**3.3 AGEs and insulin resistance in vitro**

Several evidences also suggest that AGEs affect the function of insulin-target cells in vitro. AGEs interact with CD36 in mouse 3T3 and human subcutaneous adipocytes, which is associated with down-regulation of leptin expression in adipocyte through reactive oxygen species (ROS) system 53. Miele et al showed in L6 skeletal muscle cells that AGEs affect glucose metabolism by impairing insulin-induced insulin receptor substrate (IRS) signaling through protein kinase Cα-mediated mechanism 37. The same research group also showed in the muscle cells that methylglyoxal, an essential source of intracellular AGEs, hampers a key insulin signaling molecule 54. Recent observations by Unoki et al also showed that AGEs impair insulin signaling in adipocytes by increasing generation of intracellular ROS 55. Thus, AGEs may not only induce the debilitating complications of diabetes, but may also contribute to the impairment of insulin signaling in insulin-target tissues which could be involved in pathophysiology of insulin resistance, metabolic syndrome and diabetes.

## **3.4 Endogenous RAGE ligands, insulin resistance and metabolic syndrome**

RAGE also interacts with other endogenous non-glycated peptide ligands including S100/calgranulin 56, amphoterin (also termed as high mobility group box 1 protein, HMGB1) 57, 58, amyloid fibrills 59, transthyretin 60, and a leukocyte integrin, Mac-1 61, many of which are important inflammatory regulators. Some of these inflammatory ligands for RAGE may be involved in pathogenesis of obesity and metabolic syndrome. Early studies show expression of S100B protein in pre- and mature- adipocyte and is induced during adipogenesis 62, 63. Physiological S100B levels appear to closely reflect adipose tissue mass or insulin resistance in humans 64-66. HMGB1 is also found to be expressed in human adipose tissue with the expression level associated with the fat mass and obesity-associated gene 67. Moreover, growing evidences suggest that infiltration of inflammatory cells, including macrophages, play fundamental roles in adiposity and metabolic syndrome 68-70. MAC-1, an integrin expressed in macrophage, can act as a RAGE ligand 61, and may be involved in adipogenesis through interaction with RAGE.
