Monounsaturated fatty acid diets improve glycemic tolerance through increased secretion of glucagon-like peptide-1.

Diets enriched in monounsaturated fatty acids (MUFA)s have been shown to benefit glycemic control. Furthermore, MUFAs specifically stimulate secretion of the antidiabetic hormone, Glucagon-like peptide-1 (GLP-1) in vitro. To determine whether the MUFA-induced benefit in glycemic tolerance in vivo is due to increased GLP-1 release, lean Zucker rats were pair-fed a synthetic diet containing 5% fat derived from either olive oil (OO; 74% MUFA) or coconut oil (CO; 87% saturated fatty acids; SFA) for 2 weeks. Food intake and body weight gain were similar for both groups over the feeding period. The OO group had improved glycemic tolerance compared with the CO group in both oral and duodenal glucose tolerance tests [area under curve (AUC) 121 +/- 61 vs. 290 +/- 24 mM.120 min, P < 0.05; and 112 +/- 28 vs. 266 +/- 65 mM.120 min, P < 0.05, respectively]. This was accompanied by increased secretion of gut glucagon-like immunoreactivity (gGLI; an index of GLP-1 levels) in the OO rats compared with the CO rats (402 +/- 96 vs. 229 +/- 33 pg/ml at t = 10 min, P < 0.05). Tissue levels of GLP-1 and plasma insulin and glucagon levels were not different between the two groups. To determine the total contribution of GLP-1 to the enhanced glycemic tolerance in OO rats, the GLP-1 receptor antagonist exendin(9-39) (Ex(9-39)) was infused 3 min before a duodenal glucose tolerance test. Ex(9-39) abolished the benefit in glycemic tolerance conferred by OO feeding (OO+Ex(9-39) vs. CO+Ex(9-39), P = NS), and resulted in a deterioration of glycemic tolerance in the OO+Ex(9-39) group when compared with the OO controls (AUC 331 +/- 21 vs. 112 +/- 28 mM.120 min, P < 0.05). To probe the mechanism by which the OO diet enhanced GLP-1 secretion, a GLP-1-secreting L cell line was incubated for 24 h with either 100 microM oleic acid (MUFA) or 100 microM palmitic acid (SFA) and subsequently challenged with GIP, a known stimulator of the L cell. Preexposure to oleic acid but not to palmitic acid significantly increased GIP-induced GLP-1 secretion when compared with controls (55 +/- 12% vs. 34 +/- 9%, P < 0.01). These results demonstrate that the benefit in glycemic tolerance obtained with MUFA diets occurs in association with increased GLP-1 secretion, through a mechanism of enhanced L cell sensitivity. These results suggest that diet therapy with MUFAs may be useful for the treatment of patients with impaired glucose tolerance and/or type 2 diabetes through increased GLP-1 secretion.

Oral delivery of glucagon-like peptide-1 in a modified polymer preparation normalizes basal glycaemia in diabetic db/db mice.

AIMS/HYPOTHESIS: The insulinotropic hormone, glucagon-like peptide-1 has been proposed for the treatment of patients with Type II (non-insulin-dependent) diabetes mellitus. As glucagon-like peptide-1 is rapidly cleaved at L-ala2 by dipeptidylpeptidase IV, D-ala2-glucagon-like peptide-1 was synthesized and shown to have dipeptidylpeptidase IV resistance in vitro and enhanced bioactivity in mice during an oral glucose challenge. The actions of D-ala2-glucagon-like peptide-1 were, however, lost within 4 h of injection, thus necessitating frequent and invasive treatment if it is to be used therapeutically. To circumvent this problem, a microsphere of D-ala2-glucagon-like peptide-1 that could be given orally was developed. METHODS: We encapsulated D-ala2-glucagon-like peptide-1 in poly(lactide-co-glycolide)-COOH with olive oil as a filler, using phase inversion. The microspheres were tested in vivo by oral gavage in mice at t = 0 h followed by repeated oral glucose tolerance tests at t = 0, 4 and 8 h. RESULTS: The D-ala2-glucagon-like peptide-1-microspheres lowered the glycaemic response to the 4 h oral glucose challenge in both normal CD1 and diabetic db/db mice, by 41 +/- 12% (p <0.001) and 27 +/- 5% (p < 0.001), respectively and by 19 +/- 11% (p < 0.05) and 28 +/- 4% (p < 0.001), respectively during the 8-h test. At 4 h after the oral gavage, basal glycaemia in the diabetic mice was reduced from 13 +/- 1 mmol/l to 10 +/- 1 mmol/l and was reduced further 8h after treatment from 12 +/- 1 mmol/l to 8 +/- 1 mmol/l (p < 0.05). Giving D-ala2-glucagon-like peptide-1 alone orally had no effect on glycaemia. CONCLUSION/INTERPRETATION: The data presented here suggest that a similar microsphere preparation could be useful in the delivery of glucagon-like peptide-1 to patients with Type II diabetes.

Endotoxin downregulates hepatic expression of P-glycoprotein and MRP2 in 2-acetylaminofluorene-treated rats.

In liver, the ATP-dependent transporters P-glycoprotein (PGP) and multidrug resistance protein-2 (MRP2) are involved in the secretion of numerous drugs and toxins in bile. Although constitutive levels of PGP and MRP-2 are decreased in rat liver after exposure to endotoxin, it is possible that induced forms of these transporters may be alternately affected. In vitro, the hepatocarcinogen, 2-acetylaminofluorene (AAF) induces expression of PGP and MRP2. Thus, we examined the influence of endotoxin on the expression of PGP and MRP2 in AAF-treated rats. Expression of PGP and MRP2 was analyzed on Westerns and by RT-PCR in livers obtained from endotoxin and control groups. In vivo, AAF treatment significantly induced PGP/mdr1 expression and imposed a significant reduction in the expression of spgp. MRP2 protein and mRNA levels were not altered by AAF administration. Endotoxin administration to both AAF-treated and non-AAF-treated rats elicited significant reductions in the protein and mRNA expression of MRP2 and PGP (P < 0.05). Our data indicate that endotoxin suppresses the overexpression of PGP and constitutive expression of MRP2 in AAF-treated rats. Furthermore, in vivo administration of AAF, which maximally induces PGP does not induce MRP2.

Inflammation and interleukin-6 mediate reductions in the hepatic expression and transcription of the mdr1a and mdr1b Genes.

In order to elucidate the mechanisms involved in the downregulation of mdr 1 gene expression reported in experimentally-induced inflammation, we examined the effects of experimentally-induced inflammation and interleukin-(IL-) 6 on transcriptional control of the mdr1 genes in rats. RNA, nuclear extracts, and nuclear protein fractions were isolated from livers harvested from saline or turpentine-treated male Sprague-Dawley rats or from IL-6 treated or nontreated (controls) cultured rat hepatocytes. mdr gene expression and regulation was examined by RT-PCR, mRNA stability studies, nuclear run-on analysis of transcription, and gel shift analysis of promoter-transcription factor interaction. As compared to controls, significantly lower levels of mdr1a and mdr1b mRNA and significantly decreased mdr1a and mdr1b transcription rates were observed in livers isolated from the turpentine-treated rats. In vitro treatments of cultured hepatocytes with IL-6 also suppressed mdr1a and mdr1b mRNA expression and imposed similar reductions in mdr1a and mdr1b transcriptional activity. Significant effects of IL-6 on mdr1 mRNA stability were not seen. Our results indicate that reductions in mdr1 expression in experimental models of inflammation likely occurs through altered gene transcription. Furthermore, as IL-6 was found to decrease mdr1 expression and gene transcription rates in vitro, this cytokine is likely involved in the reduction of mdr1 expression that is seen in vivo during an acute inflammatory response.