Effect of dietary restriction on hepatic and renal phosphoenolpyruvate carboxykinase induction in young and old Fischer 344 rats

HNF4α contributes to glucose formation in aged rat hepatocytes

Aging-dependent physiological conditions are attributed to parenchymal structural changes to cellular functions in aged organisms. Compared to the young animals, the primary hepatocytes from old rats showed a higher glucose output and a higher expression of the key gluconeogenesis-regulating enzyme, phosphoenol pyruvate carboxykinase (PEPCK). The primary hepatocytes from old rats showed a higher glucose output and a higher expression of the key gluconeogenesis-regulating enzyme, phosphoenol pyruvate carboxykinase (PEPCK), compared with those from the young animals. The in situ hybridization study showed increased PEPCK mRNA expression in the aged liver tissues. The livers from old rats showed loosened hexagonal hepatic lobular structures, increased collagen accumulation, and high expression of the hypoxia marker hypoxia-inducible factor 1α (HIF1α). Hypoxia increased the PEPCK mRNA and protein expression levels in accordance with the HIF1α expression. PEPCK promoter luciferase reporter assay showed that hypoxia increased PEPCK through transcriptional activation. Furthermore, the hepatocyte nuclear factor α (HNF4α) protein, but not the HNF4α mRNA level, increased in parallel with the PEPCK mRNA expression under hypoxic conditions. Glucose production increased under hypoxic conditions, but this increment diminished by HNF4α siRNA in young hepatocytes. Moreover, increased glucose production from old rat hepatocytes was reversed by the down-regulation of HNF4α through a specific siRNA. This study suggests that the mild hypoxic conditions in response to aging-dependent hepatic structural changes may contribute to the induction of the gluconeogenic enzyme PEPCK through HNF4α protein stabilization.

The role of hepatic, renal and intestinal gluconeogenic enzymes in glucose homeostasis of juvenile rainbow trout

Rainbow trout is unable to utilize high levels of dietary carbohydrates and experiences hyperglycemia after consumption of carbohydrate-rich meals. Carbohydrates stimulate hepatic glycolytic activity, but gene expression of the rate-limiting gluconeogenic enzymes glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK) remains high. Although there is significant mRNA expression and activity of gluconeogenic enzymes in trout intestine and kidney, the regulation of these enzymes by diet is not known. We tested the hypothesis that dietary carbohydrate modulates intestinal and renal G6Pase, FBPase and PEPCK. Fish were either fasted or fed isocaloric carbohydrate-free (CF) or high carbohydrate (HC) diets for 14 days. As expected, fish fed HC exhibited postprandial hyperglycemia and enhanced levels of hepatic glucokinase mRNA and activity. Dietary carbohydrates had no significant effect on the expression and activity of PEPCK, FBPase and G6Pase in all three organs. In contrast, fasting enhanced the activity, but not the mRNA expression of both hepatic and intestinal PEPCK, as well as intestinal FBPase. Therefore, the activity of rate-limiting gluconeogenic enzymes in trout can be modified by fasting, but not by the carbohydrate content of the diet, potentially causing hyperglycemia when fed high levels of dietary carbohydrates. In this species consuming low carbohydrate diets at infrequent intervals in the wild, fasting-induced increases in hepatic and intestinal gluconeogenic enzyme activities may be a key adaptation to prevent perturbations in blood glucose during food deprivation.

Conserved and tissue-specific genic and physiologic responses to caloric restriction and altered IGFI signaling in mitotic and postmitotic tissues

Caloric restriction (CR), the consumption of fewer calories without malnutrition, and reduced insulin and/or IGFI receptor signaling delay many age-related physiological changes and extend the lifespan of many model organisms. Here, we present and review microarray and biochemical studies indicating that the potent anticancer effects of CR and disrupted insulin/IGFI receptor signaling evolved as a byproduct of the role of many mitotic tissues as reservoirs of metabolic energy. We argue that the longevity effects of CR are derived from repeated cycles of apoptosis and autophagic cell death in mitotically competent tissues and protein turnover and cellular repair in postmitotic tissues. We review studies showing that CR initiated late in life can rapidly induce many of the benefits of lifelong CR, including its anticancer effects. We also discuss evidence from liver and heart indicating that many benefits of lifelong CR are recapitulated in mitotic and postmitotic tissues when CR is initiated late in life.