Direct inhibition of Indian hedgehog expression by parathyroid hormone (PTH)/PTH-related peptide and up-regulation by retinoic acid in growth plate chondrocyte cultures

Indian hedgehog (Ihh) is highly expressed in prehypertrophic chondrocytes i n vivo and has been proposed to regulate the proliferation and maturation o f chondrocytes and bone collar formation in the growth plate. In high-densi ty cultures of rabbit growth-plate chondrocytes, Ihh mRNA was also expresse d at the highest level in the prehypertrophic stage. To explore endogenous factors that regulate Ihh expression in chondrocytes, we examined the effec ts of various growth factors on Ihh mRNA expression in this system. Retinoi c acid (RA) and bone morphogenetic protein-2 enhanced Ihh mRNA expression, whereas PTH/PTH-related peptide (PTHrP) markedly suppressed Ihh expression. RCS at more than 10(-8) M induced the expression of Ihh and Patched I (Ptc 1) within 3 h, before it increased the type X collagen mRNA level at 6-24 h . Cycloheximide blocked the up-regulation of Ihh by RA, indicating the requ irement of de novo protein synthesis for this stimulation. These findings s uggest that RA is involved in the up-regulation of Ihh during endochondral bone formation. In contrast to RA, PTH (1-84) at 10(-7) M abolished the mRN A expression of Ihh and Ptc1 within 2-4 h, before it suppressed the express ion of type X collagen at 12-24 h, The inhibition of Ihh expression by PTH (1-84) did not require de novo protein synthesis. PTH (1-34), PTHrP (1-34), and (Bu)(2)cAMP also suppressed Ihh expression. On the other hand, Ihh has been reported to induce PTHrP synthesis in the perichondrium, Consequently , the direct inhibitory action of PTH/PTHrP on Ihh appears to be a negative feedback mechanism that prevents excess PTHrP accumulation in cartilage. ( C) 2001 Academic Press.