Role of Kupffer cells in peroxisome proliferator-induced hepatocyte proliferation

Investigations of adsorption behavior and anti-inflammatory activity of glycine functionalized Al12N12 and Al12ON11 fullerene-like cages

The adsorption behavior of the amino acid, glycine (Gly), via the carboxyl, hydroxyl, and amino groups onto the surfaces of Al₁₂N₁₂ and Al₁₆N₁₆ fullerene-like cages were computationally evaluated by the combination of density functional theory (DFT) and molecular docking studies. It was found that Gly can chemically bond with the Al₁₂N₁₂ and Al₁₆N₁₆ fullerene-like cages as its amino group being more favorable to interact with the aluminum atoms of the adsorbents compared to carboxyl and hydroxyl groups. Oxygen and carbon doping were reported to reduce steric hindrance for Glycine interaction at Al site of Al₁₂ON₁₁/Gly and Al₁₂CN₁₁/Gly complexes. Interaction was further enhanced by oxygen doping due to its greater electron withdrawing effect. Herein, the Al₁₂ON₁₁/Gly complex where two carbonyl groups of Gly are bonded to the aluminum atoms of the Al₁₂N₁₂ fullerene-like cage is the most stable interaction configuration showing ∆_adsH and ∆_adsG values of -81.74 kcal/mol and -66.21 kcal/mol, respectively. Computational studies also revealed the frequency shifts that occurred due to the interaction process. Molecular docking analysis revealed that the Al₁₂N₁₂/Gly (-11.7 kcal/mol) and the Al₁₂ON₁₁/Gly (-9.2 kcal/mol) complexes have a good binding affinity with protein tumor necrosis factor alpha (TNF-α). TNF-α was implicated as a key cytokine in various diseases, and it has been a validated therapeutic target for the treatment of rheumatoid arthritis. These results suggest that the Al₁₂N₁₂/Gly complex in comparison with the Al₁₆N₁₆/Gly, Al₁₂ON₁₁/Gly, and the Al₁₂CN₁₁/Gly complexes could be efficient inhibitors of TNF-α.

Glycine-Poly-L-Lactic Acid Copolymeric Nanoparticles for the Efficient Delivery of Bortezomib

PURPOSE

Bortezomib (BTZ) is a proteasome inhibitor used for multiple myeloma and mantle cell lymphoma treatment. BTZ's aqueous in solubility is the main hindrance in its successful development as a commercial formulation. The main objective of the present study is to develop and characterize folic acid-glycine-poly-L-lactic acid (FA-Gly₄-PLA) based nanoformulation (NPs) to improve solubility and efficacy of BTZ.

METHODS

BTZ loaded FA-Gly₄-PLA NPs were prepared and characterized for size, zeta potential, in vitro studies such as release, kinetics modeling, hemolytic toxicity, and cell line-based studies (Reactive Oxygen Species: ROS and cytotoxicity).

RESULTS

BTZ loaded NPs (BTZ-loaded FA-Gly₄-PLA) and blank NPs (FA-Gly₄-PLA) size, zeta, and PDI were found to be 110 ± 8.1 nm, 13.7 ± 1.01 mV, 0.19 ± 0.03 and 198 ± 9.01 nm, 8.63 ± 0.21 mV, 0.21 ± 0.08 respectively. The percent encapsulation efficiency (% EE) and percent drug loading (% DL) of BTZ loaded FA-Gly₄-PLA NPs was calculated to be 78.3 ± 4.1 and 12.38 ± 2.1. The Scanning Electron Microscopy (SEM) showed that NPs were slightly biconcave in shape. The in vitro release of BTZ from FA-Gly₄-PLA NPs resulted in the sustained manner. The prepared NPs were less hemolytic than BTZ.

CONCLUSIONS

BTZ loaded Gly₄-PLA NPs apoptotic index was found to be much higher than BTZ but lesser than BTZ loaded FA-Gly₄-PLA against breast cancer cell lines (MDA-MB-231). ROS intracellular assessment assay indicated that BTZ and BTZ loaded FA-Gly₄-PLA NPs exhibited higher ROS production. Conclusively, the BTZ loaded FA-Gly₄-PLA NPs were able to encapsulate more BTZ than BTZ loaded Gly₄-PLA NPs and were found to be more effective as per as in vitro anti-cancer effect is concerned.

Hepatocyte-specific PPARA expression exclusively promotes agonist-induced cell proliferation without influence from nonparenchymal cells

Peroxisome proliferator-activated receptor-α (PPARA) is a nuclear transcription factor and key mediator of systemic lipid metabolism. Prolonged activation in rodents causes hepatocyte proliferation and hepatocellular carcinoma. Little is known about the contribution of nonparenchymal cells (NPCs) to PPARA-mediated cell proliferation. NPC contribution to PPARA agonist-induced hepatomegaly was assessed in hepatocyte (Ppara△Hep)- and macrophage (Ppara△Mac)-specific Ppara null mice. Mice were treated with the agonist Wy-14643 for 14 days, and response of conditional null mice was compared with conventional knockout mice (Ppara-/- ). Wy-14643 treatment caused weight loss and severe hepatomegaly in wild-type and Ppara△Mac mice, and histological analysis revealed characteristic hepatocyte swelling; Ppara△Hep and Ppara-/- mice were protected from these effects. Ppara△Mac serum chemistries, as well as aspartate aminotransferase and alanine aminotransferase levels, matched wild-type mice. Agonist-treated Ppara△Hep mice had elevated serum cholesterol, phospholipids, and triglycerides when compared with Ppara-/- mice, indicating a possible role for extrahepatic PPARA in regulating circulating lipid levels. BrdU labeling confirmed increased cell proliferation only in wild-type and Ppara△Mac mice. Macrophage PPARA disruption did not impact agonist-induced upregulation of lipid metabolism, cell proliferation, or DNA damage and repair-related gene expression, whereas gene expression was repressed in Ppara△Hep mice. Interestingly, downregulation of inflammatory cytokines IL-15 and IL-18 was dependent on macrophage PPARA. Cell type-specific regulation of target genes was confirmed in primary hepatocytes and Kupffer cells. These studies conclusively show that cell proliferation is mediated exclusively by PPARA activation in hepatocytes and that Kupffer cell PPARA has an important role in mediating the anti-inflammatory effects of PPARA agonists.

Multifarious Beneficial Effect of Nonessential Amino Acid, Glycine: A Review

Glycine is most important and simple, nonessential amino acid in humans, animals, and many mammals. Generally, glycine is synthesized from choline, serine, hydroxyproline, and threonine through interorgan metabolism in which kidneys and liver are the primarily involved. Generally in common feeding conditions, glycine is not sufficiently synthesized in humans, animals, and birds. Glycine acts as precursor for several key metabolites of low molecular weight such as creatine, glutathione, haem, purines, and porphyrins. Glycine is very effective in improving the health and supports the growth and well-being of humans and animals. There are overwhelming reports supporting the role of supplementary glycine in prevention of many diseases and disorders including cancer. Dietary supplementation of proper dose of glycine is effectual in treating metabolic disorders in patients with cardiovascular diseases, several inflammatory diseases, obesity, cancers, and diabetes. Glycine also has the property to enhance the quality of sleep and neurological functions. In this review we will focus on the metabolism of glycine in humans and animals and the recent findings and advances about the beneficial effects and protection of glycine in different disease states.

Mechanistic considerations for human relevance of cancer hazard of di(2-ethylhexyl) phthalate

Di(2-ethylhexyl) phthalate (DEHP) is a peroxisome proliferator agent that is widely used as a plasticizer to soften polyvinylchloride plastics and non-polymers. Both occupational (e.g., by inhalation during its manufacture and use as a plasticizer of polyvinylchloride) and environmental (medical devices, contamination of food, or intake from air, water and soil) routes of exposure to DEHP are of concern for human health. There is sufficient evidence for carcinogenicity of DEHP in the liver in both rats and mice; however, there is little epidemiological evidence on possible associations between exposure to DEHP and liver cancer in humans. Data are available to suggest that liver is not the only target tissue for DEHP-associated toxicity and carcinogenicity in both humans and rodents. The debate regarding human relevance of the findings in rats or mice has been informed by studies on the mechanisms of carcinogenesis of the peroxisome proliferator class of chemicals, including DEHP. Important additional mechanistic information became available in the past decade, including, but not limited to, sub-acute, sub-chronic and chronic studies with DEHP in peroxisome proliferator-activated receptor (PPAR) α-null mice, as well as experiments utilizing several transgenic mouse lines. Activation of PPARα and the subsequent downstream events mediated by this transcription factor represent an important mechanism of action for DEHP in rats and mice. However, additional data from animal models and studies in humans exposed to DEHP from the environment suggest that multiple molecular signals and pathways in several cell types in the liver, rather than a single molecular event, contribute to the cancer in rats and mice. In addition, the toxic and carcinogenic effects of DEHP are not limited to liver. The International Agency for Research on Cancer working group concluded that the human relevance of the molecular events leading to cancer elicited by DEHP in several target tissues (e.g., liver and testis) in rats and mice can not be ruled out and DEHP was classified as possibly carcinogenic to humans (Group 2B).

Effect of interferon-α2b on the expression of various drug-metabolizing enzymes and transporters in co-cultures of freshly prepared human primary hepatocytes

The purpose of this study was to assess the impact of interferon-α2b (IFN-α2b) on the expression of various drug-metabolizing enzymes and transporters in freshly prepared co-cultures (parenchymal and non-parenchymal cells) of human primary hepatocytes. At therapeutically relevant concentrations (from 1000 to 3000 IU/mL), IFN-α2b up-regulated STAT1 (signal transducer and activator of transcription factor 1) mRNA expression. Conversely, three cytochrome P450s (CYP1A2, CYP2B6, CYP2E1), a UDP-glucuronosyltransferase (UGT2B7), a sulphotransferase (SULT1A1) and organic anion transporter (OAT2) were significantly down-regulated (~50%; P < 0.05). Western blot analysis of CYP1A2, UGT2B7 and OAT2 protein supported the mRNA data. Two peroxisome proliferator activator receptor alpha (PPARα)-controlled genes (pyruvate dehydrogenase kinase 4 and adipose differentiation-related protein), CYP3A4 and multidrug resistance-associated protein 2 were significantly up-regulated (up to 223%; P < 0.05). On the other hand, SULT2A1, carboxylesterase 2, organic anion transporting peptide (OATP1B1, OATP1B3, OATP2B1), organic cation transporter 1, P-glycoprotein and breast cancer resistance protein mRNA expression was not significantly affected. Western blot analysis of CYP3A4 supported the mRNA data also. The present results demonstrated complex interactions between IFN-α2b and hepatocytes and the observed down-regulation of CYP1A2, OAT2 and UGT2B7 is consistent with reports of drug interactions between IFN-α2b and drugs such as theophylline, clozapine and gemfibrozil.

Evaluation of an in vitro toxicogenetic mouse model for hepatotoxicity

Numerous studies support the fact that a genetically diverse mouse population may be useful as an animal model to understand and predict toxicity in humans. We hypothesized that cultures of hepatocytes obtained from a large panel of inbred mouse strains can produce data indicative of inter-individual differences in in vivo responses to hepato-toxicants. In order to test this hypothesis and establish whether in vitro studies using cultured hepatocytes from genetically distinct mouse strains are feasible, we aimed to determine whether viable cells may be isolated from different mouse inbred strains, evaluate the reproducibility of cell yield, viability and functionality over subsequent isolations, and assess the utility of the model for toxicity screening. Hepatocytes were isolated from 15 strains of mice (A/J, B6C3F1, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, BALB/cByJ, AKR/J, MRL/MpJ, NOD/LtJ, NZW/LacJ, PWD/PhJ and WSB/EiJ males) and cultured for up to 7 days in traditional 2-dimensional culture. Cells from B6C3F1, C57BL/6J, and NOD/LtJ strains were treated with acetaminophen, WY-14,643 or rifampin and concentration-response effects on viability and function were established. Our data suggest that high yield and viability can be achieved across a panel of strains. Cell function and expression of key liver-specific genes of hepatocytes isolated from different strains and cultured under standardized conditions are comparable. Strain-specific responses to toxicant exposure have been observed in cultured hepatocytes and these experiments open new opportunities for further developments of in vitro models of hepatotoxicity in a genetically diverse population.

Bezafibrate decreases growth stimulatory action of the sphingomyelin signaling pathway in regenerating rat liver

Liver regeneration after partial hepatectomy (PH) is achieved through proliferation of hepatocytes and non-parenchymal cells. The nuclear peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in regulation of lipid metabolism and proliferation of hepatic cells. The sphingomyelin signal transduction pathway is involved in the regulation of the cell cycle in eukaryotic organisms. Sphingosine-1-phosphate (S1P) and ceramide (CER)-- the intermediates of the pathway--are known to stimulate and to inhibit cellular proliferation. The aim of the present study was to investigate the effect of PPARalpha activation by bezafibrate on the sphingomyelin signaling pathway during the first 24h of liver regeneration after PH in the rat. The content of sphingomyelin, ceramide, sphingosine, sphinganine, sphingosine-1-phosphate and the activity of sphingomyelinases and ceramidases were determined at various time points after PH. It has been found that the activity of neutral Mg(2+)-dependent sphingomyelinase (nSMase) increased, whereas the activity of acidic sphingomyelinase (aSMase) decreased in the regenerating liver. Activation of PPARalpha by bezafibrate lower the activity of nSMase and increased the activity of aSMase in the regenerating rat liver. The content of ceramide was higher in bezafibrate-treated rats, whereas the content of sphingosine-1-phosphate was markedly lower as compared to the untreated rats. Therefore, it is concluded that activation of PPARalpha by bezafibrate decreases the growth-stimulatory activity of the sphingomyelin pathway in regenerating rat liver.

Time course investigation of PPARalpha- and Kupffer cell-dependent effects of WY-14,643 in mouse liver using microarray gene expression

Administration of peroxisome proliferators to rodents causes proliferation of peroxisomes, induction of beta-oxidation enzymes, hepatocellular hypertrophy and hyperplasia, with chronic exposure ultimately leading to hepatocellular carcinomas. Many responses associated with peroxisome proliferators are nuclear receptor-mediated events involving peroxisome proliferators-activated receptor alpha (PPARalpha). A role for nuclear receptor-independent events has also been shown, with evidence of Kupffer cell-mediated free radical production, presumably through NAPDH oxidase, induction of redox-sensitive transcription factors involved in cytokine production and cytokine-mediated cell replication following acute treatment with peroxisome proliferators in rodents. Recent studies have demonstrated, by using p47(phox)-null mice which are deficient in NADPH oxidase, that this enzyme is not related to the phenotypic events caused by prolonged administration of peroxisome proliferators. In an effort to determine the timing of the transition from Kupffer cell-to PPARalpha-dependent modulation of peroxisome proliferator effects, gene expression was assessed in liver from Pparalpha-null, p47(phox)-null and corresponding wild-type mice following treatment with 4-chloro-6-(2,3-xylidino)-pyrimidynylthioacetic acid (WY-14,643) for 8 h, 24 h, 72 h, 1 week or 4 weeks. WY-14,643-induced gene expression in p47(phox)-null mouse liver differed substantially from wild-type mice at acute doses and striking differences in baseline expression of immune related genes were evident. Pathway mapping of genes that respond to WY-14,643 in a time- and dose-dependent manner demonstrates suppression of immune response, cell death and signal transduction and promotion of lipid metabolism, cell cycle and DNA repair. Furthermore, these pathways were largely dependent on PPARalpha, not NADPH oxidase demonstrating a temporal shift in response to peroxisome proliferators. Overall, this study shows that NADPH oxidase-dependent events, while detectable following acute treatment, are transient. To the contrary, a strong PPARalpha-specific gene signature was evident in mice that were continually exposed to WY-14,643.

Hepatocellular proliferation in response to agonists of peroxisome proliferator-activated receptor alpha: a role for Kupffer cells?

Background

It has been proposed that PPARα agonists stimulate Kupffer cells in rodents which in turn, release mitogenic factors leading to hepatic hyperplasia, and eventually cancer. However, Kupffer cells do not express PPARα receptors, and PPARα agonists stimulate hepatocellular proliferation in both TNFα- and TNFα receptor-null mice, casting doubt on the involvement of Kupffer cells in the mitogenic response to PPARα agonists. This study was therefore designed to investigate whether the PPARα agonist PFOA and the Kupffer cell inhibitor methylpalmitate produce opposing effects on hepatocellular proliferation and Kupffer cell activity in vivo, in a manner that would implicate these cells in the mitogenic effects of PPARα agonists.

Methods

Male Sprague-Dawley rats were treated intravenously via the tail vein with methylpalmitate 24 hrs prior to perfluorooctanoic acid (PFOA), and were sacrificed 24 hrs later, one hr after an intraperitoneal injection of bromodeoxyuridine (BrdU). Sera were analyzed for TNFα and IL-1β. Liver sections were stained immunohistochemically and quantified for BrdU incorporated into DNA.

Results

Data show that PFOA remarkably stimulated hepatocellular proliferation in the absence of significant changes in the serum levels of either TNFα or IL-1β. In addition, methylpalmitate did not alter the levels of these mitogens in PFOA-treated animals, despite the fact that it significantly blocked the hepatocellular proliferative effect of PFOA. Correlation between hepatocellular proliferation and serum levels of TNFα or IL-1β was extremely poor.

Conclusion

It is unlikely that mechanisms involving Kupffer cells play an eminent role in the hepatic hyperplasia, and consequently hepatocarcinogenicity attributed to PPARα agonists. This conclusion is based on the above mentioned published data and the current findings showing animals treated with PFOA alone or in combination with methylpalmitate to have similar levels of serum TNFα and IL-1β, which are reliable indicators of Kupffer cell activity, despite a remarkable difference in hepatocellular proliferation.

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

Lack of direct mitogenic activity of dichloroacetate and trichloroacetate in cultured rat hepatocytes

Dichloroacetate (DCA) and trichloroacetate (TCA) are hepatocarcinogenic metabolites of the common groundwater contaminant, 1,1,2-trichloroethylene. DCA and TCA have been shown to induce hepatocyte proliferation in vivo, but it is not known if this response is the result of direct mitogenic activity or whether cell replication occurs indirectly in response to tissue injury or inflammation. In this study we used primary cultures of rat hepatocytes, a species susceptible to DCA- but not TCA-induced hepatocarcinogenesis, to determine whether DCA and TCA are direct hepatocyte mitogens. Rat hepatocytes, cultured in growth factor-free medium, were treated with 0.01-1.0 mM DCA or TCA for 10-40 h; cell replication was then assessed by measuring incorporation of 3H-thymidine into DNA and by cell counts. DCA or TCA treatment did not alter 3H-thymidine incorporation in the cultured hepatocytes. Although an increase in cell number was not observed, DCA treatment significantly abrogated the normal background cell loss, suggesting an ability to inhibit apoptotic cell death in primary hepatocyte cultures. Furthermore, treatment with DCA synergistically enhanced the mitogenic response to epidermal growth factor. The data indicate that DCA and TCA are not direct mitogens in hepatocyte cultures, which is of interest in view of their ability to stimulate hepatocyte replication in vivo. Nevertheless, the synergistic enhancement of epidermal growth factor-induced hepatocyte replication by DCA is of particular interest and warrants further study.

Aging and enhanced hepatocarcinogenicity by peroxisome proliferator-activated receptor alpha agonists

The hepatocarcinogenic effect of PPARalpha agonists is enhanced by aging. Exposure to these chemicals produces a five- to seven-fold higher yield of grossly visible hepatic tumors in old relative to young animals. This review presents current experimental evidence, which supports a mechanism involving enhanced exposure to oxidative stress, and diminished apoptosis in this age-related difference in sensitivity. In the aged liver, a decrease in hepatic antioxidant activity, coupled with a PPARalpha agonist-induced increase in the activities of various oxidases, may expose these livers to oxidative stress. Additionally, livers of senescent animals appeared more sensitive to the anti-apoptotic effect of PPARalpha agonists. Since apoptosis safeguards cells with damaged DNA from progressing to the point of tumor formation, inhibition of hepatocellular apoptosis by PPARalpha agonists could well lead to the formation of focal lesions in the aged liver. Although PPARalpha-dependent alterations in cell cycle regulatory proteins have been reported, the correlation between hepatocellular DNA replication and liver cancer caused by PPARalpha agonists is a weak one. These findings have implications for human susceptibility to these chemicals.

Interaction networks: coordinating responses to xenobiotic exposure

In the last decade the increased usage of '-omic' technologies, plus the sequencing of over 800 complete genomes has led to a vast increase in the amount of information available to the researcher for examining cellular responses to xenobiotics. Much effort has been put into the identification and analysis of expression profiles associated with pathobiological conditions and/or xenobiotic exposure. These profiles are commonly used in two applications. Firstly, comparative profile experiments are used to classify pathobiological states and for the screening of novel chemical entities to predict their action(s) on the body. Secondly, mechanistic investigations will gain information on the molecular mechanisms underlying toxic responses/pathobiological states. During the course of such analysis it has become increasingly clear that a series of highly refined interaction networks exist within the body, regulating both the sensitivity and selectivity of the body's response to pathobiological states/xenobiotic exposure. These interaction networks exist at several levels: Firstly, within individual cells, the interaction between factors that transmit xenobiotics signals will determine the overall cellular response. Secondly, intraorgan communication occurs between the different cell types/sub-types which makes up an organ, coordinating the overall organ response. Finally, interorgan interactions provide axes of response through the body.

Strategies for using in vitro screens in drug metabolism

In vitro assays are increasingly being used in drug metabolism studies to screen novel chemicals. Their advantages are twofold: first, they allow testing early in the drug discovery phase, providing important information on chemical characteristics; second, human cells or cell constituents can be utilized, increasing the relevance to man. However, the process of isolation, transformation or storage of these cell systems may alter their phenotype (and, in the case of tumour-derived cell lines, genotype as well). A review of the systems currently employed shows that, whereas all systems have their own caveats, it is possible to find an appropriate system for any particular question that is asked.

Gene expression changes in rat liver following exposure to liver growth agents: role of Kupffer cells in xenobiotic-mediated liver growth

Many xenobiotics are known to cause liver enlargement and hepatocarcinogenesis in rats, although the molecular mechanisms that underlie this effect remain largely undefined. Human exposure to several of these compounds, including glucocorticoids and peroxisome proliferators may be significant, due to their use in both pharmaceutical and industrial processes. It is therefore important to elucidate the molecular mechanisms underlying this abnormal liver enlargement in rats, as this will enable more accurate extrapolation of the possible outcomes of human exposure. Male Sprague-Dawley rats were dosed with the peroxisome proliferator Wy-14,643 and changes in liver gene expression examined using subtractive suppression hybridisation examined either 12 of 24hr later. Twenty-five transcripts were identified which showed differential gene expression in liver following exposure to Wy-14,643. Biochemical indices of liver growth (DNA synthesis, apoptosis) showed that these changes correlated with the initiation of liver enlargement. Rats were next treated with either Wy-14,643, cyproterone acetate and dexamethasone, chemically and mechanistically-distinct hepatomegalic compounds. Carboxylesterase and Kupffer cell receptor mRNA levels were seen to alter in a qualitatively similar fashion for all three compounds, and in a liver specific fashion. In addition, these changes correlated with a decrease in the density of Kupffer cells within the liver, which are known to release mitogenic cytokines, and have been linked to Wy-14,643-induced cell proliferation. We therefore propose that Kupffer cells play a role in a general mechanism of xenobiotic-mediated liver enlargement.

L-Glycine: a novel antiinflammatory, immunomodulatory, and cytoprotective agent

PURPOSE OF REVIEW

In recent years, evidence has mounted in favor of the antiinflammatory, immunomodulatory and cytoprotective effects of the simplest amino acid L-glycine. This article will focus on the recent findings about the responsible mechanisms of protection and review the beneficial effects of glycine in different disease states.

RECENT FINDINGS

Glycine protects against shock caused by hemorrhage, endotoxin and sepsis, prevents ischemia/reperfusion and cold storage/reperfusion injury to a variety of tissues and organs including liver, kidney, heart, intestine and skeletal muscle, and diminishes liver and renal injury caused by hepatic and renal toxicants and drugs. Glycine also protects against peptidoglycan polysaccharide-induced arthritis and inhibits gastric secretion and protects the gastric mucosa against chemically and stress-induced ulcers. Glycine appears to exert several protective effects, including antiinflammatory, immunomodulatory and direct cytoprotective actions. Glycine acts on inflammatory cells such as macrophages to suppress activation of transcription factors and the formation of free radicals and inflammatory cytokines. In the plasma membrane, glycine appears to activate a chloride channel that stabilizes or hyperpolarizes the plasma membrane potential. As a consequence, agonist-induced opening of L-type voltage-dependent calcium channels and the resulting increases in intracellular calcium ions are suppressed, which may account for the immunomodulatory and antiinflammatory effects of glycine. Lastly, glycine blocks the opening of relatively non-specific pores in the plasma membrane that occurs as the penultimate event leading to necrotic cell death.

SUMMARY

Multiple protective effects make glycine a promising treatment strategy for inflammatory diseases.

Dose-related effects of the peroxisome proliferator methylclofenapate in rat liver

Male Sprague-Dawley rats were fed diets containing 0 (control) and 2.5-750 ppm of the peroxisome proliferator methylclofenapate (MCP) for 1, 4 and 13 weeks. In other studies MCP has been shown to produce liver tumors at dietary levels of 50 and 250, but not 10 ppm. MCP treatment produced increases in relative liver weight and activities of peroxisomal and microsomal fatty acid oxidising enzymes at all time points at doses as low as 10 and 2.5 ppm, respectively. Replicative DNA synthesis was studied by implanting osmotic pumps containing 5-bromo-2'-deoxyuridine during study weeks 0-1, 3-4 and 12-13. Hepatocyte labelling index values were significantly increased by treatment with 10-750 ppm MCP for 1 week and 150-750 ppm MCP for 13 weeks. Treatment with 50-750 ppm MCP for 13 weeks increased hepatic peroxisome proliferator-activated receptor alpha and transforming growth factor-β1 gene expression to 150-165 and 150-170% of control, respectively. These results demonstrate that while low doses of MCP produce sustained hepatomegaly and peroxisome proliferation in rat liver, higher doses are required to produce a sustained stimulation of replicative DNA synthesis.

Hepatocellular proliferation in response to a peroxisome proliferator does not require TNFalpha signaling

Rodents exposed to peroxisome proliferator xenobiotics respond with marked increases in hepatocellular replication and growth that results in tumor formation. Recently, tumor necrosis factor-alpha (TNFalpha) was proposed as the central mediator of this maladaptive response. To define the role of TNFalpha signaling in hepatocellular growth induced by peroxisome proliferators we administered three daily gavage doses of the potent peroxisome proliferator, Wy-14 643, to mice nullizygous for TNF-receptor I (TNFR1), TNFR2, or both receptors. We demonstrate here that regardless of genotype the mice responded with almost identical increases in liver to body weight ratios and hepatocyte proliferation. Lacking evidence that TNFalpha signaling mediates these effects, we then examined the possible contribution of alternative cytokine pathways. Semi-quantitative, reverse transcriptase polymerase chain reaction analysis revealed that wild type mice acutely exposed to Wy-14 643 had increased hepatic expression of Il1beta, Il1r1, Hnf4, and Stat3 genes. Moreover, hepatic adenomas from mice chronically exposed to Wy-14 643 had increased expression of Il1beta, Il1r1, Il6, and Ppargamma1. Expression of Il1alpha, Tnfalpha, Tnfr1, Tnfr2, Pparalpha, or C/ebpalpha was not altered by acute Wy-14 643 exposure or in adenomas induced by Wy-14643. These data suggest that the hepatic mitogenesis and carcinogenesis associated with peroxisome proliferator exposure is not mediated via TNFalpha but instead may involve an alternative pathway requiring IL1beta and IL6.

Phthalates rapidly increase production of reactive oxygen species in vivo: role of Kupffer cells

The role of oxidants in the mechanism of tumor promotion by peroxisome proliferators remains controversial. The idea that induction of acyl-coenzyme A oxidase leads to increased production of H(2)O(2), which damages DNA, seems unlikely; still, free radicals might be important in signaling in specialized cell types such as Kupffer cells, which produce mitogens. Because hard evidence for increased oxidant production in vivo after treatment with peroxisome proliferators is lacking, the spin-trapping technique and electron spin resonance spectroscopy were used. Rats were given di(2-ethylhexyl) phthalate (DEHP) acutely. The spin trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone was also given and bile samples were collected for 4 h. Under these conditions, the intensity of the six-line radical adduct signal increased to a maximum value of 2.5-fold 2 h after administration of DEHP, before peroxisomal oxidases were induced. Furthermore, DEHP given with [(13)C(2)]dimethyl sulfoxide produced a 12-line electron spin resonance spectrum, providing evidence that DEHP stimulates (*)OH radical formation in vivo. Furthermore, when rats were pretreated with dietary glycine, which inactivates Kupffer cells, DEHP did not increase radical signals. Moreover, similar treatments were performed in knockout mice deficient in NADPH oxidase (p47(phox) subunit). Importantly, DEHP increased oxidant production in wild-type but not in NADPH oxidase-deficient mice. These data provide evidence for the hypothesis that the molecular source of free radicals induced by peroxisome proliferators is NADPH oxidase in Kupffer cells. On the contrary, radical adduct formation was not affected in peroxisome proliferator-activated receptor alpha knockout mice. These observations represent the first direct, in vivo evidence that phthalates increase free radicals in liver before peroxisomal oxidases are induced.

Dexamethasone selectively inhibits WY14,643-induced cell proliferation and not peroxisome proliferation in mice

It has been proposed that the hepatocellular proliferation induced by peroxisome proliferators may occur through an indirect mechanism involving cytokine release as opposed to direct regulation of cell growth genes by PPARalpha. We compared the induction of peroxisome proliferation and cell proliferation in C57Bl/6 mice treated with 100 mg/kg/day WY14,643 in the presence or absence of increasing doses of dexamethasone (DEX), an inhibitor of the release of proinflammatory cytokines. Biochemical markers of peroxisome proliferation, including fatty acyl-CoA oxidase activity, CYP4A content, and liver-to-body-weight ratios were markedly increased in the WY14,643-treated mice. DEX coadministration, up to a maximum dose of 50 mg/kg/day, did not prevent the induction of these parameters. Acyl-CoA oxidase mRNA levels increased 5-fold with WY14,643 treatment and 15-fold with DEX coadministration at 5 mg/kg/day. ApoCIII mRNA levels were decreased by 50% in WY14,643-treated mice. DEX alone at 5 mg/kg/day increased the ApoCIII mRNA 4-fold, but WY14,643 coadministration also inhibited this induction by greater than 50%. In addition, immunohistochemical detection of peroxisomes with anti-PMP-70 antibody demonstrated marked increase in hepatocellular peroxisomes in WY14,643-treated mice regardless of DEX treatment. In contrast, coadministration of DEX at 2 mg/kg/day partially inhibited the hepatocyte proliferation response (measured by BrdU incorporation or Ki-67 immunohistochemical detection). Moreover, DEX at doses of 5 mg/kg/day or higher completely inhibited the induction of cell proliferation and, at these higher doses, reduced the cell proliferation rate to levels below the vehicle-treated control mice. Our studies clearly demonstrate that the hepatocellular proliferation induced by a peroxisome proliferator can be modulated independently of the other pleiotropic effects usually induced by these agents, suggesting an indirect mechanism of hyperplasia.

Proteomic analysis of differential protein expression in primary hepatocytes induced by EGF, tumour necrosis factor alpha or the peroxisome proliferator nafenopin

Peroxisome proliferators are nongenotoxic rodent-liver carcinogens that have been shown to cause both an induction of hepatocyte proliferation and a suppression of apoptosis. Both epidermal growth factor (EGF) and the peroxisome proliferator nafenopin induce DNA replication in primary rat hepatocyte cultures, but apparently through different signalling pathways. However, both EGF and nafenopin require tumour necrosis factor alpha (TNFalpha) signalling to induce DNA replication. By examining proteins isolated from rat primary hepatocyte cultures using two-dimensional gel electrophoresis and mass spectrometry, we found that proteins showing an altered expression pattern in response to nafenopin differed from those showing altered expression in response to EGF. However, many proteins showing altered expression upon stimulation with TNFalpha were common to both the EGF and nafenopin responses. These proteome profiling experiments contribute to a better understanding of the molecular mechanisms involved in the response to peroxisome proliferators. We found 32 proteins with altered expression upon stimulation with nafenopin, including muscarinic acetylcholine receptor 3, intermediate filament vimentin and the beta subunit of the ATP synthase. These nonperoxisomal protein targets offer insights into the mechanisms of peroxisome proliferator-induced carcinogenesis in rodents and provide opportunities to identify toxicological markers to facilitate early identification of nongenotoxic carcinogens.

Promoter activity and regulation of the CYP4F2 leukotriene B(4) omega-hydroxylase gene by peroxisomal proliferators and retinoic acid in HepG2 cells

The human liver CYP4F2 gene (Accession No. AF221943) encodes a leukotriene B(4) omega-hydroxylase that metabolizes leukotriene B(4) (LTB(4)) to a less potent proinflammatory eicosanoid, 20-OH-LTB(4). We sequenced a 6.7-kb genomic fragment of the human CYP4F2 gene that has the first five exons and 500 bp of the 5'-flanking region. The major transcription start site was found to be 49 bp upstream of the 3' end of exon 1 and the ATG translation initiation codon was located in exon 2. Besides the TATA box at -39 bp and basal transcription factor binding sites, the promoter region and 412-bp intron 1 have several putative binding sites for nuclear factors that may mediate the inflammatory response and lipid homeostasis. We found two DR1 elements in the 5' promoter, a DR2 element in intron 1, and RXR/RAR binding sites in both intron 1 and the 5' promoter. DNase I footprinting revealed three protected sequences, with the region containing two CAATT boxes at -71 and -111 bp important in CYP4F2 gene expression. Luciferase reporter assays showed that the 500-bp upstream sequence has strong promoter activity. Transient transfection experiments identified two sites in the 5' promoter and intron 1 that cooperate in gene transcription while exon 1 and a GC-rich region flanking exon 1 inhibit transcription. trans-Retinoic acid and 9-cis-retinoic acid stimulate promoter activity 3- and 6-fold, respectively, while cotransfection with RXRalpha or RAR/RXRalpha further enhanced activity. Peroxisome proliferators inhibit CYP4F2 gene promoter activity and cotransfection with PPARalpha or PPARalpha/RXRalpha can slightly attenuate this inhibition. Both saturated fatty acids and 12-hydroxydodecanoic acid (12-OH-C(12)) can stimulate CYP4F2 gene promoter activity. Therefore, the CYP4F2 gene is repressed by peroxisomal proliferators and induced by retinoic acid, with RAR/RXRalpha mediating the induction while PPARalpha/RXR functions neither in the repression nor in the induction by peroxisomal proliferators or retinoic acid.

A commentary on the use of hepatocytes in drug metabolism studies during drug discovery and development

Isolated hepatocytes and liver slices, in short-term suspension or longer-term culture, offer the prospect of providing qualitative metabolic information and quantitative pharmacokinetic parameters from key animal species and man at early stages of the drug discovery-development continuum. The propensity for changes in the fidelity of drug metabolism after removal of hepatocytes from the organ has long been recognized. The many and varied approaches which have been undertaken in an attempt to compensate for physiological shortcomings of in vitro hepatocyte systems are reviewed. In this respect, short-term suspension culture may provide a baseline against which to measure the success of extended culture methods, but it should be remembered that even freshly isolated hepatocyte preparations have deficiencies and liabilities that may affect the nature of information gathered. This article discusses the current advances and shortcomings of hepatocyte suspensions and cultures, along with liver slice technology, at both quantitative and qualitative levels.

Peroxisome proliferator-activated receptor alpha is restricted to hepatic parenchymal cells, not Kupffer cells: implications for the mechanism of action of peroxisome proliferators in hepatocarcinogenesis

Peroxisome proliferators increase hepatocyte proliferation and cause liver tumors in rodents, yet the mechanism of action is not understood. Based on studies with null mice it is known that peroxisome proliferator-activated receptor-alpha (PPARalpha) is involved. There is also evidence that Kupffer cells play a central role in peroxisome proliferator-induced carcinogenesis, most likely via mechanisms involving increases in superoxide, activation of nuclear factor kappaB and production of tumor necrosis factor-alpha (TNFalpha). However, it is not known whether PPARalpha is constitutively expressed in Kupffer cells. Therefore, the expression of PPAR isoforms in rat Kupffer and parenchymal cells was examined. Kupffer cells and hepatocytes of >99% purity were isolated from rats fed either a control diet or one containing 0.1% WY-14,643 for 1 week. Protein and RNA were obtained and PPAR expression was analyzed using northern and western blots. PPARalpha, PPARbeta and PPARgamma mRNA was detected in purified hepatocytes. In Kupffer cells, mRNA encoding PPARgamma was present while transcripts for PPARalpha and PPARbeta were not detected. Immunoblots were consistent with the results found by northern analysis. Moreover, when Kupffer cells from wild-type or PPARalpha-null mice were treated with WY-14,643 in vitro, superoxide production was similar. Combined, these results show that PPARalpha is expressed in rat parenchymal cells but not in Kupffer cells. These data are consistent with the hypothesis that parenchymal cells respond to Kupffer cell-derived TNFalpha via mechanisms dependent on PPARalpha within the parenchymal cells.

Endotoxin induces structure-function alterations of rat liver peroxisomes: Kupffer cells released factors as possible modulators

We report that endotoxin treatment results in decreased amounts of peroxisomes as well as changes in structure and function of peroxisomal membranes. Peroxisomes isolated from the liver of control and treated animals showed a marked decrease in total protein, but no significant alteration in the sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) protein profile. However, the Western blot study of the peroxisomal beta-oxidation enzymes and catalase showed an increase in those enzymes in the peroxisomal peak of normal density in endotoxin-treated rats. Disintegration of peroxisomal membranes by carbonate treatment from endotoxin-treated liver and change in the fluidity of peroxisomal membranes suggests alterations in peroxisomal membrane structure. No such alterations were found in mitochondrial or microsomal membranes of endotoxin-treated livers. The lipid analysis of these organelles showed that the only organelle affected was the peroxisome, with a significant decrease in the phospholipid and cholesterol concentrations. To understand the mechanism of endotoxin-mediated alterations in peroxisomes, we studied the possible role of Kupffer cell secreted soluble factors (tumor necrosis factor alpha [TNF-alpha]) on the peroxisomal structure/function. Inactivation/elimination of Kupffer cells by gadolinium chloride before endotoxin treatment did not normalize the overall peroxisomal protein amount and the lipid composition of isolated peroxisomes. However, the levels of individual protein amount in remaining peroxisomes were normalized. Endotoxin also decreased peroxisomal beta-oxidation, and this was partially restored with gadolinium treatment. These results clearly show that peroxisomes are severely affected by endotoxin treatment and suggest that the damage to this organelle may contribute, at least in part, to endotoxin-induced hepatic cytotoxicity.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.

Corn oil rapidly activates nuclear factor-kappaB in hepatic Kupffer cells by oxidant-dependent mechanisms

N-6 polyunsaturated fatty acids (N-6 PUFAs), major constituents of corn oil and natural ligands for peroxisome proliferator-activated receptors, increase the rate of growth of established tumors. It has been proposed that chemical peroxisome proliferators increase hepatocyte proliferation by mechanisms involving activation of nuclear factor-kappaB (NF-kappaB) and production of low levels of tumor necrosis factor alpha (TNFalpha) by Kupffer cells; however, how N-6 PUFAs are involved in increased cell proliferation in liver is not well understood. Here, the hypothesis that N-6 PUFAs increase production of mitogens by activation of Kupffer cell NF-kappaB was tested. A single dose of corn oil (2 ml/kg, i.g.), but not olive oil or medium-chain triglycerides (saturated fat), caused an approximately 3-fold increase in hepatocyte proliferation. Similarly, when activity of NF-kappaB in whole rat liver or isolated hepatocytes and Kupffer cells was measured at various time intervals for up to 36 h, only corn oil activated NF-kappaB. Corn oil increased NF-kappaB activity approximately 3-fold 1-2 h after treatment exclusively in the Kupffer cell fraction. In contrast, increases were small and only occurred after approximately 8 h in hepatocytes. The activation of NF-kappaB at 2 h and increases in cell proliferation at 24 h due to corn oil were prevented almost completely when rats were pretreated for 4 days with either dietary glycine (5% w/w), an agent that inactivates Kupffer cells, or the NADPH oxidase inhibitor, diphenyleneiodonium (s.c., 1 mg/kg/day). Furthermore, arachidonic acid (100 microM) activated superoxide production approximately 4-fold when added to isolated Kupffer cells in vitro. This phenomenon was not observed with oleic or linoleic acids. Interestingly, a single dose of corn oil increased TNFalpha mRNA nearly 2-fold 8 h after treatment. It is concluded that corn oil rapidly activates NF-kappaB in Kupffer cells via oxidant-dependent mechanisms. This triggers production of low levels of TNFalpha which is mitogenic in liver and promotes growth of hepatocytes.