Extracellular-Regulated Kinases: Signaling From Ras to ERK Substrates to Control Biological Outcomes

Abstract

The extracellular-regulated kinases ERK1 and ERK2 are evolutionarily conserved, ubiquitous serine-threonine kinases that are involved in regulating cellular signaling in both normal and pathological conditions. Their expression is critical for development and their hyperactivation is a major factor in cancer development and progression. Since their discovery as one of the major signaling mediators activated by mitogens and Ras mutation, we have learned much about their regulation, including their activation, binding partners and substrates. In this review I will discuss some of what has been discovered about the members of the Ras to ERK pathway, including regulation of their activation by growth factors and cell adhesion pathways. Looking downstream of ERK activation I will also highlight some of the many ERK substrates that have been discovered, including those involved in feedback regulation, cell migration and cell cycle progression through the control of transcription, pre-mRNA splicing and protein synthesis.

Keywords: Adhesion; ERK; Feedback; MEK; Proliferation; RAS; Raf; Signaling; Substrates.

Figure 1

Schematic showing the activation of the Ras to ERK pathway by growth factor binding to a receptor tyrosine kinase. Ligand binding induces receptor dimerization and autophosphorylation. The Grb2 adapter protein binds to activated receptors and increases association of the guanine nucleotide exchange factor Sos to Ras, resulting in Ras loading of GTP and activation. Ras enhances membrane recruitment and activation of the Raf protein kinases, which activate MEKs, leading to ERK activation. Once activated, ERKs phosphorylate cytoplasmic substrates and translocate to the nucleus to phosphorylate nuclear targets.

Figure 2

ERK signaling generates multiple biological responses through direct phosphorylation of its protein substrates.

Figure 3

Activation of ERK results in negative feedback regulation of pathway activation by direct phosphorylation of upstream components of the pathway, namely MEK1, B-Raf and C-Raf, and Sos.