MONO- OR DUAL-PHOSPHORYLATION OF AKT KINASE IS REGULATED BY DISTINCT RECEPTORS THAT INVOLVE THE COMMON INSULIN RECEPTOR SUBSTRATE

ABSTRACT

We have previously shown that the interleukin (IL)-4 signal transduction involves the Insulin Receptor Substrate (IRS) in human colorectal carcinoma cells LS513. In the present study it was tested whether IL-4 counters Insulin-like Growth Factor (IGF)-1 through competition at the IRS signal transduction pathway and, thus, induces a molecular “insulin resistance” or whether IL-4 invokes an alternative signal transduction. The activated receptors of IL-4 and IGF-1 both docked to IRS-1 and IRS-2 and invoked IRS complex formation with phosphatidylinositol (PI) 3-kinase, as assessed by immunoprecipitation and detection of the precipitated compounds by immunoblot analysis. Both, IL-4 and IGF-1, signaling pathways induced phosphorylation of Akt kinase in a PI 3-kinase-dependent manner, as assessed by addition of the PI 3-kinase inhibitor Ly294002. Interleukin-4 stimulation induced mono-phosphorylation at serine residue S473 of Akt kinase but failed to activate the kinase. Insulin-like growth factor-1 stimulation invoked dual-phosphorylation at S473 and T308 of Akt kinase and subsequent activation of the kinase. When LS513 cells were treated with IL-4 to induce mono-phosphorylation of Akt, dual-phosphorylation and activation of Akt kinase in response to IGF-1 were still intact. Interleukin-4 yet reduced cell growth by at least 50% both, in the absence and presence of growth factor IGF-1. In the LS513 cells, IL-4 stimulated phosphorylation of Jak2, an adapter molecule at the IL-4 receptor, and phosphorylation of transcription factor Stat6 both, in the absence and presence of IGF-1. We found a similar IL-4 signal transduction and growth suppression in multiple human cell cultures, including primary cells. Our findings indicate that the molecular mechanism underlying growth suppression by IL-4 may depend on gene-expression but not on “insulin/growth factor resistance” at IRS.

Key Words: :

• Receptor gamma-chair
• Cytokine receptor
• Receptor substrate
• Vascular smooth muscle
• Interleukin 13