ErbB2 is a receptor tyrosine kinase whose activity in normal cells depends on dimerization with another ligand-binding ErbB receptor. In contrast, amplification of c-erbB2 in tumors results in dramatic overexpression and constitutive activation of the receptor. Breast cancer cells overexpressing ErbB2 depend on its activity for proliferation, because treatment of these cells with ErbB2-specific antagonistic antibodies or kinase inhibitors blocks tumor cells in the G 1 phase of the cell cycle. Intriguingly, loss of ErbB2 signaling is accompanied by a decrease in the phosphotyrosine content of ErbB3. On the basis of these results, it has been proposed that ErbB3 might be a partner for ErbB2 in promoting cellular transformation. To test this hypothesis and directly examine the role of the ''kinase dead'' ErbB3, we specifically ablated its expression with a designer transcription factor (E3). By infection of ErbB2-overexpressing breast cancer cells with a retrovirus expressing E3, we show that ErbB3 is an essential partner in the transformation process. Loss of functional ErbB2 or ErbB3 has similar effects on cell proliferation and cell cycle regulators. Furthermore, expression of constitutively active protein kinase B rescues the proliferative block induced as a consequence of loss of ErbB2 or ErbB3 signaling. These results demonstrate that ErbB2 overexpression and activity alone are insufficient to promote breast tumor cell division. Furthermore, we identify ErbB3's role, which is to couple active ErbB2 to the phosphatidylinositol 3-kinase͞protein kinase B pathway. Thus, the ErbB2͞ErbB3 dimer functions as an oncogenic unit to drive breast tumor cell proliferation.T he family of ErbB receptor tyrosine kinases includes four members: epidermal growth factor (EGF) receptor͞ErbB1, ErbB2, ErbB3, and ErbB4. Binding of peptides of the EGFrelated growth factor family to the extracellular domain of ErbB receptors results in the formation of homo-and heterodimers. Ligand binding induces the intrinsic receptor kinase activity, ultimately leading to stimulation of intracellular signaling cascades (1, 2). The physiological role of ErbB2, in the context of ErbB ligand signaling, is to serve as a coreceptor (3, 4). In fact, ErbB2 appears to be the preferred partner of the other ligandbound ErbBs (5, 6). The importance of heterodimer-mediated signaling in normal development is obvious from studies in genetically modified mice. This is particularly true for ErbB2͞ ErbB3 and ErbB2͞ErbB4 heterodimers. Loss of ErbB2 or ErbB3 has a similar impact on neuronal development (7), whereas loss of ErbB2 or ErbB4 has major effects on heart development (8, 9).A wealth of clinical data has demonstrated that ErbB receptor tyrosine kinases, in particular ErbB1 and ErbB2, have roles in human cancer development, thus making them attractive targets for cancer therapies (10-13). ErbB2 overexpression, generally attributable to gene amplification, occurs in 25-30% of breast cancer and correlates with shorter time to relapse and lower overall survival (1...

The epidermal growth factor (EGF)-related peptides bind the ErbB receptors, inducing the formation of different homo- and heterodimers. Receptor dimerization promotes activation of the intrinsic kinase, leading to phosphorylation of specific tyrosines located in the ErbB's cytoplasmic region. These phosphorylated residues serve as docking sites for a variety of signaling molecules whose recruitment stimulates intracellular signaling cascades, which ultimately control diverse genetic programs. Particular ligand-receptor complexes have essential roles in embryonic development as well as in the adult. Finally, ErbB receptors are being pursued as therapeutic targets because aberrant ErbB activity has been observed in many human cancers. In this review, we discuss these data in more detail, illustrating the importance of tightly regulated ErbB signaling throughout life.

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BACKGROUNDSerelaxin is a recombinant form of human relaxin-2, a vasodilator hormone that contributes to cardiovascular and renal adaptations during pregnancy. Previous studies have suggested that treatment with serelaxin may result in relief of symptoms and in better outcomes in patients with acute heart failure. METHODSIn this multicenter, double-blind, placebo-controlled, event-driven trial, we enrolled patients who were hospitalized for acute heart failure and had dyspnea, vascular congestion on chest radiography, increased plasma concentrations of natriuretic peptides, mild-to-moderate renal insufficiency, and a systolic blood pressure of at least 125 mm Hg, and we randomly assigned them within 16 hours after presentation to receive either a 48-hour intravenous infusion of serelaxin (30 μg per kilogram of body weight per day) or placebo, in addition to standard care. The two primary end points were death from cardiovascular causes at 180 days and worsening heart failure at 5 days. RESULTSA total of 6545 patients were included in the intention-to-treat analysis. At day 180, death from cardiovascular causes had occurred in 285 of the 3274 patients (8.7%) in the serelaxin group and in 290 of the 3271 patients (8.9%) in the placebo group (hazard ratio, 0.98; 95% confidence interval [CI], 0.83 to 1.15; P = 0.77). At day 5, worsening heart failure had occurred in 227 patients (6.9%) in the serelaxin group and in 252 (7.7%) in the placebo group (hazard ratio, 0.89; 95% CI, 0.75 to 1.07; P = 0.19). There were no significant differences between the groups in the incidence of death from any cause at 180 days, the incidence of death from cardiovascular causes or rehospitalization for heart failure or renal failure at 180 days, or the length of the index hospital stay. The incidence of adverse events was similar in the two groups. CONCLUSIONSIn this trial involving patients who were hospitalized for acute heart failure, an infusion of serelaxin did not result in a lower incidence of death from cardiovascular causes at 180 days or worsening heart failure at 5 days than placebo. (Funded by Novartis Pharma; RELAX-AHF-2 ClinicalTrials.gov number, NCT01870778.

Overexpression of fibroblast growth factor receptor (FGFR) tyrosine kinases has been found in many human breast cancers and has been associated with poor patient prognosis. In order to understand the mechanism by which FGFR mediates breast cancer cell proliferation, we used a low molecular weight compound, PD173074, that selectively inhibits FGFR tyrosine kinase activity and autophosphorylation. This potential anticancer agent caused a G1 growth arrest of MDA-MB-415, MDA-MB-453 and SUM 52 breast cancer cells. Our analyses revealed that FGFR signaling links to the cell cycle machinery via D-type cyclins. PD173074-mediated inhibition of FGFR activity caused downregulation of cyclin D1 and cyclin D2 expression, inhibition of cyclin D/cdk4 activity and, as a consequence, reduction of pRB phosphorylation. Retroviral-mediated ectopic expression of cyclin D1 prevented pRB hypophosphorylation and the cell cycle G1 block in PD173074-treated cells, suggesting a central role for D cyclins in proliferation of FGFR-driven breast cancer cells. The repression of FGFR activity caused downregulation of MAPK in MDA-MB-415 and MDA-MB-453 cells. In SUM 52 cells, both MAPK and PI3K signaling pathways were suppressed. In conclusion, results shown here describe a mechanism by which FGFR promotes proliferation of breast cancer cells.

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Addition of the ErbB-ligand, Heregulinb1 (HRG), to breast tumour-derived T47D cells promotes D-cyclin expression, p21 cip1 synthesis, cyclin-dependent kinase (CDK) activation through re-distribution of p27 kip1 and DNA synthesis. In contrast EGF has no e ect on T47D cell cycle progression. By comparing these two ligands and the use of speci®c inhibitors for phosphatidylinositol-3 kinase (PI3K), mitogen-activated protein kinase (MAPK) and p38MAPK, we have identi®ed several molecular mechanisms required for ErbB receptormediated proliferation. The PI3K, MAPK and p38MAPK pathways each displayed distinct activation pro®les in response to either HRG or EGF, with obvious di erences in both the intensity and duration of signal output. Through inhibition of each of these pathways it is apparent that each pathway is necessary, yet insu cient alone, to stimulate proliferation. Each pathway regulates distinct subsets of essential cell cycle regulators and integration of these signal networks is required for the timely expression of these components, which culminates in cell cycle progression. Signi®cantly, the mechanisms controlling ligand-stimulated proliferation through ErbB2 are strikingly similar to the mechanisms through which overexpressed, constitutively activated, ErbB2 orchestrates uncontrolled proliferation in cancer cells. This suggests that downstream e ectors of ErbB receptors represent good therapeutic targets for breast cancer.