Optical spectroscopic approach as a rapid tool to characterize the interactions of retinoids with human nuclear receptors

Retinoids are potent molecules that can affect a variety of fundamental biological processes including cell differentiation and proliferation and apoptosis. These molecules elicit their biological effects by activating a family of nuclear receptors which act as ligand-inducible transcription factors belonging to the steroid/thyroid receptor superfamily. Retinoic acid receptors form heterodimers in which response to ligand binding, both partners contribute to transactivation and/or DNA binding in vivo. Surface-enhanced Raman scattering (SERS), Fourier transform-SERS (FT-SERS), fluorescence and circular dichroism are proposed to rapidly give information on the interaction of the different RARs and RXRs with their specific ligands at physiological concentrations. FT-SERS data reveal a significant attenuation in intensity of the bands originating from the retinoic polyenic chain upon complexation. The spectrum is dominantly of the (Beta) - ionone ring. Fluorescence measurements supported the hydrophobic character of the ligand binding pocket and the circular dichroic data indicate that the protein helices extend upon ligand binding. These novel spectroscopic information are fully consistent with published x-ray crystallographic results and suggest that these techniques may be valuable additional tools to characterize the interactions of agonists and antagonists with residues of the ligand binding pocket retinoid receptor homo- and hetero-dimers.