MODULATION OF THE RETINOIC ACID AND RETINOID-X RECEPTOR SIGNALING PATHWAYS IN P19 EMBRYONAL CARCINOMA-CELLS BY CALRETICULIN

Calreticulin is a widely expressed calcium binding protein that can bi nd to an amino acid sequence motif, KXGFFKR, which is present in the c ytoplasmic domain of all integrin alpha-subunits. Closely related sequ ences, KXFPKR and KXFFRR, are encoded in the DNA-binding domain of all members of the steroid/thyroid/retinoid receptor superfamily and it h as recently been demonstrated that calreticulin inhibits their activit y both in vitro and in vivo. Here we present novel evidence that calre ticulin can interfere directly with the retinoic acid (RARs) and retin oid X (RXRs) receptor pathways. Calreticulin exhibits the ability to i nhibit DNA-binding activity of both heterodimeric RAR/RXR and homodime ric RXR complexes in, vitro. Inhibition of RXR binding to DNA is achie ved with a concentration of calreticulin that is approximately fourfol d lower than that required for inhibition of RAR/RXR binding to a cogn ate binding site. Coprecipitation experiments suggest a direct protein :protein interaction between calreticulin and retinoid receptors, Stab le overexpression of calreticulin in P19 embryonal carcinoma cells sig nificantly decreases the rapid activation of the endogenous RA-respons ive RAR beta gene, abrogates the ability of endogenous RAR/RXR complex es to bind to DNA, and inhibits the emergence of the RA-induced differ entiated phenotype, These data demonstrate that calreticulin can inter fere with the two distinct retinoid signaling pathways through a mecha nism likely involving direct protein:protein interactions and that dis ruption of the retinoid signal alters biological processes in vivo. (C ) 1997 Academic Press