Our research interests span the broad areas of gastroenterology. Ongoing research projects include:
•Pathogenesis of Nonalcoholic Steatohepatitis (NASH)
Non-alcoholic fatty liver disease (NAFLD) is the most common cause of elevated liver enzymes in the US. NAFLD with inflammation and fibrosis is known as non-alcoholic steatohepatitis (NASH), which accounts for about 10% of liver transplants in the US and is projected to become the most common indication for liver transplantation in the near future. The pathogenesis of NASH is a subject of intense study. Many factors are implicated in the development of steatosis, liver inflammation, fibrosis, cirrhosis and hepatocellular carcinoma.
A unique contribution we made to the understanding of NASH pathogenesis is the well-characterized microarray database, which document the altered gene expression in NASH livers, compared to healthy livers Global gene expression of NASH patients AND Increased hemoglobin expression in NASH livers . These data have been an invaluable resource to power our NASH research in various directions.
Fatty liver is a prerequisite for the development of NASH. The homeostasis of hepatic lipid depends on the dynamic balance of multiple metabolic pathways. Previous studies focusing on individual pathway or enzyme drew conflicting conclusions on the molecular mechanism for the accumulation of lipid in hepatocytes. Our microarray data allowed us to compare all the major pathways in parallel, and therefore came to a more accurate knowledge about the molecular mechanism of steatosis (1). We confirmed our findings by quantitative real-time PCR (qRT-PCR).
Oxidative stress is a driving force of the disease progression from simple steatotis to NASH. Much of our research effort focuses on the origin of liver oxidative stress and the antioxidant responses in NASH livers. We identified one source of oxidative stress to be endogenous alcohol produced in the gut. We developed the endogenous alcohol hypothesis when we found in our microarray data that all enzymes involved in alcohol catabolism are up-regulated in NASH livers (2). Previous studies by Diehl's group suggested that intestinal bacteria produced more alcohol in obese mice than lean animals. Therefore it was our priority to identify the bacteria responsible for alcohol production. Examination of the microbial composition in the gut of NASH patients allowed us to identify the alcohol producing Escherichia, among other interesting findings (3). In addition to provide a robust source of oxidative stress, endogenous alcohol also contributes to liver fat deposition. Our data is supported by and best explains the observation that NASH and alcoholic steatohepatitis share many histological features. Both NASH and alcoholic steatohepatitis patients exhibit macrovesicular and microvesicular fat in hepatocytes. The number and size of Mallory bodies, and the pattern of pericellular fibrosis are also indistinguishable between two disease groups.
Alcohol alone may not be a "hit" strong enough to cause all the NASH pathologies. Our microarray data and our newly generated microbiome data MG-rast suggest other potential sources of oxidative stress, which are our current research subjects.
The prevention or mitigation of oxidative stress in patients with simple steatosis could prevent NASH. What are the molecular mechanisms that our body takes to fight oxidative stress? Our recent finding is that hemoglobin is expressed in hepatocytes and functions as an anti-oxidant in oxidative stressed cells (4). Paraoxonase 1 (PON1) is a known anti-oxidant in the serum. A protective role of this enzyme in NASH is being investigated in our lab
•Mechanism and regulation of gastric acid secretion
Abnormal acid secretion is the reason of many GI diseases including GERD, gastric, duodenal and esophageal ulcers. The spending in treating these conditions is substantial. The gastric parietal cell, lining the lumen of the stomach, is responsible for the secretion of isotonic HCl (0.15M) into stomach. One ATP is consumed for every proton secreted into the stomach lumen and a lot of proton pump (H,K-ATPase, the alpha and beta subunits of this enzyme were discovered in 1967(5) and 1990(6)) is required for this job. To accommodate these many proton pumps, the apical plasma membrane, in the resting state, is expanded in the form of numerous invaginations which express relatively short microvilli, and a large compartment of cytoplasmic membranes, commonly called tubulovesicles, fully loaded with proton pumps. Upon stimulation by hismatine initiated PKA signaling, these tubulovesicles traffic to and fuse with apical membrane, forming densely packed microvilli comparable to those found on the brush border membrane of small intestine. This intracellular trafficking and fusion events bring proton pumps to their post for active acid secretion. In time, these proton pumps are brought back into the cytoplasm (by way of endocytosis) for a reliable mechanism to turn off acid secretion. Although the membrane recycling theory was raised long time ago(7), there are still many major gaps in the understanding of the mechanism for the regulation of acid secretion, which are the research interests of our laboratory. Techniques employed include isolation and primary culture of gastric parietal cells, measurement of acid secretion, fractionation of different membranes by differential and gradient centrifugation.
Figure 1, Schematic representation of the parietal cell in resting and stimulated states. Drastic morphological change occurs with stimulation. In the resting state the apical canaliculi extend into the cell presenting short microvilli. Tubulovesicles containing cargo H,K pumps (red) abound in the cytoplasmic space. There are also many mitochondria.
•Using gastric parietal cell model to study general cell biological questions: how membrane trafficking is regulated by small G-proteins, how filamentous actin supports the dynamic change of microvilli on apical membrane
Parietal cell has a remarkable large volume of intracellular membrane trafficking adapted to the elegant mechanism for the regulation of acid secretion. This means that this cell is abundant in those protein machineries required for membrane trafficking and fusion, exocytosis and endocytosis. For instance, no other cell types express the amount of syntaxin3 found in parietal cell. Therefore, parietal cell is the top choice for elucidating many of the core questions in cell biology. Techniques used to attack these questions include immunoabsorption, differential ultra-centrifugation, IMAC, 2D-electrophoresis, Western blot analysis, LC-MSMS, immunofluorescence and confocal microscopy.
•Pathogenesis of Inflammatory Bowel Diseases (IBD)
This is an investigator-initiated research, sponsored by the Pharmaceutical Industry.
The etiology of IBD is unknown, but a body of evidence from clinical and experimental observation indicates a role for intestinal microflora in the pathogenesis of this disease. An increasing number of both clinical and laboratory-derived observations support the importance of luminal components in driving the inflammatory response in Crohn‘s disease.
Members of the Toll-like receptor family are key regulators of both innate and adaptive immune responses. These receptors bind molecular structures that are expressed by microbes but are not expressed by the human host. Activation of these receptors initiates an inflammatory cascade that attempts to clear the offending pathogen and set in motion a specific adaptive immune response. Defects in sensing of pathogens or mediation of the inflammatory cascade may contribute to the pathophysiology of disease and injure the host by activating a deleterious immune response, such as in inflammatory bowel disease. The focus of this research is to identify specific toll-like receptor mutations that may be associated with the development of inflammatory bowel disease.
- Lixin Zhu, Susan S. Baker, Wensheng Liu, Meng-Hua Tao, Raza Patel and Robert D. Baker*, 2011, Lipid in the livers of adolescents with non-alcoholic steatohepatitis: combined effects of pathways on steatosis. Metabolism. 60(7):1001-1011.
- Susan S. Baker, Robert D. Baker, Wensheng Liu, Norma J. Nowak, and Lixin Zhu*, 2010, Role of alcohol metabolism in non-alcoholic steatohepatitis. PLoS ONE, 5(3): e9570.
- Lixin Zhu, Susan S. Baker, Chelsea Gill, Wensheng Liu, Razan Alkhouri, Robert D. Baker, Steven R. Gill, 2013, Characterization of the gut microbiome in non-alcoholic steatohepatitis (NASH) patients: a connection between endogenous alcohol and NASH, Hepatology, 57(2): 601-609.
- Wensheng Liu, Susan S. Baker, Robert D. Baker, and Lixin Zhu, 2011, Upregulation of hemoglobin expression by oxidative stress in hepatocytes and its implication in nonalcoholic steatohepatitis. PLoS ONE. 6(9): e24363
- Forte JG, Forte GM, Saltman P. 1967. K+-stimulated phosphatase of microsomes from gastric mucosa. J Cell Physiol 69: 293-304
- Okamoto CT, Karpilow JM, Smolka A, Forte JG. 1990. Isolation and characterization of gastric microsomal glycoproteins. Evidence for a glycosylated beta-subunit of the H+/K(+)-ATPase. Biochim Biophys Acta 1037: 360-72
- Forte TM, Machen TE, Forte JG. 1977. Ultrastructural changes in oxyntic cells associated with secretory function: a membrane-recycling hypothesis. Gastroenterology 73: 941-55