The role of INDY protein on hepatic insulin resistance

Type 2 diabetes (T2DM) is a metabolic disease reaching epidemic dimensions by affecting more than 220 million people worldwide. It is a highly ambitious goal to find a therapeutic target that protects from lipid-induced insulin resistance in muscle and liver. Indy (abbreviation for "I`m not dead yet") is a gene with a very prominent effect on increased life span in Drosophila and Caenorhabditis elegans. The homologous gene in mammals encodes a sodium-coupled citrate transporter NaCT (mINDY) and functions as an exchanger of dicarboxylate and tricarboxylate Krebs cycle intermediates whereby mutations in this gene affect metabolism in a manner akin to caloric restriction in favor of longevity. In our project, we plan to examine the effect of decreased whole body and liver specific expression of mINDY on lipid-induced hepatic insulin resistance in vivo using a novel 13C/ 31P NMR method. In addition, insulin-stimulated whole body and tissue specific rates of glucose metabolism will be measured. Decreased whole body and liver- specific expression of mINDY will be tested in awake rodent models of type 2 diabetes and insulin resistance. For this purpose, mice (mINDY KO) and rats (mINDY antisense oligonucleotide treated) will be used. Based on preliminary data we predict these agents will lead to decreased: i) hepatic DAG content, ii) PKCepsilon activation, iii) IRK-beta threonine phosphorylation and iv) protection from lipid-induced hepatic insulin resistance mainly by increased whole body energy expenditure. Overall we anticipate that the results from these studies will demonstrate that mINDY functions as a novel regulator of hepatic energy metabolism in mammals and that mINDY will be a novel therapeutic target for the treatment of non-alcoholic fatty liver disease and T2DM.

Non-alcoholic fatty liver disease is the most frequently diagnosed cause of chronic liver disease and up to 30% of the European population is affected by this common disease. An increase in metabolic risk factors such as obesity and the progressive aging of the population contribute to the high prevalence of this disease. In our project, we were able to show for the first time that a citrate transporter in the cell membrane of liver cells is an attractive target for pharmacological treatment of fatty liver. INDY is a transmembrane transporter, which transports citrate into the liver cell, where it serves as a building block for fatty acid synthesis (de novo lipogenesis). A reduced expression of the INDY-gene promotes longevity in flies and worms, two commonly used model organisms in biomedical science. Therefore, the acronym INDY owes its name the saying "I'm Not Dead Yet" from the movie "Monty Python and the Holy Grail" from 1975. Knock-out of a mammalian homolog of INDY in the mouse protects these animals from the adiposity, fatty liver and insulin resistance, diseases that can develop with excess calorie intake or aging. The aim of this project was therefore to test in rats the effect of inducible and liver-specific suppression of INDY by intraperitoneal administration of 2'-O-methoxyethyl chimeric antisense oligonucleotides (ASOs). This intervention is oriented towards a therapeutic approach and the physiology of the rat as compared to that of the mouse more closely resembles human physiology. Some ASOs are already on the market (for example the FDA-approved mipomersen, a drug directed against the ApoB-messenger RNA for the treatment of familial hypercholesterolemia) or are in clinical trials (Phase 2 for protein tyrosine phosphatase-1B for the treatment of type 2 diabetes) and the testing of new targets is thus a promising field. Indeed, we were able to show in our project that the use of INDY ASOs prevented high-fat diet induced hepatic steatosis and improved hepatic insulin sensitivity. Our investigations qualify INDY as a promising target for the treatment of fatty liver disease and type 2 diabetes.