D-glucose- and 3-O-methyl-D-glucose-induced upregulation of selected genes in rat hepatocytes and INS1E cells: Re-evaluation of the possible role of hexose phosphorylation

The biochemical events involved in the upregulation of selected glucose-responsive genes by 3-O-methyl-D-glucose (3-MG) remain to be elucidated. The present study mainly aimed to re-evaluate the possible role of 3-MG phosphorylation in the upregulation of the thioredoxin interacting protein (TXNIP) and liver pyruvate kinase (LPK) genes in rat hepatocytes and INS1E cells. TXNIP and LPK transcription was assessed in rat liver and INS1E cells exposed to a rise in D-glucose concentration, 2-deoxy-D-glucose (2-DG), 3-MG and, when required, D-mannoheptulose. The phosphorylation of D-[U-14C]glucose and 3-O-[14C]methyl-D-glucose (14C-labeled 3-MG) was measured in rat liver, INS1E cell and rat pancreatic islet homogenates. The utilization of D-[5-3H]glucose by intact INS1E cells was also measured. In rat hepatocytes, a rise in the D-glucose concentration increased the TXNIP/hypoxanthine-guanine phosphoribosyl transferase (HPRT) and LPK/HPRT ratios, while 2-DG and 3-MG also increased the TXNIP/HPRT ratio, but not the LPK/HPRT ratio. In INS1E cells, the TXNIP/HPRT and LPK/HPRT ratios were increased in response to the addition of D-glucose, 2-DG and 3-MG. Furthermore, D-mannoheptulose abolished the response to D-glucose and 2-DG, but not to 3-MG, in these cells. Liver cell homogenates catalyzed the phosphorylation of 3-MG to a modest extent, whilst INS1E and rat pancreatic islet cell homogenates did not. Moreover, 3-MG marginally decreased D-glucose phosphorylation in INS1E cell homogenates but not in liver cell homogenates. D-[5-3H]glucose utilization by intact INS1E cells was decreased by 2-DG, but not by 3-MG. These findings reinforce the view that the upregulation of the TXNIP and LPK genes induced by 3-MG is not attributable to its phosphorylation or any favorable effect on D-glucose metabolism.