Changes in the components of a nuclear inositide cycle during differentiation in murine erythroleukaemia cells

Nuclear Phosphoinositides as Key Determinants of Nuclear Functions

Polyphosphoinositides (PPIns) are signalling messengers representing less than five per cent of the total phospholipid concentration within the cell. Despite their low concentration, these lipids are critical regulators of various cellular processes, including cell cycle, differentiation, gene transcription, apoptosis and motility. PPIns are generated by the phosphorylation of the inositol head group of phosphatidylinositol (PtdIns). Different pools of PPIns are found at distinct subcellular compartments, which are regulated by an array of kinases, phosphatases and phospholipases. Six of the seven PPIns species have been found in the nucleus, including the nuclear envelope, the nucleoplasm and the nucleolus. The identification and characterisation of PPIns interactor and effector proteins in the nucleus have led to increasing interest in the role of PPIns in nuclear signalling. However, the regulation and functions of PPIns in the nucleus are complex and are still being elucidated. This review summarises our current understanding of the localisation, biogenesis and physiological functions of the different PPIns species in the nucleus.

When PIP2 Meets p53: Nuclear Phosphoinositide Signaling in the DNA Damage Response

The mechanisms that maintain genome stability are critical for preventing tumor progression. In the past decades, many strategies were developed for cancer treatment to disrupt the DNA repair machinery or alter repair pathway selection. Evidence indicates that alterations in nuclear phosphoinositide lipids occur rapidly in response to genotoxic stresses. This implies that nuclear phosphoinositides are an upstream element involved in DNA damage signaling. Phosphoinositides constitute a new signaling interface for DNA repair pathway selection and hence a new opportunity for developing cancer treatment strategies. However, our understanding of the underlying mechanisms by which nuclear phosphoinositides regulate DNA damage repair, and particularly the dynamics of those processes, is rather limited. This is partly because there are a limited number of techniques that can monitor changes in the location and/or abundance of nuclear phosphoinositide lipids in real time and in live cells. This review summarizes our current knowledge regarding the roles of nuclear phosphoinositides in DNA damage response with an emphasis on the dynamics of these processes. Based upon recent findings, there is a novel model for p53’s role with nuclear phosphoinositides in DNA damage response that provides new targets for synthetic lethality of tumors.

Nuclear Phosphoinositides: Their Regulation and Roles in Nuclear Functions

Polyphosphoinositides (PPIns) are a family of seven lipid messengers that regulate a vast array of signalling pathways to control cell proliferation, migration, survival and differentiation. PPIns are differentially present in various sub-cellular compartments and, through the recruitment and regulation of specific proteins, are key regulators of compartment identity and function. Phosphoinositides and the enzymes that synthesise and degrade them are also present in the nuclear membrane and in nuclear membraneless compartments such as nuclear speckles. Here we discuss how PPIns in the nucleus are modulated in response to external cues and how they function to control downstream signalling. Finally we suggest a role for nuclear PPIns in liquid phase separations that are involved in the formation of membraneless compartments within the nucleus.

Phosphatidylinositol 5 Phosphate (PI5P): From Behind the Scenes to the Front (Nuclear) Stage

Phosphatidylinositol (PI)-related signaling plays a pivotal role in many cellular aspects, including survival, cell proliferation, differentiation, DNA damage, and trafficking. PI is the core of a network of proteins represented by kinases, phosphatases, and lipases which are able to add, remove or hydrolyze PI, leading to different phosphoinositide products. Among the seven known phosphoinositides, phosphatidylinositol 5 phosphate (PI5P) was the last to be discovered. PI5P presence in cells is very low compared to other PIs. However, much evidence collected throughout the years has described the role of this mono-phosphoinositide in cell cycles, stress response, T-cell activation, and chromatin remodeling. Interestingly, PI5P has been found in different cellular compartments, including the nucleus. Here, we will review the nuclear role of PI5P, describing how it is synthesized and regulated, and how changes in the levels of this rare phosphoinositide can lead to different nuclear outputs.

Phosphoinositides in the nucleus and myogenic differentiation: how a nuclear turtle with a PHD builds muscle

Phosphoinositides are a family of phospholipid messenger molecules that control various aspects of cell biology in part by interacting with and regulating downstream protein partners. Importantly, phosphoinositides are present in the nucleus. They form part of the nuclear envelope and are present within the nucleus in nuclear speckles, intra nuclear chromatin domains, the nuclear matrix and in chromatin. What their exact role is within these compartments is not completely clear, but the identification of nuclear specific proteins that contain phosphoinositide interaction domains suggest that they are important regulators of DNA topology, chromatin conformation and RNA maturation and export. The plant homeo domain (PHD) finger is a phosphoinositide binding motif that is largely present in nuclear proteins that regulate chromatin conformation. In the present study I outline how changes in the levels of the nuclear phosphoinositide PtdIns5P impact on muscle cell differentiation through the PHD finger of TAF3 (TAF, TATA box binding protein (TBP)-associated factor), which is a core component of a number of different basal transcription complexes.

Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

A reliable method for purifying envelope-stripped nuclei from immortalized murine embryonic fibroblasts (iMEFs) was established. Quantitative profiling of the glycerophospholipids (GPLs) in envelope-free iMEF nuclei yields several conclusions. First, we find the endonuclear glycerophospholipidome differs from that of bulk membranes, and phosphatidylcholine (PtdCho) and phosphatidylethanolamine species are the most abundant endonuclear GPLs by mass. By contrast, phosphatidylinositol (PtdIns) represents a minor species. We also find only a slight enrichment of saturated versus unsaturated GPL species in iMEF endonuclear fractions. Moreover, much lower values for GPL mass were measured in the iMEF nuclear matrix than those reported for envelope-stripped IMF-32 nuclei. The collective results indicate that the nuclear matrix in these cells is a GPL-poor environment where GPL occupies only approximately 0.1% of the total nuclear matrix volume. This value suggests GPL accommodation in this compartment can be satisfied by binding to resident proteins. Finally, we find no significant role for the PtdIns/PtdCho-transfer protein, PITPα, in shuttling PtdIns into the iMEF nuclear matrix.

The basal transcription complex component TAF3 transduces changes in nuclear phosphoinositides into transcriptional output

Phosphoinositides (PI) are important signaling molecules in the nucleus that influence gene expression. However, if and how nuclear PI directly affects the transcriptional machinery is not known. We report that the lipid kinase PIP4K2B regulates nuclear PI5P and the expression of myogenic genes during myoblast differentiation. A targeted screen for PI interactors identified the PHD finger of TAF3, a TATA box binding protein-associated factor with important roles in transcription regulation, pluripotency, and differentiation. We show that the PI interaction site is distinct from the known H3K4me3 binding region of TAF3 and that PI binding modulates association of TAF3 with H3K4me3 in vitro and with chromatin in vivo. Analysis of TAF3 mutants indicates that TAF3 transduces PIP4K2B-mediated alterations in PI into changes in specific gene transcription. Our study reveals TAF3 as a direct target of nuclear PI and further illustrates the importance of basal transcription components as signal transducers.

PIP4K and the role of nuclear phosphoinositides in tumour suppression

Phosphatidylinositol-5-phosphate (PtdIns5P)-4-kinases (PIP4Ks) are stress-regulated lipid kinases that phosphorylate PtdIns5P to generate PtdIns(4,5)P₂. There are three isoforms of PIP4Ks: PIP4K2A, 2B and 2C, which localise to different subcellular compartments with the PIP4K2B isoform being localised predominantly in the nucleus. Suppression of PIP4K expression selectively prevents tumour cell growth in vitro and prevents tumour development in mice that have lost the tumour suppressor p53. p53 is lost or mutated in over 70% of all human tumours. These studies suggest that inhibition of PIP4K signalling constitutes a novel anti-cancer therapeutic target. In this review we will discuss the role of PIP4K in tumour suppression and speculate on how PIP4K modulates nuclear phosphoinositides (PPIns) and how this might impact on nuclear functions to regulate cell growth. This article is part of a Special Issue entitled Phosphoinositides.

Nuclear phospholipase C-β1 and diacylglycerol LIPASE-α in brain cortical neurons

Phosphoinositide (PtdIns) signaling involves the generation of lipid second messengers in response to stimuli in a receptor-mediated manner at the plasma membrane. In neuronal cells of adult brain, the standard model proposes that activation of metabotropic receptors coupled to Phospholipase C-β1 (PLC-β1) is linked to endocannabinoid signaling through the production of diacylglycerol (DAG), which could be systematically metabolized by 1,2-diacylglycerol Lipases (DAGL) to produce an increase of 2-arachidonoyl-glycerol (2-AG), the most abundant endocannabinoid in the brain. However, the existence of a nuclear PtdIns metabolism independent from that occurring elsewhere in the cell is now widely accepted, suggesting that the nucleus constitutes both a functional and a distinct compartment for PtdIns metabolism. In this review, we shall highlight the main achievements in the field of neuronal nuclear inositol lipid metabolism with particular attention to progress made linked to the 2-AG biosynthesis. Our aim has been to identify potential sites of 2-AG synthesis other than the neuronal cytoplasmic compartment by determining the subcellular localization of PLC-β1 and DAGL-α, which is much more abundant than DAGL-β in brain. Our data show that PLC-β1 and DAGL-α are detected in discrete brain regions, with a marked predominance of pyramidal morphologies of positive cortical cells, consistent with their role in the biosynthesis and release of 2-AG by pyramidal neurons to control their synaptic inputs. However, as novelty, we showed here an integrated description of the localization of PLC-β1 and DAGL-α in the neuronal nuclear compartment. We discuss our comparative analysis of the expression patterns of PLC-β1 and DAGL-α, providing some insight into the potential autocrine role of 2-AG production in the neuronal nuclear compartment that probably subserve additional roles to the recognized activation of the CB1 cannabinoid receptor.

Nuclear phosphoinositides and their impact on nuclear functions

Polyphosphoinositides (PPIn) are important lipid molecules whose levels are de-regulated in human diseases such as cancer, neurodegenerative disorders and metabolic syndromes. PPIn are synthesized and degraded by an array of kinases, phosphatases and lipases which are localized to various subcellular compartments and are subject to regulation in response to both extra- and intracellular cues. Changes in the activities of enzymes that metabolize PPIn lead to changes in the profiles of PPIn in various subcellular compartments. Understanding how subcellular PPIn are regulated and how they affect downstream signaling is critical to understanding their roles in human diseases. PPIn are present in the nucleus, and their levels are changed in response to various stimuli, suggesting that they may serve to regulate specific nuclear functions. However, the lack of nuclear downstream targets has hindered the definition of which pathways nuclear PPIn affect. Over recent years, targeted and global proteomic studies have identified a plethora of potential PPIn-interacting proteins involved in many aspects of transcription, chromatin remodelling and mRNA maturation, suggesting that PPIn signalling within the nucleus represents a largely unexplored novel layer of complexity in the regulation of nuclear functions.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

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.

Methods to assess changes in the pattern of nuclear phosphoinositides

Phosphatidylinositol (PtdIns) and its phosphorylated derivatives represent less than 5% of total membrane phospholipids in cells. Despite their low abundance, they form a dynamic signalling system that is regulated in response to a variety of extra and intra-cellular cues (Curr Opin Genet Dev 14:196-202, 2004). Phosphoinositides and the enzymes that synthesize them are found in many different sub-cellular compartments including the nuclear matrix, heterochromatin, and sites of active RNA splicing, suggesting that phosphoinositides may regulate specific functions within the nuclear compartment (Nat Rev Mol Cell Biol 4:349-360, 2003; Curr Top Microbiol Immunol 282:177-206, 2004; Cell Mol Life Sci 61:1143-1156, 2004). The existence of distinct sub-cellular pools has led to the challenging task of understanding how the different pools are regulated and how changes in the mass of lipids within the nucleus can modulate nuclear specific pathways. Here we describe methods to determine how enzymatic activities that modulate nuclear phosphoinositides are changed in response to extracellular stimuli.

Alternative splice variants of phospholipase C-beta2 are expressed in platelets: effect on Galphaq-dependent activation and localization

Phospholipase C (PLC) beta2 plays a pivotal role in G-protein dependent signal transduction in platelets. We have previously demonstrated in platelets, leukocytes and human erythroleukemia cells the presence of transcripts of two forms of PLC-beta2 generated by alternative splicing. They differ by 45 nucleotides in the carboxyl-terminal region and are designated as PLC-beta2a and PLC-beta2b, with and without by 15 amino acid residues (corresponding to 864-878). The presence of the two variants has not been shown at the protein level in cells. Moreover, the carboxy-terminal region of PLC-beta has been implicated in Galphaq activation, particulate association, and nuclear localization, suggesting that the PLC-beta2 splice variants may be regulated differentially. We demonstrate for the first time that both PLC-beta2 isoforms are expressed in platelets at the protein level. Studies in CV-1 cells transfected with PLC-beta2a or beta2b cDNAs, along with constitutively activated Galphaq (Q209L), showed that inositolphosphate formation was comparable between the two variants. However, the nuclear localization of the two isoforms was different with a higher cytoplasmic to nuclear ratio for PLC-beta2b compared to PLC-beta2a, suggesting that a great proportion of the total PLC-beta2a was in the nucleus relative to PLC-beta2b. There was no difference in the relative distribution of the two variants between the cytosol and particulate fractions. Both PLC-beta2 alternative splice variants are expressed at the protein level in platelets. In transfected CV-1 cells, PLC-beta2a is relatively more enriched in the nuclei than PLC-beta2b suggesting that the two variants may have different effects in cell proliferation and differentiation.

Protein kinase C inhibits binding of diacylglycerol kinase-zeta to the retinoblastoma protein

We previously showed that the retinoblastoma protein (pRB), a key regulator of G1 to S-phase transition of the cell cycle, binds to and stimulates diacylglycerol kinase-zeta (DGKzeta) to phosphorylate the lipid second messenger diacylglycerol into phosphatidic acid. pRB binds to the MARCKS phosphorylation-site domain of DGKzeta that can be phosphorylated by protein kinase C (PKC). Here, we report that activation of PKC by phorbol ester inhibits DGKzeta binding to pRB. Ro 31-8220, a specific inhibitor of PKC, alleviated this inhibition of binding. Mimicking of PKC phosphorylation of serine residues (by S/D but not S/N mutations) within the DGKzeta-MARCKS phosphorylation-site domain also prevented DGKzeta binding to pRB, suggesting that PKC phosphorylation of these residues negatively regulates the interaction between DGKzeta and pRB. In PKC overexpression studies, it appeared that activation of particularly the (wild-type) PKCalpha isoform inhibits DGKzeta binding to pRB, whereas dominant-negative PKCalpha neutralized this inhibition. PKCalpha activation thus prevents DGKzeta regulation by pRB, which may have implications for nuclear diacylglycerol and phosphatidic acid levels during the cell cycle.

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.

Nuclear PI(4,5)P(2): a new place for an old signal

Over the last decades, evidence has accumulated suggesting that there is a distinct nuclear phosphatidylinositol pathway. One of the best examined nuclear lipid pathways is the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) by PLC resulting in activation of nuclear PKC and production of inositol polyphosphates. However, there is a growing number of data that phosphoinositides are not only precursor for soluble inositol phosphates and diacylglycerol, instead they can act as second messengers themselves. They have been implicated to play a role in different important nuclear signaling events such as cell cycle progression, apoptosis, chromatin remodeling, transcriptional regulation and mRNA processing. This review focuses on the role of specifically PI4,5P(2) in the nucleus as a second messenger as well as a precursor for PI3,4,5P3, inositol polyphosphates and diacylglycerol.

Nuclear phosphoinositide kinases and inositol phospholipids

The presence of inositol phospholipids in the nuclei of mammalian cells has by now been well established, as has the presence of the enzymes responsible for their metabolism. However, our understanding of the role of these nuclear phosphoinositides in regulating cellular events has lagged far behind that for its cytosolic counterpart. It is clear, though, that the nuclear phosphoinositide pool is independent of the cytosolic pool and is, therefore, likely to be regulating a unique set of cellular events. As with its cytosolic phosphoinositides, many nuclear phosphoinositides and their metabolic enzymes are located at distinct sub-cellular structures. This arrangement spatially limits the production and activity of inositol phospholipids and is believed to be a major mechanism for regulating their function. Here, we will introduce the components of nuclear inositol phospholipid signal transduction and discuss how their spatial arrangement may dictate which nuclear functions they are modulating.

Nuclear inositol lipid metabolism: more than just second messenger generation?

A distinct polyphosphoinositide cycle is present in the nucleus, and growing evidence suggests its importance in DNA replication, gene transcription, and apoptosis. Even though it was initially thought that nuclear inositol lipids would function as a source for second messengers, recent findings strongly indicate that lipids present in the nucleus also fulfil other roles. The scope of this review is to highlight the most intriguing advances made in the field over the last few years, such as the possibility that nuclear phosphatidylinositol (4,5) bisphosphate is involved in maintaining chromatin in a transcriptionally active conformation, the new emerging roles for intranuclear phosphatidylinositol (3,4,5) trisphosphate and phosphoinositide 3-kinase, and the evidence which suggests a tight relationship between a decreased level of nuclear phosphoinositide specific phospholipase C-beta1 and the evolution of myelodisplastic syndrome into acute myeloid leukemia.

Linking lipids to chromatin

Dynamic regulation of chromatin structure is thought to be a prerequisite for nuclear functions that require accessibility to DNA such as replication, transcription and DNA repair. The phosphoinositide (PI) pathway is a second messenger signalling system regulated in response to a variety of extracellular (growth factors, differentiation signals) and intracellular (cell cycle progression, DNA damage) stimuli. The presence of a PI pathway in the nucleus together with the recent findings that specific nuclear proteins can interact with and are regulated by phosphoinositides suggest that changes in the nuclear phosphoinositide profile may have a direct role in modulating chromatin structure.

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.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Multidisciplinary utilization of dimethyl sulfoxide: pharmacological, cellular, and molecular aspects

DMSO is an amphipathic molecule with a highly polar domain and two apolar methyl groups, making it soluble in both aqueous and organic media. It is one of the most common solvents for the in vivo administration of several water-insoluble substances. Despite being frequently used as a solvent in biological studies and as a vehicle for drug therapy, the side-effects of DMSO (undesirable for these purposes) are apparent from its utilization in the laboratory (both in vivo and in vitro) and in clinical settings. DMSO is a hydrogen-bound disrupter, cell-differentiating agent, hydroxyl radical scavenger, intercellular electrical uncoupler, intracellular low-density lipoprotein-derived cholesterol mobilizing agent, cryoprotectant, solubilizing agent used in sample preparation for electron microscopy, antidote to the extravasation of vesicant anticancer agents, and topical analgesic. Additionally, it is used in the treatment of brain edema, amyloidosis, interstitial cystitis, and schizophrenia. Several systemic side-effects from the use of DMSO have been reported, namely nausea, vomiting, diarrhea, hemolysis, rashes, renal failure, hypertension, bradycardia, heart block, pulmonary edema, cardiac arrest, and bronchospasm. Looking at the multitude of effects of DMSO brought to light by these studies, it is easily understood how many researchers working with DMSO (or studying one of its specific effects) might not be fully aware of the experiences of other groups who are working with it but in a different context.

Nuclear and chromatin lipids: metabolism in normal and gamma-irradiated rats

The data on nuclear and chromatin lipid metabolism are reviewed. The amount of neutral lipids and phospholipids in nuclei of rat thymus, liver and neocortex neuron as well as the amount of lipids in rat thymus and liver chromatin are described. The metabolic responses of nuclear and chromatin lipids from thymus to different doses and dose rates of gamma-irradiation of rats are discussed. In most cases, the nuclear and chromatin lipid responses are distinct. Changes in nuclear and chromatin lipid metabolism in response to gamma-irradiation are suggested to connect with the signal transduction pathway and the regulation of the transcriptional and replicative chromatin activity. The influence of beta-carotene and picrotoxin on rat liver nuclear lipids and neocortex neuronal nuclear lipids, respectively, was analyzed. The possible involvement of the lipid traffic in the chromatin lipid responses to gamma-irradiation and other agents is suggested.

Lipid kinases and Ca(2+) signaling in Trypanosoma cruzi stimulated by a synthetic peptide

The synthetic peptide carrying residues 1-40 of chicken alpha(D)-globin, which promotes differentiation in Trypanosoma cruzi epimastigote, stimulated PPtdIns-k, DAG-k, and PA-k activities in a dose-dependent manner. A biphasic behavior only for PPtdIns-k and DAG-k was demonstrated by changes in [(32)P]PPtdIns and PtdOH levels, the earlier phase peaking at 3 min with a return to basal levels by 6 min and then a second phase with a sustained increase in time. This behavior was not observed for PA-k; the DGPP levels peaked at 6 min and were sustained in time. PMA pretreatment only abolished the first peak of PPtdIns-k, DAG-k activities, and InsPs/InsP(3) levels. There was also a transient elevation in intracellular calcium concentration, but this variation was modified only 50% by PMA. The results suggest that peptide 1-40 induces activation of the inositol cycle through lipid kinase activation in a biphasic manner. In this response, the early increase of enzymatic activities would be regulated by PKC and the InsP(3) may only be responsible, in part, for the calcium signaling.

Proliferating or differentiating stimuli act on different lipid-dependent signaling pathways in nuclei of human leukemia cells

Previous results have shown that the human promyelocytic leukemia HL-60 cell line responds to either proliferating or differentiating stimuli. When these cells are induced to proliferate, protein kinase C (PKC)-beta II migrates toward the nucleus, whereas when they are exposed to differentiating agents, there is a nuclear translocation of the alpha isoform of PKC. As a step toward the elucidation of the early intranuclear events that regulate the proliferation or the differentiation process, we show that in the HL-60 cells, a proliferating stimulus (i.e., insulin-like growth factor-I [IGF-I]) increased nuclear diacylglycerol (DAG) production derived from phosphatidylinositol (4,5) bisphosphate, as indicated by the inhibition exerted by 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and U-73122 (1-[6((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione), which are pharmacological inhibitors of phosphoinositide-specific phospholipase C. In contrast, when HL-60 cells were induced to differentiate along the granulocytic lineage by dimethyl sulfoxide, we observed a rise in the nuclear DAG mass, which was sensitive to either neomycin or propranolol, two compounds with inhibitory effect on phospholipase D (PLD)-mediated DAG generation. In nuclei of dimethyl sulfoxide-treated HL-60 cells, we observed a rise in the amount of a 90-kDa PLD, distinct from PLD1 or PLD2. When a phosphatidylinositol (4,5) bisphosphate-derived DAG pool was generated in the nucleus, a selective translocation of PKC-beta II occurred. On the other hand, nuclear DAG derived through PLD, recruited PKC-alpha to the nucleus. Both of these PKC isoforms were phosphorylated on serine residues. These results provide support for the proposal that in the HL-60 cell nucleus there are two independently regulated sources of DAG, both of which are capable of acting as the driving force that attracts to this organelle distinct, DAG-dependent PKC isozymes. Our results assume a particular significance in light of the proposed use of pharmacological inhibitors of PKC-dependent biochemical pathways for the therapy of cancer disease.

Regulation of phosphoinositide-specific phospholipase C

Eleven distinct isoforms of phosphoinositide-specific phospholipase C (PLC), which are grouped into four subfamilies (beta, gamma, delta, and epsilon), have been identified in mammals. These isozymes catalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to inositol 1,4,5-trisphosphate and diacylglycerol in response to the activation of more than 100 different cell surface receptors. All PLC isoforms contain X and Y domains, which form the catalytic core, as well as various combinations of regulatory domains that are common to many other signaling proteins. These regulatory domains serve to target PLC isozymes to the vicinity of their substrate or activators through protein-protein or protein-lipid interactions. These domains (with their binding partners in parentheses or brackets) include the pleckstrin homology (PH) domain [PtdIns(3)P, beta gamma subunits of G proteins] and the COOH-terminal region including the C2 domain (GTP-bound alpha subunit of Gq) of PLC-beta; the PH domain [PtdIns(3,4,5)P3] and Src homology 2 domain [tyrosine-phosphorylated proteins, PtdIns(3,4,5)P3] of PLC-gamma; the PH domain [PtdIns(4,5)P2] and C2 domain (Ca2+) of PLC-delta; and the Ras binding domain (GTP-bound Ras) of PLC-epsilon. The presence of distinct regulatory domains in PLC isoforms renders them susceptible to different modes of activation. Given that the partners that interact with these regulatory domains of PLC isozymes are generated or eliminated in specific regions of the cell in response to changes in receptor status, the activation and deactivation of each PLC isoform are likely highly regulated processes.

Cytoplasmic and nuclear phospholipase C-beta 1 relocation: role in resumption of meiosis in the mouse oocyte

The location of the phospholipase C beta 1-isoform (PLC-beta 1) in the mouse oocyte and its role in the resumption of meiosis were examined. We used specific monoclonal antibodies to monitor the in vitro dynamics of the subcellular distribution of the enzyme from the release of the oocyte from the follicle until breakdown of the germinal vesicle (GVBD) by Western blotting, electron microscope immunohistochemistry, and confocal microscope immunofluorescence. PLC-beta 1 became relocated to the oocyte cortex and the nucleoplasm during the G2/M transition, mainly in the hour preceding GVBD. The enzyme was a 150-kDa protein, corresponding to PLC-beta 1a. Its synthesis in the cytoplasm increased during this period, and it accumulated in the nucleoplasm. GVBD was dramatically inhibited by the microinjection of anti-PLC-beta1 monoclonal antibody into the germinal vesicle (GV) only when this accumulation was at its maximum. In contrast, PLC-gamma 1 was absent from the GV from the time of release from the follicle until 1 h later, and microinjection of anti-PLC-gamma 1 into the GV did not affect GVBD. Our results demonstrate a relationship between the relocation of PLC-beta 1 and its role in the first step of meiosis.

Evidence for two alternatively spliced forms of phospholipase C-beta2 in haematopoietic cells

Alternatively spliced forms have been reported for several phospholipase C (PLC) isozymes, but not for PLC-beta2, the most abundant PLC-beta in platelets. PLC-beta2 cDNA cloned from the HL-60-cell cDNA library is 3543 bases long, coding for 1181 amino acids. Compared with the published sequence, a deletion of 45 nucleotides (2755-2799 nt, amino acids 864-878) was detected in platelet and leucocyte mRNA amplified by reverse transcription (RT) polymerase chain reaction (PCR) using primers corresponding to 1814-1838 nt (forward) and 3328-3352 nt (reverse). Amplification of genomic DNA using primers corresponding to 2575-2596 nt and 2864-2885 nt yielded a approximately 750 bp product; restriction analysis and sequencing revealed the 45-bp exon flanked by introns of 198 bp and 118 bp. Amplification of leucocyte and platelet cDNA using the same primers yielded products of approximately 310 nt and approximately 265 nt, with (PLC-beta2a) and without (PLC-beta2b) the 45-nt sequence. Thus, two alternatively spliced forms (1181 and 1166 amino acids) of PLC-beta2 are generated in haematopoietic cells. They differ in the carboxyl terminal sequence implicated in interaction of PLC-beta enzymes with Galphaq, particulate association and nuclear localization. We propose that the PLC-beta2 splice variants may be regulated differentially with distinct roles in signal transduction.

Searching for a function for nuclear actin

The abundant cytoskeletal protein actin has numerous cytoplasmic roles. Although there are many reports of the presence of actin in the nucleus, in general they have been discounted as artifactual. However, recent work has begun to provide evidence for important roles for actin in nuclear processes ranging from chromatin remodelling to splicing. In addition, several regulators of actin polymerization are localized to the nucleus or translocate to the nucleus in a regulated manner, suggesting that there is some function of actin in the nucleus that is subject to regulation. This review discusses the evidence for actin in the nucleus and summarizes recent work suggesting that actin or actin-related proteins are involved in the regulation of nuclear processes such as chromatin remodelling.

Insulin-like growth factor-I-dependent stimulation of nuclear phospholipase C-beta1 activity in Swiss 3T3 cells requires an intact cytoskeleton and is paralleled by increased phosphorylation of the phospholipase

Swiss 3T3 mouse fibroblasts were exposed to 10 microM colchicine to disrupt microtubules, then stimulated with insulin-like growth factor-I. Immunoprecipitation experiments showed that insulin-like growth factor-I receptor and insulin receptor substrate-1 were tyrosine phosphorylated to the same extent in both cells treated with colchicine and in those not exposed to the drug. Moreover, the activity of phosphatidylinositol 3-kinase was not affected by incubation with colchicine. While in nuclei prepared from cells not exposed to colchicine it was possible to detect an insulin-like growth factor-I-dependent increase in the mass of diacylglycerol, as well as stimulation of phospholipase C activity, no similar changes were observed in nuclei obtained from cells treated with colchicine. Activation of the nuclear phospholipase activity was paralleled by an increase of its phosphorylation. Immunofluorescent studies revealed that mitogen-activated protein kinase did not translocate towards the nucleus when the cytoskeleton was depolymerized. These results show that in Swiss 3T3 cells some as yet unknown events necessary for the insulin-like growth factor-I-dependent activation of nuclear polyphosphoinositide metabolism require the presence of an intact cytoskeleton and are situated down-stream the activation of insulin receptor substrate-1 and phosphatidylinositol 3-kinase. Activation of nuclear phospholipase C-beta1 might be linked to its phosphorylation and translocation of mitogen-activated protein kinase to the nucleus.

Phosphatidylinositol 3-kinase in HL-60 nuclei is bound to the nuclear matrix and increases during granulocytic differentiation

We have used HL-60 leukemia cells to investigate phosphatidylinositol 3-kinase (PI 3-K) during granulocytic differentiation at the nuclear level. Nuclei of HL-60 cells showed a constitutive presence of PI 3-K that increased when cells were treated with differentiating doses of ATRA. PI 3-K was also detected tightly bound to nuclear matrices of HL-60 cells, isolated by nuclease treatment and high salt extraction. Four days of ATRA treatment induced a striking increase of nuclear matrix bound PI 3-K. In situ morphological analysis by confocal microscopy showed the translocation of PI 3-K to the nucleus and to the subnuclear fractions. PI 3-K enzymatic activity was stimulated during the granulocytic differentiation process and parallelled the increase in content of nuclei and subnuclear fractions. PI 3-K activity was recovered in nuclei also without the addition of exogenous substrates, consistent with the presence of both substrates and enzyme in the nucleus. These results indicate that specific intracellular localization of PI 3-K determines the production of different phosphoinositides in the sites of the enzyme translocation, and suggest that 3-phosphoinositide metabolism may play a specific role in the nucleus, candidating PI 3-K as a key enzyme in promoting granulocytic differentiation of HL-60 cells.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.

Nuclear diacylglycerol produced by phosphoinositide-specific phospholipase C is responsible for nuclear translocation of protein kinase C-alpha

It is well established that an independent inositide cycle is present within the nucleus, where it is involved in the control of cell proliferation and differentiation. Previous results have shown that when Swiss 3T3 cells are treated with insulin-like growth factor-I (IGF-I) a rapid and sustained increase in mass of diacylglycerol (DAG) occurs within the nuclei, accompanied by a decrease in the levels of both phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. However, it is unclear whether or not other lipids could contribute to this prolonged rise in DAG levels. We now report that the IGF-I-dependent increase in nuclear DAG production can be inhibited by the specific phosphatidylinositol phospholipase C inhibitor 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine or by neomycin sulfate but not by the purported phosphatidylcholine-phospholipase C specific inhibitor D609 or by inhibitors of phospholipase D-mediated DAG generation. Treatment of cells with 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine or neomycin sulfate inhibited translocation of protein kinase C-alpha to the nucleus. Moreover, exposure of cells to 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine, but not to D609, dramatically reduced the number of cells entering S-phase upon stimulation with IGF-I. These results suggest that the only phospholipase responsible for generation of nuclear DAG after IGF-I stimulation of 3T3 cells is PI-PLC. When this activity is inhibited, neither DAG rise is seen nor PKC-alpha translocation to the nucleus occurs. Furthermore, this PI-PLC activity appears to be essential for the G0/G1 to S-phase transition.

Inositides in the nucleus: taking stock of PLC beta 1

The nucleus was shown to be a site for inositol lipid cycle which can be affected by treatment of quiescent cells with growth factors such as IGF-I. In fact, the exposure of Swiss 3T3 cells to IGF-I results in a rapid and transient increase in nuclear PLC beta 1 activity. In addition, several other reports have shown the involvement of PLC beta 1 in nuclear signalling in different cell types. Indeed, PLC beta 1 differs from the PLC gamma and della isozymes in that it has a long COOH-terminal sequence which contains a cluster of lysine residues that are critical for association with the nucleus. Although the demonstration of PtInsP and PtdInsP2 hydrolysis by nuclear PLC beta 1 established the existence of nuclear PLC signalling, the significance of this autonomous pathway in the nucleus has yet to be thoroughly clarified. By inducing both the inhibition of PLC beta 1 expression by antisense RNA and its overexpression we show that this nuclear PLC is essential for the onset of DNA synthesis following IGF-I stimulation of quiescent Swiss 3T3 cells. Moreover, using a different cell system, i.e. Friend erythroleukemia cells induced to differentiate towards erythrocytes, it has been evidenced that there is a relationship between the expression and activity of nuclear PLC beta 1 and the association of PI-PT alpha with the nucleus in that, when PLC activity ceases, in differentiated and resting cells at the same time there is a dramatic decrease of the association of PI-PT alpha with the nucleus.

In vitro and in vivo inhibitory actions of morin on rat brain phosphatidylinositolphosphate kinase activity

Phosphatidylinositol-4,5-bisphosphate occupies a central role in signal transduction and in cellular transformation. Phosphatidylinositol-4,5-bisphosphate is produced by the enzymatic phosphorylation of phosphatidylinositol-4-phosphate by phosphatidylinositolphosphate kinase (EC 2.7.1.68). Inhibition of this enzyme might conceivably lowers the cellular pool of phosphatidylinositol-4,5-bisphosphate, thus constituting a feasible control point in regulating signal transduction and cellular transformation. Morin, a plant flavonoid, was demonstrated to exhibit in vitro inhibitory action on phosphatidylinositolphosphate kinase extracted from rat brain. This inhibition of enzymatic activity was found to be dose-dependent, with an IC50 value of approximately 10 microM morin. Lineweaver-Burk transformation of the inhibition data indicates that inhibition was competitive with respect to ATP. The Ki was calculated to be 5.15 x 10(-6) M. Inhibition was uncompetitive with respect to phosphatidylinositol-4-phosphate. The Ki was determined to be 0.94 x 10(-5) M. Administration of morin to rats led to a decrease in phosphatidylinositolphosphate kinase activity in brain extracts. This in vivo action of morin was found to be dose-dependent and time-dependent. These effects of morin on rat brain phosphatidylinositolphosphate kinase activity are discussed in relation to the other reported biological actions of this flavonoid.

A role for nuclear phosphatidylinositol-specific phospholipase C in the G2/M phase transition

Protein kinase C (PKC) is activated at the nucleus during the G2 phase of cell cycle, where it is required for mitosis. However, the mechanisms controlling cell cycle-dependent activation of nuclear PKC are not known. We now report that nuclear levels of the major physiologic PKC activator diacylglycerol (DAG) fluctuate during cell cycle. Specifically, nuclear DAG levels in G2/M phase cells are 2. 5-3-fold higher than in G1 phase cells. In synchronized cells, nuclear DAG levels rise to a peak coincident with the G2/M phase transition and return to basal levels in G1 phase cells. This increase in DAG level is sufficient to stimulate betaII PKC-mediated phosphorylation of its mitotic nuclear envelope substrate lamin B in vitro. Isolated nuclei from G2 phase cells contain an active phospholipase activity capable of generating DAG in vitro. Nuclear phospholipase activity is inhibited by the selective phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and neomycin sulfate, but not by the phosphatidylcholine-PLC selective inhibitor D609 or inhibitors of phospholipase D-mediated DAG generation. Treatment of synchronized cells with 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine leads to decreased nuclear PI-PLC activity and cell cycle blockade in the G2 phase, suggesting a role for nuclear PI-PLC in the G2/M phase transition. Our data are consistent with the hypothesis that nuclear PI-PLC generates DAG to activate nuclear betaII PKC, whose activity is required for mitosis.

Changes of nuclear PI-PLC gamma1 during rat liver regeneration

We have previously demonstrated that rat liver nuclei contain PI-PLC beta1 and gamma1 in the inner nuclear matrix and lamina associated with specific phosphodiesterase activity (Bertagnolo et al., 1995, Cell Signall. 7, 669-678). Since compensatory hepatic growth is an informative and well characterized model for natural cell proliferation, the presence of specific PI-PLC isoforms and their activity as well as PIP2 recovery were studied at various regenerating times, ranging from 3 to 22 h after partial hepatectomy. Three PI-PLC isoforms (beta1, gamma1, delta1) were examined in control and regenerating liver cells by using specific antibodies. By means of in situ immunocytochemistry and confocal microscopy, PI-PLC beta1 was found mainly in the nucleoplasm and this pattern was not modified after hepatectomy. On the contrary, the nuclear gamma1 isoform showed a marked decrease at 3 and 16 h after hepatectomy, but a clear increase at 22 h covering with bright intensity the whole nucleus. The PI-PLC delta1 isoform, which is exclusively cytoplasmic, was not altered during rat liver regeneration. By western blotting analysis on whole cell homogenates, none of the PI-PLC isozymes under study showed proliferation-linked modification. However, analyses of isolated nuclei identified changes in the nucleus associated PI-PLC gamma1 that paralleled the in situ observation whereas the beta1 isoform was unmodified at all the times examined. Nuclear phosphodiesterase activity on PIP2 was lower at 3 and 16 h, in comparison with sham operated rats, increased at 6 h and reached the highest value after 22 h. Consistently, the recovery of PIP2, obtained in conditions that optimise PIP-kinase activity, showed a marked decrease at 3 h and an increase up to 16 h of liver regeneration, followed by a further decrease at 22 h. These data are consistent with a close relationship between cell proliferation and the nuclear inositide cycle, depending, in rat liver, predominantly on the modulation of the gamma1 isoform of PI-PLC.

Intranuclear translocation of phospholipase C beta2 during HL-60 myeloid differentiation

Phospholipases C (PLC) beta3, gamma1, and gamma2 were detected in nuclei of HL-60 promyelocitic leukaemia cells. When HL-60 cells undergo terminal myeloid differentiation in the presence of ATRA, the beta2 isoform appeared inside nuclei and was up-regulated until 72 hours of ATRA treatment. The beta3 isozyme was also increased until 72 hours and both isoforms lowered their intranuclear amount at 96 hours and following days of treatment. By contrast PLC gamma1 and gamma2 progressively increased in the nucleus during granulocytic differentiation even after 72 hours of treatment. Terminal differentiation was characterised by the expression of high levels of PLC gamma1 and gamma2 and by low levels of PLC beta2 and beta3 in the nucleus. PIP2 and PIP hydrolysis paralleled the prevalence of the beta or gamma subfamily, respectively. Moreover, at all the examined times no changes of PLCs in the whole cell were detectable, indicating a de novo nuclear translocation of the beta2 and an increased accumulation of beta3, gamma1, and gamma2 isoforms. Thus, the intranuclear presence, expression, and activity of PLC isozymes, which are modulated during differentiation of HL-60 cells, implicate a role for nuclear phosphoinositide signalling in the process of cell maturation. In particular the nuclear translocation of PLC beta2 candidates this PLC as a key enzyme in the granulocytic differentiative commitment of HL-60 cells.

Phosphoinositide signalling in nuclei of Friend cells: DMSO-induced differentiation reduces the association of phosphatidylinositol-transfer protein with the nucleus

Friend erythroleukemia cells have a nuclear phosphoinositide cycle which is related to both mitogen-stimulated cell growth and erythorid differentiation. Because of the important role of the phosphatidylinositol-transfer protein (PI-TP) in phosphatidylinositol 4,5-bisphosphate (PtdInsP2) synthesis, we have analysed nuclei isolated from Friend cells for the presence of PI-TP. By Western Blotting it was demonstrated that both intact nuclei and nuclei deprived of the outer membrane contained the PI-TP alpha isoform. Upon induction of erythroid differentiation by DMSO, the amount of nuclear PI-TP alpha was greatly diminished. As shown previously, under these same conditions, nuclear phospholipase C beta1 (PLC beta1) is down-regulated as well.

A 104-kDa diacylglycerol kinase containing ankyrin-like repeats localizes in the cell nucleus

The cDNA corresponding to a fourth species of diacylglycerol (DG) kinase (EC 2.7.1.107) was isolated from cDNA libraries of rat retina and brain. This cDNA encoded a 929-aa, 104-kDa polypeptide termed DGK-IV. DGK-IV was different from previously identified mammalian DG kinase species, DGK-I, DGK-II, and DGK-III, in that it contained no EF-hand motifs but did contain four ankyrin-like repeats at the carboxyl terminus. These structural features of DGK-IV closely resemble the recently cloned, eye-specific DG kinase of Drosophila that is encoded by the retinal degeneration A (rdgA) gene. However, DGK-IV was expressed primarily in the thymus and brain with relatively low expression in the eye and intestine. Furthermore, the primary structure of the DGK-IV included a nuclear targeting motif, and immunocytochemical analysis revealed DGK-IV to localize in the nucleus of COS-7 cells transfected with the epitope-tagged cDNA, suggesting an involvement of DGK-IV in intranuclear processes.

The role of carboxyl-terminal basic amino acids in Gqalpha-dependent activation, particulate association, and nuclear localization of phospholipase C-beta1

The phospholipase C (PLC)-beta isozymes differ from the PLC-gamma and PLC-delta isozymes in that they possess a long COOH-terminal sequence downstream of their catalytic domain, are activated by alpha subunits of the Gq class of G proteins, associate with the particulate subcellular fraction, and are present in the nucleus. Most of the COOH-terminal domain of PLC-beta isozymes is predicted to be helical, and three regions in this domain, PLC-beta1 residues 911-928 (region 1), 1055-1072 (region 2), and 1109-1126 (region 3), contain a high proportion of basic residues that are highly conserved. Projection of the sequences of these three regions in helical wheels reveals clustering of the basic residues. The role of the COOH terminus and the clustered basic residues in PLC-beta1 was investigated by either truncating the entire COOH-terminal domain (mutant DeltaC) or replacing two or three clustered basic residues with isoleucine (or methionine), and expressing the mutant enzymes in CV-1, Rat-2, or Swiss 3T3 cells. The DeltaC mutant no longer showed the ability to be activated by Gqalpha, to translocate to the nucleus, or to associate with the particulate fraction. Substitution of clusters of basic residues in regions 1 and 2 generally reduced the extent of activation by Gqalpha, whereas substitution of a basic cluster in region 3 had no effect. Substitution of the cluster of lysine residues 914, 921, and 925 in region 1 had the most marked effect, reducing Gqalpha-dependent activity to 10% of that of wild type. All substitution mutants, with the exception of that in which lysine residues 1056, 1063, and 1070 in region 2 were substituted with isoleucine, behaved like the wild-type enzyme in showing an approximately equal distribution between cytoplasm and nucleus; only 12% of the region 2 mutant was present in the nucleus. None of the basic clusters appeared critical for particulate association; however, replacement of each cluster reduced the amount of PLC-beta1 in the particulate fraction by some extent, suggesting that all the basic residues contribute to the association, presumably by interacting with acidic residues in the particulate fraction. Membrane localization of PLC-beta isozymes is therefore likely mediated by both the COOH-terminal domain and the pleckstrin homology domain, the latter of which is known to bind phosphatidylinositol 4,5-biphosphate.

Calcium transported to isolated rat liver nuclei by nicotinamide adenine dinucleotide is insensitive to thapsigargin

Calcium uptake by isolated nuclei was mediated by nicotinamide adenine dinucleotide. Oxidized nicotinamide nucleotide analogues were more effective mediators of nuclear calcium uptake. Thapsigargin inhibited ATP-mediated nuclear calcium transport without affecting NAD-mediated nuclear calcium uptake. Whilst DBHQ did not influence ATP-induced calcium transport, it did stimulate NAD-mediated nuclear calcium entry. Calcium channel blockers did not influence the action of NAD. This study provides a further mechanism for nuclear calcium transport regulated by changes in the cytosolic NAD(+)/NADH ratio.