How do growth factors influence the cell cycle?

Growth factor signaling regulates cell growth and survival in the G0/G1 phase of the cell cycle. Once past the G1 phase, cells are committed to division and do not rely on the presence of growth factors until they re-enter G0/G1. These growth factors regulate the cell cycle by initiation of different signaling pathways: Ras/Erk pathway and PI3/Akt pathway. These pathways are regulated by the binding of growth factors to its corresponding epidermal growth factor receptor. In the Ras/Erk pathway, the phosphorylated epidermal growth factor receptor generates a cascade of events to activate Ras, which subsequently activates Raf. Raf then phosphorylates and activates protein kinase, which subsequently activates Erk. Erk then promotes the activation and translocation of RSK. The PI3k/Akt pathway generates PIP3 upon binding of the ligand to its EGFR. Then, proteins such as Akt are recruited to activate a series of processes to prevent cell death. During the G1 phase, growth factors stimulate cells to enter the cell cycle from the G0 phase and initiate the transition from G1 to S phase. This is done by promoting the expression of proteins and enzymes required for DNA replication. Growth factors do not control S-phase progression. After successful completion of DNA synthesis, cells enter the G2 phase to prepare for mitosis. Protein kinases that are activated in G2 phase prevent re-replication of DNA. Mammalian cells are typically diploid. Therefore, after DNA replication a cell must divide. Thus, control by growth factors is also unnecessary in the G2 phase.