RESUMO
The biochemical events involved in the upregulation of selected glucoseresponsive genes by 3OmethylDglucose (3MG) remain to be elucidated. The present study mainly aimed to reevaluate the possible role of 3MG 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 Dglucose concentration, 2deoxyDglucose (2DG), 3MG and, when required, Dmannoheptulose. The phosphorylation of D[U14C]glucose and 3O[14C]methylDglucose (14C-labeled 3-MG) was measured in rat liver, INS1E cell and rat pancreatic islet homogenates. The utilization of D[53H]glucose by intact INS1E cells was also measured. In rat hepatocytes, a rise in the Dglucose concentration increased the TXNIP/hypoxanthineguanine phosphoribosyl transferase (HPRT) and LPK/HPRT ratios, while 2DG and 3MG 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 Dglucose, 2DG and 3MG. Furthermore, Dmannoheptulose abolished the response to Dglucose and 2DG, but not to 3MG, in these cells. Liver cell homogenates catalyzed the phosphorylation of 3MG to a modest extent, whilst INS1E and rat pancreatic islet cell homogenates did not. Moreover, 3MG marginally decreased Dglucose phosphorylation in INS1E cell homogenates but not in liver cell homogenates. D[53H]glucose utilization by intact INS1E cells was decreased by 2DG, but not by 3MG. These findings reinforce the view that the upregulation of the TXNIP and LPK genes induced by 3MG is not attributable to its phosphorylation or any favorable effect on Dglucose metabolism.
Assuntos
3-O-Metilglucose/farmacologia , Glucose/farmacologia , Hepatócitos/efeitos dos fármacos , Hexoses/metabolismo , Regulação para Cima/efeitos dos fármacos , Animais , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Proteínas de Ciclo Celular , Células Cultivadas , Expressão Gênica , Hepatócitos/citologia , Hepatócitos/metabolismo , Hipoxantina Fosforribosiltransferase/genética , Hipoxantina Fosforribosiltransferase/metabolismo , Fosforilação/efeitos dos fármacos , Piruvato Quinase/genética , Piruvato Quinase/metabolismo , RatosRESUMO
The glucagon receptor (GLR) expression is positively regulated by glucose. This regulation is allowed by the presence, in the promotor of the rat GLR gene, of a sequence feature similar to the two E-boxes motifs constituting the carbohydrate response elements (ChoRE) described for several glycolytic and lipogenic enzyme genes. Using reporter gene assays, we demonstrated here that, despite structural homologies with these ChoREs, the GLR gene glucose response element presents various functional dissimilarities. Testing glucose analogs, we demonstrated that, as for other genes, the glucose must be first phosphorylated. However, at variance with others homologue genes, our data showed the implication of the nonoxidative branch of the pentose phosphate pathway in the transmission of the glucose signal and lack of inhibition by adenosine monophosphate (AMP)-kinase. Furthermore, the activity of our reporter gene was strongly stimulated by butyrate, propionate, and acetate. This observation contrasts with fatty-acid-induced inhibition of the glucose activation, observed for all other genes containing homolog ChoREs. We also showed that glucose and butyrate influence the reporter gene expression via different features.