IL-1β suppresses prolonged Akt activation and expression of E2F-1 and cyclin A in breast cancer cells

Cell cycle aberrations occurring at the G₁/S checkpoint often lead to uncontrolled cell proliferation and tumor growth. We recently demonstrated that IL-1β inhibits insulin-like growth factor (IGF) -I-induced cell proliferation by preventing cells from entering the S phase of the cell cycle, leading to G₀/G₁ arrest. Notably, IL-1β suppresses the ability of the IGF-I receptor tyrosine kinase to phosphorylate its major docking protein, insulin receptor substrate-1, in MCF-7 breast carcinoma cells. In this study, we extend this juxtamembrane cross-talk between cytokine and growth factor receptors to downstream cell cycle machinery. IL-1β reduces the ability of IGF-I to activate Cdk2 and to induce E2F-1, cyclin A, and cyclin A-dependent phosphorylation of a retinoblastoma tumor suppressor substrate. Long-term activation of the phosphatidylinositol 3-kinase/Akt signaling pathway, but not the mammalian target of rapamycin or mitogen-activated protein kinase pathways, is required for IGF-I to hyperphosphorylate retinoblastoma and to cause accumulation of E2F-1 and cyclin A. In the absence of IGF-I to induce Akt activation and cell cycle progression, IL-1β has no effect. IL-1β induces p21^Cip1/Waf1, which may contribute to its inhibition of IGF-I-activated Cdk2. Collectively, these data establish a novel mechanism by which prolonged Akt phosphorylation serves as a convergent target for both IGF-I and IL-1β; stimulation by growth factors such as IGF-I promotes G₁-S phase progression, whereas IL-1β antagonizes IGF-induced Akt phosphorylation to induce cytostasis. In this manner, Akt serves as a critical bridge that links proximal receptor signaling events to more distal cell cycle machinery.