Insulin and glucagon secretion in essential fatty acid deficient rats

Modulation of palmitate-induced endoplasmic reticulum stress and apoptosis in pancreatic β-cells by stearoyl-CoA desaturase and Elovl6

Induction of endoplasmic reticulum (ER) stress and apoptosis by elevated exogenous saturated fatty acids (FAs) plays a role in the pathogenesis of β-cell dysfunction and loss of islet mass in type 2 diabetes. Regulation of monounsaturated FA (MUFA) synthesis through FA desaturases and elongases may alter the susceptibility of β-cells to saturated FA-induced ER stress and apoptosis. Herein, stearoyl-CoA desaturase (SCD)1 and SCD2 mRNA expression were shown to be induced in islets from prediabetic hyperinsulinemic Zucker diabetic fatty (ZDF) rats, whereas SCD1, SCD2, and fatty acid elongase 6 (Elovl6) mRNA levels were markedly reduced in diabetic ZDF rat islets. Knockdown of SCD in INS-1 β-cells decreased desaturation of palmitate to MUFA, lowered FA partitioning into complex neutral lipids, and increased palmitate-induced ER stress and apoptosis. Overexpression of SCD2 increased desaturation of palmitate to MUFA and attenuated palmitate-induced ER stress and apoptosis. Knockdown of Elovl6 limited palmitate elongation to stearate, increasing palmitoleate production and attenuating palmitate-induced ER stress and apoptosis, whereas overexpression of Elovl6 increased palmitate elongation to stearate and palmitate-induced ER stress and apoptosis. Overall, these data support the hypothesis that enhanced MUFA synthesis via upregulation of SCD2 activity can protect β-cells from elevated saturated FAs, as occurs in prediabetic states. Overt type 2 diabetes is associated with diminished islet expression of SCD and Elovl6, and this can disrupt desaturation of saturated FAs to MUFAs, rendering β-cells more susceptible to saturated FA-induced ER stress and apoptosis.

Mechanisms of lipid malabsorption in Cystic Fibrosis: the impact of essential fatty acids deficiency

Transport mechanisms, whereby alimentary lipids are digested and packaged into small emulsion particles that enter intestinal cells to be translocated to the plasma in the form of chylomicrons, are impaired in cystic fibrosis. The purpose of this paper is to focus on defects that are related to intraluminal and intracellular events in this life-limiting genetic disorder. Specific evidence is presented to highlight the relationship between fat malabsorption and essential fatty acid deficiency commonly found in patients with cystic fibrosis that are often related to the genotype. Given the interdependency of pulmonary disease, pancreatic insufficiency and nutritional status, greater attention should be paid to the optimal correction of fat malabsorption and essential fatty acid deficiency in order to improve the quality of life and extend the life span of patients with cystic fibrosis.

Pancreatic function in the essential fatty acid deficient rat

The influence of essential fatty acid (EFA) deficiency on pancreatic endocrine and exocrine function was studied in 120-day-old rats. The plasma insulin response was determined after in vivo administration of glucose and arginine. The plasma glucagon response was assessed after infusion of arginine. Islet peptides were examined by immunocytochemistry. The exocrine function of pancreas was studied by amylase secretion in isolated pancreatic acinar cells after stimulation with the cholinergic agonist carbacholine chloride. The EFA-deficient (EFAD) rats showed higher basal plasma insulin concentrations and lower basal glucose levels than control rats (P less than .01 and P less than .01, respectively). The plasma insulin response to glucose was potentiated in the EFAD rats (P less than .001). Both insulin and glucagon responses to arginine were normal. The isolated pancreatic acinar cells showed a low basal amylase secretion, but a normal response to carbacholine chloride. There were no overt morphological changes seen in the pancreas and the immunocytochemical staining pattern of insulin, glucagon, somatostatin, and pancreatic polypeptide cells did not differ from controls. The results of the study show that the secretory function of the endocrine and exocrine pancreas is operational in EFA deficiency. The EFA deficiency was accompanied by a basal hyperinsulinemia and hypoglycemia and an exaggerated insulin response to glucose, the pathophysiology of which has to be further studied.

Relation between essential fatty acid metabolism and gastrointestinal symptoms in cystic fibrosis

Studies in our laboratory have supported the hypothesis, that the basic defect in cystic fibrosis increases the metabolism of essential fatty acids and thereby gradually gives rise to essential fatty acid deficiency, which is a well documented finding in most cases with this disease. Both the increased metabolism--giving high liberation of arachidonic acid and its metabolic products, i.e. different eicosanoids--and the subsequent essential fatty acid deficiency will cause gastrointestinal symptoms and the sequence of this development will mirror the natural history of the disease. Clinical data and results from animal research are discussed in relation to gastrointestinal symptoms and signs of cystic fibrosis.

Relation between defective regulation of arachidonic acid release and symptoms in cystic fibrosis

A hypothesis is formed about the basic defect in CF based on the findings of 1) a defective inhibition of AA release by dexamethasone stimulation of lymphocytes from patients with CF, probably due to a defect in or absence of lipocortin and 2) the essential fatty acid deficiency (EFAD) in CF reported by different authors for 25 years. Both in lymphocytes and fibroblasts from patients with CF increased AA release has been demonstrated. AA, substrate for the eicosanoid system (prostaglandins, leukotrienes, hydroxyeicosatetraenoic acids, thromboxanes), has been shown to increase mucus release, influence chloride transport and the stimulus-secretion coupling (Ca++ balance), all factors suggested as basic defects in CF. The increased release of AA might explain the development of EFAD, since AA is synthesized from linoleic acid. In animals EFAD gives rise to symptoms similar to those in CF, e.g. defect Na transport, liver steatosis, increased caloric need, disturbed insulin release, increased bacterial colonization of airways, and decreased immune response. Regular supplementation with fat emulsion (Intralipid) to CF patients normalizes the renal Na transport and prevents liver steatosis. We suggest that the basic defect in CF is a defect in or absence of lipocortin, causing an increased release of AA, which regulates synthesis in the eicosanoid system. An increase of the products in this system gives rise to the basic signs and symptoms of CF. The resulting EFAD is dependent on the rate of AA turnover and could explain most of the clinical symptoms and the progression of the disease.

Glucose-induced phospholipid hydrolysis in isolated pancreatic islets: quantitative effects on the phospholipid content of arachidonate and other fatty acids

Our recent findings indicate that glucose-induced insulin secretion from isolated pancreatic islets is temporally associated with accumulation of substantial amounts of free arachidonic acid and that arachidonate may serve as a second messenger for intracellular calcium mobilization in islets. In an effort to determine the source of this released arachidonate, the endogenous fatty acid composition of phospholipids from islets has been determined by thin-layer chromatographic separation of the phospholipids, methanolysis to the fatty acid methyl esters, and quantitative gas chromatographic analyses. The relative abundance of phospholipids in islets as judged by their fatty acid content was phosphatidylcholine (PC), 0.63; phosphatidylethanolamine (PE), 0.23; phosphatidylinositol (PI), 0.067; phosphatidylserine (PS), 0.049. Arachidonate constituted 17% of the total islet fatty acid content, and PC contained 43% of total islet arachidonate. Islets incubated with [3H]arachidonate in the presence of 28 mM D-glucose incorporated radiolabel into PC with a considerably higher specific activity than that of PE, PS or PI. The total fatty acid content of PC from islets incubated with 28 mM glucose for 30 min was significantly lower than that of islets incubated with 3 mM glucose, and smaller effects were observed with PE, PS and PI. The molar decrement in PC arachidonate was 3.2 pmol/islet under these conditions, which is sufficient to account for the previously observed accumulation of free arachidonate (2 pmol/islet). A sensitive method involving negative ion-chemical ionization-mass spectrometric analyses of the pentafluorobenzyl esters of fatty acids derived from trace amounts of lysophosphatidylcholine (lyso-PC) was developed, and glucose-stimulation was found to reduce islet lyso-PC content by about 10-fold. These findings indicate that the insulin secretagogue D-glucose induces phospholipid hydrolysis in islets and suggest that PC may be the major source of free arachidonate which accumulates in glucose-stimulated islets.