDC5 murine chondroprogenitor cell line, which has been shown to undergo in vitro differentiation and extracellular matrix mineralization, was cultured under both differentiating and non-differentiating media conditions under conditions in 1mM -0.25mM sodium phosphate monobasic (inorganic phosphate) in the presence or absence of 4mM β-glycerol phosphate (organic phosphate). In the first series of studies, overall cell growth (total DNA and protein contents), mineralization (calcium accumulation), and cell-normalized oxidative metabolism (basal respiration, maximal respiration, ATP turnover, spare capacity, proton leak, and non-mitochondrial respiration rates) were measured over a 28 day time course in cultures grown in differentiating (ascorbic acid, insulin-transferrin-selenium, and β-glycerol phosphate) conditions in 1mM phosphate. These studies found that when the cells were induced to differentiate, there was a measurable increase in protein content while DNA content decreased by 30%, indicating a fraction of the cells underwent cell death. Differentiation was further associated with an overall two-fold increase in oxidative respiration. Next we assessed how differentiation, the promotion of matrix mineralization, and inorganic phosphate availability affected oxidative respiration. When differentiation was not induced with ascorbic acid and β-glycerol phosphate, there was no over growth in the cultures nor any change in total extracellular matrix mineralization or oxidative respiration. In the absence only of β-glycerol phosphate, differentiation proceeded but matrix mineralization did not occur. However, overall protein content and oxidative respiration were statistically two- and 1.5-fold higher, respectively, independent of the inorganic phosphate contents of the growth media. These results suggest that both differentiation and overall protein accumulation are strongly associated with increased oxidative metabolism while mineralization of the matrix decreased oxidative function. Only at the lowest phosphate levels were changes in basal oxidative function observed. These results are consistent with previous in vivo findings suggesting that diminished expression of mitochondrial associated genes in callus tissues from hypophosphatemic mice were associated with an overall decrease in chondrocyte differentiation.