Retinol‐binding protein‐4 attenuates insulin‐induced phosphorylation of IRS1 and ERK1/2 in primary human adipocytes

Abstract

Reduced sensitivity to insulin in adipose, muscle, and liver tissues is a hallmark of type 2 diabetes. Animal models and patients with type 2 diabetes exhibit elevated levels of circulating retinol-binding protein (RBP4), and RBP4 can induce insulin resistance in mice. However, little is known about how RBP4 affects insulin signaling. We examined the mechanisms of action of RBP4 in primary human adipocytes. RBP4-treated adipocytes exhibited the same molecular defects in insulin signaling, via IRS1 to MAP kinase, as in adipocytes from patients with type 2 diabetes. Without affecting autophosphorylation of the insulin receptor, RBP4 blocked the insulin-stimulated phosphorylation of IRS1 at serine (307) [corresponding to serine (302) in the murine sequence] and concomitantly increased the EC50 (from 0.5 to 2 nM) for insulin stimulation of IRS1 phosphorylation at tyrosine. The phosphorylation of IRS1 at serine (312) [corresponding to serine (307) in the murine sequence] was not affected in cells from diabetic patients and was also not affected by RBP4. The EC50 for insulin stimulation of downstream phosphorylation of MAP kinase ERK1/2 was increased (from 0.2 to 0.8 nM) by RBP4. We show that ERK1/2 phosphorylation is similarly impaired in adipocytes from patients with type 2 diabetes. However, the sensitivity to insulin for downstream signaling to control of protein kinase B and glucose uptake was not affected by RBP4. When insulin-resistant adipocytes from patients with type 2 diabetes were incubated with antibodies against RBP4, insulin-induced phosphorylation of IRS1 at serine (307) was normalized and the EC50 for insulin stimulation of ERK1/2 phosphorylation was reduced. Endogenous levels of RBP4 were markedly reduced in adipocytes from obese or type 2 diabetic subjects, whereas expression levels of RBP4 mRNA were unaffected. These findings indicate that RBP4 may be released from diabetic adipocytes and act locally to inhibit phosphorylation of IRS1 at serine (307), a phosphorylation site that may integrate nutrient sensing with insulin signaling.

Short-Term Overeating Induces Insulin Resistance in Fat Cells in Lean Human Subjects

“…In contrast, the molecular mechanisms behind insulin resistance in humans are poorly understood (2,3). In T2D, insulininduced phosphorylation of IRS1 on tyrosine residues has been shown to be impaired in both human skeletal muscle (4)(5)(6)(7) and adipose tissue (8,9). In adipocytes from patients with T2D we have found that a positive feedback control loop, involving phosphorylation of IRS1 at serine-307 (human sequence, corresponding to serine-302 in the murine sequence) is not fully functional (10,11).…”

RBP4: a controversial adipokine

“…While studying altered insulin signaling in adipocytes treated with RBP4, it was proposed that RBP4 inhibits phosphorylation at the site of the insulin receptor substrate 1 that may be responsible for integrating nutrient sensing with insulin signaling (91). In a study focusing on the role of inflammation in RBP4 expression in human adipocytes, RBP4 was unexpectedly downregulated by tumor necrosis factor a (TNFa) (92).…”

Regulation of Retinol Binding Protein 4 Expression and Its Relation to Adipogenesis in Bovine Adipocytes

“…All these findings together provide an evidence for negative relationship between RBP4 expression and secretion from bovine adipocytes and the adipogenesis. This notion can be supported by the ideas stated that insulin resistance in mice is related to adipose-derived rather than liver-derived RBP4 because manipulation of adipose-derived RBP4 enhanced insulin sensitivity without affecting serum RBP4 level (Yang et al, 2005) Ost et al (2007 added, all trans RA treatment down-regulated RBP4 expression in mice adipocytes not in liver while it enhanced glucose tolerance and insulin sensitivity of the animals. Although all of these evidence indicating a negative correlation between RBP4 expression and secretion from bovine adipocytes and adipogenesis, we found that RA decreased both adipogenesis and RBP4 expression and secretion in intramuscular adipocytes cultured in normal glucose medium.…”

“…These differences in RBP4 secretion from bovine adipocytes may have impact on the role of RBP4 in adipogenesis in bovine adipocytes. Because it was indicated that RBP4, especially that produced from adipose tissue, interferes with glucose uptake and insulin sensitivity in mice adipocytes (Yang et al, 2005) and attenuates insulin signaling in human adipocytes (Ost et al, 2007). Smith et al (2009) and our previous study (data under review) reported that glucose is essential for adipogenesis in bovine intramuscular adipocytes.…”

“…Glucose has an important role in adipogenesis in bovine intramuscular adipocytes as a substrate for fatty acid biosynthesis (Smith et al, 2009). RBP4 especially that produced from adipose tissue interferes with glucose uptake and utilization in adipocytes of rodents and human (Yang et al, 2005;Ost et al, 2007). In contrast, RA enhances glucose uptake by tissues through reducing RBP4 production, especially that from adipose tissue (Mercader et al, 2008;Manolescu et al, 2010).…”