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20S proteasomes are very heterogeneous protein complexes involved in many cellular processes. In the present study, we combined an MRM-based assay with the production and purification of entire SILAC labelled proteasome to monitor absolute quantities of the different 20S proteasome subtypes in various human cells and tissues. This method applied to adipocyte-derived stem cells (ADSCs) amplified under various conditions highlights an increased expression of immunoproteasome when this type of cell is primed with IFNγ or amplified in a 20% O2 environment., Graphical Abstract Highlights Design of an MRM assay to determine the absolute quantity and stoichiometry of ubiquitous and tissue-specific human 20S proteasome subtypes. Use of purified isotopically labelled 20S proteasome as internal standard for accurate quantification. Variation in the expression of immunoproteasome in adipocyte-derived stem cells (ADSCs) grown under different O2 levels might be causal for change in cells differentiation capacity. The status of 20S proteasome during ADSCs expansion might constitute an additional relevant quality control parameter to contribute to predict, among other quality markers, their therapeutic capacity., The proteasome controls a multitude of cellular processes through protein degradation and has been identified as a therapeutic target in oncology. However, our understanding of its function and the development of specific modulators are hampered by the lack of a straightforward method to determine the overall proteasome status in biological samples. Here, we present a method to determine the absolute quantity and stoichiometry of ubiquitous and tissue-specific human 20S proteasome subtypes based on a robust, absolute SILAC-based multiplexed LC-Selected Reaction Monitoring (SRM) quantitative mass spectrometry assay with high precision, accuracy, and sensitivity. The method was initially optimized and validated by comparison with a reference ELISA assay and by analyzing the dynamics of catalytic subunits in HeLa cells following IFNγ-treatment and in range of human tissues. It was then successfully applied to reveal IFNγ- and O2-dependent variations of proteasome status during primary culture of Adipose-derived-mesenchymal Stromal/Stem Cells (ADSCs). The results show the critical importance of controlling the culture conditions during cell expansion for future therapeutic use in humans. We hypothesize that a shift from the standard proteasome to the immunoproteasome could serve as a predictor of immunosuppressive and differentiation capacities of ADSCs and, consequently, that quality control should include proteasomal quantification in addition to examining other essential cell parameters. The method presented also provides a new powerful tool to conduct more individualized protocols in cancer or inflammatory diseases where selective inhibition of the immunoproteasome has been shown to reduce side effects.

International audience; BACKGROUND & AIMS: Glucose is absorbed into intestine cells via the sodium glucose transporter 1 (SGLT-1) and glucose transporter 2 (GLUT2); various peptides and hormones control this process. Apelin is a peptide that regulates glucose homeostasis and is produced by proximal digestive cells; we studied whether glucose modulates apelin secretion by enterocytes and the effects of apelin on intestinal glucose absorption. METHODS: We characterized glucose-related luminal apelin secretion in vivo and ex vivo by mass spectroscopy and immunologic techniques. The effects of apelin on (14)C-labeled glucose transport were determined in jejunal loops and in mice following apelin gavage. We determined levels of GLUT2 and SGLT-1 proteins and phosphorylation of AMPKα2 by immunoblotting. The net effect of apelin on intestinal glucose transepithelial transport was determined in mice. RESULTS: Glucose stimulated luminal secretion of the pyroglutaminated apelin-13 isoform ([Pyr-1]-apelin-13) in the small intestine of mice. Apelin increased specific glucose flux through the gastric epithelial barrier in jejunal loops and in vivo following oral glucose administration. Conversely, pharmacologic apelin blockade in the intestine reduced the increased glycemia that occurs following oral glucose administration. Apelin activity was associated with phosphorylation of AMPKα2 and a rapid increase of the GLUT2/SGLT-1 protein ratio in the brush border membrane. CONCLUSIONS: Glucose amplifies its own transport from the intestinal lumen to the bloodstream by increasing luminal apelin secretion. In the lumen, active apelin regulates carbohydrate flux through enterocytes by promoting AMPKα2 phosphorylation and modifying the ratio of SGLT-1:GLUT2. The glucose-apelin cycle might be pharmacologically handled to regulate glucose absorption and assess better control of glucose homeostasis.

In eukaryotic cells, intracellular protein breakdown is mainly performed by the ubiquitin–proteasome system. Proteasomes are supramolecular protein complexes formed by the association of multiple sub-complexes and interacting proteins. Therefore, they exhibit a very high heterogeneity whose function is still not well understood. Here, using a newly developed method based on the combination of affinity purification and protein correlation profiling associated with high-resolution mass spectrometry, we comprehensively characterized proteasome heterogeneity and identified previously unknown preferential associations within proteasome sub-complexes. In particular, we showed for the first time that the two main proteasome subtypes, standard proteasome and immunoproteasome, interact with a different subset of important regulators. This trend was observed in very diverse human cell types and was confirmed by changing the relative proportions of both 20S proteasome forms using interferon-γ. The new method developed here constitutes an innovative and powerful strategy that could be broadly applied for unraveling the dynamic and heterogeneous nature of other biologically relevant supramolecular protein complexes.

The proteasome is the main proteolytic system involved in intracellular proteins homeostasis in eukaryotes. Although the structure of proteasome complexes has been well characterized, the distribution of its activators and associated proteins are less studied. Here, we determine the composition and the stoichiometry of proteasome complexes and their associated proteins in a wide range of human cell lines using a one-step affinity purification method and a label-free quantitative proteomic approach. We show that proteasome complexes are highly dynamic protein assemblies, the activity of which being regulated at different levels by variations in the stoichiometry of bound regulators, in the composition of catalytic subunits and associated proteins, and in the rate of the 20S catalytic core complex assembly.