GLUT-1 and GLUT-2 mRNA, protein, and glucose transporter activity in cultured fetal and adult hepatocytes

In vivo confirmation of altered hepatic glucose metabolism in patients with liver fibrosis/cirrhosis by 18F-FDG PET/CT

OBJECTIVE

The aim of this study was to assess the value of ¹⁸F-FDG PET/CT for quantitative assessment of hepatic metabolism in patients with different stages of liver fibrosis/cirrhosis.

MATERIALS AND METHODS

¹⁸F-FDG PET/CT scans of 37 patients either with or without liver fibrosis/cirrhosis, classified according to the METAVIR score (F0-F4) obtained from histopathological analysis of liver specimen, were analyzed retrospectively and classified as follows: no liver fibrosis (F0, n = 6), mild liver fibrosis (F1, n = 11), advanced liver fibrosis (F2, n = 6), severe liver fibrosis (F3, n = 5), and liver cirrhosis (F4, n = 11). The liver-to-blood ratio (LBR, scan time corrected for a reference time of 75 min) was compared between patient groups.

RESULTS

Patients with liver fibrosis or cirrhosis (≥ F1; LBR 1.53 ± 0.35) showed a significant higher LBR than patients with normal liver parenchyma (F0, 1.08 ± 0.23; P = 0.004). In direct comparison, LBR increased up to the advanced stage of liver fibrosis (F2; 2.00 ± 0.40) and decreased until liver cirrhosis is reached (F4, 1.32 ± 0.14).

CONCLUSION

Functional changes in liver parenchyma during liver fibrosis/cirrhosis affect hepatic glucose metabolism and significantly differ between stages of liver fibrosis/cirrhosis, classified according to the METAVIR scoring system, as demonstrated by LBR quantification by ¹⁸F-FDG PET/CT.

Hepatic expression and cellular distribution of the glucose transporter family

Glucose and other carbohydrates are transported into cells using members of a family of integral membrane glucose transporter (GLUT) molecules. To date 14 members of this family, also called the solute carrier 2A proteins have been identified which are divided on the basis of transport characteristics and sequence similarities into several families (Classes 1 to 3). The expression of these different receptor subtypes varies between different species, tissues and cellular subtypes and each has differential sensitivities to stimuli such as insulin. The liver is a contributor to metabolic carbohydrate homeostasis and is a major site for synthesis, storage and redistribution of carbohydrates. Situations in which the balance of glucose homeostasis is upset such as diabetes or the metabolic syndrome can lead metabolic disturbances that drive chronic organ damage and failure, confirming the importance of understanding the molecular regulation of hepatic glucose homeostasis. There is a considerable literature describing the expression and function of receptors that regulate glucose uptake and release by hepatocytes, the most import cells in glucose regulation and glycogen storage. However there is less appreciation of the roles of GLUTs expressed by non parenchymal cell types within the liver, all of which require carbohydrate to function. A better understanding of the detailed cellular distribution of GLUTs in human liver tissue may shed light on mechanisms underlying disease pathogenesis. This review summarises the available literature on hepatocellular expression of GLUTs in health and disease and highlights areas where further investigation is required.

PVHL is a regulator of glucose metabolism and insulin secretion in pancreatic beta cells

Insulin secretion from pancreatic beta cells is stimulated by glucose metabolism. However, the relative importance of metabolizing glucose via mitochondrial oxidative phosphorylation versus glycolysis for insulin secretion remains unclear. von Hippel-Lindau (VHL) tumor suppressor protein, pVHL, negatively regulates hypoxia-inducible factor HIF1alpha, a transcription factor implicated in promoting a glycolytic form of metabolism. Here we report a central role for the pVHL-HIF1alpha pathway in the control of beta-cell glucose utilization, insulin secretion, and glucose homeostasis. Conditional inactivation of Vhlh in beta cells promoted a diversion of glucose away from mitochondria into lactate production, causing cells to produce high levels of glycolytically derived ATP and to secrete elevated levels of insulin at low glucose concentrations. Vhlh-deficient mice exhibited diminished glucose-stimulated changes in cytoplasmic Ca(2+) concentration, electrical activity, and insulin secretion, which culminate in impaired systemic glucose tolerance. Importantly, combined deletion of Vhlh and Hif1alpha rescued these phenotypes, implying that they are the result of HIF1alpha activation. Together, these results identify pVHL and HIF1alpha as key regulators of insulin secretion from pancreatic beta cells. They further suggest that changes in the metabolic strategy of glucose metabolism in beta cells have profound effects on whole-body glucose homeostasis.

GLUT1 expression is increased in hepatocellular carcinoma and promotes tumorigenesis

Accelerated glycolysis is one of the biochemical characteristics of cancer cells. The glucose transporter isoform 1 (GLUT1) gene encodes a key rate-limiting factor in glucose transport into cancer cells. However, its expression level and functional significance in hepatocellular cancer (HCC) are still disputed. Therefore, we aimed to analyze the expression and function of the GLUT1 gene in cases of HCC. We found significantly higher GLUT1 mRNA expression levels in HCC tissues and cell lines compared with primary human hepatocytes and matched nontumor tissue. Immunohistochemical analysis of a tissue microarray of 152 HCC cases revealed a significant correlation between Glut1 protein expression levels and a higher Ki-67 labeling index, advanced tumor stages, and poor differentiation. Accordingly, suppression of GLUT1 expression by siRNA significantly impaired both the growth and migratory potential of HCC cells. Furthermore, inhibition of GLUT1 expression reduced both glucose uptake and lactate secretion. Hypoxic conditions further increased GLUT1 expression levels in HCC cells, and this induction was dependent on the activation of the transcription factor hypoxia-inducible factor-1alpha. In summary, our findings suggest that increased GLUT1 expression levels in HCC cells functionally affect tumorigenicity, and thus, we propose GLUT1 as an innovative therapeutic target for this highly aggressive tumor.