Retinol-binding protein is produced by rabbit chondrocytes and responds toparathyroid hormone (PTH)/PTH-related peptide-cyclic adenosine monophosphate pathway

PTH and dibutyryl cAMP [(Bu)(2)cAMP] induced the expression of a 19-kDa pro tein in the conditioned media of rabbit growth plate chondrocyte cultures. The 19-kDa protein was identified as plasma retinol-binding protein (RBP) b y aminoterminal sequence analysis and immunoblot analysis with an anti-REP monoclonal antibody. Northern blot analysis showed that PTH, PTH-related pe ptide (PTHrP), and (Bu)(2)cAMP increased the REP messenger RNA (mRNA) level in chondrocyte cultures. Further, both PTH and (Bu)(2)cAMP markedly induce d the expression of REP mRNA by about 10-fold at 3 h and by about 40-fold a t 24 h, indicating a pretranslational regulation. The level of the mRNA exp ression induced by PTH, PTHrP, and (Bu)(2)cAMP was as high as that by retin oic acid (RA), known as a potent inducer of REP in hepatoma cells. REP mRNA was also detected in cartilage tissues at higher levels than in the other tissues examined except liver. Both REP and PTH/PTHrP inhibited the dediffe rentiative activity of RA on growth plate chondrocytes when added to the cu lture medium. These results demonstrate that chondrocytes synthesize and se crete REP in vivo and in vitro and suggest that PTH/PTHrP modulates the eff ect of RA by means of REP production in chondrocytes.