Our research interests span the broad areas of gastroenterology. Ongoing research projects include:
•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(1) and 1990(2)) 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(3), 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 Nonalcoholic Steatohepatitis (NASH)
NASH research is funded by the Peter and Tommy Fund.
NASH is a disease of the liver that is associated with obesity and adult onset, or type II, diabetes. NASH is not a benign disease. Many people with NASH have a shorter life expectancy than those who no not have NASH. NASH is associated with cirrhosis and is the third most common reason for liver transplantation in adults. No one knows what causes NASH, but it is known that in obese people there is increased fat in the liver. In addition to fat, cells that cause inflammation are found in the liver in patients with NASH. It is thought that these inflammatory cells may cause liver damage that results in fibrosis, cirrhosis and ultimately liver failure. The purpose of this research is to understand the relationship between obesity and the molecular factors that control inflammation so the interaction of the two can be better understood and treatments developed.
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. Previous studies suggested that intestinal bacteria produced more alcohol in obese mice than lean animals. Therefore, we hypothesized and provided the first molecular evidence that alcohol metabolism contributes to the pathogenesis of NASH(4). To identify the exact microbiota contributing to the elevated alcohol production, a microbiome study was initiated in our group. In collaboration with Dr. Steve Gill at University of Rochester, the GI microbiomes of NASH patients were analyzed, and compared to those of normal controls and simple fatty liver patients. Our preliminary data suggest significant differences among these groups.
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. With a high through-put technique, we compared all the major pathways in parallel and therefore came to a more accurate knowledge about the molecular mechanism of steatosis(5). We are now looking into the detailed mechanisms of lipid trafficking and storage in hepatocytes.
NASH is the hepatic manifestation of metabolic syndrome. A new direction in our lab is to investigate how the abnormal carbohydrate metabolism contributes to the development of NASH (e.g., providing precursors for lipid de novo synthesis). Key enzymes in carbohydrate metabolism, their transcription factors and regulators are being investigated.
Oxidative stress is believed to be a major factor mediating the transition from simple steatosis to NASH. The prevention or mitigation of oxidative stress in patients with simple steatosis could prevent NASH. Our current research examines two facets of this problem: 1) what are the molecular mechanisms causing oxidative stress; 2) what are the molecular mechanisms that our body take to fight oxidative stress. Our recent finding is that hemoglobin is expressed in hepatocytes and functions as an anti-oxidant in oxidative stressed cells (6). 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.
•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.
- 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
- 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, Wensheng Liu, Meng-Hua Tao, Raza Patel and Robert D. Baker*, 2011, Lipid in the livers of adolescents with nonalcoholic steatohepatitis: combined effects of pathways on steatosis. Metabolism. 60(7):1001-1011
- Wensheng Liu, Susan S. Baker, Robert D. Baker, Norma J. Nowak, Lixin Zhu*, 2011, Upregulation of Hemoglobin Expression by Oxidative Stress in Hepatocytes and Its Implication in Nonalcoholic Steatohepatitis. PLoS ONE 6(9): e24363.