ABSTRACT
Recent studies have shown that hyperinsulinemia may increase the cancer risk. Moreover, many tumors demonstrate an increased activation of IR signaling pathways. Phosphatidylinositol 3-kinase (PI3K) is necessary for insulin action. In epithelial cells, which do not express GLUT4 and gluconeogenic enzymes, insulin-mediated PI3K activation regulates cell survival, growth, and motility. Although the involvement of the regulatory subunit of PI3K (p85α (PI3K)) in insulin signal transduction has been extensively studied, the function of its N-terminus remains elusive. It has been identified as a serine (S83) in the p85α (PI3K) that is phosphorylated by protein kinase A (PKA). To determine the molecular mechanism linking PKA to insulin-mediated PI3K activation, we used p85α (PI3K) mutated forms to prevent phosphorylation (p85A) or to mimic the phosphorylated residue (p85D). We demonstrated that phosphorylation of p85α (PI3K)S83 modulates the formation of the p85α (PI3K)/IRS-1 complex and its subcellular localization influencing the kinetics of the insulin signaling both on MAPK-ERK and AKT pathways. Furthermore, the p85α (PI3K)S83 phosphorylation plays a central role in the control of insulin-mediated cell proliferation, cell migration, and adhesion. This study highlights the p85α (PI3K)S83 role as a key regulator of cell proliferation and motility induced by insulin in MCF-7 cells breast cancer model.
Subject(s)
Cell Movement , Class Ia Phosphatidylinositol 3-Kinase/metabolism , Cyclic AMP-Dependent Protein Kinases/metabolism , Insulin/metabolism , Cell Movement/drug effects , Cell Proliferation , Cell Survival , Humans , Insulin/pharmacology , Insulin Receptor Substrate Proteins/metabolism , Intracellular Space/metabolism , MCF-7 Cells , Phosphorylation , Protein Binding , Protein Transport , Signal Transduction/drug effectsABSTRACT
Dual-specificity phosphatase 6 (DUSP6, mitogen-activated protein kinase (MAPK) phosphatase 3 or PYST1) dephosphorylates phosphotyrosine and phosphothreonine residues on extracellular signal-regulated kinase (ERK1/2; MAPK1/2) to inactivate the ERK1/2 kinase. DUSP6 is a critical regulator of the ERK signaling cascade and has been implicated as a tumor suppressor. We report here experimental evidences that DUSP6 is transcriptionally upregulated in primary and long-term cultures of human glioblastoma, as assayed by northern hybridization and real-time quantitative PCR, producing constitutive high level of protein expression. Functional assays were performed with adenovirus-mediated expression of DUSP6 in glioblastoma cultures. Protein overexpression inhibits growth by inducing G1-phase delay and increased mitogenic/anchorage dependence and clonogenic potential in vitro. Changes in cell morphology were associated with an increased tumor growth in vivo. Chemoresistance is a major cause of treatment failure and poor outcome in human glioblastomas. Importantly, DUSP6 overexpression increased resistance to cisplatin-mediated cell death in vitro and in vivo. Antisense-mediated depletion of DUSP6 acted in lowering the threshold to anticancer DNA-damaging drugs. We conclude that upregulation of DUSP6 exerts a tumor-promoting role in human glioblastomas exacerbating the malignant phenotype.