Molecular Mechanisms of Arrestin-Domain Containing Proteins in Metabolism

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The arrestin superfamily is composed of two subfamilies: the classical visual/beta-arrestins that were first identified as regulators of G-protein coupled receptor signaling, and the more ancient branch of arrestin domain-containing proteins that are sometimes called the “alpha-arrestins”. Recently, the roles of some members of the alpha-arrestin family have been revealed in mammalian metabolism. The most studied member of the alpha-arrestin family is thioredoxin interacting protein (TXNIP), which regulates glucose and fructose metabolism. Arrestin domain-containing 4 (ARRDC4) is a member of the alpha-arrestin family that has not been subject to extensive investigation. The arrestin domains of ARRDC4 show 41% amino acid sequence similarity to TXNIP; therefore, ARRDC4 might be expected to have functions similar to TXNIP. However, we have shown that TXNIP is the only member of the family that binds covalently to thioredoxin. Consistent with our biochemistry studies, the recently solved crystal structure revealed that the cysteine 247 residue of TXNIP is essential for binding covalently to thioredoxin. Furthermore, we have reported that mice with deletion of TXNIP have improved insulin sensitivity, indicating a potential like between redox state and glucose metabolism through thioredoxin. Here we show new unpublished experiments revealing that mice with global deletion of arrdc4 have improved insulin sensitivity. We also show improved insulin signaling in metabolic tissues from mice with deletion of arrdc4. Our preliminary data demonstrate that the ARRDC4 protein interacts directly with glucose transporter protein 4 (GLUT4) in insulin-stimulated conditions. Furthermore, we present a new mouse model with mutation of C247 of TXNIP and show that this single amino acid change in TXNIP improves insulin sensitivity in mice fed a High Fat Diet. In this project, we will use these new discoveries and mouse models to understand how arrestin domain-containing proteins regulate glucose metabolism in vivo and at the molecular mechanism level. Our Specific Aims are: Specific Aim 1 will test the hypothesis that alpha-arrestin protein ARRDC4 regulates insulin-stimulated glucose uptake in vivo. Specific Aim 2 will investigate the molecular mechanisms of alpha arrestin domain-containing protein interactions with glucose transporters and insulin receptor. Specific Aim 3 will test the hypothesis that TXNIP can regulate insulin sensitivity through thioredoxin-independent mechanisms and identify the target tissue responsible.