Different zonal distribution of the asialoglycoprotein receptor, the alpha 2-macroglobulin receptor/low-density-lipoprotein receptor-related protein and the lipoprotein-remnant receptor of rat liver parenchymal cells

Targeted Delivery of Stk25 Antisense Oligonucleotides to Hepatocytes Protects Mice Against Nonalcoholic Fatty Liver Disease

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are emerging as leading causes of liver disease worldwide. Currently, no specific pharmacologic therapy is available for NAFLD/NASH, which has been recognized as one of the major unmet medical needs of the 21st century. Our recent studies in genetic mouse models, human cell lines, and well-characterized patient cohorts have identified serine/threonine protein kinase (STK)25 as a critical regulator of hepatic lipid partitioning and NAFLD/NASH. Here, we studied the metabolic benefit of liver-specific STK25 inhibitors on NAFLD development and progression in a mouse model of diet-induced obesity.

METHODS

We developed a hepatocyte-specific triantennary N-acetylgalactosamine (GalNAc)-conjugated antisense oligonucleotide (ASO) targeting Stk25 and evaluated its effect on NAFLD features in mice after chronic exposure to dietary lipids.

RESULTS

We found that systemic administration of hepatocyte-targeting GalNAc-Stk25 ASO in obese mice effectively ameliorated steatosis, inflammatory infiltration, hepatic stellate cell activation, nutritional fibrosis, and hepatocellular damage in the liver compared with mice treated with GalNAc-conjugated nontargeting ASO, without any systemic toxicity or local tolerability concerns. We also observed protection against high-fat-diet-induced hepatic oxidative stress and improved mitochondrial function with Stk25 ASO treatment in mice. Moreover, GalNAc-Stk25 ASO suppressed lipogenic gene expression and acetyl-CoA carboxylase protein abundance in the liver, providing insight into the molecular mechanisms underlying repression of hepatic steatosis.

CONCLUSIONS

This study provides in vivo nonclinical proof-of-principle for the metabolic benefit of liver-specific inhibition of STK25 in the context of obesity and warrants future investigations to address the therapeutic potential of GalNAc-Stk25 ASO in the prevention and treatment of NAFLD.

Efficient Liver Targeting by Polyvalent Display of a Compact Ligand for the Asialoglycoprotein Receptor

A compact and stable bicyclic bridged ketal was developed as a ligand for the asialoglycoprotein receptor (ASGPR). This compound showed excellent ligand efficiency, and the molecular details of binding were revealed by the first X-ray crystal structures of ligand-bound ASGPR. This analogue was used to make potent di- and trivalent binders of ASGPR. Extensive characterization of the function of these compounds showed rapid ASGPR-dependent cellular uptake in vitro and high levels of liver/plasma selectivity in vivo. Assessment of the biodistribution in rodents of a prototypical Alexa647-labeled trivalent conjugate showed selective hepatocyte targeting with no detectable distribution in nonparenchymal cells. This molecule also exhibited increased ASGPR-directed hepatocellular uptake and prolonged retention compared to a similar GalNAc derived trimer conjugate. Selective release in the liver of a passively permeable small-molecule cargo was achieved by retro-Diels-Alder cleavage of an oxanorbornadiene linkage, presumably upon encountering intracellular thiol. Therefore, the multicomponent construct described here represents a highly efficient delivery vehicle to hepatocytes.

Mechanisms determining phenotypic heterogeneity of hepatocytes

This review summarizes results of biochemical and immunohistochemical studies indicating the existence of functional heterogeneity of hepatocytes depending on their localization in the hepatic acinus; this determines characteristic features of metabolism of carbohydrates, lipids, and xenobiotics. The physiological significance of hepatocyte heterogeneity is discussed. According to the proposed model of intercellular communication, the metabolic specialization of hepatocytes is determined by secretory activity of hepatic resident macrophages (Kupffer cells) localized mainly in the periportal zone of the liver acinus. Macrophages participate in secretion of a wide spectrum of intercellular mediators (cytokines, prostaglandins, growth factors) and also in metabolism of numerous blood metabolites and biologically active substances (hormones, lipoproteins, etc.). In the sinusoid and in the space of Disse (also known as perisinusoidal space) they form a concentration gradient of regulatory factors and metabolites inducing the phenotypic differences between hepatocytes.

The lobular expression of the rat asialoglycoprotein receptor is regulated at the posttranscriptional level

The purpose of this study was to define the distribution of the asialoglycoprotein receptor (ASGP-R) main peptide, rat hepatic lectin (RHL)-1, within the rat liver lobule and to investigate its possible modulation in physiological states characterised by marked changes of receptorial expression. In particular, we chose livers from rats partially hepatectomised or at the end of pregnancy, as models, respectively, of decreased or increased expression of the ASGP-R, and used the in situ hybridisation and immunocytochemistry techniques to analyse in parallel the lobular distributions of RHL-1 mRNA and protein. In normal rat liver, although the RHL-1 mRNA was homogeneously distributed, the RHL-1 peptide was predominantly localised on the surface of pericentral hepatocytes with a gradient of expression towards the periportal zone. This gradient of expression of RHL-1 peptide was reduced in regenerating livers, in which the positive stain was restricted to a few layers of cells around the central vein. In contrast, livers at the end of pregnancy showed an overall increase of the peptide with a concomitant flattening of the gradient across the liver plate. In all the conditions, we never observed important changes in the pattern of expression of the specific mRNA. These findings indicate that the distribution of ASGP-R is heterogeneous across the liver lobule, with a pattern of expression prevalently modulated at the posttranscriptional level.

Asialoglycoprotein receptor facilitates hemolysis in patients with alcoholic liver cirrhosis

Hemolysis in patients with advanced alcoholic liver disease is a common clinical problem and indicates an unfavorable prognosis. In many cases, the etiology of the hemolysis remains unknown. We observed three patients with alcoholic liver disease, suffering from severe hemolytic anemia, requiring multiple blood transfusions. Steroid therapy was ineffective and two of the patients died. All patients had a soluble variant of the human asialoglycoprotein receptor (s-ASGP-R) in their serum, as well as high titers of autoantibodies against this receptor (anti-ASGP-R). Consecutively, examination of 60 patients with alcoholic liver disease revealed a high incidence for s-ASGP-R (36%) and anti-ASGP-R (27%) in patients with alcoholic liver cirrhosis (ALC) compared to patients with cirrhosis due to viral hepatitis. The potential etiology of hemolysis was studied in vitro on erythrocytes from patients with ALC and from healthy donors. Isolated ASGP-R but not anti-ASGP-R bound to the surface of erythrocytes preferentially of blood group A1 and caused dose-dependent agglutination and hemolysis, while this phenomenon was much lower using erythrocytes of the blood group B and almost absent with blood group O-erythrocytes. Furthermore, agglutination and hemolysis only occurred in erythrocytes from ALC-patients or after the pre-treatment of cells with neuraminidase. ASGP-R induced agglutination and hemolysis was blocked by the competitive ASGP-R inhibitor asialofetuin. In conclusion, our results indicate a new, non-immunological mechanism for hemolysis in patients with alcoholic liver disease, mediated through agglutination by a soluble variant of the human asialoglycoprotein receptor and mechanical shear stress.

Mice expressing only the mutant APOE3Leiden gene show impaired VLDL secretion

Apolipoprotein E (apoE)-deficient mice develop hepatic steatosis and show impaired very low density lipoprotein (VLDL)-triglyceride (TG) secretion. These effects are normalized on the introduction of the human APOE3 gene. To assess whether this apoE effect is isoform specific, we studied hepatic lipid metabolism in mice expressing either APOE3 or the mutant APOE3Leiden on apoe-/- or apoe+/- backgrounds. The transgenes were expressed mainly in periportal hepatocytes, as revealed by in situ hybridization. Mice expressing APOE3Leiden, on the apoe-/- and apoe+/- backgrounds, had fatty livers, which were absent in APOE3/apoe-/- mice. APOE3Leiden/apoe-/- mice showed a strongly reduced VLDL-TG secretion compared with APOE3/apoe-/- mice (48+/-14 versus 82+/-10 micromol/kg per hour, respectively). The presence of a single mouse apoe allele increased VLDL-TG secretion in APOE3Leiden/apoe+/- mice (121+/-43 micromol/kg per hour) compared with APOE3Leiden/apoe-/- mice. These results show that APOE3Leiden does not prevent development of a fatty liver and does not normalize VLDL-TG secretion in mice with an apoE-deficient background. The presence of a single mouse apoe allele is sufficient to normalize the APOE3Leiden-associated reduction of VLDL-TG secretion but does not prevent steatosis. We conclude that apoE-mediated stimulation of VLDL secretion is isoform specific.

Hepatic zonation of the formation and hydrolysis of cholesteryl esters in periportal and perivenous parenchymal cells

The periportal (PP) and perivenous (PV) zones of the liver acinus differ in enzyme complements and capacities for cholesterol and bile acid synthesis and other metabolic processes. The aim of this investigation was to determine the acinar distribution of the catalytic activity of the enzymes governing the formation and hydrolysis of cholesteryl esters using PP and PV hepatocytes from normal or cholestyramine-fed rats. The hepatocyte subpopulations were isolated by centrifugal elutriation, characterized according to the distribution pattern of a number of cell parameters and marker enzymes, and assayed for acyl-CoA:cholesterol acyltransferase (ACAT) and lysosomal, cytosolic and microsomal cholesteryl ester hydrolase (CEH). In normally fed rats, no zonation was found in the activity of lysosomal CEH and ACAT, and the activity of both cytosolic and microsomal CEH zonated toward the PV zone of the acinus. Concentrations of free and esterified cholesterol in homogenates, cytosol, and microsomes of PP and PV cells were, however, similar. Cholestyramine raised significantly the PV/PP ratio of ACAT because of an exclusive PP reduction of activity and abolished the heterogeneity in microsomal CEH because of a greater inhibitory PV response, whereas the PV dominance of cytosolic CEH and the homogeneous distribution of lysosomal CEH were unaffected. These results demonstrated homogeneity within the liver acinus for the enzymatic degradation of endocyted lipoprotein-derived cholesteryl esters, a structural zonation of the cytosolic CEH and a dynamic zonation of ACAT and the microsomal CEH, with a PV dominance of the enzymatic capacity for the degradation of stored cholesteryl esters in normal livers.

Pathogenesis of autoimmune hepatitis

Autoimmune hepatitis (AIH) is an idiopathic disorder affecting the hepatic parenchyma. There are no morphological features that are pathognomonic of the condition but the characteristic histological picture is that of an interface hepatitis without other changes that are more typical of other liver diseases. It is associated with hypergammaglobulinaemia, high titres of a wide range of circulating auto-antibodies, often a family history of other disorders that are thought to have an autoimmune basis, and a striking response to immunosuppressive therapy. The pathogenetic mechanisms are not yet fully understood but there is now considerable circumstantial evidence suggesting that: (a) there is an underlying genetic predisposition to the disease; (b) this may relate to several defects in immunological control of autoreactivity, with consequent loss of self-tolerance to liver auto-antigens; (c) it is likely that an initiating factor, such as a hepatotropic viral infection or an idiosyncratic reaction to a drug or other hepatotoxin, is required to induce the disease in susceptible individuals; and, (d) the final effector mechanism of tissue damage probably involves auto-antibodies reacting with liver-specific antigens expressed on hepatocyte surfaces, rather than direct T-cell cytotoxicity against hepatocytes.

Differential expression of apolipoprotein E messenger RNA within the rat liver lobule determined by in situ hybridization

Apolipoprotein (Apo) E plays a key role in the metabolism of lipoproteins. It also modulates immunoregulation, cell growth and differentiation and the response to nerve injury. The liver is a major site of ApoE synthesis. Most of the circulating ApoE is thought to be of hepatic origin with most synthesized in hepatocytes. We showed that total liver ApoE messenger RNA (mRNA) levels were greater in normal adult female rats than in male and that gender-specific patterns of liver ApoE mRNA expression were present by in situ hybridization. In the male liver, the signal was strongest in the portal area, decreasing toward the central vein with the weakest signal in pericentral hepatocytes, resulting in a hepatic lobular gradient of expression. In female liver, a strong periportal signal also was observed that decreased in Zone 2, similar to that in males, but which then increased in pericentral hepatocytes resulting in a bowl-like distribution in marked contrast with that of the male. The results suggest that ApoE mRNA level is regulated differentially in hepatocytes within the liver plate and that the regulation is gender-dependent. Further, the results suggest that in males, hepatocytes in the portal area are the major contributors of ApoE to the plasma and/or sinusoidal pool, whereas in females, both portal and central area hepatocytes play an equal role.

Cytokeratin 18 is expressed on the hepatocyte plasma membrane surface and interacts with thrombin-antithrombin complexes

During experiments to identify putative hepatic receptors for thrombin-antithrombin (TAT) complexes, a 45-kDa protein was identified by ligand blotting. Following gel purification, amino acid sequencing revealed the 45-kDa TAT-binding polypeptide to be cytokeratin 18 (CK18). The presence of CK18 on the surface of intact rat hepatoma cells was demonstrated by binding of 125I-anti-CK18 antibodies. Anti-CK18 antibodies reduced the binding and internalization of 125I-TAT by rat hepatoma cells. Immunocytochemical analysis, to determine the location of CK18 in vivo, revealed a periportal gradient of CK18 staining; with hepatocytes around the portal triads demonstrating striking pericellular staining. In addition, anti-CK18 IgG associated with perfused livers to a significantly greater extent than preimmune IgG. Taken together, these data provide evidence that CK18 is found on the extracellular surface of hepatocytes and could play a role in TAT removal. Finally, these data, in conjunction with recent reports of CK8 (Hembrough, T. A., Li, L., and Gonias, S. L. (1996) J. Biol. Chem. 271, 25684-25691) and CK1 cell membrane surface expression (Schmaier, A. H. (1997) Thromb. Hemostasis 78, 101-107), indicate a novel role for these proteins as putative cellular receptors or cofactors to cellular receptors.