Cellular ATP synthesis mediated by type III sodium-dependent phosphate transporter Pit-1 is critical to chondrogenesis.

Disturbed endochondral ossification in X-linked hypophosphatemia indicates an involvement of P(i) in chondrogenesis. We studied the role of the sodium-dependent P(i) cotransporters (NPT), which are a widely recognized regulator of cellular P(i) homeostasis, and the downstream events in chondrogenesis using Hyp mice, the murine homolog of human X-linked hypophosphatemia. Hyp mice showed reduced apoptosis and mineralization in hypertrophic cartilage. Hyp chondrocytes in culture displayed decreased apoptosis and mineralization compared with WT chondrocytes, whereas glycosaminoglycan synthesis, an early event in chondrogenesis, was not altered. Expression of the type III NPT Pit-1 and P(i) uptake were diminished, and intracellular ATP levels were also reduced in parallel with decreased caspase-9 and caspase-3 activity in Hyp chondrocytes. The competitive NPT inhibitor phosphonoformic acid and ATP synthesis inhibitor 3-bromopyruvate disturbed endochondral ossification with reduced apoptosis in vivo and suppressed apoptosis and mineralization in conjunction with reduced P(i) uptake and ATP synthesis in WT chondrocytes. Overexpression of Pit-1 in Hyp chondrocytes reversed P(i) uptake and ATP synthesis and restored apoptosis and mineralization. Our results suggest that cellular ATP synthesis consequent to P(i) uptake via Pit-1 plays an important role in chondrocyte apoptosis and mineralization, and that chondrogenesis is ATP-dependent.