Dissecting the Mechanism of Insulin Resistance Using a Novel Heterodimerization Strategy to Activate Akt

Abstract

Insulin resistance can occur in response to many different external insults, including chronic exposure to insulin itself as well as other agonists such as dexamethasone. It is generally thought that such defects arise due to a defect(s) at an early stage in the insulin signaling cascade. One model suggests that this involves activation of the mammalian target of rapamycin/S6 kinase pathway, which inactivates insulin receptor substrate via Ser/Thr phosphorylation. However, we have recently shown that insulin receptor substrate is not a major node for insulin resistance defects. To explore the mechanism of insulin resistance, we have developed a novel system to activate Akt independently of its upstream effectors as well as other insulinresponsive pathways such as mitogen-activated protein kinase. 3T3-L1 adipocytes were rendered insulin-resistant either with chronic insulin or dexamethasone treatment, but conditional activation of Akt2 stimulated hemagglutinin-tagged glucose transporter 4 translocation to the same extent in these insulinresistant and control cells. However, addition of insulin to cells in which Akt was conditionally activated resulted in a reversion to the insulin-resistant state, indicating a feedforward inhibitory mechanism activated by insulin itself. This effect was overcome with wortmannin, implicating a role for phosphatidylinositol 3-kinase in this inhibitory process. We conclude that in chronic insulin-and dexamethasone-treated cells, acute activation with insulin itself is required to activate a feedforward inhibitory pathway likely emanating from phosphatidylinositol 3-kinase that converges on a target downstream of Akt to cause insulin resistance.

Antipsychotics-induced metabolic alterations: Focus on adipose tissue and molecular mechanisms

“…At the cellular level, development of insulin resistance is associated with an impairment in the insulin signaling pathway. More specifically, impaired GLUT4 translocation into the cell membrane has been attributed to defects in signaling involving the insulin receptor substrate 1 (IRS-1) (Hotamisligil et al, 1996;Morino et al, 2008), Akt (also known as protein kinase B (PKB)) (Teruel et al, 2001;Tonks et al, 2013) and effectors downstream of Akt (Ng et al, 2010). Adipose tissue also influences glucose homeostasis indirectly by regulating lipid homeostasis (Walther and Farese, 2012) and several factors related to lipid homeostasis have been proposed to contribute also to insulin resistance.…”

Selective Insulin Resistance in Adipocytes

“…To investigate the potential selectivity of insulin resistance in adipocytes, we first examined a range of insulin-regulated processes in 3T3-L1 adipocytes rendered insulin-resistant via either exposure to high levels of insulin (CI), glucocorticoids (DEX), or inflammatory cytokines (TNF␣) (24,31). In all three of these model systems, insulin-stimulated glucose transport was significantly blunted (Fig.…”

Optogenetic activation reveals distinct roles of PIP3 and Akt in adipocyte insulin action

“…Knockdown and knockout studies have shown that Akt, particularly the Akt2 isoform, is required for insulin-stimulated GLUT4 translocation (Cho et al, 2001a,b;Jiang et al, 2003;Katome et al, 2003;McCurdy and Cartee, 2005). Studies in which constitutively active Akt2 has been overexpressed, or in which chemical-genetic approaches have been used to activate Akt2 acutely, suggest that Akt2 alone is fully sufficient to stimulate GLUT4 translocation (Kohn et al, 1996;Ng et al, 2010bNg et al, , 2008. However, comparison of GLUT4 translocation after acute Akt2 activation has been made only upon submaximal insulin stimulation, and it remains unclear how the data fit with the less-well-studied non-PI3K and nonAkt signals that are required for insulin-stimulated GLUT4 translocation Chang et al, 2007;Chiang et al, 2001;Farese et al, 2007;Klip et al, 2014;Sajan et al, 2014b;Sylow et al, 2014;Ueda et al, 2010;Cheney et al, 2011;Govers et al, 2004;Martinez et al, 2010).…”