Changes in parathyroid hormone receptors during chondrocyte cytodifferentiation

The purpose of this study was to investigate the relationship between changes in parathyroid hormone (PTH) receptor levels and chondrocyte maturation during endochondral ossification. Chondrocytes were isolated from the growth plate of rabbit ribs and maintained in the presence of 10% serum in mass cultures. Treatment with PTH-(1-84) and a PTH-(1-34) fragment suppressed the increases in alkaline phosphatase activity and in type X collagen and 1α,25-dihydroxyvitamin D₃ receptor levels and abolished ⁴⁵Ca incorporation into mineral, all of which occurred in parallel untreated cultures in the hypertrophic (terminal) stage. These effects of PTH were observed at low concentrations (10^-10 to 10^-9 M) and within 24-48 h of treatment. PTH-(1-84) and PTH-(1-34) also increased [³⁵S]sulfate incorporation into newly synthesized proteoglycans. In contrast, the middle and carboxyl-terminal fragments of PTH tested had little effect on proteoglycan synthesis or terminal differentiation. The binding of ¹²⁵I- PTH-(1-34) to cells in the growth plate was greater than that to cells in liver, skin, muscle, brain, or kidney. When the correlation between binding levels and stage of maturation was examined, we found that ¹²⁵I-PTH-(1- 34) binding to its 72-kDa receptor was low in resting and proliferating chondrocytes, increased 10-fold in matrix-forming chondrocytes, and thereafter decreased in hypertrophic chondrocytes both in vitro and in situ. Scatchard analysis revealed that the changes in PTH binding were due to changes in the number, and not in the affinity, of the receptor. The changes in PTH-(1-34) binding paralleled those in [³⁵S]sulfate incorporation into proteoglycans. These findings suggest that stage-dependent increases in PTH/PTH-related peptide receptor levels localize the hormone stimulation of proteoglycan synthesis and inhibition of precocious hypertrophy in the matrix-forming zone of growth plates.