TCF7L2 rs7903146 impairs islet function and morphology in non-diabetic individuals

AIMS/HYPOTHESIS

Transcription factor 7-like 2 (TCF7L2) is a Wnt-signalling-associated transcription factor. Genetic studies have clearly demonstrated that DNA polymorphisms within TCF7L2 confer the strongest known association with increased risk of type 2 diabetes. However, the impact of the TCF7L2 type-2-diabetes-associated rs7903146 T allele on biological function and morphology of human pancreatic islets is unknown.

METHODS

Paraffin sections of pancreases from 187 brain-deceased donors (HbA(1c) <6.5% [48 mmol/mol]) were used to genotype the TCF7L2 variant rs7903146 and evaluate its impact on islet morphology and alpha and beta cell subpopulations following immunostaining for glucagon and C-peptide. Following islet isolation, we investigated the correlation between TCF7L2 genotype and in vitro islet functional variables from our in-house pancreatic database.

RESULTS

TCF7L2 rs7903146 (T/T) was associated with reduced basal and glucose-stimulated insulin secretion in isolated human islets, and reduced islet density in whole pancreas. Morphological analysis demonstrated islet size was increased in T/T carriers. Furthermore, rs7903146 was associated with an increased glucagon/C-peptide ratio, especially in bigger islets.

CONCLUSION/INTERPRETATION

The TCF7L2 variant rs7903146 risk allele is associated with impaired insulin secretion, reduction of total islet number and quantitative as well as qualitative morphological changes in human islets. Understanding how the TCF7L2 genotype modulates its activity and how TCF7L2 impacts the islet morphology may aid the design of new therapeutic approaches for the treatment of type 2 diabetes.

mTOR signaling: implications for cancer and anticancer therapy

Mounting evidence links deregulated protein synthesis to tumorigenesis via the translation initiation factor complex eIF4F. Components of this complex are often overexpressed in a large number of cancers and promote malignant transformation in experimental systems. mTOR affects the activity of the eIF4F complex by phosphorylating repressors of the eIF4F complex, the eIF4E binding proteins. The immunosuppressant rapamycin specifically inhibits mTOR activity and retards cancer growth. Importantly, mutations in upstream negative regulators of mTOR cause hamartomas, haemangiomas, and cancers that are sensitive to rapamycin treatment. Such mutations lead to increased eIF4F formation and consequently to enhanced translation initiation and cell growth. Thus, inhibition of translation initiation through targeting the mTOR-signalling pathway is emerging as a promising therapeutic option.

mTOR signaling: implications for cancer and anticancer therapy

Mounting evidence links deregulated protein synthesis to tumorigenesis via the translation initiation factor complex eIF4F. Components of this complex are often overexpressed in a large number of cancers and promote malignant transformation in experimental systems. mTOR affects the activity of the eIF4F complex by phosphorylating repressors of the eIF4F complex, the eIF4E binding proteins. The immunosuppressant rapamycin specifically inhibits mTOR activity and retards cancer growth. Importantly, mutations in upstream negative regulators of mTOR cause hamartomas, haemangiomas, and cancers that are sensitive to rapamycin treatment. Such mutations lead to increased eIF4F formation and consequently to enhanced translation initiation and cell growth. Thus, inhibition of translation initiation through targeting the mTOR-signalling pathway is emerging as a promising therapeutic option.

Effects of glutamine deprivation on protein synthesis in a model of human enterocytes in culture

To assess the effect of glutamine availability on rates of protein synthesis in human enterocytes, Caco-2 cells were grown until differentiation and then submitted to glutamine deprivation produced by exposure to glutamine-free medium or methionine sulfoximine [L-S-[3-amino-3-carboxypropyl]-S-methylsulfoximine (MSO)], a glutamine synthetase inhibitor. Cells were then incubated with (2)H(3)-labeled leucine with or without glutamine, and the fractional synthesis rate (FSR) of total cell protein was determined from (2)H(3)-labeled enrichments in protein-bound and intracellular free leucine measured by gas chromatography-mass spectrometry. Both protein FSR (28 +/- 1.5%/day) and intracellular glutamine concentration (6.1 +/- 0.6 micromol/g protein) remained unaltered when cells were grown in glutamine-free medium. In contrast, MSO treatment resulted in a dramatic reduction in protein synthesis (4.6 +/- 0.6 vs. 20.2 +/- 0.8%/day, P < 0.01). Supplementation with 0.5-2 mM glutamine for 4 h after MSO incubation, but not with glycine nor glutamate, restored protein FSR to control values (24 +/- 1%/day). These results demonstrate that in Caco-2 cells, 1) de novo glutamine synthesis is highly active, since it can maintain intracellular glutamine pool during glutamine deprivation, 2) inhibition of glutamine synthesis is associated with reduced protein synthesis, and 3) when glutamine synthesis is depressed, exogenous glutamine restores normal intestinal FSR. Due to the limitations intrinsic to the use of a cell line as an experimental model, the physiological relevance of these findings for the human intestine in vivo remains to be determined.

Protein restriction and dexamethasone as a model of protein hypercatabolism in dogs: effect of glutamine on leucine turnover

To determine (1) whether protein restriction, combined with glucocorticosteroid treatment, can be used as a hypercatabolic model and (2) if so, whether glutamine attenuates protein wasting in this model, the effects of protein restriction, dexamethasone, and glutamine on leucine metabolism were assessed in dogs. A control group (n = 8) received a maintenance diet; another group (n = 8) received a protein-restricted diet either (1) alone; (2) along with a 7-day corticoid treatment; or (3) along with a 7-day corticoid treatment and a 7-hour intravenous (IV) glutamine infusion. The last day of each regimen, dogs underwent an IV isotope infusion in the fasting state, with a 3-hour NaH(13)CO3 infusion to assess CO2 production, and immediately thereafter, a 3-hour (13)C-leucine infusion to assess leucine appearance rate (Ra), oxidation (Ox), and nonoxidative leucine disposal (NOLD), expressed as micromol x kg(-1) x h(-1). Protein restriction was associated with a 24% decline in leucine Ra (223 +/- 16 v 298 +/- 17; P <.01), an index of whole body proteolysis, and a 29% decline in NOLD (180 +/- 15 v 223 +/- 13; P <.01), an index of whole body protein synthesis. In the protein-restricted group, dexamethasone treatment was associated with a 32% increase in Ra, (295 +/- 28 v 223 +/- 16; P <.05), a 186% increase in Ox (120 +/- 14 v 43 +/- 4; P <.001), with no change in NOLD, when compared with the protein-restricted alone. After protein restriction + dexamethasone, glutamine infusion induced a 40% increase in plasma glutamine (1,090 +/- 70 v 780 +/- 29 micromol x L(-1); P <.01), but failed to alter Ra, Ox, or NOLD. These results suggest that (1) in dogs, protein restriction combined with a 7-day course of dexamethasone results in alterations in leucine kinetics similar to those observed in stress-induced protein wasting in humans, and (2) in that model, a 7-hour IV glutamine infusion in the fasting state does not significantly attenuate protein wasting.