Nuclear Lipid Signaling Research Articles

Nuclear Phosphatidylinositol Signaling: Focus on Phosphatidylinositol Phosphate Kinases and Phospholipases C

Phosphatidylinositol (PI) metabolism represents the core of a network of signaling pathways which modulate many cellular functions including cell proliferation, cell differentiation, apoptosis, and membrane trafficking. An array of kinases, phosphatases, and lipases acts on PI creating an important number of second messengers involved in different cellular processes. Although, commonly, PI signaling was described to take place at the plasma membrane, many evidences indicated the existence of a PI cycle residing in the nuclear compartment of eukaryotic cells. The discovery of this mechanism shed new light on many nuclear functions, such as gene transcription, DNA modifications, and RNA expression. As these two PI cycles take place independently of one another, understanding how nuclear lipid signaling functions and modulates nuclear output is fundamental in the study of many cellular processes. J. Cell. Physiol. 231: 1645-1655, 2016. © 2015 Wiley Periodicals, Inc.

Disentangling biological signaling networks by dynamic coupling of signaling lipids to modifying enzymes

An unresolved problem in biological signal transduction is how particular branches of highly interconnected signaling networks can be decoupled, allowing activation of specific circuits within complex signaling architectures. Although signaling dynamics and spatiotemporal mechanisms serve critical roles, it remains unclear if these are the only ways cells achieve specificity within networks. The transcription factor Steroidogenic Factor-1 (SF-1) is an excellent model to address this question, as it forms dynamic complexes with several chemically distinct lipid species (phosphatidylinositols, phosphatidylcholines and sphingolipids). This property is important since lipids bound to SF-1 are modified by lipid signaling enzymes (IPMK & PTEN), regulating SF-1 biological activity in gene expression. Thus, a particular SF-1/lipid complex can interface with a lipid signaling enzyme only if SF-1 has been loaded with a chemically compatible lipid substrate. This mechanism permits dynamic downstream responsiveness to constant upstream input, disentangling specific pathways from the full network. The potential of this paradigm to apply generally to nuclear lipid signaling is discussed, with particular attention given to the nuclear receptor superfamily of transcription factors and their phospholipid ligands.

Nuclear PTEN Promotes Neuronal Survival

The Nedd4 ubiquitin ligase adaptor Ndfip1 is needed for nuclear translocation of the lipid phosphatase Pten after cerebral ischemia.

Nuclear Phospholipase C in Biological Control and Cancer

Inositol lipids are key regulators of several cellular functions. The identification of an independent nuclear polyphosphoinositides signaling machinery has led the way to find new roles for these molecules. PI-PLC-β1 is the most extensively studied PLC isoform in the nuclear compartment and a key player in the regulation of nuclear lipid signaling. Nuclear PI-PLC-β1 is involved in cell cycle progression and differentiation in response to growth factor stimulation. A growing body of evidence has demonstrated that nuclear phosphoinositides are also involved in cancer cell generation, proliferation, and resistance to apoptosis. Evidence on ex vivo human cancer cells from patients with myelodysplastic syndromes (MDS) confirmed these observations, suggesting the involvement of PI-PLC-β1 both in the pathogenesis of the disease and in the progression of MDS to acute myeloid leukemia. These studies have offered new targets for the development of novel therapeutic strategies as well as new prognostic tools.

Plant nuclear lipid signaling

Our goal is to characterize the nuclear PI pathway in plants and understand the role of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) in regulating nuclear signaling. Because PtdIns(4,5)P2 is so low in plants, our approach was to generate a plant cell line with increased nuclear PtdInsP kinase activity. We expressed the human phosphatidylinositol 4‐phosphate 5 kinase 1α (HsPIP5K1) in Nicotiana tabacum cells, isolated nuclei from the transformed and non‐transformed cells and compared the effect of expressing HsPIP5K1 on nuclear lipids. Lipid kinase activity was assayed. The PIPK specific activity was 20‐fold higher in HsPIP5K1–expressing cell nuclei. Transformed nuclei contained an average of 32.4 pmoles of PtdInsP(4,5)P2 per mg protein while the PtdInsP(4,5)P2 in WT nuclei was not detectable. Nuclear lipids in HsPIP5K1‐expressing cells showed a decrease in total LysoPC and LysoPE; however, the percentage of unsaturated fatty acids increased in several phospholipids e.g. 18:1 in LysoPC and LysoPE. In spite of the increase in nuclear PtdInsP(4,5)P2, the HsPIP5K1 cells continue to divide and grow although the rate of DNA synthesis is less. We are currently assessing the status of the posttranslational modifications of histones in anticipation that these will be affected by the high levels of PtdInsP(4,5)P2. Funded in part by grant from NSF (WFB) and NIH training grant (CBD).

Nuclear inositide signaling in myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are defined as clonal hematopoietic stem-cell disorders characterized by ineffective hematopoiesis in one or more of the lineages of the bone marrow. Although distinct morphologic subgroups exist, the natural history of MDS is progression to acute myeloid leukemia (AML). However, the molecular the mechanisms the underlying MDS evolution to AML are not completely understood. Inositides are key cellular second messengers with well-established roles in signal transduction pathways, and nuclear metabolism elicited by phosphoinositide-specific phospholipase C (PI-PLC) beta1 and Akt plays an important role in the control of the balance between cell cycle progression and apoptosis in both normal and pathologic conditions. Recent findings evidenced the role played by nuclear lipid signaling pathways, which could become promising therapeutic targets in MDS. This review will provide a concise and updated revision of the state of art on this topic.

Kinases in the Polyadenylation Complex

After transcription of a gene, the abundance of a protein in a cell can be controlled through various mechanisms operating on the mRNA, including regulation of alternative splicing, polyadenylation, stability and degradation, nuclear export, and accessibility to translation factors. Star-PAP is a noncanonical poly(A) polymerase that has a proline-rich region (PRR), which splits the catalytic domain. Star-PAP functions as part of a larger macromolecular complex. However, in addition to the common proteins found in other polyadenylation complexes, the Star-PAP complex includes phosphatidylinositol-4-phosphate 5 kinase Iα (PIPKIα), and Star-PAP activity is stimulated by phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ]. Gonzales et al . showed that the Star-PAP complex isolated from transfected cells also includes protein kinase activity and that Star-PAP is phosphorylated by the associated kinase. The protein kinase activity of the Star-PAP complex was inhibited by the addition of PI(4,5)P 2 , which led the authors to identify the kinase as casein kinase Iα (CKIα), which is known to be inhibited by PI(4,5)P 2 . CKIα and Star-PAP colocalized in nuclear speckles, which are a site of pre-mRNA processing, and inhibitors of CKIα blocked Star-PAP phosphorylation in vitro. Star-PAP is responsible for increasing the abundance of many mRNAs based on microarray experiments; however, CKIα and PIPKIα appear to control the targeting of Star-PAP to a subset of mRNAs involved in oxidative stress. Cells in which CKIα or PIPKIα was decreased using siRNA showed a specific decrease in the abundance of a subset of mRNAs processed by Star-PAP, including two that encode enzymes involved in the response to oxidative stress (HO-1 and NQO1), without affecting other transcripts targeted by Star-PAP. Thus, the presence of a lipid kinase and a protein kinase in the Star-PAP complex appears to target Star-PAP to specific mRNA targets and connects nuclear lipid signaling to mRNA processing. M. L. Gonzales, D. L. Mellman, R. A. Anderson, CKIα is associated with and phosphorylates Star-PAP and is also required for expression of select Star-PAP target messenger RNAs. J. Biol. Chem. 283 , 12665-12673 (2008). [Abstract] [Full Text]

Nuclear phosphoinositide signaling

Nuclear lipid metabolism performs a pivotal function in multiple signaling networks that mediate a variety of cellular events, including proliferation, differentiation and cell survival. The presence of phosphoinositides in the nuclei of mammalian cells is no longer in any doubt, and this has been corroborated by the detection of the enzymes responsible for phosphoinositide metabolism, phosphoinositide kinases and phosphatases in the nucleus. The nuclear phosphoinositide pool exists independently of the cytosolic pool, thereby showing a distinctive feature of nuclear lipid signaling as opposed to its cytosolic counterpart. The principal objective of this review is to summarize our updated knowledge regarding nuclear phosphoinositides and to discuss the current theories about the roles of the nuclear phosphoinositides.

Nuclear expression of diacylglycerol kinases: possible involvement in DNA replication.

The existence of intranuclear lipid-dependent signal transduction systems has been demonstrated by several independent groups. Remarkably, intranuclear lipid-dependent signal transduction pathways are regulated independently from their membrane/cytosolic counterparts. A sizable body of evidence suggests that nuclear lipid signaling controls critical biological functions such as cell proliferation, differentiation, and apoptosis. Diacylglycerol (DG) is a fundamental lipid second messenger which is produced in the nucleus. Since the levels of nuclear DG fluctuate during the cell cycle progression, it has been suggested that this lipid second messenger has important regulatory roles. Most likely, nuclear DG serves as a chemoattractant for some isoforms of protein kinase C that migrate to the nucleus in response to a variety of agonists. The nucleus also contains diacylglycerol kinases (DGKs), i.e. the enzymes that, by converting DG into phosphatidic acid (PA), terminate DG-dependent events. This review aims at highlighting the different isozymes of DGKs present within the nucleus as well as at discussing their potential functions with particular emphasis placed on DNA replication.

PHD Finger Finds a Partner

The nuclear phosphoinositide (PI) signaling pathways remain less well characterized than the cytosolic PI pathways (see Irvine). Gozani et al. identified ING2, a protein that may link p53 to chromatin remodeling, in a screen for PI-binding proteins. They discovered that the ING2 plant homeodomain (PHD) finger motif, which is a zinc finger motif with no known binding partners, interacted with PIs and showed highest affinity for PI(3)P and PI(5)P. Structural modeling and mutational analysis of the ING2 PHD suggested that two basic regions were required for PI binding. Other PHD fingers from other proteins also bound PIs when used as glutathione S -transferase (GST) fusions to probe lipid blots. Expression of the bacterial protein IpgD, which increases the abundance of PI(5)P at the plasma membrane, along with green fluorescent protein (GFP)-tagged ING2 showed PHD-dependent relocalization of a diffuse cytosolic pool of ING2 to the plasma membrane (ING2 was also prevalent in the nucleus in these cells). In cells overexpressing phosphoinositide kinase IIβ (PIKIIβ), the interaction between endogenous ING2 and the chromatin-nuclear matrix fraction was decreased. The ability of overexpressed ING2 to stimulate p53 acetylation and gene regulatory activity and apoptotic activity was abolished by mutation of the PHD to block lipid binding. RNA interference knock down of ING2 or overexpression of the PHD finger of ING2 to act as a dominant-negative decreased etoposide-induced apoptosis and p53 acetylation. These results together identify a binding partner for the orphan PHD finger and demonstrate that ING2 serves as a link between the nuclear PI cycle and p53 activity. O. Gozani, P. Karuman, D. R. Jones, D. Ivanov, J. Cha, A. A. Lugovskoy, C. L. Baird, H. Zhu, S. J. Field, S. L. Lessnick, J. Villasenor, B. Mehrotra, J. Chen, V. R. Rao, J. S. Brugge, C. G. Ferguson, B. Payrastre, D. G. Myszka, L. C. Cantley, G. Wagner, N. Divecha, G. D. Prestwich, J. Yuan, The PHD finger of the chromatin-associated protein ING2 functions as a nuclear phosphoinositide receptor. Cell 114 , 99-111 (2003). [Online Journal] R. F. Irvine, Nuclear lipid signaling. Sci. STKE 2002 , re13 (2003). [Abstract] [Full Text]

PHD Finger Finds a Partner

The nuclear phosphoinositide (PI) signaling pathways remain less well characterized than the cytosolic PI pathways (see Irvine). Gozani et al. identified ING2, a protein that may link p53 to chromatin remodeling, in a screen for PI-binding proteins. They discovered that the ING2 plant homeodomain (PHD) finger motif, which is a zinc finger motif with no known binding partners, interacted with PIs and showed highest affinity for PI(3)P and PI(5)P. Structural modeling and mutational analysis of the ING2 PHD suggested that two basic regions were required for PI binding. Other PHD fingers from other proteins also bound PIs when used as glutathione S-transferase (GST) fusions to probe lipid blots. Expression of the bacterial protein IpgD, which increases the abundance of PI(5)P at the plasma membrane, along with green fluorescent protein (GFP)-tagged ING2 showed PHD-dependent relocalization of a diffuse cytosolic pool of ING2 to the plasma membrane (ING2 was also prevalent in the nucleus in these cells). In cells overexpressing phosphoinositide kinase IIβ (PIKIIβ), the interaction between endogenous ING2 and the chromatin-nuclear matrix fraction was decreased. The ability of overexpressed ING2 to stimulate p53 acetylation and gene regulatory activity and apoptotic activity was abolished by mutation of the PHD to block lipid binding. RNA interference knock down of ING2 or overexpression of the PHD finger of ING2 to act as a dominant-negative decreased etoposide-induced apoptosis and p53 acetylation. These results together identify a binding partner for the orphan PHD finger and demonstrate that ING2 serves as a link between the nuclear PI cycle and p53 activity.O. Gozani, P. Karuman, D. R. Jones, D. Ivanov, J. Cha, A. A. Lugovskoy, C. L. Baird, H. Zhu, S. J. Field, S. L. Lessnick, J. Villasenor, B. Mehrotra, J. Chen, V. R. Rao, J. S. Brugge, C. G. Ferguson, B. Payrastre, D. G. Myszka, L. C. Cantley, G. Wagner, N. Divecha, G. D. Prestwich, J. Yuan, The PHD finger of the chromatin-associated protein ING2 functions as a nuclear phosphoinositide receptor. Cell 114, 99-111 (2003). [Online Journal]R. F. Irvine, Nuclear lipid signaling. Sci. STKE 2002, re13 (2003). [Abstract] [Full Text]

New Concepts: Updated Reviews

This week the STKE unveils an important new feature: updated reviews. Because the STKE is a purely electronic resource, we've been able to develop a powerful new weapon in the battle against information overload. Today's issue includes an updated review on Nuclear Lipid Signaling by Robin F. Irvine (http://stke.sciencemag.org/cgi/content/full/sigtrans;2002/150/re13). Updated reviews allow the authors of an STKE review to revise their original review to keep its information up-to-date and accurate. Updating a review saves time for both the author and the reader. The author has a template into which to add or revise information as the field advances. The STKE also provides the reader who is familiar with the earlier version of the review a guide to the updated review called a "Review Update" (http://stke.sciencemag.org/cgi/content/full/sigtrans;2002/150/re13/DC1). This new type of document highlights the major changes in the updated review, allowing the reader to rapidly access the new information.

Nuclear lipid metabolism and signaling.

Evidence has been accumulating that nuclear lipid metabolism is involved in the regulation of nuclear functions. Here I describe an autonomous nuclear lipid signaling that has been found to be associated with the metabolism of such lipids as phosphoinositides, choline phospholipids, and the acylation and deacylation cycle. Some lipid signals from the plasma membrane ultimately reach the nucleus and regulate the nuclear function. In this case, however, generated lipids and their metabolites may not directly act on the nuclear factors involved in nuclear function. The unique and direct effects of nuclear lipids and their metabolites on nuclear factors are also discussed.

Diacylglycerol kinases in nuclear lipid-dependent signal transduction pathways.

Several independent groups have shown that lipid-dependent signal transduction systems operate in the nucleus and that they are regulated independently from their membrane and cytosolic counterparts. A sizable body of evidence suggests that nuclear lipid signaling controls critical biological functions such as cell proliferation and differentiation. Diacylglycerol is a fundamental lipid second messenger which is produced in the nucleus. The levels of nuclear diacylglycerol fluctuate during the cell cycle progression, suggesting that such a molecule has important regulatory roles. Most likely, nuclear diacylglycerol serves as a chemoattractant for some isoforms of protein kinase C that migrate to the nucleus in response to a variety of agonists. The nucleus also contains diacylglycerol kinases, i.e. the enzymes that, by converting diacylglycerol into phosphatidic acid, terminate diacylglycerol-dependent events. A number of diacylglycerol kinases encoded by separate genes are present in the mammalian genome. This review aims at highlighting the different isotypes of diacylglycerol kinases identified at the nuclear level as well as at discussing their potential function and regulation.

Recruiting Protein Kinase in Time for Mitosis

The nuclear lipid cycle and how it is regulated are areas of active investigation (see Irvine ). The generation of specific lipid species in the nucleus may play a key role in recruiting and activating lipid-dependent enzymes such as members of the protein kinase C (PKC) family. Deacon et al . suggest that the formation of the tetraunsaturated diacylglycerol species, 1-stearoyl, 2-arachidonyl glycerol (SAG), may be a key element for the localization and activation of PKC βII, a classical PKC isoform that is regulated by calcium and diacylgycerol. PKC βII translocated to the nucleus during the G 2 to M phase of the cell cycle and was presumably activated by its association with the membrane. Furthermore, the concentration of SAG was selectively increased in the nuclei of cells at G 2 -M. SAG activated PKC βII and PKC α in vitro. Thus, the localized increase in nuclear SAG may be sufficient to recruit and activate specific PKC isoforms during the G 2 -M phase of the cell cycle. R. Irvine, Nuclear lipid signaling. Science's STKE (2000), http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2000/48/re1 [Abstract] [Full Text] E. M. Deacon, T. R. Pettitt, P. Webb, T. Cross, H. Chahal, M. J. O. Wakelan, J. M. Lord, Generation of duacylglycerol molecular species through the cell cycle: a role for 1-stearoyl, 2-arachidonyl glycerol in the activation of nuclear protein kinase C-βII at G2/M. J. Cell Sci. 115 , 983-989 (2002). [Online Journal]

Recruiting Protein Kinase in Time for Mitosis

The nuclear lipid cycle and how it is regulated are areas of active investigation (see Irvine). The generation of specific lipid species in the nucleus may play a key role in recruiting and activating lipid-dependent enzymes such as members of the protein kinase C (PKC) family. Deacon et al. suggest that the formation of the tetraunsaturated diacylglycerol species, 1-stearoyl, 2-arachidonyl glycerol (SAG), may be a key element for the localization and activation of PKC βII, a classical PKC isoform that is regulated by calcium and diacylgycerol. PKC βII translocated to the nucleus during the G2 to M phase of the cell cycle and was presumably activated by its association with the membrane. Furthermore, the concentration of SAG was selectively increased in the nuclei of cells at G2-M. SAG activated PKC βII and PKC α in vitro. Thus, the localized increase in nuclear SAG may be sufficient to recruit and activate specific PKC isoforms during the G2-M phase of the cell cycle.R. Irvine, Nuclear lipid signaling. Science's STKE (2000), http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2000/48/re1 [Abstract] [Full Text]E. M. Deacon, T. R. Pettitt, P. Webb, T. Cross, H. Chahal, M. J. O. Wakelan, J. M. Lord, Generation of duacylglycerol molecular species through the cell cycle: a role for 1-stearoyl, 2-arachidonyl glycerol in the activation of nuclear protein kinase C-βII at G2/M. J. Cell Sci. 115, 983-989 (2002). [Online Journal]

Nuclear lipid signaling: novel role of eicosanoids.

Nuclear lipid signaling is an established, widespread mechanism that operates in multiple cellular processes including proliferative and differentiative responses to a variety of stimuli. In this literature review with key references highlighted, we put forward the hypothesis that differential flow through various intracrine mechanisms can dictate resultant cellular actions such as mitosis, differentiation, or apoptosis.

Nuclear lipid signaling.

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.