Inhibition of MEK1 Signaling Pathway in the Liver Ameliorates Insulin Resistance.

Although mitogen-activated protein kinase kinase (MEK) is a key signaling molecule and a negative regulator of insulin action, it is still uncertain whether MEK can be a therapeutic target for amelioration of insulin resistance (IR) in type 2 diabetes (T2D) in vivo. To clarify whether MEK inhibition improves T2D, we examined the effect of continuous MEK inhibition with two structurally different MEK inhibitors, RO5126766 and RO4987655, in mouse models of T2D. RO5126766 and RO4987655 were administered via dietary admixture. Both compounds decreased blood glucose and improved glucose tolerance in doses sufficient to sustain inhibition of extracellular signal-regulated kinase (ERK)1/2 phosphorylation downstream of MEK in insulin-responsive tissues in db/db mice. A hyperinsulinemic-euglycemic clamp test showed increased glucose infusion rate (GIR) in db/db mice treated with these compounds, and about 60% of the increase was attributed to the inhibition of endogenous glucose production, suggesting that the liver is responsible for the improvement of IR. By means of adenovirus-mediated Mek1 shRNA expression, we confirmed that blood glucose levels are reduced by suppression of MEK1 expression in the liver of db/db mice. Taken together, these results suggested that the MEK signaling pathway could be a novel therapeutic target for novel antidiabetic agents.

The AP-1 complex regulates intracellular localization of insulin receptor substrate 1, which is required for insulin-like growth factor I-dependent cell proliferation.

The activation of the insulin/insulin-like growth factor I (IGF-I) receptor and the subsequent tyrosine phosphorylation of insulin receptor substrates (IRSs) are key initial events in a variety of insulin/IGF bioactivities, including mitogenesis. It has been reported that IRS-1 associates with intracellular membrane compartments, and this localization is believed to be important for insulin/IGF signal transduction. However, the molecular mechanisms underlying IRS-1 localization remain unclear. Here we show that in L6 myoblasts, IRS-1 associates with µ1A of the ubiquitously expressed AP-1 complex, which packages cargo proteins into clathrin-coated vesicles derived from intracellular membranes. While wild-type IRS-1 was predominantly localized to vesicular structures, IRS-1 mutants lacking three YXXΦ motifs responsible for binding to µ1A were mislocalized to the mannose-6-phosphate receptor-positive structures, suggesting that AP-1-dependent transport to peripheral vesicles is inhibited in these mutants. Furthermore, deletion of AP-1 binding sites in IRS-1 impaired IGF-I-induced cell proliferation, accompanied by reduced tyrosine phosphorylation of IRS-1 and its association with phosphoinositide (PI) 3-kinase. These data demonstrate the importance of AP-1-dependent localization of IRS-1 in mediating IGF-I-stimulated signaling and maximum mitogenic response.