J. Mark Brown, PhD
My lab has several projects funded by the National Institutes of Health (NIH) and the American Heart Association (AHA) that focus on the interrelationship between nutrient metabolism and the development of chronic metabolic diseases such as obesity, diabetes, and coronary heart disease (CHD). To accomplish the goals of our research program, we use an interdisciplinary approach that includes genetic manipulation of mice, cell biology, molecular biology, biochemistry, microsurgery, mass spectrometry, pathology, and nutrition. My laboratory currently has two active research programs, and we are always looking for highly motivated graduate students, postdoctoral fellows, and technicians to participate in our multidisciplinary training program:
Project 1) Mechanism Regulating Non-Biliary Reverse Cholesterol Transport.
Reduction of low-density lipoprotein (LDL)-cholesterol through statin therapy has only modestly decreased CHD-associated mortality in developed countries, which has prompted the search for alternative therapeutic strategies for CHD. Major efforts are now focused on therapies that augment high-density lipoprotein (HDL)-mediated reverse cholesterol transport (RCT), and ultimately increase the fecal disposal of cholesterol. The process of RCT has long been thought to simply involve HDL-mediated delivery of peripheral cholesterol to the liver for biliary excretion out of the body. However, our work has revealed a novel pathway for RCT that does not rely on biliary secretion. This non-biliary pathway rather involves the direct excretion of cholesterol by the proximal small intestine. Compared to RCT therapies that augment biliary sterol loss, modulation of non-biliary fecal sterol loss through the intestine is a much more attractive therapeutic strategy, given that excessive biliary cholesterol secretion can promote gallstone formation. Currently, we are at an early stage in understanding the molecular mechanisms regulating the non-biliary pathway for RCT, and several projects are available to highly motivated students and postdoctoral fellows. We are currently utilizing both targeted and unbiased screening approaches to identify both protein and lipoprotein mediators of non-biliary RCT. Discoveries stemming from this research program have strong potential to open new therapeutic opportunities targeting the small intestine as an inducible cholesterol secretory organ.
Figure 1-A: Working Model For "Biliary" Fecal Sterol Loss.
Figure 1-A represents the classic “biliary” view of reverse cholesterol transport (RCT). In this model, free cholesterol is effluxed from peripheral tissues, and delivered to the liver via HDL-mediated presentation to hepatic SR-BI. This free cholesterol is immediately shunted into the bile through the actions of hepatic ABCG5/G8. A portion of this biliary cholesterol can be reabsorbd by the intestine through the actions of intestinal NPC1L1, whereas intestinal ABCG5/G8 can resecrete newly absorbed cholsterol back into the lumen. Taking into account dietary sterol, it is predicted that ~55-65% of fecal sterol loss can be attributed to biliary sterol loss in mice and humans.
Figure 1-B: Working Model For "Non-Biliary" Fecal Sterol Loss.
Figure 1-B represents our proposed mechanism of “non-biliary” fecal sterol loss in a mouse model of biliary insufficiency (NPC1L1Liver-Tg). We have previously demonstrated that by overexpressing NPC1L1 in the liver, biliary cholesterol secretion in blunted by >90%, likely resulting from NPC1L1’s ability to recapture newly secreted cholesterol. In the case of biliary insufficiency, we believe that the liver secretes the excess hepatic cholesterol onto nascent lipoproteins which are specifically targeted to the small intestine. After being internalized into intestinal enterocytes, the cholesterol is trafficked across the cell to the apical membrane and effluxed into the intestinal lumen by ABCG5/G8 or a currently unidentified cholesterol transporter. The sum of“biliary”and“non-biliary”pathways represents the total flux of peripheral cholesterol into the feces. My laboratory is currently investigating protein and lipoprotein mediators of non-biliary fecal sterol loss.
Project 2) Alpha Beta Hydrolase Domain (Abhd) Proteins in Metabolic Disease.
Another major focus of Dr. Brown’s research program surrounds functional characterization of a family of proteins known as alpha/beta hydrolase domain (Abhd) containing proteins. These proteins are highly conserved lipid metabolizing enzymes, and mutations in several of these proteins have been implicated in inherited inborn errors in lipid metabolism. We have generated several genetically modified mouse models and cell models to better understand the function of this important class of lipid metabolizing enzymes, with a focus on their role in the development of obesity, hepatic steatosis, and type II diabetes.
Figure 2: Alpha Beta Hydrolase Domain (Abhd) Proteins: Enzyme Regulators of Metabolic Disease.
Several members of the Abhd family have been genetically linked to inborn errors in lipid metabolism and intracellular signal transduction. My laboratory is currently generating genetically modified mouse models to further understand the physiological role of Abhd proteins in metabolic diseases such as obesity, type II diabetes, and atherosclerosis. The predicted serine nucleophile within the consensus GXSXG esterase motif is shown in red. Note that Abhd5 does not contain a catalytic serine nucleophile. In contrast, Abhd7 and Abhd9 contain aspartic acid nucleophiles giving them epoxide hydrolase activity.