The diurnal rhythm of liver glycogen phosphorylase: correlating changes in enzyme activity and enzymic protein

Circadian Clocks Make Metabolism Run

Most organisms adapt to the 24-h cycle of the Earth's rotation by anticipating the time of the day through light-dark cycles. The internal time-keeping system of the circadian clocks has been developed to ensure this anticipation. The circadian system governs the rhythmicity of nearly all physiological and behavioral processes in mammals. In this review, we summarize current knowledge stemming from rodent and human studies on the tight interconnection between the circadian system and metabolism in the body. In particular, we highlight recent advances emphasizing the roles of the peripheral clocks located in the metabolic organs in regulating glucose, lipid, and protein homeostasis at the organismal and cellular levels. Experimental disruption of circadian system in rodents is associated with various metabolic disturbance phenotypes. Similarly, perturbation of the clockwork in humans is linked to the development of metabolic diseases. We discuss recent studies that reveal roles of the circadian system in the temporal coordination of metabolism under physiological conditions and in the development of human pathologies.

Glucose Homeostasis: Regulation by Peripheral Circadian Clocks in Rodents and Humans

Most organisms, including humans, have developed an intrinsic system of circadian oscillators, allowing the anticipation of events related to the rotation of Earth around its own axis. The mammalian circadian timing system orchestrates nearly all aspects of physiology and behavior. Together with systemic signals, emanating from the central clock that resides in the hypothalamus, peripheral oscillators orchestrate tissue-specific fluctuations in gene expression, protein synthesis, and posttranslational modifications, driving overt rhythms in physiology and behavior. There is increasing evidence on the essential roles of the peripheral oscillators, operative in metabolically active organs in the regulation of body glucose homeostasis. Here, we review some recent findings on the molecular and cellular makeup of the circadian timing system and its implications in the temporal coordination of metabolism in health and disease.

Diurnal rhythmicity in biological processes involved in bioavailability of functional food factors

In the past few decades, many types of functional factors have been identified in dietary foods; for example, flavonoids are major groups widely distributed in the plant kingdom. However, the absorption rates of the functional food factors are usually low, and many of these are difficult to be absorbed in the intact forms because of metabolization by biological processes during absorption. To gain adequate beneficial effects, it is therefore mandatory to know whether functional food factors are absorbed in sufficient quantity, and then reach target organs while maintaining beneficial effects. These are the reasons why the bioavailability of functional food factors has been well investigated using rodent models. Recently, many of the biological processes have been reported to follow diurnal rhythms recurring every 24 h. Therefore, absorption and metabolism of functional food factors influenced by the biological processes may vary with time of day. Consequently, the evaluation of the bioavailability of functional food factors using rodent models should take into consideration the timing of consumption. In this review, we provide a perspective overview of the diurnal rhythm of biological processes involved in the bioavailability of functional food factors, particularly flavonoids.

The effects of streptozotocin-induced diabetes and insulin supplementation on expression of the glycogen phosphorylase gene in rat liver

We have previously observed that the chronic effects of streptozotocin-induced diabetes cause a decrease in the total hepatic glycogen phosphorylase activity with a corresponding reduction in the phosphorylase protein levels. These effects were normalized by insulin administration to diabetic rats. There was no change in the total glycogen synthase activity as a result of diabetes or insulin supplementation. These results are extended to examine the effects of diabetes and insulin administration to diabetic animals on the expression of phosphorylase and glycogen synthase enzymes. The expression (i.e. mRNA levels) of phosphorylase was down-regulated (45% of normal levels) in diabetic livers, and this was normalized by insulin supplementation to diabetic animals. Diabetes or insulin supplementation to diabetic rats showed no effect on the transcription rate of phosphorylase. As expected, diabetes (or insulin administration to diabetic animals) did not cause any alteration in the mRNA levels or in the transcription rate of hepatic glycogen synthase. The stability of phosphorylase mRNA was then examined using hepatocytes prepared from normal and diabetic rats. Diabetes caused a decrease in the half-life of phosphorylase mRNA from 14 h in normal hepatocytes to 6.5 h in diabetic hepatocytes. Insulin supplementation to the medium of diabetic hepatocytes increased the half-life of phosphorylase mRNA to a level comparable with normal values. This study indicates that the chronic effect of insulin on the activation of the total hepatic phosphorylase activity (and protein) is mediated through the stabilization of its mRNA levels.