A polybasic motif in ErbB3-binding protein 1 (EBP1) has key functions in nucleolar localization and polyphosphoinositide interaction

We reveal the identification of a polybasic motif necessary for polyphosphoinositide interaction and nucleolar targeting of ErbB3 binding protein 1 (EBP1). EBP1 interacts directly with phosphatidylinositol(3,4,5)-triphosphate and their association is detected in the nucleolus, implying regulatory roles of nucleolar processes.

Polyphosphoinositides (PPIns) are present in the nucleus where they participate in crucial nuclear processes, such as chromatin remodelling, transcription and mRNA processing. In a previous interactomics study, aimed to gain further insight into nuclear PPIns functions, we identified ErbB3 binding protein 1 (EBP1) as a potential nuclear PPIn-binding protein in a lipid pull-down screen. EBP1 is a ubiquitous and conserved protein, located in both the cytoplasm and nucleolus, and associated with cell proliferation and survival. In the present study, we show that EBP1 binds directly to several PPIns via two distinct PPIn-binding sites consisting of clusters of lysine residues and positioned at the N- and C-termini of the protein. Using interaction mutants, we show that the C-terminal PPIn-binding motif contributes the most to the localization of EBP1 in the nucleolus. Importantly, a K372N point mutation, located within the C-terminal motif and found in endometrial tumours, is sufficient to alter the nucleolar targeting of EBP1. Our study reveals also the presence of the class I phosphoinositide 3-kinase (PI3K) catalytic subunit p110β and its product PtdIns(3,4,5)P₃ together with EBP1 in the nucleolus. Using NMR, we further demonstrate an association between EBP1 and PtdIns(3,4,5)P₃ via both electrostatic and hydrophobic interactions. Taken together, these results show that EBP1 interacts directly with PPIns and associate with PtdIns(3,4,5)P₃ in the nucleolus. The presence of p110β and PtdIns(3,4,5)P₃ in the nucleolus indicates their potential role in regulating nucleolar processes, at least via EBP1.

FOXM1: From cancer initiation to progression and treatment

The Forkhead box protein M1 (FOXM1) transcription factor is a regulator of myriad biological processes, including cell proliferation, cell cycle progression, cell differentiation, DNA damage repair, tissue homeostasis, angiogenesis and apoptosis. Elevated FOXM1 expression is found in cancers of the liver, prostate, brain, breast, lung, colon, pancreas, skin, cervix, ovary, mouth, blood and nervous system, suggesting it has an integral role in tumorigenesis. Recent research findings also place FOXM1 at the centre of cancer progression and drug sensitivity. In this review the involvement of FOXM1 in various aspects of cancer, in particular its role and regulation within the context of cancer initiation, progression, and cancer drug response, will be summarised and discussed.

Removal or masking of phosphatidylinositol(4,5)bisphosphate from the outer mitochondrial membrane causes mitochondrial fragmentation

Mitochondria are central players in programmed cell death and autophagy. While phosphoinositides are well established regulators of membrane traffic, cellular signalling and the destiny of certain organelles, their presence and role for mitochondria remain elusive. In this study we show that removal of PtdIns(4,5)P₂ by phosphatases or masking the lipid with PH domains leads to fission of mitochondria and increased autophagy. Induction of general autophagy by amino acid starvation also coincides with the loss of mitochondrial PtdIns(4,5)P₂, suggesting an important role for this lipid in the processes that govern mitophagy. Our findings reveal that PKCα can rescue the removal or masking of PtdIns(4,5)P₂, indicating that the inositol lipid is upstream of PKC.

Physiology and pathology of nuclear phospholipase C β1

The existence and function of inositide signaling in the nucleus is well documented and we know that the existence of the inositide cycle inside the nucleus has a biological role. An autonomous lipid-dependent signaling system, independently regulated from its plasma membrane counterpart, acts in the nucleus and modulates cell cycle progression and differentiation.We and others focused on PLCβ1, which is the most extensively investigated PLC isoform in the nuclear compartment. PLCβ1 is a key player in the regulation of nuclear inositol lipid signaling, and, as discussed above, its function could also be involved in nuclear structure because it hydrolyses PtdIns(4,5)P2, a well accepted regulator of chromatin remodelling. The evidence, in a number of patients with myelodysplastic syndromes, that the mono-allelic deletion of PLCβ1 is associated with an increased risk of developing acute myeloid leukemia paves the way for an entirely new field of investigation. Indeed the genetic defect evidenced, in addition to being a useful prognostic tool, also suggests that altered expression of this enzyme could have a role in the pathogenesis of this disease, by causing an imbalance between proliferation and apoptosis. The epigenetics of PLCβ1 expression in MDS has been reviewed as well.

Calcium-dependent lateral organization in phosphatidylinositol 4,5-bisphosphate (PIP2)- and cholesterol-containing monolayers

Biological membrane function, in part, depends upon the local regulation of lipid composition. The spatial heterogeneity of membrane lipids has been extensively explored in the context of cholesterol and phospholipid acyl-chain-dependent domain formation, but the effects of lipid head groups and soluble factors in lateral lipid organization are less clear. In this contribution, the effects of divalent calcium ions on domain formation in monolayers containing phosphatidylinositol 4,5-bisphosphate (PIP2), a polyanionic, multifunctional lipid of the cytosolic leaflet of the plasma bilayer, are reported. In binary monolayers of PIP2 mixed with zwitterionic lipids, calcium induced a rapid, PIP2-dependent surface pressure drop, with the concomitant formation of laterally segregated, PIP2-rich domains. The effect was dependent upon head-group multivalency, because lowered pH suppressed the surface-pressure effect and domain formation. In accordance with previous observations, inclusion of cholesterol in lipid mixtures induced coexistence of two liquid phases. Phase separation strongly segregated PIP2 to the cholesterol-poor phase, suggesting a role for cholesterol-dependent lipid demixing in regulating PIP2 localization and local concentration. Similar to binary mixtures, subphase calcium induced contraction of ternary cholesterol-containing monolayers; however, in these mixtures, calcium induced an unexpected, PIP2- and multivalency-dependent decrease in the miscibility phase transition surface pressure, resulting in rapid dissolution of the domains. This result emphasizes the likely critical role of subphase factors and lipid head-group specificity in the formation and stability of cholesterol-dependent domains in cellular plasma membranes.

Great expectations for PIP: phosphoinositides as regulators of signaling during development and disease

Phosphoinositides function as signaling precursors as well as regulators and scaffolds of signaling molecules required for important cellular processes such as membrane trafficking. Although a picture of the biochemical and cell biological functions of phosphoinositides is emerging, less is known about how these functions impact signaling on a broader scale during development. This review summarizes recent work on the role of phosphoinositides in developmental signaling and in a number of diseases and developmental disorders.