Regulation of PTH/PTH-related protein receptor expression by endogenous PTH-related protein in the rat osteosarcoma...

We have utilized clonal lines of the rat osteoblastic cell line ROS 17/2.8 stably transfected with full-length parathyroid hormone-related protein (PTHrP) cDNA in a sense or an antisense orientation to examine the effects of alteration in the production of endogenous PTHrP on expression of the PTH/PTHrP receptor. In the stably transfected clonal cell lines, changes in PTH/PTHrP receptor expression were evaluated by Northern blot analysis, whole-cell ligand binding of 125I-[Tyr36] PTHrP (1-36), and exogenous PTHrP (1-34)-stimulated cyclic adenosine monophosphate (cAMP) accumulation. Compared to control (vector-transfected) cells, PTHP-overproducing (sense-transfected) cells exhibited a marked decrease in the expression of PTH/PTHrP receptor mRNA and PTHrP ligand binding, as well as a corresponding decrease in the PTHrP (1-34)-stimulated cAMP response. By contrast, the antisense-transfected cells showed a marked increase in expression of PTH/PTHrP receptor mRNA and PTHrP (1-34) ligand binding, but a significant increase in the PTHrP (1-34)-stimulated cAMP response was not detected. Using antisense-transfected ROS cells, PTH/PTHrP receptor mRNA expression and 125I-[Tyr36] PTHrP (1-36) binding were downregulated by treatment for 24 h with exogenous PTHrP (1-36), forskolin, or dibutyryl cAMP. The findings extend those of earlier studies showing receptor downregulation by exogenous PTH by indicating that endogenous PTHrP, as well as circulating PTH, may help regulate receptor production; and suggesting that even very low concentrations of the peptide may influence receptor production.