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

✍ Scribed by Aránzazu Sánchez; Valentina M. Factor; Luis A. Espinoza; Insa S. Schroeder; Snorri S. Thorgeirsson

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 488 KB
• Volume: 40
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.20363

✦ Synopsis

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 TNF␣-mediated cytotoxicity, a common determinant of liver injury. The data show that 50% of differentiated cells underwent apoptosis after 6 hours of TNF␣ treatment whereas control RLE cells were resistant. Both cell types displayed mitochondrial depolarization and release of cytochrome c but the TNF␣ 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 TNF␣ in differentiated RLE cells. Conversely, glutathione-depleting agents sensitized control RLE cells to TNF␣ induced apoptosis. In conclusion, efficient antioxidant defense system involving glutathione renders hepatic progenitor cells resistant to TNF␣-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). (HEPATOLOGY 2004;40:590 -599.)

H epatic stem cells have attracted great attention as potential candidates for liver-directed gene therapy and as a tool for regenerative medicine. It is well established that hepatic stem cells participate in liver regeneration when proliferation of hepatocytes is in-hibited and/or when mature parenchyma is damaged by toxic insults. 1 Despite extensive efforts to use hepatic progenitor cells as an ideal source for liver repopulation, 2,3 the mechanisms controlling lineage commitment and response of hepatic stem cells to environmental signals remain largely unknown.

Activation of hepatic stem cells has been observed not only in animal models of chemically-induced liver injury but also in a number of human liver diseases, including hepatitis, cholestasis, and alcoholic and nonalcoholic fatty liver disease. 4 -6 A common feature of these adverse conditions is marked induction of tumor necrosis factor (TNF)␣ and increased apoptosis resulting in continuous hepatocyte death. TNF␣ is a pleiotropic cytokine critical in tissue homeostasis. In general, TNF␣ is not cytotoxic for liver cells, but rather contributes to hepatocyte proliferation and is essential for liver regeneration. 7,8 However, during liver injury, hepatocytes become vulnerable to this cytokine, probably due to impairment of defense mechanisms. 9,10 Similarly, TNF␣ induces apoptosis of both hepatocytes and hepatoma cells in vitro when RNA or protein synthesis is inhibited. 11 During the last few years, understanding of the apoptotic pathway initiated by TNF␣ in hepatocytes has