Activation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) by free fatty acid receptor 1 (FFAR1/GPR40) protects from palmitate-induced beta cell death, but plays no role in insulin secretion.

GPR40/FFAR1 mediates palmitate-induced stimulation of insulin secretion but its involvement in lipotoxicity is controversial. Our previous observations suggest that FFAR1/GPR40 agonists protect against lipotoxicity although the underlying mechanism remains elusive. The present study examines the role of ERK1/2 and GPR40/FFAR1 in palmitate-induced stimulation of insulin secretion and beta cell death. Insulin secretion of INS-1E cells was measured by radioimmunoassay. Protein phosphorylation was examined on Western blots. Apoptosis was assessed by TUNEL staining. Palmitate and the GPR40/FFAR1 agonist TUG-469 increased phosphorylation of ERK1/2 at low (2.8 mmol/L) and high (12 mmol/L) glucose but stimulated insulin secretion only at high glucose. The MEK1 inhibitor PD98059 significantly reduced phosphorylation of ERK1/2 but did not reverse the stimulation of secretion induced by glucose, palmitate or TUG-469. PD98059 rather augmented glucose-induced secretion. Prolonged exposure to palmitate stimulated apoptosis, an effect counteracted by TUG-469. PD98059 accentuated palmitate-induced apoptosis and reversed TUG-469-mediated inhibition of cell death. Activation of ERK1/2 by palmitate and GPR40/FFAR1 agonist correlates neither with stimulation of insulin secretion nor with induction of apoptosis. The results suggest a significant anti-apoptotic role of ERK1/2 under conditions of lipotoxicity.

Panse M ，Gerst F ，Kaiser G ，Teutsch CA ，Dölker R ，Wagner R ，Häring HU ，Ullrich S ... -  《-》

Reevaluation of fatty acid receptor 1 as a drug target for the stimulation of insulin secretion in humans.

Wagner R ，Kaiser G ，Gerst F ，Christiansen E ，Due-Hansen ME ，Grundmann M ，Machicao F ，Peter A ，Kostenis E ，Ulven T ，Fritsche A ，Häring HU ，Ullrich S ... -  《-》

Point mutation of Ffar1 abrogates fatty acid-dependent insulin secretion, but protects against HFD-induced glucose intolerance.

The fatty acid receptor 1 (FFAR1/GPR40) mediates fatty acid-dependent augmentation of glucose-induced insulin secretion (GIIS) in pancreatic β-cells. Genetically engineered Ffar1-knockout/congenic mice univocally displayed impaired fatty acid-mediated insulin secretion, but in vivo experiments delivered controversial results regarding the function of FFAR1 in glucose homeostasis and liver steatosis. This study presents a new coisogenic mouse model carrying a point mutation in Ffar1 with functional consequence. These mice reflect the situations in humans in which point mutations can lead to protein malfunction and disease development. The Munich N-ethyl-N-nitrosourea (ENU) mutagenesis-derived F1 archive containing over 16,800 sperms and corresponding DNA samples was screened for mutations in the coding region of Ffar1. Two missense mutations (R258W and T146S) in the extracellular domain of the protein were chosen and homozygote mice were generated. The functional consequence of these mutations was examined in vitro in isolated islets and in vivo in chow diet and high fat diet fed mice. Palmitate, 50 μM, and the FFAR1 agonist TUG-469, 3 μM, stimulated insulin secretion in islets of Ffar1T146S/T146S mutant mice and of wild-type littermates, while in islets of Ffar1R258W/R258W mutant mice, these stimulatory effects were abolished. Insulin content and mRNA levels of Ffar1, Glp1r, Ins2, Slc2a2, Ppara, and Ppard were not significantly different between wild-type and Ffar1R258W/R258W mouse islets. Palmitate exposure, 600 μM, significantly increased Ppara mRNA levels in wild-type but not in Ffar1R258W/R258W mouse islets. On the contrary, Slc2a2 mRNA levels were significantly reduced in both wild-type and Ffar1R258W/R258W mouse islets after palmitate treatment. HFD feeding induced glucose intolerance in wild-type mice. Ffar1R258W/R258W mutant mice remained glucose tolerant although their body weight gain, liver steatosis, insulin resistance, and plasma insulin levels were not different from those of wild-type littermates. Worth mentioning, fasting plasma insulin levels were lower in Ffar1R258W/R258W mice. A point mutation in Ffar1 abrogates the stimulatory effect of palmitate on GIIS, an effect that does not necessarily translate to HFD-induced glucose intolerance.

Sabrautzki S ，Kaiser G ，Przemeck GKH ，Gerst F ，Lorza-Gil E ，Panse M ，Sartorius T ，Hoene M ，Marschall S ，Häring HU ，Hrabě de Angelis M ，Ullrich S ... -  《Molecular Metabolism》

Free fatty acid receptor 1 (FFAR1/GPR40) signaling affects insulin secretion by enhancing mitochondrial respiration during palmitate exposure.

Fatty acids affect insulin secretion via metabolism and FFAR1-mediated signaling. Recent reports indicate that these two pathways act synergistically. Still it remains unclear how they interrelate. Taking into account the key role of mitochondria in insulin secretion, we attempted to dissect the metabolic and FFAR1-mediated effects of fatty acids on mitochondrial function. One-hour culture of MIN6 cells with palmitate significantly enhanced mitochondrial respiration. Antagonism or silencing of FFAR1 prevented the palmitate-induced rise in respiration. On the other hand, in the absence of extracellular palmitate FFAR1 agonists caused a modest increase in respiration. Using an agonist of the M3 muscarinic acetylcholine receptor and PKC inhibitor we found that in the presence of the fatty acid mitochondrial respiration is regulated via Gαq protein-coupled receptor signaling. The increase in respiration in palmitate-treated cells was largely due to increased glucose utilization and oxidation. However, glucose utilization was not dependent on FFAR1 signaling. Collectively, these results indicate that mitochondrial respiration in palmitate-treated cells is enhanced via combined action of intracellular metabolism of the fatty acid and the Gαq-coupled FFAR1 signaling. Long-term palmitate exposure reduced ATP-coupling efficiency of mitochondria and deteriorated insulin secretion. The presence of the FFAR1 antagonist during culture did not improve ATP-coupling efficiency, however, it resulted in enhanced mitochondrial respiration and improved insulin secretion after culture. Taken together, our study demonstrates that during palmitate exposure, integrated actions of fatty acid metabolism and fatty acid-induced FFAR1 signaling on mitochondrial respiration underlie the synergistic action of the two pathways on insulin secretion.

Kristinsson H ，Bergsten P ，Sargsyan E 《-》

Palmitate and insulin counteract glucose-induced thioredoxin interacting protein (TXNIP) expression in insulin secreting cells via distinct mechanisms.

Panse M ，Kluth O ，Lorza-Gil E ，Kaiser G ，Mühlbauer E ，Schürmann A ，Häring HU ，Ullrich S ，Gerst F ... -  《PLoS One》