An intronic Ikaros-binding element mediates retinoic acid suppression of the kappa opioid receptor gene, accompanied by histone deacetylation on the promoters

The mouse kappa opioid receptor (KOR) gene is constitutively expressed in m ouse embryonal carcinoma P19 stem cells and suppressed by retinoic acid (RA ) in cells undergoing neuronal differentiation. A negative regulatory eleme nt is located within intron 1 of the KOR gene, which contains an Ikaros (Ik )-binding site (GG-GAAgGGGAT). This sequence is an Ik-1 respondive, functio nally negative element as demonstrated in the context of both natural KOR a nd heterologous promoters. The two underlined G residues of the second half -site are critical for Ik-1 binding and Ik-mediated repression of the KOR g ene. RA induces Ik-1 expression within 1 day of treatment and suppresses KO R expression between 2 and 3 days. Overexpression of Ik-1 in P19 suppresses endogenous KOR gene expression, accompanied by increased binding of Ik-1 t o the Ik-binding site and chromatin histone deacetylation on KOR promoters. It is proposed that in an RA-induced P19 differentiation model, RA elevate s Ik-1 expression, which recruits histone deacetylase to intron 1 of the KO R gene and silences KOR gene promoters.