EBP50 phosphorylation by Cdc2/Cyclin B kinase affects actin cytoskeleton reorganization and regulates functions of human breast cancer cell line MDA-MB-231

The actin cytoskeleton plays an important role in cell shape determination, adhesion and cell cycle progression. Ezrinradixin-moesin (ERM)-binding phosphoprotein 50 (EBP50), also known as Na(+)-H(+) exchanger regulatory factor 1 (NHERF1), associates with actin cytoskeleton and is related to cell cycle progression. Its Ser279 and Ser301 residues are phosphorylated by cyclin-dependent kinase 2 (cdc2)/cyclin B during the mitosis phase. However, the biological significance of EBP50 phosphorylation mediated by cdc2/cyclin B is not clear. In the present study, MDA-MB-231 cells with low levels of endogenous EBP50 protein were stably transfected with constructs of EBP50 wild type (WT), phosphodeficient (serine 279 and serine 301 mutated to alanine-S279A/S301A) or phospho-mimetic (serine 279 and serine 301 mutated to aspartic acid-S279D/S301D) mutants. Subsequently, multiple phenotypes of these cells were characterized. Failure of cdc2/cyclin B-mediated EBP50 phosphorylation in cells expressing S279A/S301A (AA cells) significantly increased F-actin content, enhanced the adherence of cells to the extracellular matrix, altered cell morphology and caused defects in cytokinesis, as reflected in the formation of giant cells with heteroploid DNA and multinucleation or giant nuclei. Furthermore, knockdown of EBP50 expression in AA cells rescued cell defects such as the cytokinesis failure and abnormal cell morphology. EBP50 S279A/ S301A had a weaker binding affinity with actin than EBP50 S279D/S301D, which might explain the increase of F-actin content in the AA cells. The present results suggest that cdc2/cyclin B-mediated EBP50 phosphorylation may play a role in the regulation of various cell functions by affecting actin cytoskeleton reorganization.

Clinical significance of EBP50 overexpression assessed by quantum dot analysis in gastric cancer

Ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) is a postsynaptic density-95/disc-large/zonula occludens-1 (PDZ) homologous domain-containing protein that is involved in cell signaling. EBP50 regulates cell apoptosis, proliferation and invasion. In the present study, the prognostic impact factor of EBP50 expression was evaluated using a quantum dot (QD)-based assay and immunohistochemistry (IHC). The EBP50 protein expression in gastric cancer (GC) tissues was evaluated using IHC and QD-IHC. The study included 101 patients with GC (29 females and 72 males, aged 24-81 years), diagnosed and treated at the General Surgery Department of Renmin Hospital of Wuhan University (Wuhan, China) between 2000 and 2005. The survival rate was calculated using the Kaplan-Meier method and log-rank tests. IHC and QD analyses of 101 GC tissue specimens revealed that EBP50-positive tumor cells were frequently present in GC. Increased EBP50 immunostaining was observed in 63 specimens (62.4%). The EBP50 expression levels were correlated with increased tumor size and the male gender. EBP50 was well distributed in the cytoplasm and nuclei of the GC cells. However, EBP50 protein expression exhibited no correlation with age, differentiation, stage or lymph node metastasis. There were no associations between the expression of EBP50 and the mean survival rates (IHC, 50.5 vs. 58.1 months, P>0.05; QD, 55.4 vs. 63.2 months, P>0.05). These findings suggest that EBP50 protein expression is not correlated with the prognosis of patients with GC. QD-IHC and IHC have similar advantages for the detection of EBP50 protein expression.