Computer simulation of ESR signals of liver paramagnetic centers

Changes of paramagnetic centres concentration characterized by g-factors values of 1.94, 2.2, and 2.03 in the rat liver were studied by ESR method under acute intoxication by diethylnitrosamine (DENA) and at preliminary threefold treatment of animals with butylhydroxytoluene (BHT). A protective effect of BHT can be explained by its stabilizing action of the membrane structures. A comparison has been carried out with a similar study of paramagnetic centres in the experiment of chronic intoxication by DENA. A simulation was performed of the liver tissue ESR spectra by means of special computer program. The parameters of simulated ESR spectra of the liver tissue with due regard for ESR signal g 2.03 corresponded to the parameters of the experimental spectra. Confirmations were obtained for the nature and number of paramagnetic centres in the liver tissue.     

The contribution of afferent signals from the liver to metabolic regulation during exercise

The crucial role of the liver as the only organ to produce glucose used by skeletal muscle during exercise is well known. Since hepatic glucose production is central to blood glucose homeostasis during exercise, it has been postulated that the liver may inform the central nervous system and other organs of its diminishing capacity to produce glucose from glycogen, before blood glucose falls. The sensory role of the liver during exercise would be similar to its role in the control of food intake. As a consequence, the experimental approaches used to test the hypothesis that afferent signals from the liver contribute to metabolic regulation during exercise are inspired by those used to test the same hypothesis in the regulation of food intake. In the present review, two questions are addressed. The existing evidence for the liver's sensory influence on metabolic adjustments to exercise is first reviewed; the nature of the initiating stimuli for the afferent contribution of the liver to physical exercise is discussed thereafter. The hypothetical construct upon which rests the contribution of the liver's afferent signals to metabolic regulation during exercise is that a decrease in liver glycogen or a related metabolic intermediate is sensed by the liver, and the signal is transduced to the central nervous system, most likely through the afferent activity of the hepatic vagus nerve, where it contributes to the orchestration of the metabolic and hormonal responses to exercise. Support in favour of this construct comes mainly from the demonstration that sectioning of the hepatic vagus nerve attenuates the normal hormonal response to exercise. It seems that the liver-glucagon axis is particularly responsive to this reflex activation. In other respects, the hepatic mechanism responsible for linking the metabolic activity in the liver to an afferent signal capable of regulating the metabolic response to exercise remains speculative. Substrates or derivatives of substrate oxidation, energy-related compounds (ATP and Pi), or changes in cell volume may all be related to changes in transmembrane potential in the liver cell, which according to the "potentiostatic" theory would determine the afferent vagal activity.     

Acetyl glutamate--a model of signals for intracellular proteolysis

The proteolysis at neutral pH of mitochondria from liver and brain is more marked in isolated preparations than "in vivo" indicating activation of proteases or inactivation of repressors during isolation. Acetyl glutamate (AG), found in liver mitochondria of ureotelic animals, plays a crucial role as activator of carbamylphosphate synthetase. Since AG levels change under a number of conditions, we checked for an AG deacylase in mitochondria, for otherwise AG must be exported and destroyed by cytosol deacylases. We noted on incubation of mitochondrial extracts with AG an increase in trichloracetic acid-soluble ninhydrin-reacting material but not in acetate liberation, indicating activation of proteases. This was checked with 14C-labelled mitochondria. Under certain conditions AG and other acyl aminoacids stimulate approximately 5 to 20% the proteolysis with rat live and with brain mitochondria.     

Endothelial signals modulate hepatocyte apicobasal polarization in zebrafish

Emerging evidence indicates that paracrine signals from endothelial cells play a role in tissue differentiation and organ formation [1-3]. Here, we identify a novel role for endothelial cells in modulating hepatocyte polarization during liver organogenesis. We find that in zebrafish, the apical domain of the hepatocytes predicts the location of the intrahepatic biliary network. The refinement of hepatocyte polarization coincides with the invasion of endothelial cells into the liver, and these endothelial cells migrate along the maturing basal surface of the hepatocytes. Using genetic, pharmacological, and transplantation experiments, we provide evidence that endothelial cells influence the polarization of the adjacent hepatocytes. This influence of endothelial cells on hepatocytes is mediated at least in part by the cell-surface protein Heart of glass and contributes to the establishment of coordinately aligned hepatocyte apical membranes and evenly spaced intrahepatic conduits.     

Shp2 controls female body weight and energy balance by integrating leptin and estrogen signals

In mammals, leptin regulates food intake and energy balance mainly through the activation of LepRb in the hypothalamus, and estrogen has a leptin-like effect in the hypothalamic control of metabolism. However, it remains to be elucidated how estrogen signaling is intertwined with the leptin pathway. We show here that Shp2, a nonreceptor tyrosine phosphatase, acts to integrate leptin and estrogen signals. The expression of a dominant-active mutant (Shp2(D61A)) in forebrain neurons conferred female, but not male, transgenic mice resistance to high-fat diet (HFD)-induced obesity and liver steatosis, accompanied by improved insulin sensitivity and glucose homeostasis. Fed with either HFD or regular chow food, Shp2(D61A) female mice showed dramatically enhanced leptin sensitivity. Microinjection of Shp2(D61A)-expressing adeno-associated virus into mediobasal hypothalamus elicited a similar antiobese effect in female mice. Biochemical analyses showed a physical association of Shp2 with estrogen receptor alpha, which is necessary for the synergistic and persistent activation of Erk by leptin and estrogen. Together, these results elucidate a mechanism for the direct cross talk of leptin and estrogen signaling and offer one explanation for the propensity of postmenopausal women to develop obesity.     

Pancreatic signals controlling food intake; insulin, glucagon and amylin

The control of food intake and body weight by the brain relies upon the detection and integration of signals reflecting energy stores and fluxes, and their interaction with many different inputs related to food palatability and gastrointestinal handling as well as social, emotional, circadian, habitual and other situational factors. This review focuses upon the role of hormones secreted by the endocrine pancreas: hormones, which individually and collectively influence food intake, with an emphasis upon insulin, glucagon and amylin. Insulin and amylin are co-secreted by B-cells and provide a signal that reflects both circulating energy in the form of glucose and stored energy in the form of visceral adipose tissue. Insulin acts directly at the liver to suppress the synthesis and secretion of glucose, and some plasma insulin is transported into the brain and especially the mediobasal hypothalamus where it elicits a net catabolic response, particularly reduced food intake and loss of body weight. Amylin reduces meal size by stimulating neurons in the hindbrain, and there is evidence that amylin additionally functions as an adiposity signal controlling body weight as well as meal size. Glucagon is secreted from A-cells and increases glucose secretion from the liver. Glucagon acts in the liver to reduce meal size, the signal being relayed to the brain via the vagus nerves. To summarize, hormones of the endocrine pancreas are collectively at the crossroads of many aspects of energy homeostasis. Glucagon and amylin act in the short term to reduce meal size, and insulin sensitizes the brain to short-term meal-generated satiety signals; and insulin and perhaps amylin as well act over longer intervals to modulate the amount of fat maintained and defended by the brain. Hormones of the endocrine pancreas interact with receptors at many points along the gut-brain axis, from the liver to the sensory vagus nerve to the hindbrain to the hypothalamus; and their signals are conveyed both neurally and humorally. Finally, their actions include gastrointestinal and metabolic as well as behavioural effects.     

Intracellular targetting signals of polymeric immunoglobulin receptors are highly conserved between species

A rat liver cDNA library, constructed in the plasmid expression vector pUEX, was immunoscreened using a rabbit polyclonal antiserum raised against rat liver Golgi membrane proteins. A sub-set of isolated clones were shown to encode the rat polymeric immunoglobulin receptor (pIgR). DNA sequence analysis of these clones provided the complete coding sequence of rat pIgR. Subsequent alignment of rat, rabbit and human predicted amino acid sequences demonstrated that the greatest degree of homology between the three pIgRs lies in their cytoplasmic tails; a region previously shown to be important for correct targetting and trancytosis of rabbit pIgR [(1984) Nature 308, 37-43].     

Inducing the liver: Understanding the signals that promote murine liver budding

The endoderm emerges as an epithelial sheet that covers the surface of the developing murine embryo. This tissue will produce the entire gut tube as well as associated digestive and respiratory organs including the thyroid, thymus, lung, liver, and pancreas. The emergence of each endodermal organ occurs in a temporally distinct manner that is dependant upon reciprocal inductive interactions between the endoderm and the underlying mesoderm. The emergence of the hepatic endoderm, which occurs using a morphological process termed liver budding, initiates during early somitogenesis in the mouse at approximately 8.25 days post‐coitum (dpc). Explant and transplant studies performed in chicken and mouse have demonstrated that secreted signals from adjacent mesodermal tissues initiate the hepatic gene program from ventral‐fated endoderm. Here, we review the data in support of the roles of members of the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), and Wnt signaling pathways in liver budding and discover that little is known about the precise endogenous signals involved in the molecular and morphological induction of liver budding in the mouse. J. Cell. Physiol. 226: 1727–1731, 2011. © 2010 Wiley‐Liss, Inc.     

Transgenic insulin released from G cells preferentially signals in the liver

We have previously produced transgenic G-InsKi mice, a model allowing regulated portal insulin delivery from gastric G cells without using beta cells. Here, we report that in G-InsKi mice portal levels of transgenic human insulin are 6-fold higher than in peripheral circulation. Peptone-induced release of transgenic human insulin from G cells preferentially stimulated signaling cascades in the liver rather than in peripheral insulin-sensitive tissues, as judged by tyrosine phosphorylation of insulin receptor beta subunit and phosphorylation of protein kinase Akt/PKB at Thr-308. G-InsKi mice provide a novel animal model for elucidating direct effects of insulin on liver functions.     

Subplasma membrane Ca2+ signals

Ca(2+) may selectively activate various processes in part by the cell's ability to localize changes in the concentration of the ion to specific subcellular sites. Interestingly, these Ca(2+) signals begin most often at the plasma membrane space so that understanding subplasma membrane signals is central to an appreciation of local signaling. Several experimental procedures have been developed to study Ca(2+) signals near the plasma membrane, but probably the most prevalent involve the use of fluorescent Ca(2+) indicators and fall into two general approaches. In the first, the Ca(2+) indicators themselves are specifically targeted to the subplasma membrane space to measure Ca(2+) only there. Alternatively, the indicators are allowed to be dispersed throughout the cytoplasm, but the fluorescence emanating from the Ca(2+) signals at the subplasma membrane space is selectively measured using high resolution imaging procedures. Although the targeted indicators offer an immediate appeal because of selectivity and ease of use, their limited dynamic range and slow response to changes in Ca(2+) are a shortcoming. Use of targeted indicators is also largely restricted to cultured cells. High resolution imaging applied with rapidly responding small molecule Ca(2+) indicators can be used in all cells and offers significant improvements in dynamic range and speed of response of the indicator. The approach is technically difficult, however, and realistic calibration of signals is not possible. In this review, a brief overview of local subplasma membrane Ca(2+) signals and methods for their measurement is provided.     

The Gata5 target, TGIF2, defines the pancreatic region by modulating BMP signals within the endoderm

Mechanisms underlying regional specification of distinct organ precursors within the endoderm, including the liver and pancreas, are still poorly understood. This is particularly true for stages between endoderm formation and the initiation of organogenesis. In this report, we have investigated these intermediate steps downstream of the early endodermal factor Gata5, which progressively lead to the induction of pancreatic fate. We have identified TGIF2 as a novel Gata5 target and demonstrate its function in the establishment of the pancreatic region within dorsal endoderm in Xenopus. TGIF2 acts primarily by restricting BMP signaling in the endoderm to allow pancreatic formation. Consistently, we found that blocking BMP signaling by independent means also perturbs the establishment of pancreatic identity in the endoderm. Previous findings demonstrated a crucial role for BMP signaling in determining dorsal/ventral fates in ectoderm and mesoderm. Our results now extend this trend to the endoderm and identify TGIF2 as the molecular link between dorsoventral patterning of the endoderm and pancreatic specification.     

Smad phosphoisoform signals in acute and chronic liver injury: similarities and differences between epithelial and mesenchymal cells

Hepatocellular carcinoma (HCC) usually arises from hepatic fibrosis caused by chronic inflammation. In chronic liver damage, hepatic stellate cells undergo progressive activation to myofibroblasts (MFB), which are important extracellular-matrix-producing mesenchymal cells. Concomitantly, perturbation of transforming growth factor (TGF)-β signaling by pro-inflammatory cytokines in the epithelial cells of the liver (hepatocytes) promotes both fibrogenesis and carcinogenesis (fibro-carcinogenesis). Insights into fibro-carcinogenic effects on chronically damaged hepatocytes have come from recent detailed analyses of the TGF-β signaling process. Smad proteins, which convey signals from TGF-β receptors to the nucleus, have intermediate linker regions between conserved Mad homology (MH) 1 and MH2 domains. TGF-β type I receptor and pro-inflammatory cytokine-activated kinases differentially phosphorylate Smad2 and Smad3 to create phosphoisoforms phosphorylated at the COOH-terminal, linker, or both (L/C) regions. After acute liver injury, TGF-β-mediated pSmad3C signaling terminates hepatocytic proliferation induced by the pro-inflammatory cytokine-mediated mitogenic pSmad3L pathway; TGF-β and pro-inflammatory cytokines synergistically enhance collagen synthesis by activated hepatic stellate cells via pSmad2L/C and pSmad3L/C pathways. During chronic liver disease progression, pre-neoplastic hepatocytes persistently affected by TGF-β together with pro-inflammatory cytokines come to exhibit the same carcinogenic (mitogenic) pSmad3L and fibrogenic pSmad2L/C signaling as do MFB, thereby accelerating liver fibrosis while increasing risk of HCC. This review of Smad phosphoisoform-mediated signals examines similarities and differences between epithelial and mesenchymal cells in acute and chronic liver injuries and considers Smad linker phosphorylation as a potential target for the chemoprevention of fibro-carcinogenesis.     

锌对缺血／再灌注肝脏自由基含量和细胞凋亡的影响

目的：观察补锌对缺血再灌注（HIR）大鼠肝脏自由基含量及细胞凋亡的影响。探讨补锌保护肝损伤的机制。方法：用荧光分光光度法测定血清MDA含量;用电子自旋共振法测定肝脏自由基浓度;用流式细胞术检测肝细胞凋亡。结果：HIR组大鼠血清MDA水平和肝自由基产生均增加,补锌后降低;肝脏缺血再灌注后肝细胞凋亡率达到57.72%,补锌后降低40.85%。结论：减少自由基产生和抑制细胞凋亡是锌保护肝缺血再灌注损伤的重要机制。     

Decoding Ca2+ signals in plants

Different input signals create their own characteristic Ca2+ fingerprints. These fingerprints are distinguished by frequency, amplitude, duration, and number of Ca2+ oscillations. Ca(2+)-binding proteins and protein kinases decode these complex Ca2+ fingerprints through conformational coupling and covalent modifications of proteins. This decoding of signals can lead to a physiological response with or without changes in gene expression. In plants, Ca(2+)-dependent protein kinases and Ca2+/calmodulin-dependent protein kinases are involved in decoding Ca2+ signals into phosphorylation signals. This review summarizes the elements of conformational coupling and molecular mechanisms of regulation of the two groups of protein kinases by Ca2+ and Ca2+/calmodulin in plants.     

Sphingosine 1-phosphate triggers both apoptotic and survival signals for human hepatic myofibroblasts

Hepatic myofibroblasts (hMFs) are central in the development of liver fibrosis during chronic liver diseases, and their removal by apoptosis contributes to the resolution of liver fibrosis. We previously identified Edg receptors for sphingosine 1-phosphate (S1P) in human hMFs. Here, we investigated the effects of S1P on hMF apoptosis. S1P reduced viability of serum-deprived hMFs by an apoptotic process that was unrelated to the conversion of S1P into sphingosine and ceramide. The apoptotic effects of S1P were receptor-independent because dihydro-S1P, an Edg agonist, had no effect. S1P also stimulated a receptor-dependent survival pathway, revealed by enhanced activation of caspase-3 by S1P in the presence of pertussis toxin. Cell survival relied on two pertussis toxin-sensitive events, activation of ERK and activation of phosphatidylinositol 3-kinase (PI3K)/Akt by S1P. Both pathways were also activated by dihydro-S1P. Blunting either ERK or PI3K enhanced caspase-3 stimulation by S1P, and simultaneous inhibition of both pathways resulted in additive effects on caspase-3 activation. In conclusion, S1P induces apoptosis of human hMFs via a receptor-independent mechanism and stimulates a survival pathway following activation of Edg receptors. The survival pathway arises from the sequential activation of G(i)/G(o) proteins and independent stimulations of ERK and PI3K/Akt. Therefore, blocking Edg receptors may sensitize hepatic myofibroblasts to apoptosis by S1P.     

USP2a protein deubiquitinates and stabilizes the circadian protein CRY1 in response to inflammatory signals

The mammalian circadian clock coordinates various physiological activities with environmental cues to achieve optimal adaptation. The clock manifests oscillations of key clock proteins, which are under dynamic control at multiple post-translational levels. As a major post-translational regulator, the ubiquitination-dependent proteasome degradation system is counterbalanced by a large group of deubiquitin proteases with distinct substrate preference. Until now, whether deubiquitination by ubiquitin-specific proteases can regulate the clock protein stability and circadian pathways remains largely unclear. The mammalian clock protein, cryptochrome 1 (CRY1), is degraded via the FBXL3-mediated ubiquitination pathway, suggesting that it is also likely to be targeted by the deubiquitination pathway. Here, we identified that USP2a, a circadian-controlled deubiquitinating enzyme, interacts with CRY1 and enhances its protein stability via deubiquitination upon serum shock. Depletion of Usp2a by shRNA greatly enhances the ubiquitination of CRY1 and dampens the oscillation amplitude of the CRY1 protein during a circadian cycle. By stabilizing the CRY1 protein, USP2a represses the Per2 promoter activity as well as the endogenous Per2 gene expression. We also demonstrated that USP2a-dependent deubiquitination and stabilization of the CRY1 protein occur in the mouse liver. Interestingly, the pro-inflammatory cytokine, TNF-α, increases the CRY1 protein level and inhibits circadian gene expression in a USP2a-dependent fashion. Therefore, USP2a potentially mediates circadian disruption by suppressing the CRY1 degradation during inflammation.