Impaired nuclear accumulation and shortened phosphorylation of ERK after growth factor stimulation in cultured hepatocytes from rats exposed to 2-acetylaminofluorene

FAK regulates Cdk2 in EGF-stimulated primary cultures of hepatocytes

In this study, we report a novel role of FAK as a regulator of Cdk2 in anchorage-dependent primary cultured hepatocytes. In response to EGF, we found that S-phase entry was reduced upon FAK inhibition. This correlated with decreased protein expression and nuclear accumulation of the G1/S-phase regulator Cdk2. Further, nuclear accumulation of the Cdk2 partner cyclinE, was reduced, but not its protein level. Also, protein levels of Cdk2 were inversely linked with increased expression of the Cdk2 inhibitor p27, known to be degraded in a Cdk2-dependent manner. Also, cyclinD1 was regulated by FAK, but to a lesser extent than Cdk2. To assess the mechanism in which FAK mediates Cdk2-regulation, FAK mutants were used: FAKY397F, mutated at its integrin-regulated site, and two others mutated at docking sites for Grb2-ERK-activation (FAKY925F) and for p130Cas-Rac1-activation (FAKY861F). All three sites were central for EGF-induced ERK-activity and Cdk2 expression. In addition, FAK was important for HGF-mediated proliferation, suggesting a general mechanism for anchorage-dependent growth. Moreover, growth factor-induced cell spreading, but not survival, required FAK. Hence, integrins and growth factors cooperate in anchorage-dependent signaling events leading to proliferation and motility. In conclusion, our data suggest that FAK acts as a central coordinator of integrin and growth factor-mediated S-phase entry by its ability to regulate Cdk2.

RAF-targeted therapy for hepatocellular carcinoma in the regenerating liver

BACKGROUND

Post-operative liver regeneration may contribute to tumor recurrence. There is a theoretical need for an adjuvant therapy that can suppress tumor growth without adversely affecting post-operative liver regeneration.

OBJECTIVE

To evaluate the effect of RAF inhibitor Sorafenib on cell viability and proliferation of hepatoma cells and hepatocytes in vitro and in an in vivo rat model.

METHODS

Cell viability, DNA synthesis, and RAF/MAPK kinase activity in the primary hepatocyte and hepatoma cell lines were investigated after Sorafenib exposure. Sequence analysis of the B-RAF gene in hepatic cells was determined. Tumor markers were compared within the rats after 70% hepatectomy with or without daily oral gavages of Sorafenib. Liver regeneration was assessed by liver function tests and proliferation markers.

RESULTS

Primary hepatocytes showed higher cell viability, proliferation rate, and stronger RAF/MAPK kinase activity compared with hepatoma cell lines. The in vivo tumor volumes, size, and metastases were significantly decreased (P < 0.05) whereas no significant change in liver regeneration related to Sorafenib exposure was found (P > 0.05). B-RAF V600E mutation was not detected neither in the hepatic cells nor untransformed hepatocytes.

CONCLUSIONS

The RAF targeted inhibitor can reduce tumor growth without retarding liver regeneration in this experiment.

Deletion of the γ-aminobutyric acid transporter 2 (GAT2 and SLC6A13) gene in mice leads to changes in liver and brain taurine contents

The GABA transporters (GAT1, GAT2, GAT3, and BGT1) have mostly been discussed in relation to their potential roles in controlling the action of transmitter GABA in the nervous system. We have generated the first mice lacking the GAT2 (slc6a13) gene. Deletion of GAT2 (both mRNA and protein) neither affected growth, fertility, nor life span under nonchallenging rearing conditions. Immunocytochemistry showed that the GAT2 protein was predominantly expressed in the plasma membranes of periportal hepatocytes and in the basolateral membranes of proximal tubules in the renal cortex. This was validated by processing tissue from wild-type and knockout mice in parallel. Deletion of GAT2 reduced liver taurine levels by 50%, without affecting the expression of the taurine transporter TAUT. These results suggest an important role for GAT2 in taurine uptake from portal blood into liver. In support of this notion, GAT2-transfected HEK293 cells transported [(3)H]taurine. Furthermore, most of the uptake of [(3)H]GABA by cultured rat hepatocytes was due to GAT2, and this uptake was inhibited by taurine. GAT2 was not detected in brain parenchyma proper, excluding a role in GABA inactivation. It was, however, expressed in the leptomeninges and in a subpopulation of brain blood vessels. Deletion of GAT2 increased brain taurine levels by 20%, suggesting a taurine-exporting role for GAT2 in the brain.

Pentoxifylline inhibits hepatic stellate cells proliferation via the Raf/ERK pathway

Pentoxifylline (PTX), which is a xanthine derivative, is a well-known suppressor of tumor necrosis factor-alpha (TNF-alpha) production in inflammatory cells and has also been shown to inhibit collagen synthesis in hepatic stellate cells (HSCs) in vitro. The present study aimed to evaluate the effects of PTX on proliferation in HSCs as mediated by the Raf/MEK/extracellular-signal-regulated kinase (ERK) signaling pathway. The rat hepatic stellate cell line T6 and activated primary rat HSCs were used in this study. The proliferation rate of the cells treated with 1 mM PTX significantly decreased compared with that of the control in T6 cells (78.3 ± 6.03% at 12 h, 61.0 ± 7.55% at 24 h, and 44.7 ± 2.08% at 48 h, p < 0.05). PTX (1 mM) also decreased the fraction of the HSC population in the S and G2/M-phases of the cell cycle in primary activated rat HSCs. The Raf-1 inhibitor GW5074 and the ERK inhibitor U0126 had inhibitory effects that were similar to those of PTX on HSC proliferation. In addition, PTX inhibited the phosphorylation of Raf-1 (p-Raf-1) and ERK (p-ERK) in a dose- and time-dependent manner in HSCs. These data provide evidence that PTX suppresses HSC proliferation via the Raf/MEK/ERK pathway.

The MAPK MEK1/2-ERK1/2 Pathway and Its Implication in Hepatocyte Cell Cycle Control

Primary cultures of hepatocytes are powerful models in studying the sequence of events that are necessary for cell progression from a G0-like state to S phase. The models mimic the physiological process of hepatic regeneration after liver injury or partial hepatectomy. Many reports suggest that the mitogen-activated protein kinase (MAPK) ERK1/2 can support hepatocyte proliferation in vitro and in vivo and the MEK/ERK cascade acts as an essential element in hepatocyte responses induced by the EGF. Moreover, its disregulation has been associated with the promotion of tumor cell growth of a variety of tumors, including hepatocellular carcinoma. Whereas the strict specificity of action of ERK1 and ERK2 is still debated, the MAPKs may have specific biological functions under certain contexts and according to the differentiation status of the cells, notably hepatocytes. In this paper, we will focus on MEK1/2-ERK1/2 activations and roles in normal rodent hepatocytes in vitro and in vivo after partial hepatectomy and in human hepatocarcinoma cells. The possible specificity of ERK1 and ERK2 in normal and transformed hepatocyte will be discussed in regard to other differentiated and undifferentiated cellular models.

EGF-induced ERK-activation downstream of FAK requires rac1-NADPH oxidase

Reactive oxygen species (ROS) function as signaling molecules mainly by reversible oxidation of redox-sensitive target proteins. ROS can be produced in response to integrin ligation and growth factor stimulation through Rac1 and its effector protein NADPH oxidase. One of the central roles of Rac1-NADPH oxidase is actin cytoskeletal rearrangement, which is essential for cell spreading and migration. Another important regulator of cell spread is focal adhesion kinase (FAK), a coordinator of integrin and growth factor signaling. Here, we propose a novel role for NADPH oxidase as a modulator of the FAK autophosphorylation site. We found that Rac1-NADPH oxidase enhanced the phosphorylation of FAK at Y397. This site regulates FAK's ability to act as a scaffold for EGF-mediated signaling, including activation of ERK. Accordingly, we found that EGF-induced activation of FAK at Y925, the following activation of ERK, and phosphorylation of FAK at the ERK-regulated S910-site depended upon NADPH oxidase. Furthermore, the inhibition of NADPH oxidase caused excessive focal adhesions, which is in accordance with ERK and FAK being modulators of focal adhesion dissociation. Our data suggest that Rac1 through NADPH oxidase is part of the signaling pathway constituted by FAK, Rac1, and ERK that regulates focal adhesion disassembly during cell spreading.

Growth of hepatocellular carcinoma in the regenerating liver

Liver resection and liver transplantation are the treatment modalities with the greatest potential for curing hepatocellular carcinoma (HCC). Tumor recurrence after resection for HCC is, however, a major problem, and an increased rate of recurrence after living donor transplantation versus cadaveric whole liver transplantation has been suggested. Factors involved in liver regeneration may stimulate the growth of occult tumors. The aim of this project was to test the hypothesis that a microscopic HCC tumor in the setting of partial hepatectomy would show enhanced growth and signs of increased invasiveness corresponding to the size of the liver resection. Hepatectomy was performed to various degrees in groups of Buffalo rats with the concomitant implantation of a fixed number of hepatoma cells in the remnant liver; a control group underwent only resection. After 21 days, the sizes and numbers of the tumors and the expression of alpha-fetoprotein (AFP), cyclin D1, calpain small subunit 1 (CAPNS1), CD34 (a microvessel density marker), vascular endothelial growth factor (VEGF), and vascular endothelial growth factor receptor 2 (VEGFR2) were evaluated and compared between the groups. The tumor volume and number increased significantly with the size of the partial hepatectomy (P < 0.05). The largest resections were also associated with increased hepatoma cell infiltration in the lungs and significant up-regulation of cyclin D1, AFP, CAPNS1, CD34, VEGF, and VEGFR2. The results suggest that liver regeneration after partial hepatectomy facilitates the growth and malignant transformation of microscopic HCC, and this could be significant for liver resection and partial liver transplantation strategies for HCC.

Distinct functions of H-Ras and K-Ras in proliferation and survival of primary hepatocytes due to selective activation of ERK and PI3K

Ras proteins mediate signals both via extracellular signal-regulated kinase 1 and 2 (ERK), and phosphoinositide 3-kinase (PI3K). These signals are key events in cell protection and compensatory cell growth after exposure to cell damaging and pro-apoptotic stimuli, thus maintaining homeostasis. By transfection techniques, we found that both H-Ras and K-Ras were expressed and appeared functionally active in primary hepatocytes. We compared the ability of H-Ras and K-Ras homologues to preferentially activate one of the two pathways, thereby differentially controlling cell survival and growth. We found that ectopic expression of dominant negative (DN) H-RasN17, but not DN K-RasN17, efficiently inhibited both phosphorylation and translocation of ERK to the nuclear compartment, which are prerequisites for cell cycle progression. Furthermore, ectopic expression of constitutive active (CA) H-RasV12, but not CA K-RasV12, potentiated EGF-induced proliferation. We also found that expression of CA mutants of either H-Ras or K-Ras protected hepatocytes from transforming growth factor-beta1 (TGF-beta1)-induced apoptosis. However, H-Ras-induced survival was mediated by ERK/RSK as well as by PI3K, whereas K-Ras-induced survival was mediated by PI3K only. In conclusion, H-Ras and K-Ras had differential functions in proliferation and survival of primary hepatocytes. H-Ras was the major mediator of ERK-induced proliferation and survival, whereas H-Ras and K-Ras both mediated PI3K-induced survival.

Activation of the p53-p21(Cip1) pathway is required for CDK2 activation and S-phase entry in primary rat hepatocytes

p53 plays a major role in the prevention of tumor development. It responds to a range of potentially oncogenic stresses by activating protective mechanisms, most notably cell-cycle arrest and apoptosis. The p53 gene is also induced during normal liver regeneration, and it has been hypothesized that p53 serve as a proliferative 'brake' to control excessive proliferation. However, it has lately been shown that p53 inhibition reduces hepatocyte growth factor-induced DNA synthesis of primary hepatocytes. Here we show that epidermal growth factor (EGF) activated p53 in a phosphatidylinositol-3 kinase-dependent way, and thus induced the cyclin-dependent kinase inhibitor p21(Cip1) in primary rat hepatocytes. p53 inactivation with a dominant-negative mutant (p53(V143A)) attenuated EGF-induced DNA synthesis and was associated with reduced CDK2 phosphorylation and retinoblastoma protein hyperphosphorylation. When p21(Cip1) was ectopically expressed in p53-inactivated cells, these effects were neutralized. In conclusion, our results demonstrate that in normal hepatocytes, EGF-induced expression of p53 is involved in regulating CDK2- and CDK4 activity, through p21(Cip1) expression.

MEK1 and MEK2 regulate distinct functions by sorting ERK2 to different intracellular compartments

In this study, we provide novel insight into the mechanism of how ERK2 can be sorted to different intracellular compartments and thereby mediate different responses. MEK1-activated ERK2 accumulated in the nucleus and induced proliferation. Conversely, MEK2-activated ERK2 was retained in the cytoplasm and allowed survival. Localization was a determinant for ERK2 functions since MEK1 switched from providing proliferation to be a mediator of survival when ERK2 was routed to the cytoplasm by the attachment of a nuclear export site. MEK1-mediated ERK2 nuclear translocation and proliferation were shown to depend on phosphorylation of S298 and T292 sites in the MEK1 proline-rich domain. These sites are phosphorylated on cellular adhesion in MEK1 but not MEK2. Whereas p21-activated kinase phosphorylates S298 and thus enhances the MEK1-ERK2 association, ERK2 phosphorylates T292, leading to release of active ERK2 from MEK1. On the basis of these results, we propose that the requirement of adhesion for cells to proliferate in response to growth factors, in part, may be explained by the MEK1 S298/T292 control of ERK2 nuclear translocation. In addition, we suggest that ERK2 intracellular localization determines whether growth factors mediate proliferation or survival and that the sorting occurs in an adhesion-dependent manner.

Cytoplasmic retention of peroxide-activated ERK provides survival in primary cultures of rat hepatocytes

Reactive oxygen species (ROS) are implicated in tissue damage causing primary hepatic dysfunction following ischemia/reperfusion injury and during inflammatory liver diseases. A potential role of extracellular signal-regulated kinase (ERK) as a mediator of survival signals during oxidative stress was investigated in primary cultures of hepatocytes exposed to ROS. Hydrogen peroxide (H(2)O(2)) induced a dose-dependent activation of ERK, which was dependent on MEK activation. The ERK activation pattern was transient compared with the ERK activation seen after stimulation with epidermal growth factor (EGF). Nuclear accumulation of ERK was found after EGF stimulation, but not after H(2)O(2) exposure. A slow import/rapid export mechanism was excluded through the use of leptomycin B, an inhibitor of nuclear export sequence-dependent nuclear export. Reduced survival of hepatocytes during ROS exposure was observed when ERK activation was inhibited. Ribosomal S6 kinase (RSK), a cytoplasmic ERK substrate involved in cell survival, was activated and located in the nucleus of H(2)O(2)-exposed hepatocytes. The activation was abolished when ERK was inhibited with U0126. In conclusion, our results indicate that activity of ERK in the cytoplasm is important for survival during oxidative stress in hepatocytes and that RSK is activated downstream of ERK. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Altered regulation of EGF receptor signaling following a partial hepatectomy

We have studied epidermal growth factor receptor (EGFR) phosphorylation and localization in the pre-replicative phase of liver regeneration induced by a 70% partial hepatectomy (PH), and how a PH affects EGFR activation and trafficking. When Western blotting was performed on livers after PH with antibodies raised against activated forms of EGFR autophosphorylation sites, no marked increase in EGFR tyrosine phosphorylation was observed. However, events associated with attenuation of EGFR signals were observed. Two hours after PH, we found increased EGFR ubiquitination and internalization, followed by receptor downregulation. Furthermore, EGFR phosphorylation following an injection of EGF was reduced after PH. This reduction correlated with an increased activation of PKC and a distinct augmentation in the phosphorylation of the PKC-regulated T654-site of EGFR. When primary cultured hepatocytes were treated with tetradecanoylphorbol acetate (TPA) to induce T654-phosphorylation of EGFR, we found colocalization of a fraction of EGFR with EEA1, downregulation of EGF-mediated EGFR autophosphorylation, altered ligand-induced intracellular sorting of EGFR, and increased mitogenic signaling through the EGFR-Ras-Raf-ERK pathway. Further, we found that both TPA and a PH enhanced EGF-induced proliferation of hepatocytes. In conclusion, our results suggest that hepatocyte priming involves modulation of EGFR that enhances its ability to mediate growth factor responses without an increase in its receptor tyrosine kinase-activity. This may be a pre-replicative competence event that increases growth factor effects during G1 progression.

Acute cytotoxicity of the chemical carcinogen 2-acetylaminofluorene in cultured rat liver epithelial cells

Acute cytotoxic effects of 2-acetylaminofluorene (AAF), a powerful chemical carcinogen, were studied in cultured rat liver epithelial F258 cells. Acute treatment of these cells by AAF resulted in inhibition of cell proliferation, through an arrest in G0/G1 phase of the cell cycle, and a loss of cell viability. By contrast, AAF failed to trigger apoptosis as demonstrated by flow cytometric analysis of hypoploid sub-G1 cells. Cytochrome P4501A1 (CYP1A1), a drug metabolizing enzyme thought to play a major role in biotransformation of AAF, was induced in AAF-treated F258 cells as assessed by Northern blotting. AAF cytotoxic effects were however not blocked by the CYP1A1 inhibitor alpha-naphtoflavone, thus suggesting that they did not require CYP1A1 activity. They were also not prevented by the antioxidant N-acetylcysteine, making unlikely a major contribution of AAF-related reactive oxygen species. These data therefore indicate that AAF can exert acute cellular toxicity, including inhibition of cell growth and cell death, in rat liver epithelial cells without triggering an apoptotic process. Such an acute toxicity may contribute to the well-known promoting effects of AAF.

Nuclear receptor and apoptosis initiator NGFI-B is a substrate for kinase ERK2

NGFI-B is an inducible orphan nuclear receptor that initiates apoptosis. Growth factors such as EGF activate the MAP kinase ERK, whose activity may determine if a cell survives or undergoes apoptosis. EGF stimulation of cells leads to phosphorylation of threonine in NGFI-B. Thr-142 of NGFI-B is comprised in a consensus MAP kinase site and was identified as a preferred substrate for ERK2 (but not ERK1) in vitro. These results suggest that NGFI-B may be a molecular target for ERK2 signals and thereby a substrate for crosstalk between a growth factor survival pathway and an inducible regulator of apoptosis.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.