Regulation of hepatic protein degradation

Liver damage and caspase-dependent apoptosis is related to protein malnutrition in mice: effect of methionine

This study aimed to determine whether the effects on the mouse liver caused by three periods of feeding a protein-free diet for 5 days followed by a normal complete diet for 5 days (3PFD-CD) are prevented by a constant methionine supply (3PFD+Met-CD). The expressions of carbonic anhydrase III (CAIII), fatty acid synthase (FAS), glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and glutathione S-transferase P1 (GSTP1) were assessed by proteomics and reverse transcriptase-polymerase chain reactions. The liver redox status was examined by measuring the activities of superoxide dismutase (SOD) and catalase (CAT), as well as protein carbonylation. Because oxidative stress can result in apoptosis, the activity and content of caspase-3, as well as the x-linked inhibitor of the apoptosis protein (XIAP) and mitochondrial caspase-independent apoptosis inducing factor (AIF) contents were assessed. In addition, the liver histomorphology was examined. Compared to the controls fed a normal complete diet throughout, feeding with 3PFD-CD increased the FAS content, decreased the CAIII content, decreased both the SOD and CAT activities, and increased protein carbonylation. It also activated caspase-3, decreased the XIAP content, decreased the AIF content, increased the number of GSTP1-positive foci and caspase-3-positive cells, and caused fatty livers. Conversely, the changes were lessened to varying degrees in mice fed 3PFD+Met-CD. The present results indicate that a regular Met supply lessens the biochemical changes, damage, and caspase-dependent apoptosis provoked by recurrent dietary amino acid deprivation in the mouse liver.

Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues

BACKGROUND & AIMS

The pathogenesis of liver fibrosis involves activation of hepatic stellate cells, which is associated with depletion of intracellular lipid droplets. When hepatocytes undergo autophagy, intracellular lipids are degraded in lysosomes. We investigated whether autophagy also promotes loss of lipids in hepatic stellate cells to provide energy for their activation and extended these findings to other fibrogenic cells.

METHODS

We analyzed hepatic stellate cells from C57BL/6 wild-type, Atg7(F/F), and Atg7(F/F)-GFAP-Cre mice, as well as the mouse stellate cell line JS1. Fibrosis was induced in mice using CCl(4) or thioacetamide (TAA); liver tissues and stellate cells were analyzed. Autophagy was blocked in fibrogenic cells from liver and other tissues using small interfering RNAs against Atg5 or Atg7 and chemical antagonists. Human pulmonary fibroblasts were isolated from samples of lung tissue from patients with idiopathic pulmonary fibrosis or from healthy donors.

RESULTS

In mice, induction of liver injury with CCl(4) or TAA increased levels of autophagy. We also observed features of autophagy in activated stellate cells within injured human liver tissue. Loss of autophagic function in cultured mouse stellate cells and in mice following injury reduced fibrogenesis and matrix accumulation; this effect was partially overcome by providing oleic acid as an energy substrate. Autophagy also regulated expression of fibrogenic genes in embryonic, lung, and renal fibroblasts.

CONCLUSIONS

Autophagy of activated stellate cells is required for hepatic fibrogenesis in mice. Selective reduction of autophagic activity in fibrogenic cells in liver and other tissues might be used to treat patients with fibrotic diseases.

Proteolytic and lipolytic responses to starvation

Mammals survive starvation by activating proteolysis and lipolysis in many different tissues. These responses are triggered, at least in part, by changing hormonal and neural statuses during starvation. Pathways of proteolysis that are activated during starvation are surprisingly diverse, depending on tissue type and duration of starvation. The ubiquitin-proteasome system is primarily responsible for increased skeletal muscle protein breakdown during starvation. However, in most other tissues, lysosomal pathways of proteolysis are stimulated during fasting. Short-term starvation activates macroautophagy, whereas long-term starvation activates chaperone-mediated autophagy. Lipolysis also increases in response to starvation, and the breakdown of triacylglycerols provides free fatty acids to be used as an energy source by skeletal muscle and other tissues. In addition, glycerol released from triacylglycerols can be converted to glucose by hepatic gluconeogenesis. During long-term starvation, oxidation of free fatty acids by the liver leads to the production of ketone bodies that can be used for energy by skeletal muscle and brain. Tissues that cannot use ketone bodies for energy respond to these small molecules by activating chaperone-mediated autophagy. This is one form of interaction between proteolytic and lipolytic responses to starvation.

Lysosomal dysfunction in muscle with special reference to glycogen storage disease type II

The importance of proper lysosomal activity in cell and tissue homeostasis is underlined by "experiments of nature", i.e. genetic defects in one of the at least 40 lysosomal enzymes/proteins present in the human cell. The complete lack of 1-4 alpha-glucosidase (glycogen storage disease type II (GSD II) or Pompe disease) is life-threatening. Patients suffering from GSD II commonly die before the age of 2 years because of cardiorespiratory insufficiency. Striated muscle cells appear to be particularly vulnerable in GSD II. The high cytoplasmic glycogen content in muscle cells most likely gives rise to a high rate of glycogen engulfment by the lysosomes. The polysaccharides become subsequently trapped in these organelles when 1-4 alpha-glucosidase activity is absent. During the course of the disease, muscle wasting occurs. It is hypothesised that the gradual loss of muscle mass is caused by a combination of disuse atrophy and lipofuscine-mediated apoptosis of myocytes. Moreover, we hypothesise that in the remaining skeletal muscle cells, longitudinal transmission of force is hampered by swollen lysosomes, clustering of non-contractile material and focal regions with degraded contractile proteins, which results in muscle weakness.

Age-related changes in the autophagic proteolysis of rat isolated liver cells: effects of antiaging dietary restrictions

Autophagy is a process that sequesters and degrades organelles and macromolecular constituents of cytoplasm for cellular restructuring and repair and as a source of nutrients for metabolic use in early starvation. The effects of two antiaging dietary regimens (initiated in rats at the age of 2 months), namely, 40% dietary restriction (DR) and every-other-day ad-libitum feeding, that exhibited different effects on metabolism and similar effects on longevity on the age-related changes in the regulation of autophagic proteolysis were studied by monitoring the rate of valine release in the incubation medium from isolated liver cells of male albino Sprague-Dawley rats aged 2, 6, 12, 18, 24, and 27 months. (The liver cells were incubated in vitro with added amino acids and 10(-7) M insulin or glucagon.) Age-matched male albino Sprague-Dawley rats fed ad libitum served as a control. Results show that in ad-libitum-fed rats, after a transient increase by age 6 months, autophagic proteolysis and regulation by amino acid exhibit a dramatic age-related decline, and that the age-related changes are prevented by dietary antiaging intervention. A comparison shows that the protective effects of DR and every-other-day ad-libitum feeding are partially different in 24-month-old rats (but the beneficial effects of the two diets on regulation of autophagic proteolysis are always similar). With regard to endocrine regulation, results confirm that the liver cell response to glucagon (but not to insulin) declines with increasing age, and they show that antiaging DRs significantly improve the effects of glucagon (and have no effect on the response to insulin). The interactions of age by diet, glucagon (and in older rats, insulin), and amino acids are significant. It is concluded that DR significantly improves the susceptibility of liver cells to lysosomal degradation, and it prevents decline with increasing age. It is suggested that improved liver autophagy and lysosomal degradation might be part of the antiaging mechanisms of DR.

Age-related changes in the regulation of autophagic proteolysis in rat isolated hepatocytes

During intervals between meals, autophagy is a major source of nutrients and may remove damaged organelles and membranes. Age-related changes in the regulation of autophagic proteolysis were studied by monitoring the rate of valine release from liver cells of 2-, 6-, 12-, 18-, and 24-month-old male Sprague-Dawley rats fed ad libitum, and incubated in vitro with added amino acids and 10(-7) M of insulin or glucagon. The maximum rate of proteolysis and its maximum inhibition by amino acids were reached at 6 months and declined thereafter. In contrast, the rate of protein degradation in the presence of high concentrations of amino acids was not affected by aging. The inhibitor effect of insulin was additive to that of amino acids and was not altered significantly by age. The conclusion is that altered regulation of autophagic proteolysis decreases susceptibility of older cells to lysosomal degradation, and it may lead to the accumulation of altered organelles and membranes.

Amino acid regulation of gene expression

The impact of nutrients on gene expression in mammals has become an important area of research. Nevertheless, the current understanding of the amino acid-dependent control of gene expression is limited. Because amino acids have multiple and important functions, their homoeostasis has to be finely maintained. However, amino-acidaemia can be affected by certain nutritional conditions or various forms of stress. It follows that mammals have to adjust several of their physiological functions involved in the adaptation to amino acid availability by regulating the expression of numerous genes. The aim of the present review is to examine the role of amino acids in regulating mammalian gene expression and protein turnover. It has been reported that some genes involved in the control of growth or amino acid metabolism are regulated by amino acid availability. For instance, limitation of several amino acids greatly increases the expression of the genes encoding insulin-like growth factor binding protein-1, CHOP (C/EBP homologous protein, where C/EBP is CCAAT/enhancer binding protein) and asparagine synthetase. Elevated mRNA levels result from both an increase in the rate of transcription and an increase in mRNA stability. Several observations suggest that the amino acid regulation of gene expression observed in mammalian cells and the general control process described in yeast share common features. Moreover, amino acid response elements have been characterized in the promoters of the CHOP and asparagine synthetase genes. Taken together, the results discussed in the present review demonstrate that amino acids, by themselves, can, in concert with hormones, play an important role in the control of gene expression.