Retinoid X receptors and retinoid response in neuroblastoma cells

Retinoic acid (RA) modulates differentiation and apoptosis of neural cells via RA receptors (RARs) and retinoid X receptors (RXRs). Neuroblastoma cells are potentially useful models for elucidating the molecular mechanisms of RA in neural cells, and responses to different isomers of RA have been interpreted in terms of differential homo- and heterodimerization of RXRs. The aim of this study was to identify the RXR types expressed in neuroblast and substrate-adherent neuroblastoma cells, and to study the participation of these RXRs in RAR heterodimers. RXRbeta was the predominant RXR type in N-type SH SY 5Y cells and S-type SH EP cells. Gel shift and supershift assays demonstrated that RARbeta and RARgamma predominantly heterodimerize with RXRbeta. In SH SY 5Y cells, RARgamma/RXRbeta was the predominant heterodimer binding to the DR5 RARE in the absence of 9-cis RA (9C), whereas the balance shifted in favor of RARbeta/RXRbeta in the presence of ligand. There was a marked difference between the N- and S-type neuroblastoma cells in retinoid receptor-DNA interactions, and this may underlie the differential effects of retinoids in these neuroblastoma cell types. There was no evidence to indicate that 9C functions via RXR homodimers in either SH SY 5Y or SH EP neuroblastoma cells. The results of this study suggest that interactions between retinoid receptors and other nuclear proteins may be critical determinants of retinoid responses in neural cells.