Histochemistry of gamma-glutamyl transpeptidase in human liver biopsies

An activatable near-infrared molecular reporter for fluoro-photoacoustic imaging of liver fibrosis

Noninvasive and accurate detection of liver fibrosis is extremely significant for well-timed intervention and treatment to prevent or reverse its progression. Fluorescence imaging probes hold great potential for imaging of liver fibrosis, but they always encounter the inherent limitation of shallow penetration depth, which compromises their ability of in vivo detection. To overcome this issue, an activatable fluoro-photoacoustic bimodal imaging probe (IP) is herein developed for specific visualization of liver fibrosis. The probe IP is constructed on a near-infrared thioxanthene-hemicyanine dye that is caged with gamma-glutamyl transpeptidase (GGT) responsive substrate and linked with integrin-targeted peptide (cRGD). Such molecular design permits IP to effectively accumulate in the liver fibrosis region through specific recognition of cRGD towards integrin and activate its fluoro-photoacoustic signal after interaction with overexpressed GGT to precisely monitor the liver fibrosis. Thus, our study presents a potential strategy to design dual-target fluoro-photoacoustic imaging probes for noninvasive detection of early-stage liver fibrosis.

The amazing universe of hepatic microstructure

An informal review is presented by the author of his 50 years of involvement in practice and research in hepatopathology. Some background for the author's attitude and meandering pathway into his professional career serves as introduction to a short discussion of the main topics of his interest and expertise. Histogenesis of liver cancer was the theme of early work for a Ph.D. thesis, the results of which were lost into oblivion due to local rules and circumstances, but were rescued three decades later. His conclusions about the cells of origin of liver cancer remain concordant with the newer concepts in the field after nearly half a century. Studies in the field of chronic hepatitis became a long saga, involving the first classification of this syndrome by "the Gnomes" in 1968, histochemical investigations of viral antigens, lymphocyte subsets and adhesion molecules, and a quarter century later, the creation of a new classification presently in use. Cholestasis was a broadening field in diagnostic entities and involved the study of liver lesions, comprising pathways of bile regurgitation (including reversed secretory polarity of hepatocytes) and so-called ductular reaction. The latter topic has a high importance for the various roles it plays in modulating liver tissue of chronic cholestasis into biliary cirrhosis, and as the territory of hepatic progenitor cells, crucial for liver regeneration in adverse conditions and in development of liver cancer. Study of the embryology of intrahepatic bile ducts helped to clarify the strange appearance of the ducts in "ductal plate configuration" in several conditions, including some forms of biliary atresia with poor prognosis and all varieties of fibrocystic bile duct diseases with "ductal plate malformation" as the basic morphologic lesion.

Gamma-glutamyl transpeptidase gene organization and expression: a comparative analysis in rat, mouse, pig and human species

Gamma-glutamyl transpeptidase (GGT) is an enzyme located at the external surface of epithelial cells. It initiates extracellular glutathione (GSH) breakdown, provides cells with a local cysteine supply and contributes to maintain intracellular GSH level. GGT expression, highly sensitive to oxidative stress, is a part of the cell antioxidant defense mechanisms. We describe recent advances in GGT gene structure and expression knowledge and put emphasis on the complex transcriptional organization of that gene and its conservation among different species. GGT gene structure has been elucidated in rat and mouse where a single gene is transcribed from multiple promoters into several transcripts which finally yield a unique polypeptidic chain. Analysis of rat, mouse, human and pig cDNA and gene sequences reveals a large conservation of the transcriptional organization of that gene. This complex structure provides flexibility in GGT expression controlled at the promoter level, through multiple regulatory sites, and at RNA level by alternate 5' untranslated sequences which may create a diversity in the stability and translational efficiency of the different transcripts. In conclusion, transcription of the GGT gene from several promoters offers multiple DNA and RNA targets for various oxidative stimuli and contributes to a broad antioxidant cell defense through GGT induction and subsequent cysteine supply from extracellular glutathione.

Glutathione in plasma, liver, and kidney in the development of CCl4-induced cirrhosis of the rat

Plasma glutathione is markedly decreased in human cirrhosis of the liver. This decrease is said to be caused by reduced concentrations of liver glutathione. However, several studies on hepatic glutathione have revealed its concentrations to be unchanged, decreased, or even elevated. To test these inconsistencies we investigated the glutathione status of plasma, liver, and kidney in rats chronically exposed to carbon tetrachloride (CCl4). After 14 weeks of CCl4 treatment, histological examination revealed progressive cirrhotic transformation. After 20 weeks, complete micro-nodular cirrhosis was present and distinct ascites had developed. Plasma reduced glutathione (GSH) decreased by 34% in the early and by 44% in the late group, paralleled by a 65% and 76% decrease of plasma oxidized glutathione (GSSG). Liver GSH in early stages of cirrhosis was reduced by 49%, but in late cirrhosis it did not differ from controls. In contrast, liver GSSG increased by 35% in the early and by 191% in the late group. Kidney GSH increased by 14% in early and 44% in late stage cirrhosis. Kidney GSSG was unchanged in the early group, but increased by 18% in the late group. The decrease of plasma GSH and GSSG is closely related to the severity of experimental cirrhosis and inversely related to an increase of hepatic oxidized glutathione. The hepatic content of reduced glutathione, however, is decreased in early cirrhosis only. According to these results the inconsistent findings in man could be due to differences in the stages of cirrhosis in the patients. The increase in kidney glutathione is a new finding that needs further investigation, but it may probably be related to kidney dysfunction in liver disease.

Serum secretory IgA and secretory component in patients with non-cirrhotic alcoholic liver diseases

Elevated levels of secretory IgA in serum have been demonstrated in several liver dysfunctions such as hepatic cytolysis and cholestasis. However, these possible alterations at an early stage of liver diseases have not yet been investigated. We studied a cohort of chronic alcoholic patients without cirrhosis in order to assess the changes in serum secretory IgA and other forms of secretory component, the split product of the polymeric Ig-receptor of epithelial cells. The possible diagnostic value of these measurements in the assessment of alcoholic disease was compared to that of serum gamma-glutamyl transpeptidase activity. Serum levels of secretory IgA and IgM and free secretory component, were quantified by an enzyme-linked immunosorbent assay in 71 patients with chronic alcoholic liver disease without cirrhosis and in 45 healthy controls. Patients were divided into two groups according to the severity of the liver abnormalities. In addition, the reversibility of serum secretory IgA, IgM and free secretory component abnormalities after alcohol withdrawal was evaluated in 15 patients. Serum levels of the three molecular forms of secretory component were significantly higher than those measured in control subjects, both in the whole population of patients and in the two groups of alcoholic patients without cirrhosis. In all groups, serum secretory IgA levels were correlated to free secretory component but not to total IgA levels. Serum secretory IgA levels were as discriminative as gammaglutamyl transferase activity in distinguishing between chronic alcoholic patients without cirrhosis and non-alcoholic subjects. The abnormalities of serum secretory IgA concentrations were reversible after alcohol withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione conjugation and conversion to mercapturic acids can occur as an intrahepatic process

By catalyzing the reaction of electrophilic compounds with the sulfhydryl group of glutathione, the glutathione S-transferases play physiologically important roles in the detoxication of potential alkylating agents. The glutathione S-conjugates thus formed are transported out of cells for further metabolism by gamma-glutamyltransferase and dipeptidases, ectoproteins that catalyze the sequential removal of the glutamyl and glycyl moieties, respectively. These ectoproteins are not found in all cells, but are localized predominantly to the apical surface of epithelial tissues. The resulting cysteine S-conjugates can be reabsorbed by specific cell types, and acetylated on the amino group of the cysteinyl residue by intracellular N-acetyl-transferases, to form the corresponding mercapturic acids (N-acetylcysteine S-conjugates). Mercapturic acids are then released into the circulation and delivered to the kidney for excretion in urine, or they may undergo further metabolism. Mercapturic acid biosynthesis is generally considered to be an interorgan process, with the liver serving as the major site of glutathione conjugation, and the kidney as the primary site for conversion of glutathione conjugates to cysteine conjugates. Cysteine conjugates formed in the kidney appear to be transported back to the liver for acetylation. This interorgan model of mercapturic acid synthesis is based largely on the interorgan distribution of the enzymes involved in their formation, and in particular of the enzyme gamma-glutamyltransferase. Rats have relatively low hepatic and high renal activities of gamma-glutamyltransferase, the only protein known to initiate the breakdown of glutathione S-conjugates. The low gamma-glutamyltransferase activity in rat liver limits the hepatic degradation of glutathione S-conjugates, particularly after large doses of xenobiotic. In contrast, hepatic gamma-glutamyltransferase is significantly higher in species such as rabbit, guinea pig, and dog, and as a consequence, nearly all of the glutathione and glutathione S-conjugates released by liver cells of these species is degraded within the liver. Recent studies demonstrate that glutathione S-conjugates synthesized within hepatocytes are secreted preferentially across the canalicular membrane into bile, and are broken down within biliary spaces to form cysteine S-conjugates. The latter are then reabsorbed by the liver, N-acetylated to form mercapturic acids, and reexcreted into bile, completing an intrahepatic pathway for mercapturic acid biosynthesis. The contribution of this intrahepatic pathway to overall mercapturate formation is dependent on dose of the electrophile, route of exposure, and the physicochemical properties of the glutathione S-conjugate formed, as well as the tissue distribution and activity of gamma-glutamyltransferase.(ABSTRACT TRUNCATED AT 400 WORDS)

Methylene dianiline: acute toxicity and effects on biliary function

4,4'-Methylene dianiline (4,4'-diaminodiphenylmethane, DAPM), which is used in the polymer industry, causes hepatobiliary damage in exposed humans. Our objectives were to characterize the acute toxicity of DAPM in liver, particularly on secretion of biliary constituents and on biliary epithelial cell gamma-glutamyl transpeptidase (GGT) activity. Biliary cannulas were positioned in Sprague-Dawley male rats under pentobarbital anesthesia. After 1 hr of control bile collection, each rat was given 250 mg DAPM/kg (50 mg/ml) po in 35% ethanol or 35% ethanol only; bile was collected for a further 4 hr. Groups of rats were also examined for liver injury and biliary function at 8 and 24 hr after DAPM. Four hours after DAPM administration, main bile duct cells were severely damaged with minimal damage to peripheral bile ductule cells. Focal periportal hepatocellular necrosis and extensive cytolysis of cortical thymocytes occurred by 24 hr. Serum indicators of liver injury were elevated by 4 hr and continued to rise through 24 hr. By 4 hr, biliary protein concentration was increased 4-fold while concentrations of biliary bile salt, bilirubin, and glutathione were decreased by approximately 80, 50, and 200%, respectively. DAPM also induced a striking effect on biliary glucose with an approximately 20-fold increase. Histochemical staining of main bile duct GGT was absent by 8 hr after DAPM. Bile flow was diminished by 40% at 4 hr; three of five rats had no bile flow by 8 hr and none had any bile flow by 24 hr. These results indicate that DAPM rapidly diminishes bile flow and alters the secretion of biliary constituents and is highly injurious to biliary epithelial cells.

Histochemical demonstration of sinusoidal gamma-glutamyltransferase activity by substrate protection fixation: comparative studies in rat and guinea pig liver

Most histochemical methods for the detection of an enzymatic activity are preceded by tissue fixation with chemical agents that partially inactivate the enzymes. It is well known that substrates exert a marked protection against fixative-induced inactivation. The conventional histochemical methods for the demonstration of hepatic gamma-glutamyltransferase activity have not been successful in detecting the activity of the enzyme on the sinusoidal side of the hepatocytes despite mounting biochemical evidence for its presence on that pole of the hepatocyte. Under conventional fixation the enzymatic activity in hepatocytes is only seen on the bile canalicular side. This may be the result of a preferential protective effect of gamma-glutamyltransferase by its normal substrate, glutathione, present in the bile canaliculus at concentrations 500 times higher than in the sinusoidal lumen (8 mmol/L vs. 10 to 20 mumol/L). To test this hypothesis and to reduce the degree of fixative-induced inhibition of the enzyme activity, glutathione was either incorporated in the fixative solution or the livers were perfused with high concentrations of glutathione (10 mmol/L) before fixation. Our results histochemically demonstrate, in the normal adult rat liver, the existence of gamma-glutamyltransferase activity not only on the bile canalicular pole but also on the sinusoidal pole of the hepatocytes. Visualization of the enzyme activity on the sinusoidal pole is dependent on glutathione protection. Guinea pig livers, which present a 10-fold higher gamma-glutamyltransferase activity than rat livers (similar to that in human beings), showed marked sinusoidal gamma-glutamyltransferase activity even in the absence of glutathione protection. Glutathione protection further increased this sinusoidal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic efflux and renal extraction of plasma glutathione: marked differences between healthy subjects and the rat

Hepatic efflux of glutathione accounts for almost 85% of the plasma level in the rat. However, the expected high concentration in the hepatic vein in man has not been demonstrated as yet. Our findings in ten patients without liver dysfunction reveal that substantial translocation of glutathione from the liver to the plasma does not occur. The removal rate of circulating glutathione by the kidney is far lower in man than in the rat (49.5% vs 84.6%). Moreover, the inferior vena cava in man contains more circulating glutathione than the artery, which is not consistent with the results obtained in the rat. Furthermore, the plasma concentration in man is about one tenth of that in the animals. These results clearly indicate that species-specific differences in the overall biosynthesis and metabolism of the tripeptide occur, resulting in marked variations in its plasma concentration.

Role of hepatic gamma-glutamyltransferase in the degradation of circulating glutathione: studies in the intact guinea pig perfused liver

The role of hepatic gamma-glutamyltransferase in the breakdown of circulating glutathione was studied in the perfused guinea pig liver. Hepatic gamma-glutamyltransferase activity in the guinea pig is sevenfold higher than in the rat and is comparable to its activity in man. Guinea pig livers were found to remove, in a single pass, 50% to 90% of glutathione (10 to 50 mumol/L) added to the portal perfusate. Removal of portal glutathione was totally dependent on the activity of gamma-glutamyltransferase and led to the near quantitative appearance of cysteinyl-glycine and cysteine in the caval perfusate. Glutathione removal by the intact liver followed saturation with a Michaelis constant (Km) of 59 mumol/L for glutathione and a maximum velocity of 235 nmol glutathione/min/gm of liver weight. The capacity of the guinea pig liver to remove circulating glutathione was estimated to be sevenfold to 10-fold higher than its net rate of output of glutathione into the circulation. Inhibition of gamma-glutamyltransferase activity in the perfused liver led to threefold to sixfold increases in the hepatic output of glutathione into the circulation, indicating that more than two thirds of glutathione transported extracellularly is broken down. Data obtained demonstrate a major role of hepatic gamma-glutamyltransferase, both in the removal of portally carried glutathione and in the degradation of glutathione molecules released by the liver itself into the sinusoids. These findings suggest the existence of an intraorgan transport of glutathione in the liver, whereby periportal cells could provide glutathione precursors to pericentral cells.

Extrahepatic obstructive cholestasis reverses the bile salt secretory polarity of rat hepatocytes

To elucidate the consequences of extrahepatic cholestasis on the structure and function of hepatocytes, we studied the effects of bile duct ligation on the turnover, surface distribution, and functional activity of the canalicular 100-kD bile salt transport protein (cBSTP). Basolateral (blLPM) and canalicular (cLPM) liver plasma membrane vesicles were purified to the same degree from normal and cholestatic rat livers and the membrane bound cBSTP identified and quantitated using polyclonal anti-cBSTP antibodies. Cholestasis of 50 h resulted in an increased release of cBSTP into bile, thereby decreasing its in vivo half-life from 65 to 25 h. Furthermore, a significant portion of cBSTP accumulated at the basolateral surface and in intracellular vesicles of cholestatic hepatocytes. This redistribution of cBSTP was functionally paralleled by decreased and increased electrogenic taurocholate anion transport in cLPM and blLPM vesicles, respectively. These results demonstrate that biliary obstruction causes a reversal of the bile salt secretory polarity of rat hepatocytes. The resulting increase in basolateral (sinusoidal) bile salt efflux might protect hepatocytes from too high an accumulation of toxic bile salts within the cell interior.

The histochemical location of three diagnostic enzymes in the marmoset liver

The histochemical locations of alkaline phosphate (ALP), leucine aminopeptidase (LAP) and gamma glutamyltransferase (GGT) were demonstrated in the liver of the marmoset (Callithrix jacchus). Although all three enzymes were located in cell membranes, the location of LAP was demonstrated by a chromogenic substrate, in the canalicular membrane. GGT was seen in a vascular network, provisionally identified as the peribiliary arterial plexus. Possible diagnostic applications in toxicology are discussed.

Rapid induction of forestomach tumors in partially hepatectomized Wistar rats given butylated hydroxyanisole

Rats subjected to two-thirds partial hepatectomy (PH) and given the antioxidant butylated hydroxyanisole (BHA) at a dietary concentration of 2% for 3 months developed forestomach lesions. Histologically, these lesions were classified as hyperplasia, dysplasia of the basal cell, papillomas, and carcinomas in situ. In intact rats forestomach carcinomas were seen by other investigators after feeding 2% BHA for 15-20 months. Histochemical studies of tumors revealed a marked increase in the phenotypic expression of the oncofetal enzyme, gamma-glutamyl transpeptidase (GGT) in the tumors of treated rats.

Liver plasma membrane: the source of high molecular weight alkaline phosphatase in human serum

This study presents biochemical, histochemical, morphological and immunological evidence that part of the high molecular weight alkaline phosphatase observed in the serum of patients with liver disease and particularly in cases of intrahepatic cholestasis or focal-, extrahepatic obstruction originates from the liver plasma membrane. The high molecular weight protein alkaline phosphatase complex contains several plasma membrane enzymes and behaves like a plasma membrane fragment after isopycnic density gradient ultracentrifugation in sucrose, cesium chloride and metrizamide. Electron microscopic examination revealed a triple-layered vesicle which retained alkaline phosphatase activity. Incubation of human liver cells with anti-serum against purified high molecular weight multienzyme complex resulted in fixation of antibodies on the plasma membrane as shown by positive plasma membrane fluorescence. These plasma membrane fragments in the serum are not of biliary origin.