Minimization of parathyroid hormone - Novel amino-terminal parathyroid hormone fragments with enhanced potency in activating the type-1 parathyroid hormone receptor

The amino-terminal and carboxyl-terminal portions of the 1-34 fragment of parathyroid hormone (PTH) contain the major determinants of receptor activation and receptor binding, respectively. We investigated how the amino-terminal signaling portion of PTH interacts with the receptor by utilizing analogs of the weakly active fragment, rat (r) PTH(1-14)NH2, and cells transfected with the wild-type human PTH-1 receptor (hP1R-WT) or a truncated PTH-1 receptor which lacked most of the amino-terminal extracellular domain (hP1R-delNt). Of 132 mono-substituted PTH(1-14) analogs, most having substitutions in the (1-9) region were inactive in assays of cAMP formation in LLC-PK1 cells stably expressing hP1R-WT, whereas most having substitutions in the (10-14) region were active, Several substitutions (e.g. Ser(3) --> Ala, Asn(10) --> Ala or Gin, Leu(11) --> Arg, Gly(12) --> Ala, Kis(14) --> Trp) enhanced activity 2-10-fold. These effects were additive, as [Ala(3,10,12),Arg(11),Trp(14)] rPTH(1-14)NH2 was 220-fold more potent than rPTH(1-14)NH2 (EC50 = 0.6 +/- 0.1 and 133 +/- 16 mu m respectively). Native rPTH(1-11) was inactive, but [Ala(3,10),Arg(11)]rPTH(1-11)NH2 achieved maximal cAMP stimulation (EC50 = 17 mu M). The modified PTH fragments induced cAMP formation with hP1R-delNt in COS-7 cells as potently as they did with hP1R-WT; PTH(1-34) was 6,000-fold weaker with hP1R-delNt than with hP1R-WT. The most potent analog, [Ala(3,10,12),Arg(11),Trp(14)]rPTH(1-14)NH2, stimulated inositol phosphate production with hP1R-WT, The results show that short NH2-terminal peptides of PTH can be optimized for considerable gains in signaling potency through modification of interactions involving the regions of the receptor containing the transmembrane domains and extracellular loops.