Involvement of all-trans-retinoic acid in the breakdown of retinoic acid receptors α and γ through proteasomes in mcf-7 human breast cancer cells 33Abbreviations: RA, all-trans retinoic acid; RAR, retinoic acid receptor; RXR, retinoid X receptor; SUMO-1, small ubiquitin-related modifier; 4HPR, 4-hydroxyphenyl retinamide; EMSA, electrophoretic mobility shift assay; SSC buffer, 150 mM NaCl and 15 mM sodium citrate, pH 7.0; DTT, dithiothreitol; and PMSF, phenylmethylsulfonyl fluoride.