Diacylglycerol kinase delta regulates protein kinase C and epidermal growth factor receptor signaling

Diacylglycerol kinases are essential for hepatocyte growth factor-dependent proliferation and motility of Kaposi's sarcoma cells

Hepatocyte growth factor (HGF) is involved in the pathogenesis of Kaposi's sarcoma (KS), the most frequent neoplasia in patients with AIDS, characterized by proliferating spindle cells, infiltrating inflammatory cells, angiogenesis, edema, and invasiveness. In vitro, this factor sustains the biological behavior of KS derived cells, after activation of its receptor and the downstream MAPK and AKT signals. In other cell types, namely endothelial and epithelial cells, movement, proliferation, and survival stimulated by HGF and other growth factors and cytokines depend on diacylglycerol kinases (DGK). In an effort to identify new intracellular transducers operative in KS cells, which could represent therapeutic targets, we investigated the role of DGK in KS cell movement and proliferation by treating cells with the DGK pharmacological inhibitor R59949. We report that R59949 strongly inhibits HGF-induced KS motility, proliferation, and anchorage-independent growth with only a partial effect on cell adhesion and spreading. R59949 does not affect cell survival, HGF receptor activation, or the classical MAPK and AKT signalling pathways. Furthermore, we carried out an siRNA screen to characterize the DGK isoforms involved in KS motility and anchorage independent growth. Our data indicate a strong involvement of DGK-δ in KS motility and of DGK-ι in anchorage-independent growth. These results indicate that DGK inhibition is sufficient to impair in vitro KS cell proliferation and movement and suggest that selected DGK represent new pharmacological targets to interfere with the malignant properties of KS, independently from the well-known RAS/MAPK and PI3K/AKT pathways.

Signal transduction pathways in the pathophysiology of bipolar disorder

Signal transduction pathways and genes associated with cellular life and death have received much attention in bipolar disorder (BPD) and provide scientists with molecular targets for understanding the biological basis of BPD. In this chapter, we describe the signal transduction pathways involved in the molecular biology of BPD and the indications for the mechanisms of disease and treatment. We discuss the BPD literature with respect to the disease itself and the effects of mood stabilizer treatment on cellular receptors, including G-protein-coupled receptors, glutamate receptors, and tyrosine receptor kinase. We also discuss the intracellular alterations observed in BPD to second messenger systems, such as cyclic adenosine monophosphate (cAMP), protein kinase A, phosphoinositide pathways, glycogen synthase kinase-3, protein kinase B, Wnt, and arachidonic acid. We describe how receptor activation and modulation of second messengers occurs, and how transcription factors are activated and altered in this disease (e.g., the transcription factors ?-catenin, cAMP response element binding protein, heat shock transcription factor-1, and activator protein-1). Abnormalities in intracellular signal transduction pathways could generate a functional discrepancy in numerous neurotransmitter systems, which may explain the varied clinical symptoms observed in BPD. The influence of mood stabilizers on transcription factors may be important in connecting the regulation of gene expression to neuroplasticity and cellular resilience.

Diacylglycerol kinase delta and protein kinase C(alpha) modulate epidermal growth factor receptor abundance and degradation through ubiquitin-specific protease 8

Many human epithelial cancers are characterized by abnormal activation of the epidermal growth factor receptor (EGFR), which is often caused by its excessive expression in tumor cells. The abundance of EGFR is modulated, in part, by its ubiquitination, which targets it for degradation. The components responsible for adding ubiquitin to EGFR are well characterized, but this is a reversible process, and the mechanisms that modulate the removal of ubiquitin from the EGFR are not well known. We found that de-ubiquitination of EGFR was regulated by diacylglycerol kinase delta (DGKdelta), a lipid kinase that terminates diacylglycerol signaling. In DGKdelta-deficient cells, ubiquitination of EGFR was enhanced, which attenuated the steady-state levels of EGFR and promoted its ligand-induced degradation. These effects were not caused by changes in the ubiquitinating apparatus, but instead were due to reduced expression of the de-ubiquitinase, ubiquitin-specific protease 8 (USP8). Depletion of protein kinase Calpha (PKCalpha), a target of diacylglycerol, rescued the levels of USP8 and normalized EGFR degradation in DGKdelta-deficient cells. Moreover, the effects of PKCalpha were caused by its inhibition of Akt, which stabilizes USP8. Our data indicate a novel mechanism where DGKdelta and PKCalpha modulate the levels of ubiquitinated EGFR through Akt and USP8.

Lipid oversupply, selective insulin resistance, and lipotoxicity: molecular mechanisms

The accumulation of fat in tissues not suited for lipid storage has deleterious consequences on organ function, leading to cellular damage that underlies diabetes, heart disease, and hypertension. To combat these lipotoxic events, several therapeutics improve insulin sensitivity and/or ameliorate features of metabolic disease by limiting the inappropriate deposition of fat in peripheral tissues (i.e. thiazolidinediones, metformin, and statins). Recent advances in genomics and lipidomics have accelerated progress towards understanding the pathogenic events associated with the excessive production, underutilization, or inefficient storage of fat. Herein we review studies applying pharmacological or genetic strategies to manipulate the expression or activity of enzymes controlling lipid deposition, in order to gain a clearer understanding of the molecular mechanisms by which fatty acids contribute to metabolic disease.

Diacylglycerol kinase eta augments C-Raf activity and B-Raf/C-Raf heterodimerization

The Ras/B-Raf/C-Raf/MEK/ERK signaling cascade is critical for the control of many fundamental cellular processes, including proliferation, survival, and differentiation. This study demonstrated that small interfering RNA-dependent knockdown of diacylglycerol kinase eta (DGKeta) impaired the Ras/B-Raf/C-Raf/MEK/ERK pathway activated by epidermal growth factor (EGF) in HeLa cells. Conversely, the overexpression of DGKeta1 could activate the Ras/B-Raf/C-Raf/MEK/ERK pathway in a DGK activity-independent manner, suggesting that DGKeta serves as a scaffold/adaptor protein. By determining the activity of all the components of the pathway in DGKeta-silenced HeLa cells, this study revealed that DGKeta activated C-Raf but not B-Raf. Moreover, this study demonstrated that DGKeta enhanced EGF-induced heterodimerization of C-Raf with B-Raf, which transmits the signal to C-Raf. DGKeta physically interacted with B-Raf and C-Raf, regulating EGF-induced recruitment of B-Raf and C-Raf from the cytosol to membranes. The DGKeta-dependent activation of C-Raf occurred downstream or independently of the already known C-Raf modifications, such as dephosphorylation at Ser-259, phosphorylation at Ser-338, and interaction with 14-3-3 protein. Taken together, the results obtained strongly support that DGKeta acts as a novel critical regulatory component of the Ras/B-Raf/C-Raf/MEK/ERK signaling cascade via a previously unidentified mechanism.

Cyclooxygenase-2 (COX-2) negatively regulates expression of epidermal growth factor receptor and causes resistance to gefitinib in COX-2-overexpressing cancer cells

Overexpression of cyclooxygenase-2 (COX-2) and epidermal growth factor receptor (EGFR) has been detected in many types of cancer. Although COX-2 and EGFR are closely related to each other, the exact mechanism of COX-2 in tumors has not been well understood. In this study, we investigated the relationship between COX-2 and EGFR in cancer cells. Using two cell lines stably overexpressing COX-2 (HCT-116-COX-2 and H460-COX-2) and a stable line of COX-2 knockdown MOR-P cells, we analyzed patterns of COX-2 and EGFR expression. To observe the effects of COX-2 on EGFR expression and activity, we did comparative analyses after treatment with various drugs (EGF, celecoxib, prostaglandin E(2), gefitinib, Ro-31-8425, PD98059, and SP600125) in HCT-116-Mock versus HCT-116-COX-2 cells and H460-Mock versus H460-COX-2 cells. Overexpression of COX-2 specifically down-regulated EGFR expression at the level of transcription. COX-2-overexpressing cells have a decreased sensitivity to gefitinib. COX-2 induced activation of extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK) but suppressed Akt activation. JNK inhibition by SP600125, a specific JNK inhibitor, resulted in restoration of EGFR levels in COX-2-overexpressing cells, whereas ERK inhibition by PD98059 did not. Overexpressed COX-2 negatively regulates EGFR expression via JNK activation, leading to gefitinib resistance. COX-2 may also regulate ERK activity independently of EGFR. Therefore, resistance of COX-2-overexpressing cells to gefitinib may be due to decreased expression of EGFR by JNK activation and EGFR-independent elevation of ERK activity by COX-2. The ability of COX-2 to inhibit EGFR expression and gefitinib effects may have significance in clinical cancer therapy.

Diacylglycerol kinases as sources of phosphatidic acid

There are ten mammalian diacylglycerol kinases (DGKs) whose primary role is to terminate diacylglycerol (DAG) signaling. However, it is becoming increasingly apparent that DGKs also influence signaling events through their product, phosphatidic acid (PA). They do so in some cases by associating with proteins and then modifying their activity by generating PA. In other cases, DGKs broadly regulate signaling events by virtue of their ability to provide PA for the synthesis of phosphatidylinositols (PtdIns).

Diacylglycerol kinase delta associates with receptor for activated C kinase 1, RACK1

The delta-isozyme (type II) of diacylglycerol kinase (DGK) is known to positively regulate growth factor receptor signaling. DGKdelta, which is distributed to clathrin-coated vesicles, interacts with DGKdelta itself, protein kinase C and AP2alpha. To search for additional DGKdelta-interacting proteins, we screened a yeast two-hybrid cDNA library from HepG2 cells using aa 896-1097 of DGKdelta as a bait. We identified aa 184-317 (WD40 repeats 5-7) of receptor for activated C kinase 1 (RACK1), which interacts with various important signaling molecules, as a novel binding partner of DGKdelta. Co-immunoprecipitation analysis, using COS-7 cells co-expressing RACK1 and DGKdelta, revealed that RACK1 selectively interacted with DGKdelta, but not with type I DGKs, in mammalian cells. The interaction was dynamically regulated by phorbol ester. Intriguingly, DGKdelta appeared to recruit RACK1 to clathrin-coated vesicles and co-localized with RACK1. These results suggest that DGKdelta serves as an adaptor protein to regulate the localization of the versatile scaffold protein, RACK1.

Mammalian diacylglycerol kinases: molecular interactions and biological functions of selected isoforms

The mammalian diacylglycerol kinases (DGK) are a group of enzymes having important roles in regulating many biological processes. Both the product and the substrate of these enzymes, i.e. diacylglycerol and phosphatidic acid, are important lipid signalling molecules. Each DGK isoform appears to have a distinct biological function as a consequence of its location in the cell and/or the proteins with which it associates. This review discusses three of the more extensively studied forms of this enzyme, DGKalpha, DGKvarepsilon, and DGKzeta. DGKalpha has an important role in immune function and its activity is modulated by several mechanisms. DGKvarepsilon has several unique features among which is its specificity for arachionoyl-containing substrates, suggesting its importance in phosphatidylinositol cycling. DGKzeta is expressed in many tissues and also has several mechanisms to regulate its functions. It is localized in several subcellular organelles, including the nucleus. The current state of our understanding of the properties and functions of these proteins is reviewed.

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

ERBBs in the gastrointestinal tract: recent progress and new perspectives

The gastrointestinal epithelium does much more than provide a physical barrier between the intestinal lumen and our internal milieu. It is actively engaged in absorption and secretion of salt and water via ion transporters, exchangers and selective ion channels. It is also a continuously self-renewing epithelium that undergoes ordered growth and differentiation along its vertical axis. From this dual perspective, we will consider the actions of the ERBB family of ligands and receptors in the maintenance of gastrointestinal homeostasis and discuss instances when the actions of this family go awry such as in cancer and Ménétrier's disease.

Diacylglycerol kinases in immune cell function and self-tolerance

Both diacylglycerol (DAG) and phosphatidic acid (PA) are important second messengers involved in signal transduction from many immune cell receptors and can be generated and metabolized through multiple mechanisms. Recent studies indicate that diacylglycerol kinases (DGKs), the enzymes that catalyze phosphorylation of DAG to produce PA, play critical roles in regulating the functions of multiple immune cell lineages. In T cells, two DGK isoforms, alpha and zeta, inhibit DAG-mediated signaling following T-cell receptor engagement and prevent T-cell hyperactivation. DGK alpha and zeta synergistically promote T-cell anergy and are critical for T-cell tolerance. In mast cells, DGKzeta plays differential roles in their activation by promoting degranulation but attenuating cytokine production following engagement of the high affinity receptor for immunoglobulin E. In dendritic cells and macrophages, DGKzeta positively regulates Toll-like receptor-induced proinflammatory cytokine production through its product PA and is critical for host defense against Toxoplasma gondii infection. These studies demonstrate pivotal roles of DGKs in regulating immune cell function by acting both as signal terminator and initiator.

The life and death of protein kinase C

Protein kinase C (PKC) is a family of kinases that plays diverse roles in many cellular functions, notably proliferation, differentiation, and cell survival. PKC is processed by phosphorylation and regulated by cofactor binding and subcellular localization. Extensive detail is available on the molecular mechanisms that regulate the maturation, activation, and signaling of PKC. However, less information is available on how signaling is terminated both from a global perspective and isozyme-specific differences. To target PKC therapeutically, various ATP-competitive inhibitors have been developed, but this method has problems with specificity. One possible new approach to developing novel, specific therapeutics for PKC would be to target the signaling termination pathways of the enzyme. This review focuses on the new developments in understanding how PKC signaling is terminated and how current drug therapies as well as information obtained from the recent elucidation of various PKC structures and down-regulation pathways could be used to develop novel and specific therapeutics for PKC.

Regulation of enzyme localization by polymerization: polymer formation by the SAM domain of diacylglycerol kinase delta1

The diacylglycerol kinase (DGK) enzymes function as regulators of intracellular signaling by altering the levels of the second messengers, diacylglycerol and phosphatidic acid. The DGK delta and eta isozymes possess a common protein-protein interaction module known as a sterile alpha-motif (SAM) domain. In DGK delta, SAM domain self-association inhibits the translocation of DGK delta to the plasma membrane. Here we show that DGK delta SAM forms a polymer and map the polymeric interface by a genetic selection for soluble mutants. A crystal structure reveals that DGKSAM forms helical polymers through a head-to-tail interaction similar to other SAM domain polymers. Disrupting polymerization by polymer interface mutations constitutively localizes DGK delta to the plasma membrane. Thus, polymerization of DGK delta regulates the activity of the enzyme by sequestering DGK delta in an inactive cellular location. Regulation by dynamic polymerization is an emerging theme in signal transduction.

Regulation of clathrin-dependent endocytosis by diacylglycerol kinase delta: importance of kinase activity and binding to AP2alpha

DGKdelta (diacylglycerol kinase delta), which phosphorylates DAG (diacylglycerol) and converts it into PA (phosphatidic acid), has an important role in signal transduction. In the present study, we have demonstrated the molecular mechanism of DGKdelta-mediated regulation of clathrin-dependent endocytosis that controls the internalization, recycling and degradation of receptors. Involvement of DGKdelta in the regulation of clathrin-dependent endocytosis was previously proposed following genome-wide RNAi (RNA interference) screening. Clathrin-coated pits are mainly formed by clathrin and AP-2 (adaptor protein 2) complex. These proteins assemble a polyhedral lattice at the membrane and gather several endocytic accessory proteins. As the intracellular localization of DGKdelta2 overlapped with clathrin-coated pits, we predicted the possible regulation of clathrin-dependent endocytosis by DGKdelta2 and its interaction with some endocytosis-regulatory proteins. DGKdelta2 contained the DXF-type binding motifs, and DGKdelta2 bound to AP2alpha, a subunit of the AP-2 complex. DGKdelta2 interacted with the platform subdomain in the AP2alpha ear domain via F369DTFRIL and D746PF sequences in the catalytic domain of DGKdelta2. For further insight into the role for DGKdelta2 in clathrin-dependent endocytosis, we measured the transferrin and EGF (epidermal growth factor) uptake-expressing wild-type or mutant DGKdelta2 under knockdown of endogenous DGKdelta. Mutants lacking binding ability to AP2alpha as well as kinase-negative mutants could not compensate for the uptake of transferrin inhibited by siRNA (small interfering RNA) treatment, whereas overexpression of wild-type DGKdelta2 completely recovered the transferrin uptake. These results demonstrate that binding between DGKdelta2 and AP2alpha is involved in the transferrin internalization and that DGK activity is also necessary for the regulation of the endocytic process.

Diacylglycerol kinases: at the hub of cell signalling

DGKs (diacylglycerol kinases) are members of a unique and conserved family of intracellular lipid kinases that phosphorylate DAG (diacylglycerol), catalysing its conversion into PA (phosphatidic acid). This reaction leads to attenuation of DAG levels in the cell membrane, regulating a host of intracellular signalling proteins that have evolved the ability to bind this lipid. The product of the DGK reaction, PA, is also linked to the regulation of diverse functions, including cell growth, membrane trafficking, differentiation and migration. In multicellular eukaryotes, DGKs provide a link between lipid metabolism and signalling. Genetic experiments in Caenorhabditis elegans, Drosophila melanogaster and mice have started to unveil the role of members of this protein family as modulators of receptor-dependent responses in processes such as synaptic transmission and photoreceptor transduction, as well as acquired and innate immune responses. Recent discoveries provide new insights into the complex mechanisms controlling DGK activation and their participation in receptor-regulated processes. After more than 50 years of intense research, the DGK pathway emerges as a key player in the regulation of cell responses, offering new possibilities of therapeutic intervention in human pathologies, including cancer, heart disease, diabetes, brain afflictions and immune dysfunctions.

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

Glucose regulates diacylglycerol intracellular levels and protein kinase C activity by modulating diacylglycerol kinase subcellular localization

Although chronic hyperglycemia reduces insulin sensitivity and leads to impaired glucose utilization, short term exposure to high glucose causes cellular responses positively regulating its own metabolism. We show that exposure of L6 myotubes overexpressing human insulin receptors to 25 mm glucose for 5 min decreased the intracellular levels of diacylglycerol (DAG). This was paralleled by transient activation of diacylglycerol kinase (DGK) and of insulin receptor signaling. Following 30-min exposure, however, both DAG levels and DGK activity returned close to basal levels. Moreover, the acute effect of glucose on DAG removal was inhibited by >85% by the DGK inhibitor R59949. DGK inhibition was also accompanied by increased protein kinase C-alpha (PKCalpha) activity, reduced glucose-induced insulin receptor activation, and GLUT4 translocation. Glucose exposure transiently redistributed DGK isoforms alpha and delta, from the prevalent cytosolic localization to the plasma membrane fraction. However, antisense silencing of DGKdelta, but not of DGKalpha expression, was sufficient to prevent the effect of high glucose on PKCalpha activity, insulin receptor signaling, and glucose uptake. Thus, the short term exposure of skeletal muscle cells to glucose causes a rapid induction of DGK, followed by a reduction of PKCalpha activity and transactivation of the insulin receptor signaling. The latter may mediate, at least in part, glucose induction of its own metabolism.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

Diacylglycerol kinases: Why so many of them?

Diacylglycerol (DAG) kinase (DGK) modulates the balance between the two signaling lipids, DAG and phosphatidic acid (PA), by phosphorylating DAG to yield PA. To date, ten mammalian DGK isozymes have been identified. In addition to the C1 domains (protein kinase C-like zinc finger structures) conserved commonly in all DGKs, these isoforms possess a variety of regulatory domains of known and/or predicted functions, such as a pair of EF-hand motifs, a pleckstrin homology domain, a sterile alpha motif domain and ankyrin repeats. Beyond our expectations, recent studies have revealed that DGK isozymes play pivotal roles in a wide variety of signal transduction pathways conducting development, neural and immune responses, cytoskeleton reorganization and carcinogenesis. Moreover, there has been rapidly growing evidence indicating that individual DGK isoforms exert their specific roles through interactions with unique partner proteins such as protein kinase Cs, Ras guanyl nucleotide-releasing protein, chimaerins and phosphatidylinositol-4-phosphate 5-kinase. Therefore, an emerging paradigm for DGK is that the individual DGK isoforms assembled in their own signaling complexes should carry out spatio-temporally segregated tasks for a wide range of biological processes via regulating local, but not global, concentrations of DAG and/or PA.

Protein kinase C and other diacylglycerol effectors in cancer

Almost three decades after the discovery of protein kinase C (PKC), we still have only a partial understanding of how this family of serine/threonine kinases is involved in tumour promotion. PKC isozymes - effectors of diacylglycerol (DAG) and the main targets of phorbol-ester tumour promoters - have important roles in cell-cycle regulation, cellular survival, malignant transformation and apoptosis. How do PKC isozymes regulate these diverse cellular processes and what are their contributions to carcinogenesis? Moreover, what is the contribution of all phorbol-ester effectors, which include PKCs and small G-protein regulators? We now face the challenge of dissecting the relative contribution of each DAG signal to cancer progression.

Disruption of diacylglycerol kinase delta (DGKD) associated with seizures in humans and mice

We report a female patient with a de novo balanced translocation, 46,X,t(X;2)(p11.2;q37)dn, who exhibits seizures, capillary abnormality, developmental delay, infantile hypotonia, and obesity. The 2q37 breakpoint observed in association with the seizure phenotype is of particular interest, because it lies near loci implicated in epilepsy in humans and mice. Fluorescence in situ hybridization mapping of the translocation breakpoints showed that no known genes are disrupted at Xp11.2, whereas diacylglycerol kinase delta (DGKD) is disrupted at 2q37. Expression studies in Drosophila and mouse suggest that DGKD is involved in central nervous system development and function. Electroencephalographic assessment of Dgkd mutant mice revealed abnormal epileptic discharges and electrographic seizures in three of six homozygotes. These findings implicate DGKD disruption by the t(X;2)(p11.2;q37)dn in the observed phenotype and support a more general role for DGKD in the etiology of seizures.

Protein kinase Calpha mediates feedback inhibition of EGF receptor transactivation induced by Gq-coupled receptor agonists

While a great deal of attention has been focused on G-protein-coupled receptor (GPCR)-induced epidermal growth factor receptor (EGFR) transactivation, it has been known for many years that the tyrosine kinase activity of the EGFR is inhibited in cells treated with tumor-promoting phorbol esters, a process termed EGFR transmodulation. Because many GPCR agonists that elicit EGFR transactivation also stimulate the Gq/phospholipase C (PLC)/protein kinase C (PKC) pathway, we hypothesized that PKC-mediated inhibition of EGFR transactivation operates physiologically as a feedback loop that regulates the intensity and/or duration of GPCR-elicited EGFR transactivation. In support of this hypothesis, we found that treatment of intestinal epithelial IEC-18 cells with the PKC inhibitors GF 109203X or Ro 31-8220 or chronic exposure of these cells to phorbol-12,13-dibutyrate (PDB) to downregulate PKCs, markedly enhanced the increase in EGFR tyrosine phosphorylation induced by angiotensin II or vasopressin in these cells. Similarly, PKC inhibition enhanced EGFR transactivation in human colonic epithelial T84 cells stimulated with carbachol, as well as in bombesin-stimulated Rat-1 fibroblasts stably transfected with the bombesin receptor. Furthermore, cell treatment with inhibitors with greater specificity towards PKCalpha, including Gö6976, Ro 31-7549 or Ro 32-0432, also increased GPCR-induced EGFR transactivation in IEC-18, T84 and Rat-1 cells. Transfection of siRNAs targeting PKCalpha also enhanced bombesin-induced EGFR tyrosine phosphorylation in Rat-1 cells. Thus, multiple lines of evidence support the hypothesis that conventional PKC isoforms, especially PKCalpha, mediate feedback inhibition of GPCR-induced EGFR transactivation.