PKCdelta blues for the beta-cell

Metabolic implications of pancreatic fat accumulation

Fat accumulation outside subcutaneous adipose tissue often has unfavourable effects on systemic metabolism. In addition to non-alcoholic fatty liver disease, which has received considerable attention, pancreatic fat has become an important area of research throughout the past 10 years. While a number of diagnostic approaches are available to quantify pancreatic fat, multi-echo Dixon MRI is currently the most developed method. Initial studies have shown associations between pancreatic fat and the metabolic syndrome, impaired glucose metabolism and type 2 diabetes mellitus. Pancreatic fat is linked to reduced insulin secretion, at least under specific circumstances such as prediabetes, low BMI and increased genetic risk of type 2 diabetes mellitus. This Review summarizes the possible causes and metabolic consequences of pancreatic fat accumulation. In addition, potential therapeutic approaches for addressing pancreatic fat accumulation are discussed.

Protein kinase C-δ signaling regulates glucagon secretion from pancreatic islets

Accumulating evidence supports the "glucagonocentric hypothesis", in which antecedent α-cell failure and inhibition of glucagon secretion are responsible for diabetes progression. Protein kinase C (PKC) is involved in glucagon secretion from α-cells, although which PKC isozyme is involved and the mechanism underlying this PKC-regulated glucagon secretion remains unknown. Here, the involvement of PKCδ in the onset and progression of diabetes was elucidated. Immunofluorescence studies revealed that PKCδ was expressed and activated in α-cells of STZ-induced diabetic model mice. Phorbol 12-myristate 13-acetate (PMA) stimulation significantly augmented glucagon secretion from isolated islets. Pre-treatment with quercetin and rottlerin, PKCδ signaling inhibitors, significantly suppressed the PMA-induced elevation of glucagon secretion. While Go6976, a Ca²⁺-dependent PKC selective inhibitor did not suppress glucagon secretion. Quercetin suppressed PMA-induced phosphorylation of Tyr³¹¹ of PKCδ in isolated islets. However, quercetin itself had no effect on either glucagon secretion or glucagon mRNA expression. Our data suggest that PKCδ signaling inhibitors suppressed glucagon secretion. Elucidation of detailed signaling pathways causing PKCδ activation in the onset and progression of diabetes followed by the augmentation of glucagon secretion could lead to the identification of novel therapeutic target molecules and the development of novel therapeutic drugs for diabetes. J. Med. Invest. 64: 122-128, February, 2017.

PKCδ knockdown inhibits free fatty acid induction of endothelial cell apoptosis

The mechanisms whereby free fatty acids induce endothelial cell apoptosis are not yet understood. The present study aimed to investigate the role of PKCδ in free fatty acid-induced endothelial cell apoptosis. In addition, we looked for evidence of apoptosis-related interactions between PKCδ and Fas signal pathway. Human umbilical vein endothelial cells were treated with various concentrations of free fatty acids and transiently transfected with PKCδ siRNA or Fas siRNA to inhibit PKCδ or Fas expression. Cell proliferation was determined through colorimetric assays, and apoptosis was quantified using flow cytometry. Protein expression was determined from cell lysates using Western blots with antibodies against p-PKCδTyr512, PKCδ, and Fas. Statistical analyses were performed. Free fatty acids had multiple effects on human umbilical vein endothelial cells, including concentration-dependent inhibition of cell proliferation, induction of apoptosis, increased Fas expression, and increased PKCδ expression and phosphorylation. Inhibition of PKCδ mRNA expression by PKCδ siRNA led to a reduction in both free fatty acid-induced apoptosis and Fas expression. However, Fas siRNA treatment inhibited Fas, but not PKCδ, expression in human umbilical vein endothelial cells. The free fatty acid-induced apoptosis in endothelial cells are possibly mediated by PKCδ and may involve upregulation of its downstream Fas.

A view on sphingolipids and disease

Sphingolipid and glycosphingolipid levels and expression of sphingolipid metabolizing enzymes are altered in a variety of diseases or in response to drug treatment. Inherited defects of enzymes and other proteins required for the lysosomal degradation of these lipids lead to human sphingolipidoses. Also genetic defects that affect sphingolipid biosynthesis are known. Although the molecular details are often far from clear, (glyco)sphingolipids have been implicated to play a role in atherosclerosis, insulin resistance, cancer, and infections by pathogens. More general aspects of selected diseases are discussed.

Glucolipotoxicity alters lipid partitioning and causes mitochondrial dysfunction, cholesterol, and ceramide deposition and reactive oxygen species production in INS832/13 ss-cells

Elevated glucose and saturated fatty acids synergize in inducing apoptosis in INS832/13 cells and in human islet cells. In order to gain insight into the molecular mechanism(s) of glucolipotoxicity (Gltox), gene profiling and metabolic analyses were performed in INS832/13 cells cultured at 5 or 20 mm glucose in the absence or presence of palmitate. Expression changes were observed for transcripts involved in mitochondrial, lipid, and glucose metabolism. At 24 h after Gltox, increased expression of lipid partitioning genes suggested a promotion of fatty acid esterification and reduced lipid oxidation/detoxification, whereas changes in the expression of energy metabolism genes suggested mitochondrial dysfunction. These changes were associated with decreased glucose-induced insulin secretion, total insulin content, ATP levels, AMP-kinase activity, mitochondrial membrane potential and fat oxidation, unchanged de novo fatty acid synthesis, and increased reactive oxygen species, cholesterol, ceramide, and triglyceride levels. However, the synergy between elevated glucose and palmitate to cause ss-cell toxicity in term of apoptosis and reduced glucose-induced insulin secretion only correlated with triglyceride and ceramide depositions. Overexpression of endoplasmic reticulum glycerol-3-phosphate acyl transferase to enhance lipid esterification amplified Gltox at intermediate glucose (11 mm), whereas reducing acetyl-coenzyme A carboxylase 1 expression by small interfering RNA to shift lipid partitioning to fat oxidation reduced Gltox. The results suggest that Gltox entails alterations in lipid partitioning, sterol and ceramide accumulation, mitochondrial dysfunction, and reactive oxygen species production, all contributing to altering ss-cell function. The data also suggest that the early promotion of lipid esterification processes is instrumental in the Gltox process.