Glucagon-like peptide-1(7-36NH2) (GLP-1) is secreted by the intestinal L cell in response to both nutrient and neural stimulation, resulting in enhanced glucose-dependent insulin secretion. GLP-1 is therefore an attractive therapeutic for the treatment of type 2 diabetes. The antidiabetic drug, metformin, is known to increase circulating GLP-1 levels, although its mechanism of action is unknown. Direct effects of metformin (5-2000 μm) or another AMP kinase activator, aminoimidazole carboxamide ribonucleotide (100-1000 μm) on GLP-1 secretion were assessed in murine human NCI-H716, and rat FRIC L cells. Neither agent stimulated GLP-1 secretion in any model, despite increasing AMP kinase phosphorylation (P < 0.05-0.01). Treatment of rats with metformin (300 mg/kg, per os) or aminoimidazole carboxamide ribonucleotide (250 mg/kg, sc) increased plasma total GLP-1 over 2 h, reaching 37 ± 9 and 29 ± 9 pg/ml (P < 0.001), respectively, compared with basal (7 ± 1 pg/ml). Plasma activity of the GLP-1-degrading enzyme, dipeptidylpeptidase-IV, was not affected by metformin treatment. Pretreatment with the nonspecific muscarinic antagonist, atropine (1 mg/kg, iv), decreased metformin-induced GLP-1 secretion by 55 ± 11% (P < 0.05). Pretreatment with the muscarinic (M) 3 receptor antagonist, 1-1-dimethyl-4-diphenylacetoxypiperidinium iodide (500 μg/kg, iv), also decreased the GLP-1 area under curve, by 48 ± 8% (P < 0.05), whereas the antagonists pirenzepine (M1) and gallamine (M2) had no effect. Furthermore, chronic bilateral subdiaphragmatic vagotomy decreased basal secretion compared with sham-operated animals (7 ± 1 vs. 13 ± 1 pg/ml, P < 0.001) but did not alter the GLP-1 response to metformin. In contrast, pretreatment with the gastrin-releasing peptide antagonist, RC-3095 (100 μg/kg, sc), reduced the GLP-1 response to metformin, by 55 ± 6% (P < 0.01) at 30 min. These studies elucidate the mechanism underlying metformin-induced GLP-1 secretion and highlight the benefits of using metformin with dipeptidylpeptidase-IV inhibitors in patients with type 2 diabetes.

Homocystinuria, an inherited disorder associated with premature atherosclerosis, represents a severe form of methionine intolerance. To analyze the importance of milder forms of methionine intolerance in the genesis of vascular disease, the relation between provokable methionine intolerance and coronary artery disease was investigated. In a group of 138 men, aged 31 to 65 years (mean 53), referred for cardiac catheterization, plasma homocystine was measured before and 6 hours after an oral l-methionine load (0.1 g/kg). Thirty-nine subjects found to have normal coronary arteries had a mean post-load plasma homocystine level of 0.59 +/- 0.37 mumol/liter. A criterion at the 95th percentile (1.64 SD above the mean) was selected and applied to the remaining 99 subjects with coronary artery disease (0.70 +/- 0.68 mumol/liter). Sixteen (16%) of 99 subjects with coronary artery disease exceeded this level as compared with 1 (2%) of 39 subjects without coronary artery disease (p less than 0.04). The risk of coronary artery disease in men with provokable methionine intolerance was increased sevenfold as estimated by the odds ratio. By correlation matrix and multivariate regression analyses, provokable homocystinemia was predictive of coronary artery disease and was independent of tobacco smoking, hypertension, diabetes mellitus, serum cholesterol and age. It is proposed that men with mild methionine intolerance exposed to the high methionine content of the Western diet may develop intermittent homocystinemia and thus may be at greater risk for the development of coronary artery disease.

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Genome-wide association studies recently identified 32 loci that associate with the age at menarche (AAM) in humans. Because the locus most robustly associated with AAM is in/near LIN28B, the goal of this study was to investigate how the Lin28 pathway might modulate pubertal timing by examining expression of Lin28b, and its homologue, Lin28a, across the pubertal transition in female mice. Quantitative reverse-transcriptase PCR data indicate that, prior to the onset of puberty, expression of both Lin28b and Lin28a decreases in the ovary, while expression of only Lin28a decreases in the hypothalamus; the expression of Lin28a increases after the onset of puberty in the pituitary. Immunohistochemistry in ovarian tissue verified that Lin28a protein levels decreased in parallel with gene expression. Although these data do not demonstrate cause and effect, they do suggest that decreased expression of Lin28a/Lin28b may facilitate the transition into puberty, consistent with previous data showing that overexpression of Lin28a in transgenic mice leads to delayed puberty. In addition, although Lin28b and/or Lin28a expression significantly decreased prior to puberty, neither Let-7a nor Let-7g miRNA levels changed significantly, raising the possibility that some effects of Lin28b and Lin28a within the hypothalamic-pituitary-gonadal (HPG) axis may be Let-7 miRNA independent. Subsequent studies, such as tissue and age specific modulation of Lin28b and Lin28a expression, could determine whether the expression patterns observed are responsible for modulating the onset of puberty and delineate further the role of this pathway in the HPG axis.