Treatment of insulin resistance

#Insulin resistance or glucose intolerance is a condition characterized by the body's inability to properly use insulin or blood sugar. In this condition, normal amounts of insulin are inadequate to produce a normal insulin response from fat, muscle, or liver cells. Insulin resistance in fat cells results in hydrolysis of stored triglycerides, which elevates free fatty acids in the blood plasma. Insulin resistance in muscle reduces glucose uptake whereas insulin resistance in liver reduces glucose storage, with both effects serving to elevate blood glucose. High plasma levels of insulin and glucose due to insulin resistance often lead to the metabolic syndrome, such as abdominal obesity, atherogenic dyslipidemia, elevated blood pressure, prothrombotic state (e.g., high fibrinogen or plasminogen activator inhibitor-1 in the blood), and proinflammatory state (e.g., elevated C-reactive protein in the blood). People with the metabolic syndrome are at increased risk of cardiovascular diseases and type 2 diabetes. The metabolic syndrome has become increasingly common in the United States. There is a need for effective treatment of insulin resistance.

#This invention relates to treating insulin resistance and identifying a compound for treating insulin resistance.

#In one aspect, the invention features a method of identifying a compound for treating insulin resistance. The method includes contacting a first cell expressing an acid-sensing ion channel 3 (ASIC3) with a test compound, and determining an expression or activity level of the acid-sensing ion channel 3 in the cell. The test compound is determined to be a candidate compound for treating insulin resistance if the expression or activity level is lower than that determined in the same manner from a second cell except that the second cell is not contacted with the compound. The activity of the acid-sensing ion channel 3 can be determined by electrophysical analysis. The ion channel activity of the acid-sensing ion channel 3 can be determined by whole-cell patch recording, a voltage-sensitive dye, an ion-sensitive dye, or a cytotoxicity assay. The method can further include administering the candidate compound to a non-human animal to confirm an efficacy thereof to treat insulin-resistance. The first cell or second cell can also be a non-neuronal and AdiproR2+ cell. Examples of the cells include NIH3T3 cells, NIH3T3 L1 cells, differentiated NIH3T3 L1 cells, adipose cells, and fibroblast cells. The first cell or second cell can be positive for one or more of FAS, FGF10, aP2, ETO, PPARγ, and AdipoR1. In one example, the first or second cell is an adipose tissue cell, such as a white adipose tissue cell.

#This invention is based, at least in part, on the unexpected discoveries that ASIC3−/− mice were protected against age-dependent glucose intolerance with enhanced insulin sensitivity and that age-dependent glucose intolerance was associated with the up-regulation of ASIC3 expressed in white adipose tissue (WAT) but not sensory neurons.

#Acid-sensing ion channels (ASICs) mediate inward currents and depolarize cells when the extracellular pH drops (Krishtal, 2003, Trends Neurosci. 26, s477-483). They belong to the epithelial sodium channel/degenerin superfamily, which are characterized by two membrane-spanning domains with intracellular N- and C-termini and a large extracellular loop (Waldmann and Laadunski, 1998, Curr. Opin. Neurobiol. 8, 418-424 and Kellenberger and Schild, 2002, Physiol. Rev. 82, 735-767). There are seven ASIC subunits, including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, ASIC4, and ASIC5, which are encoded by five genes. Multiple subunits assemble to form a functional ion channel. Homomeric or heteromeric ASICs has various sensitivity to extracellular pH, ranging from near physiological pH (˜pH 7.0 for ASIC3) to very acidic status (˜pH 5.0 for ASIC2a). ASICs express throughout neurons of mammalian central and peripheral nervous systems (Krishtal, 2003, Trends Neurosci. 26, 477-483). Accordingly, the role of ASICs has been implicated in detecting tissue acidosis, ischemia, and modulating synaptic activity (Molliver et al., 2005, Molecular Pain 1, 35, and Xiong et al, 2004, Cell 118, 687-698).

#ASIC3 is the most sensitive acid-sensing ion channel (pH0.5 activation ˜6.7) and predominantly expressed in primary sensory neurons, especially in metaboreceptive sensory neurons or ischemia-sensing neurons (Waldmann and Lasdunski, 1998, Curr. Opin. Neurobiol. 8, 418-424). It responds better to lactic acid than to other acids to depolarize ischemia-sensing neurons. Therefore, ASIC3-expressing neurons might function as metaboreceptors to sense the anaerobic metabolism of tissues and trigger acid-linked pain sensation in muscle and heart (Molliver et al., 2005, Molecular Pain 1, 35).