In situ detection of phospholipid and phosphoinositide metabolism

In vivo detection of phospholipase C by enzyme-activated near-infrared probes

In this article, the characterization of the first near-infrared (NIR) phospholipase-activated molecular beacon is reported, and its utility for in vivo cancer imaging is demonstrated. The probe consists of three elements: a phospholipid (PL) backbone to which the NIR fluorophore, pyropheophorbide a (Pyro), and the NIR Black Hole Quencher 3 (BHQ) were conjugated. Because of the close proximity of BHQ to Pyro, the Pyro-PtdEtn-BHQ probe is self-quenched until enzyme hydrolysis releases the fluorophore. The Pyro-PtdEtn-BHQ probe is highly specific to one isoform of phospholipase C, phosphatidylcholine-specific phospholipase C (PC-PLC), responsible for catabolizing phosphatidylcholine directly to phosphocholine. Incubation of Pyro-PtdEtn-BHQ in vitro with PC-PLC demonstrated a 150-fold increase in fluorescence that could be inhibited by the specific PC-PLC inhibitor tricyclodecan-9-yl xanthogenate (D609) with an IC(50) of 34 ± 8 μM. Since elevations in phosphocholine have been consistently observed by magnetic resonance spectroscopy in a wide array of cancer cells and solid tumors, we assessed the utility of Pyro-PtdEtn-BHQ as a probe for targeted tumor imaging. Injection of Pyro-PtdEtn-BHQ into mice bearing DU145 human prostate tumor xenografts followed by in vivo NIR imaging resulted in a 4-fold increase in tumor radiance over background and a 2 fold increase in the tumor/muscle ratio. Tumor fluorescence enhancement was inhibited with the administration of D609. The ability to image PC-PLC activity in vivo provides a unique and sensitive method of monitoring one of the critical phospholipase signaling pathways activated in cancer, as well as the phospholipase activities that are altered in response to cancer treatment.

Single-cell analysis of phosphoinositide 3-kinase and phosphatase and tensin homolog activation

A single-cell assay was developed to measure the activation of phosphoinositide 3-kinase (PI3K) using microanalytical chemical separations and a fluorescently labeled lipid substrate. Phosphatidyl-inositol 4,5 bisphosphate labeled on its acyl chain with Bodipy fluorescein (Bodipy Fl PIP(2)) was utilized as a substrate for both in vitro and cell-based assays. Detection limits for the substrate and product of the PI3K reaction were 10 to 20 zeptomol. In vitro assays with PI3K with and without pharmacologic inhibitors demonstrated that Bodipy Fl PIP(2) was converted to phosphatidyl-inositol 3,4,5 trisphosphate (Bodipy Fl PIP(3)). Bodipy Fl PIP(3) could be back converted to Bodipy Fl PIP(2) by the phosphatase PTEN. When Bodipy Fl PIP(2) was added to a cell lysate, 1.4 fmol of the Bodipy Fl PIP(3) were produced per ng of protein in the cytoplasmic extract in 10 min. Addition of Bodipy Fl PIP(3) to a cell lysate yielded 3 fmol of Bodipy Fl PIP(2) per ng of protein in 8 min. Both Bodipy Fl PIP(2) and Bodipy Fl PIP(3) were measureable in single cells and the two species could be inter-converted. Under the appropriate conditions, a fluorescent diacylglycerol was also detected in single cells. When the FcepsilonR 1 receptor on the cells loaded with the fluorescent lipid was cross-linked, the amount of Bodipy Fl PIP(3) generated per cell increased 4-fold over that of unstimulated cells. This production of Bodipy Fl PIP(3) was blocked by wortmannin. Chemical cytometry utilizing the fluorescent lipids will be of value in understanding lipid metabolism at the single-cell level.

Preparation of an antibody that recognizes and neutralizes Dictyostelium differentiation-inducing factor-1

In the development of the cellular slime mold Dictyostelium discoideum, the differentiation-inducing factor-1 (DIF-1; 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one) plays an important role in the regulation of cell differentiation and chemotaxis; however, the cellular signaling systems involving DIF-1 remain to be elucidated. To obtain a probe for DIF-1, we synthesized a DIF derivative (DIF-1-NH(2); 6-amino-1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one), and prepared an anti-DIF-1 antibody using a DIF-1-NH(2)-conjugated macromolecule as the immunogen. A 100-fold dilution of the antibody bound to DIF-1-NH(2)-conjugated resin, and this binding was inhibited by co-addition of 20 microM DIF-1 or DIF-1-NH(2). In a monolayer culture of HM44 cells, a DIF-deficient D. discoideum strain, 0.5 nM exogenous DIF-1 induced stalk cell formation in approximately 60% of the cells; this induction was dose-dependently inhibited by the antibody (diluted 12.5- or 25-fold). Furthermore, this inhibition by the antibody was recovered by co-addition of 2.5 or10 nM DIF-1. The results indicate that the anti-DIF-1 antibody recognizes DIF-1 and neutralizes its function.

PKC-ζ expression is lower in osteoblasts from arthritic patients: IL1-β and TNF-α induce a similar decrease in non-arthritic human osteoblasts

Protein kinase C (PKC) is a family of enzymes detected in a diverse range of cell types where they regulate various cellular functions such as proliferation, differentiation, cytoskeletal remodelling, cytokine production, and receptor-mediated signal transduction. In this study we have analyzed the expression of 11 PKC isoforms (-alpha, -beta(I), -beta(II), -gamma, -delta, -eta, -theta, -epsilon, -zeta, -iota/lambda, and -micro) in osteoblasts from patients with osteoarthritis (OA) and rheumatoid arthritis (RA) in comparison with osteoblasts from post-traumatic (PT) patients. By Western blotting analysis, nine isoforms, -alpha, -beta(I), -beta(II), -delta, -theta, - epsilon, -zeta, - iota/lambda, and -micro, were detected in osteoblasts. In RA and OA patients, PKC -theta and -micro were greater expressed whereas PKC-epsilon and -zeta decreased when compared with normal cells. The subcellular distribution and quantitative differences were confirmed by immuno-electron microscopy. Furthermore, we demonstrated that treatment with the proinflammatory cytokines, IL-1beta and TNF-alpha, significantly decreased PKC-zeta expression in PT osteoblasts. This suggests that proinflammatory cytokines can modulate the expression of this PKC isoform in osteoblasts in a way which is similar to changes detected in arthritic patients.

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Role of lipids in the MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) signaling pathway is activated in response to a variety of extracellular stimuli such as growth factor stimulation. The best-characterized MAPK pathway involves the sequential activation of Raf, MEK and ERK proteins, capable of regulating the gene expression required for cell proliferation. Binding to specific lipids can regulate both the subcellular localization of these MAPK signaling proteins as well as their kinase activities. More recently it has become increasingly clear that the majority of MAPK signaling takes place intracellularly on endosomes and that the perturbation of endocytic pathways has dramatic effects on the MAPK pathway. This review highlights the direct effects of lipids on the localization and regulation of MAPK pathway proteins. In addition, the indirect effects lipids have on MAPK signaling via their regulation of endocytosis and the biophysical properties of different membrane lipids as a result of growth factor stimulation are discussed. The ability of a protein to bind to both lipids and proteins at the same time may act like a "ZIP code" to target that protein to a highly specific microlocation and could also allow a protein to be "handed off" to maintain tight control over its binding partners and location.

Quantitative immunodetection of key elements of polyphosphoinositide signal transduction in osteoblasts from arthritic patients shows a direct correlation with cell proliferation

Phosphoinositides play an essential role in diverse cellular functions such as cell proliferation, cytoskeletal regulation, intracellular vesicle trafficking, motility, cell metabolism and death. Alteration of these pathways is common to many diseases. In this study, we show that osteoblasts from patients affected by osteoarthritis (OA) and by rheumatoid arthritis (RA) present a decreased cell proliferation and a reduced expression of the key elements of polyphosphoinositide signal transduction such as phosphatidylinositol-3-kinase (PI 3K), phospholipase C gamma1 (PLCgamma1), and protein kinase C zeta (PKCzeta) compared to the post-traumatic (PT) patients. Our results suggest that a correlation may exist between the reduced osteoblast proliferation observed in OA and RA patients and the lowered expression of PI 3K, PLCgamma1, and PKCzeta enzymes. The reduced proliferation rate of osteoblasts in response to these signal transduction effectors could counteract the evolution of arthritic disease.

Visualization and perturbation of phosphoinositide and phospholipid signaling

Cells signal through lipids that are produced by phospholipid (PL) and phosphoinositide (PIPn) metabolism involve three enzymatic processes: (i) ester and phosphodiester hydrolysis by phospholipases, (ii) monophosphate hydrolysis by phosphatases, and (iii) phosphorylation of hydroxy groups by kinases. Unregulated enzyme activity correlates with specific pathologies, which are specific targets for therapeutic intervention. A variety of reagents now permit monitoring of in vitro enzyme activity, spatiotemporal changes of intracellular lipid concentrations, and identification of lipid-protein interactions. This minireview summarizes a chemical biology approach that illustrates how chemically synthesized affinity probes can be used to characterize changes in lipid signaling in cellular and molecular biology.

Modification of protein sub-nuclear localization by synthetic phosphoinositides: Evidence for nuclear phosphoinositide signaling mechanisms

PtdInsPs are critical signaling molecules that regulate diverse cellular functions. One method to study PtdInsP biology involves using synthetic PtdInsP analogs to activate endogenous PtdInsP-mediated events in living cells. Such methodology has been successfully employed to explore the role of several PtdInsP-biological outcomes in the cytoplasm. However, this strategy has not previously been used to examine the function of PtdInsPs in the nucleus of live cells, primarily because there has not been a well-defined PtdInsP-binding protein to provide functional nuclear readouts. Here we have shown that synthetic PtdIns(5)P analogs access and function in the nucleus. We have found that these molecules modify the sub-nuclear localization of PHD finger-containing proteins in live cells and in real time. This work demonstrates that synthetic PtdInsPs and PtdInsP derivatives may be powerful tools for probing nuclear PtdInsP functions. Finally, our work supports a model that endogenous PtdInsPs regulate sub-nuclear localization and function of endogenous nuclear PtdInsP-binding proteins.

Phosphoinositide Signaling

Lipid signaling by phosphoinositides (PIP(n)s) involves an array of proteins with lipid recognition, kinase, phosphatase, and phospholipase functions. Understanding PIP(n) pathway signaling requires identification and characterization of PIP(n)-interacting proteins. Moreover, spatiotemporal localization and physiological function of PIP(n)-protein complexes must be elucidated in cellular and organismal contexts. For protein discovery to functional elucidation, reporter-linked phosphoinositides or tethered PIP(n)s have been essential. The phosphoinositide 3-kinase (PI 3-K) signaling pathway has recently emerged as an important source of potential "druggable" therapeutic targets in human pathophysiology in both academic and pharmaceutical environments. This review summarizes the chemistry of PIP(n) affinity probes and their use in identifying macromolecular targets. The process of target validation will be described, i.e., the use of tethered PIP(n)s in determining PIP(n) selectivity in vitro and in establishing the function of PIP(n)-protein complexes in living cells.

Antigenic Prenylated Peptide Conjugates and Polyclonal Antibodies To Detect Protein Prenylation

Posttranslational modification of proteins with farnesyl and geranylgeranyl groups is a required modification for signaling proteins that includes the small GTPases in the Ras, Rho, and Rab families, heterotrimeric G proteins, and nuclear lamin proteins. To develop antibodies capable of detecting and distinguishing prenylated proteins, we synthesized two antigens, succinylglycine-(geranylgeranyl)cysteine methyl ester (SuccG-(gg)CMe, 1) and succinylglycine-(farnesyl)cysteine methyl ester (SuccG-(f)CMe, 2). These prenylated peptides were covalently coupled to bovine serum albumin (BSA) and to keyhole limpet hemocyanin (KLH) to produce polyvalent, immunogenic bioconjugates. Immunization of rabbits with these immunogens generated polyclonal antisera that contained significant titers of anti-geranylgeranyl and anti-farnesyl antibodies. The selectivity of the polyclonal antisera was examined using ELISA and dot blotting methods. The anti-farnesyl and anti-geranylgeranyl antisera crossreacted with both antigens. Attempts to purify the polyclonal antisera by either positive or negative immunoaffinity selection protocols failed to produce selective anti-geranylgeranyl and anti-farnesyl antibodies. Moreover, both crude antisera and purified antibodies also crossreacted with myristoylated and palmitoylated BSA conjugates. Immunofluorescence staining of EYFP-CVLL or EYFP-CVIM transfected CHO-K1 cells with rabbit polyclonal antisera showed colocalized membrane fluorescence. Thus, an important caveat for the use of antibodies raised against aliphatic antigens is that extensive controls must be performed to determine selectivity.

Nuclear inositides: facts and perspectives

Strong evidence has been accumulating over the last 15 years suggesting that phosphoinositides, which are involved in the regulation of a large variety of cellular processes in the cytoplasm and in the plasma membrane, are present within the nucleus. Several advances have resulted in the discovery that nuclear phosphoinositides are involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Although nuclear inositol lipids generate second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. This review aims at highlighting the most significant and updated findings about inositol lipid metabolism in the nucleus.

IL1-beta and TNF-alpha induce changes in the nuclear polyphosphoinositide signalling system in osteoblasts similar to that occurring in patients with rheumatoid arthritis: an immunochemical and immunocytochemical study

Osteoarthritis (OA) and rheumatoid arthritis (RA) are common joint diseases that can lead to destruction of cartilage and structural changes in the subchondral bone. In this study we show by western blot and quantitative immunocytochemistry that nuclear phospholipase C beta(1) (PLC beta(1)) and phosphatidylinositol 4,5-bisphosphate (PIP(2)), two key elements of the polyphosphoinositide signal transduction system that regulate different cellular processes, increase in primary osteoblast cultures of RA patients when compared with post-traumatic after fall (PT) patients, whilst those of OA are not significantly affected. Moreover, we demonstrate that these alterations could be induced in PT osteoblasts by proinflammatory cytokines IL-1 beta and TNF-alpha. This suggests that proinflammatory cytokines, highly produced by RA infiltrating mononuclear cells, can modulate the nuclear polyphosphoinositide signalling pathway of the osteoblasts involved in bone remodelling.

Diacylglycerol kinase-theta is localized in the speckle domains of the nucleus

It is well established that the nucleus is endowed with enzymes that are involved in lipid-dependent signal transduction pathways. Diacylglycerol (DAG) is a fundamental lipid second messenger that is produced in the nucleus. Previous reports have shown that the nucleus contains diacylglycerol kinases (DGKs), i.e., the enzymes that, by converting DAG into phosphatidic acid (PA), terminate DAG-dependent events. Here, we show, by immunofluorescence staining and confocal analysis, that DGK-theta localizes mainly to the nucleus of various cell lines, such as MDA-MB-453, MCF-7, PC12, and HeLa. Nuclear DGK-theta co-localizes with phosphatidylinositol 4,5-bisphosphate (PIP(2)) in domains that correspond to nuclear speckles, as revealed by the use of an antibody to the splicing factor SC-35, a well-established marker for these structures. The spatial distribution of nuclear DGK-theta was dynamic in that it was affected by inhibition of mRNA transcription with alpha-amanitin. Immuno-electron microscopy analysis demonstrated that DGK-theta, PIP(2), and phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) associated with electron-dense particles within the nucleus that correspond to interchromatin granule clusters. Cell fractionation experiments performed in MDA-MB-453, HeLa, and PC12 cells showed a preferential association of DGK-theta with the nucleus. Western blots demonstrated that DGK-theta was enriched in the nuclear matrix fraction prepared from MDA-MB-453 cells. Immunoprecipitation experiments with an antibody to PLCbeta1 revealed in MDA-MB-453 cells an association between this enzyme and both DGK-theta and phosphatidylinositol phosphate kinase Ialpha (PIPKIalpha). Our findings strengthen the contention that speckles represent a crucial site for the nuclear-based inositol lipid cycle. We may speculate that nuclear speckle-located DGK-theta, on cell stimulation with an agonist, converts to PA the DAG derived from PLCbeta1-dependent PIP(2) hydrolysis.

Nuclear lipids: new functions for old molecules?

It is becoming increasingly evident that stimulation of nuclear lipid metabolism plays a central role in many signal transduction pathways that ultimately result in various cell responses including proliferation and differentiation. Nuclear lipid metabolism seems to be at least as complex as that existing at the plasma membrane. However, a distinctive feature of nuclear lipid biochemical pathways is their operational independence from their cell periphery counterparts. Although initially it was thought that nuclear lipids would serve as a source for second messengers, recent evidence points to the likelihood that lipids present in the nucleus also fulfil other roles. The aim of this review is to highlight the most intriguing advances made in the field over the last year, such as the production of new probes for the in situ mapping of nuclear phosphoinositides, the identification of two sources for nuclear diacylglycerol production, the emerging details about the peculiar regulation of nuclear phosphoinositide synthesizing enzymes, and the distinct possibility that nuclear lipids are involved in processes such as chromatin organization and pre-mRNA splicing.