Retinoic acid-induced heparin-binding factor (RIHB) mRNA and protein are strongly induced in chick embryo chondrocytes treated with retinoic acid

Abstract

Retinoic acid-induced heparin-binding factor (RIHB) is a highly basic polypeptide expressed during early chick embryogenesis. We have examined the induction of RIHB by retinoic acid in chondrocytes isolated from the sterna of Day 15 chick embryos and the effects of exogenous RIHB on these cells. There is an induction of RIHB mRNA in chondrocytes which is dose dependent, with maximal levels of expression observed with concentrations of retinoic acid in the 10(-6) M range. RIHB mRNA is first observed 16 h after commencement of treatment, is maximal after 24-48 h, and is completely attenuated after 5 days. This transient pattern of expression is very similar to that of type X collagen; however, RIHB induction precedes that of type X collagen by about 24 h. The expression of both RIHB and type X collagen precedes the drop in keratan sulfate:chondroitin sulfate proteoglycan and type II collagen expression and the surge of fibronectin expression. The induction of RIHB mRNA is accompanied by an increased synthesis of the protein. No RIHB can be detected in untreated chondrocytes; however, large amounts are produced by cells treated with 5 x 10(-7) M retinoic acid. The protein is recovered mainly in the culture medium and bound to the extracellular matrix. Only a small amount can be detected in cell extracts. RIHB can be detected in the culture medium after 16-24 h and, unlike the mRNA, persists over the 5-day period examined. The effect of exogenous RIHB (purified from chick embryos) on chondrocyte proliferation and morphology was examined. When added to the culture medium in concentrations of up to 500 ng/ml RIHB had no effect on [3H]thymidine incorporation or cell morphology. Thus, RIHB is not the direct mediator of retinoic acid for these cells, but is strongly induced during the treatment.