ABSTRACT
Rho GDP-dissociation inhibitors (RhoGDIs) are regulators of Rho family GTPases. RhoGDIß has been implicated in cancer progression, but its precise role remains unclear. We determined the subcellular localization of RhoGDIß and examined the effects of its overexpression and RNAi knockdown in cancer cells. Immunofluorescence staining showed that RhoGDIß localized to centrosomes in human cancer cells. In HeLa cells, exogenous GFP-tagged RhoGDIß localized to centrosomes and its overexpression caused prolonged mitosis and aberrant cytokinesis in which the cell shape was distorted. RNAi knockdown of RhoGDIß led to increased incidence of monopolar spindle mitosis resulting in polyploid cells. These results suggest that RhoGDIß has mitotic functions, including regulation of cytokinesis and bipolar spindle formation. The dysregulated expression of RhoGDIß may contribute to cancer progression by disrupting these processes.
Subject(s)
Centrosome/ultrastructure , Mitosis , rho Guanine Nucleotide Dissociation Inhibitor beta/genetics , Cytokinesis/genetics , Gene Knockdown Techniques , HeLa Cells , Humans , RNA Interference , Spindle Apparatus/genetics , rho Guanine Nucleotide Dissociation Inhibitor beta/chemistryABSTRACT
Poly(ADP-ribose) polymerase (PARP) is an enzyme that mediates post-translational modification of proteins. Seventeen known members of the PARP superfamily can be grouped into three classes based on catalytic activity: (i) classical poly(ADP-ribose) polymerases, (ii) mono(ADPribosyl) transferases and (iii) catalytically inactive members. PARP6 belongs to the mono(ADP-ribosyl) transferase class, and here we have found that PARP6 is a negative regulator of cell proliferation. Forced expression of PARP6 in HeLa cells induced growth suppression, but a PARP6 mutant with a C-terminal deletion lacking the catalytic domain had no effect. The PARP6-expressing cells accumulated in the S-phase, and the magnitude of S-phase accumulation was observed to be greater in cells expressing a PARP6 mutant with an N-terminal deletion, lacking a putative regulatory domain. Immunohistochemical analysis revealed that PARP6 positivity was found at higher frequencies in colorectal cancer tissues with well-differentiated histology compared to those with poorly differentiated histology. Furthermore, PARP6 positivity negatively correlated with the Ki-67 proliferation index. Kaplan-Meier analysis showed that PARP6-positive colorectal cancer had a good prognosis. Based on these results, we propose that PARP6 acts as a tumor suppressor through its role in cell cycle control.
