Systems‐level interactions between insulin–EGF networks amplify mitogenic signaling

Abstract

Crosstalk mechanisms have not been studied as thoroughly as individual signaling pathways. We exploit experimental and computational approaches to reveal how a concordant interplay between the insulin and epidermal growth factor (EGF) signaling networks can potentiate mitogenic signaling. In HEK293 cells, insulin is a poor activator of the Ras/ERK (extracellular signal-regulated kinase) cascade, yet it enhances ERK activation by low EGF doses. We find that major crosstalk mechanisms that amplify ERK signaling are localized upstream of Ras and at the Ras/Raf level. Computational modeling unveils how critical network nodes, the adaptor proteins GAB1 and insulin receptor substrate (IRS), Src kinase, and phosphatase SHP2, convert insulin-induced increase in the phosphatidylinositol-3,4,5-triphosphate (PIP 3 ) concentration into enhanced Ras/ERK activity. The model predicts and experiments confirm that insulin-induced amplification of mitogenic signaling is abolished by disrupting PIP 3 -mediated positive feedback via GAB1 and IRS. We demonstrate that GAB1 behaves as a non-linear amplifier of mitogenic responses and insulin endows EGF signaling with robustness to GAB1 suppression. Our results show the feasibility of using computational models to identify key target combinations and predict complex cellular responses to a mixture of external cues.

Complexity of Receptor Tyrosine Kinase Signal Processing

“…The data and computational modeling results suggest that major cross talk mechanisms that amplify ERK signaling by insulin are localized upstream of Ras and at the Ras/Raf level (Borisov et al 2009). Among the variety of cross talk interactions affecting multiple Ras activation and inactivation routes, five key network nodes are shown to be crucial for the EGFinsulin synergy.…”

“…RTK cross talk can also be mediated solely by downstream signaling interactions without affecting the receptors themselves. In this case, costimulation by two ligands, which often occurs in vivo, can lead to synergic or mutually inhibiting responses (Borisov et al 2009;Martin et al 2009). …”

“…Feedforward and feedback loops embracing interacting RTK pathways make the input -output response relationships of one pathway dependent on the activity of the other pathway, thereby creating context-dependent signaling output. A recent study explored how a concordant interplay between RTK pathways stimulated by EGF and insulin can potentiate signaling by the ERK/MAPK at physiological, low EGF levels in HEK293 cells (Borisov et al 2009). Although the EGFR and IR networks share many downstream components, their responses to cognate stimuli are different.…”

“…The main insulin function is metabolic, including the control of glucose metabolism, and stimulation of protein and lipid syntheses. Using a combined experimental and computa-tional analysis, it was shown that at low physiological concentration of EGF, cross talk between the EGFR and IR networks converts the insulin-induced increase in the PIP 3 concentration into enhanced ERK activation, whereas at saturating EGF levels the insulin effect becomes insignificant (Borisov et al 2009). …”

Insulin signaling dynamics in human adipocytes : Mathematical modeling reveals mechanisms of insulin resistance in type 2 diabetes

“…One such target is Ets-like gene 1 (Elk1) [92], a nuclear localized transcription factor that is phosphorylated at multiple residues, whereof Ser 383 and Thr 363 have been shown to be important for transcriptional activity [93,94]. The ERK signaling branch is highly activated by growth factors like the epidermal growth factor (EGF), but is also activated to a smaller extent by insulin [95,96]. The insulin stimulated phosphorylation of ERK occurs via the proteins Shc [97] and/or IRS1, which both are phosphorylated by the insulin receptor in response to insulin [98] (Figure 12).…”