Search

Inositol polyphosphate 4-phosphatase-Il (INPP4B) is a regulator of the phosphoinositide 3-kinase (P13K) signaling pathway and is implicated as a tumor suppressor in epithelial carcinomas. INPP4B loss of heterozygosity (LOH) is detected in some human breast cancers; however, the expression of INPP4B protein in breast cancer subtypes and the normal breast is unknown. We report here that INPP4B is expressed in nonproliferative estrogen receptor (ER)-positive cells in the normal breast, and in ER-positive, but not negative, breast cancer cell lines. INPP4B knockdown in ER-positive breast cancer cells increased Akt activation, cell proliferation, and xenograft tumor growth. Conversely, reconstitution of INPP4B ex- pression in ER-negative, INPP4B-null human breast cancer cells reduced Akt activation and anchorage-independent growth. INPP4B protein expression was frequently lost in primary human breast carcinomas, associated with high clinical grade and tumor size and loss of hormone receptors and was lost most commonly in aggressive basal-like breast carcinomas. INPP4B protein loss was also frequently observed in phosphatase and tensin homolog (PTEN)-null tumors. These studies provide evidence that INPP4B functions as a tumor suppressor by negatively regulating normal and malignant mammary epithelial cell proliferation through regulation of the PI3K/Akt signaling pathway, and that loss of INPP4B protein is a marker of aggressive basal-like breast carcinomas. [ABSTRACT FROM AUTHOR]

Phosphoinositides direct membrane trafficking, facilitating the recruitment of effectors to specific membranes. In yeast phosphatidylinositol 4,5-bisphosphate (Ptdlns(4,5)P2) is proposed to regulate vacuolar fusion; however, in intact cells this phosphoinositide can only be detected at the plasma membrane. In Saccharomyces cerevisiae the 5-phosphatase, Inp54p, dephosphorylates Ptdlns(4,5)P2 forming Ptdlns(4)P, a substrate for the phosphatase Sac1p, which hydrolyzes (Ptdlns(4)P). We investigated the role these phosphatases in regulating Ptdlns(4,5)P2 subcellular distribution. Ptdlns(4,5)P2 bioprobes exhibited loss of plasma membrane localization and instead labeled a subset of fragmented vacuoles in Δsac1 Δinp54 and sac1ts Δinp54 mutants. Furthermore, sac1ts Δinp54 mutants exhibited vacuolar fusion defects, which were rescued by Iatrunculin A treatment, or by inactivation of Mss4p, a Ptdlns(4)P 5-kinase that synthesizes plasma membrane Ptdlns(4,5)P2. Under these conditions Ptdlns(4,5)P2 was not detected on vacuole membranes, and vacuole morphology was normal, indicating vacuolar Ptdlns(4,5)P2 derives from Mss4p-generated plasma membrane Ptdlns(4,5)P2. Δsac1 Δinp54 mutants exhibited delayed carboxypeptidase Y sorting, cargo-selective secretion defects, and defects in vacuole function. These studies reveal Ptdlns(4,5)P2 hydrolysis by lipid phosphatases governs its spatial distribution, and loss of phosphatase activity may result in PtdIns(4,5)P2 accumulation on vacuole membranes leading to vacuolar fragmentation/fusion defects. [ABSTRACT FROM AUTHOR]

Identifies the cellular functions of inositol polyphosphate 5-phosphatases. Regulation of hemapoietic cell proliferation and activation; Recruitment of various effector proteins to membrane; Distinction of subcellular localization of various isoforms.

Phosphoinositide 3-kinase (PI3K) regulates cell polarity and migration by generating phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) at the leading edge of migrating cells. The serine-threonine protein kinase Akt binds to PI(3,4,5)P3, resulting in its activation. Active Akt promotes spatially regulated actin cytoskeletal remodeling and thereby directed cell migration. The inositol polyphosphate 5-phosphatases (5-ptases) degrade PI(3,4,5)P3 to form PI(3,4)P2, which leads to diminished Akt activation. Several 5-ptases, including SKIP and SHIP2, inhibit actin cytoskeletal reorganization by opposing PI3K/Akt signaling. In this current study, we identify a molecular co-chaperone termed silencer of death domains (SODD/BAG4) that forms a complex with several 5-ptase family members, including SKIP, SHIP1, and SHIP2. The interaction between SODD and SKIP exerts an inhibitory effect on SKIP PI(3,4,5)P3 5-ptase catalytic activity and consequently enhances the recruitment of PI(3,4,5)P3-effectors to the plasma membrane. In contrast, SODD-/- mouse embryonic fibroblasts exhibit reduced Akt-Ser473 and -Thr308 phosphorylation following EGF stimulation, associated with increased SKIP PI(3,4,5)P3-5-ptase activity. SODD-/- mouse embryonic fibroblasts exhibit decreased EGF-stimulated F-actin stress fibers, lamellipodia, and focal adhesion complexity, a phenotype that is rescued by the expression of constitutively active Akt1. Furthermore, reduced cell migration was observed in SODD-/- macrophages, which express the three 5-ptases shown to interact with SODD (SKIP, SHIP1, and SHIP2). Therefore, this study identifies SODD as a novel regulator of PI3K/Akt signaling to the actin cytoskeleton. [ABSTRACT FROM AUTHOR]