Dissociation of second messenger activation by parathyroid hormone fragments in osteosarcoma cells

PTH activates multiple second messengers in its target cells, but the level at which the hormonal signal splits into different pathways is still unknown. To achieve insights on this issue, we have studied the structure-function relationship of PTH by analyzing the effects of bovine PTH-(1-34) [bPTH- (1-34)] and PTH fragments truncated at the N-terminus on the intracellular calcium concentration ([Ca²⁺]i) and cAMP production in the rat osteogenic sarcoma cell line UMR 106-01. [Ca²⁺]i was measured in single cells using fura-2. When exposed to 10^-7M bPTH-(1-34), 20% of the cells responded with a transient increase in [Ca²⁺]i of variable amplitude. Equimolar doses of bPTH-(2-34), propionyl bPTH-(2-34) [(pbPTH-(2- 34)], and bPTH-(3-34) also transiently increased [Ca²⁺]i, whereas both [tyrosine34]bPTH-(7-34) amide [bPTH-(7-34)j and bPTH-(30-34) were ineffective. The amplitude of the [Ca²⁺] i transients was dose-dependent, with threshold concentrations of 1010 M for bPTH-(1-34) and bPTH-(2-34), and 10^-9M for bPTH-(3-34). The response rate to the active peptides ranged between 10-30%, without a clear dose-relatedness. A second addition of 10^-7M bPTH-(1-34) to cells prestimulated with equimolar doses of bPTH-(2-34), pbPTH-(2-34), or bPTH-(3-34) produced another transient, whereas after exposure to 10^-7M bPTH-(1-34), the cells were completely desensitized to a second homologous stimulation, suggesting that the binding affinity of the truncated peptides for the PTH receptor is lower than that of the intact bPTH-(1-34) fragment. In addition, both bPTH-(1-34) and bPTH-(2-34) dose-dependently stimulated cAMP production, but the former was more potent (ED₅₀= 10^-9us. 10^-7M, respectively). On the contrary, pbPTH-(2-34), bPTH-(3-34), and bPTH-(7-34) had no effect on cAMP. Pretreating the cells with pertussis toxin to enhance cAMP responses via inhibition of Gi potentiated the effect of bPTH-(l- 34) and bPTH-(2-34) and disclosed weak but detectable agonist action of pbPTH-(2-34). These results indicate that specific domains of the PTH molecule are linked to activation of different second messenger pathways; while the first two amino acids are indispensable for activating the cAMP system, generation of the [Ca²⁺]i signal appears to involve a longer domain, including the amino acid residue in position 3.