title: Phosphoinositide 3-kinase activation in late G1 is required for c-Myc stabilization and S phase entry.
abstract: Phosphoinositide 3-kinase (PI3K) is one of the early-signaling molecules induced by growth factor (GF) receptor stimulation that are necessary for cell growth and cell cycle entry. PI3K activation occurs at two distinct time points during G(1) phase. The first peak is observed immediately following GF addition and the second in late G(1), before S phase entry. This second activity peak is essential for transition from G(1) to S phase; nonetheless, the mechanism by which this peak is induced and regulates S phase entry is poorly understood. Here, we show that activation of Ras and Tyr kinases is required for late-G(1) PI3K activation. Inhibition of late-G(1) PI3K activity results in low c-Myc and cyclin A expression, impaired Cdk2 activity, and reduced loading of MCM2 (minichromosome maintenance protein) onto chromatin. The primary consequence of inhibiting late-G(1) PI3K was c-Myc destabilization, as conditional activation of c-Myc in advanced G(1) as well as expression of a stable c-Myc mutant rescued all of these defects, restoring S phase entry. These results show that Tyr kinases and Ras cooperate to induce the second PI3K activity peak in G(1), which mediates initiation of DNA synthesis by inducing c-Myc stabilization.
authors.affiliation: Department of Immunology and Oncology, Centro Nacional de Biotecnología/CSIC, Darwin 3, Campus de Cantoblanco, Madrid E-28049, Spain.
authors.forename: Amit
authors.initials: A
authors.lastname: Kumar
authors.forename: Miriam
authors.initials: M
authors.lastname: Marqués
authors.forename: Ana C
authors.initials: AC
authors.lastname: Carrera
mesh_terms: D000818:Animals; D018929:Cell Culture Techniques; D003470:Culture Media / Q000737:chemistry; D016895:Culture Media, Serum-Free; D004789:Enzyme Activation; D016193:G1 Phase; D051379:Mice; D041681:NIH 3T3 Cells; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000502:physiology*; D011960:Receptors, Estrogen / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012190:Retroviridae / Q000235:genetics; D016196:S Phase; D018631:ras Proteins / Q000378:metabolism
chemical_list: D003470:Culture Media; D016895:Culture Media, Serum-Free; D016271:Proto-Oncogene Proteins c-myc; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D019869:Phosphatidylinositol 3-Kinases; D011505:Protein-Tyrosine Kinases; D018631:ras Proteins