The liver performs numerous metabolic and homeostatic functions in the body, including xenobiotic metabolism, energy storage/production, urea formation, glutamine synthesis and cholesterol homeostasis. While some liver functions (such as albumin secretion) can be carried out by all hepatocytes, other functions are limited to a subset of hepatocytes and determined by cell position within the lobule. Specifically, certain enzymes are expressed only in cells surrounding the central vein whereas others are expressed only in hepatocytes surrounding the portal triads. This phenomenon is called positional (or zonal) heterogeneity or metabolic zonation. This compartmentalization of function enables the liver to perform multiple, and sometimes opposing, metabolic pathways in distinct hepatocyte subpopulations. Zonal gene regulation is essential for normal liver function. Disruption of zonally regulated metabolic processes can alter carbohydrate and lipid metabolism, ammonia detoxification and biliary function. A number of agents (alcohol, drugs and toxins) preferentially damage hepatocytes surrounding the central veins due to restricted expression of P450 enzymes in these cells. Thus, zonal control of gene expression has significant clinical implications. The overall objective of this proposal is to develop a better understanding of zonal gene regulation. Based on our studies on mouse alpha-fetoprotein (AFP) gene regulation, we have developed a model of zonal regulation. Specifically, we have found that AFP enhancer element E3 exhibits highly restricted activity in hepatocytes surrounding the central vein in the adult liver. Building on studies performed by our lab and others, we will investigate the role of a subfamily of orphan nuclear receptors and the Wnt/b-catenin/TCF signaling pathway in the control of E3. These studies utilize a unique set of reagents and transgenic mouse lines developed in our lab. The long-term goal of these studies is to understand the basic mechanisms that govern the zonal regulation of many liver genes which will lead to better treatment of liver disease in humans.
The liver is a complex organ that must coordinately express numerous proteins to facilitate proper development and maintain normal physiological homeostasis. In the adult liver, appropriate gene expression, including zonal gene expression, must be maintained during normal conditions, when hepatocytes are generally quiescent, and during disease, when some hepatocytes may be damaged and others may be proliferating. Our ongoing studies on the control of alpha-fetoprotein enhancer activity described in this proposal will help elucidate aspects of zonal gene expression in the adult liver.
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