The human ATP-binding cassette (ABC) transporter, ABCG2 (BCRP ⁄ MXR ⁄ ABCP), is a plasma membrane protein containing intramolecular and intermolecular disulfide bonds and an N-linked glycan at Asn596. We have recently reported that the intramolecular disulfide bond is a critical checkpoint for determining the degradation fates of ABCG2. In the present study, we aimed to analyze quantitatively the impact of the N-linked glycan on the protein stability of ABCG2. For this purpose, we incorporated one single copy of ABCG2 cDNA into a designated site of genomic DNA in Flp-In-293 cells to stably express ABCG2 or its variant proteins. When ABCG2 wild type-expressing cells were incubated with various N-linked glycosylation inhibitors, tunicamycin profoundly suppressed the protein expression level of ABCG2 and, accordingly, reduced the ABCG2-mediated cellular resistance to the cancer chemotherapeutic SN-38. When Asn596 was converted to Gln596, the resulting variant protein was not glycosylated, and its protein level was about one-third of the wild type level in Flp-In-293 cells. Treatment with MG132, a proteasome inhibitor, increased the level of the variant protein. Immunoblotting with anti-ubiquitin IgG1k after immunoprecipitation of ABCG2 revealed that the N596Q protein was ubiquitinated at levels that were significantly enhanced by treatment with MG132. Immunofluorescence microscopy demonstrated that treatment with MG132 increased the level of ABCG2 N596Q protein both in intracellular compartments and in the plasma membrane. In conclusion, we propose that the N-linked glycan at Asn596 is important for stabilizing de novo-synthesized ABCG2 and that disruption of this linkage results in protein destabilization and enhanced ubiquitin-mediated proteasomal degradation.

Aims: To investigate whether preoperative enteral diets ­enriched in eicosapentaenoic acid (EPA) supplements could reduce the incidence of hypercytokinemia after pancreatoduodenectomy (PD) in a double-blinded randomized ­controlled trial. Methods: Patients with resectable periampullary cancer were randomized into either the control group or the treatment group. Patients in the treatment group received oral supplementation (600 kcal/day) containing EPA for 7 days before surgery. Patients in the control group received isocaloric isonitrogenous standard nutrition (600 kcal/day) without EPA for 7 days before surgery. The primary endpoint was postoperative serum concentrations of interleukin-6 (IL-6). The secondary endpoints were the postoperative nutritional status and the incidence of postoperative infectious complications. Results: Twenty-four patients were enrolled in the present study. After exclusion, 20 patients (control group, n = 9; treatment group, n = 11) were analyzed. There were no significant differences in the curves for the serum concentration of IL-6 (p = 0.68) or the incidence of infectious complications between the 2 groups (control group: 78%, treatment group: 55%, p = 0.37). Conclusions: The results of a double-blinded randomized controlled trial indicated that preoperative immunonutrition had no marked impact on the rates of postoperative hypercytokinemia or infectious complications after PD.

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Exosomes are small vesicles secreted from cells that transport their embedded molecules through bidirectional exocytosis- and endocytosis-like pathways. Expression patterns of exosomal molecules such as proteins and RNAs can be indicative of cell type since their signature is thought to be unique among cells. Using human primary (AZ-521) and metastatic (AZ-P7a) duodenal cancer cell lines, we conducted a comparative exosomal proteome analysis to identify proteins with metastatic marker potential. As determined by LC-MS/MS and Western blot analyses, polyadenylate-binding protein 1 (PABP1) was found to be predominantly abundant in AZ-P7a exosomes. The amount of exosomal PABP1 in AZ-P7a cells increased by treating the cells with inhibitors for the classical ER/Golgi secretory pathway (brefeldin A and monensin) and the ubiquitin-proteasome pathway (MG-132 and PYR-41). Treatment of AZ-P7a cells with the neutral sphingomyelinase inhibitor GW4869, which suppresses exosome release, not only reduced the amount of exosomal PABP1 but also produced PABP1-immunoreactive products cleaved via a proteolysis-like process. Taken together, these results suggest that AZ-P7a cells do not tolerate intracellular PABP1 accumulation and are thus exported into the extracellular milieu by the exosome-mediated pathway. In addition, PABP1 has a potential use as a biomarker for metastatic duodenal cancer.

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