The Ras-ERK and PI3K-mTOR pathways: cross-talk and compensation

Abstract

The Ras-extracellular signal-regulated kinase (Ras-ERK) and phosphatidylinositol 3-kinase-mammalian target of rapamycin (PI3K-mTOR) signaling pathways are the chief mechanisms for controlling cell survival, differentiation, proliferation, metabolism, and motility in response to extracellular cues. Components of these pathways were among the first to be discovered when scientists began cloning proto-oncogenes and purifying cellular kinase activities in the 1980s. Ras-ERK and PI3K-mTOR were originally modeled as linear signaling conduits activated by different stimuli, yet even early experiments hinted that they might intersect to regulate each other and co-regulate downstream functions. The extent of this cross-talk and its significance in cancer therapeutics are now becoming clear.

Figure 1

The Core Pathway Components. The RAS-MAPK and PI3K-mTOR pathways respond to extracellular and intracellular cues to control cell survival, proliferation, motility, and metabolism. (a) The Ras-ERK-MAPK Pathway. In quiescent cells, inactive Ras-GDP associates with the plasma membrane and inactive Raf, MEK, and ERK are largely cytoplasmic. GF (growth factor) binding activates RTK auto-phosphorylation, generating binding sites for the SHC and GRB2 adaptor molecules that recruit SOS, the RasGEF (GTPase exchange factor), to the membrane. SOS catalyzes Ras GTP exchange and Ras-GTP then recruits Raf to the membrane, where it gets activated [1]. HNC (polypeptide hormone, neurotransmitter, and chemokine) activation of GPCRs feed into the MAPK cascade by trans-activating upstream RTKs, thereby inducing SOS translocation, and/or Raf activation [2]. Cell-permeable phorbol esters such as PMA directly bind and activate PKC by mimicking the natural PKC ligand diacylglycerol. The mechanism by which PKC activates ERK is not resolved and could be through activation of SOS or Raf [2]. Raf activates MEK and MEK activates ERK via activation loop phosphorylation. ERK also feeds back to negatively regulate the pathway. (b) The PI3K-mTOR Pathway. In quiescent cells, the lipid phosphatase PTEN maintains low levels of PIP3, resulting in AKT inactivation. TSC2, in complex with TSC1, maintains RHEB in the GDP-bound state. Insulin and IGF1 bind their cognate RTKs, and subsequent receptor autophosphorylation creates binding sites that then recruit IRS, an adaptor protein for PI3K. Different RTKs activate PI3K through distinct docking proteins, such as FRS (FGF Receptor Substrate) or GAB (c-Met or EGFR), or via direct binding of PI3K (Platelet-derived Growth Factor Receptor) [11]. Activated PI3K phosphorylates PIP2 to generate membrane-bound PIP3. Pleckstrin homology (PH) domains in AKT and PDK1 recognize PIP3 and translocate to the membrane. PDK1 phosphorylates the activation loop and mTORC2 phosphorylates the hydrophobic motif of AKT, thus promoting AKT activation and phosphorylation of TSC2. This TSC2 phosphorylation inhibits TSC2 GAP activity. RHEB-GTP localizes to the lysosome and activates mTORC1 following its recruitment by the Rag GTPases [6].

Figure 2

Pathway Crosstalk. The Ras-MAPK and PI3K-mTORC1 pathways regulate each other via cross-inhibition (red) and cross-activation (green). Each pathway has a mechanism to negatively feed onto the other: ERK phosphorylation of GAB and AKT phosphorylation of Raf. Components of the Ras-ERK pathway (Ras, Raf, ERK, and RSK) also positively regulate the PI3K-mTORC1 pathway. TSC2 and mTORC1 are key integration points that receive many inputs from both the Ras-ERK and PI3K signaling. Positive regulation of the substrate protein is shown as an arrow. Negative regulation of the substrate protein is depicted as a blunt-ended line.

Figure 3

Pathway Convergence and AGC Kinase Promiscuity. ERK and the AGC kinases often regulate the same substrates to bring about the same phenotypic effects. Situations in which the same residue is phosphorylated by multiple AGC kinases are symbolized in bold blue. Representative substrates for different combination of ERK and AGC kinase inputs are depicted in purple. FOXO and GSK3 are examples of Ras-ERK and PI3K-mTORC1 convergence at different residues on the same substrate via ERK and AGC kinase inputs. The Myc–Mad1 and Myc–Max dimers are examples of the two pathways converging on different members of the same complex. BAD and S6 are examples of AGC kinases regulating different motifs in the same substrate. YB1 and ERα are examples of AGC kinase promiscuity, in which several AGC kinases phosphorylate the same residue. For simplicity, inputs from SGK and PKC are not included.