D-galactosamine hepatotoxicity. IV. Further studies of the pathogenesis of fatty liver

Assessment of animal experimental models of toxic liver injury in the context of their potential application as preclinical models for cell therapy

Preclinical animal models allow to study development and progression of several diseases, including liver disorders. These studies, for ethical reasons and medical limits, are impossible to carry out in human patients. At the same time, such experimental models constitute an important source of knowledge on pathomechanisms for drug- and virus-induced hepatotoxicity, both acute and chronic. Carbon tetrachloride, D-Galactosamine, and retrorsine are xenobiotics that can be used in immunocompetent animal models of hepatotoxicity, where chemical-intoxicated livers present histological features representative of human viruses-related infection. A prolonged derangement into liver architecture and functions commonly lead to cirrhosis, eventually resulting in hepatocellular carcinoma. In human, orthotopic liver transplantation commonly resolve most the problems related to cirrhosis. However, the shortage of donors does not allow all the patients in the waiting list to receive an organ on time. A promising alternative treatment for acute and chronic liver disease has been advised in liver cell transplantation, but the limited availability of hepatocytes for clinical approaches, in addition to the immunosuppressant regiment required to sustain cellular long-term engraftment have been encouraging the use of alternative cell sources. A recent effective source of stem cells have been recently identified in the human amnion membrane. Human amnion epithelial cells (hAEC) have been preclinically tested and proven sufficient to rescue immunocompetent rodents lethally intoxicated with drugs. The adoption of therapeutic procedures based on hAEC transplant in immunocompetent recipients affected by liver diseases, as well as patients with immune-related disorders, may constitute a successful new alternative therapy in regenerative medicine.

Utility of hepatic phosphorus-31 magnetic resonance spectroscopy in a rat model of acute liver failure

The ability to document the extent of hepatic injury and predict the outcome of fulminant hepatic failure would be helpful in identifying those patients who might benefit from liver transplantation. The aim of the present study was to determine whether in vivo phosphorus-31 magnetic resonance spectroscopy (31P MRS) accurately assesses the severity of liver damage and is of prognostic value in a D-galactosamine (D-galN)-induced model of acute liver failure. Adult male Sprague-Dawley rats (n = 36) received an intraperitoneal dose of D-galN (1.0 g/kg), and MRS examinations were performed at peak (48 hours) and in subsequent experiments, just prior to peak (30 hours) hepatic injury. Rats not exposed to D-galN served as controls. The concentration of hepatic phosphorylated metabolites decreased in proportion to the severity of liver injury at 48 hours. Significant correlations were detected between hepatic adenosine triphosphate (ATP) and serum aspartate aminotransferase, bilirubin, and percentage of hepatocyte necrosis identified histologically (r = -.91, -.74, and -.92, respectively; p < .001). Prior to peak hepatic injury (30 hours), 31P MRS was able to predict with 100% accuracy those rats that would survive (ATP > 2.3 mM) and those that would not (ATP < 1.5 mM). When an intermediate cutoff value of 2.0 mM was selected, ATP levels were able to correctly predict survival and death with 80% and 60% accuracy, respectively. These findings indicate that hepatic ATP levels as measured by 31P MRS provide a noninvasive indication of the severity of liver damage and serve as a useful prognostic indicator of outcome in this model of acute liver failure.

Hepatic D-galactosamine toxicity studied with localized in vivo 31P magnetic resonance spectroscopy in intact rats

Spatially resolved 31P magnetic resonance spectroscopy (MRS) at 4.7 T was applied to noninvasively assess liver phosphorus metabolites in a biochemically well-characterized model of hepatotoxicity induced by injection of a sublethal dose of D-galactosamine (galN). A newly developed hybrid method based on spectral localization with B0 and B1 gradients was employed to obtain multivoxel spectra in intact anesthesized rats. Spatially localized in vivo spectra were recorded 0 to 26 h after galN injection of female rats. In response to galN exposure, diphosphodiester peaks ascribed to UDP-hexosamines became detectable by 4 h and persisted up to 26 h. A metabolite coresonating with inorganic phosphate increased rapidly in intensity by 2 h after galN and returned to baseline by 18 h; this resonance was shown not to be Pi and was assigned to galN-1-phosphate by subsequent high resolution MRS experiments on extracts prepared from these livers. These results confirmed in vivo the metabolic perturbations described previously for this model of hepatotoxicity following biochemical studies based on classical extraction methods. Unlike the in vitro studies, however, these noninvasive experiments provided additional information on the time course of metabolic alterations on the same animal.

Peroxisomes in guinea pig liver: their peculiar morphological features may reflect certain aspects of lipoprotein metabolism in this species

We have studied the ultrastructural characteristics and the distribution of peroxisomes in guinea pig liver using electron-microscopic cytochemistry for catalase and morphometry. By light microscopy, peroxisomes appear as dark 0.2-0.5 microns granules in the cytoplasm of liver parenchymal cells, often forming large clusters that measure up to 5 microns across. Rows of single peroxisomes or their aggregates line the sinusoidal surface of hepatocytes. Electron microscopy reveals that clusters of up to 25 individual peroxisomes are usually located in the subsinusoidal region of parenchymal cells. The mean diameter and the volume density of peroxisomes are larger in pericentral than in periportal regions of the liver lobule. Whereas large amounts of lipoprotein particles with a mean diameter of 160 nm (chylomicrons) are present in the Disse space, the cytoplasm of parenchymal cells contains multivesicular bodies and abundant lipid droplets. In addition, the Golgi complexes show distended lipoprotein-filled vesicles suggesting active biosynthesis of lipoproteins. We propose that the unique features of peroxisomes in guinea pig liver, such as cluster formation and alignment along the sinusoidal surface, may be related to the high levels of lipoproteins in the portal circulation and their hepatic catabolism in this species.

Rumen holotrich ciliate protozoa

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Effect of D-galactosamine in vitro on [U-14C]palmitate oxidation, triacylglycerol synthesis and secretion in isolated hepatocytes

Isolated rat hepatocytes were used to study in vitro effects of 10 mM D-galactosamine (GalN) on hepatic fatty acids metabolism. At this concentration, membrane integrity and biochemical competence (i.e., gluconeogenesis and ureogenesis) remained unaffected. Protein synthesis and secretion, as measured by the incorporation of [U-14C]leucine into total and medium protein, was significantly inhibited when incubated for more than 2 h. GalN activated the incorporation of [U-14C]palmitate into triacylglycerols and depressed its utilization in the formation of labelled ketone bodies and 14CO2. Hepatocytes isolated from fasted rats exposed to GalN in vitro did not show any variation in prelabelled triacylglycerol secretion. GalN induced a rapid inhibition of prelabelled triacylglycerol secretion by hepatocytes isolated from fed rats in which this secretion occurred to a larger extent than in hepatocytes isolated from fasted rats. The data reported here suggest that GalN induces a rise of triacylglycerol synthesis by inhibiting the palmitate oxidation pathway and a decrease of triacylglycerol secretion through an early derangement of the secretory pathway.

Methionine metabolism and ultrastructural changes with D-galactosamine in isolated rat hepatocytes

The biochemical and morphological effects of 2, 10 and 100 mM of D-galactosamine (GalN) were studied in isolated rat hepatocytes during 2 h of incubation. Lactate dehydrogenase (LDH), alanine aminotransferase (ALAT) and cell viability did not change, whatever the concentration used. The variations observed, which were dose dependent, included a large drop in ATP levels and inhibition of RNA and protein synthesis. A very high concentration of GalN was necessary, however, to induce a significant decline in methionine adenosyltransferase activity compared to control cells. The use of L-[methyl-14C]methionine during cell incubation with GalN demonstrated a decrease of S-adenosyl-L-methionine (SAMe) and an accumulation of L-methionine content related to the GalN concentration. These results suggested that an hepatotoxic agent such as GalN was able to induce disturbances of methionine metabolism. Some of the ultrastructural changes observed were different from those previously found in vivo, in rats given GalN intraperitoneally, underlining the marked difference between in vivo and in vitro intoxication.

Lipoprotein lipase and hepatic lipase deficiencies associated with impaired chylomicron clearance in D-(+) galactosamine hepatitis

D-(+) galactosamine (GaIN) produces a reversible from of hepatic injury in the rat, accompanied by alterations in morphology and composition of plasma lipoproteins in the fasting state. Lipoprotein lipase (LPL) and hepatic lipase (HL) activities were measured in fasting control and GaIN rats 24 hr after GaIN injection and initiation of fasting. Significant (p less than 0.001) deficiencies of both enzymes were noted in GaIN animals as compared to controls with LPL activity decreasing to 37.6% and HL activity to 23.2% of control values in GaIN animals. Serial enzyme determinations performed in both GaIN and control animals after gastric fat loading revealed an early persistent HL deficiency (p less than 0.025) at 9 hr after GaIN injection and initiation of fasting which persisted after the fat loading at 15 hr, and a later appearing LPL deficiency (p less than 0.025) was noted at 24 hr after GaIN injection and at 9 hr after fat loading. Serial compositional studies of plasma lipoproteins in pooled specimens after a gastric fat load revealed a marked chylomicronemia in GaIN animals compared to controls which reached a maximum at 12 hr after fat loading. A slight increase in VLDL (very low density lipoprotein) triglyceride and total cholesterol (CH) and a late-appearing (16 hr after fat loading) LDL (low density lipoprotein) CH peak, consisting mostly of unesterified CH, were also noted in GaIN rats as compared to control animals. These data demonstrate a defect in chylomicron (CM) catabolism in GaIN hepatopathy in the rat which is probably secondary to the observed severe LPL and HL deficiencies, although other factors such as activator deficiency, plasma inhibitory substances, and a defective CM particle may be important.

Lipoproteins and apolipoprotein patterns in rat plasma after liver injury induced by D-galactosamine

The effect of galactosamine administration on the concentration and composition of plasma lipoproteins was studied in female rats. After a single intraperitoneal injection of 3.48 mmol galactosamine/kg the following alterations occurred within 24 h: Disappearance of alpha-lipoproteins as shown by agarose electrophoresis, increase of abnormal lipoproteins in the beta- and pre-beta-region. These changes corresponded to a two- to three-fold increase of VLDL and LDL and a decrease of HDL to 20-30% of normal after sequential ultracentrifugation. When compared to control animals, galactosamine-treated rats exhibited an abnormal chemical composition of their plasma lipoproteins: A nearly total loss of cholesterol esters, but an increase of free cholesterol and phospholipids, particularly in the LDL and HDL density classes. Moreover, polyacrylamide gel electrophoresis in 8M urea revealed a clear diminution of all C-apoproteins in the VLDL and HDL density class and a complete disappearance of the A II band in the HDL fraction 24-48 h after galactosamine administration. These lacking apoprotein bands reappeared 72-168 h after galactosamine treatment. The same effects could be obtained by lower doses of galactosamine (2.32 and 1.16 mmol/kg, respectively) which led only to slight increases of hepatocellular enzymes in the plasma. It is concluded therefore, that galactosamine administration in experimental animals leads to profound, but reversible alterations in the synthesis and/or secretion of plasma lipoproteins.

Reduced insulin binding to hepatic plasma membranes in D-galactosamine-treated rats

Six to 12 hr after IP injection of 400 mg/kg of D-galactosamine in rats a 5-fold increase in plasma insulin was observed. In addition, impaired glucose assimilation was present after an IV Load in spite of unchanged fasting glucose levels. In streptozotocin-diabetic rats (100 mg/kg IV) plasma insulin remained diminished 12 h after induction of D-galactosamine hepatitis. Under identical conditions of preparation and incubation, the liver plasma membranes of D-galactosamine-treated rats, in both normal and diabetic states, bound only 40--60% as much insulin per mg of membrane protein as those of the control rats. Scatchard analysis suggested that this was due to a decrease in the number of receptor sites in the membranes of the D-galactosamine-injected rats. No difference in the insulin degrading capacity and in insulin-receptor dissociation of the plasma membranes between control and D-galactosamine-treated groups was found. These data suggest that a reduction in the number of hepatic insulin receptors in galactosamine hepatitis can lead to insulin resistance and hyperinsulinaemia.

Loss of the lipoprotein lipase activating ability of rat serum after administration of some fatty liver inducing drugs

The effects of the administration of different fatty liver inducing drugs on the serum lipoprotein lipase activating ability was investigated in rats. Addition of serum from 2-mercaptoethanol-, 2-mercaptoacetate-, ethionine- or D-galactosamine- treated rats failed to activate heart and adipose tissue lipoprotein lipase from control rats. The activating effect of serum was only slightly reduced in isopropanol-treated rats, whereas it was found unaffected in ethanol-treated ones. Electrophoresis of the lipoproteins and of the very low density lipoproteins (VLDL) fraction of sera from 2-mercaptoethanol-, 2-mercaptoacetate-, isopropanol-, ethionine- and D-galactosamine-treated rats suggest that the lack of lipoprotein lipase activation ability of these sera is most probably related to the impairing effects of these drugs upon VLDL metabolism, i.e. reduction of VLDL secretion in the case of 2-mercaptoethanol, 2-mercaptoacetate and isopropanol, production of abnormal VLDL in the case of D-galactosamine and both decreased VLDL secretion and production of abnormal VLDL in the case of ethionine.