Oxidative stress and oval cell accumulation in mice and humans with alcoholic and nonalcoholic fatty liver disease

Progenitor cell expansion: an important source of hepatocyte regeneration in chronic hepatitis

BACKGROUND/AIMS

Progenitor cell activation with subsequent maturation to hepatocytes and cells of the biliary lineage has been demonstrated in a variety of chronic liver diseases but the kinetics and magnitude of the progenitor cell response has not been adequately studied in detail in chronic hepatitis. We undertook this study to evaluate factors responsible for the progenitor cell/ductular response and further dissect the role of disease grade and stage as determinants of hepatocellular differentiation of bipotential progenitor cells in chronic hepatitis.

METHODS

Cytokeratin 7 (and 19) stained biopsies from patients with chronic hepatitis C (n = 47), hepatitis B (n = 20), and autoimmune hepatitis (n = 20) were studied. Ploidy analysis and proliferation indices were evaluated in a subset of cases.

RESULTS

Ductular reactions were present in the majority of cases (97%), appeared early in disease, and correlated with disease activity, while progenitor cell derived hepatocyes appeared later in disease and their extent correlated with disease stage. Proliferation indices of all cell types correlated with disease activity.

CONCLUSIONS

Progenitor cell derived hepatocytes accrue in chronic hepatitis, possibly related to native hepatocellular dysfunction. However, the fate of these hepatocytes is unclear.

Obesity and hepatocellular carcinoma

Epidemiological studies have shown that obesity is a risk factor for hepatocellular carcinoma. Similar studies further indicate that diabetes is also a major risk factor. Both obesity and diabetes are frequently associated with nonalcoholic fatty liver disease, and case reports have shown progression of nonalcoholic fatty liver disease to cirrhosis and hepatocellular carcinoma. Although no study has clearly tied all of these variables together, it is likely that the association of hepatocellular carcinoma with obesity represents the progression of underlying nonalcoholic fatty liver disease to cirrhosis. The mechanism most likely involves replicative senescence of steatotic mature hepatocytes and compensatory hyperplasia of progenitor (oval) cells as a reaction to chronic injury due to ongoing nonalcoholic steatohepatitis and resultant hepatic fibrosis. Growth factors associated with chronic inflammation, type 2 diabetes, and DNA mutations as a result of lipid peroxidation probably play significant roles in clonal expansion and hepatocellular carcinoma progression. It remains unclear whether cirrhosis is a prerequisite for the development of hepatocellular carcinoma or whether hepatocellular carcinoma can develop in fatty liver in the absence of cirrhosis. However, well-documented case reports suggest that most cases of hepatocellular carcinoma arise in the setting of nonalcoholic steatohepatitis with cirrhosis. Whether therapy aimed at nonalcoholic fatty liver disease reduces the risk of hepatocellular carcinoma remains to be shown. Prophylactic measures and the role of cancer surveillance have not been adequately investigated, but current evidence suggests a risk for hepatocellular carcinoma in nonalcoholic steatohepatitis-related cirrhosis that rivals that of hepatocellular carcinoma in hepatitis C virus-related cirrhosis, particularly in older male patients.

In vitro differentiation of rat liver derived stem cells results in sensitization to TNFalpha-mediated apoptosis

Hepatic stem cells are activated after liver damage and have a critical role in tissue homeostasis and repair. Characterization of molecular and cellular events accompanying the expansion and differentiation of liver stem cells is essential for understanding the basic biology of stem cells and for facilitating clinical application of the stem cells. We assessed whether in vitro differentiation of putative hepatic progenitor (rat liver epithelial [RLE]) cells toward hepatocytic lineage affects the response to TNFalpha-mediated cytotoxicity, a common determinant of liver injury. The data show that 50% of differentiated cells underwent apoptosis after 6 hours of TNFalpha treatment whereas control RLE cells were resistant. Both cell types displayed mitochondrial depolarization and release of cytochrome c but the TNFalpha treatment resulted in activation of caspases 9 and 3 and the execution of apoptosis only in differentiated RLE cells. Apoptotic death was associated with increased ROS production and depletion of glutathione. Antioxidants completely prevented both glutathione depletion and apoptosis induced by TNFalpha in differentiated RLE cells. Conversely, glutathione-depleting agents sensitized control RLE cells to TNFalpha induced apoptosis. In conclusion, efficient antioxidant defense system involving glutathione renders hepatic progenitor cells resistant to TNFalpha-mediated apoptosis and acquisition of sensitivity to death stimuli is an implicit feature of the differentiation process. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Tumor necrosis factor and its potential role in insulin resistance and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a spectrum of hepatic pathology that resembles alcohol-induced fatty liver disease(AFLD), but which develops in individuals who are not heavy drinkers. In people, NAFLD is associated strongly with obesity,insulin resistance, and dysmetabolic syndrome, but the exact mechanisms that promote liver disease in this clinical context remain poorly understood. The proinflammatory cytokine, funor necrosis factor alpha is known to be a key mediator of AFLD. This article discusses clinical and experimental evidence that tumor necrosis factor plays a role in the pathogenesis of insulin resistance syndromes, including nonalcoholic fatty syndromes, including nonalcoholic fatty liver disease.

Influence of serum from liver-damaged rats on differentiation tendency of bone marrow-derived stem cells

AIM: Recent studies in both rodents and humans indicated that bone marrow (BM)-derived stem cells were able to home to the liver after they were damaged and demonstrated plasticity in becoming hepatocytes. However, the question remains as to how these stem cells are activated and led to the liver and where the signals initiating the mechanisms of activation and differentiation of stem cells originate. The aim of this study was to investigate the influence of serum from liver-damaged rats on differentiation tendency of bone marrow-derived stem cells.

METHODS: Serum samples were collected from rats treated with a 2-acetylaminofluorene (2-AAF) /carbon tetrachloride (CCl4) program for varying time points and then used as stimulators of cultured BM stem cells. Expression of M₂- and L-type isozymes of rat pyruvate kinase, albumin as well as integrin-β 1 were then examined by reverse transcription polymerase chain reaction (RT-PCR) to estimate the differentiation state of BM stem cells.

RESULTS: Expression of M₂-type isozyme of pyruvate kinase (M2-PK), a marker of immature hepatocytes, was detected in each group stimulated with experimental serum, but not in controls including mature hepatocytes, BM stem cells without serum stimulation, and BM stem cells stimulated with normal control serum. As a marker expressed in the development of liver, the expression signal of integrin-β 1 was also detectable in each group stimulated with experimental serum. However, expression of L-type isozyme of pyruvate kinase (L-PK) and albumin, marker molecules of mature hepatocytes, was not detected in groups stimulated with experimental serum.

CONCLUSION: Under the influence of serum from rats with liver failure, BM stem cells begin to differentiate along a direction to hepatocyte lineage and to possess some features of immature hepatocytes.

Nomenclature of the finer branches of the biliary tree: canals, ductules, and ductular reactions in human livers

The work of liver stem cell biologists, largely carried out in rodent models, has now started to manifest in human investigations and applications. We can now recognize complex regenerative processes in tissue specimens that had only been suspected for decades, but we also struggle to describe what we see in human tissues in a way that takes into account the findings from the animal investigations, using a language derived from species not, in fact, so much like our own. This international group of liver pathologists and hepatologists, most of whom are actively engaged in both clinical work and scientific research, seeks to arrive at a consensus on nomenclature for normal human livers and human reactive lesions that can facilitate more rapid advancement of our field.

Neuroregulation of the neuroendocrine compartment of the liver

Liver progenitor cells as well as hepatic stellate cells have neuroendocrine features. Progenitor cells express chromogranin-A and neural cell adhesion molecule, parathyroid hormone-related peptide, S-100 protein, neurotrophins, and neurotrophin receptors, while hepatic stellate cells express synaptophysin, glial fibrillary acidic protein, neural cell adhesion molecule, nestin, neurotrophins, and their receptors. This phenotype suggests that these cell types form a neuroendocrine compartment of the liver, which could be under the control of the central nervous system. We recently showed that the parasympathetic nervous system promotes progenitor cell expansion after liver injury, since selective vagotomy reduces the number of progenitor cells after chemical injury in the rat. Similarly, after transplantation, which surgically denervates the liver, human livers that develop hepatitis have fewer progenitor cells than native, fully innervated livers with similar degrees of liver injury. There is also accumulating experimental evidence linking the autonomic system, in particular the sympathetic nervous system (SNS), with the pathogenesis of cirrhosis and its complications. Recently, it has been shown that hepatic stellate cells themselves respond to neurotransmitters. Moreover, inhibition of the SNS reduced fibrosis in carbon tetrachloride-induced liver injury. In view of the denervated state of transplanted livers, it is very important to unravel the neural control mechanisms of regeneration and fibrogenesis. Moreover, since there is a shortage of donor organs, a better understanding of the mechanisms of regeneration could have therapeutic possibilities, which could even obviate the need for orthotopic liver transplantation.