Scientists report new insights into the mechanisms by which a high-fat diet can lead to type 2 diabetes. Mouse and human studies by researchers in the U.S. and Japan suggest that elevated levels of free fatty acids leads to both reduced expression of the transcription factors FOXA2 and HNF1A in pancreatic beta cells and their exclusion from the cell nuclei. This in turn results in a deficit of GnT-4a glycosyltransferase expression in beta cells, leading to the characteristic signs of metabolic disease including hyperglycemia, impaired glucose tolerance, hyperinsulinemia, hepatic steatosis, and diminished insulin action in muscle and adipose tissues.
The scientists, from the Sanford-Burnham Medical Research Institute at the University of California Santa Barbara, the RIKEN Advanced Science Institute in Japan, and the University of California San Diego, further showed that protection from disease could be provided by enforced beta cell-specific GnT-4a protein glycosylation, which supported the maintenance of glucose transporter expression and the preservation of glucose transport.
Co-authors Kazuaki Ohtsubo, Ph.D., Mark Z. Chen, Ph.D., Jerrold M. Olefsky, Ph.D., and Jamey D. Marth, Ph.D., report their findings in Nature Medicine in a paper titled “Pathway to diabetes through attenuation of pancreatic beta cell glycosylation and glucose transport.”
Insulin resistance is a metabolic hallmark of type 2 diabetes, and pancreatic beta cell dysfunction represents a diagnostic determinant of the disease, causing the loss of glucose-stimulated insulin secretion (GSIS), the authors explain. Clues as to the molecular pathways involved in type 2 diabetes development are embodied in a mouse model of the disease that lacks GnT-4a glycosyltransferase, an enzyme encoded by the Mgat4a gene. GnT-4a is involved in positioning of the Slc2a2-encoded glucose transporter-2 (Glut-2) glycoprotein on the cell surface.
The Sanford-Burnham-led team set out to search for factors that control GnT-4a function and glucose transporter expression, and to see whether a deficiency in GnT-4a protein glycosylation and glucose transport are truly causal in disease pathogenesis in the mouse model, and potentially in humans.
The researchers first showed that wild-type mice fed a high-fat diet (HFD) became deficient in Mgat4a and Slc2a2 RNA in pancreatic islet cells, when compared with animals fed a regular diet. Through promoter region DNA sequence analyses of the mouse Mgat4a and Slc2a2 genes, the orthologous human genes MGAT4A and SLC2A1 (also known as GLUT1) and the human SLC2A2 gene, they identified potential binding sites on multiple transcription factors including mouse and human FOXA2 and HNF1A.
In mice Foxa2 and Hnf1a binding to the Mgat4a and Slc2a2 genes was significantly reduced in pancreatic islets from mice eating a HFD. This reduced binding coincided with decreased abundance of the Foxa2 and Hnf1a proteins, a marked reduction in their nuclear localization of both, and increased cytoplasmic localization. Decreased protein abundance and changes in localization combined to varying degrees, resulting in an 80% drop in the levels of both Foxa2 and Hnf1a in the cell nuclei.
Studies in mouse islet cells confirmed that Foxa2 and Hnf1a normally contribute to Mgat4a and Slc2a2 transactivation in normal mice fed the standard diet, but that this transactivation was impaired in islets of mice fed HFD. Significantly, when cultured islet cells from mice fed a normal diet were treated with palmitic acid (a lipid used to model the diabetogenic effect of increased free fatty acids) nuclear exclusion of Foxa2 and Hnf1a again resulted, and was associated with the downregulation of Mgat4a and Slc2a2 gene expression.
Importantly, similar studies in human islets from healthy nondiabetic donors showed that in the absence of exogenouse palmitic acid, FOXA2 and HNF1A proteins localized primarily in the nucleus, whereas the addition of palmitic acid caused nuclear exclusion and increased cytoplasmic localization of both factors. The addition of palmitic acid also diminished FOXA2 and HNF1A binding to the promotor regions of MGAT4A, SLC2A1, and SLC2A2 genes, and attenuated mRNA expression of the three genes, which coincided with the loss of GSIS.
“The similarities we observed in the responses of normal mouse and human islet cells to palmitic acid suggested that islets from human type 2 diabetes donors may show defects comparable to those of mice rendered diabetic by high-fat diet administration,” the authors note.
Further analyses showed that FOXA2 and HNF1A were excluded by more than 70% from the nuclei of islet cells in tissue from type 2 diabetic patients. In the diabetic islet tissue expression of SLC2A1 and SLC2A2 mRNAs encoding the human GLUT1 and GLUT2 glucose transporters was also decreased by 70–90%, and MGAT4A mRNA expression was reduced by 60%. The reduction in MGAT4A expression translated to a 10-50-fold reduction of the GnT-4a glycan product. Importantly, islet cells from donors with type 2 diabetes were 80–90% deficient in cell surface expression of both GLUT1 and GLUT2 glycoproteins, had very low glucose transport activity, and lacked the GSIS response.
The team moved on to evaluate the impact of diminished GnT-4a glycosylation on glucose transporter expression and the onset of disease signs in a HFD-induced mouse model of type 2 diabetes. They generated transgenic mice that constitutively expressed the human MGAT4A gene specifically in beta cells, but in no other cell type or tissue. Both wild-type and transgenic mice fed a HFD became obese, but while the HFD wild-type mice displayed hyperglycemia and hyperinsulinemia, MGAT4A transgenic littermates maintained much lower concentrations of blood glucose and insulin.
Fasted MGAT4A transgenic animals also displayed markedly greater glucose tolerance, reduced insulinemia, and preserved GSIS. Circulating free fatty acid and triglyceride concentrations in these animals were in addition markedly reduced in comparison with wild-type littermates, “indicating that enforced beta cell GnT-4a protein glycosylation diminished multiple and systemic disease signs,” the authors write.
Moreover, insulin tolerance tests in mice fed a HFD indicated greater glucose clearance in MGAT4A transgenic mice than in wild-type animals, and glucose infusion rate studies suggested whole-body insulin sensitivity was significantly greater in MGAT4A transgenic mice. Hepatic steatosis was evident in wild-type mice that received the high-fat diet, whereas the livers of HFD MGAT4A transgenic mice notably lacked signs of steatosis.
To test whether the disease protection afforded by GnT-4a involves maintenance of beta cell glucose sensing, the researchers then generated transgenic mice bearing constitutive beta cell-specific expression of the human SLC2A2 gene. In these animals there was greater beta cell GLUT-2 glycoprotein expression and glucose transport in comparison with beta cells from wild-type and MGAT4A transgenic mice that received the standard diet.
In fact, the SLC2A2 transgene conferred an intermediate degree of improvement at the biochemical level between wild-type and MGAT4A transgenic mice. For example, the SLC2A2 transgene provided an intermediate reduction in hyperglycemia and hyperinsulinemia, whereas the development of obesity was unaffected. Glucose transporter glycosylation by GnT-4a remained optimal in islet cells of MGAT4A transgenic mice receiving the high-fat diet, whereas corresponding SLC2A2 transgenic beta cells showed a decrease in GLUT-2 glycosylation among mice fed the high-fat diet, consistent with the downmodulation of endogenous MGAT4A expression, diminished GnT-4a activity and decreased GLUT-2 abundance.
The overall results suggest a molecular and pathogenic pathway that includes a key role of pancreatic beta cell GnT-4a glycosylation and glucose transport in the origin and severity of disease signs including insulin resistance that are together diagnostic of type 2 diabetes, the authors conclude.
“A pathogenic tipping point in this pathway may occur when elevated free fatty acid (FFA) concentrations impair the expression and function of FOXA2 and HNF1A transcription factors sufficiently in beta cells to deplete GnT-4a glycosylation and glucose transporter expression. The resulting dysfunction of beta cells leads to impaired glucose tolerance and failure of GSIS and further contributes to hyperglycemia, hepatic steatosis, and systemic insulin resistance. Preservation of beta cell GnT-4a glycosylation and glucose transporter expression breaks this pathogenic cycle and its link to diet and obesity.”
The impact of enforced beta cell-specific expression of MGAT4A and SLC2A2 on metabolic abnormalities including GSIS and insulin resistance was particularly notable, they add. “Constitutive beta cell expression of GnT-4a or Glut-2 preserved considerable systemic insulin sensitivity, indicating that beta cell function influences insulin action on these peripheral target tissues.”
The results were something of a surprise, however, Dr. Marth notes. “The observation that beta cell malfunction significantly contributes to multiple disease signs, including insulin resistance, was unexpected.” However, the findings do provide pointers to new potential therapeutic approaches, he adds. “Now that we know more fully how states of overnutrition can lead to type 2 diabetes, we can see more clearly how to intervene. The identification of the molecular players in this pathway to diabetes suggests new therapeutic targets and approaches toward developing an effective preventative or perhaps curative treatment. This may be accomplished by beta cell gene therapy or by drugs that interfere with this pathway in order to maintain normal beta cell function.”