The principal focus of our laboratory has been to delineate the molecular mechanisms underlying the process of hepatocyte injury and death. In any human liver disease, the clinical outcome depends largely on the amount of hepatic function remaining after the death of injured hepatocytes. A better understanding of how hepatocyte injury results in cell death may lead to the development of new therapies for human liver disease.
Investigations into toxin-induced liver injury have centered on the finding that cell death occurs not from the direct biochemical effects of toxins, but as the result of the effects of inflammatory cell products including cytokines and reactive oxygen species (ROS). Using in vitro and in vivo models of hepatocyte injury, we have shown that toxic liver injury occurs in large part from activation of the apoptotic mitochondrial death pathway by the cytokine tumor necrosis factor-alpha (TNF). Our studies have demonstrated that apoptosis from TNF results from a failure of the transcription factor NF-KB to down regulate c-Jun N-terminal kinase (JNK). Similar studies into the mechanism of death from ROS have demonstrated that ROS-induced hepatocyte death also occurs from JNK/c-Jun overactivation. Current investigations are identifying the mechanisms by which JNK overactivation triggers hepatocyte death focusing on the identification of novel substrates of JNK. Additional studies are defining the protective functions of the lysosomal degradative pathway of autophagy in these hepatocellular death pathways.
A second focus of our studies in liver injury is an attempt to understand the mechanisms by which simple fatty liver (steatosis) progresses to nonalcoholic steatohepatitis (NASH), a disease characterized by hepatic injury, inflammation and fibrosis. These investigations examine how factors such as chronic oxidative stress, which accompanies hepatic steatosis, alters cell signaling pathways and sensitizes steatotic hepatocytes to injury. Overactivation of the JNK signaling pathway has been demonstrated to promote both steatosis and hepatocellular injury, and the mechanism of this effect and the ability of JNK inhibition to prevent and reverse steatohepatitis in vivo are currently under investigation.
A final area of laboratory interest unrelated to liver injury is on the role of autophagy, specifically the impairment in autophagy that accompanies aging, in cellular responses to excess lipids. This work is defining how autophagy regulates cellular lipid storage and function in the setting of increased lipids as occurs in the metabolic syndrome. These investigations are being conducted in immune cells as well as hepatocytes, with the goal of delineating how the decrease in autophagy with aging mediates hepatic and immune system dysfunction.
Wang, Y., Singh, R., Lefkowitch, J.H., Rigoli, R., and Czaja, M.J. TNF-induced toxic liver injury results from JNK2-dependent activation of caspase-8 and the mitochondrial death pathway. J. Biol. Chem. 2006; 281: 15258-15267.
Wang, Y., Singh, R., Massey, A.C., Kane, S.S., Kaushik, S., Grant, T., Xiang, Y., Cuervo, A.M., and Czaja, M.J. Loss of macroautophagy promotes or prevents fibroblast apoptosis depending on the death stimulus. J. Biol. Chem. 2008; 283:4766-4777.
Singh, R., Wang, Y., Xiang, Y., Tanaka, K.E., Gaarde, W.A. and Czaja, M.J. Differential effects of JNK1 and JNK2 inhibition on murine steatohepatitis and insulin resistance. Hepatology 2009; 49:87-96.
Singh, R., Kaushik, S., Wang, Y., Xiang, Y., Novak, I., Komatsu, M., Tanaka, K., Cuervo, A.M., and Czaja, M.J. Autophagy regulates lipid metabolism. Nature 2009; 458:1131-1135.
Singh, R., Xiang, Y., Wang, Y., Baikati, K., Cuervo, A.M., Luu, Y.K., Tang, Y., Pessin, J.E., Schwartz, G.J., and Czaja, M.J. Autophagy regulates adipose mass and differentiation in mice. J. Clin. Invest. 2009; 119:3329-3339.
Wang, Y., Singh, R., Xiang, Y., and Czaja, M.J. Macroautophagy and chaperone-mediated autophagy are required for hepatocyte resistance to oxidant stress. Hepatology 2010; 52:266-277.
Amir, M., Zhao, E., Fontana, L., Rosenberg, H.J., Tanaka, K., Gao, G., and Czaja, M.J. Inhibition of hepatocyte autophagy increases tumor necrosis factor-dependent liver injury by promoting caspase 8 activation. Cell Death Diff. 2013, 20:878-887.
Material in this section is provided by individual faculty members who are solely responsible for its accuracy and content.