Impact of cell swelling on proliferative signal transduction in the liver

Adaptive growth changes in the liver remnant are affected by the size of hepatectomy in rats

In this study we investigated the dynamics of hepatocyte hyperplasia and hypertrophy in rats subjected to increasing sizes of partial hepatectomy (PH). A total of 104 rats were randomized according to the size of PH. On postoperative days (PODs) 1, 3 and 5, blood was drawn and the remnant liver removed for stereological analysis. Liver parameters and regeneration rate were significantly affected by size of PH. On POD 1, hepatocyte volumes had increased significantly in all PH groups. On POD 3, all groups showed hepatocyte volumes approximating baseline. On POD 5, hepatocyte volumes were significantly lower in PH (90) than in baseline, sham and PH (30) rats. Increasing hepatocyte proliferation was not observed following PH (30). Following PH (70), cell proliferation was significantly elevated on PODs 1 and 3, and following PH (90) on PODs 3 and 5. In conclusion, general hypertrophy of hepatocytes after different size of PH was followed by hepatocyte proliferation only in the liver remnant of PH (70) and PH (90).

KIAA0101 mRNA expression in the peripheral blood of hepatocellular carcinoma patients: Association with some clinicopathological features

OBJECTIVES

The development of hepatocellular carcinoma (HCC) is multi-factorial, multi-step and involving many genes. Recent studies have revealed the involvement of KIAA0101 in HCC development and progression. KIAA0101 is involved in the regulation of DNA repair, cell cycle progression, and cell proliferation. This study aims to elucidate the clinicopathological significance of KIAA0101 mRNA expression in the whole blood of HCC patients.

DESIGN AND METHODS

This study was conducted on 77 patients with proven HCC who presented to the outpatient clinic at the National Cancer Institute - Cairo University over a period of 8 consecutive months. Thirty patients with cirrhosis and forty apparently healthy volunteers were included as control groups. Detection of KIAA0101 mRNA was done on whole blood collected on EDTA for all patients and control subjects using real-time PCR.

RESULTS

KIAA0101 mRNA was over-expressed in the HCC group compared to the control groups. Overexpression of KIAA0101 mRNA was significantly associated with distant metastasis, advanced stage, high serum alkaline phosphatase and low serum albumin levels. Both sensitivity and specificity of KIAA0101 mRNA were higher than those of AFP and CEA.

CONCLUSION

Being associated with some of the prognostic factors of HCC which reflect tumor progression; as advanced stage, distant metastasis, hypoalbuminemia and elevated serum alkaline phosphatase, together with its relatively high diagnostic performance; KIAA0101 mRNA might be nominated to play a probable role in the diagnosis and prognosis prediction of HCC. Further studies on a wider scale are recommended to confirm these results.

[Contralateral hepatic hypertrophy following unilateral yttrium-90 radioembolization : Implications for liver surgery]

BACKGROUND

Preservation of an adequate future liver remnant (FLR) is the principal limitation to liver surgery in patients with primary or secondary liver malignancies. Hence, methods to increase the volume of the FLR in preparation for liver resection are gaining in importance.

OBJECTIVE

In addition to the traditional methods for induction of FLR hypertrophy, such as portal vein embolization (PVE) or portal vein ligation (PVL) with or without parenchymal dissection (ALPPS, in situ split), radioembolization (RE) using yttrium-90 microspheres also leads to a volume increase of non-embolized liver parenchyma. This review outlines its potential role as an alternative procedure for induction of liver hypertrophy.

MATERIAL AND METHODS

Synopsis and critical discussion of the available literature on the mechanisms of induction of liver hypertrophy, the advantages and drawbacks of the traditional methods, and current research on volume changes associated with RE as well as their implications for possible clinical use in preparation for liver surgery.

RESULTS

Both PVE and PVL can achieve a substantial contralateral volume gain of up to 70 %. The development of contralateral hypertrophy can be accelerated by dissecting the liver parenchyma along the intended plane of resection in addition to PVL (in situ split). Compared to these methods, RE achieves less contralateral liver hypertrophy; however, this effect should not be disregarded as RE provides effective treatment of ipsilateral liver tumors along with induction of hypertrophy and may be associated with a reduced risk of tumor progression compared to PVE and PVL.

CONCLUSION

The available data suggest that RE can complement the armamentarium of methods for induction of FLR hypertrophy in specific situations. Further studies are needed to establish its definitive role for this indication and are in preparation.

Effects of immunosuppressive drugs on oral mucosa in patients with Behçet's disease: cytomorphological and cytopathological assessment

BACKGROUND/AIM

The aim of this study was to investigate cytomorphological and cytopathological changes in oral exfoliated smears collected from immunosuppressed patients with Behçet's disease (BD) using stereological methods.

MATERIALS AND METHODS

For cytomorphometric analysis, mucosal cell smears were obtained from the buccal mucosa and the floor of the mouths of BD patients treated with immunosuppressive drugs and from healthy volunteers. All mucosal smears from the patients and the healthy volunteers were stained using the Papanicolaou method and examined cytopathologically under light microscopy and cytomorphologically via the stereological nucleator method.

RESULTS

The cytomorphological analysis revealed 3 types of mucosal cells, with numbers of particularly pink cells lower in the aphthous areas of the patients with BD compared to the healthy controls (P < 0.05). The nuclear volumes (NVs) and cytoplasmic volume (CVs) were significantly higher in the BD patients (P < 0.05), but the NV/CV ratio was higher only in the drug-use patient groups (P > 0.05). There was lower apoptotic activity in the nondrug-use patients with BD and in the immunosuppressive-taking BD patients.

CONCLUSION

The findings suggest that quantifiably morphological and morphometric changes in oral mucosa can be detected by stereological techniques. Changes in these parameters may indicate malignant transformation in the oral mucosa.

Albumin suppresses human hepatocellular carcinoma proliferation and the cell cycle

Many investigations have revealed that a low recurrence rate of hepatocellular carcinoma (HCC) is associated with high serum albumin levels in patients; therefore, high levels of serum albumin are a major indicator of a favorable prognosis. However, the mechanism inhibiting the proliferation of HCC has not yet been elucidated, so we investigated the effect of serum albumin on HCC cell proliferation. Hep3B was cultured in MEM with no serum or containing 5 g/dL human albumin. As control samples, Prionex was added to generate the same osmotic pressure as albumin. After 24-h incubation, the expressions of α-fetoprotein (AFP), p53, p21, and p57 were evaluated with real-time PCR using total RNA extracted from the liver. Protein expressions and the phosphorylation of Rb (retinoblastoma) were determined by Western blot analysis using total protein extracted from the liver. For flow cytometric analysis of the cell cycle, FACS analysis was performed. The percentages of cell cycle distribution were evaluated by PI staining, and all samples were analyzed employing FACScalibur (BD) with appropriate software (ModFit LT; BD). The cell proliferation assay was performed by counting cells with using a Scepter handy automated cell counter (Millipore). The mRNA levels of AFP relative to Alb(−): Alb(−), Alb(+), and Prionex, were 1, 0.7 ± 0.2 (p < 0.001 for Alb(−)), and 1 ± 0.3, respectively. The mRNA levels of p21 were 1, 1.58 ± 0.4 (p = 0.007 for Alb(−) and p = 0.004 for Prionex), and 0.8 ± 0.2, respectively. The mRNA levels of p57 were 1, 4.4 ± 1.4 (p = 0.002 for Alb(−) and Prionex), and 1.0 ± 0.1, respectively. The protein expression levels of Rb were similar in all culture media. The phosphorylation of P807/811 and P780 of Rb protein was reduced in Alb(+). More cells in the G0/G1 phase and fewer cells in S and G2/M phases were obtained in Alb(+) than in Alb(−) (G0/G1: 60.9%, 67.7%, 61.5%; G2/M: 16.5%, 13.1%, 15.6%; S: 22.6%, 19.2%, 23.0%, Alb(−), Alb(+), Prionex, respectively). The same results were obtained in HepG2. Cell proliferation was inhibited in 5 g/dL albumin medium in both HepG2 cells and Hep3B cells in 24 h culture by counting cell numbers. The presence of albumin in serum reduces the phosphorylation of Rb proteins and enhances the expression of p21 and p57, following an increase in the G0/G1 cell population, and suppresses cell proliferation. These results suggest that albumin itself suppresses the proliferation of hepatocellular carcinoma.

The role of hyperosmotic stress in inflammation and disease

Hyperosmotic stress is an often overlooked process that potentially contributes to a number of human diseases. Whereas renal hyperosmolarity is a well-studied phenomenon, recent research provides evidence that many non-renal tissues routinely experience hyperosmotic stress that may contribute significantly to disease initiation and progression. Moreover, a growing body of evidence implicates hyperosmotic stress as a potent inflammatory stimulus by triggering proinflammatory cytokine release and inflammation. Under physiological conditions, the urine concentrating mechanism within the inner medullary region of the mammalian kidney exposes cells to high extracellular osmolarity. As such, renal cells have developed many adaptive strategies to compensate for increased osmolarity. Hyperosmotic stress is linked to many maladies, including acute and chronic, as well as local and systemic, inflammatory disorders. Hyperosmolarity triggers cell shrinkage, oxidative stress, protein carbonylation, mitochondrial depolarization, DNA damage, and cell cycle arrest, thus rendering cells susceptible to apoptosis. However, many adaptive mechanisms exist to counter the deleterious effects of hyperosmotic stress, including cytoskeletal rearrangement and up-regulation of antioxidant enzymes, transporters, and heat shock proteins. Osmolyte synthesis is also up-regulated and many of these compounds have been shown to reduce inflammation. The cytoprotective mechanisms and associated regulatory pathways that accompany the renal response to hyperosmolarity are found in many non-renal tissues, suggesting cells are commonly confronted with hyperosmotic conditions. Osmoadaptation allows cells to survive and function under potentially cytotoxic conditions. This review covers the pathological consequences of hyperosmotic stress in relation to disease and emphasizes the importance of considering hyperosmolarity in inflammation and disease progression.

Liver regeneration and the atrophy-hypertrophy complex

The atrophy-hypertrophy complex (AHC) refers to the controlled restoration of liver parenchyma following hepatocyte loss. Different types of injury (e.g., toxins, ischemia/reperfusion, biliary obstruction, and resection) elicit the same hypertrophic response in the remnant liver. The AHC involves complex anatomical, histological, cellular, and molecular processes. The signals responsible for these processes are both intrinsic and extrinsic to the liver and involve both physical and molecular events. In patients in whom resection of large liver malignancies would result in an inadequate functional liver remnant, preoperative portal vein embolization may increase the remnant liver sufficiently to permit aggressive resections. Through continued basic science research, the cellular mechanisms of the AHC may be maximized to permit curative resections in patients with potentially prohibitive liver function.

Role of kinases and G-proteins in the hyposmotic stimulation of cardiac IKs

Exposure of cardiac myocytes to hyposmotic solution stimulates slowly-activating delayed-rectifying K(+) current (I(Ks)) via unknown mechanisms. In the present study, I(Ks) was measured in guinea-pig ventricular myocytes that were pretreated with modulators of cell signaling processes, and then exposed to hyposmotic solution. Pretreatment with compounds that (i) inhibit serine/threonine kinase activity (10-100 microM H89; 200 microM H8; 50 microM H7; 1 microM bisindolylmaleimide I; 10 microM LY294002; 50 microM PD98059), (ii) stimulate serine/threonine kinase activity (1-5 microM forskolin; 0.1 microM phorbol-12-myristate-13-acetate; 10 microM acetylcholine; 0.1 microM angiotensin II; 20 microM ATP), (iii) suppress G-protein activation (10 mM GDPbetaS), or (iv) disrupt the cytoskeleton (10 microM cytochalasin D), had little effect on the stimulation of I(Ks) by hyposmotic solution. In marked contrast, pretreatment with tyrosine kinase inhibitor tyrphostin A25 (20 microM) strongly attenuated both the hyposmotic stimulation of I(Ks) in myocytes and the hyposmotic stimulation of current in BHK cells co-expressing Ks channel subunits KCNQ1 and KCNE1. Since attenuation of hyposmotic stimulation was not observed in myocytes and cells pretreated with inactive tyrphostin A1, we conclude that TK has an important role in the response of cardiac Ks channels to hyposmotic solution.

Oxidative stress induces actin-cytoskeletal and tight-junctional alterations in hepatocytes by a Ca2+ -dependent, PKC-mediated mechanism: protective effect of PKA

Oxidative stress elevates Ca2+ and, presumably, activates Ca2+ -dependent PKCs. We analyzed the participation of Ca2+ -dependent PKCs in actin disorganization and tight-junctional impairment induced by the pro-oxidant tert-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets. tBOOH (100 microM) augmented radical oxygen species (ROS), as indicated by increased lipid peroxidation (+217%, p < 0.05) and intracellular production of 2',7'-dichlorofluorescein (+36%, p < 0.05). Cytosolic Ca2+ and PKCalpha translocation to membrane, an indicator of PKCalpha activation, were also elevated by tBOOH (+100 and +79%, respectively, p < 0.05). tBOOH increased the number of couplets displaying membrane blebs (+278%, p < 0.001) and caused redistribution of F-actin. tBOOH induced tight-junctional impairment, as indicated by a reduction in the percentage of couplets retaining presecreted cholyllysylfluorescein in their canalicular vacuoles (-54%, p < 0.001). tBOOH induced redistribution of the tight-junctional-associated protein ZO-1. All these events were prevented by the panspecific PKC inhibitors H7 and staurosporine, the Ca2+ -dependent PKC inhibitor Gö6976, the intracellular Ca2+ chelator BAPTA/AM, and the PKA activator dibutyryl-cyclic AMP. Furthermore, PKC inhibition and PKA activation not only prevented but also fully reversed tBOOH-induced blebbing. Conversely, tBOOH-induced ROS formation and Ca2+ elevation remained unchanged. We conclude that ROS induce hepatocellular actin-cytoskeleton rearrangement and tight-junctional impairment by a PKC-mediated, Ca2+ -dependent mechanism, which is counteracted by PKA.

Recruitment of the tyrosine phosphatase Src homology 2 domain tyrosine phosphatase-2 to the p85 subunit of phosphatidylinositol-3 (PI-3) kinase is required for insulin-like growth factor-I-dependent PI-3 kinase activation in smooth muscle cells

IGF-I stimulates smooth muscle cell (SMC) migration and the phosphatidylinositol-3 (PI-3) kinase pathway plays an important role in mediating the IGF-I-induced migratory response. Prior studies have shown that the tyrosine phosphatase Src homology 2 domain tyrosine phosphatase (SHP)-2 is necessary to activate PI-3 kinase in response to growth factors and expression of a phosphatase inactive form of SHP-2 (SHP-2/C459S) impairs IGF-I-stimulated cell migration. However, the mechanism by which SHP-2 phosphatase activity or the recruitment of SHP-2 to other signaling molecules contributes to IGF-I stimulated PI-3 kinase activation has not been determined. SMCs that had stable expression of SHP-2/C459S had reduced cell migration and Akt activation in response to IGF-I, compared with SMC-expressing native SHP-2. Similarly in cells expressing native SHP-2, IGF-I induced SHP-2 binding to p85, whereas in cells expressing SHP-2/C459S, there was no increase. Because the C459S substitution results in loss of the ability of SHP-2 to disassociate from its substrates, making it inaccessible not only to p85 but also the other proteins, a p85 mutant in which tyrosines 528 and 556 were changed to phenylalanines was prepared to determine whether this would disrupt the p85/SHP-2 interaction and whether the loss of this specific interaction would alter IGF-I stimulated the cell migration. Substitution for these tyrosines in p85 resulted in loss of SHP-2 recruitment and was associated with a reduction in association of the p85/p110 complex with insulin receptor substrate-1. Cells stably expressing this p85 mutant also showed a decrease in IGF-I-stimulated PI-3 kinase activity and cell migration. Preincubation of cells with a cell-permeable peptide that contains the tyrosine556 motif of p85 also disrupted SHP-2 binding to p85 and inhibited the IGF-I-induced increase in cell migration. The findings indicate that tyrosines 528 and 556 in p85 are required for SHP-2 association. SHP-2 recruitment to p85 is required for IGF-I-stimulated association of the p85/p110 complex with insulin receptor substrate-1 and for the subsequent activation of the PI-3 kinase pathway leading to increased cell migration.

Oxidative Stress Induces Internalization of the Bile Salt Export Pump, Bsep, and Bile Salt Secretory Failure in Isolated Rat Hepatocyte Couplets: A Role for Protein Kinase C and Prevention by Protein Kinase A

We have shown that Ca2+-mediated protein kinase C (PKC) activation induces impairment of bile salt secretory function and F-actin redistribution in hepatocyte couplets. Because oxidative stress induces Ca2+ elevation, we tested here whether PKC inhibition or protein kinase A (PKA) activation, which often counteracts PKC-dependent effects, can prevent and reverse these alterations. The pro-oxidant compounds tert-butylhydroperoxide (tBOOH, 100 microM) and 2,3-dimethoxy-1,4-naphthoquinone (30 microM), reduced by -41% and -29%, respectively, the percentage of couplets accumulating the fluorescent bile salt analog, cholyl-lysylfluorescein in their canalicular vacuoles (p < 0.01). tBOOH-induced bile salt secretory failure was accompanied by internalization of the canalicular bile salt export pump (Bsep), and disarrangement of cytoskeletal F-actin. All these deleterious effects were fully prevented by the intracellular Ca2+ chelator BAPTA/AM (20 microM), the pan-specific PKC inhibitors H7 (100 microM) and staurosporine (1 microM), the inhibitor of Ca2+-dependent PKCs, Gö6976 (2 microM), and the PKA activator dibutyryl-cAMP (500 microM). H7, Gö6976, and dibutyryl-cAMP not only prevented but also fully reversed the decrease in the cholyl-lysyl-fluorescein accumulation. In conclusion, these results suggest that low levels of oxidative stress impair bile salt secretion by internalizing Bsep through a Ca2+-dependent, PKC-mediated mechanism, and that inhibition of PKC, or activation of PKA, prevents and reverses these effects. Alterations in actin organization may be a causal factor.

Distinct endoplasmic reticulum stress responses are triggered during human liver transplantation

Injury due to cold ischaemia-reperfusion (IR) represents a major cause of primary graft non-function following human liver transplantation. This major cellular response translates into a dramatic decrease in intracellular ATP concentration during the ischaemic phase, thus sensitizing cells to reperfusion shock. We postulated that IR-induced cellular damage might cause alterations of the secretory pathway, particularly at the level of endoplasmic reticulum (ER) function. Under these circumstances, the ER triggers an adaptive response named the 'unfolded protein response' (UPR). In this study, we show that the expression of BiP, CHOP/GADD153 and GADD34, known to be induced specifically upon ER stress, are differentially affected upon IR, thus suggesting that distinct ER stress responses are activated during each phase of transplantation. With an approach combining semi-quantitative RT-PCR and immunoblotting using phospho-specific antibodies, we show that the IRE-1 pathway is activated upon early ischaemia and, in a second phase, upon early reperfusion. This occurs through the atypical splicing of XBP-1 mRNA, its translation into a transcriptionally active XBP-1 protein and the subsequent increase in EDEM mRNA expression, and may also contribute to the observed reperfusion-induced activation of MAPK/SAPK. In contrast, we demonstrate that the PERK pathway, leading to inhibition of cap-dependent translation, is mainly activated upon reperfusion, as shown by PERK and eIF2alpha phosphorylation. PERK activation is detected restrictively in sinusoidal endothelial cells and could contribute to the exaggerated sensivity of this liver cell type to IR injury. These results correlate well with the observed defect in protein secretion and suggest that the biphasic ER stress response may influence liver secretory functions and, as a consequence, condition liver transplantation outcomes.

Cytoplasmic-to-nuclear volume ratio affects AP-1 complex formation as an indicator of cell cycle responsiveness

Cytoplasmic volume undergoes a series of changes during mitosis in eukaryotes; in turn, signaling events such as osmotic stress can regulate the cytoplasmic volume in cells. In some organisms, increase in cytoplasmic-to-nuclear volume ratio was seen to affect the growth potential in cells, however, the mechanistics of such a regulation, if at all present, was unclear. In a computational model, we have constructed a growth factor-induced signaling pathway leading to AP-1 heterodimer formation through transcriptional regulation, and analyzed the effects of increasing the cytoplasmic-to-nuclear ratio on c-jun transcription and AP-1 complex. We have observed that larger cytoplasmic volumes caused both an increase in the final AP-1 product and a delay in the time of AP-1 accumulation.

Signal pathways underlying homocysteine-induced production of MCP-1 and IL-8 in cultured human whole blood

AIM

To elucidate the mechanisms underlying homocysteine (Hcy)-induced chemokine production.

METHODS

Human whole blood was pretreated with inhibitors of calmodulin (CaM), protein kinase C (PKC), protein tyrosine kinase (PTK), mitogen-activated protein kinase (MAPK), and NF-kappaB and activators of PPARgamma for 60 min followed by incubation with Hcy 100 micromol/L for 32 h. The levels of mitogen chemokine protein (MCP)-1 and interleukin-8 (IL-8) were determined by enzyme-linked immunosorbant assay (ELISA).

RESULTS

Inhibitors of PKC (calphostin C, 50-500 nmol/L and RO-31-8220, 10-100 nmol/L), CaM (W7, 28-280 micromol/L), ERK1/2 MAPK (PD 98059, 2-20 micromol/L), p38 MAPK (SB 203580, 0.6-6 micromol/L), JNK MAPK (curcumin, 2-10 micromol/L), and NF-kappaB (PDTC, 10-100 nmol/L) markedly reduced Hcy 100 micromol/L-induced production of MCP-1 and IL-8 in human cultured whole blood, but the inhibitors of PTK (genistein, 2.6-26 micromol/L and tyrphostin, 0.5-5 micromol/L) had no obvious effect on MCP-1 and IL-8 production. PPARgamma activators (ciglitazone 30 micromol/L and troglitazone 10 micromol/L) depressed the Hcy-induced MCP-1 production but not IL-8 production in the cultured whole blood.

CONCLUSION

Hcy-induced MCP-1 and IL-8 production is mediated by activated signaling pathways such as PKC, CaM, MAPK, and NF-kappaB. Our results not only provide clues for the signal transduction pathways mediating Hcy-induced chemokine production, but also offer a plausible explanation for a pathogenic role of hyperhomocysteinemia in these diseases.

Role of oxidative stress in the increased activation of signal transducers and activators of transcription-3 in the fatty livers of obese Zucker rats

BACKGROUND

Fatty livers have chronic oxidative stress, which could activate several transcription factors. We hypothesized that fatty livers of obese rats have increased activation of signal transducers and activators of transcription-1 and transcription-3 (Stat-1 and Stat-3) and that tocopherol treatment will decrease Stat activation.

METHODS

Obese (Ob) and lean (Ln) Zucker rats with or without tocopherol treatment were used. Western blots of liver nuclear and cytoplasmic extracts to assess phosphorylated and total Stat-3 and tyrosine kinases Jak-2 and Tyk-2, immunohistochemistry to assess distribution of phosphoStat-3, and gel shift assays to assess Stat and nuclear factor kappa B binding were performed. Interleukin-6 serum levels and hepatic transcripts were determined by immunoassay and reverse polymerase chain reaction with Southern blotting, respectively.

RESULTS

Livers of Ob animals had increased nuclear phosphoStat-3, decreased cytoplasmic Stat-3, and increased Stat-3 binding. Serum interleukin-6 was not measurable in either Ob or Ln animals and hepatic transcript levels were not significantly different. Tocopherol administration decreased nuclear phosphoStat-3, increased cytoplasmic Stat-3, and decreased Stat-3 binding activity.

CONCLUSIONS

Chronic oxidative stress in fatty livers is associated with increased Stat-3 activation and decreased cytosolic Stat-3. Tocopherol treatment decreases Stat-3 activation and increases cytosolic Stat-3. Tocopherol-induced changes in Stat-3 may play a role in its beneficial effects in hepatic ischemia in fatty livers.

Extracellular ATP activates c-jun N-terminal kinase signaling and cell cycle progression in hepatocytes

Partial hepatectomy leads to an orchestrated regenerative response, activating a cascade of cell signaling events necessary for cell cycle progression and proliferation of hepatocytes. However, the identity of the humoral factors that trigger the activation of these pathways in the concerted regenerative response in hepatocytes remains elusive. In recent years, extracellular ATP has emerged as a rapidly acting signaling molecule that influences a variety of liver functions, but its role in hepatocyte growth and regeneration is unknown. In this study, we sought to determine if purinergic signaling can lead to the activation of c-jun N-terminal kinase (JNK), a known central player in hepatocyte proliferation and liver regeneration. Hepatocyte treatment with ATPgammaS, a nonhydrolyzable ATP analog, recapitulated early signaling events associated with liver regeneration-that is, rapid and transient activation of JNK signaling, induction of immediate early genes c-fos and c-jun, and activator protein-1 (AP-1) DNA-binding activity. The rank order of agonist preference, UTP>ATP>ATPgammaS, suggests that the effects of extracellular ATP is mediated through the activation of P2Y2 receptors in hepatocytes. ATPgammaS treatment alone and in combination with epidermal growth factor (EGF) substantially increased cyclin D1 and proliferating cell nuclear antigen (PCNA) protein expression and hepatocyte proliferation in vitro. Extracellular ATP as low as 10 nM was sufficient to potentiate EGF-induced cyclin D1 expression. Infusion of ATP by way of the portal vein directly activated hepatic JNK signaling, while infusion of a P2 purinergic receptor antagonist prior to partial hepatectomy inhibited JNK activation. In conclusion, extracellular ATP is a hepatic mitogen that can activate JNK signaling and hepatocyte proliferation in vitro and initiate JNK signaling in regenerating liver in vivo. These findings have implications for enhancing our understanding of novel factors involved in the initiation of regeneration, liver growth, and development.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Osmotic swelling induces p75 neurotrophin receptor (p75NTR) expression via nitric oxide

Brain injuries by physical trauma, epileptic seizures, or microbial infection upset the osmotic homeostasis resulting in cell swelling (cerebral edema), inflammation, and apoptosis. Expression of the neurotrophin receptor p75NTR is increased in the injured tissue and axon regeneration is repressed by the Nogo receptor using p75NTR as the signal transducer. Hence, p75NTR seems central to the injury response and we wished to determine the signals that regulate its expression. Here, we demonstrate that tonicity mediated cell swelling rapidly activates transcription of the endogenous p75NTR gene and of a p75NTR promoter-reporter gene in various cell types. Transcription activation is independent of de novo protein synthesis and requires the activities of phospholipase C, protein kinase C, and nitric-oxide synthase. Hence, p75NTR is a nitric oxide effector gene regulated by osmotic swelling, thereby providing a strategy for therapeutic intervention to modulate p75NTR functions following injury.

Hepatitis viruses and the MAPK pathway: is this a survival strategy?

The viruses that cause hepatitis comprise of at least five different agents, which share the ability to cause inflammation and necrosis of the liver. The disease spectrum is quite diverse and the outcome of infection by the different hepatitis viruses can be rationalized based on virus-host cell interactions. New insights into the molecular basis of viral hepatitis reveal that three of these agents - the hepatitis B, C and E viruses (HBV, HCV and HEV) modulate the mitogen-activated protein kinase (MAPK) signaling pathway. In this review we briefly describe the structural organization of the MAPK cascade and emphasize its importance as a central pathway in the signaling network. Selected mechanisms through which HBV, HCV and HEV proteins target various steps in the MAPK pathway are discussed and used to propose a pro-survival outcome for the host cell. In addition, we offer an insight into how the common theme of MAPK activation and its downstream effects may be used to rationalize the different outcomes of hepatitis B, C and E.

Osmomechanical stress selectively regulates translocation of protein kinase C isoforms

Osmomechanical stress, resulting in cell swelling and activation/regulation of numerous cellular processes, may play a critical role in cell signaling by selectively regulating translocation of protein kinase C (PKC) isoforms from cytosol to membrane compartments. Western blotting of renal epithelial cell fractions demonstrated the expression of five PKC isoforms. Three of these isoforms (PKCalpha, PKCepsilon, PKCzeta) translocated to the membrane fraction upon exposure of cells to osmomechanical stress (hypotonic medium). Immunohistochemical staining of cells using isoform-specific antibodies further demonstrated translocation of the phorbol ester-sensitive isoforms, PKCalpha and PKCepsilon, to both the plasma membrane and perinuclear sites, reflecting potential initial steps in regulation of specific effector pathways. Indeed, selective inhibition of PKCs indicates a potential role for PKCalpha in modulating a calcium influx channel. It is concluded that osmomechanical stress induces selective translocation of specific PKC isoforms, demonstrating a key role of osmomechanical stress in selectively regulating PKC-dependent signaling pathways.

A scientist revisits the atrophy-hypertrophy complex: hepatic apoptosis and regeneration

In 1898, Cantlie demonstrated his deep insight into the AHC when he wrote. "It is theoretically possible to tie the vessels of one side at the gate of the liver...leaving the other side to do the work. That one-half of the liver can hypertrophy, so as to perform the function of the whole, is attested by pathological study...I commend this subject to all those who are working on the surgery of the liver; and I believe that if, in the hands of future observers, the statements I have made receive closer investigation, surgery of the liver will be advanced a step." More than 100 years later, his foresight is coming to fruition, but the understanding of the AHC remains rudimentary. Further insight into the cellular and molecular mechanisms that initiate hepatocyte replication after contralateral atrophy may allow the development of novel therapeutic treatments or adjuncts.