dRetinol-binding protein 4 (RBP4), the sole retinol transporter in blood, is secreted from adipocytes and liver. Serum RBP4 levels correlate highly with insulin resistance, other metabolic syndrome factors, and cardiovascular disease. Elevated serum RBP4 causes insulin resistance, but the molecular mechanisms are unknown. Here we show that RBP4 induces expression of proinflammatory cytokines in mouse and human macrophages and thereby indirectly inhibits insulin signaling in cocultured adipocytes. This occurs through activation of c-Jun N-terminal protein kinase (JNK) and Toll-like receptor 4 (TLR4) pathways independent of the RBP4 receptor, STRA6. RBP4 effects are markedly attenuated in JNK1 ؊/؊ JNK2 ؊/؊ macrophages and TLR4؊/؊ macrophages. Because RBP4 is a retinol-binding protein, we investigated whether these effects are retinol dependent. Unexpectedly, retinol-free RBP4 (apo-RBP4) is as potent as retinol-bound RBP4 (holo-RBP4) in inducing proinflammatory cytokines in macrophages. Apo-RBP4 is likely to be physiologically significant since RBP4/retinol ratios are increased in serum of lean and obese insulin-resistant humans compared to ratios in insulin-sensitive humans, indicating that higher apo-RBP4 is associated with insulin resistance independent of obesity. Thus, RBP4 may cause insulin resistance by contributing to the development of an inflammatory state in adipose tissue through activation of proinflammatory cytokines in macrophages. This process reveals a novel JNK-and TLR4-dependent and retinol-and STRA6-independent mechanism of action for RBP4.O besity is a major risk factor for insulin resistance, which is a critical pathogenic factor in type 2 diabetes (50). Determination of the physiologic and cellular mechanisms linking obesity to type 2 diabetes could lead to development of new prevention and treatment approaches. Multiple mechanisms may contribute, including abnormal production of adipocyte-secreted proteins (adipokines) (1,15,29), infiltration of white adipose tissue (WAT) with proinflammatory macrophages (42), and aberrant lipid deposition in tissues such as muscle and liver (51). These mechanisms are not mutually exclusive. For example, adipokines can affect inflammation and lipid deposition in tissues (15).Serum retinol-binding protein 4 (RBP4) is an adipokine and is also secreted by liver. RBP4 levels are increased in obese and insulin-resistant humans and mouse models, and genetic or pharmacologic elevation of serum RBP4 causes insulin resistance in normal mice (19,31,65). Although many studies show strong correlations of serum RBP4 levels with obesity and the severity of insulin resistance (9,16,27,35), others do not (8,17,32,46), as reviewed in reference 32. This may result from the use of different populations of human subjects or from methodological issues with RBP4 assays (18,32,64). Many studies also show that serum RBP4 levels correlate with other components of the metabolic syndrome in humans, including hypertension (47, 54, 64), dyslipidemia (41, 47, 64, 67), waist/hip ratio (31...

Insulin resistance is a major cause of diabetes and is highly associated with adipose tissue (AT) inflammation in obesity. RBP4, a retinol-transporter, is elevated in insulin resistance and contributes to increased diabetes risk. We aimed to determine the mechanisms for RBP4-induced insulin resistance. Here we show that RBP4 elevation causes AT inflammation by activating innate immunity which elicits an adaptive immune-response. RBP4-overexpressing mice (RBP4-Ox) are insulin-resistant and glucose-intolerant and have increased AT macrophage and CD4 T-cell infiltration. In RBP4-Ox, AT CD206+ macrophages express pro-inflammatory markers and activate CD4 T-cells while maintaining alternatively-activated macrophage markers. These effects result from direct activation of AT antigen-presenting cells (APCs) by RBP4 through a JNK-dependent pathway. Transfer of RBP4-activated APCs into normal mice is sufficient to induce AT inflammation, insulin resistance and glucose intolerance. Thus, RBP4 causes insulin resistance, at least partly, by activating AT APCs which induce CD4 T-cell Th1 polarization and AT inflammation.

We studied M8-B, a selective and potent antagonist of the transient receptor potential melastatin-8 (TRPM8) channel. In vitro, M8-B blocked cold-induced and TRPM8-agonist-induced activation of rat, human, and murine TRPM8 channels, including those on primary sensory neurons. In vivo, M8-B decreased deep body temperature (Tb) in Trpm8+/+ mice and rats, but not in Trpm8−/− mice, thus suggesting an on-target action. The intravenous administration of M8-B was more effective in decreasing Tb in rats than the intrathecal or intracerebroventricular administration, indicating a peripheral action. M8-B attenuated cold-induced c-Fos expression in the lateral parabrachial nucleus, thus indicating a site of action within the cutaneous cooling neural pathway to thermoeffectors, presumably on sensory neurons. A low intravenous dose of M8-B did not affect Tb at either a constantly high or a constantly low ambient temperature (Ta), but the same dose readily decreased Tb if rats were kept at a high Ta during the M8-B infusion and transferred to a low Ta immediately thereafter. These data suggest that both a successful delivery of M8-B to the skin (high cutaneous perfusion) and the activation of cutaneous TRPM8 channels (by cold) are required for the hypothermic action of M8-B. At tail skin temperatures < 23°C, the magnitude of the M8-B-induced decrease in Tb was inversely related to skin temperature, thus suggesting that M8-B blocks thermal (cold) activation of TRPM8. M8-B affected all thermoeffectors studied (thermopreferendum, tail skin vasoconstriction, and brown fat thermogenesis), thus suggesting that TRPM8 is a universal cold receptor in the thermoregulation system.