TRANSFORMING GROWTH-FACTOR-BETA - A DOWN-REGULATOR OF THE PARATHYROIDHORMONE-RELATED PROTEIN-RECEPTOR IN RENAL EPITHELIAL-CELLS

We have recently provided evidence for the ability of transforming gro wth factor-beta 1 (TGF beta) to modulate PTH-related protein (PTHrP)-m ediated responses in opossum kidney (OK) cells through reducing the nu mber of PTHrP receptor-binding sites. In the present studies, we inves tigated the possible mechanisms by which TGF beta might regulate PTHrP receptor density in OK cells, an area that has remained largely unexp lored. The steady state level of PTHrP receptor mRNA was time dependen tly reduced by TGF beta treatment, with the nadir (similar to 3-fold d ecrease) between 6-10 h, preceding the maximal inhibition on PTHrP rec eptor binding at 18 h. We then assessed whether the 41% reduction in b inding consequent to 18-h TGF beta exposure was reversible. PTHrP-bind ing activity recovered considerably after 24 h (23% decrease compared with controls) and almost completely by 48 h. However, the addition of monensin or cycloheximide, but not actinomycin (at a dose effective i n preventing TGF beta action in this system) during the 24-h recovery period prevented restoration of PTHrP binding. Upon removal of TGF bet a, the PTHrP receptor message showed a trend toward recovery in the en suing 24 h. Therefore, TGF beta provides an example of heterologous de sensitization of the PTHrP receptor in OK epithelial cells by decreasi ng the expression of the receptor message. The desensitization was rev ersible, and the first 24-h recovery phase was dependent on synthesis and processing of new receptor proteins.