Beta2-chimaerin is a high affinity receptor for the phorbol ester tumor promoters

The emerging roles of srGAPs in cancer

GTPase activating proteins (GAPs) were initially considered as the inhibitors of cell signaling pathways because of their nature to activate the intrinsic GTPase activity of the RhoGTPases. But recent studies of dysregulated GAPs in many cancers such as glioblastoma, colorectal cancer, breast cancer, and renal cancer have elucidated the important roles of GAPs in carcinogenesis and GAPs have been shown to perform multiple nonconventional functions in different contexts. We have discussed the recent developments in the roles played by different types of srGAPs (SLIT-ROBO Rho GTPase-activating proteins) in cancer.

Sphingomyelin synthase-related protein generates diacylglycerol via the hydrolysis of glycerophospholipids in the absence of ceramide

Diacylglycerol (DG) is a well-established lipid second messenger. Sphingomyelin synthase (SMS)-related protein (SMSr) produces DG and ceramide phosphoethanolamine (CPE) by the transfer of phosphoethanolamine from phosphatidylethanolamine (PE) to ceramide. We previously reported that human SMSr overexpressed in COS-7 cells significantly increased DG levels, particularly saturated and/or monounsaturated fatty acid-containing DG molecular species, and provided DG to DG kinase (DGK) δ, which regulates various pathophysiological events, including epidermal growth factor-dependent cell proliferation, type 2 diabetes, and obsessive-compulsive disorder. However, mammalian SMSr puzzlingly produces only trace amounts of CPE/DG. To clarify this discrepancy, we highly purified SMSr and examined its activities other than CPE synthase. Intriguingly, purified SMSr showed a DG-generating activity via hydrolysis of PE, phosphatidic acid (PA), phosphatidylinositol (PI), and phosphatidylcholine (PC) in the absence of ceramide. DG generation through the PA phosphatase (PAP) activity of SMSr was approximately 300-fold higher than that with PE and ceramide. SMSr hydrolyzed PI ten times stronger than PI(4,5)bisphosphate (PI(4,5)P₂). The PAP and PC-phospholipase C (PLC) activities of SMSr were inhibited by propranolol, a PAP inhibitor, and by D609, an SMS/PC-PLC inhibitor. Moreover, SMSr showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PA molecular species, but not arachidonic-acid-containing PA, which is exclusively generated in the PI(4,5)P₂ cycle. We confirmed that SMSr expressed in COS-7 cells showed PAP and PI-PLC activities. Taken together, our study indicated that SMSr possesses previously unrecognized enzyme activities, PAP and PI/PE/PC-PLC, and constitutes a novel DG/PA signaling pathway together with DGKδ, which is independent of the PI(4,5)P₂ cycle.

Identification of a truncated β1-chimaerin variant that inactivates nuclear Rac1

β1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which also encodes the β2- and β3-chimaerin isoforms. All chimaerin isoforms have a C1 domain that binds diacylglycerol as well as tumor-promoting phorbol esters and a catalytic GAP domain that inactivates the small GTPase Rac. Nuclear Rac has emerged as a key regulator of various cell functions, including cell division, and has a pathological role by promoting tumorigenesis and metastasis. However, how nuclear Rac is regulated has not been fully addressed. Here, using several approaches, including siRNA-mediated gene silencing, confocal microscopy, and subcellular fractionation, we identified a nuclear variant of β1-chimaerin, β1-Δ7p-chimaerin, that participates in the regulation of nuclear Rac1. We show that β1-Δ7p-chimaerin is a truncated variant generated by alternative splicing at a cryptic splice site in exon 7. We found that, unlike other chimaerin isoforms, β1-Δ7p-chimaerin lacks a functional C1 domain and is not regulated by diacylglycerol. We found that β1-Δ7p-chimaerin localizes to the nucleus via a nuclear localization signal in its N terminus. We also identified a key nuclear export signal in β1-chimaerin that is absent in β1-Δ7p-chimaerin, causing nuclear retention of this truncated variant. Functionally analyses revealed that β1-Δ7p-chimaerin inactivates nuclear Rac and negatively regulates the cell cycle. Our results provide important insights into the diversity of chimaerin Rac-GAP regulation and function and highlight a potential mechanism of nuclear Rac inactivation that may play significant roles in pathologies such as cancer.

Development of a liquid chromatography-mass spectrometry based enzyme activity assay for phosphatidylcholine-specific phospholipase C

Phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) hydrolyzes PC to generate the second messenger 1,2-diacylglycerol (DG) and phosphocholine. PC-PLC plays pivotal roles in inflammation, carcinogenesis, tumor progression, atherogenesis, and subarachnoid hemorrhage. Although the activity of PC-PLC in mammalian tissues was discovered approximately 40 years ago, neither the protein nor its gene has been identified. In the present study, we developed a non-radioactive enzyme activity assay for PC-PLC based on mass spectrometric detection of DG following HPLC separation. This new liquid chromatography-mass spectrometry (LC-MS) assay directly determines a specific reaction product, 1-palmitoyl-2-oleoyl-DG, that is generated from 1-palmitoyl-2-oleoyl-PC by purified Bacillus cereus PC-PLC. The LC-MS assay offers several advantages including a lower background (0.02% versus 91%), higher signal background ratio (4242 versus 1.06)/signal noise ratio (7494 versus 4.4), higher sensitivity (≥32-fold), and lower limit of quantitation (0.04 pmol versus 0.69 pmol of PC-PLC), than a conventional fluorometric assay, which indirectly detects phosphocholine produced in the reaction. In addition to Bacillus cereus PC-PLC, the LC-MS assay was applicable to the measurement of mammalian PC-PLC prepared from the mouse brain. The radioisotope-free, highly sensitive and precise LC-MS assay for PC-PLC would be useful for the purification and identification of PC-PLC protein.

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.

Assessment of Jatropha curcas L. biodiesel seed cake toxicity using the zebrafish (Danio rerio) embryo toxicity (ZFET) test

Consequent to the growing demand for alternative sources of energy, the seeds from Jatropha curcas remain to be the favorite for biodiesel production. However, a significant volume of the residual organic mass (seed cake) is produced during the extraction process, which raises concerns on safe waste disposal. In the present study, we assessed the toxicity of J. curcas seed cake using the zebrafish (Danio rerio) embryotoxicity test. Within 1-h post-fertilization (hpf), the fertilized eggs were exposed to five mass concentrations of J. curcas seed cake and were followed through 24, 48, and 72 hpf. Toxicity was evaluated based on lethal endpoints induced on zebrafish embryos namely egg coagulation, non-formation of somites, and non-detachment of tail. The lowest concentration tested, 1 g/L, was not able to elicit toxicity on embryos whereas 100 % mortality (based also on lethal endpoints) was recorded at the highest concentration at 2.15 g/L. The computed LC50 for the J. curcas seed cake was 1.61 g/L. No further increase in mortality was observed in the succeeding time points (48 and 72 hpf) indicating that J. curcas seed cake exerted acute toxicity on zebrafish embryos. Sublethal endpoints (yolk sac and pericardial edema) were noted at 72 hpf in zebrafish embryos exposed to higher concentrations. The observed lethal endpoints induced on zebrafish embryos were discussed in relation to the active principles, notably, phorbol esters that have remained in the seed cake even after extraction.

Evidence for a role of MRCK in mediating HeLa cell elongation induced by the C1 domain ligand HMI-1a3

Diacylglycerol (DAG) is a central mediator of signaling pathways that regulate cell proliferation, survival and apoptosis. Therefore, C1 domain, the DAG binding site within protein kinase C (PKC) and other DAG effector proteins, is considered a potential cancer drug target. Derivatives of 5-(hydroxymethyl)isophthalic acid are a novel group of C1 domain ligands with antiproliferative and differentiation-inducing effects. Our previous work showed that these isophthalate derivatives exhibit antiproliferative and elongation-inducing effects in HeLa human cervical cancer cells. In this study we further characterized the effects of bis(3-trifluoromethylbenzyl) 5-(hydroxymethyl)isophthalate (HMI-1a3) on HeLa cell proliferation and morphology. HMI-1a3-induced cell elongation was accompanied with loss of focal adhesions and actin stress fibers, and exposure to HMI-1a3 induced a prominent relocation of cofilin-1 into the nucleus regardless of cell phenotype. The antiproliferative and morphological responses to HMI-1a3 were not modified by pharmacological inhibition or activation of PKC, or by RNAi knock-down of specific PKC isoforms, suggesting that the effects of HMI-1a3 were not mediated by PKC. Genome-wide gene expression microarray and gene set enrichment analysis suggested that, among others, HMI-1a3 induces changes in small GTPase-mediated signaling pathways. Our experiments revealed that the isophthalates bind also to the C1 domains of β2-chimaerin, protein kinase D (PKD) and myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK), which are potential mediators of small GTPase signaling and cytoskeletal reorganization. Pharmacological inhibition of MRCK, but not that of PKD attenuated HMI-1a3-induced cell elongation, suggesting that MRCK participates in mediating the effects of HMI-1a3 on HeLa cell morphology.

β3-chimaerin, a novel member of the chimaerin Rac-GAP family

Chimaerins are a family of diacylglycerol- and phorbol ester-regulated GTPase activating proteins (GAPs) for the small G-protein Rac. Extensive evidence indicates that these proteins play important roles in development, axon guidance, metabolism, cell motility, and T cell activation. Four isoforms have been reported to-date, which are products of CHN1 (α1- and α2-chimaerins) and CHN2 (β1- and β2-chimaerins) genes. Although these gene products are assumed to be generated by alternative splicing, bioinformatics analysis of the CHN2 gene revealed that β1- and β2-chimaerins are the products of alternative transcription start sites (TSSs) in different promoter regions. Furthermore, we found an additional TSS in CHN2 gene that leads to a novel product, which we named β3-chimaerin. Expression profile analysis revealed predominantly low levels for the β3-chimaerin transcript, with higher expression levels in epididymis, plasma blood leucocytes, spleen, thymus, as well as various areas of the brain. In addition to the prototypical SH2, C1, and Rac-GAP domains, β3-chimaerin has a unique N-terminal domain. Studies in cells established that β3-chimaerin has Rac-GAP activity and is responsive to phorbol esters. The enhanced responsiveness of β3-chimaerin for phorbol ester-induced translocation relative to β2-chimaerin suggests differential ligand accessibility to the C1 domain.

Coordinated activation of the Rac-GAP β2-chimaerin by an atypical proline-rich domain and diacylglycerol

Chimaerins, a family of GTPase activating proteins (GAPs) for the small G-protein Rac, have been implicated in development, neuritogenesis, and cancer. These Rac-GAPs are regulated by the lipid second messenger diacylglycerol (DAG) generated by tyrosine-kinases such as the epidermal growth factor receptor (EGFR). Here we identify an atypical Pro-rich motif in chimaerins that binds to the adaptor protein Nck1. Unlike most Nck1 partners, chimaerins bind to the third SH3 domain of Nck1. This association is mediated by electrostatic interactions of basic residues within the Pro-rich motif with acidic clusters in the SH3 domain. EGF promotes the binding of β2-chimaerin to Nck1 in the cell periphery in a DAG-dependent manner. Moreover, β2-chimaerin translocation to the plasma membrane and its peripheral association with Rac1 requires Nck1. Our studies underscore a coordinated mechanism for β2-chimaerin activation that involves lipid interactions via the C1 domain and protein-protein interactions via the N-terminal Pro-rich region.

Activation of nuclear factor κB (NF-κB) in prostate cancer is mediated by protein kinase C epsilon (PKCepsilon)

Protein kinase C ε (PKCε) has emerged as an oncogenic kinase and plays important roles in cell survival, mitogenesis and invasion. PKCε is up-regulated in most epithelial cancers, including prostate, breast, and lung cancer. Here we report that PKCε is an essential mediator of NF-κB activation in prostate cancer cells. A strong correlation exists between PKCε overexpression and NF-κB activation status in prostate cancer cells. Moreover, transgenic overexpression of PKCε in the mouse prostate causes preneoplastic lesions that display significant NF-κB hyperactivation. PKCε RNAi depletion or inhibition in prostate cancer cells diminishes NF-κB translocation to the nucleus with subsequent impairment of both activation of NF-κB transcription and induction of NF-κB responsive genes in response to the proinflammatory cytokine tumor necrosis factor α (TNFα). On the other hand, PKCε overexpression in normal prostate cells enhances activation of the NF-κB pathway. A mechanistic analysis revealed that TNFα activates PKCε via a C1 domain/diacylglycerol-dependent mechanism that involves phosphatidylcholine-phospholipase C. Moreover, PKCε facilitates the assembly of the TNF receptor-I signaling complex to trigger NF-κB activation. Our studies identified a molecular link between PKCε and NF-κB that controls key responses implicated in prostate cancer progression.

The RacGAP β2-Chimaerin selectively mediates axonal pruning in the hippocampus

Axon pruning and synapse elimination promote neural connectivity and synaptic plasticity. Stereotyped pruning of axons that originate in the hippocampal dentate gyrus (DG) and extend along the infrapyramidal tract (IPT) occurs during postnatal murine development by neurite retraction and resembles axon repulsion. The chemorepellent Sema3F is required for IPT axon pruning, dendritic spine remodeling, and repulsion of DG axons. The signaling events that regulate IPT axon pruning are not known. We find that inhibition of the small G protein Rac1 by the Rac GTPase-activating protein (GAP) β2-Chimaerin (β2Chn) mediates Sema3F-dependent pruning. The Sema3F receptor neuropilin-2 selectively binds β2Chn, and ligand engagement activates this GAP to ultimately restrain Rac1-dependent effects on cytoskeletal reorganization. β2Chn is necessary for axon pruning both in vitro and in vivo, but it is dispensable for axon repulsion and spine remodeling. Therefore, a Npn2/β2Chn/Rac1 signaling axis distinguishes DG axon pruning from the effects of Sema3F on repulsion and dendritic spine remodeling.

p23/Tmp21 associates with protein kinase Cdelta (PKCdelta) and modulates its apoptotic function

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKCδ, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKCδ C1b domain associates with p23 and identified two key residues (Asp(245) and Met(266)) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKCδ-dependent apoptosis and activation of PKCδ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKCδ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKCδ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKCδ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli.

p23/Tmp21 differentially targets the Rac-GAP beta2-chimaerin and protein kinase C via their C1 domains

The ER/Golgi protein p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β-chimaerin. C1 domains from PKC isozymes can also interact with p23/Tmp21. Our study highlights the relevance of C1 domains in protein-protein interactions in addition to their well-established lipid-binding properties.

The C1 domains in protein kinase C (PKC) isozymes and other signaling molecules are responsible for binding the lipid second messenger diacylglycerol and phorbol esters, and for mediating translocation to membranes. Previous studies revealed that the C1 domain in α- and β-chimaerins, diacylglycerol-regulated Rac-GAPs, interacts with the endoplasmic reticulum/Golgi protein p23/Tmp21. Here, we found that p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β2-chimaerin. Glu227 and Leu248 in the β2-chimaerin C1 domain are crucial for binding p23/Tmp21 and perinuclear targeting. Interestingly, isolated C1 domains from individual PKC isozymes differentially interact with p23/Tmp21. For PKCε, it interacts with p23/Tmp21 specifically via its C1b domain; however, this association is lost in response to phorbol esters. These results demonstrate that p23/Tmp21 acts as an anchor that distinctively modulates compartmentalization of C1 domain-containing proteins, and it plays an essential role in β2-chimaerin relocalization. Our study also highlights the relevance of C1 domains in protein–protein interactions in addition to their well-established lipid-binding properties.

A novel cross-talk in diacylglycerol signaling: the Rac-GAP beta2-chimaerin is negatively regulated by protein kinase Cdelta-mediated phosphorylation

Although the family of chimaerin Rac-GAPs has recently gained significant attention for their involvement in development, cancer, and neuritogenesis, little is known about their molecular regulation. Chimaerins are activated by the lipid second messenger diacylglycerol via their C1 domain upon activation of tyrosine kinase receptors, thereby restricting the magnitude of Rac signaling in a receptor-regulated manner. Here we identified a novel regulatory mechanism for beta2-chimaerin via phosphorylation. Epidermal growth factor or the phorbol ester phorbol 12-myristate 13-acetate caused rapid phosphorylation of beta2-chimaerin on Ser(169) located in the SH2-C1 domain linker region via protein kinase Cdelta, which retained beta2-chimaerin in the cytosol and prevented its C1 domain-mediated translocation to membranes. Furthermore, despite the fact that Ser(169) phosphorylation did not alter intrinsic Rac-GAP activity in vitro, a non-phosphorylatable beta2-chimaerin mutant was highly sensitive to translocation, and displayed enhanced association with activated Rac, enhanced Rac-GAP activity, and anti-migratory properties when expressed in cells. Our results not only revealed a novel regulatory mechanism that facilitates Rac activation, but also identified a novel mechanism of cross-talk between diacylglycerol receptors that restricts beta2-chimaerin relocalization and activation.

Biochemical characterization of hyperactive beta2-chimaerin mutants revealed an enhanced exposure of C1 and Rac-GAP domains

Recent studies established that the Rac-GAP beta2-chimaerin plays important roles in development, neuritogenesis, and cancer progression. A unique feature of beta2-chimaerin is that it can be activated by phorbol esters and the lipid second messenger diacylglycerol (DAG), which bind with high affinity to its C1 domain and promote beta2-chimaerin translocation to membranes, leading to the inactivation of the small G-protein Rac. Crystallographic evidence and cellular studies suggest that beta2-chimaerin remains in an inactive conformation in the cytosol with the C1 domain inaccessible to ligands. We developed a series of beta2-chimaerin point mutants in which intramolecular contacts that occlude the C1 domain have been disrupted. These mutants showed enhanced translocation in response to phorbol 12-myristate 13-acetate (PMA) in cells. Binding assays using [(3)H]phorbol 12,13-dibutyrate ([(3)H]PDBu) revealed that internal contact mutants have a reduced acidic phospholipid requirement for phorbol ester binding. Moreover, disruption of intramolecular contacts enhances binding of beta2-chimaerin to acidic phospholipid vesicles and confers enhanced Rac-GAP activity in vitro. These studies suggest that beta2-chimaerin must undergo a conformational rearrangement in order to expose its lipid binding sites and become activated.

Effect of phorbol esters on the macrophage-mediated biodegradation of polyurethanes via protein kinase C activation and other pathways

It was previously found that re-seeding monocyte-derived macrophages (MDM) on polycarbonate-based polyurethanes (PCNUs) in the presence of the protein kinase C (PKC) activator phorbol myristate acetate (PMA) inhibited MDM-mediated degradation of PCNUs synthesized with 1,6-hexane diisocyanate (HDI), as well as esterase activity and monocyte-specific esterase (MSE) protein. However, no effect on the degradation of a 4,4'-methylene bisphenyl (MDI)-derived PCNU (MDI321) occurred. This finding suggested that oxidation, a process linked to the PKC pathway, was not activated in the same manner for all PCNUs. In the current study MDM were re-seeded onto the above PCNU surfaces with PMA, PKC-inactive 4alphaPMA and the PKC inhibitor bisindolylmaleimide I hydrochloride (BIM) for 48 h before assaying for PCNU degradation, esterase activity, MSE protein, DNA, cell viability and cell morphology. 4alphaPMA did not alter MDM-mediated HDI PCNU degradation but MDI321 degradation increased in this condition. BIM alone had no effect on any parameter; however, when BIM and PMA were added together, the PMA inhibition of biodegradation, esterase activity and MSE protein was partially reversed for MDM on HDI PCNUs only. Adding PMA to MDM on HDI PCNUs increased intercellular connections, whereas 4alphaPMA or BIM+PMA increased cell size. Although this study demonstrated a role for oxidation via a PKC-activated pathway in MDM-mediated PCNU degradation, phorbol esters appear to also activate non-PKC pathways that have roles in biodegradation. Moreover, the sensitivity to material surface chemistry in the MDM response to each PCNU dictates a multi-factorial degradative process involving alternate material specific oxidative and hydrolytic mechanisms.

Identification of an autoinhibitory mechanism that restricts C1 domain-mediated activation of the Rac-GAP alpha2-chimaerin

Chimaerins are a family of GTPase activating proteins (GAPs) for the small G-protein Rac that have gained recent attention due to their important roles in development, cancer, neuritogenesis, and T-cell function. Like protein kinase C isozymes, chimaerins possess a C1 domain capable of binding phorbol esters and the lipid second messenger diacylglycerol (DAG) in vitro. Here we identified an autoinhibitory mechanism in alpha2-chimaerin that restricts access of phorbol esters and DAG, thereby limiting its activation. Although phorbol 12-myristate 13-acetate (PMA) caused limited translocation of wild-type alpha2-chimaerin to the plasma membrane, deletion of either N- or C-terminal regions greatly sensitize alpha2-chimaerin for intracellular redistribution and activation. Based on modeling analysis that revealed an occlusion of the ligand binding site in the alpha2-chimaerin C1 domain, we identified key amino acids that stabilize the inactive conformation. Mutation of these sites renders alpha2-chimaerin hypersensitive to C1 ligands, as reflected by its enhanced ability to translocate in response to PMA and to inhibit Rac activity and cell migration. Notably, in contrast to PMA, epidermal growth factor promotes full translocation of alpha2-chimaerin in a phospholipase C-dependent manner, but not of a C1 domain mutant with reduced affinity for DAG (P216A-alpha2-chimaerin). Therefore, DAG generation and binding to the C1 domain are required but not sufficient for epidermal growth factor-induced alpha2-chimaerin membrane association. Our studies suggest a role for DAG in anchoring rather than activation of alpha2-chimaerin. Like other DAG/phorbol ester receptors, including protein kinase C isozymes, alpha2-chimaerin is subject to autoinhibition by intramolecular contacts, suggesting a highly regulated mechanism for the activation of this Rac-GAP.

Role of chimaerins, a group of Rac-specific GTPase activating proteins, in T-cell receptor signaling

Chimaerins are GTPase-activating proteins that inactivate the GTP-hydrolase Rac1 in a diacylglycerol-dependent manner. To date, the study of chimaerins has been done mostly in neuronal cells. Here, we show that alpha2- and beta2-chimaerin are expressed at different levels in T-cells and that they participate in T-cell receptor signaling. In agreement with this, we have observed that alpha2- and beta2-chimaerins translocate to the T-cell/B-cell immune synapse and, using both gain- and loss-of-function approaches, demonstrated that their catalytic activity is important for the inhibition of the T-cell receptor- and Vav1-dependent stimulation of the transcriptional factor NF-AT. Mutagenesis-based approaches have revealed the molecular determinants that contribute to the biological program of chimaerins during T-cell responses. Unexpectedly, we have found that the translocation of chimaerins to the T-cell/B-cell immune synapse does not rely on the canonical binding of diacylglycerol to the C1 region of these GTPase-activating proteins. Taken together, these results identify chimaerins as candidates for the downmodulation of Rac1 in T-lymphocytes and, in addition, uncover a novel regulatory mechanism that mediates their activation in T-cells.

S-Phase-specific activation of PKC alpha induces senescence in non-small cell lung cancer cells

Protein kinase C (PKC) has been widely implicated in positive and negative control of cell proliferation. We have recently shown that treatment of non-small cell lung cancer (NSCLC) cells with phorbol 12-myristate 13-acetate (PMA) during G1 phase inhibits the progression into S phase, an effect mediated by PKC delta-induced up-regulation of the cell cycle inhibitor p21 Cip1. However, PMA treatment in asynchronously growing NSCLC cells leads to accumulation of cells in G2/M. Studies in post-G1 phases revealed that PMA induced an irreversible G2/M cell cycle arrest in NSCLC cells and conferred morphological and biochemical features of senescence, including elevated SA-beta-Gal activity and reduced telomerase activity. Remarkably, this effect was phase-specific, as it occurred only when PKC was activated in S, but not in G1, phase. Mechanistic analysis revealed a crucial role for the classical PKC alpha isozyme as mediator of the G2/M arrest and senescence, as well as for inducing p21(Cip1) an obligatory event for conferring the senescence phenotype. In addition to the unappreciated role of PKC isozymes, and specifically PKC alpha, in senescence, our data introduce the paradigm that discrete PKCs trigger distinctive responses when activated in different phases of the cell cycle via a common mechanism that involves p21 Cip1 up-regulation.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Chimaerins: GAPs that bridge diacylglycerol signalling and the small G-protein Rac

Chimaerins are the only known RhoGAPs (Rho GTPase-activating proteins) that bind phorbol ester tumour promoters and the lipid second messenger DAG (diacylglycerol), and show specific GAP activity towards the small GTPase Rac. This review summarizes our knowledge of the structure, biochemical and biological properties of chimaerins. Recent findings have established that chimaerins are regulated by tyrosine kinase and GPCRs (G-protein-coupled receptors) via PLC (phospholipase C) activation and DAG generation to promote Rac inactivation. The finding that chimaerins, along with some other proteins, are receptors for DAG changed the prevalent view that PKC (protein kinase C) isoenzymes are the only cellular molecules regulated by DAG. In addition, vigorous recent studies have begun to decipher the critical roles of chimaerins in the central nervous system, development and tumour progression.

Proteomics analysis of the expression of neurogranin in murine neuroblastoma (Neuro-2a) cells reveals its involvement for cell differentiation

Neurogranin (Ng) is a neural-specific, calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC). Although its biochemical property has been well characterized, the physiological function of Ng needs to be elucidated. In the present study, we performed proteomics analysis of the induced compositional changes due to the expression of Ng in murine neuroblastoma (Neuro-2a) cells using isotope coded affinity tags (ICAT) combined with 2-dimensional liquid chromatography/tandem mass spectrometry (2D-LC/MS/MS). We found that 40% of identified proteins were down-regulated and most of these proteins are microtubule components and associated proteins that mediated neurite outgrowth. Western blot experiments confirmed the expression of α-tubulin and microtubule- associated protein 1B (MAP 1B) was dramatically reduced in Neuro-2a-Ng cells compared to control. Cell morphology of Neuro-2a-Ng showed far less neurites than the control. Serum deprivation induced the extension of only one or two long neurites per cell in Neuro-2a-Ng, contrasting to the extension of multiple neurites per control cell. Ng may be linked to neurite formation by affecting expression of several microtubule related proteins. Furthermore, the PKC activator (PMA) induced an enhanced ERK1/2 activity in the cells that expressed Ng. The mutation of Ng at S36A caused sustained increase of ERK1/2 activity, whereas the ERK1/2 activity in mutation at I33Q showed no difference compared to wild type Ng, suggesting the phosphorylation of Ng but not the CaM /Ng interaction plays an important role in ERK activation. Ng may be involved in neuronal growth and differentiation via PKC and ERK1/2 signaling pathways.

The lipid second messenger diacylglycerol as a negative regulator of Rac signalling

We have established a novel role for the second messenger DAG (diacylglycerol), a product of PtdIns(4,5)P2 hydrolysis by PLC (phospholipase C). In addition to its well-known function as a protein kinase C activator, DAG produced by stimulation of the epidermal growth factor receptor causes the redistribution of the Rac-GAP (GTPase-activating protein) beta2-chimaerin to the plasma membrane, where it associates with the active form of Rac1 and promotes the inactivation of this small G-protein. This represents the first example of a Rac-GAP regulated directly by DAG in response to the activation of a tyrosine kinase receptor, and suggests a previously unappreciated role for this lipid as a negative modulator of Rac signalling.

Cytotoxic action of phorbol esters on human pancreatic cancer cells

We previously showed that phorbol esters are cytotoxic to human thyroid epithelial cells expressing a mutant RAS oncogene. Here we explore the generality of this finding using cells derived from pancreatic cancer, which, like thyroid, shows a high frequency of RAS mutation, but is a much greater cause of cancer mortality. The response to phorbol myristate acetate (PMA) and related agents was assessed on a panel of 9 pancreatic cancer cell lines, using a range of assays for cell growth and death in vitro and in vivo. In most lines, PMA induced non-apoptotic cell death which was, surprisingly, independent of its "classic" target, protein kinase C. With 24 hr exposure, the IC(50) in the most sensitive line (Aspc-1) was <1 ng/ml (1.6 nM), with survival undetectable at concentrations >/=>/=16 nM, and after only 1 hr exposure the IC(50) was still </=</=16 nM. Interestingly, the efficacy of a second phorbol ester, phorbol dibutyrate, was much lower, and the PMA analogue bryostatin-1, which is in clinical trials against other tumour types, was totally inactive. Pre-treatment of Aspc-1 cells with PMA before subcutaneous inoculation into nude mice prevented, or greatly retarded, subsequent xenograft tumour growth. Furthermore, treatment of established tumours with a single peri-tumoral injection of PMA induced extensive cell death and arrested tumour development. Taken together with recent Phase 1 clinical studies, these data suggest that activity against pancreatic cancer will be attainable by systemic administration of PMA, and point to potential novel therapeutic targets for this highly aggressive cancer.

Protein Kinase C (PKC)α and PKCθ Are the Major PKC Isotypes Involved in TCR Down-Regulation

It is well known that protein kinase C (PKC) plays an important role in regulation of TCR cell surface expression levels. However, eight different PKC isotypes are present in T cells, and to date the particular isotype(s) involved in TCR down-regulation remains to be identified. The aim of this study was to identify the PKC isotype(s) involved in TCR down-regulation and to elucidate the mechanism by which they induce TCR down-regulation. To accomplish this, we studied TCR down-regulation in the human T cell line Jurkat, in primary human T cells, or in the mouse T cell line DO11.10 in which we either overexpressed constitutive active or dominant-negative forms of various PKC isotypes. In addition, we studied TCR down-regulation in PKC knockout mice and by using small interfering RNA-mediated knockdown of specific PKC isotypes. We found that PKCalpha and PKCtheta were the only PKC isotypes able to induce significant TCR down-regulation. Both isotypes mediated TCR down-regulation via the TCR recycling pathway that strictly depends on Ser(126) and the di-leucine-based receptor-sorting motif of the CD3gamma chain. Finally, we found that PKCtheta was mainly implicated in down-regulation of directly engaged TCR, whereas PKCalpha was involved in down-regulation of nonengaged TCR.

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.

The zebrafish homologue of mammalian chimerin Rac-GAPs is implicated in epiboly progression during development

In this paper, we report an in vivo model for the chimerins, a family of Rac GTPase-activating proteins (Rac-GAPs) that are uniquely regulated by the lipid second messenger diacylglycerol and have been implicated in the control of actin dynamics, migration, and proliferation. We cloned the zebrafish homologue of mammalian alpha2-chimerin (chn1) and determined that it possesses Rac-GAP activity and a C1 domain with phorbol ester/diacylglycerol-binding capability. chn1 morpholino knockdown embryos exhibit severe abnormalities, including the development of round somites, lack of yolk extension, and a kinked posterior notochord. These zebrafish morphants show Rac hyperactivation and progress faster through epiboly, leading to tailbud-stage embryos that have a narrow axis and an enlarged tailbud with expanded bmp4 and shh expression. Phenotypic rescue was achieved by mRNA microinjection of chn1 or an active chimerin Rac-GAP domain into the yolk syncytial layer but not by a chn1 mutant deficient in Rac-GAP activity, suggesting that the lack of chn1 Rac-GAP activity in the yolk syncytial layer was causative of the misbalance in morphogenetic movements. Our results reveal a crucial role for chn1 in early development and implicate Rac as a key regulator of morphogenetic movements during zebrafish epiboly.

Protein kinase C enhances glycine-insensitive desensitization of NMDA receptors independently of previously identified protein kinase C sites

Protein kinase C (PKC) phosphorylates the NR1 and NR2A subunits of NMDARs at consensus sites located within their intracellular C-terminal tails. However, the functional consequences of these biochemical events are not well understood. In HEK293 cells expressing NR1/NR2A, activation of endogenous PKC by 4beta-phorbol 12-myristate 13-acetate (PMA) increased NMDAR desensitization as evidenced by a reduced steady-state current without any change in peak. The effects of PMA on NMDAR-mediated responses were prevented by specific PKC inhibitors and were not mimicked by an inactive enantiomer of PMA. The effects of PMA were preserved despite mutagenesis of the major PKC sites on the NR1 subunit (S889A, S890A, S896A and S897A) or removal of the entire NR1 C-terminal tail (NR1(stop838)). When co-expressing NR1(stop838)/NR2A the effects of PMA could only be observed with agonist concentrations sufficient to induce glycine-insensitive desensitization. Moreover, the effects of PMA were observed in receptors composed of NR1/NR2A and NR1/NR2B, but not NR1/NR2C, a subunit combination in which desensitization is absent. The NR2 subunit dependence suggested that the actions of PMA might require specific PKC sites previously identified within NR2A. However, a C-terminal truncated form of NR2A (NR2A(stop905)) remained responsive to PMA. We conclude that activation of PKC increases NMDAR glycine-insensitive desensitization independently of previously identified sites located within the NR1 C-terminus and distal segment of the NR2A C-terminus.

β2-chimaerin provides a diacylglycerol-dependent mechanism for regulation of adhesion and chemotaxis of T cells

The small GTPase Rac contributes to regulation of cytoskeletal rearrangement during chemokine-induced lymphocyte adhesion and migration in a multi-step process that is very precisely coordinated. Chimaerins are Rac1-specific GTPase-activating proteins of unknown biological function, which have a canonical diacylglycerol C1-binding domain. Here we demonstrate endogenous expression of β2-chimaerin in T lymphocytes and study the functional role of this protein in phorbol ester and chemokine (CXCL12)-regulated T-cell responses. We used green fluorescent protein-tagged β2-chimaerin and phorbol ester stimulation to investigate changes in protein localization in living lymphocytes. Our results demonstrate that active Rac cooperates with C1-dependent phorbol ester binding to induce sustained GFP-β2-chimaerin localization to the membrane. Subcellular distribution of GFP β2-chimaerin in living cells showed no major changes following CXCL12 stimulation. Nonetheless Rac1-GTP levels were severely inhibited in GFP-β2-chimaerin-expressing cells, which displayed reduced CXCL12-induced integrin-dependent adhesion and spreading. This effect was dependent on chimaerin GTPase-activating protein function and required diacylglycerol generation. Whereas β2-chimaerin overexpression decreased static adhesion, it enhanced CXCL12-dependent migration via receptor-dependent diacylglycerol production. These studies demonstrate that β2-chimaerin provides a novel, diacylglycerol-dependent mechanism for Rac regulation in T cells and suggest a functional role for this protein in Rac-mediated cytoskeletal remodeling.

C1, see them all

Recent structural analysis of crystalline beta2-chimaerin shows the central protein kinase C-like, diacylglycerol (DAG)-binding C1 domain to be masked by its intramolecular interactions with the N-terminal SH2 and GAP domains, and linker regions. A mechanism of activation has been derived from modelling of a GAP-Rac GTPase complex--the auto-inhibitory constraints are released via membrane engagement, unmasking the C1 domain to enable DAG binding and subsequent GAP stimulation of Rac GTPase catalytic activity.

Targeting protein kinase C and "non-kinase" phorbol ester receptors: emerging concepts and therapeutic implications

Phorbol esters, natural compounds that mimic the action of the lipid second messenger diacylglycerol (DAG), are known to exert their biological actions through the activation of classical and novel protein kinase C (PKC) isozymes. Phorbol esters, via binding to the PKC C1 domains, cause major effects on mitogenesis by controlling the activity of cyclin-cdk complexes and the expression of cdk inhibitors. In the last years it became clear that phorbol esters activate other molecules having a C1 domain in addition to PKCs. One of the most interesting families of "non-kinase" phorbol ester receptors is represented by the chimaerins, lipid-regulated Rac-GAPs that modulate actin cytoskeleton reorganization, migration, and proliferation. The discovery of the chimaerins and other "non-kinase" phorbol ester receptors has major implications in the design of agents for cancer therapy.

Tumor promoter binding of the protein kinase C C1 homology domain peptides of RasGRPs, chimaerins, and Unc13s

Recent investigations discovered nonkinase-type phorbol ester receptors, RasGRPs, chimaerins, and Unc13s. Phorbol ester binding occurs at the cysteine-rich sequences of about 50 residues in the C1 domains of these receptors. Fifty-one-residue RasGRP C1 peptides except for RasGRP2 showed significant phorbol 12,13-dibutyrate (PDBu) binding, but the K(d) values of the RasGRP1 and RasGRP3 C1 peptides were about 10-fold larger than those for the corresponding whole enzymes. Addition of the C-terminal basic amino acid cluster decreased their K(d) values about 10-fold, suggesting that the positive charges of these C1 peptides play an important role in the PDBu binding in the presence of negatively-charged phosphatidylserine. The 51-mer chimaerin C1 peptides showed potent PDBu binding, while the Unc13 and Munc13-1 C1 peptides without sufficient positive charges hardly bound PDBu. By the rapid screening system using this C1 peptide library, 5-prenyl-indolactam-V was identified as a promising lead for the novel protein kinase C isozyme specific ligands.

Rac-GAP-dependent inhibition of breast cancer cell proliferation by {beta}2-chimerin

beta2-Chimerin is a member of the "non-protein kinase C" intracellular receptors for the second messenger diacylglycerol and the phorbol esters that is yet poorly characterized, particularly in the context of signaling pathways involved in proliferation and cancer progression. beta2-Chimerin possesses a C-terminal Rac-GAP (GTPase-activating protein) domain that accelerates the hydrolysis of GTP from the Rac GTPase, leading to its inactivation. We found that beta2-chimerin messenger levels are significantly down-regulated in human breast cancer cell lines as well as in breast tumors. Adenoviral delivery of beta2-chimerin into MCF-7 breast cancer cells leads to inhibition of proliferation and G(1) cell cycle arrest. Mechanistic studies show that the effect involves the reduction in Rac-GTP levels, cyclin D1 expression, and retinoblastoma dephosphorylation. Studies using the mutated forms of beta2-chimerin revealed that these effects were entirely dependent on its C-terminal GAP domain and Rac-GAP activity. Moreover, MCF-7 cells stably expressing active Rac (V12Rac1) but not RhoA (V14RhoA) were insensitive to beta2-chimerin-induced inhibition of proliferation and cell cycle progression. The modulation of G(1)/S progression by beta2-chimerin not only implies an essential role for Rac in breast cancer cell proliferation but also raises the intriguing possibility that diacylglycerol-regulated non-protein kinase C pathways can negatively impact proliferation mechanisms controlled by Rho GTPases.

Immunophenotypic changes induced on human HL60 leukaemia cells by 1alpha,25-dihydroxyvitamin D3 and 12-O-tetradecanoyl phorbol-13-acetate

1alpha,25-Dihydroxyvitamin D3 (1,25D3) induces HL60 cells to acquire a monocyte-like phenotype, while cells treated with 12-O-tetradecanoyl phorbol-13-acetate (TPA) resemble macrophages. Using a microarray of 82 CD antibodies, 24 cluster of differentiation (CD) antigens were detected on HL60 cells. 1,25D3 induced the following antigens in decreasing order of the change: CD14, CD11c, CD11b, CD54, CD86, CD38 and CD66c, with repression of CD117, CD71, CD95, CD45 and CD64. TPA induced the following antigens in decreasing order of the change: CD11c, CD9, CD11b, CD54, CD38, CD45RO and CD66c, with repression of CD4, CD117, CD95, CD71 and CD64. The results presented provide a basis for monitoring differentiation therapy of myeloid leukaemias in patients.

Alpha2-chimaerin, cyclin-dependent Kinase 5/p35, and its target collapsin response mediator protein-2 are essential components in semaphorin 3A-induced growth-cone collapse

Neurite outgrowth is influenced by positive and negative signals that include the semaphorins, an important family of axonal outgrowth inhibitors. Here we report that the Rac GTPase activating protein (GAP)alpha2-chimaerin is involved in Semaphorin 3A (Sema 3A) signaling. In dorsal root ganglion neurons, Sema 3A-induced growth cone collapse was inhibited by alpha2-chimaerin mutated to eliminate GAP activity or interaction with phosphotyrosine. Activation of alpha2-chimaerin by phorbol ester caused growth cone collapse. Active alpha2-chimaerin interacts with collapsin response mediator protein-2 (CRMP-2) and cyclin-dependent kinase (Cdk) 5/p35 kinase through its SH2 and GAP domains, respectively. Cdk5 phosphorylates CRMP-2 at serine 522, possibly facilitating phosphorylation of serine 518 and threonine 514 by glycogen synthase kinase 3beta (GSK3beta), a kinase previously implicated in Sema 3A signaling. Phosphorylation of CRMP-2 serine 522 was essential for Sema 3A-induced growth cone collapse, which is dependent on Cdk5 but not Rho kinase activity. alpha2-chimaerin, like CRMP-2, can associate with the Sema 3A receptor. These results indicate that active alpha2-chimaerin Rac GAP, Cdk5/p35, and its substrate CRMP-2, are implicated in the dynamics of growth cone guidance initiated through Sema 3A signaling.

Characterization of a novel GTPase-activating protein associated with focal adhesions and the actin cytoskeleton

In the present study we characterize a novel RhoGAP protein (RC-GAP72) that interacts with actin stress fibers, focal adhesions, and cell-cell adherens junctions via its 185-amino acid C-terminal region. Overexpression of RC-GAP72 in fibroblasts induces cell rounding with partial or complete disruption of actin stress fibers and formation of membrane ruffles, lamellipodia, and filopodia. RC-GAP72 mutant truncated downstream of the GTPase-activating protein (GAP) domain retains the ability to stimulate membrane protrusions but fails to affect stress fiber integrity or induce cell retraction. A mutant protein consisting of the C terminus of RC-GAP72 and lacking the GAP domain does not exert any visible effect on cellular morphology. Inactivation of the GAP domain by a point mutation does not abolish the effect of RC-GAP72 on actin stress fibers but moderates its capability to induce membrane protrusions. Our data imply that the cytoskeletal localization of RC-GAP72 and its interaction with GTPases are essential for its effect on the integrity of actin stress fibers, whereas the induction of lamellipodia and filopodia depends on the activity of the GAP domain irrespective of binding to the actin cytoskeleton. We propose that RC-GAP72 affects cellular morphology by targeting activated Cdc42 and Rac1 GTPases to specific subcellular sites, triggering local morphological changes. The overall physiological functions of RC-GAP72 are presently unknown, yet our data suggest that RC-GAP72 plays a role in regulating cell morphology and cytoskeletal organization.

Structural Mechanism for Lipid Activation of the Rac-Specific GAP, β2-Chimaerin

The lipid second messenger diacylglycerol acts by binding to the C1 domains of target proteins, which translocate to cell membranes and are allosterically activated. Here we report the crystal structure at 3.2 A resolution of one such protein, beta2-chimaerin, a GTPase-activating protein for the small GTPase Rac, in its inactive conformation. The structure shows that in the inactive state, the N terminus of beta2-chimaerin protrudes into the active site of the RacGAP domain, sterically blocking Rac binding. The diacylglycerol and phospholipid membrane binding site on the C1 domain is buried by contacts with the four different regions of beta2-chimaerin: the N terminus, SH2 domain, RacGAP domain, and the linker between the SH2 and C1 domains. Phospholipid binding to the C1 domain triggers the cooperative dissociation of these interactions, allowing the N terminus to move out of the active site and thereby activating the enzyme.

Divergence and complexities in DAG signaling: looking beyond PKC

For many years protein kinase C (PKC) has been the subject of extensive studies as a molecular target for the treatment of cancer and other diseases. To better define the role of PKC isozymes in the control of cell proliferation, survival and transformation, the examination of PKC-mediated signal transduction pathways by isozyme-specific intervention has become essential. However, issues related to the selectivity of activators and inhibitors of PKC isozymes, in addition to convoluted cross-talks between phorbol ester-regulated pathways, have greatly complicated our understanding of PKC-mediated responses. An additional level of complexity is provided by the fact diacylglycerol (DAG) signals can be transduced by phorbol ester receptors other than PKC. These receptors include chimaerins, RasGRPs, MUNC13s, PKD (PKC mu) and DAG kinases beta and gamma. Thus, it is conceivable that some of the effects that were originally attributed to PKC isozymes in response to phorbol esters might be mediated by PKC-independent pathways. A key issue for the design of novel therapeutic strategies that target PKC isozymes is a comprehensive analysis of isozyme-specific signal transduction pathways in different cell types and the development of pharmacological and molecular tools that can distinguish between the various PKC and 'non-PKC' phorbol ester receptors.

Phosphoinositide involvement in phagocytosis and phagosome maturation

Cells of the innate immune system engulf invading microorganisms into plasma membrane-derived vacuoles called phagosomes. Newly formed phagosomes gradually acquire microbicidal properties by a maturation process which involves sequential and coordinated rounds of fusion with endomembranes and concomitant fission. Some pathogens interfere with this maturation sequence and thereby evade killing by the immune cells, managing to survive intracellularly as parasites. Phosphoinositides seem to be intimately involved in the processes of phagosome formation and maturation, and initial observations suggest that the ability of some microorganisms to survive intracellularly is associated with alterations in phosphoinositide metabolism. This chapter presents a brief overview of phosphoinositides in cells of the immune system, their metabolism in the context of phagocytosis and phagosome maturation and their possible derangements during infectious pathogenosis.

Ligand Structure-Activity Requirements and Phospholipid Dependence for the Binding of Phorbol Esters to Protein Kinase D

Although protein kinase D (PKD), like protein kinase C (PKC), possesses a C1 domain that binds phorbol esters and diacylglycerol, the structural differences from PKC within this and other domains of PKD imply differential regulation by lipids and ligands. We characterized the phorbol ester and phospholipid binding properties of a glutathione S-transferase-tagged full-length PKD and compared them with those of PKC-alpha and -delta. We found that PKD is a high-affinity phorbol ester receptor for a range of structurally and functionally divergent phorbol esters and analogs and showed both similarities and differences in structure-activity relations compared with the PKCs examined. In particular, PKD had lower affinity than PKC for certain diacylglycerol analogs, which might be caused by a lysine residue at the 22 position of the PKD-C1b domain in place of the tryptophan residue at this position conserved in the PKCs. The membrane-targeting domains in PKD are largely different from those in PKC; among these differences, PKD contains a pleckstrin homology (PH) domain that is absent in PKC. However, phosphatidylinositol-4,5-bisphosphate PIP2, a lipid ligand for some PH domains, reconstitutes phorbol 12,13-dibutyrate (PDBu) binding to PKD similarly as it does to PKC-alpha and -delta, implying that the PH domain in PKD may not preferentially interact with PIP2. Overall, the requirement of anionic phospholipids for the reconstitution of [3H]PDBu binding to PKD was intermediate between those of PKC-alpha and -delta. We conclude that PKD is a high-affinity phorbol ester receptor; its lipid requirements for ligand binding are approximately comparable with those of PKC but may be differentially regulated in cells through the binding of diacylglycerol to the C1 domain.

Characterization of the Rac-GAP (Rac-GTPase-activating protein) activity of beta2-chimaerin, a 'non-protein kinase C' phorbol ester receptor

The regulation and function of beta2-chimaerin, a novel receptor for the phorbol ester tumour promoters and the second messenger DAG (diacylglycerol), is largely unknown. As with PKC (protein kinase C) isoenzymes, phorbol esters bind to beta2-chimaerin with high affinity and promote its subcellular distribution. beta2-Chimaerin has GAP (GTPase-activating protein) activity for the small GTP-binding protein Rac1, but for not Cdc42 or RhoA. We show that acidic phospholipids enhanced its catalytic activity markedly in vitro, but the phorbol ester PMA had no effect. beta2-Chimaerin and other chimaerin isoforms decreased cellular levels of Rac-GTP markedly in COS-1 cells and impaired GTP loading on to Rac upon EGF (epidermal growth factor) receptor stimulation. Deletional and mutagenesis analysis determined that the beta2-chimaerin GAP domain is essential for this effect. Interestingly, PMA has a dual effect on Rac-GTP levels in COS-1 cells. PMA increased Rac-GTP levels in the absence of a PKC inhibitor, whereas under conditions in which PKC activity is inhibited, PMA markedly decreased Rac-GTP levels and potentiated the effect of beta2-chimaerin. Chimaerin isoforms co-localize at the plasma membrane with active Rac, and these results were substantiated by co-immunoprecipitation assays. In summary, the novel phorbol ester receptor beta2-chimaerin regulates the activity of the Rac GTPase through its GAP domain, leading to Rac inactivation. These results strongly emphasize the high complexity of DAG signalling due to the activation of PKC-independent pathways, and cast doubts regarding the selectivity of phorbol esters and DAG analogues as selective PKC activators.

Interaction of palmitoylcarnitine with protein kinase C in neuroblastoma NB-2a cells

As reported previously [Acta Neurobiol. Exp. 57 (1997) 263], palmitoylcarnitine was observed to promote differentiation of neuroblastoma NB-2a cells with a concomitant inhibition of proliferation and of the phorbol ester stimulated activity of the protein kinase C (PKC). In the present study, palmitoylcarnitine was observed to inhibit phosphorylation of the PKC peptide substrate and to completely diminish binding of phorbol 12-myristate-13-acetate (PMA), although the effect was found to be uncompetitive. The exposure of NB-2a cells to palmitoylcarnitine in the presence of PMA resulted in a dramatic decrease in phosphorylation of the conventional and novel isozymes of PKC, mainly on serine. This effect was observed to be dose dependent. Inhibitors of serine/threonine phosphatases were not influencing the effect of palmitoylcarnitine what can point to an interaction between PKC and palmitoylcarnitine, affecting the process of autophosphorylation. These findings suggest that pamitoylcarnitine could be a natural modulator of PKC activity, thus regulating the process of cell differentiation.

Move over protein kinase C, you've got company: alternative cellular effectors of diacylglycerol and phorbol esters

Diacylglycerol is an essential second messenger in mammalian cells. The most prominent intracellular targets of diacylglycerol and of the functionally analogous phorbol esters belong to the protein kinase C (PKC) family. However, at least five alternative types of high-affinity diacylglycerol/phorbol-ester receptor are known: chimaerins, protein kinase D, RasGRPs, Munc13s and DAG kinase gamma. Recent evidence indicates that these have functional roles in diacylglycerol second messenger signalling in vivo and that several cellular processes depend on these targets rather than protein kinase C isozymes. These findings contradict the still prevalent view according to which all diacylglycerol/phorbol-ester effects are caused by the activation of protein kinase C isozymes. RasGRP1 (in Ras/Raf/MEK/ERK signalling) and Munc13-1 (in neurotransmitter secretion) are examples of non-PKC diacylglycerol/phorbol-ester receptors that mediate diacylglycerol and phorbol-ester effects originally thought to be caused by PKC isozymes. In the future, pharmacological studies on PKC must be complemented with alternative experimental approaches to allow the separation of PKC-mediated effects from those caused by alternative targets of the diacylglycerol second messenger pathway. The examples of RasGRP1 and Munc13-1 show that detailed genetic analyses of C(1)-domain-containing non-PKC diacylglycerol/phorbol-ester receptors in mammals are ideally suited to achieve this goal.

The use of fluorescent phorbol esters in studies of protein kinase C-membrane interactions

The family of protein kinase C (PKC) isozymes belongs to a growing class of proteins that become active by associating with membranes containing anionic phospholipids, such as phosphatidylserine. Depending on the particular PKC isoform, this process is mediated by Ca(2+)-binding to a C2 domain and interaction of activators such as 1,2-diacyl-sn-glycerol or phorbol esters with tandem C1 domains. This cooperation between the C1 and C2 domains in inducing the association of PKC with lipid membranes provides the energy for a conformational change that consists of the release of a pseudosubstrate sequence from the active site, culminating in activation. Thus, the properties of the interactions of the C1 and C2 domains with membranes, both as isolated domains, and as modules in the full length PKC isoforms, have been the subject of intense scrutiny. Here, we review the findings of studies in which fluorescent phorbol esters have been utilized to probe the properties of the C1 domains of PKC with respect to the interaction with activators, the subsequent interaction with membranes, and the role of the activating conformational change that leads to activation.

Novel "nonkinase" phorbol ester receptors: the C1 domain connection

In recent years, there have been great advances in our understanding of the pharmacology and biology of the receptors for the phorbol ester tumor promoters and the second messenger diacylglycerol (DAG). The traditional view of protein kinase C (PKC) as the sole receptor for the phorbol esters has been challenged with the discovery of proteins unrelated to PKC that bind phorbol esters with high affinity, suggesting a high degree of complexity in the signaling pathways activated by DAG. These novel "nonkinase" phorbol ester receptors include chimaerins (a family of Rac GTPase activating proteins), RasGRPs (exchange factors for Ras/Rap1), and Munc13 isoforms (scaffolding proteins involved in exocytosis). In all cases, phorbol ester binding occurs at the single C1 domain present in these proteins and, as in PKC isozymes, ligand binding is a phospholipid-dependent event. Moreover, the novel phorbol ester receptors are also subject to subcellular redistribution or "translocation" by phorbol esters, leading to their association to different effector and/or regulatory molecules. Clearly, the use of phorbol esters as specific activators of PKC in cellular models is questionable. Alternative pharmacological and molecular approaches are therefore needed to dissect the involvement of each receptor class as a mediator of phorbol ester/DAG responses.

Structural basis of RasGRP binding to high-affinity PKC ligands

The Ras guanyl releasing protein RasGRP belongs to the CDC25 class of guanyl nucleotide exchange factors that regulate Ras-related GTPases. These GTPases serve as switches for the propagation and divergence of signaling pathways. One interesting feature of RasGRP is the presence of a C-terminal C1 domain, which has high homology to the PKC C1 domain and binds to diacylglycerol (DAG) and phorbol esters. RasGRP thus represents a novel, non-kinase phorbol ester receptor. In this paper, we investigate the binding of indolactam(V) (ILV), 7-(n-octyl)-ILV, 8-(1-decynyl)benzolactam(V) (benzolactam), and 7-methoxy-8-(1-decynyl)benzolactam(V) (methoxylated benzolactam) to RasGRP through both experimental binding assays and molecular modeling studies. The binding affinities of these lactams to RasGRP are within the nanomolar range. Homology modeling was used to model the structure of the RasGRP C1 domain (C1-RasGRP), which was subsequently used to model the structures of C1-RasGRP in complex with these ligands and phorbol 13-acetate using a computational docking method. The structural model of C1-RasGRP exhibits a folding pattern that is nearly identical to that of C1b-PKCdelta and is comprised of three antiparallel-strand beta-sheets capped against a C-terminal alpha-helix. Two loops A and B comprising residues 8-12 and 21-27 form a binding pocket that has some positive charge character. The ligands phorbol 13-acetate, benzolactam, and ILV are recognized by C1-RasGRP through a number of hydrogen bonds with loops A and B. In the models of C1-RasGRP in complex with phorbol 13-acetate, benzolactam, and ILV, common hydrogen bonds are formed with two residues Thr12 and Leu21, whereas other hydrogen bond interactions are unique for each ligand. Furthermore, our modeling results suggest that the shallower insertion of ligands into the binding pocket of C1-RasGRP compared to C1b-PKCdelta may be due to the presence of Phe rather than Leu at position 20 in C1-RasGRP. Taken together, our experimental and modeling studies provide us with a better understanding of the structural basis of the binding of PKC ligands to the novel phorbol ester receptor RasGRP.

Chimaerins, novel non-protein kinase C phorbol ester receptors, associate with Tmp21-I (p23): evidence for a novel anchoring mechanism involving the chimaerin C1 domain

The regulation and function of chimaerins, a family of "non-protein kinase C" (PKC) phorbol ester/diacylglycerol receptors with Rac-GAP activity, is largely unknown. In a search for chimaerin-interacting proteins, we isolated Tmp21-I (p23), a protein localized at the perinuclear Golgi area. Remarkably, phorbol esters translocate beta2-chimaerin to the perinuclear region and promote its association with Tmp21-I in a PKC-independent manner. A deletional analysis revealed that the C1 domain in chimaerins is required for the interaction with Tmp21-I, thereby implying a novel function for this domain in protein-protein associations in addition to its role in lipid and phorbol ester binding. Our results support the emerging concept that multiple pathways transduce signaling by phorbol esters and revealed that, like PKC isozymes, chimaerins are subject to a positional regulation. In this setting, Tmp21-I serves as an anchoring protein that determines the intracellular localization of these novel phorbol ester receptors.