Phosphate ions mediate chondrocyte apoptosis through a plasma membrane transporter mechanism

In a previous investigation we showed that phosphate ions (Pi) induced apop tosis of terminally differentiated hypertrophic chondrocytes, To explore th e mechanism by which Pi induces cell death, we asked the following two ques tions, First, can we prevent Pi-induced apoptosis by inhibiting plasma memb rane Na-Pi cotransport? Second, which specific Na-Pi transporters are expre ssed in chondrocytes and are they developmentally regulated? Terminally dif ferentiated hypertrophic chondrocytes were isolated from chick tibial carti lage and cell death was measured in the presence of 3-7 mmol/L Pi. To ascer tain whether apoptosis was linked to a rise in cellular Pi loading, we exam ined the effect of phosphonoformic acid (PFA), a competitive inhibitor of N a-Pi cotransport on Pi-induced apoptosis in chondrocytes. We found that 1 m mol/L PFA blocked anion-induced cell death and prevented an increase in the cell Pi content. In a parallel study, we determined that the bisphosphonat e, alendronate, also protected chondrocytes from death, albeit at a lower c oncentration than PFA, Using a DNA end-labeling procedure, we showed that t he Pi-treated cells were apoptotic and, as might be predicted, the presence of PFA blocked induction of the death sequence, Next, we examined the expr ession of two Pi transporters in relation to chondrocyte maturation and ani on treatment. We noted that there was expression of the constitutive transp orter, Glvr-1, and a type II cotransporter in chick growth plate cells. Alt hough these transport systems are active in terminally differentiated cells , it is probable that the initiation of apoptosis may require the induction of other Pi-transport systems, It is concluded that, at the mineralization front, cell death is linked directly to the elevation in environmental ani on concentration and the concomitant rise in intracellular Pi levels. (C) 2 001 by Elsevier Science Inc. All rights reserved.