Disulfide isomerase glucose-regulated protein 58 is required for the nuclear localization and degradation of retinoic acid receptor alpha.

Retinoic acid receptor alpha (RARA) is critical for spermatogenesis, as shown by a sterility phenotype observed in Rara knockout mice. RARA is important in both Sertoli and germ cells of the testis. Here, we demonstrate that a disulfide isomerase glucose-regulated protein 58 (GRp58) participates in the nuclear import and degradation of RARA in Sertoli cells. GRp58 interacted with RARA in the presence of all-trans retinoic acid (ATRA) ligand and, as a complex, it was translocated from the cytoplasm to the nucleus and, then with time, GRp58 dissociated from RARA and was found in the cytoplasm. The GRp58 RNAi treatment disrupted ATRA-dependent RARA nuclear localization, indicating the requirement of GRp58 for RARA nuclear localization. Moreover, treatment with sulfhydryl-modifying agents that oxidize SH-groups of cysteine residues to disulfide bonds abolished ATRA-mediated RARA nuclear localization, suggesting that the thiol oxidoreductase activity of GRp58 may be required for RARA nuclear import. Additionally, the proteasome inhibitor treatment resulted in the co-localization of GRp58 and RARA at the endoplasmic reticulum (ER), suggesting that GRp58 may bring RARA to the ER for the ER-associated degradation (ERAD) of RARA before it is de-coupled from RARA for recycling. In this regard, proteasome inhibitor treatment also increased the interaction of RARA with UBE2J2, an ERAD-associated ubiquitin E2 enzyme. Collectively, the results indicate that GRp58 may act as a molecular chaperone that alters the protein conformation of RARA for its delivery to the nucleus and, then with time, accompanies RARA to the ER for RARA ubiquitination and proteasome-mediated ERAD.