RETINOIC ACID UP-REGULATES NUCLEAR RETINOIC ACID RECEPTOR-ALPHA EXPRESSION IN HUMAN NEUROBLASTOMA-CELLS

Retinoic acid (RA) nuclear receptors (RARs) are thought to mediate the cellular and molecular effects of RA on a wide variety of tissues. In most cell types, RAR alpha expression remains relatively constant fol lowing exposure to RA, while that of RAR beta is rapidly induced. In t his study, we show that in human neuroblastoma, a cell type exceptiona lly sensitive to RA-induced differentiation, RAR alpha as well as RAR beta is markedly up-regulated by RA treatment. This effect was consist ent in all 5 neuroblastoma cell lines tested and was reflected in a 2- to 5-fold increase in receptor mRNA levels as assessed by Northern-bl ot analysis. Using LA-N-5 human neuroblastoma cells, we found that rec eptor up-regulation occurred in a time- and dose-dependent fashion wit h increases in both RAR alpha and beta mRNA detectable 1-2 hr after th e addition of RA. These inductions were not abrogated by cycloheximide , indicating that protein synthesis was not required for the RA respon ses. Nuclear run-off experiments combined with Northern-blot analysis of RAR alpha stability directly demonstrated that the up-regulation of RAR alpha mRNA levels reflected an increased rate of transcription wi thout changes in message half-life. These findings, showing direct act ivation by RA of RAR alpha gene transcription in human neuroblastoma c ells, suggest differences in the overall regulation of this receptor f rom that found in most other RA-inducible tissue. (C) 1994 Wiley-Liss, Inc.