RETINOIC ACID REGULATES INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN EXPRESSION IN HUMAN OSTEOBLAST CELLS

Retinoic acid (RA) regulates the growth and differentiation of numerou s cells types and plays a key role in skeletal development. Previous s tudies have demonstrated that insulin-like growth factors (IGFs) are i mportant local regulators of bone cell proliferation and differentiati on and that IGF-binding proteins (IGFBPs) modulate their activities. I n an attempt to test the hypothesis that RA mediates its effects on bo ne cells in part by regulating IGFBP expression, we first examined the effect of RA on IGFBP expression in human osteoblast model systems an d then compared these responses to the effects of RA on IGFBPs in huma n skin fibroblasts. The most dramatic effect of RA on IGFBP expression in all cell types tested was to increase IGFBP-6 messenger RNA (mRNA) abundance more than 1000% of the control value. Significant effects o n IGFBP-5 mRNA abundance were also found, with maximal reductions to 3 5% of control within 24 h of treatment. In addition, RA maximally incr eased IGFBP-3 and -4 mRNA to 580% and 390% of the control value, respe ctively, in SaOS-2 cells, but had variable effects on IGFBP-5 and -4 m RNA levels in human bone cells, U2-OS, and human skin fibroblasts. The levels of the 24-, 29- to 32-, and 38- to 42-kDa IGFBPs in the condit ioned medium of RA-treated cultures increased, as determined by ligand blot analysis, whereas the amount of IGFBP-5 was reduced, as determin ed by RIA. Cycloheximide abolished the RA-stimulated increase in IGFBP -6 mRNA and reduced baseline IGFBP-6 mRNA levels, but did not affect R A-modulated mRNA levels of the other IGFBPs. RA modestly increased the stabilities of all four IGFBP mRNAs, which could contribute to the ob served increases in IGFBP-3 and IGFBP-4 mRNA levels; however, the 217% increase in the IGFBP-5 mRNA half-life in the presence of RA could no t contribute to the reduction in mRNA levels. In addition, the small i ncrease in the IGFBP-6 mRNA half-life could not account for the 1900% increase in the mRNA level. These data suggest that RA stimulated chan ges in IGFBP-5 and -6 mRNA levels may in part be mediated by alteratio ns in transcription or other early posttranscription regulatory mechan isms. In conclusion, RA significantly regulates IGFBP expression in hu man osteoblast cells.