Up-regulation of the Pit-2 Phosphate Transporter/Retrovirus Receptor by Protein Kinase C ε

Abstract

The membrane receptors for the gibbon ape leukemia retrovirus and the amphotropic murine retrovirus serve normal cellular functions as sodium-dependent phosphate transporters (Pit-1 and Pit-2, respectively). Our earlier studies established that activation of protein kinase C (PKC) by treatment of cells with phorbol 12-myristate 13-acetate (PMA) enhanced sodium-dependent phosphate (Na/Pi) uptake. Studies now have been carried out to determine which type of Na/Pi transporter (Pit-1 or Pit-2) is regulated by PKC and which PKC isotypes are involved in the up-regulation of Na/Pi uptake by the Na/Pi transporter/viral receptor. It was found that the activation of short term (2-min) Na/Pi uptake by PMA is abolished when cells are infected with amphotropic murine retrovirus (binds Pit-2 receptor) but not with gibbon ape leukemia retrovirus (binds Pit-1 receptor), indicating that Pit-2 is the form of Na/Pi transporter/viral receptor regulated by PKC. The PKC-mediated activation of Pit-2 was blocked by pretreating cells with the pan-PKC inhibitor bisindolylmaleimide but not with the conventional PKC isotype inhibitor Gö 6976, suggesting that a novel PKC isotype is required to regulate Pit-2. Overexpression of PKCepsilon, but not of PKCalpha, -delta, or -zeta, was found to mimic the activation of Na/Pi uptake. To further establish that PKCepsilon is involved in the regulation of Pit-2, cells were treated with PKCepsilon-selective antisense oligonucleotides. Treatment with PKCepsilon antisense oligonucleotides decreased the PMA-induced activation of Na/Pi uptake. These results indicate that PMA-induced stimulation of Na/Pi uptake by Pit-2 is specifically mediated through activation of PKCepsilon.