STRUCTURE-FUNCTION REQUIREMENTS OF PARATHYROID-HORMONE FOR STIMULATION OF 1,25-DIHYDROXYVITAMIN-D(3) PRODUCTION BY RAT RENAL PROXIMAL TUBULES

PTH stimulates synthesis and secretion of 1,25-dihydroxyvitamin D3 [1, 25-(OH)2D3] in renal proximal tubule cells through activation of the p rotein kinase-A (PKA) or the protein kinase-C (PKC) signaling pathway. The relative contribution of the two transducing systems was explored using PTH fragments with selective activation of either PKA or PKC. R at renal proximal tubules were isolated by Percoll centrifugation, and PKA and PKC activities were measured after treatment with synthetic f ragments and analogs of PTH. Rat PTH-(1-34), [Nle8,Nle15,Tyr34]bovine PTH-(3-34), and human PTH-(13-34) increased PKC activity in a dose-dep endent manner. All fragments tested stimulated PKC at physiological co ncentrations (10(-11)-10(-10) m). Rat PTH-(1-34) (10(-7) m) increased PKA activity 4.5-fold, but other fragments failed to stimulate PKA bet ween 10(-12)-10(-6) M. Human PTH-(28-34) stimulation of PKC was variab le from experiment to experiment. All four PTH fragments tested increa sed 1,25-(OH)2D3 secretion by perifused renal proximal tubules at the lowest concentrations that stimulated PKC activity. The adenylate cycl ase inhibitor 2',5'-dideoxyadenosine (10(-4) m) reduced PTH-(1-34)-sti mulated PKA activity by 60%, but failed to block the rise in 1,25-(OH) 2D3 secretion. The results of these studies demonstrate that PTH fragm ents that contain the PKC translocating domain stimulate 1,25-(OH)2D3 secretion, whereas elimination of the PKA activation domain does not a lter the potency of the analogs' 1,25-(OH)2D3-stimulating activity. Th ese results support the concept that PKC translocation may be required for PTH stimulation of 1,25-(OH)2D3 secretion.