Protein markers for insulin-producing beta cells with higher glucose sensitivity

Background and Methodology: Pancreatic beta cells show intercellular differences in their metabolic glucose sensitivity and associated activation of insulin production. To identify protein markers for these variations in functional glucose sensitivity, rat beta cell subpopulations were flow-sorted for their level of glucose-induced NAD(P) H and their proteomes were quantified by label-free data independent alternate scanning LC-MS. Beta cell-selective proteins were also identified through comparison with rat brain and liver tissue and with purified islet alpha cells, after geometrical normalization using 6 stably expressed reference proteins. Principal Findings: All tissues combined, 943 proteins were reliably quantified. In beta cells, 93 out of 467 quantifiable proteins were uniquely detected in this cell type; several other proteins presented a high molar abundance in beta cells. The proteome of the beta cell subpopulation with high metabolic and biosynthetic responsiveness to 7.5 mM glucose was characterized by (i) an on average 50% higher expression of protein biosynthesis regulators such as 40S and 60S ribosomal constituents, NADPH-dependent protein folding factors and translation elongation factors; (ii) 50% higher levels of enzymes involved in glycolysis and in the cytosolic arm of the malate/aspartate-NADH-shuttle. No differences were noticed in mitochondrial enzymes of the Krebs cycle, beta-oxidation or respiratory chain. Conclusions: Quantification of subtle variations in the proteome using alternate scanning LC-MS shows that beta cell metabolic glucose responsiveness is mostly associated with higher levels of glycolytic but not of mitochondrial enzymes

Exposure of human islets to cytokines can result in disproportionately elevated proinsulin release

Infiltration of immunocytes into pancreatic islets precedes loss of β cells in type 1 diabetes. It is conceivable that local release of cytokines affects the function of β cells before their apoptosis. This study examines whether the elevated proinsulin levels that have been described in prediabetes can result from exposure of β cells to cytokines. Human β-cell preparations were cultured for 48 or 72 hours with or without IL-1β, TNF-α, or IFN-γ, alone or in combination. None of these conditions were cytotoxic, nor did they reduce insulin biosynthetic activity. Single cytokines did not alter medium or cellular content in insulin or proinsulin. Cytokine combinations, in particular IL-1β plus IFN-γ, disproportionately elevated medium proinsulin levels. This effect expresses an altered functional state of the β cells characterized by preserved proinsulin synthesis, a slower hormone conversion, and an increased ratio of cellular proinsulin over insulin content. The delay in proinsulin conversion can be attributed to lower expression of PC1 and PC2 convertases. It is concluded that disproportionately elevated proinsulin levels in pre–type 1 diabetic patients might result from exposure of their β cells to cytokines released from infiltrating immunocytes. This hormonal alteration expresses an altered functional state of the β cells that can occur independently of β-cell death