Cell Biol. this kinase was present in the BT474 and SKBR3 cell lines, which also expressed activated forms of ErbB2, ErbB3, and ErbB4. Treatments aimed at decreasing the activity of these receptors caused Erk5 inactivation, indicating that the active form of Erk5 present in BT474 and SKBR3 cells was due to a persistent positive stimulus originating at the ErbB receptors. In BT474 cells expression of the dominant negative form of Erk5 resulted in reduced proliferation, indicating that in these cells Erk5 was also involved in the control of proliferation. Taken together, these results suggest that Erk5 may play a role in the regulation of cell proliferation by NRG receptors and indicate that constitutively active NRG receptors may induce proliferative responses in cancer cells through this MAPK pathway. Receptor tyrosine kinases of the ErbB family play essential functions in several physiological processes, such as cell growth (11, 36, 66), differentiation, and tissue development (8, 55, 61), and have been implicated in pathological processes, such as tumor generation and/or progression (36, 66). This Moxalactam Sodium family comprises four structurally related transmembrane receptors, the epidermal growth factor (EGF) receptor (EGFR or ErbB1/HER1), ErbB2 (neu/HER2), ErbB3 (HER3), and ErbB4 (HER4) (36, 66). Activation of ErbB receptors may occur by ligand binding (67, 68) or by overexpression of the receptor (36, 57), the latter mechanism being particularly relevant in certain pathologic instances such as malignancy (30, 62C64). Ligand-mediated activation of ErbB receptors occurs by interaction of the ectodomain of these receptors with specific members of the EGF family of ligands (11, 48). This family includes EGF, transforming growth factor , amphiregulin, betacellulin, and epiregulin, which preferentially bind to and activate the EGFR (3, 48, 65). A second group of EGF-like ligands, the neuregulins (NRGs), bind to ErbB3 and ErbB4 (6, 38, 53). Ligand-induced activation of ErbB receptors is usually complex and often includes oligomeric interactions between different ErbB receptors (19, 54). Thus, upon ligand binding, ErbB receptors oligomerize and this results in transphosphorylation of the receptors on tyrosine residues. While ligand-induced homooligomerization of EGFR or ErbB4 results in its activation, heterooligomerization is usually expected to Moxalactam Sodium play a major role in the function of ErbB3 and ErbB2. In fact, ErbB3 contains an inefficient kinase activity in its intracellular domain name that would prevent activation of ErbB3 homooligomers (32). On the other Moxalactam Sodium hand, ErbB2 does not bind any EGF family ligand with enough affinity, thus preventing its activation by ligands (65). Cooperation between these receptors has been exhibited in cell lines expressing Moxalactam Sodium ErbB2 and ErbB3 (29, 50, 52, 69). In these models, signal transduction occurs by the combined action of ErbB3 acting as a receptor for the ligand that is then presented to ErbB2, which acts as a signal transducer and phosphorylates ErbB3 in heterodimeric ErbB2-ErbB3 complexes. Tyrosine phosphorylation of specific residues within the intracellular domain name of the receptors results in the binding of signaling molecules with enzymatic activity or adaptor molecules that allow activation of specific intracellular targets (68). Important downstream pathways that are activated by these receptors and have been linked to the regulation of cell proliferation are the mitogen-activated protein kinase (MAPK) routes (58, 70). MAPK routes are characteristically organized into a three-kinase module that includes a MAPK; the upstream kinase MEK or MKK, which phosphorylates and activates the MAPK; and the MEK kinase, which is responsible for the activation of MEK (26). Three major MAPK pathways in mammals have been described (13). The extracellular signal-regulated kinase 1 (Erk1) and Erk2 (Erk1/2) route is activated by receptors for polypeptide growth factors (26), by G protein-coupled receptors (31), or by direct stimulation of intracellular pathways such as the protein kinase C messenger system (59). Growth factor receptor stimulation results in activation of the Mouse monoclonal to INHA Ras pathway, which then triggers the activity of Raf kinases, which phosphorylate and activate the dual-specificity MEKs responsible for the tyrosine and threonine phosphorylation and activation of Erk1/2 (46, 60). Erks then phosphorylate cytoplasmic substrates (26, 70) and are also translocated to the nucleus (27, 44). As a result of Erk translocation, several nuclear proteins, such as transcription factors (37, 58, 70) Moxalactam Sodium or other structural proteins.