Alterations of hepatocellular peroxisomes in viral hepatitis in the mouse

Peroxisomes in Immune Response and Inflammation

The immune response is essential to protect organisms from infection and an altered self. An organism’s overall metabolic status is now recognized as an important and long-overlooked mediator of immunity and has spurred new explorations of immune-related metabolic abnormalities. Peroxisomes are essential metabolic organelles with a central role in the synthesis and turnover of complex lipids and reactive species. Peroxisomes have recently been identified as pivotal regulators of immune functions and inflammation in the development and during infection, defining a new branch of immunometabolism. This review summarizes the current evidence that has helped to identify peroxisomes as central regulators of immunity and highlights the peroxisomal proteins and metabolites that have acquired relevance in human pathologies for their link to the development of inflammation, neuropathies, aging and cancer. This review then describes how peroxisomes govern immune signaling strategies such as phagocytosis and cytokine production and their relevance in fighting bacterial and viral infections. The mechanisms by which peroxisomes either control the activation of the immune response or trigger cellular metabolic changes that activate and resolve immune responses are also described.

Fission and proliferation of peroxisomes

Peroxisomes are remarkably dynamic, multifunctional organelles, which react to physiological changes in their cellular environment and adopt their morphology, number, enzyme content and metabolic functions accordingly. At the organelle level, the key molecular machinery controlling peroxisomal membrane elongation and remodeling as well as membrane fission is becoming increasingly established and defined. Key players in peroxisome division are conserved in animals, plants and fungi, and key fission components are shared with mitochondria. However, the physiological stimuli and corresponding signal transduction pathways regulating and modulating peroxisome maintenance and proliferation are, despite a few exceptions, largely unexplored. There is emerging evidence that peroxisomal dynamics and proper regulation of peroxisome number and morphology are crucial for the physiology of the cell, as well as for the pathology of the organism. Here, we discuss several key aspects of peroxisomal fission and proliferation and highlight their association with certain diseases. We address signaling and transcriptional events resulting in peroxisome proliferation, and focus on novel findings concerning the key division components and their interplay. Finally, we present an updated model of peroxisomal growth and division. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.

Secondary alterations of human hepatocellular peroxisomes

The morphological and morphometric characteristics of peroxisomes in normal human liver and the peroxisomal alterations in the liver of patients with acquired or congenital non-peroxisomal diseases are reviewed. Secondary peroxisomal changes are observed in steatosis, hepatitis and cirrhosis induced by various agents (viruses, alcohol, drugs, etc.), in cholestasis, in hepatomas, in extra-hepatic cancer with or without liver metastasis, in extrahepatic inflammatory processes, in metabolic disorders affecting metabolism of carbohydrates, lipids and lipoproteins, glycoproteins, amino acids, bilirubin or copper, and in altered thyroid hormone levels. They are recognized as a proliferation of peroxisomes (increased in number and to a lesser extent in surface density and volume density) often accompanied by a minor reduction in size (at most to 68% of the mean diameter in control livers) but very rarely by an increase in mean peroxisomal diameter, and as proliferation-related changes in shape (tails, gastruloid cisternae, funnel-like constrictions, elongation, protrusions) in at least a few of the peroxisomes. These secondary alterations of the peroxisomes are clearly distinguishable from the primary changes in peroxisomes observed in the liver of patients with congenital peroxisomal disorders.

Modification of the amount of cholesterol in hepatic steatosis induced in susceptible and resistant mice infected with MHV3: a biochemical and ultrastructural study

A mouse hepatitis virus-3 strain subcultured in our laboratory is a unique experimental model in which to study virus-induced liver steatosis. This strain produces massive lipid deposition not only in sensitive adult BALB/c mice but also (though less extensive) in virus-resistant adult A/J mice. Biochemical determinations have shown that this steatosis is characterized by an increased amount of neutral lipids (sterols and triglycerides) in infected livers of BALB/c mice and by a smaller increase in those of A/J mice. However, the relative percentage of cholesterol and triglycerides is similar in both strains. Liver phospholipid content was significantly decreased in both strains of mice. To discriminate between cytoplasmic and membrane cholesterol content in different types of liver cells, an ultrastructural study was performed with filipin, a specific cholesterol marker. This study shows on one hand an important increase in the cholesterol in the hepatocytes of BALB/c mice and a smaller increase in those of A/J mice, in agreement with biochemical data. However, marked cholesterol decrease and abnormal cholesterol distribution were observed in the endothelial liver cells of infected BALB/c mice. This decreased cholesterol content probably led to higher fluidity of these membranes, which could be related to the important drop in the number of endothelial cell fenestrae observed after mouse hepatitis virus-3 infection. Because in A/J infected mice neither a decrease in the amount and distribution of cholesterol nor decreased fenestration were observed in endothelial liver cells, these findings could be correlated with the resistance of these mice to the infection.

Hepatocellular peroxisomes in human alcoholic and drug-induced hepatitis: a quantitative study

The peroxisomes in the liver of four patients with alcoholic hepatitis and in six patients with drug-induced hepatitis are compared to eight control livers by catalase cytochemistry and morphometry. A decrease of catalase activity is observed in alcoholic, amitriptyline, aprindine, clomipramine and methiomazole hepatitis. Peroxisomes with a heterogeneous distribution of the catalase reaction product are found in most hepatitis livers. The number of organelles is increased 1.5 to 4.2 times in alcoholic, aprindine, methimazole and phenytoin hepatitis livers. In the last case, peroxisomes are also smaller. Changes in shape are seen in all hepatitis livers; they include invaginations, tails, funnel-like constrictions and gastruloid cisternae. In aprindine, phenytoin, methimazole and two alcoholic hepatitis livers, surface density exceeds the upper control value. These data indicate a loss of catalase activity in most hepatitis livers but also peroxisomal proliferation and shape modifications. It has been proposed that the latter changes are favorable for metabolic activity.

Altered pathogenicity in the liver induced by a mouse hepatitis virus type 3 thermosensitive mutant

Intraperitoneal inoculation into sensitive BALB/c mice of D85, a thermosensitive (ts) mutant, provokes acute hepatitis followed by recovery of the mice. The ts mutant was able to replicate in the liver. However, the maximal viral titre was obtained 2 days later than was the case with the wild-type (wt) MHV 3 infection; the viral antigens remained localized within small foci and no invasion of the entire liver was observed. The hepatocytes infected with D85 showed strong steatosis similar to that induced by wt virus, but the other lesions induced by MHV 3 (closing of endothelial cell fenestrae and hepatocytolysis) were not seen. An important feature noticed with the D85 mutant concerned the establishment, in the surviving animals, of persistent infection: this phenomenon was demonstrated by the decrease of viral titre in the liver, viral RNA detection, and the fact that viral antigens gradually decreased until the 3rd month post-infection.

Liver pathology and immunocytochemistry in congenital peroxisomal diseases: a review

Diagnostic and pathogenetic investigations of peroxisomal disorders should include the study of the macroscopic and microscopic pathology of the liver, in addition to careful clinical observations, skeletal X-ray and brain CT scan, assays of very long-chain fatty acids and bile acid intermediates, and selected enzyme activities. This review of the literature also contains novel observations about the following syndromes: cerebro-hepato-renal (Zellweger) syndrome, X-linked and neonatal adrenoleukodystrophies (ALD, NALD), NALD-like syndromes, infantile phytanic acid storage, classical Refsum disease, rhizomelic and other forms of chondrodysplasia punctata (XD, XR, AR), hyperpipecolic acidaemia, primary hyperoxaluria I, pseudo-Zellweger and Zellweger-like syndromes, and single enzyme deficiencies. Microscopic data include catalase staining and morphometry of peroxisomes, immunolocalization of beta-oxidation enzymes, detection of trilamellar, polarizing inclusions in PAS-positive macrophages, fibrosis and iron storage. Peroxisomal enlargement appears to be related to functional deficit in beta-oxidation disorders as well as in rhizomelic chondrodysplasia punctata. Because normal peroxisomal localization of active beta-oxidation enzymes can accompany a C26 beta-oxidation deficit, other mechanisms such as impaired transport of metabolites should be investigated. 'Ghost'-like organelles are shown in the liver of an infantile Refsum patient and in an NALD-like case; immuno-gold labelling of membrane proteins did not reveal ghosts in Zellweger livers.