Diurnal rhythm of rat liver glycogen synthetase

A Txnrd1-dependent metabolic switch alters hepatic lipogenesis, glycogen storage, and detoxification

Besides helping to maintain a reducing intracellular environment, the thioredoxin (Trx) system impacts bioenergetics and drug metabolism. We show that hepatocyte-specific disruption of Txnrd1, encoding Trx reductase-1 (TrxR1), causes a metabolic switch in which lipogenic genes are repressed and periportal hepatocytes become engorged with glycogen. These livers also overexpress machinery for biosynthesis of glutathione and conversion of glycogen into UDP-glucuronate; they stockpile glutathione-S-transferases and UDP-glucuronyl-transferases; and they overexpress xenobiotic exporters. This realigned metabolic profile suggested that the mutant hepatocytes might be preconditioned to more effectively detoxify certain xenobiotic challenges. Hepatocytes convert the pro-toxin acetaminophen (APAP, paracetamol) into cytotoxic N-acetyl-p-benzoquinone imine (NAPQI). APAP defenses include glucuronidation of APAP or glutathionylation of NAPQI, allowing removal by xenobiotic exporters. We found that NAPQI directly inactivates TrxR1, yet Txnrd1-null livers were resistant to APAP-induced hepatotoxicity. Txnrd1-null livers did not have more effective gene expression responses to APAP challenge; however, their constitutive metabolic state supported more robust GSH biosynthesis, glutathionylation, and glucuronidation systems. Following APAP challenge, this effectively sustained the GSH system and attenuated damage.

Circadian transcription of the cholesterol 7 alpha hydroxylase gene may involve the liver-enriched bZIP protein DBP

The liver-enriched transcription factor DBP is expressed with a stringent circadian rhythm. We present evidence that DBP is a regulator of the circadian expression of the rat gene encoding cholesterol 7 alpha hydroxylase (C7 alpha H), the rate-limiting enzyme in the conversion of cholesterol to bile acids. As with DBP, C7 alpha H mRNA reaches peak levels in the evening, and its cycling is independent of daily food and light cues. As predicted for a DBP target gene, the primary level of C7 alpha H circadian expression is at the transcriptional level. DBP can activate the C7 alpha H promoter in cotransfection assays through a cognate DNA site centered around -225. In nuclear extracts prepared by a novel method that, in contrast to conventional techniques, yields near-quantitative recovery of DBP and other non-histone proteins, the DNA site required for DBP activation is the predominant site of occupancy by nuclear factors on the C7 alpha H promoter. At this site, the predominant binding activity is an evening-specific complex of which DBP is a component. These data suggest that DBP may play an important role in cholesterol homeostasis through circadian transcriptional regulation of cholesterol 7 alpha hydroxylase.

Diurnal rhythms in liver carbohydrate metabolism. Comparative aspects and critical review

Literature data on the diurnal rhythms of blood glucose, liver glycogen levels and key hepatic enzyme activities of glycolysis, gluconeogenesis, glycogen metabolism and lipogenesis in animals are reviewed. Materials on the diurnal rhythms of the activities of other enzymes involved in carbohydrate metabolism and related pathways such as the equilibrium glycolytic enzymes are also given. Interspecies comparison and analysis of the results and their interpretation are given.

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

The diurnal rhythm of phosphorylase activity in mouse liver extracts was correlated with the 24 h fluctuations in phosphorylase protein. This last measurement was made using rocket immunoelectrophoresis. The peak activity of phosphorylase appeared coincident (at 18:00 h) with the greatest amount of phosphorylase protein detected. Conversely, the lowest activity measured and lowest enzymic protein content both occurred at 02:00 h. Regression analysis revealed a significant positive correlation between enzyme activity and protein. Thus changes in the cellular concentration of this enzyme are implicated in the diurnal rhythm of liver glycogen.

Diurnal variations in the activities of the glycogen metabolizing enzymes in mouse liver

The glycogen level in mouse liver was maximal during the night and decreased to the lowest level during the light period. The peak activity of phosphorylase alpha was observed during the light hours and thus paralleled the decline of hepatic glycogen concentrations. The period of rapid glycogen synthesis (1800-2200 hr) was immediately preceded by maximum glycogen synthase alpha activity. Significant diurnal rhythms for phosphorylase kinase and phosphorylase phosphatase activities were also observed and appear to play a role in regulating the diurnal rhythm of phosphorylase alpha activity.

Diurnal rhythmicity in folic acid metabolism of rat liver and kidney

In rat liver and kidney, tetrahydrofolate formylase activities show diurnal rhythms with maxima in the middle of the dark phase. The rhythms are not generated by de novo synthesis of enzyme protein. Dihydrofolate reductase activity is arrhythmic in both liver and kidney.

The influence of diurnal rhythms of carbohydrate metabolism in adult rat liver on the metabolic characteristics of isolated liver parenchymal cells

Rats trained to the "8 + 16" controlled feeding cycle where food is only available for the first 8 h of the 12 h dark period exhibit a pronounced diurnal rhythm of hepatic glycogen metabolism. Glycogen is stored within the liver parenchymal cells during the dark period and subsequently mobilized for energy production during the light period. Hepatocytes, isolated by collagenase perfusion, from livers of such animals have differing capacities for glycogen synthesis when incubated with glucose. Cells prepared at the end of the 16 h period without food have very little capacity for synthesis compared with much higher rates obtained in cells obtained during the feeding period. Cells obtained from liver containing a large glycogen concentration produce a net breakdown of glycogen during incubations with glucose, however experiments using radioactively labelled glucose indicate that synthesis does occur in these cells. The changes in the capacity of the cells for glycogen synthesis appear to be due, in part, to changes in the percentage of the cell population involved in synthesis and in the activity of glycogen synthetase a. Attempts of influence the rate of glycogen synthesis at any time of day with insulin or dexamethasone were unsuccessful.

Protein synthesis during endogenous rhythmic leaflet movement in Albizzia

A rhythm was found in protein synthesis accompanying rhythmic leaflet opening and closing in the dark in the plant Albizzia. More protein was synthesized during the opening of the leaflets than during the closing. Furthermore, an inhibitor of protein synthesis, cycloheximide, prevented rhythmic opening of leaflets but had no effect on rhythmic closing. It is suggested that protein synthesis is involved in the movement across membranes of K(+) ions that cause turgor changes and leaflet movement.