Dietary fatty acids modify insulin secretion of rat pancreatic islet cells in vitro

Mechanisms of impaired pancreatic β‑cell function in high‑fat diet‑induced obese mice: The role of endoplasmic reticulum stress

The aim of the study was to examine whether there was excessive endoplasmic reticulum stress (ERs) in the islets of high-fat diet (HFD)-induced obese mice, as well as the effects of ERs on β-cell function. Male C57BL/6J mice were fed a HFD for 16 weeks. Pancreatic β-cell function was evaluated using intraperitoneal glucose tolerance and insulin release tests, and via electron microscopy. The expression of activating transcription factor 6 (ATF6) and phosphorylated (p)-eukaryotic initiation factor 2α (eIF2α) were detected via immunofluorescence staining to determine whether exposure to a HFD induced ERs in pancreatic islets. In vitro, ERs was induced by palmitate (PA) in INS-1 cells, and the protein expression of ATF6, and mRNA expression of ATF6 and insulin were examined via western blot and quantitative PCR (qPCR) analyses, respectively. The nuclear localization of ATF6 was examined by immunofluorescence. Finally, small interfering (si)RNA was used to downregulate ATF6 expression in INS-1 cells to further determine whether ATF6 mediated the ERs-induced impairment of insulin gene transcription. After 16 weeks of induction, the obese mice showed impaired glucose tolerance, insulin resistance and hyperinsulinemia. Immunohistochemistry staining showed increased p-eIF2α and ATF6 expression in pancreatic islets in the obesity group compared with the normal group. Electron microscopy indicated that the microstructures and secretory functions of β-cells were impaired. After 24 h of incubation, ATF mRNA and protein expression in the PA group was significantly higher compared with the control group. However, the insulin mRNA expression in the PA group was significantly decreased. Furthermore, qPCR showed that the insulin mRNA expression was significantly increased 24 h after PA treatment in cells transfected with ATF6-siRNA compared with the negative control group. The present suggested shows that ERs-induced activation of ATF6 may play an important role in the development of β-cell dysfunction in obese mice.

Soybean oil treatment impairs glucose-stimulated insulin secretion and changes fatty acid composition of normal and diabetic islets

We investigated the effect of sub-chronic soybean oil (SO) treatment on the insulin secretion and fatty acid composition of islets of Langerhans obtained from Goto-Kakizaki (GK), a model of type 2 diabetes, and normal Wistar rats. We observed that soybean-treated Wistar rats present insulin resistance and defective islet insulin secretion when compared with untreated Wistar rats. The decrease in insulin secretion occurred at all concentrations of glucose and arginine tested. Furthermore we observed that soybean-treated normal islets present a significant decrease in two saturated fatty acids, myristic and heneicosanoic acids, and one monounsaturated eicosenoic acid, and the appearance of the monounsaturated erucic acid. Concerning diabetic animals, we observed that soybean-treated diabetic rats, when compared with untreated GK rats, present an increase in plasma non-fasting free fatty acids, an exacerbation of islet insulin secretion impairment in all conditions tested and a significant decrease in the monounsaturated palmitoleic acid. Altogether our results show that SO treatment results in a decrease of insulin secretion and alterations on fatty acid composition in normal and diabetic islets. Furthermore, the impairment of insulin secretion, islet erucic acid and fasting plasma insulin levels are similar in treated normal and untreated diabetic rats, suggesting that SO could have a deleterious effect on beta-cell function and insulin sensitivity.

Dietary fatty acids and insulin secretion: a population-based study

OBJECTIVE

Few epidemiological studies have examined the relationship of dietary fatty acids, especially MUFA, with the interrelation between insulin secretion and insulin resistance. We assessed the relation of dietary fatty acids with insulin secretion in a free-living population.

DESIGN AND SETTING

This cross-sectional, population-based study was undertaken in Pizarra, a small town in Spain.

SUBJECTS AND METHODS

Anthropometrical data were collected for 1226 persons selected randomly from the municipal census, 538 of whom (randomly chosen) were given a prospective, quantitative, 7-day nutritional questionnaire. The fatty acid composition of the serum phospholipids was used as a biological marker of the type of fat consumed. Beta-cell function (betaCFI) and insulin-resistance index (IRI) were estimated by the Homeostasis Model Assessment.

RESULTS

To determine which factors influence the variability of the betaCFI, we analyzed the variance of the betaCFI according to sex, the presence of carbohydrate metabolism disorders and the different components of the diet, adjusting the models for age, body mass index (BMI) and IRI. The dietary MUFA and polyunsaturated fatty acids (PUFA) contributed to the variability of the betaCFI, whereas only the proportion of serum phospholipid MUFA, but neither the saturated fatty acids nor the PUFA accounted for part of the variability of the betaCFI in a multiple regression analysis.

CONCLUSION

The results of this population-based study corroborate the results of other clinical and experimental studies suggesting a favorable relationship of MUFA with beta-cell insulin secretion.

SPONSORSHIP

Fondo de Investigación Sanitaria, Junta de Andalucía and the Asociación Maimónides.

Fatty acid composition of erythrocyte phospholipids is related to insulin levels, secretion and resistance in obese type 2 diabetics on Metformin

BACKGROUND

Relationships between fatty acids in erythrocyte phospholipids and insulin parameters have been described in healthy and overweight individuals, but not in obese diabetics. We assessed whether erythrocyte phospholipid fatty acids are related to insulin parameters in obese type 2 diabetics on Metformin.

METHODS

In 23 diabetics, the fractions of the different fatty acids in erythrocyte phospholipids were correlated with insulin levels, secretion, sensitivity, resistance and insulinemic response following a standardised breakfast.

RESULTS

Fasting insulin levels and insulin resistance correlated positively with the fraction of alpha-linolenic and dihomo-gamma-linolenic acid and with the ratios of stearic to palmitic and dihomo-gamma-linolenic to linoleic acid and negatively with the fraction of palmitic acid in erythrocyte phospholipids. Insulin secretion correlated negatively with the fraction of palmitic acid. For this parameter, a positive correlation was also found with the sum of uneven fatty acids. Insulinemic response following a meal was negatively associated with the fraction of oleic acid in erythrocyte phospholipids. Insulin sensitivity did not correlate with erythrocyte fatty acids.

CONCLUSIONS

The relationships found differ from those described in healthy and overweight individuals and may be characteristic for type 2 diabetics. They concur with the recommendations that saturated fat intake should be reduced and monounsaturated increased.