Beta-arrestin 2 functions as a G-protein-coupled receptor-activated regulator of oncoprotein Mdm2

Oncoprotein Mdm2 is a master negative regulator of the tumor suppressor p53 and has been recently shown to regulate the ubiquitination of beta-arrestin 2, an important adapter and scaffold in signaling of G-protein-coupled receptors (GPCRs). However, whether beta-arrestin 2 has any effect on the function of Mdm2 is still unclear. Our current results demonstrated that the binding of Mdm2 to beta-arrestin 2 was significantly enhanced by stimulation of GPCRs. Activation of GPCRs led to formation of a ternary complex of Mdm2, beta-arrestin 2, and GPCRs and thus recruited Mdm2 to GPCRs at plasma membrane. Moreover, the binding of beta-arrestin 2 to Mdm2 suppressed the self-ubiquitination of Mdm2 and consequently reduced the Mdm2-mediated p53 degradation and ubiquitination. Further experiments revealed that overexpression of beta-arrestin 2 enhanced the p53-mediated apoptosis while suppression of endogenous beta-arrestin 2 expression by RNA interference technology considerably attenuated the p53-mediated apoptosis. Our study thus suggests that beta-arrestin 2 may serve as a cross-talk linker between GPCR and p53 signaling pathways.     

Novel oxidative stress-responsive gene ERS25 functions as a regulator of the heat-shock and cell death response

Members of the yeast p24 family, including Emp24p and Erv25p, exist as heteromeric complexes that have been proposed to cycle between the endoplasmic reticulum (ER) and Golgi compartments. The specific functions and sites of action of p24 proteins are still unknown. Here we identified a human homolog of the yeast p24 family of proteins, named ERS25 (endoplasmic reticulum stress-response protein 25), and investigated its role in stress response. ERS25 is predicted to have an ER localization signal peptide, a GOLD (Golgi dynamics) domain, which is found in several eukaryotic Golgi and lipid-trafficking proteins, a coiled-coil region, and a transmembrane domain. We demonstrate that ERS25 is localized to the ER and is induced by ER-specific stress, heat shock, and oxidative stress. The selective induction of ERS25 by brefeldin A, but not tunicamycin, implicates the involvement of ERS25 in protein trafficking between the ER and the Golgi. Small interfering RNA-mediated inhibition of ERS25 results in a significant decrease in apoptosis as well as a reduction of reactive oxygen species induced by oxidative stress. Moreover, ERS25 depletion results in a significant increase in the levels of the ER chaperone HSP70 in response to heat-shock stress through increased levels of HSF-1. We also found that inhibition of ERS25 induction in response to heat shock enhanced the binding of HSP70 to Apaf-1, which is likely to interfere in stress-mediated apoptosis. Together, the data presented here demonstrate that ERS25 may play a critical role in regulation of heat-shock response and apoptosis.