Obesity is a global pandemic of enormous medical, economic, and social concern affecting a significant portion of the world's population. The consequences of obesity in human can be severe and there is increasing evidence linking obesity to diabetes, heart disease, and hypertension. The knowledge and the exact identity of genes that act together in fat metabolism could speed discovery of the molecular causes of and therapies for obesity. Studies in our lab focus on Kruppel-like family of transcription factors, KLFs. We demonstrate that a member of this family, klf-3 is one of the important regulators of two key processes in lipid metabolism; fatty acid b-oxidation and lipoprotein assembly and transport. Our efforts include investigation of how klf interacts with lipid metabolism regulatory machinery, characterization of specific pathway targets, and genetic manipulation of these targets to better understand the pathology of excess fat accumulation in human. Identifying the defined mechanisms by which various pathways coordinate and interruption in these pathways may result in abnormal metabolic regulation is a major area in obesity research. Given the significance of excess body fat accumulation to human health, our primary objective is to identify and characterize signaling molecules that are involved in fat storage. Our project is altogether pre-clinical, yet the science is very elaborate and promising. All efforts to get to the root of obesity should be pursued, including our studies on mammalian Kruppel-like family of transcription factors, KLFs. Our on-going project has a significant implication for diabetes, obesity and cardiovascular disease.
Mammals have developed a mechanism to store excessive amounts of energy substrates in the form of intracellular triglyceride (TG) deposits in lipid droplets, which are most prominent in mammalian adipose tissues; the major storage site for fat. During starvation, TG is hydrolyzed into fatty acids (FAs) to provide energy. Breakdown of fatty acyl-CoAs to acetyl-CoA occurs via beta-oxidation enzymes, where several parallel pathways with intersecting substrate specificities are used. We take advantage of the availability of Caenorhabditis elegans models, its genomic databases, and the availability of mutants associated with fat metabolism in order to understand the complexity of fat metabolism. We found that many metabolic genes are potential targets under the control of klf-3. Klf-3 mutations cause accumulation of enlarged, neutral lipid rich intestinal droplets; however, it does not change the normal feeding behavior of the mutant animal. The KLF-3 activity appeared to be necessary for maintaining the normal expression of enzymes that are essential in FA synthesis and mitochondrial or peroxisomal b-oxidation pathway and may therefore modulate these processes that mediate lipid metabolism. Our results also suggest that klf-3 may contribute in normal reproductive behavior and fecundity. To reconcile with these seemingly unrelated phenotypes of lipid utilization and reproductive behavior in klf-3 mutants, we are examining the mechanistic basis of klf-3 regulation of FA b-oxidation and elucidate how this pathway intersects reproduction.
The next question we address is the role of klf-3 in lipoprotein assembly and transport
Dietary lipids are absorbed from the small intestines and transported to various organs and tissues to maintain lipid/cholesterol homeostasis. Because of non-polar nature of lipids, mammals have evolved a mechanism that allows the conversion of insoluble lipids into lipoproteins so that it can be transported and delivered to its destination. For transportation the assembly and secretion of lipoprotein particles is important, which is primarily achieved in the liver as very low density lipoprotein (VLDL) and in the intestine as chylomicrons. The assembly of triglyceride-rich lipoproteins requires the formation of a complex between apolipoprotein B (apoB), a structural protein, and microsomal triglyceride transfer protein (MTP), an endoplasmic resident chaperone and protein disulfide isomerase (PDI). In humans, there are two important apoB proteins; apoB-100 is made exclusively by the liver, whereas apoB-48 is made in the intestine. It is believed that MTP transports lipid by a shuttle mechanism suggesting that MTP acts as carrier to transfer lipids from their site of synthesis to nascent lipoproteins within the ER and thus able to transfer TG and other lipids between membranes. MTP and apoB interact during lipoprotein assembly to facilitate lipoprotein production and therefore genetic loss of either apoB or MTP results in the inability of both the liver and intestine to secrete VLDL. Regulation of the assembly and secretion of apoB-containing lipoproteins has become an active area of investigation as it is known that overproduction of apoB-containing lipoproteins may be responsible for coronary artery disease and hyperlipidemia. The process involved in the assembly and secretion of VLDL or intestinal chylomicrons is complex and the cellular and molecular mechanisms by which various lipid and protein components are brought together for VLDL assembly are poorly understood. We are interested in the mechanisms underlying lipoprotein assembly and transport and to better understand the regulatory role of klf-3 in these processes, and identify novel targets for new therapies, which may be tested in vertebrate models. We found that mutation in C. elegans dsc-4 (mq920), the homolog of MTP, and suppression of vit1-6 genes activity that are homologous to human apoB increases fat accumulation in the intestine of dsc-4 (mq920) mutant and vit RNAi worm respectively suggesting that similar to klf-3 mutation, dsc-4 mutants or vit RNAi worms retain fat in their intestine and perhaps unable to efficiently transport stored fat for energy. We further found that Klf-3 mutations reduce expression of dsc-4, and all vitellogenin (vit1-6) genes and that klf-3 genetically interact with both dsc-4 and vit genes. Our data suggest that klf-3, dsc-4 and vit genes coordinate lipid transport but how these molecules act together to synchronize this process is under investigation. We are also investigating whether the molecules and mechanisms involved in fat metabolism is conserved across species including mammals. We have begun to test our paradigm in mammalian HepG2 cell line. We believe that our study will have a considerable impact on obesity research because it will lead to a significant understanding of the lipid metabolism and perhaps will allow us to lay the groundwork for translational research for treatment of obesity and its related cardiovascular and other diseases involving KLF-3 as therapeutic agent. We also believe that all efforts to get to the root of obesity should be pursued, including our studies on mammalian KLFs.
The C. elegans KLFs in sickle cell biologyThe beta-thalassemias and sickle cell disease (SCD) are genetic blood disorders that result from mutations in the b-globin gene causing the imbalanced synthesis of globin chains and the formation of structurally abnormal hemoglobin, HbS, respectively. The erythroid Kruppel-like factor (EKLF or KLF1) of the C2H2 zinc-finger class is required for normal globin production in mammals and is implicated in hereditary hemoglobinopathies b-thalassemias and SCD. We are interested in identifying the conserved proteins targets with critical roles in modulating klfs activity and henceforth answer some of the important questions related to mammalian KLF1. For example, how does inactivation of KLF1 result in defective erythropoiesis that leads to a severe anemic state?