Liver cells

p63 controls metabolic activation of hepatic stellate cells and fibrosis via an HER2-ACC1 pathway

The p63 protein has pleiotropic functions and, in the liver, participates in the progression of nonalcoholic fatty liver disease (NAFLD). However, its functions in hepatic stellate cells (HSCs) have not yet been explored. TAp63 is induced in HSCs from animal models and patients with liver fibrosis and its levels positively correlate with NAFLD activity score and fibrosis stage. In mice, genetic depletion of TAp63 in HSCs reduces the diet-induced liver fibrosis. In vitro silencing of p63 blunts TGF-β1-induced HSCs activation by reducing mitochondrial respiration and glycolysis, as well as decreasing acetyl CoA carboxylase 1 (ACC1). Ectopic expression of TAp63 induces the activation of HSCs and increases the expression and activity of ACC1 by promoting the transcriptional activity of HER2. Genetic inhibition of both HER2 and ACC1 blunt TAp63-induced activation of HSCs. Thus, TAp63 induces HSC activation by stimulating the HER2-ACC1 axis and participates in the development of liver fibrosis.

Hepatic levels of S-adenosylmethionine regulate the adaptive response to fasting

There has been an intense focus to uncover the molecular mechanisms by which fasting triggers the adaptive cellular responses in the major organs of the body. Here, we show that in mice, hepatic S-adenosylmethionine (SAMe)-the principal methyl donor-acts as a metabolic sensor of nutrition to fine-tune the catabolic-fasting response by modulating phosphatidylethanolamine N-methyltransferase (PEMT) activity, endoplasmic reticulum-mitochondria contacts, β-oxidation, and ATP production in the liver, together with FGF21-mediated lipolysis and thermogenesis in adipose tissues. Notably, we show that glucagon induces the expression of the hepatic SAMe-synthesizing enzyme methionine adenosyltransferase α1 (MAT1A), which translocates to mitochondria-associated membranes. This leads to the production of this metabolite at these sites, which acts as a brake to prevent excessive β-oxidation and mitochondrial ATP synthesis and thereby endoplasmic reticulum stress and liver injury. This work provides important insights into the previously undescribed function of SAMe as a new arm of the metabolic adaptation to fasting.

N-Acetyl-2-Aminofluorene (AAF) Processing in Adult Rat Hepatocytes in Primary Culture Occurs by High-Affinity Low-Velocity and Low-Affinity High-Velocity AAF Metabolite-Forming Systems

N-acetyl-2-aminofluorene (AAF) is a procarcinogen used widely in physiological investigations of chemical hepatocarcinogenesis. Its metabolic pathways have been described extensively, yet little is known about its biochemical processing, growth cycle expression, and pharmacological properties inside living hepatocytes-the principal cellular targets of this hepatocarcinogen. In this report, primary monolayer adult rat hepatocyte cultures and high specific-activity [ring G-3 H]-N-acetyl-2-aminofluorene were used to extend previous observations of metabolic activation of AAF by highly differentiated, proliferation-competent hepatocytes in long-term cultures. AAF metabolism proceeded by zero-order kinetics. Hepatocytes processed significant amounts of procarcinogen (≈12 μg AAF/106 cells/day). Five ring-hydroxylated and one deacetylated species of AAF were secreted into the culture media. Extracellular metabolite levels varied during the growth cycle (days 0-13), but their rank quantitative order was time invariant: 5-OH-AAF > 7-OH-AAF > 3-OH-AAF > N-OH-AAF > aminofluorene (AF) > 1-OH-AAF. Lineweaver-Burk analyses revealed two principal classes of metabolism: System I (high-affinity and low-velocity), Km[APPARENT] = 1.64 × 10-7  M and VMAX[APPARENT] = 0.1 nmol/106 cells/day and System II (low-affinity and high-velocity), Km[APPARENT] = 3.25 × 10-5  M and VMAX[APPARENT] = 1000 nmol/106 cells/day. A third system of metabolism of AAF to AF, with Km[APPARENT] and VMAX[APPARENT] constants of 9.6 × 10-5  M and 4.7 nmol/106 cells/day, was also observed. Evidence provided in this report and its companion paper suggests selective roles and intracellular locations for System I- and System II-mediated AAF metabolite formation during hepatocarcinogenesis, although some of the molecules and mechanisms responsible for multi-system processing remain to be fully defined.

Role of Aramchol in steatohepatitis and fibrosis in mice

Nonalcoholic steatohepatitis (NASH) is the advanced form of nonalcoholic fatty liver disease (NAFLD) that sets the stage for further liver damage. The mechanism for the progression of NASH involves multiple parallel hits, including oxidative stress, mitochondrial dysfunction, inflammation, and others. Manipulation of any of these pathways may be an approach to prevent NASH development and progression. Arachidyl‐amido cholanoic acid (Aramchol) is presently in a phase IIb NASH study. The aim of the present study was to investigate Aramchol's mechanism of action and its effect on fibrosis using the methionine‐ and choline‐deficient (MCD) diet model of NASH. We collected liver and serum from mice fed an MCD diet containing 0.1% methionine (0.1MCD) for 4 weeks; these mice developed steatohepatitis and fibrosis. We also collected liver and serum from mice receiving a control diet, and metabolomes and proteomes were determined for both groups. The 0.1MCD‐fed mice were given Aramchol (5 mg/kg/day for the last 2 weeks), and liver samples were analyzed histologically. Aramchol administration reduced features of steatohepatitis and fibrosis in 0.1MCD‐fed mice. Aramchol down‐regulated stearoyl‐coenyzme A desaturase 1, a key enzyme involved in triglyceride biosynthesis and the loss of which enhances fatty acid β‐oxidation. Aramchol increased the flux through the transsulfuration pathway, leading to a rise in glutathione (GSH) and the GSH/oxidized GSH ratio, the main cellular antioxidant that maintains intracellular redox status. Comparison of the serum metabolomic pattern between 0.1MCD‐fed mice and patients with NAFLD showed a substantial overlap. Conclusion: Aramchol treatment improved steatohepatitis and fibrosis by 1) decreasing stearoyl‐coenyzme A desaturase 1 and 2) increasing the flux through the transsulfuration pathway maintaining cellular redox homeostasis. We also demonstrated that the 0.1MCD model resembles the metabolic phenotype observed in about 50% of patients with NAFLD, which supports the potential use of Aramchol in NASH treatment. (Hepatology Communications 2017;1:911–927)

Methionine and S-adenosylmethionine levels are critical regulators of PP2A activity modulating lipophagy during steatosis

BACKGROUND & AIMS

Glycine N-methyltransferase (GNMT) expression is decreased in some patients with severe non-alcoholic fatty liver disease. Gnmt deficiency in mice (Gnmt-KO) results in abnormally elevated serum levels of methionine and its metabolite S-adenosylmethionine (SAMe), and this leads to rapid liver steatosis development. Autophagy plays a critical role in lipid catabolism (lipophagy), and defects in autophagy have been related to liver steatosis development. Since methionine and its metabolite SAMe are well known inactivators of autophagy, we aimed to examine whether high levels of both metabolites could block autophagy-mediated lipid catabolism.

METHODS

We examined methionine levels in a cohort of 358 serum samples from steatotic patients. We used hepatocytes cultured with methionine and SAMe, and hepatocytes and livers from Gnmt-KO mice.

RESULTS

We detected a significant increase in serum methionine levels in steatotic patients. We observed that autophagy and lipophagy were impaired in hepatocytes cultured with high methionine and SAMe, and that Gnmt-KO livers were characterized by an impairment in autophagy functionality, likely caused by defects at the lysosomal level. Elevated levels of methionine and SAMe activated PP2A by methylation, while blocking PP2A activity restored autophagy flux in Gnmt-KO hepatocytes, and in hepatocytes treated with SAMe and methionine. Finally, normalization of methionine and SAMe levels in Gnmt-KO mice using a methionine deficient diet normalized the methylation capacity, PP2A methylation, autophagy, and ameliorated liver steatosis.

CONCLUSIONS

These data suggest that elevated levels of methionine and SAMe can inhibit autophagic catabolism of lipids contributing to liver steatosis.

Liver damage, inflammation, and enhanced tumorigenesis after persistent mTORC1 inhibition

Obesity can result in insulin resistance, hepatosteatosis, and nonalcoholic steatohepatitis (NASH) and increases liver cancer risk. Obesity-induced insulin resistance depends, in part, on chronic activation of mammalian target of rapamycin complex 1 (mTORC1), which also occurs in human and mouse hepatocellular carcinoma (HCC), a frequently fatal liver cancer. Correspondingly, mTORC1 inhibitors have been considered as potential NASH and HCC treatments. Using a mouse model in which high-fat diet enhances HCC induction by the hepatic carcinogen DEN, we examined whether mTORC1 inhibition attenuates liver inflammation and tumorigenesis. Notably, rapamycin treatment or hepatocyte-specific ablation of the specific mTORC1 subunit Raptor resulted in elevated interleukin-6 (IL-6) production, activation of signal transducer and activator of transcription 3 (STAT3), and enhanced HCC development, despite a transient reduction in hepatosteatosis. These results suggest that long-term rapamycin treatment, which also increases IL-6 production in humans, is unsuitable for prevention or treatment of obesity-promoted liver cancer.

Hepatoma cells from mice deficient in glycine N-methyltransferase have increased RAS signaling and activation of liver kinase B1

BACKGROUND & AIMS

Patients with cirrhosis are at high risk for developing hepatocellular carcinoma (HCC), and their liver tissues have abnormal levels of S-adenosylmethionine (SAMe). Glycine N-methyltransferase (GNMT) catabolizes SAMe, but its expression is down-regulated in HCC cells. Mice that lack GNMT develop fibrosis and hepatomas and have alterations in signaling pathways involved in carcinogenesis. We investigated the role of GNMT in human HCC cell lines and in liver carcinogenesis in mice.

METHODS

We studied hepatoma cells from GNMT knockout mice and analyzed the roles of liver kinase B1 (LKB1, STK11) signaling via 5'-adenosine monophosphate-activated protein kinase (AMPK) and Ras in regulating proliferation and transformation.

RESULTS

Hepatoma cells from GNMT mice had defects in LKB1 signaling to AMPK, making them resistant to induction of apoptosis by adenosine 3',5'-cyclic monophosphate activation of protein kinase A and calcium/calmodulin-dependent protein kinase kinase 2. Ras-mediated hyperactivation of LKB1 promoted proliferation of GNMT-deficient hepatoma cells and required mitogen-activated protein kinase 2 (ERK) and ribosomal protein S6 kinase polypeptide 2 (p90RSK). Ras activation of LKB1 required expression of RAS guanyl releasing protein 3 (RASGRP3). Reduced levels of GNMT and phosphorylation of AMPKα at Thr172 and increased levels of Ras, LKB1, and RASGRP3 in HCC samples from patients were associated with shorter survival times.

CONCLUSIONS

Reduced expression of GNMT in mouse hepatoma cells and human HCC cells appears to increase activity of LKB1 and RAS; activation of RAS signaling to LKB1 and RASGRP3, via ERK and p90RSK, might be involved in liver carcinogenesis and be used as a prognostic marker. Reagents that disrupt this pathway might be developed to treat patients with HCC.

Murine double minute 2 regulates Hu antigen R stability in human liver and colon cancer through NEDDylation

Hu antigen R (HuR) is a central RNA-binding protein regulating cell dedifferentiation, proliferation, and survival, which are well-established hallmarks of cancer. HuR is frequently overexpressed in tumors correlating with tumor malignancy, which is in line with a role for HuR in tumorigenesis. However, the precise mechanism leading to changes in HuR expression remains unclear. In the liver, HuR plays a crucial role in hepatocyte proliferation, differentiation, and transformation. Here, we unraveled a novel mean of regulation of HuR expression in hepatocellular carcinoma (HCC) and colon cancer. HuR levels correlate with the abundance of the oncogene, murine double minute 2 (Mdm2), in human HCC and colon cancer metastases. HuR is stabilized by Mdm2-mediated NEDDylation in at least three lysine residues, ensuring its nuclear localization and protection from degradation.

CONCLUSION

This novel Mdm2/NEDD8/HuR regulatory framework is essential for the malignant transformation of tumor cells, which, in turn, unveils a novel signaling paradigm that is pharmacologically amenable for cancer therapy.

S-Adenosylmethionine regulates connexins sub-types expressed by hepatocytes

Intercellular communication via GAP Junctions plays an important role in tissue homeostasis, apoptosis, carcinogenesis, cell proliferation and differentiation. Hepatocyte connexins (Cx) 26 and 32 levels are decreased during the de-differentiation process of primary hepatocytes in culture, a situation that is also characterized by a decrease in S-Adenosylmethionine (SAMe) levels. In this current study, we show that SAMe supplementation in cultured hepatocytes every 12h, leads to an up-regulation of Cx26 and 32 mRNA and protein levels and blocks culture-induced Cx43 expression, although it failed to increase Cx26 and 32 membrane localization and GAP junction intracellular communication. SAMe reduced nuclear β-catenin accumulation, which is known to stimulate the TCF/LEF-dependent gene transcription of Cx43. Moreover SAMe-induced reduction in Cx43 and β-catenin was prevented by the proteasome inhibitor MG132, and was not mediated by GSK3 activity. SAMe, and its metabolite 5'-methylthioadenosine (MTA) increased Cx26 mRNA in a process partially mediated by Adenosine A(2A) receptors but independent of PKA. Finally livers from MAT1A knockout mice, characterized by low hepatic SAMe levels, express higher Cx43 and lower Cx26 and 32 protein levels than control mice. These results suggest that SAMe maintains a characteristic expression pattern of the different Cxs in hepatocytes by differentially regulating their levels.

Activation of LKB1-Akt pathway independent of phosphoinositide 3-kinase plays a critical role in the proliferation of hepatocellular carcinoma from nonalcoholic steatohepatitis

LKB1, originally considered a tumor suppressor, plays an important role in hepatocyte proliferation and liver regeneration. Mice lacking the methionine adenosyltransferase (MAT) gene MAT1A exhibit a chronic reduction in hepatic S-adenosylmethionine (SAMe) levels, basal activation of LKB1, and spontaneous development of nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC). These results are relevant for human health because patients with liver cirrhosis, who are at risk to develop HCC, have a marked reduction in hepatic MAT1A expression and SAMe synthesis. In this study, we isolated a cell line (SAMe-deficient [SAMe-D]) from MAT1A knockout (MAT1A-KO) mouse HCC to examine the role of LKB1 in the development of liver tumors derived from metabolic disorders. We found that LKB1 is required for cell survival in SAMe-D cells. LKB1 regulates Akt-mediated survival independent of phosphoinositide 3-kinase, adenosine monophosphate protein-activated kinase (AMPK), and mammalian target of rapamycin complex (mTORC2). In addition, LKB1 controls the apoptotic response through phosphorylation and retention of p53 in the cytoplasm and the regulation of herpesvirus-associated ubiquitin-specific protease (HAUSP) and Hu antigen R (HuR) nucleocytoplasmic shuttling. We identified HAUSP as a target of HuR. Finally, we observed cytoplasmic staining of p53 and p-LKB1(Ser428) in a NASH-HCC animal model (from MAT1A-KO mice) and in liver biopsies obtained from human HCC derived from both alcoholic steatohepatitis and NASH.

CONCLUSION

The SAMe-D cell line is a relevant model of HCC derived from NASH disease in which LKB1 is the principal conductor of a new regulatory mechanism and could be a practical tool for uncovering new therapeutic strategies.

Hepatocyte IKKbeta/NF-kappaB inhibits tumor promotion and progression by preventing oxidative stress-driven STAT3 activation

The NF-kappaB activating kinase IKKbeta suppresses early chemically induced liver tumorigenesis by inhibiting hepatocyte death and compensatory proliferation. To study IKKbeta's role in late tumor promotion and progression, we developed a transplant system that allows initiated mouse hepatocytes to form hepatocellular carcinomas (HCC) in host liver after a long latency. Deletion of IKKbeta long after initiation accelerated HCC development and enhanced proliferation of tumor initiating cells. These effects of IKKbeta/NF-kappaB were cell autonomous and correlated with increased accumulation of reactive oxygen species that led to JNK and STAT3 activation. Hepatocyte-specific STAT3 ablation prevented HCC development. The negative crosstalk between NF-kappaB and STAT3, which is also evident in human HCC, is a critical regulator of liver cancer development and progression.

Hepatocyte IKKbeta/NF-kappaB inhibits tumor promotion and progression by preventing oxidative stress-driven STAT3 activation

The NF-kappaB activating kinase IKKbeta suppresses early chemically induced liver tumorigenesis by inhibiting hepatocyte death and compensatory proliferation. To study IKKbeta's role in late tumor promotion and progression, we developed a transplant system that allows initiated mouse hepatocytes to form hepatocellular carcinomas (HCC) in host liver after a long latency. Deletion of IKKbeta long after initiation accelerated HCC development and enhanced proliferation of tumor initiating cells. These effects of IKKbeta/NF-kappaB were cell autonomous and correlated with increased accumulation of reactive oxygen species that led to JNK and STAT3 activation. Hepatocyte-specific STAT3 ablation prevented HCC development. The negative crosstalk between NF-kappaB and STAT3, which is also evident in human HCC, is a critical regulator of liver cancer development and progression.

Impaired liver regeneration in mice lacking glycine N-methyltransferase

Hepatic S-adenosylmethionine (SAMe) is maintained constant by the action of methionine adenosyltransferase I/III (MATI/III), which converts methionine into SAMe and glycine N-methyltransferase (GNMT), which eliminates excess SAMe to avoid aberrant methylation reactions. During liver regeneration after partial hepatectomy (PH) MATI/III activity is inhibited leading to a decrease in SAMe. This injury-related reduction in SAMe promotes hepatocyte proliferation because SAMe inhibits hepatocyte DNA synthesis. In MATI/III-deficient mice, hepatic SAMe is reduced, resulting in uncontrolled hepatocyte growth and impaired liver regeneration. These observations suggest that a reduction in SAMe is crucial for successful liver regeneration. In support of this hypothesis we report that liver regeneration is impaired in GNMT knockout (GNMT-KO) mice. Liver SAMe is 50-fold higher in GNMT-KO mice than in control animals and is maintained constant following PH. Mortality after PH was higher in GNMT-KO mice than in control animals. In GNMT-KO mice, nuclear factor kappaB (NFkappaB), signal transducer and activator of transcription-3 (STAT3), inducible nitric oxide synthase (iNOS), cyclin D1, cyclin A, and poly (ADP-ribose) polymerase were activated at baseline. PH in GNMT-KO mice was followed by the inactivation of STAT3 phosphorylation and iNOS expression. NFkappaB, cyclin D1 and cyclin A were not further activated after PH. The LKB1/AMP-activated protein kinase/endothelial nitric oxide synthase cascade was inhibited, and cytoplasmic HuR translocation was blocked despite preserved induction of DNA synthesis in GNMT-KO after PH. Furthermore, a previously unexpected relationship between AMPK phosphorylation and NFkappaB activation was uncovered.

CONCLUSION

These results indicate that multiple signaling pathways are impaired during the liver regenerative response in GNMT-KO mice, suggesting that GNMT plays a critical role during liver regeneration, promoting hepatocyte viability and normal proliferation.

Humanin: a novel central regulator of peripheral insulin action

Background

Decline in insulin action is a metabolic feature of aging and is involved in the development of age-related diseases including Type 2 Diabetes Mellitus (T2DM) and Alzheimer's disease (AD). A novel mitochondria-associated peptide, Humanin (HN), has a neuroprotective role against AD-related neurotoxicity. Considering the association between insulin resistance and AD, we investigated if HN influences insulin sensitivity.

Methods and Findings

Using state of the art clamp technology, we examined the role of central and peripheral HN on insulin action. Continuous infusion of HN intra-cerebro-ventricularly significantly improved overall insulin sensitivity. The central effects of HN on insulin action were associated with activation of hypothalamic STAT-3 signaling; effects that were negated by co-inhibition of hypothalamic STAT-3. Peripheral intravenous infusions of novel and potent HN derivatives reproduced the insulin-sensitizing effects of central HN. Inhibition of hypothalamic STAT-3 completely negated the effects of IV HN analog on liver, suggesting that the hepatic actions of HN are centrally mediated. This is consistent with the lack of a direct effect of HN on primary hepatocytes. Furthermore, single treatment with a highly-potent HN analog significantly lowered blood glucose in Zucker diabetic fatty rats. Based upon the link of HN with two age-related diseases, we examined if there were age associated changes in HN levels. Indeed, the amount of detectable HN in hypothalamus, skeletal muscle, and cortex was decreased with age in rodents, and circulating levels of HN were decreased with age in humans and mice.

Conclusions

We conclude that the decline in HN with age could play a role in the pathogenesis of age-related diseases including AD and T2DM. HN represents a novel link between T2DM and neurodegeneration and along with its analogues offers a potential therapeutic tool to improve insulin action and treat T2DM.

Evidence for LKB1/AMP-activated protein kinase/ endothelial nitric oxide synthase cascade regulated by hepatocyte growth factor, S-adenosylmethionine, and nitric oxide in hepatocyte proliferation

S-adenosylmethionine (SAMe) is involved in numerous complex hepatic processes such as hepatocyte proliferation, death, inflammatory responses, and antioxidant defense. One of the most relevant actions of SAMe is the inhibition of hepatocyte proliferation during liver regeneration. In hepatocytes, SAMe regulates the levels of cytoplasmic HuR, an RNA-binding protein that increases the half-life of target messenger RNAs such as cyclin D1 and A2 via inhibition of hepatocyte growth factor (HGF)-mediated adenosine monophosphate-activated protein kinase (AMPK) phosphorylation. Because AMPK is activated by the tumor suppressor kinase LKB1, and AMPK activates endothelial nitric oxide (NO) synthase (eNOS), and NO synthesis is of great importance for hepatocyte proliferation, we hypothesized that in hepatocytes HGF may induce the phosphorylation of LKB1, AMPK, and eNOS through a process regulated by SAMe, and that this cascade might be crucial for hepatocyte growth. We demonstrate that the proliferative response of hepatocytes involves eNOS phosphorylation via HGF-mediated LKB1 and AMPK phosphorylation, and that this process is regulated by SAMe and NO. We also show that knockdown of LKB1, AMPK, or eNOS with specific interference RNA (iRNA) inhibits HGF-mediated hepatocyte proliferation. Finally, we found that the LKB1/AMPK/eNOS cascade is activated during liver regeneration after partial hepatectomy and that this process is impaired in mice treated with SAMe before hepatectomy, in knockout mice deficient in hepatic SAMe, and in eNOS knockout mice.

CONCLUSION

We have identified an LKB1/AMPK/eNOS cascade regulated by HGF, SAMe, and NO that functions as a critical determinant of hepatocyte proliferation during liver regeneration after partial hepatectomy.

Hepatocyte necrosis induced by oxidative stress and IL-1 alpha release mediate carcinogen-induced compensatory proliferation and liver tumorigenesis

Hepatocyte I kappaB kinase beta (IKK beta) inhibits hepatocarcinogenesis by suppressing accumulation of reactive oxygen species (ROS) and liver damage, whereas JNK1 activation promotes ROS accumulation, liver damage, and carcinogenesis. We examined whether hepatocyte p38 alpha, found to inhibit liver carcinogenesis, acts similarly to IKK beta in control of ROS metabolism and cell death. Hepatocyte-specific p38 alpha ablation enhanced ROS accumulation and liver damage, which were prevented upon administration of an antioxidant. In addition to elevated ROS accumulation, hepatocyte death, augmented by loss of either IKK beta or p38 alpha, was associated with release of IL-1 alpha. Inhibition of IL-1 alpha action or ablation of its receptor inhibited carcinogen-induced compensatory proliferation and liver tumorigenesis. IL-1 alpha release by necrotic hepatocytes is therefore an important mediator of liver tumorigenesis.

Differential regulation of the JNK/AP-1 pathway by S-adenosylmethionine and methylthioadenosine in primary rat hepatocytes versus HuH7 hepatoma cells

S-adenosylmethionine (AdoMet) and 5'-methylthioadenosine (MTA) exert a protective action on apoptosis induced by okadaic acid in primary rat hepatocytes but not in human transformed HuH7 cells. In the present work, we analyzed the role played by the JNK/activator protein (AP)-1 pathway in this differential effect. Okadaic acid induced the phosphorylation of JNK and c-Jun and the binding activity of AP-1 in primary hepatocytes, and pretreatment with either AdoMet or MTA prevented those effects. In HuH7 cells, pretreatment with either AdoMet or MTA did not affect JNK and c-Jun activation or AP-1 binding induced by okadaic acid. In both cell types, p38 was activated by okadaic acid, but neither AdoMet nor MTA presented a significant effect on its activity. Therefore, the differential effect of both AdoMet and MTA on the JNK/AP-1 pathway could explain their antiapoptotic effect on primary hepatocytes and the lack of protection they show against okadaic acid-induced apoptosis in hepatoma cells.

Glucocorticoids regulate transcription of the gene for phosphoenolpyruvate carboxykinase in the liver via an extended glucocorticoid regulatory unit

The hepatic transcriptional regulation by glucocorticoids of the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK-C) gene is coordinated by interactions of specific transcription factors at the glucocorticoid regulatory unit (GRU). We propose an extended GRU that consists of four accessory sites, two proximal AF1 and AF2 sites and their distal counterpart dAF1 (-993) and a new site, dAF2 (-1365); together, these four sites form a palindrome. Sequencing and gel shift binding assays of hepatic nuclear proteins interacting with these sites indicated similarity of dAF1 and dAF2 sites to the GRU proximal AF1 and AF2 sites. Chromatin immunoprecipitation assays demonstrated that glucocorticoids enhanced the binding of FOXO1 and peroxisome proliferator-activated receptor-alpha to AF2 and dAF2 sites and not to dAF1 site but enhanced the binding of hepatic nuclear transcription factor-4alpha only to the dAF1 site. Insulin inhibited the binding of these factors to their respective sites but intensified the binding of phosphorylated FOXO1. Transient transfections in HepG2 human hepatoma cells showed that glucocorticoid receptor interacts with several non-steroid nuclear receptors, yielding a synergistic response of the PEPCK-C gene promoter to glucocorticoids. The synergistic stimulation by glucocorticoid receptor together with peroxisome proliferator-activated receptor-alpha or hepatic nuclear transcription factor-4alpha requires all four accessory sites, i.e. a mutation of each of these markedly affects the synergistic response. Mice with a targeted mutation of the dAF1 site confirmed this requirement. This mutation inhibited the full response of hepatic PEPCK-C gene to diabetes by reducing PEPCK-C mRNA level by 3.5-fold and the level of circulating glucose by 25%.

Liver-specific inactivation of the Nrf1 gene in adult mouse leads to nonalcoholic steatohepatitis and hepatic neoplasia

Knockout studies have shown that the transcription factor Nrf1 is essential for embryonic development. Nrf1 has been implicated to play a role in mediating activation of oxidative stress response genes through the antioxidant response element (ARE). Because of embryonic lethality in knockout mice, analysis of this function in the adult knockout mouse was not possible. We report here that mice with somatic inactivation of nrf1 in the liver developed hepatic cancer. Before cancer development, mutant livers exhibited steatosis, apoptosis, necrosis, inflammation, and fibrosis. In addition, hepatocytes lacking Nrf1 showed oxidative stress, and gene expression analysis showed decreased expression of various ARE-containing genes, and up-regulation of CYP4A genes. These results suggest that reactive oxygen species generated from CYP4A-mediated fatty acid oxidation work synergistically with diminished expression of ARE-responsive genes to cause oxidative stress in mutant hepatocytes. Thus, Nrf1 has a protective function against oxidative stress and, potentially, a function in lipid homeostasis in the liver. Because the phenotype is similar to nonalcoholic steatohepatitis, these animals may prove useful as a model for investigating molecular mechanisms of nonalcoholic steatohepatitis and liver cancer.

IKK-beta links inflammation to obesity-induced insulin resistance

Inflammation may underlie the metabolic disorders of insulin resistance and type 2 diabetes. IkappaB kinase beta (IKK-beta, encoded by Ikbkb) is a central coordinator of inflammatory responses through activation of NF-kappaB. To understand the role of IKK-beta in insulin resistance, we used mice lacking this enzyme in hepatocytes (Ikbkb(Deltahep)) or myeloid cells (Ikbkb(Deltamye)). Ikbkb(Deltahep) mice retain liver insulin responsiveness, but develop insulin resistance in muscle and fat in response to high fat diet, obesity or aging. In contrast, Ikbkb(Deltamye) mice retain global insulin sensitivity and are protected from insulin resistance. Thus, IKK-beta acts locally in liver and systemically in myeloid cells, where NF-kappaB activation induces inflammatory mediators that cause insulin resistance. These findings demonstrate the importance of liver cell IKK-beta in hepatic insulin resistance and the central role of myeloid cells in development of systemic insulin resistance. We suggest that inhibition of IKK-beta, especially in myeloid cells, may be used to treat insulin resistance.

Resistance to TGF-beta-induced apoptosis in regenerating hepatocytes

Treatment with transforming growth factor beta (TGF-beta) of hepatocytes from two different proliferative conditions, such as fetal development and adult liver regeneration, shows that regenerating cells respond to this cytokine in terms of growth inhibition, but are less sensitive than the fetal ones to the apoptosis induced by this factor. Regenerating TGF-beta treated cells show higher cell viability and lower percentage of apoptotic cells than the fetal treated ones. Furthermore, TGF-beta treated regenerating hepatocytes maintain a well-preserved parenchyma-like organization. Treatment with TGF-beta induces the loss of mitochondrial transmembrane potential in fetal but not in regenerating hepatocytes and activation of caspase-3 is lower in regenerating than in fetal cells. Regenerating hepatocytes show higher intracellular levels of some antiapoptotic proteins, such as Bcl-x(L) and c-IAP-1 and, interestingly, they present higher intracellular glutathione levels, which might provide of mechanisms to avoid potential dangerous effects of the oxidative stress-mediated apoptosis induced by TGF-beta. In fact, treatment with BSO (a glutathione synthesis inhibitor) restores the response of regenerating hepatocytes to TGF-beta in terms of cell death. In conclusion, increased levels of Bcl-x(L) and cIAP-1 and higher intracellular glutathione levels could confer resistance to the apoptosis induced by TGF-beta during liver regeneration.

Transforming growth factor-beta activates both pro-apoptotic and survival signals in fetal rat hepatocytes

Transforming growth factor-beta (TGF-beta) induces apoptosis in fetal rat hepatocytes. However, a subpopulation of these cells survives concomitant with changes in morphology and phenotype, reminiscent of an epithelial mesenchymal transition (EMT) [Exp. Cell Res. 252 (1999) 281-291]. In this work, we have isolated the subpopulation that survives to TGF-beta-induced apoptosis, showing that these cells maintain the response to TGF-beta in terms of Smads activation and growth inhibition. Analyses of the intracellular signals that could impair the apoptotic effects of TGF-beta have indicated that the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway is activated in these resistant cells. Experiments in fetal rat hepatocytes have shown that TGF-beta is able to transiently activate PI 3-K/Akt by a mechanism independent of protein synthesis but dependent on a tyrosine kinase activity. Pro-apoptotic signals, such as oxidative stress and caspases, contribute to the loss of Akt at later times. Inhibiting PI 3-K sensitizes fetal hepatocytes (FH) to the apoptosis induced by TGF-beta and causes spontaneous death in the resistant cells. In conclusion, TGF-beta, as it is known for other cytokines, could be inducing pro-apoptotic and survival signals in hepatocytes, the balance among them will decide cell fate.

Steroid 5alpha reductase mRNA type 1 is differentially regulated by androgens and glucocorticoids in the rat liver

Testosterone and 5alpha-dihydrotestosterone (DHT) are the principal male hormones (androgens) in mammals. The enzyme, steroid 5alpha reductase catalyzes the conversion of testosterone (T) to its biologically potent steroid (DHT) in androgen dependent tissues. Two 5alpha reductase isoenzymes have been identified in rat tissues. The type I isoenzyme has been shown to be predominately expressed in the rat liver, whereas androgen target tissues of the genital tract express mainly isoenzyme II. The effects of androgens and glucocorticoids on the abundance of steroid 5alpha reductase type I (5alphaR1) messenger RNA in the rat liver were examined. Steady state levels of 5alphaR1 mRNA decreased dramatically to 1.5% of control levels 14 days following adrenalectomy (ADX), whereas dexamethasone (Dex) administered (0.5 mg/100 g) to ADX animals enhanced the expression of 5alphaR1 to twice its' normal values within 40 hours. Bilateral orchiectomy induced, within eight days, the expression of 5alphaR1 mRNA in the rat liver to 1.75 fold the normal value while testosterone injection failed to reduce this enhanced expression in castrated animals. Addition of Dex (1 microM) to primary cultures of rat hepatocyte resulted in a five- and three-fold reduction in the mRNA expression of 5alphaR1 after 24 and 48 hours, respectively. DHT (0.5 microM) however, induced the expression of 5alphaR1 mRNA by two- and seven-fold 24 and 48 hours post-treatment, respectively. In vitro nuclear run-on analysis of the 5alphaR1 gene showed no correlation between the rate of synthesis and steady state levels of this mRNA either in the intact liver or in cultured hepatocytes. These results appear to suggest that glucocorticoids and androgens differentially regulate 5alphaR1 mRNA in the rat liver. Moreover, our findings appear to indicate that regulation of 5alphaR1 gene is primarily at the post-transcriptional level.

SREBP-1c and Sp1 interact to regulate transcription of the gene for phosphoenolpyruvate carboxykinase (GTP) in the liver

The sterol regulatory element-binding protein-1c (SREBP-1c), as well as SREBP-1a and SREBP-2, inhibit transcription of the gene encoding the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C). There are two SREBP regulatory elements (SREs) in the PEPCK-C gene promoter (-322 to -313 and -590 to -581). The SRE at -590 overlaps an Sp1 site on the opposite strand of the DNA. These SREs bound SREBP-1a and SREBP-1c with low affinity but the addition of purified upstream stimulatory activity enhanced the binding of SREBP-1 to both of these sites. Mutating these SREs increased both unstimulated (5-fold) and protein kinase A-stimulated transcription (8-27-fold) from the PEPCK-C gene promoter; this was lost when both SREs were mutated. The SRE at -590 differs by a single base pair from the SRE in the low density lipoprotein (LDL) receptor gene (T in the PEPCK-C gene promoter at -582, compared with an A in the SRE of the gene for the LDL receptor promoter). Introduction of the LDL receptor SRE into the PEPCK-C gene promoter increased SREBP-1c binding and caused a 10-fold enhancement of basal transcription from the promoter, rather than an inhibition as observed with the SRE in the PEPCK-C gene promoter. The T/A change does not alter the binding of Sp1 to its site on the opposite strand of the DNA. Sp1 bound to the promoter independently of SREBP-1c but competed with SREBP-1c for binding. Sp1 does not bind to the SRE at -322. Chromatin immunoprecipitation analysis, using rat hepatocytes, demonstrated that SREBP-1 and Sp1 were associated in vivo with putative regulatory regions corresponding to the SREs in the PEPCK-C gene promoter. We propose that insulin represses transcription of the gene for PEPCK-C by inducing SREBP-1c production in the liver, which interferes with the stimulatory effect of Sp1 at -590 of the PEPCK-C gene promoter.

IKKbeta is required for prevention of apoptosis mediated by cell-bound but not by circulating TNFalpha

IkappaB kinase beta (IKKbeta) is required for NF-kappaB activation and suppression of TNFalpha-mediated liver apoptosis. To investigate how IKKbeta suppresses apoptosis, we generated hepatocyte-specific Ikkbeta knockout mice, Ikkbeta(Deltahep), which exhibit little residual NF- kappaB activity but are healthy with normal liver function. Unexpectedly, Ikkbeta(Deltahep) mice are slightly more sensitive than controls to LPS-induced liver apoptosis but are highly susceptible to liver destruction following concanavalin A (ConA)-induced T cell activation. Unlike LPS, a potent inducer of circulating TNFalpha, ConA exerts cytotoxic effects through cell-bound TNFalpha, which activates type 1 and 2 TNF receptors (TNFR). While TNFR2 does not contribute to NF-kappaB activation, it is important for ConA-induced JNK activation, which is augmented by the absence of IKKbeta. Using JNK-deficient mice we show that JNK is required for ConA-induced liver damage. Thus, the antiapoptotic function of IKKbeta, which is most critical in situations that involve cell-bound TNFalpha, is mediated partially through attenuation of JNK activity.

Hydroxylation and glycosylation of the four conserved lysine residues in the collagenous domain of adiponectin. Potential role in the modulation of its insulin-sensitizing activity

It has recently been shown that the fat-derived hormone adiponectin has the ability to decrease hyperglycemia and to reverse insulin resistance. However, bacterially produced full-length adiponectin is functionally inactive. Here, we show that endogenous adiponectin secreted by adipocytes is post-translationally modified into eight different isoforms, as shown by two-dimensional gel electrophoresis. Carbohydrate detection revealed that six of the adiponectin isoforms are glycosylated. The glycosylation sites were mapped to several lysines (residues 68, 71, 80, and 104) located in the collagenous domain of adiponectin, each having the surrounding motif of GXKGE(D). These four lysines were found to be hydroxylated and subsequently glycosylated. The glycosides attached to each of these four hydroxylated lysines are possibly glucosylgalactosyl groups. Functional analysis revealed that full-length adiponectin produced by mammalian cells is much more potent than bacterially generated adiponectin in enhancing the ability of subphysiological concentrations of insulin to inhibit gluconeogenesis in primary rat hepatocytes, whereas this insulin-sensitizing ability was significantly attenuated when the four glycosylated lysines were substituted with arginines. These results indicate that full-length adiponectin produced by mammalian cells is functionally active as an insulin sensitizer and that hydroxylation and glycosylation of the four lysines in the collagenous domain might contribute to this activity.

NO sensitizes rat hepatocytes to proliferation by modifying S-adenosylmethionine levels

BACKGROUND & AIMS

Liver regeneration is a fundamental response of this organ to injury. Hepatocyte proliferation is triggered by growth factors, such as hepatocyte growth factor. However, hepatocytes need to be primed to react to mitogenic signals. It is known that nitrous oxide (NO), generated after partial hepatectomy, plays an important role in hepatocyte growth. Nevertheless, the molecular mechanisms behind this priming event are not completely known. S-adenosylmethionine (AdoMet) synthesis by methionine adenosyltransferase is the first step in methionine metabolism, and NO regulates hepatocyte S-adenosylmethionine levels through specific inhibition of this enzyme. We have studied the modulation of hepatocyte growth factor-induced proliferation by NO through the regulation of S-adenosylmethionine levels.

METHODS

Studies were conducted in cultured rat hepatocytes isolated by collagenase perfusion, which triggers NO synthesis.

RESULTS

The mitogenic response to hepatocyte growth factor was blunted when inducible NO synthase was inhibited; this process was overcome by the addition of an NO donor. This effect was dependent on methionine concentration in culture medium and intracellular S-adenosylmethionine levels. Accordingly, we found that S-adenosylmethionine inhibits hepatocyte growth factor-induced cyclin D1 and D2 expression, activator protein 1 induction, and hepatocyte proliferation.

CONCLUSIONS

Together our findings indicate that NO may switch hepatocytes into a hepatocyte growth factor-responsive state through the down-regulation of S-adenosylmethionine levels.

Enhanced gene delivery in vitro and in vivo by improved transferrin-lipoplexes

Cationic liposomes and the complexes they form with DNA (lipoplexes) constitute the most promising alternative to the use of viral vectors for gene therapy. One of the limitations to their application in vivo, however, is the inhibition of gene delivery by serum. In a previous study, we demonstrated that transferrin (Tf)-lipoplexes were superior to plain lipoplexes in transfecting HeLa cells in the presence of high concentrations of serum. With the goal of obtaining efficient gene expression in vivo, we evaluated the efficacy of Tf-lipoplexes (containing DOTAP and cholesterol) in transfecting primary hepatocytes and adipocytes in the presence of high serum concentrations. The association of transferrin with cationic liposomes increased luciferase expression compared to plain lipoplexes in primary cells as well as in HepG2 and 3T3-L1 differentiated adipocytes. The complexes were not cytotoxic and were highly effective in protecting DNA from attack by DNase I. An efficient and reliable method was developed to prepare lipoplexes containing both Tf and protamine sulfate, where the latter was mixed with transferrin, followed by the addition of cationic liposomes and DNA. The resulting protamine-Tf-lipoplexes increased significantly the levels of gene expression in cultured cells and in various tissues in mice following i.v. administration.

Derivation, characterization, and phenotypic variation of hepatic progenitor cell lines isolated from adult rats

Liver progenitor cells (LPCs) cloned from adult rat livers following allyl alcohol injury express hematopoietic stem cell and early hepatic lineage markers when cultured on feeder layers; under these conditions, neither mature hepatocyte nor bile duct, Ito, stellate, Kupffer cell, or macrophage markers are detected. These phenotypes have remained stable without aneuploidy or morphological transformation after more than 100 population doublings. When cultured without feeder layers, the early lineage markers disappear, and mature hepatocyte markers are expressed; mature hepatocytic differentiation and cell size are also augmented by polypeptide and steroidal growth factors. In contrast to hepatocytic potential, duct-like structures and biliary epithelial markers are expressed on Matrigel. Because they were derived without carcinogens or mutagens, these bipotential LPC lines provide novel tools for models of cellular plasticity and hepatocarcinogenesis, as well as lines for use in cellular transplantation, gene therapy, and bioreactor construction.

The adipocyte-secreted protein Acrp30 enhances hepatic insulin action

Acrp30 is a circulating protein synthesized in adipose tissue. A single injection in mice of purified recombinant Acrp30 leads to a 2-3-fold elevation in circulating Acrp30 levels, which triggers a transient decrease in basal glucose levels. Similar treatment in ob/ob, NOD (non-obese diabetic) or streptozotocin-treated mice transiently abolishes hyperglycemia. This effect on glucose is not associated with an increase in insulin levels. Moreover, in isolated hepatocytes, Acrp30 increases the ability of sub-physiological levels of insulin to suppress glucose production. We thus propose that Acrp30 is a potent insulin enhancer linking adipose tissue and whole-body glucose metabolism.

Regulation of the glucose-6-phosphatase gene by glucose occurs by transcriptional and post-transcriptional mechanisms. Differential effect of glucose and xylitol

To understand how glucose regulates the expression of the glucose-6-phosphatase gene, the effect of glucose was studied in primary cultures of rat hepatocytes. Glucose-6-phosphatase mRNA levels increased about 10-fold when hepatocytes were incubated with 20 mm glucose. The rate of transcription of the glucose-6-phosphatase gene increased about 3-fold in hepatocytes incubated with glucose. The half-life of glucose-6-phosphatase mRNA was estimated to be 90 min in the absence of glucose and 3 h in its presence. Inhibition of the oxidative and the nonoxidative branches of the pentose phosphate pathway blocked the stimulation of glucose-6-phosphatase expression by glucose but not by xylitol or carbohydrates that enter the glycolytic/gluconeogenic pathways at the level of the triose phosphates. These results indicate that (i) the glucose induction of the mRNA for the catalytic unit of glucose-6-phosphatase occurs by transcriptional and post-transcriptional mechanisms and that (ii) xylitol and glucose increase the expression of this gene through different signaling pathways.

Dilinoleoylphosphatidylcholine selectively modulates lipopolysaccharide-induced Kupffer cell activation

Polyenylphosphatidylcholine (PPC), a mixture of polyunsaturated phosphatidylcholines extracted from soybeans, protects against alcoholic and non-alcoholic liver injury. Because Kupffer cells mediate liver injury, we hypothesized that PPC may modulate their activation. The activation of Kupffer cells by lipopolysaccharide (LPS) leads to an enhanced production of cytokines. Among these, tumor necrosis factor-alpha(TNF-alpha) exerts mainly a hepatotoxic effect, whereas interleukin-1beta (IL-1beta) appears to be hepatoprotective. The present study evaluated whether dilinoleoylphosphatidylcholine (DLPC), the main component of PPC (40% to 52%), affects LPS-induced Kupffer cell activation in vitro. For comparison, palmitoyl-linoleoylphosphatidylcholine (PLPC), the other major component of PPC (23% to 24%), and distearoylphosphatidylcholine (DSPC), the saturated counterpart of DLPC, were also tested. Rat Kupffer cells were cultured in serum-free RPMI-1640 medium containing 10 micromol/L of either DLPC, PLPC, or DSPC in the presence or absence of LPS (1 microg/mL). After 20 hours in culture, the media were collected for cytokine measurements by enzyme-linked immunosorbent assays. LPS significantly stimulated TNF-alpha and IL-1beta production by 62% and 328%, respectively. Treatment of Kupffer cells with LPS plus DLPC decreased the production of TNF-alpha by 23% (12.17+/-1.83 pg/ng DNA vs 15.72 +/-2.74 pg/ng DNA, P < .05, n = 6) and increased that of IL-1beta by 17% (1.80 +/- 0.16 pg/ng DNA vs 1.54 +/- 0.08 pg/ng DNA, P< .05, n = 6). No effect of PLPC or DSPC on LPS-induced TNF-alpha or IL-1beta generation was observed, thereby illustrating the selective effect of DLPC in this process. Thus DLPC selectively modulates the LPS-induced activation of Kupffer cells by decreasing the production of the cytotoxic TNF-alpha while increasing that of the protective IL-1beta. This dual action of DLPC on cytokines may provide a mechanism for the protective effect against liver injury, but its significance still needs to be determined by in vivo studies.

Fatty acid desaturation: effect of alphafetoprotein on alpha-linolenic acid conversion by fetal rat hepatocytes

Freshly isolated fetal hepatocytes transformed 4.3, 8.5 and 19.2 pmol/min/10(6) cells of stearic, linoleic and alpha-linolenic acids, respectively, complexed to albumin or alpha-fetoprotein (AFP), to more unsaturated derivatives. Thus, fetal hepatocytes displayed high elongase and delta9, delta6, delta5-desaturase activities, as well as an ability to synthesize hexaene derivatives. Desaturase activities decreased when the time of culture of fetal hepatocytes (previous to incubation with the substrate) was prolonged, being practically undetectable after 24 h of culture. However, the rate of fatty acid uptake remained nearly constant. When AFP was used as the carrier the amount of hexaene fatty acid derivatives of alpha-linolenic acid recovered in cells was reduced up to 50% by albumin. This effect was associated with an increase of radioactivity found in the culture medium of hepatocytes incubated with AFP compared to albumin. Both observations taken together could be explained by an efflux of hexaene derivatives from cells caused by AFP.

Hepatocyte primary culture bioassay: a simplified tool to assess the initiation of the liver regeneration cascade

The objective was to develop and optimize a hepatocyte primary culture bioassay to detect proliferative factors (PF) in the serum or plasma of partially hepatectomized (PHX) rats to serve as a tool to assess the initiation of the liver regeneration cascade. The bioassay detects PF by measuring hepatocyte proliferation through directly counting increases in viable cell number over the culture period using a hemocytometer. Hepatocytes were obtained using a two-step collagenase perfusion procedure. The purified hepatocytes (>80% viability, >95% parenchymal cells) were seeded into 6-well culture plates and allowed to attach overnight. The unattached cells were washed out, and the starting cell count was determined from three randomly selected wells after trypsin digestion. Sera from 2/3 PHX rats at 1-6 h postPHX was added to the culture. With a medium change at 24 h, the final cell counting was performed at 48 h. The net cell proliferation was expressed as the difference between the counts at 48 h and starting h. The optimized assay conditions could detect an increase of PF in PHX rat serum between 1 and 4 h after PHX (peaking at 4 h). The bioassay showed both a qualitative and quantitative sensitivity to distinguish between the PF levels in 1/3 and 2/3 PHX rats.

Differentiation of pancreatic epithelial progenitor cells into hepatocytes following transplantation into rat liver

The ability to identify, isolate, and transplant progenitor cells from solid tissues would greatly facilitate the treatment of diseases currently requiring whole organ transplantation. In this study, cell fractions enriched in candidate epithelial progenitor cells from the rat pancreas were isolated and transplanted into the liver of an inbred strain of Fischer rats. Using a dipeptidyl dipeptidase IV genetic marker system to follow the fate of transplanted cells in conjunction with albumin gene expression, we provide conclusive evidence that, after transplantation to the liver, epithelial progenitor cells from the pancreas differentiate into hepatocytes, express liver-specific proteins, and become fully integrated into the liver parenchymal structure. These studies demonstrate the presence of multipotent progenitor cells in the adult pancreas and establish a role for the liver microenvironment in the terminal differentiation of epithelial cells of foregut origin. They further suggest that such progenitor cells might be useful in studies of organ repopulation following acute or chronic liver injury.

Correlation of expression of connexin mRNA isoforms with degree of cellular differentiation

Examination of rat hepatic cell lines has revealed a correlation between the differentiated state of the cells and the gap junctional proteins, or connexins, they express. The cell lines RLC (Gershenson et al, 1970) and FTO.2B (Killary et al, 1984) were examined and compared to primary adult hepatocytes for expression of fetal and adult hepatic antigens under various tissue culture conditions. Maximal expression of fetal antigens was observed in cells grown in serum-supplemented medium, on either tissue culture plastic or type IV collagen. Maximal expression of adult specific antigens was seen in cells grown in a hormonally defined medium containing heparin, on type I or type IV collagen. The cell line RLC strongly expressed fetal antigens, while FTO.2B expressed both fetal and adult antigens. These cell lines and another poorly differentiated hepatic cell line, WB-F344 (Tsao et al., 1984) were used to assess the developmental profile of mRNAs encoding isoforms of gap junctions: connexins 26, 32, and 43. The cell lines each transcribed mRNAs of all three connexins, as determined by transcriptional elongation analysis. By contrast, only certain of the connexin mRNAs could be detected in specific cell lines by Northern analysis: RLC expressed only connexin 43 mRNA; WB-F344 expressed connexin 32 and 43 mRNAs. Selection among the connexin mRNAs appears to occur post-transcriptionally. Culture of the cell lines in hormonally defined medium vs. serum supplemented medium did not affect the patterns of connexin mRNA abundance. When the cell lines were cultured in hormonally defined medium containing heparin, however, the level of connexin mRNAs did vary: Connexin 26 mRNA increased in WB-F344 cells, and connexins 32 and 43 mRNAs increased in FTO.2B, but connexin 43 mRNA decreased in WB-F344 and RLC. The abundance of connexin mRNAs also varied when the cell lines were analyzed at different cell densities: connexin 43 mRNA increased with cell density in RLC and WB-F344, and connexin 26 mRNA peaked at an intermediate density and fell at higher cell densities in WB-F344. The differences in connexin mRNA expression among cell lines characteristic of different stages of hepatic differentiation, and the differences in regulation of connexin mRNAs in the hepatic cell lines, suggest distinct biological roles of the highly homologous proteins. Moreover, connexin gene expression may be a marker of hepatic development: as hepatocytes differentiate the proportions of connexin 43 then 26 mRNAs decrease while that of connexin 32 mRNA increases.

Secretion, surface localization, turnover, and steady state expression of protein disulfide isomerase in rat hepatocytes

Protein disulfide isomerase in isolated rat hepatocytes was present at a concentration of 7 micrograms/mg cell protein, representing a approximately 2-fold enrichment compared to isolated hepatic non-parenchymal cells. Though localized mainly in microsomal fractions of hepatocytes, direct immunofluorescence and cell surface radioiodination followed by immunoprecipitation revealed the presence of M(r) 57,000 disulfide isomerase at the cell surface. Electrostatic interaction of the protein with the cell surface was suggested by susceptibility to carbonate washing. Metabolic radiolabeling and immunoprecipitation studies also indicated that some of the newly synthesized M(r) 57,000 disulfide isomerase was secreted. Treatment of cells with colchicine markedly reduced the recovery of disulfide isomerase from the media, indicating microtubular-directed secretion of the protein. Partial staphlococcal V8 proteolytic digestion of the secreted protein revealed a peptide pattern similar to that of the cellular protein. Immunoprecipitation with antibody specific to the -KDEL peptide retention sequence confirmed the presence of this sequence in the secreted protein. Studies of the turnover of disulfide isomerase revealed a half-life of approximately 96 h. Treatment of cells with tunicamycin or heat shock resulted in an increased recovery of newly synthesized disulfide isomerase from cell lysates but diminished recovery from the media. The secretion and cell surface distribution of disulfide isomerase in hepatocytes may be important for the pathogenesis of immune mediated liver injury.

Transcriptional and posttranscriptional regulation of hepatic beta 2-adrenergic receptor gene expression during development

Hepatic responsiveness to beta 2-adrenergic stimulation is dynamically regulated during early development as well as following hepatic injury and disease. In the present study, the molecular mechanisms that underlie the decline in the steady-state levels of hepatic beta 2-adrenergic receptor mRNA that occurs during development in the male rat were investigated. As determined by nuclear run-on assays, an age-associated reduction in beta 2-adrenergic receptor gene transcription was observed. The transcription rate of the beta 2-adrenergic receptor gene in postnatal day 18 liver was approximately 50% lower than that of fetal liver. Stability of beta 2-adrenergic receptor gene transcripts was highest (t1/2 approximately 6h) in hepatocytes isolated from fetal rats and was lowest (t1/2 approximately 6h) in hepatocytes from postnatal day 14 rats. In fetal hepatocytes, but not postnatal day 2 hepatocytes, cycloheximide appeared to stabilize beta 2-adrenergic receptor gene transcripts in the presence of actinomycin D. These findings establish the molecular basis of reduced steady-state levels of beta 2-adrenergic receptor mRNA in liver during early postnatal development and suggest multilevel regulatory control of hepatic beta 2-adrenergic receptor gene expression.

Ito cell expression of a nuclear retinoic acid receptor

Although it has been suggested that retinoids regulate Ito cell proliferation and collagen synthesis, little is known about the ability of Ito cells to respond to retinoids in vivo. Because retinoids may mediate their molecular effects through nuclear receptors, Ito cells were examined for the presence of one of these receptors, nuclear retinoic acid receptor-beta. The modulation of nuclear retinoic acid receptor-beta expression was also studied during cell culture and hepatic fibrogenesis. Northern hybridization analysis revealed that Ito cells freshly isolated from normal rat liver contained nuclear retinoic acid receptor-beta messenger RNA at levels significantly higher than those found in other hepatic cell types. Ito cells also contained messenger RNA for two other nuclear retinoic acid receptors, nuclear retinoic acid receptor-alpha and nuclear retinoic acid receptor-gamma. Using an antibody to human nuclear retinoic acid receptor-beta, the nuclear presence of this receptor was demonstrated in normal Ito cells. In contrast, Ito cells cultured for at least 7 days had no detectable messenger RNA or nuclear staining for nuclear retinoic acid receptor-beta despite a 20 +/- 5-fold increase in the messenger RNA level of another retinoid binding protein, cellular retinol binding protein. Analysis of Ito cells isolated from rats with carbon tetrachloride-induced hepatic fibrosis revealed an 81% +/- 3% decrease in nuclear retinoic acid receptor-beta messenger RNA levels in these cells when compared with normal Ito cells. No difference in the messenger RNA levels of cellular retinol binding protein was found in Ito cells isolated from either normal or fibrotic liver.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of hepatic fibrosis on Ito cell gene expression

While Ito cells appear to be a major source of increased matrix synthesis during hepatic fibrogenesis, the cellular changes that occur in these cells during liver fibrosis have not been well delineated. In this study we examined Ito cell gene expression in isolated cells from normal rats, and rats with carbon tetrachloride-induced fibrosis, in order to better define the changes occurring in these cells during this pathologic process. Specifically, we addressed three questions: (1) which matrix genes are over expressed in Ito cells in fibrotic liver; (2) do these cells increase their expression of the fibrogenic cytokine transforming growth factor-beta 1 (TGF-beta 1); and (3) do Ito cells change their phenotype during hepatic fibrogenesis as reflected by alterations in the expression of their intermediate filament genes? Northern hybridization analysis revealed that Ito cells isolated from fibrotic livers had significant increases in mRNA levels of types I, III and IV procollagen compared to normal cells, while no increases were found in hepatocytes, and Kupffer/endothelial cells had only an increase in type I procollagen mRNA. Analysis of other matrix proteins which increase during hepatic fibrogenesis revealed elevations in laminin B and fibronectin mRNA levels only in Ito cells. Increased Ito cell matrix gene expression was also associated with a 4-fold increase in TGF-beta 1 levels in these cells. No increase in TGF-beta 1 mRNA was found in hepatocytes, and less than a 2-fold increase was found in Kupffer/endothelial cells isolated from fibrotic livers.(ABSTRACT TRUNCATED AT 250 WORDS)

Towards liver-directed gene therapy: retrovirus-mediated gene transfer into human hepatocytes

Liver-directed gene therapy is being considered in the treatment of inherited metabolic diseases. One approach we are considering is the transplantation of autologous hepatocytes that have been genetically modified with recombinant retroviruses ex vivo. We describe, in this report, techniques for isolating human hepatocytes and efficiently transducing recombinant genes into primary cultures. Hepatocytes were isolated from tissue of four different donors, plated in primary culture, and exposed to recombinant retroviruses expressing either the LacZ reporter gene or the cDNA for rabbit LDL receptor. The efficiency of gene transfer under optimal conditions, as determined by Southern blot analysis, varied from a maximum of one proviral copy per cell to a minimum of 0.1 proviral copy per cell. Cytochemical assays were used to detect expression of the recombinant derived proteins, E. coli beta-galactosidase and rabbit LDL receptor. Hepatocytes transduced with the LDL receptor gene expressed levels of receptor protein that exceeded the normal endogenous levels. The ability to isolate and genetically modify human hepatocytes, as described in this report, is an important step towards the development of liver-directed gene therapies in humans.

The role of capillarization in hepatic failure: studies in carbon tetrachloride-induced cirrhosis

During the cirrhotic process, the hepatic microvascular phenotype is transformed from sinusoids (discontinuous capillaries) into continuous capillaries. This transformation has been termed capillarization. Many hepatic functions depend on the rapid, bidirectional exchange of macromolecules between plasma and hepatocytes. To determine whether capillarization contributes to hepatic failure in cirrhosis, we decided to study the plasma clearance (125I) and hepatocyte uptake (electron microscopy) of three tracers in normal and cirrhotic rats. The tracers chosen were a hemeundecapeptide with peroxidatic activity (fluid-phase pinocytosis), asialofetuin (receptor-mediated endocytosis of a medium size protein) and ferritin (receptor-mediated endocytosis of a large size protein). The results demonstrate a decreased hepatocyte uptake of hemeundecapeptide; a significant delay in plasma clearance of asialofetuin; and a minor delay in plasma clearance of ferritin, but a striking trapping of ferritin in the cirrhotic capillary basement membrane. The delayed plasma clearance in cirrhosis cannot be ascribed to a decreased number of surface receptors because, in isolated hepatocytes, the number of molecules bound per cell was equivalent in normal and cirrhotic livers. These findings support the concept of capillarization, with the formation of continuous diffusion and filtration barriers between plasma and hepatocytes, representing a significant hindrance to the bidirectional macromolecular exchange normally taking place between these two compartments. Furthermore, at least in the case of ferritin, the capillary basement membrane of cirrhotic livers seems to be the major filtration barrier. This hindrance to hepatocyte uptake, and presumably also to secretion, may be the cause (or at least a major determinant) of the hepatic failure characteristic of cirrhosis.

Effect of alpha-fetoprotein and albumin on the uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal rat hepatocytes

The uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal hepatocytes has been studied using albumin and alpha-fetoprotein (AFP) as carrier proteins. Both types of cells took up linoleic (18:2(n-6)) and linolenic (18:3(n-3)) fatty acids at the same rate when they were added complexed either to albumin or AFP at a 1:1 molar ratio. At 37 degrees C a greater incorporation of arachidonic acid (20:4(n-6)) and mainly docosahexaenoic acid (22:6(n-3)) was observed when these acids were bound to albumin (6.5 nmol 20:4(n-6); 6.4 nmol 22:6(n-3) /million cells) as compared to AFP (5.5 nmol 20:4(n-6); 4.3 nmol 22:6(n-3)/million cells). The 20:4(n-6) and 22:6(n-3) uptake seems to be inversely related to the apparent association constants (k'a) between these fatty acids and both proteins (1.3 and 1.5 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 6.4 and 54.0 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP). Experiments carried out at 4 degrees C indicated that binding of 20:4(n-6) (0.83 nmol/million cells in presence of albumin; 2.16 nmol/million cells in presence of AFP) and 22:6(n-3) (0.83 nmol/million cells in presence of albumin; 1.32 nmol/million cells in presence of AFP) by cell membranes was also inversely related to the k'a of these proteins. At 4 degrees C, the k'a of AFP and albumin for 20:4(n-6) and 22:6(n-3) changed considerably (12.7 and 9.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 3.9 and 14.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP) with respect to the k'a calculated at 37 degrees C. Hence, k'a values were higher for albumin and lower for AFP than the corresponding values at 37 degrees C. It was concluded that uptake by cells and interaction of fatty acids with cell membranes depend mainly on the k'a of fatty acids and carrier proteins at equilibrium.

Defective ATP-dependent bile canalicular transport of organic anions in mutant (TR-) rats with conjugated hyperbilirubinemia

TR- mutant Wistar rats secrete markedly fewer organic anions other than bile acids from the liver into the bile than do control rats. Fluorescence-image analysis of isolated normal and TR- hepatocyte "doublets", which retain a bile canaliculus between them, revealed that normal hepatocytes readily transport a fluorescent bile acid (fluorescein isothiocyanate glycocholate) and a nonbile acid organic anion (carboxydichlorofluorescein diacetate) into the canaliculus. Hepatocyte doublets from TR- rats also transported fluorescein isothiocyanate glycocholate normally, but transport of carboxydichlorofluorescein diacetate into the canaliculus was negligible. Vesicles derived from the canicular domain of the plasma membrane of hepatocytes (CMV) from control and TR- rats were used to characterize the transport process for 35S-labeled bromosulphthalein and 35S-labeled bromosulphthalein glutathione, which represent nonbile acid organic anions. CMV from normal rat hepatocytes had an ATP- and temperature-dependent, saturable transport process for these 35S-labeled compounds that was absent in CMV from TR- rats. CMV from TR- rats retained normal ATP-dependent transport of daunomycin, and immunologic blots with a monoclonal antibody against the multidrug resistance gene product, P-glycoprotein, revealed no difference between normal and TR-CMV. These studies reveal that the bile canaliculus in normal rats contains an ATP-dependent organic anion transport system that is functionally absent in TR- mutant rats. The defect in TR- mutant rats is phenotypically similar to that seen in mutant Corriedale sheep and in the Dubin-Johnson syndrome in man.

Gamma-interferon treatment inhibits collagen deposition in murine schistosomiasis

Since interferons have been shown to affect the synthesis of matrix proteins such as collagen in several in vitro systems, the potential role of gamma-interferon in inhibiting hepatic fibrosis was investigated. Hepatic cells, consisting primarily of hepatocytes, were treated with recombinant gamma-interferon for 24 hr. Northern blot hybridization showed that gamma-interferon treatment caused a profound decrease in pro-alpha 2(I)collagen mRNA levels but an increase in beta-actin mRNA content. The effects of gamma-interferon were then studied in an in vivo model of hepatic fibrogenesis, murine schistosomiasis. Schistosoma-infected mice were treated with daily i.m. injections of gamma-interferon for a 4-week period starting 4 weeks after the initial infection. gamma-Interferon treatment decreased collagen deposition as determined by histologic evaluation and measurement of total liver collagen content. Northern blots showed Types I and III procollagen mRNA levels for treated, infected animals to be only 32 and 29% that of infected controls, but beta-actin mRNA levels were significantly elevated. These results indicate a potential role for gamma-interferon as an antifibrogenic agent in vivo.

Regulation of beta-adrenergic receptor expression in rat liver

To begin defining the factors regulating neurotransmitter receptor expression, we examined beta-adrenergic receptors in rat liver in vivo and in primary liver cultures under defined hormonal conditions. beta-receptors described a remarkable developmental profile in vivo, increasing fivefold between embryonic days 16 and 20, and decreasing tenfold by early adulthood. The developmental decrease reflected reduced receptor number without a change in receptor properties. The ontogenetic decrease appeared to be specific for beta-receptors; alpha-receptors developed in a hyperbolic fashion, reaching high plateau values by the third postnatal week. Adult rat liver cells plated into culture re-expressed high beta-receptor levels, exhibiting a 4-8-fold increase. A similar pattern of expression of the beta-receptors, having similar pharmacological properties, was observed in primary liver cultures maintained in serum-free medium, in a serum-supplemented medium or in several variations of a serum-free, hormonally defined medium designed for primary liver cultures. Thus, the degree of expression of the beta-receptors was not found affected by various hormones, by serum, or by any medium condition. By contrast, the degree of expression of the beta-receptors was markedly sensitive to cell density. High expression of the beta-receptors was observed at low cell densities (1-3 x 10(6) cells/150 mm dish), and low expression or no expression was observed in confluent cultures (10-20 x 10(6) cells/150 mm dish). Our experiments suggest that beta-receptor expression does not follow an immutable program, but may be regulated by density-dependent cell-cell interactions.