CHARACTERIZATION OF THE PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASE-CISOZYMES PRESENT IN THE BOVINE PARATHYROID AND IN HUMAN KIDNEY HEK293CELLS STABLY TRANSFECTED WITH THE HUMAN PARATHYROID CA2-SENSING RECEPTOR()

The regulation of parathyroid hormone secretion by the chief cells of the parathyroid is mediated by a 7-transmembrane (7-TM) Ca2+-sensing r eceptor (CaR), which signals via activation of pertussis toxin-insensi tive G proteins, causing stimulation of phosphatidylinositol-specific phospholipase C (PI-PLC). We have identified the PI-PLC isoforms expre ssed in two model systems utilized for studying CaR signal transductio n, i.e. dispersed bovine parathyroid cells and a human embryonic kidne y cell line (HEK 293) stably transfected with the human parathyroid Ca R-cDNA. All of the eight PI-PLC isozymes examined in this study were f ound to be expressed to varying extents in the bovine parathyroid glan d and in the CaR-transfected HEK cells as assessed by immunoblotting. We localized the expression of the more abundant isozymes (beta 1, bet a 2, beta 3, gamma l, gamma 2, delta 2) to the chief cells of the bovi ne parathyroid by immunocytochemistry, while the two less abundant iso zymes (delta 1, beta 4) were not detectable in parathyroid sections. G proteins activated by 7-TM receptors are known to activate mainly PI- PLC of the beta class. Therefore, beta 1, beta 2, beta 3 and beta 4, a ll expressed in the bovine parathyroid, are candidate isozymes for cou pling to the CaR. A comparison of the levels of expression of PI-PLC i sozymes between CaR-transfected HEK cells and non-transfected HEK cell s suggested that the expression of the CaR in this human cell line doe s not cause a significant up-regulation of any of the PLC beta and PLC gamma isozymes. PLC delta 2, showing predominantly nuclear localizati on in the parathyroid, was the sole PI-PLC isozyme with higher levels of expression in CaR-transfected HEK cells.