The Akt substrate Girdin is a regulator of insulin signaling in myoblast cells

Abstract

Akt kinases are important mediators of the insulin signal, and some Akt substrates are directly involved in glucose homeostasis. Recently, Girdin has been described as an Akt substrate that is expressed ubiquitously in mammals. Cells overexpressing Girdin show an enhanced Akt activity. However, not much is known about Girdin's role in insulin signaling. We therefore analyzed the role of Girdin in primary human myotubes and found a correlation between Girdin expression and insulin sensitivity of the muscle biopsy donors, as measured by a hyperinsulinemic-euglycemic clamp. To understand this finding on a cellular level, we then investigated the function of Girdin in C2C12 mouse myoblasts. Girdin knock-down reduced Akt and insulin receptor substrate-1 phosphorylation. In contrast, stable overexpression of Girdin in C2C12 cells strikingly increased insulin sensitivity through a massive upregulation of the insulin receptor and enhanced tyrosine phosphorylation of insulin receptor substrate-1. Furthermore, Akt and c-Abl kinases were constitutively activated. To investigate medium-term insulin responses we measured glucose incorporation into glycogen. The Girdin overexpressing cells showed a high basal glycogen synthesis that peaked already at 1nM insulin. Taken together, we characterized Girdin as a new and major regulator of the insulin signal in myoblasts and skeletal muscle.

Heterotrimeric G protein signaling via GIV/Girdin: Breaking the rules of engagement, space, and time

“…Finally, in the case of type II diabetes levels of GIV expression correlate with insulin sensitivity, high GIV equals better metabolic insulin response . Additionally, the degree of phosphoinhibition of GIV's GEF function (by PKCθ at S1674) in skeletal muscle biopsies of patients with insulin resistance correlated with clinical response to treatment with insulin sensitizer thiazolidinediones : high degrees of pre‐treatment inhibition or post‐treatment residual inhibition was associated with little or no clinical response to the anti‐diabetic drug.…”

The untapped potential of tyrosine-based G protein signaling

“…More importantly, efforts to target these pathways has been rewarded with tremendous success: GPCRs represent the target for ~40% of currently marketed drugs (60), whereas PTKs, both receptor and non-receptor TKs are the targets for another ~15% of drugs (61, 62). Therefore, it is not surprising that a signaling pathway that functions at the crossroads of tyrosine-based signaling and G proteins impacts a rapidly growing list of diseases, e.g., multiple aspects of cancer progression [tumor cell migration, invasion and metastasis (50, 56, 63–71); stemness (72), drug resistance (73), tumor-stroma interplay (74), tumor angiogenesis (75)], organ (liver) fibrosis (76), dermal wound healing (68), nephrotic syndrome (77), insulin resistance/type II diabetes (78, 79), disorders of the blood vessels (80–83), and neuronal plasticity and memory formation (84). The role of GIV’s GEF function has been investigated in some, but not all of these disease states [summarized in (8)], and where investigated, a clear therapeutic goal has emerged (see Figure 3).…”

“…Finally, in the case of type II diabetes, levels of GIV expression correlate with insulin sensitivity: high GIV, equals better metabolic insulin response (78). Additionally, the degree of phosphoinhibition of GIV’s GEF function (by PKCθ at S1674) in skeletal muscles biopsies of patients with insulin resistance correlated with clinical response to treatment with insulin sensitizer Thiazolidinediones (86): high degrees of pre-treatment inhibition or post-treatment residual inhibition was associated with little or no clinical response to the anti-diabetic drug.…”

“…Thus, the pharmacologic goal to halt and reverse fibrogenic progression is to selectively turn ‘OFF’ GIV’s GEF function (using small molecule inhibitors). C) Schematic summarizing our and others’ work (78, 86, 91) on how GIV regulates several steps of the insulin signaling cascade, e.g., receptor autophosphorylation, the recruitment and activation of IRS1, activation of the PI3K-Akt signaling pathway, exocytosis of the glucose transporter, GLUT4 and glucose uptake into skeletal muscles, all of which together constitute the metabolic insulin response. While an intact and efficient metabolic insulin response is required for physiology (i.e., insulin sensitivity), aberrant signals in response to circulating free fatty acids (among other causes) dampen such metabolic response and contribute to insulin resistance, the hallmark of type II diabetes.…”

“…While an intact and efficient metabolic insulin response is required for physiology (i.e., insulin sensitivity), aberrant signals in response to circulating free fatty acids (among other causes) dampen such metabolic response and contribute to insulin resistance, the hallmark of type II diabetes. The metabolic insulin response is driven more efficiently and consequently, the insulin sensitivity is highest when GIV expression is elevated (78) and its GEF function is ‘ON’ (86). Fatty acids trigger insulin resistant states by phosphoinhibiting GIV’s GEF function via the DAG→PKCθ pathway, whereas insulin sensitizers like Pioglitazone act by potentiating tyrosine phosphorylation of GIV and by reversing phosphoinhibition and restoring GIV’s GEF function.…”