Search

Cytokinetic abscission is the cellular process leading to physical separation of two postmitotic sister cells by severing the intercellular bridge. The most noticeable structural component of the intercellular bridge is a transient organelle termed as midbody, localized at a central region marking the site of abscission. Despite its major role in completion of cytokinesis, our understanding of spatiotemporal regulation of midbody assembly is limited. Here, we report the first characterization of coiled-coil domain-containing protein-124 (Ccdc124), a eukaryotic protein conserved from fungi-to-man, which we identified as a novel centrosomal and midbody protein. Knockdown of Ccdc124 in human HeLa cells leads to accumulation of enlarged and multinucleated cells; however, centrosome maturation was not affected. We found that Ccdc124 interacts with the Ras-guanine nucleotide exchange factor 1B (RasGEF1B), establishing a functional link between cytokinesis and activation of localized Rap2 signaling at the midbody. Our data indicate that Ccdc124 is a novel factor operating both for proper progression of late cytokinetic stages in eukaryotes, and for establishment of Rap2 signaling dependent cellular functions proximal to the abscission site. [ABSTRACT FROM AUTHOR]

Small GTPases belonging to Ras family of proteins have key roles in regulating nearly every aspect of cell biology, such as cell division, cellular differentiation, vesicular transport and localization of cargo proteins, cell morphology, and gene expression. Depending on the extra- cellular signals, Ras family members oscillate between GTP-bound (active) and GDP-bound (inactive) conformations, and in the active form they interact with downstream effector molecules, leading to conformational changes in those effectors. As a result, extracellular signals trigger cellular responses, by means of initiation of a phosphorylation cascade. Functional cycles of Ras family of proteins include interactions with GTPase Activating Proteins (GAPs), and with Guanine Nucleotide Exchange Factors (GEFs) that regulate these small GTPases. GAP proteins activate intrinsic GTP hydrolysis activities of these small G-proteins, and convert active Ras-GTP to inactive Ras-GTP. On the other hand, GEF proteins interact with inactivated Ras-GDP molecules, and convert those inactivated molecules back to activated Ras-GTP, by triggering dissociation of GDP, and reassociation of Ras with GTP which is found at higher concentrations as compared to GDP in the cell cytoplasm. It is well known that mutations in Ras that block these molecules in GTP-bound forms by impairing GTP hydrolysis activity could trigger an uncontrolled and aberrant cellular proliferation. This type of mutations in Ras genes causing uncontrolled activation of the protein, are found in approximately 30% of human cancers overall, and they indicate the significance of mutations in genes encoding Ras family members in cellular proliferation related to de novo oncogenesis in human. [ABSTRACT FROM AUTHOR]

Background: Breast cancer is a remarkably heterogeneous disease. Luminal, basal-like, ''normal-like'', and ERBB2+ subgroups were identified and were shown to have different prognoses. The mechanisms underlying this heterogeneity are poorly understood. In our study, we explored the role of cellular differentiation and senescence as a potential cause of heterogeneity. Methodology/Principal Findings: A panel of breast cancer cell lines, isogenic clones, and breast tumors were used. Based on their ability to generate senescent progeny under low-density clonogenic conditions, we classified breast cancer cell lines as senescent cell progenitor (SCP) and immortal cell progenitor (ICP) subtypes. All SCP cell lines expressed estrogen receptor (ER). Loss of ER expression combined with the accumulation of p21Cip1 correlated with senescence in these cell lines. p21Cip1 knockdown, estrogen-mediated ER activation or ectopic ER overexpression protected cells against senescence. In contrast, tamoxifen triggered a robust senescence response. As ER expression has been linked to luminal differentiation, we compared the differentiation status of SCP and ICP cell lines using stem/progenitor, luminal, and myoepithelial markers. The SCP cells produced CD24+ or ER+ luminal-like and ASMA+ myoepithelial-like progeny, in addition to CD44+ stem/progenitor-like cells. In contrast, ICP cell lines acted as differentiation-defective stem/progenitor cells. Some ICP cell lines generated only CD44+/ CD24-/ER-/ASMA- progenitor/stem-like cells, and others also produced CD24+/ER- luminal-like, but not ASMA+ myoepitheliallike cells. Furthermore, gene expression profiles clustered SCP cell lines with luminal A and ''normal-like'' tumors, and ICP cell lines with luminal B and basal-like tumors. The ICP cells displayed higher tumorigenicity in immunodeficient mice. Conclusions/Significance: Luminal A and ''normal-like'' breast cancer cell lines were able to generate luminal-like and myoepithelial-like progeny undergoing senescence arrest. In contrast, luminal B/basal-like cell lines acted as stem/ progenitor cells with defective differentiation capacities. Our findings suggest that the malignancy of breast tumors is directly correlated with stem/progenitor phenotypes and poor differentiation potential. [ABSTRACT FROM AUTHOR]