Identification of a functional nuclear export sequence in diacylglycerol kinase-zeta

Localization of phospholipid-related signal molecules in salivary glands of rodents: A review

BACKGROUND

In the 1950s, Hokin conducted initial studies on phosphoinositide turnover/cycle in salivary glandular cells. From these studies, the idea emerged that receptor-mediated changes in intramembranous levels of phosphoinositides represent an early step in the stimulus-response pathway. Based on this idea and the general view that knowledge of the exact localization of a given endogenous molecule in cells in situ is important for understanding its functional significance, we have reviewed available information about the localization of several representative phosphoinositide-signaling molecules in the salivary glands in situ in mice.

HIGHLIGHT

We focused on phosphatidylinositol 4-kinase, phosphatidylinositol 4 phosphate 5-kinase α, β, γ, phospholipase Cβ, muscarinic cholinoceptors 1 and 3, diacylglycerol kinase ζ, phospholipase D1 and 2, ADP-ribosylation factor 6 and its exchange factors for Arf6, and cannabinoid receptors. These molecules individually exhibit differential localization in a spatiotemporal manner in the exocrine glands, making it possible to deduce their functional significance, such as their involvement in secretion and cell differentiation.

CONCLUSION

Although phosphoinositide-signaling molecules whose in situ localization in glandular cells has been clarified are still limited, the obtained information on their localization suggests that their functional significance is more valuable in glandular ducts than in acini. It thus suggests the necessity of greater attention to the ducts in their physio-pharmacological analyses. The purpose of this review is to encourage more in situ localization studies of phosphoinositide-signaling molecules with an aim to further understand their possible involvement in the pathogenesis of salivary gland diseases.

Localization of phosphatidylinositol phosphate 5 kinase γ, phospholipase β3 and diacylglycerol kinase ζ in corneal epithelium in comparison with conjunctival epithelium of mice

Based on the theory that the phosphoinositide (PI) signal is involved in the physiology of cornea and conjunctiva, we examined the localization in the mouse anterior ocular epithelia of immunoreactivities for phosphatidylinositol 4-phosphate 5-kinase (PIP5K), phospholipase C (PLC) and diacylglycerol kinase (DGK), enzymes that work sequentially in PI cycle. Immunoreactivity for PIP5Kγ in the corneal epithelium, including the limbus, was distinct in adults in contrast to faint or negligible immunoreactivity in the conjunctival epithelium in neonatal mice. This adult localization pattern was first recognized at the postnatal time of eyelid opening. Immunoreactivity for PLCβ3 was rather equally distinct throughout the entire corneal and conjunctival epithelia in adults. DGKζ-immunoreactive nuclei were mainly localized in the basal half domain of the corneal epithelium but in both basal and apical domains of the conjunctival epithelium in adults. This nuclear immunoreactivity was at weak or negligible levels in the peripheral and limbus cornea and in a considerable portion of the bulbar conjunctival epithelium continuous with the limbus. The adult patterns for PLCβ3 and DGKζ were already present at birth. The present findings suggest the following possibilities on the functional significance of the three enzyme molecules. PIP5Kγ is involved in cornea-specific functions such as bright-field vision, including corneal transparency, and in the stability of epithelial junctions, for which there seems to be a much higher requirement in the corneal epithelium than in the conjunctival epithelium. PLCβ3 is involved from birth in as-yet undefined functions exerted ubiquitously from birth in both corneal and conjunctival epithelia. DGKζ is involved in regulation from birth of the transcription in epithelial cells, including apoptosis as well as regulation of mitosis of epithelial cells in both cornea and conjunctiva, with the transcription involvement more apparent in the conjunctiva, although it does not work in stem cells of the corneal limbus.

Regulation of p53 and NF-κB transactivation activities by DGKζ in catalytic activity-dependent and -independent manners

Diacylglycerol kinase (DGK) constitutes a family of enzymes that phosphorylate diacylglycerol to phosphatidic acid (PA). These lipids serve as second messengers, thereby activating distinct downstream cascades and different cellular responses. Therefore, DG-to-PA conversion activity induces a phase transition of signaling pathways. One member of the family, DGKζ, is involved closely with stress responses. Morphological data showing that DGKζ localizes predominantly to the nucleus and that it shuttles between the nucleus and the cytoplasm implicate DGKζ in the regulation of transcription factors during stress responses. Tumor suppressor p53 and NF-κB are major stress-responsive transcription factors. They exert opposing effects on cellular pathophysiology. Herein, we summarize DGKζ catalytic activity-dependent and -independent regulatory mechanisms of p53 and NF-κB transactivation activities, including p53 degradation and NF-κB nuclear translocation. We also discuss how each component of DGKζ-interacting protein complex modulates the specificity and selectivity of target gene expression.

Negative control of diacylglycerol kinase ζ-mediated inhibition of T cell receptor signaling by nuclear sequestration in mice

Diacylglycerol kinases (DGKs) play important roles in restraining diacylglycerol (DAG)-mediated signaling. Within the DGK family, the ζ isoform appears to be the most important isoform in T cells for controlling their development and function. DGKζ has been demonstrated to regulate T cell maturation, activation, anergy, effector/memory differentiation, defense against microbial infection, and antitumor immunity. Given its critical functions, DGKζ function should be tightly regulated to ensure proper signal transduction; however, mechanisms that control DGKζ function are still poorly understood. We report here that DGKζ dynamically translocates from the cytosol into the nuclei in T cells after TCR stimulation. In mice, DGKζ mutant defective in nuclear localization displayed enhanced ability to inhibit TCR-induced DAG-mediated signaling in primary T cells, maturation of conventional αβT and iNKT cells, and activation of peripheral T cells compared with WT DGKζ. Our study reveals for the first time nuclear sequestration of DGKζ as a negative control mechanism to spatially restrain it from terminating DAG mediated signaling in T cells. Our data suggest that manipulation of DGKζ nucleus-cytosol shuttling as a novel strategy to modulate DGKζ activity and immune responses for treatment of autoimmune diseases and cancer.

Diacylglycerol kinase control of protein kinase C

The diacylglycerol kinases (DGK) are lipid kinases that transform diacylglycerol (DAG) into phosphatidic acid (PA) in a reaction that terminates DAG-based signals. DGK provide negative regulation to conventional and novel protein kinase C (PKC) enzymes, limiting local DAG availability in a tissue- and subcellular-restricted manner. Defects in the expression/activity of certain DGK isoforms contribute substantially to cognitive impairment and mental disorders. Abnormal DGK overexpression in tumors facilitates invasion and resistance to chemotherapy preventing tumor immune destruction by tumor-infiltrating lymphocytes. Effective translation of these findings into therapeutic approaches demands a better knowledge of the physical and functional interactions between the DGK and PKC families. DGKζ is abundantly expressed in the nervous and immune system, where physically and functionally interacts with PKCα. The latest discoveries suggest that PDZ-mediated interaction facilitates spatial restriction of PKCα by DGKζ at the cell–cell contact sites in a mechanism where the two enzymes regulate each other. In T lymphocytes, DGKζ interaction with Sorting Nexin 27 (SNX27) guarantees the basal control of PKCα activation. SNX27 is a trafficking component required for normal brain function whose deficit has been linked to Alzheimer's disease (AD) pathogenesis. The enhanced PKCα activation as the result of SNX27 silencing in T lymphocytes aligns with the recent correlation found between gain-of-function PKCα mutations and AD and suggests that disruption of the mechanisms that provides a correct spatial organization of DGKζ and PKCα may lie at the basis of immune and neuronal synapse impairment.

Expression and localization of diacylglycerol kinase ζ in guinea pig cochlea and its functional implication under noise-exposure stress conditions

Cochlear hair cells are essential for the mechanotransduction of hearing. Sensorineural hearing loss can be irreversible because hair cells have a minimal ability to repair or regenerate themselves once damaged. In order to develop therapeutic interventions to prevent hair cell loss, it is necessary to understand the signaling pathway operating in cochlear hair cells and its alteration upon damage. Diacylglycerol kinase (DGK) regulates intracellular signal transduction through phosphorylation of lipidic second messenger diacylglycerol. We have previously reported characteristic expression and localization patterns of DGKs in various organs under pathophysiological conditions. Nevertheless, little is known about morphological and functional aspects of this enzyme family in the cochlea. First RT-PCR analysis reveals predominant mRNA expression of DGKα, DGKε and DGKζ. Immunohistochemical analysis shows that DGKζ localizes to the nuclei of inner hair cells (IHCs), outer hair cells (OHCs), supporting cells and spiral ganglion neurons in guinea pig cochlea under normal conditions. It is well known that loud noise exposure induces cochlear damage, thereby resulting in hair cell loss. In particular, OHCs are highly vulnerable to noise exposure than IHCs. We found that after 1 week of noise exposure DGKζ translocates from the nucleus to the cytoplasm in damage-sensitive OHCs and gradually disappears thereafter. In sharp contrast, DGKζ remains to the nucleus in damage-resistant IHCs. These results suggest that DGKζ cytoplasmic translocation is well correlated with cellular damage under noise-exposure stress conditions and is involved in delayed cell death in cochlear outer hair cells.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

DGKζ ablation engenders upregulation of p53 level in the spleen upon whole-body ionizing radiation

The tumor suppressor gene product p53, which coordinates the cellular response to various stresses, is subject to tight regulation by a complex network of signal transduction. The DGK family metabolizes lipidic second messenger diacylglycerol to produce phosphatidic acid. Our earlier studies showed that one isozyme, DGKζ, is involved in the regulatory mechanism of p53. In a cellular model of doxorubicin-induced DNA damage, overexpression of wild-type DGKζ suppresses p53 protein induction and reduces apoptosis, whereas knockdown of DGKζ upregulates p53 protein level and promotes apoptosis. Further examination reveals that DGKζ facilitates p53 degradation via ubiquitin-proteasome system in the cytoplasm. However, it remains undetermined whether the regulatory mechanism of DGKζ on p53 function found in cell-based experiments is also functional at the animal level. This study was conducted to elucidate this point using an experiment with DGKζ-KO mice under DNA damage induced by whole-body ionizing radiation. Our results reveal that p53 protein is induced robustly in the spleen of DGKζ-KO mice upon exposure to ionizing radiation, thereby promoting apoptosis in this organ. Taken together, the results demonstrate that DGKζ plays a sentinel role in p53 expression at the cellular and organismal levels after DNA damaging stress conditions.

Reciprocal regulation of p53 and NF-κB by diacylglycerol kinase ζ

Diacylglycerol kinase (DGK) participates in lipid mediated-signal transduction. It phosphorylates diacylglycerol (DG) to phosphatidic acid (PA), thereby regulating the balanced control of these second messenger actions. Previous reports have described that one DGK family, DGKζ, is closely involved in stress responses under various conditions. Cellular stress response, a physiological process enabling cells to cope with an altered environment, is finely tuned through various signaling cascades and their molecular crosstalk. The major components of stress response are p53 and NF-κB. p53 generally serves as a proapoptotic transcriptional factor, whereas NF-κB promotes resistance to programmed cell death under most circumstances. Recent studies have suggested that DGKζ facilitates p53 degradation in cytoplasm through ubiquitin proteasome system and that DGKζ deletion upregulates p53 protein levels under basal and DNA-damage conditions. Counter-intuitively, however, DGKζ deletion suppresses p53 transcriptional activity despite increased p53 levels. In contrast, DGKζ knockdown engenders enhancement of NF-κB pathway in response to cytokines such as TNF-α and IL-1β. In response to these cytokines, DGKζ downregulation accelerates phosphorylation of the p65 subunit and its nuclear translocation, thereby enhancing NF-κB transcriptional activity. Furthermore, DGKζ deficiency is shown to promote increased association of p65 subunit with the transcriptional cofactor CBP. It is particularly interesting that this association is observed even under basal conditions in the absence of stimulation. These findings suggest that DGKζ plays a role in sequestration of the limiting pool of CBP/p300 between the NF-κB p65 subunit and p53, and that DGKζ downregulation shifts CBP/p300 toward the NF-κB subunit to regulate reciprocally antagonistic phenotypes of these transcription factors.

Downregulation of diacylglycerol kinase ζ enhances activation of cytokine-induced NF-κB signaling pathway

The transcription factor NF-κB family serves as a key component of many pathophysiological events such as innate and adaptive immune response, inflammation, apoptosis, and oncogenesis. Various cell signals trigger activation of the regulatory mechanisms of NF-κB, resulting in its nuclear translocation and transcriptional initiation. The diacylglycerol kinase (DGK) family, a lipid second messenger-metabolizing enzyme in phosphoinositide signaling, is shown to regulate widely various cellular processes. Results of recent studies suggest that one family member, DGKζ, is closely involved in immune and inflammatory responses. Nevertheless, little is known about the regulatory mechanism of DGKζ on NF-κB pathway in cytokine-induced inflammatory signaling. This study shows that siRNA-mediated DGKζ knockdown in HeLa cells facilitates degradation of IκB, followed by nuclear translocation of NF-κB p65 subunit. In addition, DGKζ-deficient MEFs show upregulation of p65 subunit phosphorylation at Serine 468 and 536 and its interaction with CBP transcriptional coactivator upon TNF-α stimulation. These modifications of p65 subunit might engender enhanced NF-κB transcriptional reporter assay of DGKζ knockdown cells. These findings provide further insight into the regulatory mechanisms of cytokine-induced NF-κB activation.

DGKζ under stress conditions: “to be nuclear or cytoplasmic, that is the question”

Eukaryotic cells have evolved to possess a distinct subcellular compartment, the nucleus, separated from the cytoplasm in a manner that allows the precise operation of the chromatin, thereby permitting controlled access to the regulatory elements in the DNA for transcription and replication. In the cytoplasm, genetic information contained in the DNA sequence is translated into proteins, including enzymes that catalyze various reactions, such as metabolic processes, energy control, and responses to changing environments. One mechanism that regulates these events involves phosphoinositide turnover signaling, which generates a lipid second messenger, diacylglycerol (DG). Since DG acts as a potent activator of several signaling molecules, it should be tightly regulated to keep cellular responsiveness within a physiological range. DG kinase (DGK) metabolizes DG by phosphorylating it to generate phosphatidic acid, thus serving as a critical regulator of DG signaling. Phosphoinositide turnover is employed differentially in the nucleus and the cytoplasm. A member of the DGK family, DGKζ, localizes to the nucleus in various cell types and is considered to regulate nuclear DG signaling. Recent studies have provided evidence that DGKζ shuttles between the nucleus and the cytoplasm in neurons under pathophysiological conditions. Transport of a signal regulator between the nucleus and the cytoplasm should be a critical function for maintaining basic processes in the nucleus, such as cell cycle regulation and gene expression, and to ensure communication between nuclear processes and cytoplasmic functions. In this review, a series of studies on nucleocytoplasmic translocation of DGKζ have been summarized, and the functional implications of this phenomenon in postmitotic neurons and cancer cells under stress conditions are discussed.

Harnessing the PI3K/Akt/mTOR pathway in T-cell acute lymphoblastic leukemia: eliminating activity by targeting at different levels

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignant hematological disorder arising in the thymus from T-cell progenitors. T-ALL mainly affects children and young adults, and remains fatal in 20% of adolescents and 50% of adults, despite progress in polychemotherapy protocols. Therefore, innovative targeted therapies are desperately needed for patients with a dismal prognosis. Aberrant activation of PI3K/Akt/mTOR signaling is a common event in T-ALL patients and portends a poor prognosis. Preclinical studies have highlighted that modulators of PI3K/Akt/mTOR signaling could have a therapeutic relevance in T-ALL. However, the best strategy for inhibiting this highly complex signal transduction pathway is still unclear, as the pharmaceutical companies have disclosed an impressive array of small molecules targeting this signaling network at different levels. Here, we demonstrate that a dual PI3K/PDK1 inhibitor, NVP-BAG956, displayed the most powerful cytotoxic effects against T-ALL cell lines and primary patients samples, when compared with a pan class I PI3K inhibitor (GDC-0941), an allosteric Akt inhibitor (MK-2206), an mTORC1 allosteric inhibitor (RAD-001), or an ATP-competitive mTORC1/mTORC2 inhibitor (KU-63794). Moreover, we also document that combinations of some of the aforementioned drugs strongly synergized against T-ALL cells at concentrations well below their respective IC₅₀. This observation indicates that vertical inhibition at different levels of the PI3K/Akt/mTOR network could be considered as a future innovative strategy for treating T-ALL patients.

Cytoplasmic localization of DGKζ exerts a protective effect against p53-mediated cytotoxicity

The transcription factor p53 plays a crucial role in coordinating the cellular response to various stresses. Therefore, p53 protein levels and activity need to be kept under tight control. We report here that diacylglycerol kinase ζ (DGKζ) binds to p53 and modulates its function both in the cytoplasm and nucleus. DGKζ, one of the DGK family that metabolizes a lipid second messenger diacylglycerol, localizes primarily to the nucleus in various cell types. Recently, reports have described that excitotoxic stress induces DGKζ nucleocytoplasmic translocation in hippocampal neurons. In this study, we found that cytoplasmic DGKζ attenuates p53-mediated cytotoxicity against doxorubicin-induced DNA damage by facilitating cytoplasmic anchoring and degradation of p53 through a ubiquitin–proteasome system. Concomitantly, decreased levels of nuclear DGKζ engender down-regulation of p53 transcriptional activity. Consistent with these in vitro cellular experiments, DGKζ-deficient brain exhibits high levels of p53 protein after kainate-induced seizures and even under normal conditions. These findings provide novel insights into the regulation of p53 function and suggest that DGKζ serves as a sentinel to control p53 function both during normal homeostasis and in stress responses.

Conflict of interests: multiple signal peptides with diverging goals

Peptide signal sequences attached to or embedded into a core protein sequence control its cellular localization and several post-translational modifications. However, misleading or cumbersome results may be generated when expressing recombinant proteins with modified signal peptides or single domains of larger proteins.

Altered expression of diacylglycerol kinase isozymes in regenerating liver

The liver possesses the capacity to restore its function and mass after injury. Liver regeneration is controlled through complicated mechanisms, in which the phosphoinositide (PI) cycle is shown to be activated in hepatocytes. Using a rat partial hepatectomy (PH) model, the authors investigated the expression of the diacylglycerol kinase (DGK) family, a key enzyme in the PI cycle, which metabolizes a lipid second-messenger diacylglycerol (DG). RT-PCR analysis shows that DGKζ and DGKα are the major isozymes in the liver. Results showed that in the process of regeneration, the DGKζ protein, which is detected in the nucleus of a small population of hepatocytes in normal liver, is significantly increased in almost all hepatocytes. However, the mRNA levels remain largely unchanged. Double labeling with bromodeoxyuridine (BrdU), an S phase marker, reveals that DGKζ is expressed independently of DNA synthesis or cell proliferation. However, DGKα protein localizes to the cytoplasm in normal and regenerating livers, but immunoblot analysis reveals that the expected (80 kDa) and the lower (70 kDa) bands are detected in normal liver, whereas at day 10 after PH, the expected band is solely recognized, showing a different processing pattern of DGKα in liver regeneration. These results suggest that DGKζ and DGKα are involved, respectively, in the nucleus and the cytoplasm of hepatocytes in regenerating liver.

Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR inhibitors: rationale and importance to inhibiting these pathways in human health

The Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades are often activated by genetic alterations in upstream signaling molecules such as receptor tyrosine kinases (RTK). Integral components of these pathways, Ras, B-Raf, PI3K, and PTEN are also activated/inactivated by mutations. These pathways have profound effects on proliferative, apoptotic and differentiation pathways. Dysregulation of these pathways can contribute to chemotherapeutic drug resistance, proliferation of cancer initiating cells (CICs) and premature aging. This review will evaluate more recently described potential uses of MEK, PI3K, Akt and mTOR inhibitors in the proliferation of malignant cells, suppression of CICs, cellular senescence and prevention of aging. Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt/mTOR pathways play key roles in the regulation of normal and malignant cell growth. Inhibitors targeting these pathways have many potential uses from suppression of cancer, proliferative diseases as well as aging.