General characteristics of protein degradation in diabetes and starvation

Neuroprotective effects of chaperone-mediated autophagy in neurodegenerative diseases

ABSTRACT

Chaperone-mediated autophagy is one of three types of autophagy and is characterized by the selective degradation of proteins. Chaperone-mediated autophagy contributes to energy balance and helps maintain cellular homeostasis, while providing nutrients and support for cell survival. Chaperone-mediated autophagy activity can be detected in almost all cells, including neurons. Owing to the extreme sensitivity of neurons to their environmental changes, maintaining neuronal homeostasis is critical for neuronal growth and survival. Chaperone-mediated autophagy dysfunction is closely related to central nervous system diseases. It has been shown that neuronal damage and cell death are accompanied by chaperone-mediated autophagy dysfunction. Under certain conditions, regulation of chaperone-mediated autophagy activity attenuates neurotoxicity. In this paper, we review the changes in chaperone-mediated autophagy in neurodegenerative diseases, brain injury, glioma, and autoimmune diseases. We also summarize the most recent research progress on chaperone-mediated autophagy regulation and discuss the potential of chaperone-mediated autophagy as a therapeutic target for central nervous system diseases.

Chaperone-mediated autophagy: Molecular mechanisms, biological functions, and diseases

Chaperone-mediated autophagy (CMA) is a lysosomal degradation pathway that eliminates substrate proteins through heat-shock cognate protein 70 recognition and lysosome-associated membrane protein type 2A-assisted translocation. It is distinct from macroautophagy and microautophagy. In recent years, the regulatory mechanisms of CMA have been gradually enriched, including the newly discovered NRF2 and p38-TFEB signaling, as positive and negative regulatory pathways of CMA, respectively. Normal CMA activity is involved in the regulation of metabolism, aging, immunity, cell cycle, and other physiological processes, while CMA dysfunction may be involved in the occurrence of neurodegenerative disorders, tumors, intestinal disorders, atherosclerosis, and so on, which provides potential targets for the treatment and prediction of related diseases. This article describes the general process of CMA and its role in physiological activities and summarizes the connection between CMA and macroautophagy. In addition, human diseases that concern the dysfunction or protective role of CMA are discussed. Our review deepens the understanding of the mechanisms and physiological functions of CMA and provides a summary of past CMA research and a vision of future directions.

Activation of Disulfide Redox Switch in REDD1 Promotes Oxidative Stress Under Hyperglycemic Conditions

The stress response protein regulated in development and DNA damage response 1 (REDD1) has been implicated in visual deficits in patients with diabetes. The aim here was to investigate the mechanism responsible for the increase in retinal REDD1 protein content that is observed with diabetes. We found that REDD1 protein expression was increased in the retina of streptozotocin-induced diabetic mice in the absence of a change in REDD1 mRNA abundance or ribosome association. Oral antioxidant supplementation reduced retinal oxidative stress and suppressed REDD1 protein expression in the retina of diabetic mice. In human retinal Müller cell cultures, hyperglycemic conditions increased oxidative stress, enhanced REDD1 expression, and inhibited REDD1 degradation independently of the proteasome. Hyperglycemic conditions promoted a redox-sensitive cross-strand disulfide bond in REDD1 at C150/C157 that was required for reduced REDD1 degradation. Discrete molecular dynamics simulations of REDD1 structure revealed allosteric regulation of a degron upon formation of the disulfide bond that disrupted lysosomal proteolysis of REDD1. REDD1 acetylation at K129 was required for REDD1 recognition by the cytosolic chaperone HSC70 and degradation by chaperone-mediated autophagy. Disruption of REDD1 allostery upon C150/C157 disulfide bond formation prevented the suppressive effect of hyperglycemic conditions on REDD1 degradation and reduced oxidative stress in cells exposed to hyperglycemic conditions. The results reveal redox regulation of REDD1 and demonstrate the role of a REDD1 disulfide switch in development of oxidative stress.

Exploring the Role of Skeletal Muscle in Insulin Resistance: Lessons from Cultured Cells to Animal Models

Skeletal muscle is essential to maintain vital functions such as movement, breathing, and thermogenesis, and it is now recognized as an endocrine organ. Muscles release factors named myokines, which can regulate several physiological processes. Moreover, skeletal muscle is particularly important in maintaining body homeostasis, since it is responsible for more than 75% of all insulin-mediated glucose disposal. Alterations of skeletal muscle differentiation and function, with subsequent dysfunctional expression and secretion of myokines, play a key role in the pathogenesis of obesity, type 2 diabetes, and other metabolic diseases, finally leading to cardiometabolic complications. Hence, a deeper understanding of the molecular mechanisms regulating skeletal muscle function related to energy metabolism is critical for novel strategies to treat and prevent insulin resistance and its cardiometabolic complications. This review will be focused on both cellular and animal models currently available for exploring skeletal muscle metabolism and endocrine function.

Inhibiting mTOR activity using AZD2014 increases autophagy in the mouse cerebral cortex

Autophagy is a catabolic process that collects and degrades damaged or unwanted cellular materials such as protein aggregates. Defective brain autophagy has been linked to diseases such as Alzheimer's disease. Autophagy is regulated by the protein kinase mTOR (mechanistic target of rapamycin). Although already demonstrated in vitro, it remains contentious whether inhibiting mTOR can enhance autophagy in the brain. To address this, mice were intraperitoneally injected with the mTOR inhibitor AZD2014 for seven days. mTOR complex 1 (mTORC1) activity was decreased in liver and brain. Autophagic activity was increased by AZD2014 in both organs, as measured by immunoblotting for LC3 (microtubule-associated proteins-1A/1B light chain 3B) and measurement of autophagic flux in the cerebral cortex of transgenic mice expressing the EGFP-mRFP-LC3B transgene. mTOR activity was shown to correlate with changes in LC3. Thus, we show it is possible to promote autophagy in the brain using AZD2014, which will be valuable in tackling conditions associated with defective autophagy, especially neurodegeneration.

Tracing the footsteps of autophagy in computational biology

Autophagy plays a crucial role in maintaining cellular homeostasis through the degradation of unwanted materials like damaged mitochondria and misfolded proteins. However, the contribution of autophagy toward a healthy cell environment is not only limited to the cleaning process. It also assists in protein synthesis when the system lacks the amino acids’ inflow from the extracellular environment due to diet consumptions. Reduction in the autophagy process is associated with diseases like cancer, diabetes, non-alcoholic steatohepatitis, etc., while uncontrolled autophagy may facilitate cell death. We need a better understanding of the autophagy processes and their regulatory mechanisms at various levels (molecules, cells, tissues). This demands a thorough understanding of the system with the help of mathematical and computational tools. The present review illuminates how systems biology approaches are being used for the study of the autophagy process. A comprehensive insight is provided on the application of computational methods involving mathematical modeling and network analysis in the autophagy process. Various mathematical models based on the system of differential equations for studying autophagy are covered here. We have also highlighted the significance of network analysis and machine learning in capturing the core regulatory machinery governing the autophagy process. We explored the available autophagic databases and related resources along with their attributes that are useful in investigating autophagy through computational methods. We conclude the article addressing the potential future perspective in this area, which might provide a more in-depth insight into the dynamics of autophagy.

White light emitting diode induces autophagy in hippocampal neuron cells through GSK-3-mediated GR and RORα pathways

Autophagy plays an important role in cell survival under diverse stress conditions. Here, we show that white LED light exposure for 24 h significantly activated autophagy-related genes and increased autophagosome formation in hippocampal neural cells (HT-22). Concurrently, the rhythmic pattern of clock-related gene expression was disrupted, which was associated with augmented expression of SIRT1, AMPK and retinoid-related orphan receptor alpha (RORα). SR1001, a specific inhibitor of RORα, protected the cells from light-induced activation of autophagy. Moreover, light exposure increased glucocorticoid receptor (GR) phosphorylation and nuclear translocation. GR inhibitor RU486 prevented light-induced up-regulation of RORα and the activation of autophagy. These changes were associated with enhanced glycogen synthase kinase-3 (GSK-3) activity and its specific inhibitor CHIR-99021 significantly rescued light-induced autophagy and augmented GR, RORα and autophagy-related proteins. Furthermore, GSK-3 was identified as an upstream regulator of GR/RORα signaling as it was not affected by GR or RORα inhibitors. Taken together, our data demonstrate that GSK-3-mediated GR/RORα signaling pathway is involved in white LED light-induced autophagy in hippocampal neuron cells.

Chaperone mediated autophagy in aging: Starve to prosper

The major lysosomal proteolytic pathways essential for maintaining proper cellular homeostasis are macroautophagy, chaperone-mediated autophagy (CMA) and microautophagy. What differentiates CMA from the other types of autophagy is the fact that it does not involve vesicle formation; the unique feature of this pathway is the selective targeting of substrate proteins containing a CMA-targeting motif and the direct translocation into the lysosomal lumen, through the aid of chaperones/co-chaperones localized both at the cytosol and the lysosomes. CMA operates at basal conditions in most mammalian cell models analyzed so far, but it is mostly activated in response to stressors, such as trophic deprivation or oxidative stress. The activity of CMA has been shown to decline with age and such decline, correlating with accumulation of damaged/oxidized/aggregated proteins, may contribute to tissue dysfunction and, possibly, neurodegeneration. Herein, we review the recent knowledge regarding the molecular components, regulation and physiology of the CMA pathway, the contribution of impaired CMA activity to poor cellular homeostasis and inefficient response to stress during aging, and discuss the therapeutic opportunities offered by the restoration of CMA-dependent proteolysis in age-associated degenerative diseases.

mTORC1-independent reduction of retinal protein synthesis in type 1 diabetes

Poorly controlled diabetes has long been known as a catabolic disorder with profound loss of muscle and fat body mass resulting from a simultaneous reduction in protein synthesis and enhanced protein degradation. By contrast, retinal structure is largely maintained during diabetes despite reduced Akt activity and increased rate of cell death. Therefore, we hypothesized that retinal protein turnover is regulated differently than in other insulin-sensitive tissues, such as skeletal muscle. Ins2(Akita) diabetic mice and streptozotocin-induced diabetic rats exhibited marked reductions in retinal protein synthesis matched by a concomitant reduction in retinal protein degradation associated with preserved retinal mass and protein content. The reduction in protein synthesis depended on both hyperglycemia and insulin deficiency, but protein degradation was only reversed by normalization of hyperglycemia. The reduction in protein synthesis was associated with diminished protein translation efficiency but, surprisingly, not with reduced activity of the mTORC1/S6K1/4E-BP1 pathway. Instead, diabetes induced a specific reduction of mTORC2 complex activity. These findings reveal distinctive responses of diabetes-induced retinal protein turnover compared with muscle and liver that may provide a new means to ameliorate diabetic retinopathy.

Regulation of autophagy by amino acid availability in S. cerevisiae and mammalian cells

Autophagy is a catabolic membrane-trafficking process that occurs in all eukaryotic organisms analyzed to date. The study of autophagy has exploded over the last decade or so, branching into numerous aspects of cellular and organismal physiology. From basic functions in starvation and quality control, autophagy has expanded into innate immunity, aging, neurological diseases, redox regulation, and ciliogenesis, to name a few roles. In the present review, I would like to narrow the discussion to the more classical roles of autophagy in supporting viability under nutrient limitation. My aim is to provide a semblance of a historical overview, together with a concise, and perhaps subjective, mechanistic and functional analysis of the central questions in the autophagy field.

Streptozotocin-induced diabetes mellitus affects lysosomal enzymes in rat liver

It has been previously shown that dextran sulfate administered to diabetic rats accumulates in the liver and kidney, and this could be due to a malfunction of the lysosomal digestive pathway. The aim of the present study was to evaluate the expression and activities of lysosomal enzymes that act upon proteins and sulfated polysaccharides in the livers of diabetic rats. Diabetes mellitus was induced by streptozotocin in 26 male Wistar rats (12 weeks old), while 26 age-matched controls received only vehicle. The livers were removed on either the 10^th or the 30^th day of the disease, weighed, and used to evaluate the activity, expression, and localization of lysosomal enzymes. A 50-60% decrease in the specific activities of cysteine proteases, especially cathepsin B, was observed in streptozotocin-induced diabetes mellitus. Expression (mRNA) of cathepsins B and L was also decreased on the 10^th, but not on the 30^th day. Sulfatase decreased 30% on the 30^th day, while glycosidases did not vary (or presented a transitory and slight decrease). There were no apparent changes in liver morphology, and immunohistochemistry revealed the presence of cathepsin B in hepatocyte granules. The decrease in sulfatase could be responsible for the dextran sulfate build-up in the diabetic liver, since the action of sulfatase precedes glycosidases in the digestive pathway of sulfated polysaccharides. Our findings suggest that the decreased activities of cathepsins resulted from decreased expression of their genes, and not from general lysosomal failure, because the levels of glycosidases were normal in the diabetic liver.

Effects of insulin resistance on skeletal muscle growth and exercise capacity in type 2 diabetic mouse models

Type 2 diabetes mellitus is associated with an accelerated muscle loss during aging, decreased muscle function, and increased disability. To better understand the mechanisms causing this muscle deterioration in type 2 diabetes, we assessed muscle weight, exercise capacity, and biochemistry in db/db and TallyHo mice at prediabetic and overtly diabetic ages. Maximum running speeds and muscle weights were already reduced in prediabetic db/db mice when compared with lean controls and more severely reduced in the overtly diabetic db/db mice. In contrast to db/db mice, TallyHo muscle size dramatically increased and maximum running speed was maintained during the progression from prediabetes to overt diabetes. Analysis of mechanisms that may contribute to decreased muscle weight in db/db mice demonstrated that insulin-dependent phosphorylation of enzymes that promote protein synthesis was severely blunted in db/db muscle. In addition, prediabetic (6-wk-old) and diabetic (12-wk-old) db/db muscle exhibited an increase in a marker of proteasomal protein degradation, the level of polyubiquitinated proteins. Chronic treadmill training of db/db mice improved glucose tolerance and exercise capacity, reduced markers of protein degradation, but only mildly increased muscle weight. The differences in muscle phenotype between these models of type 2 diabetes suggest that insulin resistance and chronic hyperglycemia alone are insufficient to rapidly decrease muscle size and function and that the effects of diabetes on muscle growth and function are animal model-dependent.

Historical landmarks of autophagy research

The year of 2013 marked the 50th anniversary of C de Duve's coining of the term "autophagy" for the degradation process of cytoplasmic constituents in the lysosome/vacuole. This year we regretfully lost this great scientist, who contributed much during the early years of research to the field of autophagy. Soon after the discovery of lysosomes by de Duve, electron microscopy revealed autophagy as a means of delivering intracellular components to the lysosome. For a long time after the discovery of autophagy, studies failed to yield any significant advances at a molecular level in our understanding of this fundamental pathway of degradation. The first breakthrough was made in the early 1990s, as autophagy was discovered in yeast subjected to starvation by microscopic observation. Next, a genetic effort to address the poorly understood problem of autophagy led to the discovery of many autophagy-defective mutants. Subsequent identification of autophagy-related genes in yeast revealed unique sets of molecules involved in membrane dynamics during autophagy. ATG homologs were subsequently found in various organisms, indicating that the fundamental mechanism of autophagy is well conserved among eukaryotes. These findings brought revolutionary changes to research in this field. For instance, the last 10 years have seen remarkable progress in our understanding of autophagy, not only in terms of the molecular mechanisms of autophagy, but also with regard to its broad physiological roles and relevance to health and disease. Now our knowledge of autophagy is dramatically expanding day by day. Here, the historical landmarks underpinning the explosion of autophagy research are described with a particular focus on the contribution of yeast as a model organism.

Chaperone-mediated autophagy: roles in disease and aging

This review focuses on chaperone-mediated autophagy (CMA), one of the proteolytic systems that contributes to degradation of intracellular proteins in lysosomes. CMA substrate proteins are selectively targeted to lysosomes and translocated into the lysosomal lumen through the coordinated action of chaperones located at both sides of the membrane and a dedicated protein translocation complex. The selectivity of CMA permits timed degradation of specific proteins with regulatory purposes supporting a modulatory role for CMA in enzymatic metabolic processes and subsets of the cellular transcriptional program. In addition, CMA contributes to cellular quality control through the removal of damaged or malfunctioning proteins. Here, we describe recent advances in the understanding of the molecular dynamics, regulation and physiology of CMA, and discuss the evidence in support of the contribution of CMA dysfunction to severe human disorders such as neurodegeneration and cancer.

Chaperones in autophagy

Cells continuously turn over proteins through cycles of synthesis and degradation in order to maintain a functional proteome and to exert a tight control in the levels of regulatory proteins. Selective degradation of proteins was initially thought to be an exclusive function of the ubiquitin-proteasome system, however, over the years, the contribution of lysosomes to this selective degradation, through the process of autophagy, has become consolidated. In this context, molecular chaperones, classically associated with protein folding, unfolding and assembling have been revealed as important modulators of selectivity during the autophagic process. Here, we review this relatively new role of chaperones in mediating selective autophagy and comment on how alterations of this function can lead to human pathologies associated to proteotoxicity.

Role of macroautophagy in nutrient homeostasis during fungal development and pathogenesis

Macroautophagy is a non-selective, bulk degradation process conserved in eukaryotes. Response to starvation stress and/or regulation of nutrient breakdown/utilization is the major intracellular function of macroautophagy. Recent studies have revealed requirement for autophagy in diverse functions such as nutrient homeostasis, organelle degradation and programmed cell death in filamentous fungal pathogens, for proper morphogenesis and differentiation during critical steps of infection. In this review, we aim to summarize the physiological functions of autophagy in fungal virulence, with an emphasis on nutrient homeostasis in opportunistic human fungal pathogens and in the rice-blast fungus, Magnaporthe oryzae. We briefly summarize the role of autophagy on the host side: for resistance to, or subversion by, the pathogens.

Autophagy, nutrition and immunology

Turnover of cellular components in lysosomes or autophagy is an essential mechanism for cellular quality control. Added to this cleaning role, autophagy has recently been shown to participate in the dynamic interaction of cells with the surrounding environment by acting as a point of integration of extracellular cues. In this review, we focus on the relationship between autophagy and two types of environmental factors: nutrients and pathogens. We describe their direct effect on autophagy and discuss how the autophagic reaction to these stimuli allows cells to accommodate the requirements of the cellular response to stress, including those specific to the immune responses.

Chaperone-mediated autophagy: Dice's 'wild' idea about lysosomal selectivity

A little over 1 year ago, we lost a bright scientist and a dear colleague who, in his younger years, proposed the 'heretical' idea that lysosomes could selectively degrade cytosolic proteins. That scientist was J. Fred Dice, and his lifetime's discovery was the degradative pathway that we now know as chaperone-mediated autophagy.

Coordination of protein synthesis and degradation

The degree of coordination between protein synthesis and protein degradation in developing and mature cels is considered. Studies on specific enzyme and general protein turnover in developing liver and differentiating mammary gland are presented. In the mature liver mitochondrion average protein degradation rates are higher for outer membrane and intermembrane space proteins than for matrix and inner membrane proteins. Significant heterogeneity of protein degradation rates was observed only in the outer mitochondrial membrane. During postnatal development the rates of degradation of proteins in many liver cellular fractions are increased. In the mitochondrion only the average rates of degradation of proteins in the outer membrane and intermembrane space fractions increase during development. Evidence for hormonally regulated changes in both protein synthesis and degradation during mammary cell differentiation is given. The data indicate that a transitory decrease in protein degradation accompanies the increase in protein synthesis on hormonal stimulation of the tissue. The results from the two model systems are collated and used to formulate a phenomenological hypothesis of protein degradation and its integration with protein synthesis in steady-state and non-steady-state conditions.

Protein degradation in metabolic and nutritional disorders

The increased protein degradation associated with diabetes appears to be different in many respects from protein catabolism in normal, well-nourished cells. In all normal eukaryotic cells examined, degradation of cytosolic proteins exhibits several striking features. Large proteins tend to be degraded more rapidly than small proteins, acidic proteins tend to be degraded more rapidly than neutral or basic proteins, and glycoproteins are degraded more rapidly than non-glycoproteins. Furthermore, a general correlation exists between protein half-life in vivo and susceptibility to proteolytic attack in vitro. In streptozotocin-diabetic rats the relationships between degradative rate and protein size, net charge, and carbohydrate content are absent or markedly reduced among cytosolic proteins of the liver. However, the correlation between protein half-life and susceptibility to proteinase in vitro is unaltered. Therefore, the enhanced protein degradation in diabetes shows little or no selectivity towards large, acidic, glycoproteins but does show specificity for proteins than tend to be sensitive to proteinases. Similar studies using other tissues from diabetic rats are reported and general characteristics of the enhanced liver protein catabolism in starvation and hyperthyroidism are briefly discussed. The biochemical reasons for the increased protein catabolism in diabetes are unclear although several possible explanations are presented. The enhanced breakdown is probably not due to cellular proteins becoming more proteinase sensitive in diabetes since experiments with a variety of endoproteinases including pronase, chymotrypsin, pepsin, and lysosomal cathepsins have failed to demonstrate more rapid digestion of liver proteins from diabetic animals.

Effects of streptozotocin-induced diabetes on rough endoplasmic reticulum and lysosomes of rat liver

In the absence of amino acids and insulin, ribosome-free regions of the rough endoplasmic reticulum (RER) invaginate to form an autophagosome, which matures into an autolysosome (W. A. Dunn, Jr., J. Cell Biol. 110: 1923-1933, 1990). In this study, biochemical and morphological methods were used to examine the structure and integrity of the RER and the lysosome-vacuolar system in livers of untreated (normal serum insulin) and streptozotocin (STZ)-treated (depressed serum insulin) fed and fasted rats. Degradation of endogenous proteins was increased by 70% in STZ-treated animals. Proteolysis was further enhanced when these animals were deprived of food for 24 h. These alterations in protein turnover were accompanied by increases in the fractional volume of autophagic vacuoles and in the hepatic amounts of three lysosomal hydrolases. These effects of STZ were prevented on administration of insulin. In addition, there was an insulin-dependent 50% loss of RER surface area in livers from STZ-treated rats. This loss of structural RER was accompanied by comparable decreases in the cellular amounts of two RER membrane proteins and one luminal protein, suggesting that the RER was degraded as a unit. Additional losses of RER were observed when STZ-treated rats were fasted. Furthermore, the hepatic amounts of two serum proteins decreased, suggesting the functional capacity of the RER was reduced. Combined, the data suggest that in STZ-induced diabetes the losses in RER are related to enhanced autophagy.

System A transport activity is stimulated in skeletal muscle in response to diabetes

We have studied the activity of system A transport in skeletal muscle during experimental diabetes. Five days after streptozotocin injection, rats showed a marked hyperglycemia and a substantial decrease in the content of GLUT-4 protein in skeletal muscle and adipose tissue. Under these conditions, basal uptake of 2-(methyl)aminoisobutyric acid (MeAIB), an index of system A transport activity, was enhanced in extensor digitorum longus (EDL) muscles from diabetic rats compared to controls. Furthermore, insulin-stimulated MeAIB uptake by the incubated EDL and soleus muscles was markedly greater in diabetic than in control rats. The derepressive phase of adaptive regulation was partially blocked in the diabetic muscle, and incubation of muscles for 3 h in the absence of amino acids led to a lower stimulation of system A transport activity in muscles from diabetic groups compared to controls. We propose that the activated system A might participate in the enhanced alanine release from muscle cells that occurs in diabetes.

Dietary omega 3 fatty acid alters prostaglandin synthesis, glucose transport and protein turnover in skeletal muscle of healthy and diabetic rats

The present study was designed to determine if dietary-fat-induced alterations in the fatty acid composition of skeletal-muscle lipid alters insulin-dependent and basal muscle metabolism, including glucose and amino acid transport, prostaglandin (PG) synthesis and protein turnover. Rats were fed on high-fat semi-purified diets providing 19% or 1% omega 3 fatty acids in the form of fish oil, for 6 weeks. After 3 weeks, half of the rats were made diabetic by a single injection of streptozotocin (50 mg/kg body wt.). After a further 3 weeks, contralateral epitrochlearis and extensor digitorum longus (EDL) muscles from each rat were incubated in vitro. High levels of dietary omega 3 fatty acids decreased PGE2 and PGF2 alpha synthesis in EDL and epitrochlearis muscle (P less than 0.0001). Diabetes and insulin had no effect on PG synthesis. Diet did not alter basal glucose or amino acid transport in EDL muscle from healthy or diabetic rats. Insulin increased glucose and amino acid transport (P less than 0.0001); the increase in glucose transport by insulin was significantly greater in muscles of rats fed on high levels of omega 3 fatty acids (P less than 0.05). Epitrochlearis from rats fed on high levels of omega 3 fatty acids showed decreased net protein degradation in the presence and absence of insulin, owing to decreased rates of protein degradation and synthesis. The data suggest that high levels of dietary omega 3 fatty acids that alter muscle membrane composition also result in alterations in glucose transport and the metabolism of muscle protein.

Effect of osmotic stress on protein turnover in Lemna minor fronds

Evidence is presented that although many proteins from the fronds of Lemna minor L. undergo enhanced degradation during osmotic stress, ribulose-1,5-bisphosphate carboxylase (RuBPCase) is not degraded. Instead RuBPCase is converted in a series of steps to a very high-molecular-weight form. The first step involves the induction of an oxidase system which after 24 h of stress converts RuBPCase to an acidic and catalytically inactive form. Subsequently, the oxidised RuBPCase protein is gradually polymerized to a number of very large aggregates (molecular weight of several million).The conversion of RuBPCase to a high-molecular-weight form appears to be correlated with (i) a reduction in the number of-SH residues and (ii) the susceptibility to in-vitro proteolysis. Indeed, the number of-SH groups per RuBPCase molecule decreases from 89 in the native enzyme to 54 and 22 in the oxidised and polymerized forms, respectively. On the other hand, the oxidised enzyme is more susceptible to in-vitro proteolysis than the native form. However, it is the polymerized form of RuBPCase which is particularly susceptible to in-vitro proteolysis.Western-blotting experiments and anti-ubiquitin antibodies were used to detect the presence of ubiquitin conjugates in extracts from osmotically stressed Lemna fronds. The possible involvement of ubiquitin in the formation of the aggregates is discussed.

Effect of streptozotocin-diabetes on rat liver mitochondrial adenosine triphosphatase turnover

The apparent turnover rates of some mitochondrial enzymes can be modified in diabetes. We studied the effect of streptozotocin-diabetes on the half-life of a protein tightly bound to the inner membrane, ATPase. The half-life (t 1/2), measured by the double-isotope technique, decreased by approx. 20% in diabetes (from approximately equal to 2.56 days in controls to approximately equal to 2.06 days in diabetic rats). These results suggest that diabetes produces an increase in degradation of ATPase by a mechanism which is not yet clear, possibly influenced by alterations induced by diabetes in hepatic lysosomes that are associated with hepatic autophagy.

Proteinases in catabolism and malnutrition

Protein synthesis and degradation are particularly sensitive to malnutrition and catabolic states. Intracellular protein degradation is determined by the conformation, molecular weight, isoelectric point, and carbohydrate content of the proteins. ATP-stimulated endoproteases appear to catalyse the rate-limiting steps. In the liver, proteolysis is reduced by amino acids and/or insulin, whereas glucagon stimulates protein degradation, probably due to depletion of intracellular gluconeogenic amino acids. In the muscle, protein degradation is promoted by interleukin-1 and inhibited by Ep-475, which specifically inactivates cathepsin B,H, and L. Myofibrillar alkaline proteinase activity increases postoperatively and in patients suffering from malignant tumors, whereas normal proteinase values were observed in these patients following total parenteral nutrition. Increased alkaline proteinase activity is also observed in diabetes mellitus and is normalized by insulin. Extracellular proteolysis has been reported in patients with hypercatabolic acute renal failure and in patients with sepsis or acute pancreatitis. Plasma fractions obtained from hypercatabolic patients with postoperative acute renal failure were proteolytic. Plasma proteinase activity decreases during hemodialysis due to elimination of a metallo-proteinase. Plasma alpha 2-macroglobulin decreases in patients with acute renal failure and also during acute pancreatitis. Proteolytic degradation of parathyroid hormone by sera obtained from patients with acute pancreatitis has been observed. Also, there is a decrease of high molecular weight kininogen during experimental acute pancreatitis. Granulocyte elastase increases postoperatively, mainly in patients with sepsis. Sepsis also causes increased proteolytic activity in the urine. In conclusion, intracellular protein degradation can supply important precursors for hepatic and renal gluconeogenesis during malnutrition.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein Degradation in Lemna with Particular Reference to Ribulose Bisphosphate Carboxylase: II. The Effect of Nutrient Starvation

The concept of ribulose bisphosphate carboxylase as a storage protein is not supported in the case of Lemna minor, where the enzyme appears to be particularly stable under conditions of nitrogen starvation. Total nutrient starvation in light and in the dark induced the degradation of this enzyme, but not at an enhanced rate compared with other leaf proteins and, surprisingly, darkness inhibited the degradation of chlorophyll which occurs with total nutrient starvation in the light. The data suggest that Lemna is not programmed to senesce in response to nutrient starvation. Differences in the pattern of protein degradation, which occurred under the stress conditions employed, are not consistent with a simple model of protein degradation in which the degradative system is assumed to be located in the vacuole. The data is best explained by a dual system in which cytosolic proteins are degraded by a vacuolar/lysosomal system and chloroplast proteins are degraded within the chloroplast. Whatever the system of degradation, our data do not support the proposed correlation between the rate of protein degradation and either protein charge or size.

Protein degradation in lemna with particular reference to ribulose bisphosphate carboxylase: I. The effect of light and dark

Ribulose bisphosphate carboxylase from Lemna minor resembles the structure reported for the enzyme from other plants. When grown in the light, the enzyme appears to undergo little or no degradation, as measured by a double-isotope method. This situation is similar to that reported for wheat and barley, but is unlike that reported for maize, where the enzyme degrades at the same rate as total protein. Prolonged periods of darkness usually induce leaf senescence, characterized by the rapid degradation of chlorophyll and protein, with ribulose bisphosphate carboxylase undergoing preferential degradation. In L. minor there is selective protein degradation in the dark, but chlorophyll and ribulose bisphosphate carboxylase are stable when fronds are kept in the darkness for up to 8 days. It appears that Lemna is not programmed to senesce, or at least that darkness does not induce senescence in Lemna. Although there is no evidence for in vivo degradation or modification of ribulose bisphosphate carboxylase during prolonged periods of darkness, extracts from fronds which have been kept in the dark for periods in excess of 24 hours convert ribulose bisphosphate carboxylase to a more acidic form. The properties of the dark-induced system which acts on ribulose bisphosphate carboxylase, suggest that it may be a mixed function oxidase. The proposition that the selectivity of protein degradation is genetically determined, so that the rate at which a protein is degraded is determined by its charge or size, was tested for fronds grown in the light or maintained in the dark. There was no significant correlation between protein degradation and either charge or size, in light or dark.

Regulation of Ca2+-dependent protein turnover in skeletal muscle by thyroxine

Dantrolene, an agent that inhibits Ca2+ mobilization, improved protein balance in skeletal muscle, as thyroid status was increased, by altering rates of protein synthesis and degradation. Thyroxine (T4) caused increases in protein degradation that were blocked by leupeptin, a proteinase inhibitor previously shown to inhibit Ca2+-dependent non-lysosomal proteolysis in these muscles. In addition, T4 abolished sensitivity to the lysosomotropic agent methylamine and the autophagy inhibitor 3-methyladenine, suggesting that T4 inhibits autophagic/lysosomal proteolysis.

Urinary profiles of organic acids and volatile metabolites during the starvation process in rats

Capillary gas chromatographic procedures were used to quantify the volatile and acidic compound profiles in the urinary samples of Sprague-Dawley rats during the starvation and refeeding periods. Numerous metabolites, identified through mass spectrometry, showed significant variations due to these physiological processes. Correlations are attempted with the previously studied biochemical processes in diabetic animals.

Amino acid incorporation in relation to molecular weight of proteins in young and adult brain

Rates of protein synthesis were studied in immature and adult rat brain tissue. After an amino acid incorporation period, in vivo or in incubated slices from brain, the soluble protein was fractionated according to molecular weight by column chromatography. In examining soluble whole proteins, no direct correlation between molecular weights and synthesis rates could be established; the highest synthesis rates were found in fractions around 70,000 MW and below 10,000. Incorporation into the subunits after fractionation by SDS gel electrophoresis was proportional to subunit molecular weight, with rates of incorporation into the largest subunits being the highest. The results suggest a relationship between turnover rate and structure of subunits of brain proteins.

Reduced serum proteins in diabetic children on a twice-daily insulin schedule

The concentrations of selected proteins of transport and nutrition were investigated in 27 diabetic children and 13 healthy controls by an immunodiffusion technique. The diabetic children had significantly lower mean pre-albumin (p less than 0.001), albumin (p less than 0.01) and orosomucoid levels (p less than 0.05) than the healthy controls. No correlation was observed between age or sex and the blood concentrations of the specific proteins analyzed in this series. Haptoglobin and hemopexin showed positive correlations with serum triglycerides (both p less than 0.01) and slight positive correlations with some of the variables of carbohydrate control. The reduced levels of prealbumin and albumin were not correlated to diabetic control as measured by HbA1, fasting plasma glucose or urinary glucose excretion. The non-physiological distribution site and the abnormal temporal pattern of insulin offered to diabetic children might be the reason for the protein abnormalities found in this study. This is seemingly the first time reduced serum levels of proteins have been reported in diabetic children.

Effects of hypothyroidism on the DNA, carbohydrate, soluble protein and sialic acid contents of rat submandibular glands

Submandibular glands are a target organ of thyroid hormones. This study examined the effects of hypothyroidism on the biochemical characteristics of these glands in the rat. There were no effects on the neutral sugar and DNA contents. However, soluble protein concentrations (micrograms/mg wet weight) were significantly decreased and sialic acid concentrations micrograms/mg soluble protein) were significantly elevated.