JNK(MAPK8-10), RAC1, EGFR, EPS8, c-CBL, K-RAS, MLK2(MAP3K10), PtdIns(4,5)P2, c-Myc, GRB2, ERBB2, SOS, H-Ras, MEK1/2, SOS, 188.8.131.52, MKK4/7, p120GAP, RNTRE, Shc, PI3K reg class IA, N-RAS, DOK2, c-Fos, E3b1(ABI-1), PtdIns(3,4,5)P3, EGF, PI3K cat class IA, c-Raf-1, Elk-1, Rab-5A, ERK1/2, VAV-2
EGFR signaling via small GTPase
The Epidermal growth factor receptor ( EGFR ) belongs to the ERBB family of receptor tyrosine kinases, which consists of four closely related members: EGFR and v-erb-b2 erythroblastic leukemia viral oncogene homolog 2, neuro/glioblastoma derived oncogene homolog ( ERBB2 ), ERBB3 and ERBB4. These receptors couple binding of extracellular growth factor ligands to intracellular signaling pathways and regulate diverse biologic responses, including proliferation, differentiation, cell motility, and survival .
Six EGFR ligands have been identified including Epidermal growth factor ( EGF ), Amphiregulin, TGF-alpha; Betacellulin, HB-EGF (heparin binding EGF-like growth factor), and Epiregulin .
ERBB2 is a unique member of the ERBB family in that it does not bind any of the known ligands with high affinity, but it is the preferred heterodimeric partner for other EGFR .
The ligand-induced receptor dimerization and subsequent autophosphorylation of distinct tyrosine residues creates docking sites for various membrane-targeted proteins, including adaptor proteins Growth factor receptor-bound protein 2 ( Grb2 ), Cas-Br-M (murine) ecotropic retroviral transforming sequence ( c-Cbl ), GRB2-associated binding protein 1 ( GAB1 ), SHC (Src homology 2 domain containing) transforming protein 1 ( Shc ), Docking protein 2, 56kDa ( DOK2 ), and Epidermal growth factor receptor pathway substrate 8 ( EPS8 ) . Adaptor proteins mediate various signaling cascades, initiated by EGFR.
A set of cascades are mediated by small GTPases. One such pathway leads to MAP kinase activation. This pathway involves Shc, Grb2, Son of Sevenless homologs ( SOS ), Harvey rat sarcoma viral oncogene homolog ( H-Ras ), Neuroblastoma RAS viral (v-ras) oncogene homolog ( N-Ras), v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog ( K-Ras ).
Adaptors Shc and Grb2 recruit exchange factors on of SOS, forming protein complex consisting of Shc/ Grb2/ SOS. Activated SOS activates small GTPases H-RAS, N-RAS, K-RAS by convertingit from inactive GDP-bound state to active GTP-bound state. Activated RAS-proteins stimulate v-raf-1 murine leukemia viral oncogene homolog 1 ( c-Raf-1 )/ Mitogen-activated protein kinase kinases 1 and 2 ( MEK1(MAP2K1) MEK2(MAP2K2) ) / Mitogen-activated protein kinase 1-3 ( ERK1/2 ) cascade, which lead to activation transcription factors ELK1, member of ETS oncogene family ( Elk-1 ), v-myc myelocytomatosis viral oncogene homolog ( c-Myc ), v-fos FBJ murine osteosarcoma viral oncogene homolog ( c-Fos ) .
The adaptor DOK2 associates with the GTPase-activating protein RAS p21 protein activator (GTPase activating protein) 1 ( p120GAP ), which reinforces intrinsic GTPase activity of RAS-proteins, thereby inactivating them. Thus DOK2 could attenuate EGF -stimulated mitogen-activated protein (MAP) kinase activation .
Another pathway involving the Ras-related C3 botulinum toxin substrate 1 ( Rac1 ), includes mitogen-induced cytoskeletal changes and Mitogen-activated protein kinases 8-10 ( JNK(MAPK8-10) ). EPS8 is a substrate of the EGFR. EPS8 complexes with SOS by the Abl-interactor 1 ( E3b1(ABI-1) ) and mediates activation of Rac1.
EGFRs regulate their own internalization via GTP-binding proteins Rab-family. EPS8 interacts with USP6 N-terminal like ( RNTRE ) , which is a RAB5A, member RAS oncogene family ( Rab-5A ) GTPase-activating protein. By entering in a complex with EPS8, RNTRE acts on Rab-5A and inhibits internalization of the EGFR. Furthermore, RNTRE diverts EPS8 from its Rac-activating function, resulting in the attenuation of Rac signaling. Thus, depending on its state of association with E3b1(ABI-1) or RNTRE, EPS8 participates in both EGFR signaling through Rac1, and trafficking through Rab-5A .
Another pathway mediated by Rac1 activation includes the activation of the Phosphatidylinositol 3-kinase (PI3K) cascade. EGF stimulation induces association of c-Cbl, complexed to the adapter protein Grb2, with Phosphoinositide-3-kinase, regulatory ( PI3K reg class IA ) thereby activating Phosphoinositide-3-kinase catalytic subunit ( PI3K cat class IA ) .
Activated PI3K cat class IA converts inositol 4,5-biphosphate ( PtdIns(4,5)P2 ) into inositol 3,4,5-trisphosphate ( PtdIns(3,4,5)P3 ), which is a secondary messenger involved in regulation various processes . PtdIns(3,4,5)P3 associates with the inner lipid bilayer of the plasma membrane promoting the recruitment of proteins with pleckstrin homology (PH) domains, including the Vav 2 guanine nucleotide exchange factor ( VAV-2 ), which activates the Rho family of Ras-related GTPases, such as Rac1 .