Differential expression of guanine nucleotide-binding proteins enhances cAMP synthesis in regenerating rat liver

Liver Regeneration and Immunity: A Tale to Tell

The physiological importance of the liver is demonstrated by its unique and essential ability to regenerate following extensive injuries affecting its function. By regenerating, the liver reacts to hepatic damage and thus enables homeostasis to be restored. The aim of this review is to add new findings that integrate the regenerative pathway to the current knowledge. An optimal regeneration is achieved through the integration of two main pathways: IL-6/JAK/STAT3, which promotes hepatocyte proliferation, and PI3K/PDK1/Akt, which in turn enhances cell growth. Proliferation and cell growth are events that must be balanced during the three phases of the regenerative process: initiation, proliferation and termination. Achieving the correct liver/body weight ratio is ensured by several pathways as extracellular matrix signalling, apoptosis through caspase-3 activation, and molecules including transforming growth factor-beta, and cyclic adenosine monophosphate. The actors involved in the regenerative process are numerous and many of them are also pivotal players in both the immune and non-immune inflammatory process, that is observed in the early stages of hepatic regeneration. Balance of Th17/Treg is important in liver inflammatory process outcomes. Knowledge of liver regeneration will allow a more detailed characterisation of the molecular mechanisms that are crucial in the interplay between proliferation and inflammation.

Mice depleted for Exchange Proteins Directly Activated by cAMP (Epac) exhibit irregular liver regeneration in response to partial hepatectomy

The exchange proteins directly activated by cAMP 1 and 2 (Epac1 and Epac2) are expressed in a cell specific manner in the liver, but their biological functions in this tissue are poorly understood. The current study was undertaken to begin to determine the potential roles of Epac1 and Epac2 in liver physiology and disease. Male C57BL/6J mice in which expression of Epac1 and/or Epac2 are deleted, were subjected to partial hepatectomy and the regenerating liver was analyzed with regard to lipid accumulation, cell replication and protein expression. In response to partial hepatectomy, deletion of Epac1 and/or Epac2 led to increased hepatocyte proliferation 36 h post surgery, and the transient steatosis observed in wild type mice was virtually absent in mice lacking both Epac1 and Epac2. The expression of the protein cytochrome P4504a14, which is implicated in hepatic steatosis and fibrosis, was substantially reduced upon deletion of Epac1/2, while a number of factors involved in lipid metabolism were significantly decreased. Moreover, the number of Küpffer cells was affected, and Epac2 expression was increased in the liver of wild type mice in response to partial hepatectomy, further supporting a role for these proteins in liver function. This study establishes hepatic phenotypic abnormalities in mice deleted for Epac1/2 for the first time, and introduces Epac1/2 as regulators of hepatocyte proliferation and lipid accumulation in the regenerative process.

Altered lysophosphatidic acid (LPA) receptor expression during hepatic regeneration in a mouse model of partial hepatectomy

BACKGROUND

Hepatic regeneration requires coordinated signal transduction for efficient restoration of functional liver mass. This study sought to determine changes in lysophosphatidic acid (LPA) and LPA receptor (LPAR) 1-6 expression in regenerating liver following two-thirds partial hepatectomy (PHx).

METHODS

Liver tissue and blood were collected from male C57BL/6 mice following PHx. Circulating LPA was measured by enzyme-linked immunosorbent assay (ELISA) and hepatic LPAR mRNA and protein expression were determined.

RESULTS

Circulating LPA increased 72 h after PHx and remained significantly elevated for up to 7 days post-PHx. Analysis of LPAR expression after PHx demonstrated significant increases in LPAR1, LPAR3 and LPAR6 mRNA and protein in a time-dependent manner for up to 7 days post-PHx. Conversely, LPAR2, LPAR4 and LPAR5 mRNA were barely detected in normal liver and did not significantly change after PHx. Changes in LPAR1 expression were confined to non-parenchymal cells following PHx.

CONCLUSIONS

Liver regeneration following PHx is associated with significant changes in circulating LPA and hepatic LPAR1, LPAR3 and LPAR6 expression in a time- and cell-dependent manner. Furthermore, changes in LPA-LPAR post-PHx occur after the first round of hepatocyte division is complete.

Gamma aminobutyric acid B and 5-hydroxy tryptamine 2A receptors functional regulation during enhanced liver cell proliferation by GABA and 5-HT chitosan nanoparticles treatment

Liver is one of the major organs in vertebrates and hepatocytes are damaged by many factors. The liver cell maintenance and multiplication after injury and treatment gained immense interest. The present study investigated the role of Gamma aminobutyric acid (GABA) and serotonin or 5-hydroxytryptamine (5-HT) coupled with chitosan nanoparticles in the functional regulation of Gamma aminobutyric acid B and 5-hydroxy tryptamine 2A receptors mediated cell signaling mechanisms, extend of DNA methylation and superoxide dismutase activity during enhanced liver cell proliferation. Liver injury was achieved by partial hepatectomy of male Wistar rats and the GABA and 5-HT chitosan nanoparticles treatments were given intraperitoneally. The experimental groups were sham operated control (C), partially hepatectomised rats with no treatment (PHNT), partially hepatectomised rats with GABA chitosan nanoparticle (GCNP), 5-HT chitosan nanoparticle (SCNP) and a combination of GABA and 5-HT chitosan nanoparticle (GSCNP) treatments. In GABA and 5-HT chitosan nanoparticle treated group there was a significant decrease (P<0.001) in the receptor expression of Gamma aminobutyric acid B and a significant increase (P<0.001) in the receptor expression of 5-hydroxy tryptamine 2A when compared to PHNT. The cyclic adenosine monophosphate content and its regulatory protein, presence of methylated DNA and superoxide dismutase activity were decreased in GCNP, SCNP and GSCNP when compared to PHNT. The Gamma aminobutyric acid B and 5-hydroxy tryptamine 2A receptors coupled signaling elements played an important role in GABA and 5-HT chitosan nanoparticles induced liver cell proliferation which has therapeutic significance in liver disease management.

Lysophosphatidic acid receptor expression and function in human hepatocellular carcinoma

BACKGROUND

Lysophosphatidic acid (LPA) is a ubiquitously expressed phospholipid that regulates diverse cellular functions. Previously identified LPA receptor subtypes (LPAR1-5) are weakly expressed or absent in the liver. This study sought to determine LPAR expression, including the newly identified LPAR6, in normal human liver (NL), hepatocellular carcinoma (HCC), and non-tumor liver tissue (NTL), and LPAR expression and function in human hepatoma cells in vitro.

METHODS

We determined LPAR1-6 expression by quantitative reverse transcriptase polymerase chain reaction, Western blot, or immunohistochemistry in NL, NTL, and HCC, and HuH7, and HepG2 cells. Hepatoma cells were treated with LPA in the absence or presence of LPAR1-3 (Ki16425) or pan-LPAR (α-bromomethylene phosphonate) antagonists and proliferation and motility were measured.

RESULTS

We report HCC-associated changes in LPAR1, 3, and 6 mRNA and protein expression, with significantly increased LPAR6 in HCC versus NL and NTL. Analysis of human hepatoma cells demonstrated significantly higher LPAR1, 3, and 6 mRNA and protein expression in HuH7 versus HepG2 cells. Treatment with LPA (0.05-10 μg/mL) led to dose-dependent HuH7 growth and increased motility. In HepG2 cells, LPA led to moderate, although significant, increases in proliferation but not motility. Pretreatment with α-bromomethylene phosphonate inhibited LPA-dependent proliferation and motility to a greater degree than Ki16425.

CONCLUSIONS

Multiple LPAR forms are expressed in human HCC, including the recently described LPAR6. Inhibition of LPA-LPAR signaling inhibits HCC cell proliferation and motility, the extent of which depends on LPAR subtype expression.

Impaired activities of cyclic adenosine monophosphate-responsive element binding protein, protein kinase A and calcium-independent phospholipase A2 are involved in deteriorated regeneration of cirrhotic liver after partial hepatectomy in rats

AIMS

  This study is to elucidate whether cyclic adenosine monophosphate (cAMP)-mediated signal is involved in lower regenerative potential of cirrhotic liver.

METHODS

  Hepatic cAMP concentration, activities of protein kinase A (PKA), c-AMP responsive element binding protein (CREB) and Ca(2+) -independent phospholipase A(2) (iPLA2) and regeneration rate were compared between rats with thioacetamide-induced cirrhotic and normal livers after two-third hepatectomy.

RESULTS

  The liver regeneration estimated by the rates of [(3) H]-thymidine incorporation and staining of proliferating cell nuclear antigen was significantly lower in the cirrhotic group. CREB, PKA and iPLA2 activities, assessed by western blots and electromobility shift assay, were significantly impaired after hepatectomy in the cirrhosis group. PKA and iPLA2 silencing by siRNA transfection significantly inhibited CREB activity and cell growth in transformed hepatocytes in vitro.

CONCLUSIONS

  CREB dysfunction, mediated by PKA and iPLA2 suppression, may be involved in the deteriorated liver regeneration in the cirrhotic rats.

Altered expression and function of regulator of G-protein signaling-17 (RGS17) in hepatocellular carcinoma

Guanine nucleotide regulatory proteins (G-proteins) are central to normal hepatocyte function and are implicated in hepatic disease initiation and progression. Regulators of G-protein signaling (RGS) are critical to defining G-protein-dependent signal fidelity, yet the role of RGS proteins in the liver is poorly defined. The aims of this study were to determine RGS17 expression in normal and transformed hepatic tissue and cells, and address the function of RGS17 in hepatic tumorgenicity. RGS17 expression was determined in human and rat HCC tissue and cell lines. Molecular approaches were used to alter RGS17 expression in HCC cells, effects on cell function measured, and RGS17 association with specific Gα-subunits determined. Using these approaches RGS17 mRNA, but not protein, was detectable in human and rat HCC tissue and cells. Conversely, RGS17 mRNA was not detected in normal tissue, isolated hepatocytes, or non-tumorigenic hepatic cells. Subsequent studies using transfected cells demonstrated that RGS17 proteins were not post-translationally modified in HCC cells, and RGS17 expression is governed by protein degradation and not via miRNAs. Notwithstanding inherently low RGS17 protein levels, altering RGS17 expression profoundly affected HCC cell mitogenesis and migration. Analysis of RGS17-G-protein interaction demonstrated RGS17 associates with both Giα- and Gqα-subunits in HCC cells of human and rat origin. In conclusion, these data demonstrate that, despite difficulties in measuring endogenous RGS protein expression, RGS17 is differentially expressed in HCC and plays a central role in regulating transformed hepatocyte tumorgenicity.

Hepatocyte proliferation during liver regeneration is impaired in mice with methionine diet-induced hyperhomocysteinemia

Elevated homocysteine levels are defined as hyperhomocysteinemia (HHcy), a disorder that is associated with cardiovascular and neurodegenerative diseases as well as with hepatic fibrosis. Recent studies have shown that HHcy promotes hepatic injury by increasing oxidative stress. Although homocysteine induces cell cycle arrest in a variety of different cell types, it is not known whether HHcy has a definitive role in hepatocyte proliferation during liver regeneration. In this report, we investigated the effect of homocysteine on liver regeneration. Our results demonstrated that mice with HHcy exhibited an impairment in liver regeneration after partial hepatectomy, as measured by immunohistochemical staining of proliferation cell nuclear antigen and bromodeoxyuridine incorporation. Impaired proliferation was also correlated with reduced cyclin D1 induction and elevated expression levels of both p53 and p21Cip1. In addition, the phosphorylation of Akt, which plays an essential role in normal regeneration responses, was attenuated during the early phases of liver regeneration in HHcy mice. Our results also indicated that the cAMP/protein kinase A pathway mediated the inhibitory effect of homocysteine on liver regeneration. These findings provide evidence that impairment of liver regeneration by HHcy may result in delayed recovery from liver injury induced by homocysteine itself.

Identification and characterization of a novel peptide interacting with cAMP-responsive elements binding and cAMP-responsive elements modulator in mouse liver

BACKGROUND/AIMS

Transcription factors coupled to cyclic adenosine mono phosphate (cAMP) signalling in the cAMP-responsive elements binding (CREB)/ATF family constitute a family of activators or repressors that bind to cAMP-responsive promoter elements (CREs) in the regulatory regions of cAMP-inducible genes. A role for CREB/ATF family has been advocated in the control of hepatocellular carcinoma progression. CREB appears to be activated by the X protein of hepatitis B virus, which links to the unphosphorylated form of CREB and activates transcription, thus obviating an otherwise indispensable Ser-133 phosphorylation. Identification of factors capable of triggering transcription via cAMP-responsive elements modulator (CREM)/CREB signalling in the absence of Ser phosphorylation will improve our knowledge of the molecular mechanism of liver cell proliferation.

METHODS

To isolate and study proteins binding and activating CREB and/or CREM in the liver, we performed the screening of a mouse liver cDNA library using the Two-Hybrid System.

RESULTS

We report the identification and characterization of a novel peptide, VTIP-peptide (VTIP-P), which binds and enhances the activation of CREM/CREB, obviating the need for transcription factor phosphorylation. We demonstrated that VTIP-P physically interacts with the activation domain (AD) of the transcription factors CREB/CREM and activates transcription by modifying their phosphorylation pattern in hepatoma cells. The data allowed the conclusion that VTIP-P binds the AD of CREB and CREM by stabilizing their phosphorylation.

CONCLUSION

The characterization of molecules capable of interfering in the liver with an important pathway such as CREB could be significant in designing and/or developing new therapeutic approaches to the control of liver cell proliferation.

Decreased aquaporin expression leads to increased resistance to apoptosis in hepatocellular carcinoma

Cells undergoing apoptosis are characterized by decreased cell size due to changes in intracellular ion concentration and rapid, aquaporin (AQP)-dependent water movement out of the cell, events required for the activation of pro-apoptotic enzymes. The current study demonstrates AQP 8 and 9 expression is significantly decreased in hepatocellular carcinoma (HCC) versus normal liver. Isolation of hepatic tumor cells (H4IIE) and hepatocytes confirmed a lack of water movement across the H4IIE cell membrane via AQPs and identified an inherent resistance of H4IIE cells to apoptotic stimuli. In contrast, hepatocytes rapidly responded to osmotic challenge through AQP-dependent water movement and underwent cell death following apoptotic stimulation.

Role of cyclic-AMP responsive element binding (CREB) proteins in cell proliferation in a rat model of hepatocellular carcinoma

The role of cyclic adenosine monophosphate (cAMP) is poorly understood in the regulation of normal and abnormal hepatic cell growth. In this study, we examined the regulation of intracellular cAMP levels and its effect on nuclear cAMP responsive elements (CREs) in a rat model of hepatocellular carcinoma (HCC). Tumorigenic liver cells were cultured from an in vivo model of HCC and the role of cAMP in cell mitogenesis determined. These data demonstrated agents that elevate intracellular cAMP ([cAMP]i) levels caused significant dose-dependent inhibition of serum-stimulated mitogenesis in HCC cells. Cells were next analyzed for transcription factor expression and activity following increased [cAMP]i. These data demonstrated time- and dose-dependent increases in CRE binding protein (pCREB) activity, a maximal response occurring after 10-20 min before returning to basal levels within 60 min. In contrast, increased [cAMP]i levels led to sustained inducible cAMP early repressor (ICER) II/IIgamma mRNA and protein induction. To understand these data in relation to the in vivo setting, HCC tumors were analyzed and compared to pair-matched normal liver (NL) samples. These studies demonstrated significantly elevated Gsalpha-protein expression in HCC versus NL in the absence of significant changes in basal cAMP levels. Analysis of total and active CREB demonstrated significantly increased total CREB/pCREB in HCC versus NL. Further analysis of CRE expression demonstrated significantly increased expression of ICER mRNA and protein in HCC versus sham operated (Sh). These data demonstrate cAMP, while capable of stimulating promitogenic CREB activation inhibits cell mitogenesis in HCC possibly via ICER induction.

C/EBP alpha and C/EBP beta regulate haptoglobin gene expression during rat liver development and the acute-phase response

The participation of C/EBP alpha and C/EBP beta in the transcriptional regulation of the haptoglobin (Hp) gene throughout liver development and the acute-phase (AP) response was examined. Western immunoblot analysis revealed that the relative concentrations of C/EBP alpha and C/EBP beta increased during differentiation in two nuclear protein fractions - the nuclear extract and nuclear matrix. The AP reaction was accompanied by a decrease of the relative concentration of C/EBP alpha and an increase of C/EBP beta during development in both protein fractions. Using Western analysis after DNA-affinity chromatography it was observed that a 45 kDa C/EBP alpha isoform displayed a binding affinity towards the Hp gene hormone responsive element (HRE) in both pre- and postnatal livers. In the course of the AP response DNA binding of the 45 kDa isoform was detected only in the adult, when its binding affinity decreased. The 35 kDa C/EBP beta isoform exhibited a binding affinity towards the Hp HRE after the second week from birth, whereas the AP response promoted an enhanced binding of 35 kDa isoform after the first postnatal week. These results indicate that Hp gene transcription is regulated by C/EBP alpha during normal liver development, whereas C/EBP beta is involved in the AP regulation during the later phase of differentiation and in the adult.

Identification of expressed genes in regenerating rat liver in 0-4-8-12 h short interval successive partial hepatectomy

AIM: To identify the genes differentially expressed in the regenerating rat liver of 0-4-8-12 h short interval successive partial hepatectomy (SISPH) and to analyze their expression profiles.

METHODS: Five hundred and fifty-one elements screened from subtractive cDNA libraries were made into a cDNA microarray (cDNA chip). Extensive gene expression analysis following 0-4-8-12 h SISPH was conducted by microarray.

RESULTS: One hundred and eighty-three elements were selected, which were either up- or down-regulated more than 2-fold at one or more time points after SISPH. Cluster analysis and generalization analysis showed that there were five distinct temporal patterns of gene expression. Eighty-six genes were unreported, associated with liver regeneration (LR).

CONCLUSION: Microarray analysis shows that the down regulated genes are much more than the up-regulated ones in SISPH; the numbers of genes expressed consistently are fewer than that expressed immediately; the genes expressed in high abundance are much fewer than that increased 2-5-fold. The comparison of SISPH with partial hepatectomy (PH) shows that the expression trends of most genes in SISPH and in PH are similar, but the expression of 43 genes is specifically altered in SISPH.

Alcohol and liver cancer

Hepatocellular carcinoma is the eighth most frequent cancer in the world, accounting for approximately 500,000 deaths per year. Unlike many malignancies, hepatocellular carcinoma occurs predominantly within the context of known risk factors, with hepatic cirrhosis being the most common precursor to the development of hepatocellular carcinoma. After ethanol ingestion, the liver represents the major site of metabolism. Ethanol metabolism by alcohol dehydrogenase leads to the generation of acetaldehyde and free radicals that bind rapidly to numerous cellular targets, including components of cell signaling pathways and DNA. In addition to direct DNA damage, acetaldehyde depletes glutathione, an antioxidant involved in detoxification. Chronic ethanol abuse leads to induction of hepatocyte microsomal cytochrome P450 2E1, an enzyme that metabolizes ethanol to acetaldehyde and, in doing so, causes further free radical production and aberrant cell function. Cytochrome P450 2E1-dependent ethanol metabolism is also associated with activation of procarcinogens, changes in cell cycle, nutritional deficiencies, and altered immune system responses. The identification of oxidative stress in mediating many deleterious effects of ethanol in the liver has led to renewed interest in the use of dietary antioxidants as therapeutic agents. Included in this group are S-adenosyl-L-methionine and plant-derived flavanoids.

Coupling cAMP signaling to transcription in the liver: pivotal role of CREB and CREM

Transcriptional factors binding to cAMP-responsive elements (CREs) in the promoters of various genes belong to the basic domain-leucine zipper superfamily and are composed of three genes in mammals, CREB, CREM, and ATF-1. A large number of CREB, CREM, and ATF-1 proteins are generated by posttranscriptional events, mostly alternative splicing, and regulate gene expression by acting as activators or repressors. Activation is classically brought about by signaling-dependent phosphorylation of a key acceptor site (Ser133 in CREB) by a number of possible kinases, including PKA, CamKIV, and Rsk-2. Phosphorylation is the prerequisite for the interaction of CBP (CREB-binding protein), a co-activator that has also histone acetyltransferase activity. Repression may involve dynamic dephosphorylation of the activators and thus decreased association with CBP. Another pathway of transcriptional repression on CRE sites implicates the inducible repressor ICER (inducible cAMP early repressor), a product of the CREM gene. Being an inducible repressor, ICER is involved in autoregulatory feedback loops of transcription that govern the down-regulation of early response genes, such as the proto-oncogene c-fos. The liver represents a remarkable physiological setting where cAMP-responsive signaling plays a major role. Indeed, a finely tuned program of gene expression is triggered by partial hepatectomy, so that through specific checkpoints a coordinated regeneration of the tissue is obtained. Temporal kinetics of transcriptional activation after hepatectomy reveals a pattern of early induction for several genes, some of them controlled by the CREB/CREM transcription factors. An important role of CREM in liver physiology was suggested by the robust induction of ICER after partial hepatectomy. The delay in tissue regeneration in CREM-deficient mice confirmed the important function of this factor in regulating hepatocyte proliferation. As gene induction is accompanied by critical changes in chromatin organization, the deciphering of the specific modification codes that histones display during liver regeneration and physiology will provide exciting new insights into the dynamics of chromatin architecture.

Acid-labile subunit regulation during the early stages of liver regeneration: implications for glucoregulation

The initiation of liver regeneration is regulated by endogenously produced growth factors and cytokines and is accompanied by suppression of growth hormone (GH) binding to hepatocytes. We have demonstrated some of these factors, particularly GH, which modulate acid-labile subunit (ALS) expression in vitro. Consequently, we investigated ALS hepatic mRNA and serum levels in rats for 24 h after partial hepatectomy (PHx). There was a significant suppression of ALS gene expression (approximately 50%, P < 0.005) and serum levels (approximately 30%, P < 0.02) by 12 h in PHx rats relative to controls. Relative to intact animals, hepatic mRNA and serum levels of ALS were suppressed by approximately 60% at 24 h. Similarly, hepatic GH receptor mRNA levels were significantly reduced in PHx animals. Moreover, hepatocytes isolated from PHx animals were less responsive to GH than those from controls. Overall, our results demonstrate that suppression of ALS gene expression and serum levels during liver regeneration relates to lowered hepatic GH sensitivity. Suppressed circulating ALS may alter insulin-like growth factor bioavailability and constitute a mechanism to maintain relatively normal glucoregulation after loss of liver mass.

Hepatitis B virus X protein activates transcription by bypassing CREB phosphorylation, not by stabilizing bZIP-DNA complexes

Although previous work has shown that the hepatitis B virus X protein (pX) stabilizes complexes between basic region leucine zipper (bZIP) proteins and target DNA, the relationship between enhanced binding and transcriptional activation has not been established. Here we show that interactions between CREB and pX, which coincidentally enhance DNA affinity, are necessary but not sufficient for increased transcriptional potency. Further, we show that transcriptional activation by pX requires a form of CREB in which Ser-133 is not phosphorylated. By stimulating the transcriptional potency of unphosphorylated CREB, pX can up-regulate the expression of cAMP-responsive genes implicated in hepatocyte proliferation, leading ultimately to the development of liver cancer after viral infection.

Effect of ethanol on tumor necrosis factor signaling during liver regeneration

OBJECTIVES

The liver has tremendous regenerative capacity but can be damaged by toxins, such as ethanol (EtOH). It has long been known that EtOH inhibits liver regeneration. Recent work demonstrates that the proinflammatory cytokine, tumor necrosis factor ( (TNF), is required for normal liver regeneration, as well as for EtOH-related liver damage. Therefore, it is conceivable that EtOH promotes liver damage by altering TNF signal transduction in such a way that proliferative signals are aborted and death signals predominate.

DESIGN AND METHODS

Anti-TNF antibodies were used to characterize the TNF signals that are induced in the regenerating liver after two-thirds (partial) hepatectomy (PH) in normal mice and rats. Then, these TNF-regulated processes were evaluated in animals that had been fed nutritionally replete, EtOH-containing diets for several weeks before PH.

RESULTS

During normal liver regeneration, TNF induces potentially dangerous responses, such as increased mitochondrial ROS production, but also promotes the activation of several factors, including NF kappa B, Jun N-terminal Kinase (JNK), and various mitochondrial membrane proteins, which are likely to permit hepatocytes to survive apoptotic and oxidant stress. Previous EtOH exposure inhibits the normal regenerative induction of NF kappa B and JNK.

CONCLUSIONS

These finding are consistent with the possibility that potential hepatotoxins compromise the balanced induction of toxic and trophic signals by TNF.

Basic fibroblast growth factor stimulates repair of wounded hepatocyte monolayer: modulatory role of protein kinase A and extracellular matrix

The two important repair mechanisms after hepatocyte injury are proliferation and migration of the nearby healthy hepatocytes. Although previous studies have shown that basic fibroblast growth factor (bFGF) levels are markedly elevated after liver injury, the role of bFGF in the repair of the wounded hepatocytes is not well understood. The aim of this study was to delineate the role of bFGF in the repair of the wounded hepatocyte monolayers. Specifically, we examined the role of bFGF in cellular proliferation and migration of hepatocytes with an in vitro wound model. Standardized excisional wounds were created in clone 9 rat hepatocyte monolayers by a razor blade, and the extent of epithelial proliferation and migration was measured. After wound formation, bFGF (30 ng/mL) significantly stimulated proliferation of hepatocytes at the wound margin. bFGF also stimulated the migration of hepatocytes at the wound front. bFGF stimulation of hepatocyte migration correlated with increased formation of actin stress fibers and bFGF-receptor protein level. The bFGF stimulation of hepatocyte migration was abolished by various protein kinase A activating agents including 3-isobutyl-1-methylxanthine, 8-bromoadenosine-3', 5'-cyclic monophosphate, forskolin, and cholera toxin. In addition, protein kinase A activating agents almost completely prevented bFGF-induced actin stress fiber formation in the cells at the wound front. Varying the basement membrane composition of the extracellular matrix had a selective enhancing effect on the basal rates of hepatocyte migration (collagen IV > or = laminin > collagen I > fibronectin > control (plastic)). bFGF treatment resulted in a similar additive increase in hepatocyte migration across all coated surfaces studied. We conclude that bFGF promotes hepatocyte wound repair by stimulating both proliferation and migration of the hepatocyte at the margin of the wound.

Enhanced Gi-protein-mediated mitogenesis following chronic ethanol exposure in a rat model of experimental hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is associated with increased expression and function of inhibitory guanine nucleotide regulatory proteins (Gi-proteins). This study addresses the effects of chronic ethanol exposure on the expression and function of adenylyl cyclase (AC)-linked G-proteins (Gs and Gi) and growth in experimental HCC. G-protein expression and function was determined by immunoblot in the hepatic tumorigenic H4IIE cell line and isolated cultured hepatocytes in the absence or presence of ethanol (5-100 mmol/L). Chronic exposure (24 hours) to ethanol dose-dependently increased Gialpha1/2 expression in the H4IIE cell line, but not in cultured hepatocytes. Gsalpha-protein expression remained unchanged in both H4IIE cells and cultured hepatocytes following ethanol treatment. In addition, ethanol directly activated a Gi-protein, because pertussis toxin (PTx)-catalyzed, adenosine diphosphate (ADP)-dependent ribosylation of Gialpha substrates decreased following ethanol treatment. The increased functional activity of Gialpha1/2-protein expression was confirmed by demonstrating that ethanol dose-dependently inhibited basal and stimulated AC activity in H4IIE cells, while not significantly altering basal AC activity in isolated cultured hepatocytes. Furthermore, while ethanol had no significant effect on basal mitogenesis in H4IIE cells or hepatocytes, increased mitogenesis caused by direct Gialpha-protein stimulation (mastoparan M7; 10-5,000 nmol/L) was further enhanced in the presence of ethanol, an effect that was completely blocked following Gi-protein inhibition (PTx; 100 ng/mL). In contrast, activation of Gi-proteins using M7 failed to alter cellular mitogenesis in isolated cultured hepatocytes, whether in the absence or presence of ethanol. Finally, analysis of mitogen-activated protein kinase (MAPK) activity demonstrated that chronic ethanol treatment further enhanced Gi-protein-stimulated MAPK activity in hepatic tumorigenic cells. In conclusion, these data demonstrate that ethanol enhances cellular mitogenesis in experimental HCC as a result of, at least in part, a Gi-MAPK-dependent pathway. Furthermore, this effect may be caused by ethanol's direct up-regulation of the expression and activity of Gi-proteins in HCC.

Transcription factor CREM coordinates the timing of hepatocyte proliferation in the regenerating liver

The liver regenerates upon partial hepatectomy (PH) as terminally differentiated hepatocytes undergo a tremendous proliferative process. CREM gene expression is powerfully induced during liver regeneration. We show that cell proliferation is significantly reduced upon PH in CREM-/- mice. There is a reduction in DNA synthesis, in the number of mitosis and of phosphorylated histone H3-positive cells. The post-PH proliferation peak is delayed by 10 hr, indicating an altered hepatocyte cell cycle. Expression of cyclins A, B, D1, E, and cdc2, of c-fos and tyrosine aminotransferase is deregulated. CREM mutation results in delayed S-phase entry, impairing the synchronization of proliferation.

Effect of transient cyclic AMP elevation on DNA synthesis in rat hepatocytes at G1 phase

Maintaining high levels of intracellular cyclic AMP (cAMP) is known to inhibit the growth of various proliferating cells including hepatocytes. We show here that transient (30 min) elevations of cAMP induced by addition of 8-bromo-cAMP (1 mmol/L) to rat hepatocytes in primary culture at three time points (12 h, 16 h and 20 h) after seeding stimulated DNA synthesis. Sustained levels of cAMP stimulated DNA synthesis to a lesser degree at a lower concentration (1 mumol/L), but inhibited it at concentrations higher than 100 mumol/L. We also determined cyclin-dependent kinase 2 (cdk2) activity in the hepatocytes during this incubation period. The transient addition of 8-bromo-cAMP at the late G1 phase increased cdk2 activity. This suggests that transient cAMP elevation in hepatocytes at the late G1 phase has a growth stimulation effect. Up-regulation of cdk2 activity may have a role in this process.

CREB controls LAP/C/EBP beta transcription

LAP/C/EBP beta is a member of the C/EBP family of transcription factors and is involved in hepatocyte-specific gene expression. Recently we showed that, besides its posttranscriptional regulation, LAP/C/EBP beta mRNA is modulated during liver regeneration. Therefore, in this study we investigated mechanisms which control LAP/C/EBP beta gene transcription. Deletion analysis of the 5'-flanking region, located upstream of the start site of transcription in the LAP/C/EBP beta gene, demonstrated that a small region in close proximity to the TATA box is important in maintaining a high level of transcription of the luciferase reporter gene constructs. In gel shift experiments two sites were identified which are important for specific complex formation within this region. Further analysis by cross-linking, super shift, and competition experiments was performed with liver cell nuclear extracts, hepatoma cell nuclear extracts, or recombinant CREB protein. These experiments conclusively demonstrated that CREB binds to both sites in the LAP/C/EBP beta promoter with an affinity similar to that with the CREB consensus sequence. Transfection experiments with promoter constructs where the CREB sites were mutated showed that these sites are important to maintain both basal promoter activity and LAP/C/EBP beta inducibility through CREB. Northern blot analysis and runoff transcription assays demonstrated that the protein kinase A pathway not only stimulated the activity of the luciferase reporter construct but also the transcription of the endogenous LAP/C/EBP beta gene in different cell types. Western blot analysis of rat liver cell nuclear extracts and runoff transcription assays of rat liver cell nuclei after two-thirds hepatectomy showed a functional link between the induction of CREB phosphorylation and LAP/C/EBP beta mRNA transcription during liver regeneration. These results demonstrate that the two CREB sites are important to control LAP/C/EBP beta transcription in vivo. As several pathways control CREB phosphorylation, our results provide evidence for the transcriptional regulation of LAP/C/EBP beta via CREB under different physiological conditions.

Differential regulation of the mitogen-activated protein and stress-activated protein kinase cascades by adrenergic agonists in quiescent and regenerating adult rat hepatocytes

To study the mechanisms by which catecholamines regulate hepatocyte proliferation after partial hepatectomy (PHX), hepatocytes were isolated from adult male rats 24 h after sham operation or two-thirds PHX and treated with catecholamines and other agonists. In freshly isolated sham cells, p42 mitogen-activated protein (MAP) kinase activity was stimulated by the alpha1-adrenergic agonist phenylephrine (PHE). Activation of p42 MAP kinase by growth factors was blunted by pretreatment of sham hepatocytes with glucagon but not by that with the beta2-adrenergic agonist isoproterenol (ISO). In PHX cells, the ability of PHE to activate p42 MAP kinase was dramatically reduced, whereas ISO became competent to inhibit p42 MAP kinase activation. PHE treatment of sham but not PHX and ISO treatment of PHX but not sham hepatocytes also activated the stress-activated protein (SAP) kinases p46/54 SAP kinase and p38 SAP kinase. These data demonstrate that an alpha1- to beta2-adrenergic receptor switch occurs upon PHX and results in an increase in SAP kinase versus MAP kinase signaling by catecholamines. In primary cultures of hepatocytes, ISO treatment of PHX but not sham cells inhibited [3H]thymidine incorporation. In contrast, PHE treatment of sham but not PHX cells stimulated [3H]thymidine incorporation, which was reduced by approximately 25 and approximately 95% with specific inhibitors of p42 MAP kinase and p38 SAP kinase function, respectively. Inhibition of the p38 SAP kinase also dramatically reduced basal [3H]thymidine incorporation. These data suggest that p38 SAP kinase plays a permissive role in liver regeneration. Alterations in the abilities of catecholamines to modulate the activities of protein kinase A and the MAP and SAP kinase pathways may represent one physiological mechanism by which these agonists can regulate hepatocyte proliferation after PHX.

Cyclic AMP signalling and cellular proliferation: regulation of CREB and CREM

In eukaryotes, transcriptional regulation upon stimulation of the adenylyl cyclase signalling pathway is mediated by a family of cAMP-responsive nuclear factors. This family consists of a large number of members which may act as activators or repressors. These factors contain the basic domain/leucine zipper motifs and bind as dimers to cAMP-response elements (CRE). The function of CRE-binding proteins (CREB) is modulated by phosphorylation by the cAMP-dependent protein kinase. The ICER (inducible cAMP early repressor) protein is the only inducible member of this family and is a product of the CREM gene. The induction of this powerful repressor is likely to be important for the transient nature of cAMP-induced gene expression. CREB proteins have been found to play an important role in the physiology of neuroendocrine functions. In addition, recent results indicate that CREB and CREM could be involved in the proliferation of hepatocytes which follows partial hepatectomy.

Differences in the regulation of cytochrome P450 family members during liver regeneration

BACKGROUND/AIMS

Cytochrome P450 enzymes (P450s) metabolise endogenous substances and a vast variety of drugs. Little is known about the regulation of P450s during pathophysiological conditions in the liver. Therefore we studied the regulation of P450 1A1, 1A2, 2E1 and 3A during liver regeneration after two-thirds hepatectomy.

METHODS

Partial hepatectomy or sham surgery was performed in Sprague-Dawley rats. At different time points after surgery, microsomal proteins were isolated and the RNA was prepared. Northern blot analysis, Western blot analysis and enzyme assays for the different P450s were performed.

RESULTS

Northern blot analysis showed a transient downregulation of cytochromes P450 1A2 and 2E1 after hepatectomy, while the expression of cytochrome P450 3A remained unaffected. Western blot analysis of microsomal proteins showed that changes of the mRNA levels are not reflected in the protein level, most likely because the half-life of the P450 proteins in hepatocytes is long, and thus a transient mRNA downregulation has little impact on the total amount of protein detected. Differences in the regulation of the enzymatic activities were found for P450 1A2 and 2E1. Interestingly, the metabolic activity of cytochrome P450 2E1 decreased dramatically post-hepatectomy, while the P450 2A1 activity remained unchanged.

CONCLUSIONS

Regulatory mechanisms were found on the RNA level and by post-translational mechanisms which downregulate P450 expression and activity during liver regeneration. These results indicate prolonged half-life of drugs during hepatocyte proliferation, and thus also have important implications for therapy in humans.

Adenovirus-mediated transfer of CCAAT/enhancer-binding protein-alpha identifies a dominant antiproliferative role for this isoform in hepatocytes

CCAAT/enhancer-binding protein (C/EBP) isoforms are thought to be important regulators of the hepatocyte phenotype. However, the specific physiological roles of different isoforms are poorly understood because hepatocytes express multiple C/EBPs, and various isoforms have overlapping functions. To identify the functions of C/EBPalpha in mature hepatocytes, replication-defective adenovirus vectors were used to efficiently and homogeneously overexpress the mouse C/EBPalpha gene in a SV40 virus-conditionally transformed rat hepatocyte line that can be induced to express C/EBPbeta and C/EBPdelta but that has little endogenous C/EBPalpha expression. Hepatocytes were infected with a recombinant adenovirus vector carrying the cDNA for C/EBPalpha driven by Rous sarcoma virus promoter elements (AdCEBPalpha) or a similar vector carrying the Escherichia coli lacZ gene (Adbetagal). Staining for beta-galactosidase demonstrated an infection efficiency of 100% at a multiplicity of infection of 25 plaque-forming units/cell and persistence of foreign gene expression for at least 9 days. Cultures infected with AdCEBPalpha had 50-fold higher levels of C/EBPalpha mRNA and protein than those infected with Ad-beta-gal, but similar expression of C/EBP-beta. Infection with AdCEBPalpha inhibited proliferation in cells expressing little C/EBPbeta, even when proliferation was driven by the SV40 transforming antigen, and also blunted mitogenic induction of the c-myc proto-oncogene in nontransformed cells with high levels of C/EBPbeta. Although overexpression of C/EBPalpha consistently increased C/EBPalpha DNA binding activity, it was not sufficient for albumin expression. Infection with AdCEBPalpha only increased albumin mRNA levels in nontransformed cells that also expressed relatively high levels of C/EBPbeta. Thus, in hepatocytes, C/EBPalpha has a dominant antiproliferative function, but must interact with other factors to regulate hepatocyte-specific gene expression.

Adenylate cyclase activity during exocrine pancreatic proliferation in the rat

Adenylate cyclase activity in pancreatic acinar cell membranes was determined in rats that had undergone a treatment with pentagastrin (250 micrograms/kg, intraperitoneal three times daily) for 1 week or that had undergone small bowel resection (90%) and were sacrified at 2 weeks, 1 month and 6 months after intervention. Both treatments are potent stimulators of pancreatic acinar cell proliferation. Adenylate cyclase activity was similar under basal conditions and after the diterpene forskolin stimulation in pancreatic acinar membranes from all groups studied. The ability of low concentrations of the stable GTP analogue, 5'-guanylylimidodiphosphate (Gpp[NH]p) to inhibit forskolin-stimulated adenylate cyclase activity was decreased in pancreatic acinar membranes from enterectomized rats at 2 weeks and 1 month after the operation and returned to control values at 6 months after enterectomy. Stimulation of adenylate cyclase by high concentration of Gpp[NH]p or by secretin (10(-8) M) was higher in both pancreatic hyperplasia conditions as compared with control animals. These findings suggest that the coupling efficiency of the Gs protein to adenylate cyclase from pancreatic acinar membranes is enhanced without any alterations in the catalytic activity of the enzyme during pancreatic proliferation. In addition, it is possible that the highly regulated pancreatic acinar adenylate cyclase activity may be necessary to regulate pancreatic acinar cell proliferation.

Effect of ethanol on p36 protein kinase substrate and insulin receptor substrate 1 expression and tyrosyl phosphorylation in human hepatocellular carcinoma cells

Ethanol inhibits insulin (IN) and epidermal growth factor (EGF)-induced hepatocyte DNA synthesis. Growth factor receptor kinases, such as IN and EGF, phosphorylate insulin receptor substrate (IRS-1) and p36 protein kinase substrate, respectively, on tyrosine residues. IRS-1 and p36 are thought to be important intracellular signal transduction molecules involved in the regulation of cell growth. These investigations explored the effect of ethanol additions on the expression and tyrosyl phosphorylation (TP) of p36 and IRS-1 in a human hepatocellular carcinoma cell line (FOCUS) in relationship to cell proliferation induced by IN and serum growth factor stimulation. It was found that p36 was constitutively and highly expressed in serum-starved cells and protein, and mRNA levels did not change with cell proliferation induced by growth factors. However, exposure of FOCUS cells to ethanol additions substantially inhibited TP of p36. The early TP of IRS-1 induced by IN stimulation was also reduced by ethanol additions. Finally, there was a parallel decrease of FOCUS cell proliferation in ethanol-exposed cultures. These studies suggest that one possible mechanism of ethanol inhibitory effect on cell proliferation is through reduced TP of putative intracellular signal transduction molecules, such as p36 and IRS-1.

Dexamethasone inversely regulates DNA synthesis and phosphoenolpyruvate carboxykinase mRNA levels in cultured rat hepatocytes: interactions with insulin, glucagon, and transforming growth factor beta 1

In hepatocytes, glucocorticoids control the expression of several genes and exert significant, but complex, regulation of the proliferation. To shed more light on the growth responses to glucocorticoids in these cells, we treated adult rat hepatocytes in primary culture with dexamethasone, in various combinations with other hormones (insulin, glucagon, transforming growth factor beta 1 (TGF beta 1)), and examined the relationship between the effects on the DNA synthesis and the mRNA level of phosphoenolpyruvate carboxykinase, a gene typically expressed in differentiated hepatocytes. Insulin exhibited the previously observed suppressing effect on the glucocorticoid-induced phosphoenolpyruvate carboxykinase mRNA level, and also reversed growth-inhibitory effects of the glucocorticoid. Dexamethasone and glucagon (via cAMP) acted strongly synergistically both in enhancing the phosphoenolpyruvate carboxykinase expression and inhibiting the growth, the inhibitory effect of glucagon on DNA synthesis being totally dependent on dexamethasone. The effects of dexamethasone plus glucagon on both the phosphoenolpyruvate carboxykinase mRNA abundance and the DNA synthesis were partially counteracted by insulin. Dexamethasone is permissive for a promoting effect of TGF beta 1 on phosphoenolpyruvate carboxykinase expression, and was found to increase the maximal inhibitory effect of (but reduced the sensitivity to) TGF beta 1 on the DNA synthesis. The results indicate that there is an inverse glucocorticoid-induced regulation of the DNA synthesis and the expression of a liver-typical gene.

The efficiency of nuclear processing of the tyrosine aminotransferase mRNA transcript increases after partial hepatectomy

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotransferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operated livers from adrenalectomized rats, was established 16 h post-hepatectomy. The increase in mRNA levels was not accounted for by a comparable change in the rate of transcription of the gene which, at 0.5 h post-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quiescent and 16-h-regenerating liver, the rate of transcription of the gene in quiescent liver was threefold higher than the rate in 16-h-regenerating liver. The maintenance of a higher rate of gene transcription in quiescent liver, as compared to regenerating liver, was shown to depend on ongoing protein synthesis. The possibility that the high rate of gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the efficiency of nuclear processing of the primary transcript.

Selective induction of CCAAT/enhancer binding protein isoforms occurs during rat liver development

BACKGROUND/AIMS

Recent evidence suggests that CCAAT/enhancer binding protein (C/EBP) transcription factors may regulate hepatocyte terminal differentiation.

METHODS

To explore this possibility, the present study looked for variations in the expression or DNA binding activity of different C/EBP isoforms during rat postnatal liver development and determined which of the C/EBPs were expressed by adult hepatocytes in primary culture.

RESULTS

In intact rats, hepatocyte proliferation is active for 2-3 weeks after birth. During this period of postnatal liver growth, several liver-specific functions emerge and C/EBP alpha, beta, and delta isoforms are induced. Nuclear expression of the 36-kilodalton C/EBP delta protein increases immediately after birth, followed first by increases in the 38-kilodalton C/EBP beta protein expression and then by increases in the 42-kilodalton C/EBP alpha protein expression. Changes in C/EBP DNA binding activity accompany developmental increases in C/EBP proteins. Messenger RNAs of all three C/EBP isoforms are expressed by mature hepatocytes in primary culture.

CONCLUSIONS

Specific C/EBP isoforms are induced differentially during the course of rat postnatal liver development. Young adult rats and cultured adult hepatocytes express all three C/EBP isoforms. These results are consistent with (but do not prove) the theory that variations in C/EBP expression and function help regulate hepatocyte terminal differentiation.

Alterations in the stimulatory G protein of the rat liver after partial hepatectomy

In adult male rat livers, cAMP generation in response to beta-adrenergic agonists was dramatically stimulated after partial hepatectomy. Quantitation of the alpha subunits of the stimulatory G protein (Gs alpha) using ADP-ribosylation catalyzed by cholera toxin revealed the increment in the amounts of two forms of Gs alpha, Gs alpha-S and Gs alpha-L, during liver regeneration. These increases in the amounts of both Gs alpha proteins were associated with the stimulation in their mRNA levels. In addition, partial hepatectomy gave rise to a shift in the proportion of beta-adrenergic receptor (beta-AR) in the high affinity state produced by beta-AR-Gs complex. The susceptibility of Gs alpha to trypsin was used as a probe for beta-AR-Gs coupling. The GTP-bound forms of both Gs alpha-S and Gs alpha-L were more trypsin-sensitive than their GDP-bound forms. Preincubation of liver plasma membranes prepared from partially hepatectomized rats with the agonist isoproterenol resulted in an enhancement of trypsin-sensitivity of Gs alpha-L, but not Gs alpha-S. This effect was retarded by the addition of the antagonist propranolol. We conclude that the increase in the amount of Gs alpha can be contributed to the rise in beta-response after partial hepatectomy, and suggest that beta-AR is preferentially coupled with Gs alpha-L rather than Gs alpha-S.

Effects of transforming growth factor beta 1 on the adenylyl cyclase-cAMP pathway in prostate cancer

We reported previously that MATLyLu rat prostate cancer cells engineered to overproduce transforming growth factor beta 1 (TGF beta 1) produce larger, more metastatic tumors in vivo. We recognized that this ability of TGF beta 1 to act as a positive modulator of prostate tumor behavior might be due to effects of TGF beta 1 on the host and/or on the tumor cells. In this study we demonstrated that the cells themselves respond to endogenously produced TGF beta 1, and that the adenylyl cyclase (AC)-cAMP pathway is affected. TGF beta 1-overproducing cells had lower membrane AC activity, lower intracellular cAMP content, and a lower Gs alpha protein level than did control cells. Prostate cancer cells were growth inhibited by 8-bromo-cAMP or forskolin, agents that elevate intracellular cAMP. Thus, TGF beta 1 overproduction affects the phenotype of the tumor cells, deliberate activation of endogenously produced latent TGF beta 1 is not required (indicating that the cells themselves are capable of activating latent TGF beta 1), and TGF beta 1 overproduction lowers the cellular concentration of the growth inhibitor cAMP. Therefore, TGF beta 1 overproduction could affect tumor behavior in vivo in part via a direct effect on the tumor cells.

Differential regulation of multiple gap junction transcripts and proteins during rat liver regeneration

The mRNA and protein expression of alpha 1 (connexin 43), beta 1 (connexin 32), and beta 2 (connexin 26) gap junction genes were examined in the regenerating rat liver after 70% partial hepatectomy (PH). Expression of beta 1 and beta 2 steady-state mRNA levels changed minimally until 12 h after PH when both transcripts decreased to approximately 15% of baseline values. A similar decrease in assembled connexin levels was detected by immunoblot and indirect immunofluorescence at 18 h after PH. Both transcripts simultaneously increased between 24 and 42 h and again rapidly decreased by 48 h post-PH. beta 1 and beta 2 assembled gap junction protein expression increased at 48 h post-PH and rapidly decreased by 56 h. By 72 to 84 h post-PH, beta 1 and beta 2 mRNA and assembled protein expression returned to near baseline levels and were maintained. Interestingly, inhibition of protein synthesis with cycloheximide completely inhibited disappearance of the beta 2 transcript, in contrast to beta 1 mRNA which was unaffected. Nuclear run-on assays showed no change in transcriptional rates for either gene during the regenerative period. However, both beta 1 and beta 2 transcripts exhibited significantly decreased mRNA half-lives at 12 h post-PH (3.8 and 3.7 h, respectively) relative to those at 0 h (10.9 and 6.1 h, respectively). Surprisingly, although the transcriptional rate for alpha 1 was similar to that observed for beta 2, no alpha 1 transcripts were detectable by northern or RNase protection analysis. The results suggest that in the regenerating rat liver, beta 1 and beta 2 gap junction genes are not regulated at the transcriptional level. Rather, the cyclical modulation of their steady-state transcripts is regulated primarily by posttranscriptional events of which mRNA stability is at least one critical factor in the control process.

Prostaglandin E2 and rat liver regeneration

BACKGROUND

Prostaglandin (PG) is reported to be involved in hepatic regeneration. However, little is known about the detailed relation in PG-induced stimulation of the proliferation. The present study was attempted to elucidate the relation.

METHODS

The serial change of PGE2 level released from the regenerating rat liver and the effect of PGE2 on the proliferation of rat hepatocytes were studied, with special reference to PGE2 binding and cyclic AMP (cAMP).

RESULTS

The PGE2 level increased 3 hours and 10 hours after partial hepatectomy. Timings of these increases seemed to coincide with those of the first and second increase of cAMP in the liver before the initiation of DNA synthesis. DNA synthesis of hepatocytes in primary culture between 24 and 36 hours, 36 and 48 hours, and 48 and 60 hours of culture were significantly enhanced by addition of PGE2 between 4 and 24 hours of culture at concentrations of 2 nmol/L to 1 mumol/L, 2 nmol/L to 200 nmol/L, and 5 mumol/L to 10 mumol/L, and 2 nmol/L and 10 mumol/L, respectively. Enhancement of DNA synthesis with PGE2 at concentrations less than 1 mumol/L seemed to be associated with the high-affinity binding and that at high concentrations with the low-affinity binding. Intracellular cAMP level in the hepatocytes increased during culture, and its increase was enhanced by PGE2 addition.

CONCLUSIONS

It is suggested that PGE2 production in the liver increases biphasically during hepatic regeneration and that PGE2 enhances the proliferation of hepatocytes by a specific receptor-mediated process, which is largely associated with cAMP-dependent process.

Modulation of thioacetamide-induced hepatocellular necrosis by prostaglandins is associated with novel histologic changes

Cytoprotective effects of the prostaglandins 16,16-dimethyl PGE2 (dmPGE2) and PGF2 alpha tromethamine (PGF2 alpha) were evaluated in the rat model of acute hepatocellular necrosis induced by thioacetamide (TAA). dmPGE2 (100 micrograms/kg SC 8 hourly) did not induce a significant increase in survival when started after the onset of TAA-induced fulminant hepatic failure. However, priming with dmPGE2 (100 micrograms/kg SC 30 min before TAA) reduced TAA-induced elevations in serum ALT (684 +/- 68 (SEM) vs 274 +/- 135 IU/1, p less than 0.01). This phenomenon did not occur if dmPGE2 was administered after TAA or by the IP route. Modulation of TAA-induced centrizonal hepatocellular necrosis by dmPGE2 was associated with a striking increase in centrizonal ballooning of hepatocytes (p less than 0.01), and, as assessed by stereology, less hepatocellular necrosis and degenerative changes. PGF2 alpha, which in contrast to dmPGE2 does not act via cAMP, had no effect on TAA-induced changes in serum ALT or hepatic histology. These findings suggest that dmPGE2 decreases hepatocellular necrosis by activating surface membrane adenylate cyclase and consequently stimulating cAMP. Ballooning of hepatocytes could occur secondary to these membrane events and appears to be a marker of dmPGE2-induced cytoprotection in this model.