Parathyroid hormone‐related protein stimulates prostaglandin E₂ release from human osteoblast‐like cells: Modulating effect of peptide length

Parathyroid hormone‐related protein (PTHrP) is a potent bone‐resorbing protein that frequently mediates the humoral hypercalcemia of malignancy syndrome. Since prostaglandins may mediate the bone‐resorptive action of certain hormones, we examined the effect of PTHrP on prostaglandin E₂ (PGE₂) secretion by human osteoblast‐like cells. There was low‐level basal secretion of PGE₂ by Saos‐2 cells (8.1 + 0.6 pg/ml). Using four different preparations of PTHrP, it was observed that with increasing peptide length, from 36 to 141 amino acids, a significant increase in efficacy for PGE₂ release was seen in these cells. All forms of PTHrP were agonists for PGE₂ release, with effects seen at concentrations as low as 10⁻¹² M in 48 h conditioned media. The amino terminus of the molecule appeared critical for this effect since the truncated derivative PTHrP‐(7–34) did not induce significant PGE₂ secretion. However, the influence of peptide length could not be explained by differential activation of adenylate cyclase since [Tyr²⁶]PTHrP‐(1–36)amide was equipotent to the longest peptide preparation, PTHrP‐(1–141), in stimulating cyclic AMP accumulation in the Saos‐2 cells. In contrast, PTHrP‐(1–141) was significantly more effective than [Tyr³⁵]PTHrP‐(1–36)‐amide in inducing a rise in cytosolic calcium. Further, this effect was noted at concentrations lower than those that caused significant cyclic AMP accumulation in the Saos‐2 cells. PTHrP‐(1–141) induced the release of PGE₂ from primary human bone cell cultures to levels entirely comparable to those seen in the Saos‐2 cells. PTHrP‐(1–141) also induced PGE₂ release by cultured fetal rat long bones at 72 h. We conclude that the carboxy‐terminal region of PTHrP has important effects on cellular signal transduction pathways and on the release of a potent bone‐active cytokine, PGE₂.