Citations
Citations
Emerging evidence suggests that sphingolipids may be involved in type 2 diabetes. However, the exact signaling defect through which disordered sphingolipid metabolism induces β-cell dysfunction remains unknown. The current study demonstrated that sphingosine-1-phosphate (S1P), the product of sphingosine kinase (SphK), is an essential factor for maintaining β-cell function and survival via regulation of mitochondrial action, as mediated by prohibitin (PHB).
We examined β-cell function and viability, as measured by mitochondrial function, in mouse insulinoma 6 (MIN6) cells in response to manipulation of cellular S1P and PHB levels.
Lack of S1P induced by sphingosine kinase inhibitor (SphKi) treatment caused β-cell dysfunction and apoptosis, with repression of mitochondrial function shown by decreases in cellular adenosine triphosphate content, the oxygen consumption rate, the expression of oxidative phosphorylation complexes, the mitochondrial membrane potential, and the expression of key regulators of mitochondrial dynamics (mitochondrial dynamin-like GTPase [OPA1] and mitofusin 1 [MFN1]). Supplementation of S1P led to the recovery of mitochondrial function and greatly improved β-cell function and viability. Knockdown of SphK2 using small interfering RNA induced mitochondrial dysfunction, decreased glucose-stimulated insulin secretion (GSIS), and reduced the expression of PHB, an essential regulator of mitochondrial metabolism. PHB deficiency significantly reduced GSIS and induced mitochondrial dysfunction, and co-treatment with S1P did not reverse these trends.
Altogether, these data suggest that S1P is an essential factor in the maintenance of β-cell function and survival through its regulation of mitochondrial action and PHB expression.
Citations
The nuclear receptor peroxisome proliferator-activator gamma (PPARγ) is a useful therapeutic target for obesity and diabetes, but its role in protecting β-cell function and viability is unclear.
To identify the potential functions of PPARγ in β-cells, we treated mouse insulinoma 6 (MIN6) cells with the PPARγ agonist pioglitazone in conditions of lipotoxicity, endoplasmic reticulum (ER) stress, and inflammation.
Palmitate-treated cells incubated with pioglitazone exhibited significant improvements in glucose-stimulated insulin secretion and the repression of apoptosis, as shown by decreased caspase-3 cleavage and poly (adenosine diphosphate [ADP]-ribose) polymerase activity. Pioglitazone also reversed the palmitate-induced expression of inflammatory cytokines (tumor necrosis factor α, interleukin 6 [IL-6], and IL-1β) and ER stress markers (phosphor-eukaryotic translation initiation factor 2α, glucose-regulated protein 78 [GRP78], cleaved-activating transcription factor 6 [ATF6], and C/EBP homologous protein [CHOP]), and pioglitazone significantly attenuated inflammation and ER stress in lipopolysaccharide- or tunicamycin-treated MIN6 cells. The protective effect of pioglitazone was also tested in pancreatic islets from high-fat-fed KK-Ay mice administered 0.02% (wt/wt) pioglitazone or vehicle for 6 weeks. Pioglitazone remarkably reduced the expression of ATF6α, GRP78, and monocyte chemoattractant protein-1, prevented α-cell infiltration into the pancreatic islets, and upregulated glucose transporter 2 (Glut2) expression in β-cells. Moreover, the preservation of β-cells by pioglitazone was accompanied by a significant reduction of blood glucose levels.
Altogether, these results support the proposal that PPARγ agonists not only suppress insulin resistance, but also prevent β-cell impairment via protection against ER stress and inflammation. The activation of PPARγ might be a new therapeutic approach for improving β-cell survival and insulin secretion in patients with diabetes mellitus
Citations
Selenoprotein P (SEPP1) and fetuin-A, both circulating liver-derived glycoproteins, are novel biomarkers for insulin resistance and nonalcoholic fatty liver disease. However, the effect of exendin-4 (Ex-4), a glucagon-like peptide-1 receptor agonist, on the expression of hepatokines, SEPP1, and fetuin-A, is unknown.
The human hepatoma cell line HepG2 was treated with palmitic acid (PA; 0.4 mM) and tunicamycin (tuni; 2ug/ml) with or without exendin-4 (100 nM) for 24 hours. The change in expression of PA-induced SEPP1, fetuin-A, and endoplasmic reticulum (ER) stress markers by exendin-4 treatment were evaluated using quantitative real-time reverse transcription polymerase chain reaction and Western blotting. Transfection of cells with AMP-activated protein kinase (AMPK) small interfering RNA (siRNA) was performed to establish the effect of exendin-4-mediated AMPK in the regulation of SEPP1 and fetuin-A expression.
Exendin-4 reduced the expression of SEPP1, fetuin-A, and ER stress markers including PKR-like ER kinase, inositol-requiring kinase 1α, activating transcription factor 6, and C/EBP homologous protein in HepG2 cells. Exendin-4 also reduced the expression of SEPP1 and fetuin-A in cells treated with tunicamycin, an ER stress inducer. In cells treated with the AMPK activator 5-aminoidazole-4-carboxamide ribonucleotide (AICAR), the expression of hepatic SEPP1 and fetuin-A were negatively related by AMPK, which is the target of exendin-4. In addition, exendin-4 treatment did not decrease SEPP1 and fetuin-A expression in cells transfected with AMPK siRNA.
These data suggest that exendin-4 can attenuate the expression of hepatic SEPP1 and fetuin-A via improvement of PA-induced ER stress by AMPK.
Citations
Tumor necrosis factor (TNF)-α and AMP-activated protein kinase (AMPK) are known to stimulate and repress lipolysis in adipocytes, respectively; however, the mechanisms regulating these processes have not been completely elucidated.
The key factors and mechanism of action of TNF-α and AMPK in lipolysis were investigated by evaluating perilipin expression and activity of protein kinase RNA-like endoplasmic reticulum kinase (PERK)/eukaryotic initiation factor 2 α (eIF2α) by Western blot and an immunofluorescence assay in 24-hour TNF-α-treated 3T3-L1 adipocytes with artificial manipulation of AMPK activation.
Enhancement of AMPK activity by the addition of activator minoimidazole carboxamide ribonucleotide (AICAR) suppressed TNF-α-induced lipolysis, whereas the addition of compound C, an inhibitor of AMPK phosphorylation, enhanced lipolysis. Perilipin, a lipid droplet-associated protein, was decreased by TNF-α and recovered following treatment with AICAR, showing a correlation with the antilipolytic effect of AICAR. Significant activation of PERK/eIF2α, a component of the unfolded protein response signaling pathway, was observed in TNF-α or vesicle-treated 3T3-L1 adipocytes. The antilipolytic effect and recovery of perilipin expression by AICAR in TNF-α-treated 3T3-L1 adipocytes were significantly diminished by treatment with 2-aminopurine, a specific inhibitor of eIF2α.
These data indicated that AICAR-induced AMPK activation attenuates TNF-α-induced lipolysis via preservation of perilipin in 3T3-L1 adipocytes. In addition, PERK/eIF2α activity is a novel mechanism of the anti-lipolytic effect of AICAR.
Citations