Distinct sensitivity of neuroblastoma cells for retinoid receptor agonists: evidence for functional receptor heterodimers.

Retinoic acid (RA) plays a major role in embryogenesis of the nervous system and has been reported to induce differentiation in neuroblastoma cell lines. To identify RA signaling pathways involved in such differentiation processes, two RA-sensitive neuroblastoma cell lines (LA-N-5 and SH-SY5Y) were extensively studied. Northern blot experiments determined that of the three RAR mRNAs, only RARalpha was significantly expressed, with respectively weak or undetectable levels of RARgamma and RARbeta. RXRs (alpha and beta) receptors were weakly expressed. Western blotting analysis confirmed the constitutive expression of RARalpha and absence of RARbeta and weak levels of RXRalpha. Treatment with all-trans-RA up-regulated RARalpha and induced a drastic increase of RARbeta (both at the RNA and protein level). To further characterize the function of RARalpha, RARbeta and RXRalpha in NB cells, nuclear extracts from LA-N-5 cells were analysed by EMSA studies. Three specific retarded complexes were observed which were significantly decreased or shifted in the presence of monoclonal antibodies to RARalpha, RARbeta and RXRalpha. RA treatment dramatically induced a DR5-binding RXRalpha-RARbeta heterodimer. Treatment with combinations of RARalpha or RARbeta agonists with a RXRalpha agonist or with a RARalpha agonist alone, induced neurite-outgrowth supporting the probability that both RXRalpha-RARalpha or RXRalpha-RARbeta heterodimers are involved in RA-mediated differentiation of NB cells. The availability of novel synthetic RA-specific receptor ligands should provide the possibility of tissue specific therapeutic regimes.