New insights on the functional role of URG7 in the cellular response to ER stress.

BACKGROUND INFORMATION: Up-regulated Gene clone 7 (URG7) is an ER resident protein, whose expression is upregulated in the presence of hepatitis B virus X antigen (HBxAg) during HBV infection. In virus-infected hepatocytes, URG7 shows an anti-apoptotic activity due to the PI3K/AKT signaling activation, does not seem to have tumorigenic properties, but it appears to promote the development and progression of fibrosis. However, the molecular mechanisms underlying URG7 activity remain largely unknown. RESULTS: To shed light on URG7 activity, we first analyzed its interactome in HepG2 transfected cells: this analysis suggests that URG7 could have a role in affecting protein synthesis, folding and promoting proteins degradation. Moreover, keeping into account its subcellular localization in the ER and that several viral infections give rise to ER stress, a panel of experiments was performed to evaluate a putative role of URG7 in ER stress. Our main results demonstrate that in ER stressed cells URG7 is able to modulate the expression of Unfolded Protein Response (UPR) markers toward survival outcomes, upregulating GRP78 protein and downregulating the pro-apoptotic protein CHOP. Furthermore, URG7 reduces the ER stress by decreasing the amount of unfolded proteins, by increasing both the total protein ubiquitination and the AKT activation and reducing caspase 3 activation. CONCLUSIONS: All together these data suggest that URG7 plays a pivotal role as a reliever of ER stress-induced apoptosis. SIGNIFICANCE: This is the first characterization of URG7 activity under ER stress conditions. The results presented here will help to hypothesize new strategies to counteract the antiapoptotic activity of URG7 in the context of the viral infection. This article is protected by copyright. All rights reserved

The hepatitis B x antigen anti-apoptotic effector URG7 is localized to the endoplasmic reticulum membrane

Hepatitis B x antigen up-regulates the liver expression of URG7 that contributes to sustain chronic virus infection and to increase the risk for hepatocellular carcinoma by its anti-apoptotic activity. We have investigated the subcellular localization of URG7 expressed in HepG2 cells and determined its membrane topology by glycosylation mapping in vitro. The results demonstrate that URG7 is N-glycosylated and located to the endoplasmic reticulum membrane with an Nlumen-Ccytosol orientation. The results imply that the anti-apoptotic effect of URG7 could arise from the C-terminal cytosolic tail binding a pro-apoptotic signaling factor and retaining it to the endoplasmic reticulum membrane

Fibroblast growth factor receptor 2 (FGFR2) is a potential activator of the MLL-AF4 leukemogenic protein.

The mixed-lineage leukemia (MLL) gene rearrangements are frequent in acute lymphoblastic leukemias. The t(4;11)(q21;q23) translocation fuses the 5′ portion of MLL to the 3′ portion of AF4 and leads to the synthesis of the chimeric oncoprotein, MLL-AF4. It mis-regulates the expression of MLL target genes that are implicated in cell differentiation and proliferation (HOXA9, MEIS1, p27kip1) (1). Indeed, the MLL-AF4–driven transactivation crucially depends on epigenetic modifications, e.g. H3-K79 methylation (1). However, there are also evidences that MLL-AF4 oncogenic pathways could involve some receptor tyrosine kinases (RTKs) (2). We previously demonstrated that, in HEK293 cells, both recombinant AF4 (Flag_AF4) and MLL-AF4 (Flag_MLL-AF4) interact with fibroblast growth factor receptor 2 (FGFR2) (4). Therefore, we further studied this interaction and demonstrated that it occurs in the nucleus, where both partners localize. We also confirmed that FGFR2 interacts with endogenous MLL-AF4 chimera, in RS4;11 lymphoblastic cells. Lastly, in HEK293 cells stably transfected with MLL-AF4, silencing of FGFR2 reduces HOXA9 transcript level. Our data strongly suggest that the oncogenic potential of MLL-AF4 chimera crucially depends on its nuclear interaction with FGFR2