The fusion oncoprotein PML-RARalpha induces endoplasmic reticulum (ER)-associated degradation of N-CoR and ER stress.

PML-RARalpha, a fusion protein of promyelocytic leukemia (PML) and the retinoic acid receptor-alpha (RARalpha), causes acute promyelocytic leukemias (APL). Although the role of nuclear PML-RARalpha has been extensively studied, a significant amount of PML-RARalpha is in the cytoplasm. The role cytoplasmic PML-RARalpha plays in leukemogenesis is unknown. Here we report that PML-RARalpha induces the N-CoR accumulation in the endoplasmic reticulum (ER), leading to the induction of ER stress and the processing of activating transcription factor 6 (ATF6), the unfolded protein response. PML-RARalpha stimulates the ubiquitylation of N-CoR via Ubc6 that is involved in the protein quality control. This ER-associated degradation (ERAD) of N-CoR reduces the soluble N-CoR protein levels in the nucleus. The two N-CoR-interacting sites in PML-RARalpha are required for the ERAD of N-CoR, suggesting the aberrant binding of PML-RARalpha to N-CoR may induce the ERAD of N-CoR. Overexpression of N-CoR induces the differentiation of APL-derived NB4 cells, suggesting that the low levels of N-CoR in the nucleus may contribute at least partly to PML-RARalpha-mediated leukemogenesis.