Molecular Identification and Characterization of a Novel Nuclear Protein Whose Expression Is Up-regulated in Insulin-resistant Animals

Abstract

Energy metabolism is the most fundamental capacity for mammals, impairment of which causes a variety of diseases such as type 2 diabetes and insulin resistance. Here, we identified a novel gene, termed diabetes-related ankyrin repeat protein (DARP) that is up-regulated in the heart of KKA y mouse, a type 2 diabetes and insulin resistance model animal. DARP contains putative nuclear localization signals and four tandem ankyrin-like repeats. Its expression is restricted in heart, skeletal muscle, and brown adipose. Western blot analysis and immunocytochemistry of DARP-transfected Chinese hamster ovary (CHO) and COS-7 cells reveal that DARP is a nuclear protein. When DARP is expressed in CHO cells, [1-14 C]palmitate uptake is significantly decreased, whereas the palmitate oxidation does not show significant change. Furthermore, DARP expression is altered by the change of energy supply induced by excess fatty acid treatment of skeletal myotube in vitro and fasting treatment of C57 mouse in vivo. We confirmed that DARP expression is also altered in Zucker fatty rat, another insulin resistance model animal. Taken together, these data suggest that DARP is a novel nuclear protein potentially involved in the energy metabolism. Detailed analysis of DARP may provide new insights in the energy metabolism.Metabolic disorders cause wide variety of diseases including hyperlipidemia, hyperuricaemia, diabetes, and insulin resistance. Among these diseases, diabetes and insulin resistance are epidemic worldwide and are expected to affect 300 million people by 2025 (1). Recently, abnormalities of fatty-acid metabolism are recognized as key components of the pathogenesis of type 2 diabetes and insulin resistance (2-5). High fat diet and raised levels of circulating free fatty acids are sufficient to induce insulin resistance that is related to the fat content of skeletal muscle in rats (6). Accumulation of lipids inside muscle cells and, specifically, an increase in muscle long chain fatty acyl-CoA content are reported to cause insulin resistance. This suggests that abnormal fatty acid metabolism and the accumulation of lipid in skeletal muscle play crucial roles in the pathogenesis of insulin resistance (7,8). Moreover, the relation between insulin resistance and muscle triglyceride content is independent of total adiposity. Although the details of the mechanisms connecting lipid accumulation and insulin resistance are still unclear, studies of insulin receptor signaling reveal that the accumulation of lipid products causes the phosphorylation of insulin receptor as well as insulin receptor substrate (IRS)-1 through protein kinase C activation. This results in the inhibition of insulin receptor signaling (4).Recently identified molecules involved in the pathogenesis of insulin resistance act at least partially through the alteration of fatty acid metabolism. Adiponectin, whose secretion from white adipose tissue is reduced in insulin-resistant animal models, induces tissue fatty acid oxidation, leading to a reduction of tissue stea...

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