Phospholipase C-related catalytically inactive protein regulates cytokinesis by protecting phosphatidylinositol 4,5-bisphosphate from metabolism in the cleavage furrow

Lipid Polarization during Cytokinesis

The plasma membrane of eukaryotic cells is composed of a large number of lipid species that are laterally segregated into functional domains as well as asymmetrically distributed between the outer and inner leaflets. Additionally, the spatial distribution and organization of these lipids dramatically change in response to various cellular states, such as cell division, differentiation, and apoptosis. Division of one cell into two daughter cells is one of the most fundamental requirements for the sustenance of growth in all living organisms. The successful completion of cytokinesis, the final stage of cell division, is critically dependent on the spatial distribution and organization of specific lipids. In this review, we discuss the properties of various lipid species associated with cytokinesis and the mechanisms involved in their polarization, including forward trafficking, endocytic recycling, local synthesis, and cortical flow models. The differences in lipid species requirements and distribution in mitotic vs. male meiotic cells will be discussed. We will concentrate on sphingolipids and phosphatidylinositols because their transbilayer organization and movement may be linked via the cytoskeleton and thus critically regulate various steps of cytokinesis.

Vasoactive intestinal peptide–VIPR2 signaling regulates tumor cell migration

Phosphoinositide metabolism is critically involved in human cancer cell migration and metastatic growth. The formation of lamellipodia at the leading edge of migrating cells is regulated by metabolism of the inositol phospholipid PI(4,5)P₂ into PI(3,4,5)P₃. The synthesized PI(3,4,5)P₃ promotes the translocation of WASP family verprolin homologous protein 2 (WAVE2) to the plasma membrane and regulates guanine nucleotide exchange factor Rac-mediated actin filament remodeling. Here, we investigated if VIPR2, a receptor for vasoactive intestinal peptide (VIP), has a potential role in regulating cell migration via this pathway. We found that silencing of VIPR2 in MDA-MB-231 and MCF-7 human breast cancer cells inhibited VIP-induced cell migration. In contrast, stable expression of exogenous VIPR2 promoted VIP-induced tumor cell migration, an effect that was inhibited by the addition of a PI3-kinase (PI3K)γ inhibitor or a VIPR2-selective antagonist. VIPR2 stably-expressing cells exhibited increased PI3K activity. Membrane localization of PI(3,4,5)P₃ was significantly attenuated by VIPR2-silencing. VIPR2-silencing in MDA-MB-231 cells suppressed lamellipodium extension; in VIPR2-overexpressing cells, VIPR2 accumulated in the cell membrane on lamellipodia and co-localized with WAVE2. Conversely, VIPR2-silencing reduced WAVE2 level on the cell membrane and inhibited the interaction between WAVE2, actin-related protein 3, and actin. These findings suggest that VIP–VIPR2 signaling controls cancer migration by regulating WAVE2-mediated actin nucleation and elongation for lamellipodium formation through the synthesis of PI(3,4,5)P₃.

Phospholipase C-related catalytically inactive protein enhances cisplatin-induced apoptotic cell death

Cisplatin is one of the most widely used chemotherapeutic agents and induces caspase-9-mediated apoptosis. Here, we examined whether phospholipase C-related catalytically inactive protein (PRIP) enhances cisplatin-induced apoptosis of breast cancer cells. PRIP depletion increased expression of X-linked inhibitor of apoptosis protein (XIAP) by inhibiting protein degradation, which is downstream of the phosphatidylinositol 3-kinase/AKT pathway and inhibits apoptotic signaling by blocking caspase-9 activation. Conversely, the viability of MCF-7 cells transfected with Prip1 was significantly lower than that of control cells in the presence of cisplatin. The number of apoptotic nuclei and expression levels of cleaved caspase-9 and downstream cleaved caspase-7 and poly-ADP ribose polymerase were greater in PRIP1-expressing MCF-7 cells treated with cisplatin than in control cells. XIAP was decreased by expression of pleckstrin homology domain of PRIP1 (PRIP1-PH domain) that blocked phosphatidylinositol 4,5 bisphosphate metabolism. In an orthotopic transplantation model, combined administration of PRIP1-PH domain-containing liposomes and cisplatin reduced the size of MCF-7 tumors compared with cisplatin alone. Our findings demonstrate that PRIP promotes XIAP degradation by inhibiting PI(3,4,5)P₃/AKT signaling and enhances cisplatin-induced apoptotic cell death. Therefore, we propose that PRIP1-PH liposomes are a novel agent to avoid cisplatin resistance.

Expression of PRIP, a phosphatidylinositol 4,5-bisphosphate binding protein, attenuates PI3K/AKT signaling and suppresses tumor growth in a xenograft mouse model

Cancer is characterized by uncontrolled proliferation resulting from aberrant cell cycle progression. The activation of phosphatidylinositol 3-kinase (PI3K)/AKT signaling, a regulatory pathway for the cell cycle, stabilizes cyclin D1 in the G1 phase by inhibiting the activity of glycogen synthase kinase 3β (GSK3β) via phosphorylation. We previously reported that phospholipase C-related catalytically inactive protein (PRIP), a phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] binding protein, regulates PI3K/AKT signaling by competitively inhibiting substrate recognition by PI3K. Therefore, in this study, we investigated whether PRIP is involved in cell cycle progression. PRIP silencing in MCF-7 cells, a human breast cancer cell line, demonstrated PI(3,4,5)P3 signals accumulated at the cell periphery compared to that of the control. This suggests that PRIP reduction enhances PI(3,4,5)P3-mediated signaling. Consistently, PRIP silencing in MCF-7 cells exhibited increased phosphorylation of AKT and GSK3β which resulted in cyclin D1 accumulation. In contrast, the exogenous expression of PRIP in MCF-7 cells evidenced stronger downregulation of AKT and GSK3β phosphorylation, reduced accumulation of cyclin D1, and diminished cell proliferation in comparison to control cells. Flow cytometry analysis indicated that MCF-7 cells stably expressing PRIP attenuate cell cycle progression. Importantly, tumor growth of MCF-7 cells stably expressing PRIP was considerably prevented in an in vivo xenograft mouse model. In conclusion, PRIP expression downregulates PI3K/AKT/GSK3β-mediated cell cycle progression and suppresses tumor growth. Therefore, we propose that PRIP is a new therapeutic target for anticancer therapy.