Transforming growth factor beta 1 inhibits epidermal growth factor receptor endocytosis and down-regulation in cultured fetal rat hepatocytes

EGFR and HER2 receptor kinase signaling mediate epithelial cell invasion by Candida albicans during oropharyngeal infection

The fungus Candida albicans is the major cause of oropharyngeal candidiasis (OPC). A key feature of this disease is fungal invasion of oral epithelial cells, a process that can occur by active penetration and fungal-induced endocytosis. Two invasins, Als3 and Ssa1, induce epithelial cell endocytosis of C. albicans, in part by binding to E-cadherin. However, inhibition of E-cadherin function only partially reduces C. albicans endocytosis, suggesting that there are additional epithelial cell receptors for this organism. Here, we show that the EGF receptor (EGFR) and HER2 function cooperatively to induce the endocytosis of C. albicans hyphae. EGFR and HER2 interact with C. albicans in an Als3- and Ssa1-dependent manner, and this interaction induces receptor autophosphorylation. Signaling through both EGFR and HER2 is required for maximal epithelial cell endocytosis of C. albicans in vitro. Importantly, oral infection with C. albicans stimulates the phosphorylation of EGFR and HER2 in the oral mucosa of mice, and treatment with a dual EGFR and HER2 kinase inhibitor significantly decreases this phosphorylation and reduces the severity of OPC. These results show the importance of EGFR and HER2 signaling in the pathogenesis of OPC and indicate the feasibility of treating candidal infections by targeting the host cell receptors with which the fungus interacts.

Oncogenic cell cycle start control

The ability to proliferate in the absence of anchorage is a fundamental attribute of cancer cells, yet how it is acquired is one central problem in cancer biology. By utilizing growth factor-transformable NRK cells and its insensitive mutants, we recently found that oncogenic stimulation invokes Cdk6 to participate in a critical step of the cell cycle start, but not via the regulation of its catalytic activity and that Cdk6 participation closely correlates with the anchorage-independent growth ability. Since many hematopoietic cells employ predominantly Cdk6 for the cell cycle start and perform anchorage-independent growth by nature, this finding raises the possibility that the mechanism by which oncogenic stimulation invokes anchorage-independent growth of NRK cells is similar to the one used for hematopoietic cell proliferation. We discuss this novel mechanism and its implication.

Endocytosis switch controlled by transmembrane osmotic pressure and phospholipid number asymmetry

The dynamics of endocytosis in living K562 cells was investigated after the osmotic pressure of the external medium was decreased and the transmembrane phospholipid number asymmetry was increased. When the external pressure was decreased by a factor of 0.54, a sudden inhibition of endocytosis was observed. Under these conditions, the endocytosis suddenly recovered after the phospholipid number asymmetry was increased. The phospholipid asymmetry was generated by the addition of exogenous phosphatidylserine, which is translocated by the endogenous flippase activity to the inner layer of the membrane. The recovery of endocytosis is thus consistent with the view that the phospholipid number asymmetry can act as a budding force for endocytosis. Moreover, we quantitatively predict both the inhibition and recovery of endocytosis as first-order phase transitions, using a general model that assumes the existence of a transmembrane surface tension asymmetry as the budding driving force. In this model, the tension asymmetry is considered to be elastically generated by the activity of phospholipid pumping. We finally propose that cells may trigger genetic transcription responses after the internalization of cytokine-receptor complexes, which could be controlled by variations in the cytosolic or external pressure.

Differential modulation of hepatocyte growth factor-stimulated motility by transforming growth factor beta1 on rat liver epithelial cells in vitro

We have previously shown that transforming growth factor-beta1 (TGF-beta1) enhances the epidermal growth factor- (EGF) and transforming growth factor-alpha (TGF-alpha)-stimulated motility of rat hepatocytes in an extracellular matrix (ECM)-dependent fashion (Stolz and Michalopoulos, 1997, J. Cell. Physiol., 170:57-68). We have extended this study to examine the effects of TGF-beta1 on hepatocyte growth factor (HGF) and EGF-stimulated motility of rat nonparenchymal liver epithelial cells (RLECs) in vitro and determined that chemotaxis, scattering, and monolayer wound healing by EGF was synergistically enhanced by TGF-beta1 on all ECMs examined. However, HGF-based motility, unlike EGF-stimulated motility, was modulated in an assay-dependent manner by TGF-beta1. HGF-stimulated chemotaxis was dramatically decreased by addition of TGF-beta1, but wound healing was synergistically enhanced by TGF-beta1 on all ECMs examined. HGF-based scattering was not consistently affected by TGF-beta1 on any ECM tested except on laminin, where scattering was often reduced by the concomitant addition of TGF-beta1. TGF-beta1 enhanced the motility associated with monolayer wound healing by HGF or EGF independent of DNA synthesis, because tritiated thymidine uptake was consistently reduced by 60% in the presence of TGF-beta1. The data indicate that HGF and EGF motility do not follow redundant signal-transduction pathways and that specific growth factor motility-related events, as measured by wound healing, scattering, and chemotaxis, are modulated independently by ECM and TGF-beta1.

Mitogenic-factor-dependent regulation of lipopolysaccharide and cytokine mixture-mediated hepatocyte nitric oxide synthesis in vitro

Hepatocellular mitogen (HGF and EGF) inhibited lipopolysaccharide and cytokine mixture (referred as LPS/CM)-induced NO synthesis and cellular injury in hepatocytes. Mitogenic inhibitors such as hydroxyurea and Wortmannin could not reverse EGF or HGF-inhibited NO production, whereas both of them showed some inhibitory effect on hepatocyte NO synthesis. Although activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) had no effect on hepatocyte NO synthesis, deletion of PKC activity by long-term treatment of hepatocytes with PMA abolished LPS/CM-induced NO production. In addition, pretreatment of hepatocytes with HGF and EGF also blocked LPS/CM-induced NO synthesis in the hepatocyte. These results suggest that proliferating signal is not directly involved in mitogen-inhibited NO synthesis in the hepatocyte, and LPS/CM-mediated NO synthesis is associated with the metabolic/redox state of hepatocytes.

Transforming growth factor beta increases the activity of phosphatidate phosphohydrolase-1 in rat hepatocytes

Phosphatidic acid (PA) is a potent second messenger arising from growth factor-induced stimulation of phospholipase D which hydrolyses phosphatidylcholine. PA is hydrolysed to diacylglycerol by PA phosphohydrolase (PAP) which exists in two forms: PAP-1 and PAP-2. In rat hepatocyte cultures, overnight (20h) incubation with transforming growth factor (TGF) beta (1 ng/ml) increased PAP-1 activity two-fold. This effect was concentration and time dependent and was greatest at low cell density. The TGFbeta effect on PAP-1 was additive to stimulation induced by dexamethasone but not by glucagon and it reversed the inhibition by insulin. Epidermal growth factor had no effect on PAP-1 activity. None of the above hormones or growth factors affected the subcellular distribution of PAP-1. Stimulation of PAP-1 by TGFbeta may be involved in mediating some of its biological effects.

Synergistic enhancement of EGF, but not HGF, stimulated hepatocyte motility by TGF-beta 1 in vitro

The ability of TGF-beta 1 (transforming growth factor-beta 1) to suppress growth factor induced proliferation of many cell types in vitro is well documented; however, TGF-beta 1 increases within a similar time frame as the hepatocyte mitogens HGF (hepatocyte growth factor), EGF (epidermal growth factor), and TGF-alpha (transforming growth factor-alpha) prior to hepatocyte proliferation during liver regeneration. This has raised the issue that TGF-beta 1 may have effects on hepatocytes additional to mito-inhibition and that these effects may be relevant to the regenerative process. To this end, we examined the effect of TGF-beta 1 on both the mitogenesis and the motility of growth factor stimulated primary rat hepatocytes and the hepatoblastoma cell line HepG2 in vitro. TGF-beta 1 significantly enhanced the chemotactic motility of EGF or TGF-alpha, and not HGF, stimulated hepatocytes on a collagen I substratum. TGF-beta 1 was not chemotactic when added alone and decreased the DNA synthesis of all hepatocyte cultures to near control levels. HepG2 cells were chemotactic toward HGF, EGF, and TGF-beta 1 alone and displayed an additive chemotactic response when TGF-beta 1 was added to either HGF or EGF. Additionally, HepG2 cells were refractory to the growth stimulatory effects of HGF or EGF and the growth inhibitory effects of TGF-beta 1. Hepatocytes plated onto other collagen-containing substrates (collagen IV, Matrigel, or ECL, an entactin-collagen IV-laminin matrix), but not on fibronectin or laminin alone, also displayed enhanced EGF stimulated motility by TGF-beta 1. The data indicate that an additional, novel role for TGF-beta 1 during liver tissue remodeling following PHx may include the synergistic enhancement EGF stimulated hepatocyte motility responses, and this enhancement is observed only on collagen-containing extracellular matrices.

Effects of octreotide on liver regeneration and tumour growth in the regenerating liver

The ability of the liver to regenerate following resection is remarkable. However, there is evidence to suggest that tumour growth within the regenerating liver is significantly increased. As octreotide (a synthetic analogue of somatostatin) inhibits the growth and development of hepatic tumour in rats, we have investigated its effects on liver regeneration, liver blood flow, hepatic reticuloendothelial system activity and tumour growth in the rat following partial hepatectomy (PH). Octreotide significantly inhibited liver regeneration in the rat 1 and 2 weeks following PH when compared with controls (regeneration index: 1.0 and 1.14 cf. 1.14 and 1.4, respectively). There was no significant difference in hepatic arterial or portal venous blood flow following PH in control or octreotide-treated rats. However, portal pressure was significantly reduced in octreotide-treated rats. Hepatic reticuloendothelial system activity was significantly increased in octreotide-treated rats compared with control animals 1 and 2 weeks after hepatectomy (uptake of radiolabelled technetium-99m albumin colloid: 2.2 and 3.9 cf. 1.6 and 1.9). The growth of both HSN (fibrosarcoma) and K12-Tr (colonic adenocarcinoma) cells in the regenerating liver was significantly decreased by octreotide treatment compared with controls (median percentage hepatic replacement: HSN control 71.3%, Octreotide 8.4%, K12-Tr Control 38.3%, Octreotide 4.5%). The results of the present study demonstrate that octreotide inhibits both liver regeneration and tumour growth following PH, possibly via a similar mechanism.

Transforming growth factor beta 1 induces mitogenesis in fetal rat brown adipocytes

The presence of transforming growth factor beta 1 (TGF-beta 1) for 24 or 48 h stimulated DNA synthesis, the percentage of cells in the S + G2/M phases of the cell cycle, and cell number, as compared to quiescent cells. The mitogenic capacity of TGF-beta 1 (1 pM) was similar to that shown by 10% fetal calf serum (FCS). TGF-beta 1 for 48 h increased by 5-fold the percentage of cells containing (3H)thymidine-labeled nuclei as compared to quiescent cels. In addition, single fetal brown adipocytes, showing their typical multilocular fat droplets phenotype, become positive for (3H)thymidine-labeled nuclei in response to TGF-beta 1. Moreover, TGF-beta 1 induced the mRNA expression of a complete set of proliferation-related genes, such as c-fos (30 min), c-myc and beta-actin (2 h), and H-ras, cdc2 kinase, and glucose 6-phosphate dehydrogenase (G6PD) at 4 and 8 h, as compared to quiescent cells. Concurrently, TGF-beta 1 for 12 h increased the protein content of proliferating cellular nuclear antigen (PCNA) by 6-fold and p21-ras by 2-fold. Although our results demonstrate that TGF-beta 1 induces the expression of very early genes related to cell proliferation, TGF-beta 1 could be acting either as a mitogen or as a survival factor in induce proliferation to fetal brown adipocytes.

Transforming growth factor-beta 1 modulates adenylyl cyclase signaling elements and epidermal growth factor signaling in cardiomyocytes

Studies presented in this report were designed to investigate the effects of transforming growth factor-beta 1 (TGF-beta 1) on epidermal growth factor (EGF)-mediated stimulation of cAMP accumulation in cardiac myocytes and elucidate the mechanism(s) involved in this modulation. TGF-beta 1 (20 pM) treatment of cardiac myocytes, in a time-dependent manner, decreased the ability of EGF (100 nM) to increase cAMP accumulation. Significant attenuation of EGF-elicited cAMP accumulation was observed 2 h after exposure to TGF-beta 1 and 18 h after addition of TGF-beta 1, the ability of EGF to increase cAMP accumulation was completely obliterated. TGF-beta 1 neither decreased immunoprecipitable EGF receptors in membranes from cardiomyocytes nor altered the specific binding of [125I]EGF to cardiomyocyte membranes. However, TGF-beta 1 decreased the ability of EGF to phosphorylate membrane proteins on tyrosine residues. TGF-beta 1 treatment of cardiomyocytes also decreased the ability of forskolin to augment cAMP accumulation in intact cells and stimulate adenylyl cyclase activity. Similarly, in membranes of TGF-beta 1-treated cells, neither isoproterenol nor EGF stimulated adenylyl cyclase activity. Interestingly, as assessed by the ability of A1F4- to stimulate adenylyl cyclase, TGF-beta 1 did not alter the coupling between Gs and catalytic subunits. Likewise, TGF-beta 1 did not alter the functional activity of the inhibitory regulatory element of the system, Gi. Western analysis of cellular proteins revealed that TGF-beta 1 did not alter the amounts of Ga alpha, Gi alpha 2, and Gi alpha 3. We conclude that TGF-beta 1 attenuates EGF-elicited cAMP accumulation in cardiomyocytes, in part, by decreasing the EGF receptor kinase function and that TGF-beta 1-mediated alterations in the activity of adenylyl cyclase catalytic subunit also contribute toward the regulation of adenylyl cyclase by various agonists.

Effect of acute ethanol exposure on cultured fetal rat hepatocytes: relation to mitochondrial function

Studies from our laboratory have shown that short-term ethanol exposure inhibits epidermal growth factor-dependent replication of cultured fetal rat hepatocytes, along with a drop in ATP level, and that these effects could be caused, at least in part, by ethanol-induced oxidative stress. In these prior studies, mitochondrial morphology was abnormal and membrane lipid peroxidation products were increased, along with reduced transmembrane potential and enhanced permeability to sucrose. To define the effects of ethanol on mitochondrial function further, the present study examines the impact of ethanol exposure on mitochondrial electron transport chain components. A 24-hr exposure of cultured fetal rat hepatocytes to ethanol (2.5 mg/ml) reduced mitochondrial complex I activity by 16% (p < 0.05), complex IV by 28% (p < 0.05), and succinate dehydrogenase by 23% (p < 0.05). This reduction was paralleled by lower ADP translocase activity (24%, p < 0.05) and diminished mitochondrial glutathione (GSH) (20%, p < 0.05). Pretreatment with 0.1 mM S-adenosyl methionine, before ethanol exposure, normalized mitochondrial GSH along with activities of complex I, complex IV, and succinate dehydrogenase. A 3-hr exposure of isolated mitochondria (which do not metabolize ethanol) to ethanol (2.5 mg/ml), inhibited the activities of complex I (19%, p < 0.05), complex IV (24%, p < 0.05), and of ATP synthesis (20%, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effect of micronodular and biliary cirrhosis on epidermal growth factor receptor expression in the rat

Cirrhosis is characterized by fibrogenesis, hepatocyte necrosis and the formation of regenerative nodules. Modulation of the epidermal growth factor receptor is an early event during regeneration. We have recently demonstrated alterations in the epidermal growth factor receptor during the development of biliary cirrhosis. The aim of the present study was to compare epidermal growth factor receptor distribution, expression and binding in biliary cirrhosis to that occurring in micronodular cirrhosis induced by phenobarbital/CCl4 exposition. Biliary cirrhosis and micronodular cirrhosis had similar functional impairment as assessed by the aminopyrine breath test. Epidermal growth factor receptor binding capacity was reduced in both models (control vs micronodular cirrhosis vs biliary cirrhosis: (mean +/- 1 SD) 60 +/- 22 vs 16 +/- 12 vs 27 +/- 9 fmol/mg protein, p < 0.05), while the binding constant was increased in biliary cirrhosis only. The receptor mass in plasma membrane, determined by Western blotting, was not changed. Distribution of epidermal growth factor receptor was assessed immunohistochemically on tissue sections. In both models, cytoplasmic staining was decreased and basolateral plasma membrane labeling was maintained. Nuclear localization was found in biliary cirrhosis only. In conclusion, in both models, cirrhosis induces an alteration in the binding properties, but not in the number of epidermal growth factor receptors in the plasma membrane. The loss of cytoplasmic epidermal growth factor receptor could reflect alterations in expression and/or in intracellular trafficking. This is supported by the reduced mRNA steady state levels for epidermal growth factor receptor which were found in both models, presumably representing down-regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

EGF receptor in neoplasia and metastasis

EGFR is a member of the tyrosine kinase family of cell surface receptors with a wide range of expression throughout development and in a variety of different cell types. The receptor can transmit signals to cells: i) upon interaction with ligands such as EGF, TGF alpha, amphiregulin or heparin binding EGF, ii) upon truncation or mutation of extracellular and/or intracellular domains, iii) upon amplification of a basal receptor activity (in the absence of ligand) through cooperation with other cellular signaling pathways or nuclear events (e.g. expression of v-erbA). The activated EGFR can exert pleiotropic functions on cells, depending on their tissue origin and state of differentiation. Under certain conditions it can also contribute to neoplasia and development of metastases. Such conditions can exist upon aberrant receptor/ligand expression and activation (e.g. in the wrong cell; at the wrong time; in the wrong amounts). Aberrant signalling can also occur through constitutive EGFR activation. Oncogenic potential of EGFR has been demonstrated in a wide range of experimental animals. EGFR is also implicated in human cancer, where it may contribute both to the initiation (glioblastoma) and progression (epithelial tumors) of the disease. EGFR may influence key steps in the processes of tumor invasion and dissemination. Involvement of EGFR in tumor spread may indicate a potential use of this receptor as a target for antimetastatic therapy.

Transforming growth factor beta and the cell surface in tumor progression

Type 1 transforming growth beta (TGF-beta 1) is a multifunctional regulator of cellular differentiation, motility and growth. It is capable of inhibiting or stimulating these processes depending on cell type, cell density, culture conditions and TGF-beta 1 concentration. TGF-beta 1 regulates growth, in part, by inducing the expression and secretion of various types of collagen, which participate in the control of cell adhesion and migration, as well as growth. TGF-beta 1 also regulates cell growth by controlling the response to epidermal growth factor (EGF) and other growth factors, in ways that can either decrease or increase their growth-promoting effects. Alterations in both negative and positive growth responses to TGF-beta 1 play important roles in tumor progression. Loss of sensitivity to growth inhibition by TGF-beta 1 can occur as a result of decreased expression of collagen. Acquisition of sensitivity to growth stimulation, and autocrine transformation by TGF-beta 1, are associated with aberrant EGF receptor regulation. Aberrant growth factor receptor regulation by TGF-beta 1 may be mediated by a protein kinase C (PKC)-dependent pathway which inhibits degradation of growth factor receptor/ligand complexes. The evidence reviewed is consistent with a minimal two-step mechanism for autocrine transformation, which involves production of growth factor and enhanced cellular response as a result of aberrant membrane traffic. Defects in membrane traffic regulation may provide an explanation for common alterations in tumor cell response to both multiple growth inhibitors and growth stimulators, and may also suggest novel approaches to cancer chemotherapy.

Differential proliferative response of cultured fetal and regenerating hepatocytes to growth factors and hormones

Upon epidermal growth factor (EGF) stimulation, fetal (20 days of gestation) and regenerating (44-48 h after partial hepatectomy) rat hepatocytes, isolated and cultured under identical conditions, increased DNA synthesis and entered into S-phase and mitosis, measured as [3H]thymidine incorporation and DNA content per nucleus in a flow cytometer, respectively. Fetal hepatocytes consisted of a homogeneous population of diploid (2C) cells. Two different populations of cells were present in regenerating liver, diploid (2C) and tetraploid (4C) cells, that responded to EGF. Glucagon or norepinephrine did not affect EGF stimulation of DNA synthesis in fetal liver cells, but they potentiated EGF response in regenerating hepatocyte cultures. Glucocorticoid hormones (dexamethasone) inhibited DNA synthesis in fetal hepatocyte cultures, an effect potentiated by the presence of glucagon or norepinephrine. In contrast, in regenerating hepatocytes, dexamethasone increased EGF-induced proliferation. EGF-dependent DNA synthesis was inhibited by TGF-beta in both fetal and regenerating cultured hepatocytes. TGF-beta action was partially suppressed by norepinephrine in regenerating hepatocytes, but was without effect in fetal hepatocyte cultures, whereas a synergistic action between TGF-beta and dexamethasone inhibiting growth in fetal but not in regenerating hepatocytes was found. Taken together, these results may suggest that there are significant differences between fetal and regenerating hepatocyte growth in their response to various hormones.

Transforming growth factor-beta 1 inhibitory effect of platelet-derived growth factor-induced signal transduction on human bone marrow fibroblasts: possible involvement of protein phosphatases

Transforming growth factor-beta 1 (TGF-beta 1) is a potent growth inhibitor for many cell types. On fibroblasts, TGF-beta 1 has been shown to inhibit human platelet-derived growth factor (PDGF)-induced mitogenicity. The mechanism implicated in this growth inhibition is unknown. In this work, we show on human bone marrow fibroblasts that TGF-beta 1, which inhibited PDGF-BB mitogenicity, was able to block PDGF-BB-induced early events such as polyphosphoinositide (PtdIns 4,5-P2, PtdIns 4-P, and PtdIns) breakdown and Ins 1,4,5-P3 formation. No significant modification by TGF-beta 1 of PDGF-BB binding (n1 = 200,000 vs. n2 = 195,000 sites per cell with TGF-beta 1; Kd1 = Kd2 = 0.5 x 10(-9) M) and of internalization kinetics was observed. In addition, TGF-beta 1 was shown to inhibit PDGF-BB receptor autophosphorylation either in intact cells or in partially isolated membranes and to partially inhibit PDGF-R tyrosine kinase activity. Since a dephosphorylation mechanism through protein phosphatases could be implicated, we used okadaic acid, a potent inhibitor of type 1 and 2A serine/threonine phosphatases and showed that okadaic acid restored PDGF-receptor autophosphorylation on tyrosine residues. Based on these data, we suggest that an alternative regulatory mechanism of PDGF tyrosine phosphorylation seems to involve serine/threonine phosphatase activation.