Abstract
The objective of this study was to compare the properties of 9-cis and all-trans retinoic acid with respect to the induction of expression of retinoic acid receptor beta (RAR-beta) and cellular retinoic acid-binding protein (CRABP) II in human neuroblastoma SH SY 5Y cells. RAR-beta and CRABP II mRNA was induced by both all-trans and 9-cis retinoic acid in SH SY 5Y cells. Induction was rapid, detectable within 2-4 h, and inhibited by actinomycin D. Time-courses of induction for RAR-beta and CRABP II differed: RAR-beta mRNA levels reached a maximum 4-6 h after adding all-trans or 9-cis retinoic acid, whereas CRABP II mRNA levels increased over at least 18 h. These differences were attributed to the longer half-life of CRABP II mRNA (20 h) compared with RAR-beta mRNA (3.9 h). The dose-response characteristics of all-trans and 9-cis retinoic acid were different: all-trans was effective at nanomolar concentrations, whereas 10-fold higher levels of 9-cis retinoic acid were required to achieve comparable induction of RAR-beta and CRABP II. Conversely, at high concentrations, 9-cis retinoic acid gave a greater induction of RAR-beta and CRABP II than all-trans. The induction of RAR-beta and CRABP II by all-trans retinoic acid was maintained in the subsequent absence of all-trans retinoic acid, whereas induction by 9-cis retinoic acid was dependent on its continued presence in the culture medium. These results suggest that, at high concentrations, 9-cis retinoic acid may produce its transcriptional effects via retinoid X receptor (RXR) homodimers. This has implications for the cellular functions of 9-cis retinoic acid and its use as a biological response modifier.