A Novel Diacylglycerol-lactone Shows Marked Selectivity in Vitro among C1 Domains of Protein Kinase C (PKC) Isoforms α and δ as Well as Selectivity for RasGRP Compared with PKCα

The cell biology of HIV-1 latency and rebound

Transcriptionally latent forms of replication-competent proviruses, present primarily in a small subset of memory CD4+ T cells, pose the primary barrier to a cure for HIV-1 infection because they are the source of the viral rebound that almost inevitably follows the interruption of antiretroviral therapy. Over the last 30 years, many of the factors essential for initiating HIV-1 transcription have been identified in studies performed using transformed cell lines, such as the Jurkat T-cell model. However, as highlighted in this review, several poorly understood mechanisms still need to be elucidated, including the molecular basis for promoter-proximal pausing of the transcribing complex and the detailed mechanism of the delivery of P-TEFb from 7SK snRNP. Furthermore, the central paradox of HIV-1 transcription remains unsolved: how are the initial rounds of transcription achieved in the absence of Tat? A critical limitation of the transformed cell models is that they do not recapitulate the transitions between active effector cells and quiescent memory T cells. Therefore, investigation of the molecular mechanisms of HIV-1 latency reversal and LRA efficacy in a proper physiological context requires the utilization of primary cell models. Recent mechanistic studies of HIV-1 transcription using latently infected cells recovered from donors and ex vivo cellular models of viral latency have demonstrated that the primary blocks to HIV-1 transcription in memory CD4+ T cells are restrictive epigenetic features at the proviral promoter, the cytoplasmic sequestration of key transcription initiation factors such as NFAT and NF-κB, and the vanishingly low expression of the cellular transcription elongation factor P-TEFb. One of the foremost schemes to eliminate the residual reservoir is to deliberately reactivate latent HIV-1 proviruses to enable clearance of persisting latently infected cells-the "Shock and Kill" strategy. For "Shock and Kill" to become efficient, effective, non-toxic latency-reversing agents (LRAs) must be discovered. Since multiple restrictions limit viral reactivation in primary cells, understanding the T-cell signaling mechanisms that are essential for stimulating P-TEFb biogenesis, initiation factor activation, and reversing the proviral epigenetic restrictions have become a prerequisite for the development of more effective LRAs.

Activation of Munc13-1 by Diacylglycerol (DAG)-Lactones

Munc13-1 is a key protein necessary for vesicle fusion and neurotransmitter release in the brain. Diacylglycerol (DAG)/phorbol ester binds to its C1 domain in the plasma membrane and activates it. The C1 domain of Munc13-1 and protein kinase C (PKC) are homologous in terms of sequence and structure. In order to identify small-molecule modulators of Munc13-1 targeting the C1 domain, we studied the effect of three DAG-lactones, (R,Z)-(2-(hydroxymethyl)-4-(3-isobutyl-5-methylhexylidene)-5-oxotetrahydrofuran-2-yl)methyl pivalate (JH-131e-153), (E)-(2-(hydroxymethyl)-4-(3-isobutyl-5-methylhexylidene)-5-oxotetrahydrofuran-2-yl)methyl pivalate (AJH-836), and (E)-(2-(hydroxymethyl)-4-(4-nitrobenzylidene)-5-oxotetrahydrofuran-2-yl)methyl 4-(dimethylamino)benzoate (130C037), on Munc13-1 activation using the ligand-induced membrane translocation assay. JH-131e-153 showed higher activation than AJH-836, and 130C037 was not able to activate Munc13-1. To understand the role of the ligand-binding site residues in the activation process, three alanine mutants were generated. For AJH-836, the order of activation was wild-type (WT) Munc13-1 > R592A > W588A > I590A. For JH-131e-153, the order of activation was WT > I590 ≈ R592A ≈ W588A. Overall, the Z isomer of DAG-lactones showed higher potency than the E isomer and Trp-588, Ile-590, and Arg-592 were important for its binding. When comparing the activation of Munc13-1 and PKC, the order of activation for JH-131e-153 was PKCα > Munc13-1 > PKCε and for AJH-836, the order of activation was PKCε > PKCα > Munc13-1. Molecular docking supported higher binding of JH-131e-153 than AJH-836 with the Munc13-1 C1 domain. Our results suggest that DAG-lactones have the potential to modulate neuronal processes via Munc13-1 and can be further developed for therapeutic intervention for neurodegenerative diseases.

Gluconolactone Alleviates Myocardial Ischemia/Reperfusion Injury and Arrhythmias via Activating PKCε/Extracellular Signal-Regulated Kinase Signaling

Gluconolactone (D-glucono-1,5-lactone or GDL) is a food additive which presents in dietary products such as tofu, yogurt, cheese, bread, wine, etc. GDL has long been considered as a free radical scavenger; however, its role in cardioprotection remains elusive. In this study, using a mouse model of myocardial ischemia/reperfusion (I/R) injury and a model of hypoxia/reoxygenation (H/R) in neonatal rat cardiomyocytes (NRCM), we explored the role of GDL in I/R injury. We found that GDL (5 mg/kg, i.p.) attenuated myocardial I/R injury as evidenced by decreased infarct size, release of cardiac injury markers and apoptosis. Additionally, GDL decreased reperfusion-induced arrhythmias and oxidative stress. These effects were also observed in parallel in vitro studies. Mechanistically, we found that GDL treatment was strongly associated with activation of pro-survival extracellular signal-regulated kinase (ERK) signaling both in vivo and in vitro, and pharmacological inhibition of ERK signaling via U0126 attenuated GDL-induced cardioprotection against H/R injury in NRCM cells. To reveal how GDL regulates ERK signaling, we predicted the putative targets of GDL by Swiss Target Prediction, and protein kinase C (PKC) emerged as the most promising target for GDL. By pharmacological intervention and immunofluorescence, we found that PKCε, an important member of the PKC family, was activated after GDL treatment in heart, thereby leading to ERK activation and cardioprotection against I/R injury. Taken together, our results demonstrated that GDL acts as a potent activator of PKCε and, thus, provides cardioprotection against I/R injury via activation of ERK signaling.

Design, Synthesis, and Characterization of Novel sn-1 Heterocyclic DAG-Lactones as PKC Activators

DAG-lactones represent useful templates for the design of potent and selective C1 domain ligands for PKC isozymes. The ester moiety at the sn-1 position, a common feature in this template, is relevant for C1 domain interactions, but it represents a labile group susceptible to endogenous esterases. An interesting challenge involves replacing the ester group of these ligands while still maintaining biological activity. Here, we present the synthesis and functional characterization of novel diacylglycerol-lactones containing heterocyclic ring substituents at the sn-1 position. Our results showed that the new compound 10B12, a DAG-lactone with an isoxazole ring, binds PKCα and PKCε with nanomolar affinity. Remarkably, 10B12 displays preferential selectivity for PKCε translocation in cells and induces a PKCε-dependent reorganization of the actin cytoskeleton into peripheral ruffles in lung cancer cells. We conclude that introducing a stable isoxazole ring as an ester surrogate in DAG-lactones emerges as a novel structural approach to achieve PKC isozyme selectivity.

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

α-Arylidene Diacylglycerol-Lactones (DAG-Lactones) as Selective Ras Guanine-Releasing Protein 3 (RasGRP3) Ligands

Diacylglycerol-lactones have proven to be a powerful template for the design of potent ligands targeting C1 domains, the recognition motif for the cellular second messenger diacylglycerol. A major objective has been to better understand the structure activity relations distinguishing the seven families of signaling proteins that contain such domains, of which the protein kinase C (PKC) and RasGRP families are of particular interest. Here, we synthesize a series of aryl- and alkyl-substituted diacylglycerol-lactones and probe their relative selectivities for RasGRP3 versus PKC. Compound 96 showed 73-fold selectivity relative to PKCα and 45-fold selectivity relative to PKCε for in vitro binding activity. Likewise, in intact cells, compound 96 induced Ras activation, a downstream response to RasGRP stimulation, with 8-29 fold selectivity relative to PKCδ S299 phosphorylation, a measure of PKCδ stimulation.

Characterization of AJH-836, a diacylglycerol-lactone with selectivity for novel PKC isozymes

Diacylglycerol (DAG) is a key lipid second messenger downstream of cellular receptors that binds to the C1 domain in many regulatory proteins. Protein kinase C (PKC) isoforms constitute the most prominent family of signaling proteins with DAG-responsive C1 domains, but six other families of proteins, including the chimaerins, Ras-guanyl nucleotide-releasing proteins (RasGRPs), and Munc13 isoforms, also play important roles. Their significant involvement in cancer, immunology, and neurobiology has driven intense interest in the C1 domain as a therapeutic target. As with other classes of targets, however, a key issue is the establishment of selectivity. Here, using [³H]phorbol 12,13-dibutyrate ([³H]PDBu) competition binding assays, we found that a synthetic DAG-lactone, AJH-836, preferentially binds to the novel PKC isoforms PKCδ and PKCϵ relative to classical PKCα and PKCβII. Assessment of intracellular translocation, a hallmark for PKC activation, revealed that AJH-836 treatment stimulated a striking preferential redistribution of PKCϵ to the plasma membrane relative to PKCα. Moreover, unlike with the prototypical phorbol ester phorbol 12-myristate 13-acetate (PMA), prolonged exposure of cells to AJH-836 selectively down-regulated PKCδ and PKCϵ without affecting PKCα expression levels. Biologically, AJH-836 induced major changes in cytoskeletal reorganization in lung cancer cells, as determined by the formation of membrane ruffles, via activation of novel PKCs. We conclude that AJH-836 represents a C1 domain ligand with PKC-activating properties distinct from those of natural DAGs and phorbol esters. Our study supports the feasibility of generating selective C1 domain ligands that promote novel biological response patterns.

Importance of the REM (Ras exchange) domain for membrane interactions by RasGRP3

RasGRP comprises a family of guanine nucleotide exchange factors, regulating the dissociation of GDP from Ras GTPases to enhance the formation of the active GTP-bound form. RasGRP1 possesses REM (Ras exchange), GEF (catalytic), EF-hand, C1, SuPT (suppressor of PT), and PT (plasma membrane-targeting) domains, among which the C1 domain drives membrane localization in response to diacylglycerol or phorbol ester and the PT domain recognizes phosphoinositides. The homologous family member RasGRP3 shows less plasma membrane localization. The objective of this study was to explore the role of the different domains of RasGRP3 in membrane translocation in response to phorbol esters. The full-length RasGRP3 shows limited translocation to the plasma membrane in response to PMA, even when the basic hydrophobic cluster in the PT domain, reported to be critical for RasGRP1 translocation to endogenous activators, is mutated to resemble that of RasGRP1. Moreover, exchange of the C-termini (SuPT-PT domain) of the two proteins had little effect on their plasma membrane translocation. On the other hand, while the C1 domain of RasGRP3 alone showed partial plasma membrane translocation, truncated RasGRP3 constructs, which contain the PT domain and are missing the REM, showed stronger translocation, indicating that the REM of RasGRP3 was a suppressor of its membrane interaction. The REM of RasGRP1 failed to show comparable suppression of RasGRP3 translocation. The marked differences between RasGRP3 and RasGRP1 in membrane interaction necessarily will contribute to their different behavior in cells and are relevant to the design of selective ligands as potential therapeutic agents.

Exploring the influence of indololactone structure on selectivity for binding to the C1 domains of PKCα, PKCε, and RasGRP

C1 domain-containing proteins, such as protein kinase C (PKC), have a central role in cellular signal transduction. Their involvement in many diseases, including cancer, cardiovascular disease, and immunological and neurological disorders has been extensively demonstrated and has prompted a search for small molecules to modulate their activity. By employing a diacylglycerol (DAG)-lactone template, we have been able to develop ultra potent analogs of diacylglycerol with nanomolar binding affinities approaching those of complex natural products such as phorbol esters and bryostatins. One current challenge is the development of selective ligands capable of discriminating between different protein family members. Recently, structure-activity relationship studies have shown that the introduction of an indole ring as a DAG-lactone substituent yielded selective Ras guanine nucleotide-releasing protein (RasGRP1) activators when compared to PKCα and PKCε. In the present work, we examine the effects of ligand selectivity relative to the orientation of the indole ring and the nature of the DAG-lactone template itself. Our results show that the indole ring must be attached to the lactone moiety through the sn-2 position in order to achieve RasGRP1 selectivity.

Protein kinase C in cancer: The top five unanswered questions

Few kinases have been studied as extensively as protein kinase C (PKC), particularly in the context of cancer. As major cellular targets for the phorbol ester tumor promoters and diacylglycerol (DAG), a second messenger generated by stimulation of membrane receptors, PKC isozymes play major roles in the control of signaling pathways associated with proliferation, migration, invasion, tumorigenesis, and metastasis. However, despite decades of research, fundamental questions remain to be answered or are the subject of intense controversy. Primary among these unresolved issues are the role of PKC isozymes as either tumor promoter or tumor suppressor kinases and the incomplete understanding on isozyme-specific substrates and effectors. The involvement of PKC isozymes in cancer progression needs to be reassessed in the context of specific oncogenic and tumor suppressing alterations. In addition, there are still major hurdles in addressing isozyme-specific function due to the limited specificity of most pharmacological PKC modulators and the lack of validated predictive biomarkers for response, which impacts the translation of these agents to the clinic. In this review we focus on key controversial issues and upcoming challenges, with the expectation that understanding the intricacies of PKC function will help fulfill the yet unsuccessful promise of targeting PKCs for cancer therapeutics.

Expression and purification of the N-terminal regulatory domain of Protein Kinase C for biophysical studies

We report the protocol for heterologous expression and purification of the N-terminal regulatory region of two Protein Kinase C (PKC)(1) isozymes, one conventional and one novel. Previous studies of these domains relied almost exclusively on the fusion constructs with high-molecular-weight solubility fusion partners such as GST and MBP. We developed experimental procedures that enabled us to overcome challenges associated with the amphiphilic character of the regulatory domain and generate sufficient quantities of fusion partner-free proteins for biophysical work. The key features of the protocol are the identity of the cleavable fusion partner, expression conditions, growth medium additives, introduction of mutation/solubility tags, and incorporation of osmolytes. The protein yields are sufficient for cost-effective production of isotopically enriched proteins for NMR work and biophysical studies in general. Our work opens up an avenue for the structural studies of these challenging proteins with high amphiphilic character.

Charge density influences C1 domain ligand affinity and membrane interactions

The C1 domain, which represents the recognition motif on protein kinase C for the lipophilic second messenger diacylglycerol and its ultrapotent analogues, the phorbol esters, has emerged as a promising therapeutic target for cancer and other indications. Potential target selectivity is markedly enhanced both because binding reflects ternary complex formation between the ligand, C1 domain, and phospholipid, and because binding drives membrane insertion of the C1 domain, permitting aspects of the C1 domain surface outside the binding site, per se, to influence binding energetics. Here, focusing on charged residues identified in atypical C1 domains which contribute to their loss of ligand binding activity, we showed that increasing charge along the rim of the binding cleft of the protein kinase C δ C1 b domain raises the requirement for anionic phospholipids. Correspondingly, it shifts the selectivity of C1 domain translocation to the plasma membrane, which is more negatively charged than internal membranes. This change in localization is most pronounced in the case of more hydrophilic ligands, which provide weaker membrane stabilization than do the more hydrophobic ligands and thus contributes an element to the structure-activity relations for C1 domain ligands. Coexpressing pairs of C1-containing constructs with differing charges each expressing a distinct fluorescent tag provided a powerful tool to demonstrate the effect of increasing charge in the C1 domain.

Synthesis, biological, and biophysical studies of DAG-indololactones designed as selective activators of RasGRP

The development of selective agents capable of discriminating between protein kinase C (PKC) isoforms and other diacylglycerol (DAG)-responsive C1 domain-containing proteins represents an important challenge. Recent studies have highlighted the role that Ras guanine nucleotide-releasing protein (RasGRP) isoforms play both in immune responses as well as in the development of prostate cancer and melanoma, suggesting that the discovery of selective ligands could have potential therapeutic value. Thus far, the N-methyl-substituted indololactone 1 is the agonist with the highest reported potency and selectivity for RasGRP relative to PKC. Here we present the synthesis, binding studies, cellular assays and biophysical analysis of interactions with model membranes of a family of regioisomers of 1 (compounds 2-5) that differ in the position of the linkage between the indole ring and the lactone moiety. These structural variations were studied to explore the interaction of the active complex (C1 domain-ligand) with cellular membranes, which is believed to be an important factor for selectivity in the activation of DAG-responsive C1 domain containing signaling proteins. All compounds were potent and selective activators of RasGRP when compared to PKCα with selectivities ranging from 6 to 65 fold. However, the parent compound 1 was appreciably more selective than any of the other isomers. In intact cells, modest differences in the patterns of translocation of the C1 domain targets were observed. Biophysical studies using giant vesicles as model membranes did show substantial differences in terms of molecular interactions impacting lipid organization, dynamics and membrane insertion. However, these differences did not yield correspondingly large changes in patterns of biological response, at least for the parameters examined.

Physicochemical characterization of diacyltetrol-based lipids consisting of both diacylglycerol and phospholipid headgroups

Synthesis and physicochemical properties of a family of diacyltetrol-based hybrid lipids, containing both diacylglycerol and anionic lipid headgroups within the same moiety, have been reported for the first time.

RasGRPs are targets of the anti-cancer agent ingenol-3-angelate

Ingenol-3–angelate (I3A) is a non-tumor promoting phorbol ester-like compound identified in the sap of Euphoria peplus. Similar to tumor promoting phorbol esters, I3A is a diacylglycerol (DAG) analogue that binds with high affinity to the C1 domains of PKCs, recruits PKCs to cellular membranes and promotes enzyme activation. Numerous anti-cancer activities have been attributed to I3A and ascribed to I3A’s effects on PKCs. We show here that I3A also binds to and activates members of the RasGRP family of Ras activators leading to robust elevation of Ras-GTP and engagement of the Raf-Mek-Erk kinase cascade. In response to I3A, recombinant proteins consisting of GFP fused separately to full-length RasGRP1 and RasGRP3 were rapidly recruited to cell membranes, consistent with direct binding of the compound to RasGRP’s C1 domain. In the case of RasGRP3, IA3 treatment led to positive regulatory phosphorylation on T133 and activation of the candidate regulatory kinase PKCδ. I3A treatment of select B non-Hodgkin’s lymphoma cell lines resulted in quantitative and qualitative changes in Bcl-2 family member proteins and induction of apoptosis, as previously demonstrated with the DAG analogue bryostatin 1 and its synthetic analogue pico. Our results offer further insights into the anticancer properties of I3A, support the idea that RasGRPs represent potential cancer therapeutic targets along with PKC, and expand the known range of ligands for RasGRP regulation.

Peptide-based ligand screening and functional analysis of protein kinase C

Protein kinase C (PKC) plays an important role in cell signaling pathways and is implicated in disorders ranging from cancer to Alzheimer's disease. Highly potent PKC ligands as therapeutic drugs have not been developed to date and useful methodologies for controlling PKC activation in defined areas are necessary to analyze precise PKC functions in cells. Studies focused on the development of PKC ligand screening systems and methods for regulation of PKC activation have been performed in our laboratory. In this review, our ligand screening methods involving synthetic peptides and solvatochromic fluorescent dye-labeled small compounds are summarized and the technique of spatio-temporal manipulation of PKC activation by caging strategies is introduced.

Molecular basis for failure of "atypical" C1 domain of Vav1 to bind diacylglycerol/phorbol ester

C1 domains, the recognition motif of the second messenger diacylglycerol and of the phorbol esters, are classified as typical (ligand-responsive) or atypical (not ligand-responsive). The C1 domain of Vav1, a guanine nucleotide exchange factor, plays a critical role in regulation of Vav activity through stabilization of the Dbl homology domain, which is responsible for exchange activity of Vav. Although the C1 domain of Vav1 is classified as atypical, it retains a binding pocket geometry homologous to that of the typical C1 domains of PKCs. This study clarifies the basis for its failure to bind ligands. Substituting Vav1-specific residues into the C1b domain of PKCδ, we identified five crucial residues (Glu(9), Glu(10), Thr(11), Thr(24), and Tyr(26)) along the rim of the binding cleft that weaken binding potency in a cumulative fashion. Reciprocally, replacing these incompatible residues in the Vav1 C1 domain with the corresponding residues from PKCδ C1b (δC1b) conferred high potency for phorbol ester binding. Computer modeling predicts that these unique residues in Vav1 increase the hydrophilicity of the rim of the binding pocket, impairing membrane association and thereby preventing formation of the ternary C1-ligand-membrane binding complex. The initial design of diacylglycerol-lactones to exploit these Vav1 unique residues showed enhanced selectivity for C1 domains incorporating these residues, suggesting a strategy for the development of ligands targeting Vav1.

Regulation and Function of the RasGRP Family of Ras Activators in Blood Cells

Ras guanyl nucleotide releasing proteins (RasGRPs) are guanyl nucleotide exchange factors that activate Ras and related GTPases such as Rap. Like Sos proteins, RasGRPs have a catalytic region composed of a Ras exchange motif (REM) and a CDC25 domain. RasGRPs also possess a pair of atypical EF hands that may bind calcium in vivo and a C1 domain resembling the diacylglycerol (DAG)-binding domain of protein kinase C. DAG directly activates RasGRPs by a membrane recruitment mechanism as well as indirectly by PKC-mediated phosphorylation. RasGRPs are prominently expressed in blood cells. RasGRP1 acts downstream of TCR, while RasGRP1 and RasGRP3 both act downstream of BCR. Together, they regulate Ras in adaptive immune cells. RasGRP2, through Rap, plays a role in controlling platelet adhesion, while RasGRP4 controls Ras activation in mast cells. RasGRP malfunction likely contributes to autoimmunity and may contribute to blood malignancies. RasGRPs might prove to be viable drug targets. The intracellular site of RasGRP action and the relationship between RasGRPs and other Ras regulatory mechanisms are subjects of lively debate.

RasGRP3, a Ras activator, contributes to signaling and the tumorigenic phenotype in human melanoma

RasGRP3, an activator for H-Ras, R-Ras and Rap1/2, has emerged as an important mediator of signaling downstream from receptor coupled phosphoinositide turnover in B and T cells. Here, we report that RasGRP3 showed a high level of expression in multiple human melanoma cell lines as well as in a subset of human melanoma tissue samples. Suppression of endogenous RasGRP3 expression in these melanoma cell lines reduced Ras-GTP formation as well as c-Met expression and Akt phosphorylation downstream from HGF or EGF stimulation. RasGRP3 suppression also inhibited cell proliferation and reduced both colony formation in soft agar and xenograft tumor growth in immunodeficient mice, demonstrating the importance of RasGRP3 for the transformed phenotype of the melanoma cells. Reciprocally, overexpression of RasGRP3 in human primary melanocytes altered cellular morphology, markedly enhanced cell proliferation, and rendered the cells tumorigenic in a mouse xenograft model. Suppression of RasGRP3 expression in these cells inhibited downstream RasGRP3 responses and suppressed cell growth, confirming the functional role of RasGRP3 in the altered behavior of these cells. The identification of the role of RasGRP3 in melanoma highlights its importance, as a Ras activator, in the phosphoinositide signaling pathway in human melanoma and provides a new potential therapeutic target.

Some phorbol esters might partially resemble bryostatin 1 in their actions on LNCaP prostate cancer cells and U937 leukemia cells

Phorbol 12-myristate 13-acetate (PMA) and bryostatin 1 are both potent protein kinase C (PKC) activators. In LNCaP human prostate cancer cells, PMA induces tumor necrosis factor alpha (TNFα) secretion and inhibits proliferation; bryostatin 1 does not, and indeed blocks the response to PMA. This difference has been attributed to bryostatin 1 not localizing PKCδ to the plasma membrane. Since phorbol ester lipophilicity influences PKCδ localization, we have examined in LNCaP cells a series of phorbol esters and related derivatives spanning some eight logs in lipophilicity (logP) to see if any behave like bryostatin 1. The compounds showed marked differences in their effects on proliferation and TNFα secretion. For example, maximal responses for TNFα secretion relative to PMA ranged from 97 % for octyl-indolactam V to 24 % for phorbol 12,13-dibenzoate. Dose-response curves ranged from monophasic for indolactam V to markedly biphasic for sapintoxin D. The divergent patterns of response, however, correlated neither to lipophilicity, to plasma membrane translocation of PKCδ, nor to the ability to interact with model membranes. In U937 human leukemia cells, a second system in which PMA and bryostatin 1 have divergent effects, viz. PMA but not bryostatin 1 inhibits proliferation and induces attachment, all the compounds acted like PMA for proliferation, but several induced a reduced level or a biphasic dose-response curve for attachment. We conclude that active phorbol esters are not all equivalent. Depending on the system, some might partially resemble bryostatin 1 in their behavior; this encourages the concept that bryostatin-like behavior may be obtained from other structural templates.

Protein kinase C regulates mitochondrial targeting of Nur77 and its family member Nor-1 in thymocytes undergoing apoptosis

Nur77 orphan steroid receptor and its family member Nor-1 are required for apoptosis of developing T cells. In thymocytes, signals from the TCR complex induce Nur77 and Nor-1 expression followed by translocation from the nucleus to mitochondria. Nur77 and Nor-1 associate with Bcl-2 in the mitochondria, resulting in a conformation change that exposes the Bcl-2 BH3 domain, a presumed pro-apoptotic molecule of Bcl-2. As Nur77 and Nor-1 are heavily phosphorylated, we examined the requirement of Nur77 and Nor-1 phosphorylation in mitochondria translocation and Bcl-2 BH3 exposure. We found that HK434, a PKC agonist, in combination with calcium ionophore, can induce Nur77 and Nor-1 phosphorylation, translocation, Bcl-2 BH3 exposure and thymocyte apoptosis. Inhibitors of both classical and novel forms of PKC were able to block this process. In contrast, only the general but not classical PKC-specific inhibitors were able to block the same process initiated by PMA, a commonly used PKC agonist. These data demonstrate a differential activation of PKC isoforms by PMA and HK434 in thymocytes, and show the importance of PKC in mitochondria translocation of Nur77/Nor-1 and Bcl-2 conformation change during TCR-induced thymocyte apoptosis.

RasGRP3 contributes to formation and maintenance of the prostate cancer phenotype

RasGRP3 mediates the activation of the Ras signaling pathway that is present in many human cancers. Here, we explored the involvement of RasGRP3 in the formation and maintenance of the prostate cancer phenotype. RasGRP3 expression was elevated in multiple human prostate tumor tissue samples and in the human androgen-independent prostate cancer cell lines PC-3 and DU 145 compared with the androgen-dependent prostate cancer cell line LNCaP. Downregulation of endogenous RasGRP3 in PC-3 and DU 145 cells reduced Ras-GTP formation, inhibited cell proliferation, impeded cell migration, and induced apoptosis. Anchorage-independent growth of the PC-3 cells and tumor formation in mouse xenografts of both cell lines were likewise inhibited. Inhibition of RasGRP3 expression reduced AKT and extracellular signal-regulated kinase 1/2 phosphorylation and sensitized the cells to killing by carboplatin. Conversely, exogenous RasGRP3 elevated Ras-GTP, stimulated proliferation, and provided resistance to phorbol 12-myristate 13-acetate-induced apoptosis in LNCaP cells. RasGRP3-overexpressing LNCaP cells displayed a markedly enhanced rate of xenograft tumor formation in both male and female mice compared with the parental line. Suppression of RasGRP3 expression in these cells inhibited downstream RasGRP3 responses, caused the cells to resume the LNCaP morphology, and suppressed growth, confirming the functional role of RasGRP3 in the altered behavior of these cells. We conclude that RasGRP3 contributes to the malignant phenotype of the prostate cancer cells and may constitute a novel therapeutic target for human prostate cancer.

Membrane anchoring of diacylglycerol lactones substituted with rigid hydrophobic acyl domains correlates with biological activities

Synthetic diacylglycerol lactones (DAG lactones) are effective modulators of critical cellular signaling pathways downstream of the lipophilic second messenger diacylglycerol that activate a host of protein kinase C (PKC) isozymes as well as other non-kinase proteins that share with PKC similar C1 membrane-targeting domains. A fundamental determinant of the biological activity of these amphiphilic molecules is the nature of their interactions with cellular membranes. This study characterizes the membrane interactions and bilayer anchoring of a series of DAG lactones in which the hydrophobic moiety is a 'molecular rod', namely a rigid 4-[2-(R-phenyl)ethynyl]benzoate moiety in the acyl position. Use of assays employing chromatic biomimetic vesicles and biophysical techniques revealed that the mode of membrane anchoring of the DAG lactone derivatives was markedly affected by the presence of the hydrophobic diphenyl rod and by the size of the functional unit at the terminus of the rod. Two primary mechanisms of interaction were observed: surface binding of the DAG lactones at the lipid/water interface and deep insertion of the ligands into the alkyl core of the lipid bilayer. These membrane-insertion properties could explain the different patterns of the PKC translocation from the cytosol to membranes that is induced by the molecular-rod DAG lactones. This investigation emphasizes that the side residues of DAG lactones, rather than simply conferring hydrophobicity, profoundly influence membrane interactions, and thus may further contribute to the diversity of biological actions of these synthetic biomimetic ligands.

Design, synthesis, and biological activity of isophthalic acid derivatives targeted to the C1 domain of protein kinase C

Protein kinase C (PKC) is a widely studied molecular target for the treatment of cancer and other diseases. We have approached the issue of modifying PKC function by targeting the C1 domain in the regulatory region of the enzyme. Using the X-ray crystal structure of the PKC delta C1b domain, we have discovered conveniently synthesizable derivatives of dialkyl 5-(hydroxymethyl)isophthalate that can act as potential C1 domain ligands. Structure-activity studies confirmed that the important functional groups predicted by modeling were indispensable for binding to the C1 domain and that the modifications of these groups diminished binding. The most promising compounds were able to displace radiolabeled phorbol ester ([(3)H]PDBu) from PKC alpha and delta at K(i) values in the range of 200-900 nM. Furthermore, the active isophthalate derivatives could modify PKC activation in living cells either by inducing PKC-dependent ERK phosphorylation or by inhibiting phorbol-induced ERK phosphorylation. In conclusion, we report here, for the first time, that derivatives of isophthalic acid represent an attractive novel group of C1 domain ligands that can be used as research tools or further modified for potential drug development.

A ceramide-binding C1 domain mediates kinase suppressor of ras membrane translocation

Genetic and biochemical data support Kinase Suppressor of Ras 1 (KSR1) as a positive regulator of the Ras-Raf-MAPK pathway, functioning as a kinase and/or scaffold to regulate c-Raf-1 activation. Membrane translocation mediated by the KSR1 CA3 domain, which is homologous to the atypical PKC C1 lipid-binding domain, is a critical step of KSR1-mediated c-Raf-1 activation. In this study, we used an ELISA to characterize the KSR1 CA3 domain as a lipid-binding moiety. Purified GST-KSR1-CA3 protein effectively binds ceramide but not other lipids including 1,2-diacylglyceol, dihydroceramide, ganglioside GM1, sphingomyelin and phosphatidylcholine. Upon epidermal growth factor stimulation of COS-7 cells, KSR1 translocates into and is activated within glycosphingolipid-enriched plasma membrane platforms. Pharmacologic inhibition of ceramide generation attenuates KSR1 translocation and KSR1 kinase activation in COS-7 cells. Disruption of two cysteines, which are indispensable for maintaining ternary structure of all C1 domains and their lipid binding capability, mitigates ceramide-binding capacity of purified GST-KSR1-CA3 protein, and inhibits full length KSR1 membrane translocation and kinase activation. These studies provide evidence for a mechanism by which the second messenger ceramide can target proteins to subcellular compartments in the process of transmembrane signal transduction.

Characterization of the differential roles of the twin C1a and C1b domains of protein kinase C-delta

Classic and novel protein kinase C (PKC) isozymes contain two zinc finger motifs, designated "C1a" and "C1b" domains, which constitute the recognition modules for the second messenger diacylglycerol (DAG) or the phorbol esters. However, the individual contributions of these tandem C1 domains to PKC function and, reciprocally, the influence of protein context on their function remain uncertain. In the present study, we prepared PKCdelta constructs in which the individual C1a and C1b domains were deleted, swapped, or substituted for one another to explore these issues. As isolated fragments, both the deltaC1a and deltaC1b domains potently bound phorbol esters, but the binding of [(3)H]phorbol 12,13-dibutyrate ([(3)H]PDBu) by the deltaC1a domain depended much more on the presence of phosphatidylserine than did that of the deltaC1b domain. In intact PKCdelta, the deltaC1b domain played the dominant role in [(3)H]PDBu binding, membrane translocation, and down-regulation. A contribution from the deltaC1a domain was nonetheless evident, as shown by retention of [(3)H]PDBu binding at reduced affinity, by increased [(3)H]PDBu affinity upon expression of a second deltaC1a domain substituting for the deltaC1b domain, and by loss of persistent plasma membrane translocation for PKCdelta expressing only the deltaC1b domain, but its contribution was less than predicted from the activity of the isolated domain. Switching the position of the deltaC1b domain to the normal position of the deltaC1a domain (or vice versa) had no apparent effect on the response to phorbol esters, suggesting that the specific position of the C1 domain within PKCdelta was not the primary determinant of its activity.

Self-assembly and lipid interactions of diacylglycerol lactone derivatives studied at the air/water interface

Synthetic diacylglycerol lactones (DAG-lactones) have been shown to be effective modulators of critical cellular signaling pathways. The biological activity of these amphiphilic molecules depends in part upon their lipid interactions within the cellular plasma membrane. This study explores the thermodynamic and structural features of DAG-lactone derivatives and their lipid interactions at the air/water interface. Surface-pressure/area isotherms and Brewster angle microscopy revealed the significance of specific side-groups attached to the terminus of a very rigid 4-(2-phenylethynyl)benzoyl chain of the DAG-lactones, which affected both the self-assembly of the molecules and their interactions with phospholipids. The experimental data highlight the formation of different phases within mixed DAG-lactone/phospholipid monolayers and underscore the relationship between the two components in binary mixtures of different mole ratios. Importantly, the results suggest that DAG-lactones are predominantly incorporated within fluid phospholipid phases rather than in the condensed phases that form, for example, by cholesterol. Moreover, the size and charge of the phospholipid headgroups do not seem to affect DAG-lactone interactions with lipids.

Conformationally constrained analogues of diacylglycerol. 30. An investigation of diacylglycerol-lactones containing heteroaryl groups reveals compounds with high selectivity for Ras guanyl nucleotide-releasing proteins

Using a diacylglycerol-lactone (DAG-lactone) template previously developed in our laboratory as a scaffold with high binding affinity for C1 domains, we describe herein a series of novel DAG-lactones containing heterocyclic moieties (pyridines, quinolines, and indoles) as alpha-arylidene fragments. Some of the DAG-lactones obtained show selective binding to RasGRP3 as compared to PKCalpha by more than 2 orders of magnitude and possess subnanomolar affinities. Because activated C1 domains bound to their ligands (DAG or DAG-lactones) insert into membranes, the lipid composition of membranes (cellular, nuclear, and those of internal organelles) is an important determinant for specificity. Therefore, reaching a proper hydrophilic/lipophilic balance for these molecules is critical. This was achieved by carefully selecting partnering acyl fragments for the DAG-lactones with the appropriate lipophilicity. The results clearly show that the combination of chemical and physical properties in these molecules needs to be perfectly balanced to achieve the desired specificity.

Conformationally constrained analogues of diacylglycerol. 29. Cells sort diacylglycerol-lactone chemical zip codes to produce diverse and selective biological activities

Diacylglycerol-lactone (DAG-lactone) libraries generated by a solid-phase approach using IRORI technology produced a variety of unique biological activities. Subtle differences in chemical diversity in two areas of the molecule, the combination of which generates what we have termed "chemical zip codes", are able to transform a relatively small chemical space into a larger universe of biological activities, as membrane-containing organelles within the cell appear to be able to decode these "chemical zip codes". It is postulated that after binding to protein kinase C (PKC) isozymes or other nonkinase target proteins that contain diacylglycerol responsive, membrane interacting domains (C1 domains), the resulting complexes are directed to diverse intracellular sites where different sets of substrates are accessed. Multiple cellular bioassays show that DAG-lactones, which bind in vitro to PKCalpha to varying degrees, expand their biological repertoire into a larger domain, eliciting distinct cellular responses.

Wealth of opportunity - the C1 domain as a target for drug development

The diacylglycerol-responsive C1 domains of protein kinase C and of the related classes of signaling proteins represent highly attractive targets for drug development. The signaling functions that are regulated by C1 domains are central to cellular control, thereby impacting many pathological conditions. Our understanding of the diacylglycerol signaling pathways provides great confidence in the utility of intervention in these pathways for treatment of cancer and other conditions. Multiple compounds directed at these signaling proteins, including compounds directed at the C1 domains, are currently in clinical trials, providing strong validation for these targets. Extensive understanding of the structure and function of C1 domains, coupled with detailed insights into the molecular details of ligand - C1 domain interactions, provides a solid basis for rational and semi-rational drug design. Finally, the complexity of the factors contributing to ligand - C1 domain interactions affords abundant opportunities for manipulation of selectivity; indeed, substantially selective compounds have already been identified.

Characterization of the interaction of phorbol esters with the C1 domain of MRCK (myotonic dystrophy kinase-related Cdc42 binding kinase) alpha/beta

C1 domains mediate the recognition and subsequent signaling response to diacylglycerol and phorbol esters by protein kinase C (PKC) and by several other families of signal-transducing proteins such as the chimerins or RasGRP. MRCK (myotonic dystrophy kinase-related Cdc42 binding kinase), a member of the dystrophia myotonica protein kinase family that functions downstream of Cdc42, contains a C1 domain with substantial homology to that of the diacylglycerol/phorbol ester-responsive C1 domains and has been reported to bind phorbol ester. We have characterized here the interaction of the C1 domains of the two MRCK isoforms alpha and beta with phorbol ester. The MRCK C1 domains bind [20-(3)H]phorbol 12,13-dibutyrate with K(d) values of 10 and 17 nm, respectively, reflecting 60-90-fold weaker affinity compared with the protein kinase C delta C1b domain. In contrast to binding by the C1b domain of PKCdelta, the binding by the C1 domains of MRCK alpha and beta was fully dependent on the presence of phosphatidylserine. Comparison of ligand binding selectivity showed resemblance to that by the C1b domain of PKCalpha and marked contrast to that of the C1b domain of PKCdelta. In intact cells, as in the binding assays, the MRCK C1 domains required 50-100-fold higher concentrations of phorbol ester for induction of membrane translocation. We conclude that additional structural elements within the MRCK structure are necessary if the C1 domains of MRCK are to respond to phorbol ester at concentrations comparable with those that modulate PKC.

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.

Regulation of Ras in lymphocytes: get a GRP

RasGRPs (guanine nucleotide releasing proteins) are a family of four GEFs (guanine nucleotide-exchange factors) (Ras GEFs) that positively regulate Ras and related small GTPases. RasGRP1 possesses a catalytic region consisting of a REM (Ras exchange motif) and a CDC25 (cell division cycle 25) domain. RasGRP1 also possesses a DAG (diacylglycerol)-binding C1 domain and a pair of EF hands that bind calcium. RasGRP1 is selectively expressed in lymphocytes as well as in some cells of the brain, kidney and skin. Functional analysis supports the hypothesis that RasGRP1 serves to couple TCR (T-cell receptor) stimulation and phospholipase C activation with Ras signalling. In B-cells, both RasGRP1 and RasGRP3 play a similar role downstream of the B-cell receptor. RasGRP2 acts on the Ras-related protein Rap and functions in platelet adhesion. RasGRP4 is expressed in mast cells and certain myeloid leukaemia cells. Membrane DAG regulates RasGRPs directly by recruitment to cellular membranes, as well as indirectly by protein kinase C-mediated phosphorylation. The properties of RasGRPs provide a novel view of Ras regulation in lymphocytes and explain several earlier observations. Many experimental results obtained with DAG analogues could be reviewed in light of these findings.

Identification of an alcohol binding site in the first cysteine-rich domain of protein kinase Cdelta

Protein kinase C (PKC) is an important signal transduction protein whose cysteine-rich regulatory domain C1 has been proposed to interact with general anesthetics in both of its diacylglycerol/phorbol ester-binding subdomains, the tandem repeats C1A and C1B. Previously, we identified an allosteric binding site on one of the two cysteine-rich domains, PKCdelta C1B. To test the hypothesis that there is an additional anesthetic site on the other cysteine-rich subdomain, C1A, we subcloned, expressed in Escherichia coli, purified, and characterized mouse PKCdelta C1A. Octanol and butanol both quenched the intrinsic fluorescence of PKCdelta C1A in a saturable manner, suggesting the presence of a binding site. To locate this site, PKCdelta C1A was photolabeled with three diazirine-containing alkanols, 3-azioctanol, 7-azioctanol, and 3-azibutanol. Mass spectrometry revealed that at low concentrations all three photoincorporated into PKCdelta C1A with a stoichiometry of 1:1 in the labeled fraction, but higher stoichiometries occurred at higher concentrations, particularly with azibutanol. Photocomplexes of PKCdelta C1A with azioctanols were separated from the unlabeled protein by HPLC, reduced, alkylated, digested with trypsin, and sequenced by mass spectrometry. All the azioctanols photolabeled PKCdelta C1A at residue Tyr-29, corresponding to Tyr-187 of the full-length PKCdelta, and at a neighboring residue, Lys-40, suggesting there is an alcohol site in this vicinity. In addition, Glu-2 was photolabeled more efficiently by 3-azibutanol than by the azioctanols, suggesting the existence of a second, smaller site.

Effects on Ligand Interaction and Membrane Translocation of the Positively Charged Arginine Residues Situated along the C1 Domain Binding Cleft in the Atypical Protein Kinase C Isoforms

The C1 domain zinc finger structure is highly conserved among the protein kinase C (PKC) superfamily members. As the interaction site for the second messenger sn-1,2-diacylglycerol (DAG) and for the phorbol esters, the C1 domain has been an important target for developing selective ligands for different PKC isoforms. However, the C1 domains of the atypical PKC members are DAG/phorbol ester-insensitive. Compared with the DAG/phorbol ester-sensitive C1 domains, the rim of the binding cleft of the atypical PKC C1 domains possesses four additional positively charged arginine residues (at positions 7, 10, 11, and 20). In this study, we showed that mutation to arginines of the four corresponding sites in the C1b domain of PKCdelta abolished its high potency for phorbol 12,13-dibutyrate in vitro, with only marginal remaining activity for phorbol 12-myristate 13-acetate in vivo. We also demonstrated both in vitro and in vivo that the loss of potency to ligands was cumulative with the introduction of the arginine residues along the rim of the binding cavity rather than the consequence of loss of a single, specific residue. Computer modeling reveals that these arginine residues reduce access of ligands to the binding cleft and change the electrostatic profile of the C1 domain surface, whereas the basic structure of the binding cleft is still maintained. Finally, mutation of the four arginine residues of the atypical PKC C1 domains to the corresponding residues in the deltaC1b domain conferred response to phorbol ester. We speculate that the arginine residues of the C1 domain of atypical PKCs may provide an opportunity for the design of ligands selective for the atypical PKCs.

RasGRP1 confers the phorbol ester-sensitive phenotype to EL4 lymphoma cells

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to the tumor promoter phorbol 12-myristate 13-acetate (PMA). In sensitive EL4 cells, PMA causes robust Erk mitogen-activated protein kinase activation that results in growth arrest. In resistant cells, PMA induces minimal Erk activation, without growth arrest. PMA stimulates IL-2 production in sensitive, but not resistant, cells. The role of RasGRP1, a PMA-activated guanine nucleotide exchange factor for Ras, in EL4 phenotype was examined. Endogenous RasGRP1 protein is expressed at much higher levels in sensitive than in resistant cells. PMA-induced Ras activation is observed in sensitive cells but not in resistant cells lacking Ras-GRP1. PMA induces down-regulation of RasGRP1 protein in sensitive cells but increases RasGRP1 in resistant cells. Transfection of RasGRP1 into resistant cells enhances PMA-induced Erk activation. In the reverse experiment, introduction of small interfering RNA (siRNA) for RasGRP1 suppresses PMA-induced Ras and Erk activations in sensitive cells. Sensitive cells incubated with siRNA for RasGRP1 exhibit the PMA-resistant phenotype, in that they are able to proliferate in the presence of PMA and do not secrete IL-2 when stimulated with PMA. These studies indicate that the PMA-sensitive phenotype, as previously defined for the EL4 cell line, is conferred by endogenous expression of RasGRP1 protein.

The exchange factor and diacylglycerol receptor RasGRP3 interacts with dynein light chain 1 through its C-terminal domain

RasGRP3 is an exchange factor for Ras-like small GTPases that is activated in response to the second messenger diacylglycerol. As with other diacylglycerol receptors, RasGRP3 is redistributed upon diacylglycerol or phorbol ester binding. Several factors are important in determining the pattern of translocation, including the potency of the diacylglycerol analog, the affinity of the receptor for phospholipids, and in some cases, protein-protein interactions. However, little is known about the mechanisms that play a role in RasGRP3 redistribution aside from the nature of the ligand. To discover potential protein binding partners for RasGRP3, we screened a human brain cDNA library using a yeast two-hybrid approach. We identified dynein light chain 1 as a novel RasGRP3-interacting protein. The interaction was confirmed both in vitro and in vivo and required the C-terminal domain encompassing the last 127 amino acids of RasGRP3. A truncated mutant form of RasGRP3 that lacked this C-terminal domain was unable to interact with dynein light chain 1 and displayed a dramatically altered subcellular localization, with a strong reticular distribution and perinuclear and nuclear localization. These findings suggest that dynein light chain 1 represents a novel anchoring protein for RasGRP3 that may regulate subcellular localization of the exchange factor and, as such, may participate in the signaling mediated by diacylglycerol through RasGRP3.

Differential effect of bryostatin 1 and phorbol 12-myristate 13-acetate on HOP-92 cell proliferation is mediated by down-regulation of protein kinase Cdelta

Bryostatin 1 is currently in clinical trials as a cancer chemotherapeutic agent. Although bryostatin 1, like phorbol 12-myristate 13-acetate (PMA), is a potent activator of protein kinase C (PKC), it induces only a subset of those responses induced by PMA and antagonizes others. We report that, in the HOP-92 non-small cell lung cancer line, bryostatin 1 induced a biphasic proliferative response, with maximal proliferation at 1 to 10 nmol/L. This biphasic response mirrored a biphasic suppression of the level of PKCdelta protein, with maximal suppression likewise at 1 to 10 nmol/L bryostatin 1. The typical phorbol ester PMA, in contrast to bryostatin 1, had no effect on the level of PKCdelta and modest suppression of cell proliferation, particularly evident at later treatment times. Flow cytometric analysis revealed changes in the fraction of cells in the G0-G1 and S phases corresponding to the effects on proliferation. Cells overexpressing PKCdelta exhibited a lower rate of cell proliferation compared with control untreated cells and showed neither a proliferative response nor a loss of PKCdelta in response to bryostatin 1. Conversely, treatment with PKCdelta small interfering RNA significantly increased the cellular growth compared with controls. We conclude that the differential effect on cellular proliferation induced by bryostatin 1 compared with PMA reflects the differential suppression of PKCdelta.

Protein kinase C regulatory domains: the art of decoding many different signals in membranes

Protein kinase C (PKC) is a member of a family of Ser/Thr phosphotransferases that are involved in many cellular signaling pathways. These enzymes possess two regulatory domains, C1 and C2, that are the targets of different second messengers. The purpose of this review is to describe in molecular terms the diverse mechanisms of activation of PKCs in the light of very significant advances made in this field over recent years. The role of some critical amino acid residues concerning activation of the enzymes and their location within known structures of isolated domains will be presented. For example, the recently deduced 3D structures of the C2 domains show that these domains can additionally act as PtdIns(4,5)P(2)-binding or phosphotyrosine-binding modules depending on the isoenzyme. All these capacities to play different roles in the cell wide web of signals underline the notion that we are dealing with a multifunctional family of enzymes which, after 30 years of investigation, we are just beginning to understand.

PKD at the crossroads of DAG and PKC signaling

Diacylglycerol (DAG) and its primary target protein kinase C (PKC) regulate many important cellular responses, yet the molecular mechanisms that control the specificity of DAG and PKC signaling are not fully understood. As such, targeting the PKC pathway for therapeutic purposes has been challenging. Protein kinase D (PKD), a novel DAG receptor, has been the subject of intense investigation in recent years. DAG regulates the intracellular localization of PKD and also activates PKD through PKC by phosphorylation. The PKC-PKD signaling cascade is crucial to PKD function in cells. Important discoveries have been made regarding the roles of PKD in cell growth, gene expression, survival, motility, protein trafficking and lymphocyte biology. This kinase is implicated in pathological processes such as cardiac hypertrophy, tumor cell proliferation and metastasis. Thus, PKD represents a novel therapeutic target for the DAG-PKC signaling network.