The effect of HACE1 on RAR protein stability

All-trans retinoic acid (RA), as a ligand for retinoic acid receptors (RAR) and retinoid X receptors (RXR), modulates their transcriptional activity. The AF-1 and AF-2 domains mediate the transcriptional activity. The ligand dependent activation of the AF-2 domain by RA is well understood to involve chromosome decompaction in the presence of ligand with the aid of coactivators. The mechanism of the ligand independent action of the AF-1 domain is less clear. The AF-1 domain of RARs may be regulated by interacting proteins such as HACE1.;In vitro and in vivo studies in our lab have shown that HACE1 interacts with RARalpha1, - beta 1, -beta2, -beta3, and --gamma 1 at the variable AF-1 domain. Transactivation studies have shown that HACE1 represses RA dependent transcriptional activity of RARgamma1 , but not RARbeta3 and RARalpha1. Our original hypothesis proposed that HACE1 represses RAR transcriptional activity by inhibiting RA-dependent degradation of RARs. Current data confirms previous observations that the half life of RARbeta3 is shortened in the presence of RA, compared to a vehicle control. Protein stability assays show that HACE1 does not have an effect on degradation of RARbeta3 and RARgamma 1; however, it increases the ligand independent degradation of RARalpha 1.;This data suggests the A/B domain of RARgamma1 recruits HACE1 for binding which results in transcriptional repression. Also, in a separate mechanism, the A/B domain of RARalpha1 binds to HACE1 which then accelerates its degradation in a ligand independent manner. The mechanisms behind these novel roles of HACE1 will need to be studied further and may help in understanding the method of AF-1 transcactivation function.