Fenretinide (FEN), a ligand of retinol binding protein 4 (RBP4), has been suggested as a measure to reduce insulin resistance and its associated disorders such as obesity, and fatty liver by reducing serum RBP4. We investigated whether there is another possible mechanism by which fenretinide reduces insulin resistance and fatty liver in genetically obese (ob/ob) mice. Male obese mice fed a high-fat diet (45% of calories from fat) were divided into two groups (n 13 each). One (FEN) received fenretinide (20 mg/kg body weight, intraperitoneally) and the other (O) received vehicle three times weekly for 24 d. C57BL/6J mice fed a normal-fat diet (16% of calories from fat) were used as a control (C; n 13). No changes in fat weight and serum leptin level could be observed in FEN mice. Lower plasma RBP4 was observed in FEN mice compared with O mice. Fenretinide improved whole-body insulin sensitivity based on glucose and insulin tolerance tests and the homeostasis model assessment of insulin resistance. Fenretinide decreased the plasma lipid (triglyceride, cholesterol, and free-fatty acid) levels, hepatic TG level, and histological steatosis score. The mechanism by which fenretinide prevents fatty liver may be explained by an increased plasma adiponectin level, increased activation of hepatic AMP-activated protein kinase, and the expression of peroxisome proliferator-activated protein-α and peroxisomal acyl-CoA oxidase, which promote fat oxidation. FEN alleviated insulin resistance and fatty liver in obese mice and thus may act as an anti-lipidemic and anti-diabetic drug.Key words fenretinide; retinol binding protein-4; insulin resistance; fatty liver; fatty acid oxidation Obesity is a worldwide public health problem and is associated with hypertension, dyslipidemia, nonalcoholic fatty liver disease (NAFLD), and type 2 diabetes, with insulin resistance being a common factor. The pathogenesis of these conditions occurs in insulin-acting peripheral tissues, including skeletal muscle, liver, and adipose tissue. [1][2][3] In particular, insulin resistance-related adipose tissue dysfunction leads to the release of a high level of free-fatty acids and perturbs the levels of various adipokines, such as leptin, adiponectin, and retinol binding protein 4 (RBP4), which directly or indirectly affect insulin sensitivity by modulating insulin signaling and molecules involved in glucose and lipid metabolism in many tissues. [4][5][6] RBP4 is a compact, globular, 21-kDa protein and was known only as a retinol carrier synthesized and secreted by the liver, until recently. 6) Although serum RBP4 originates largely from the liver under physiological conditions, it is secreted considerably from fat tissues in an insulin-resistant state.7) Yang et al. showed that adipose tissue-specific ablation of glucose transporter 4 increased RBP4 mRNA expression in adipocytes and serum RBP4 levels in rats, and focused on the link between the serum RBP4 level and insulin sensitivity.
8)Despite some reports that RBP4 is not associated with insulin resis...