ras-transformed cells: altered levels of phosphatidylinositol-4,5-bisphosphate and catabolites

Phosphatidylinositol 4-phosphate; A minor lipid with multiple personalities

Phosphorylated products of phosphatidylinositol (PI), named Diphosphoinositide (DPI) and triphosphoinositide (TPI) were identified long time ago and found to exhibit high turnover rates based on their rapid 32P-phosphate labeling. The PI kinase activities that were responsible for their production were subsequently identified and found to be associated with different organelle membranes, including the plasma membrane. These activities were then linked with a certain group of cell surface receptors that activated phospholipase C enzymes to hydrolyze PI and used calcium or cGMP as a second messenger. This visionary concept was introduced in the seminal BBA review written by Robert Michell, exactly 50 years ago. The enzymology and functional diversity of PI 4-phosphate (PI4P) (the term that has replaced DPI) has since underwent an expansion that could not have been foreseen. In this review I will attempt to revisit this expansion with some historical reflections celebrating the 50th anniversary of the Michell review.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

Structure and regulation of phospholipase Cβ and ε at the membrane

Phospholipase C (PLC) β and ε enzymes hydrolyze phosphatidylinositol (PI) lipids in response to direct interactions with heterotrimeric G protein subunits and small GTPases, which are activated downstream of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). PI hydrolysis generates second messengers that increase the intracellular Ca²⁺ concentration and activate protein kinase C (PKC), thereby regulating numerous physiological processes. PLCβ and PLCε share a highly conserved core required for lipase activity, but use different strategies and structural elements to autoinhibit basal activity, bind membranes, and engage G protein activators. In this review, we discuss recent structural insights into these enzymes and the implications for how they engage membranes alone or in complex with their G protein regulators.

MBOAT7-driven phosphatidylinositol remodeling promotes the progression of clear cell renal carcinoma

Objective

The most common kidney cancer, clear cell renal cell carcinoma (ccRCC), is closely associated with obesity. The “clear cell” variant of RCC gets its name from the large lipid droplets that accumulate in the tumor cells. Although renal lipid metabolism is altered in ccRCC, the mechanisms and lipids driving this are not well understood.

Methods

We used shotgun lipidomics in human ccRCC tumors and matched normal adjacent renal tissue. To assess MBOAT7s gene expression across tumor severity, we examined histologically graded human ccRCC samples. We then utilized genome editing in ccRCC cell lines to understand the role of MBOAT7 in ccRCC progression.

Results

We identified a lipid signature for ccRCC that includes an increase in arachidonic acid-enriched phosphatidylinositols (AA-PI). In parallel, we found that ccRCC tumors have increased expression of acyltransferase enzyme membrane bound O-acyltransferase domain containing 7 (MBOAT7) that contributes to AA-PI synthesis. In ccRCC patients, MBOAT7 expression increases with tumor grade, and increased MBOAT7 expression correlates with poor survival. Genetic deletion of MBOAT7 in ccRCC cells decreases proliferation and induces cell cycle arrest, and MBOAT7^−/− cells fail to form tumors in vivo. RNAseq of MBOAT7^−/− cells identified alterations in cell migration and extracellular matrix organization that were functionally validated in migration assays.

Conclusions

This study highlights the accumulation of AA-PI in ccRCC and demonstrates a novel way to decrease the AA-PI pool in ccRCC by limiting MBOAT7. Our data reveal that metastatic ccRCC is associated with altered AA-PI metabolism and identify MBOAT7 as a novel target in advanced ccRCC.

Computational Modeling Reveals that Signaling Lipids Modulate the Orientation of K-Ras4A at the Membrane Reflecting Protein Topology

The structural, dynamical, and functional characterization of the small GTPase K-Ras has become a research area of intense focus due to its high occurrence in human cancers. Ras proteins are only fully functional when they interact with the plasma membrane. Here we present all-atom molecular dynamics simulations (totaling 5.8 μs) to investigate the K-Ras4A protein at membranes that contain anionic lipids (phosphatidyl serine or phosphatidylinositol bisphosphate). We find that similarly to the homologous and highly studied K-Ras4B, K-Ras4A prefers a few distinct orientations at the membrane. Remarkably, the protein surface charge and certain lipids can strongly modulate the orientation preference. In a novel analysis, we reveal that the electrostatic interaction (attraction but also repulsion) between the protein's charged residues and anionic lipids determines the K-Ras4A orientation, but that this is also influenced by the topology of the protein, reflecting the geometry of its surfaces.

Proteins kinase Cɛ is required for non-small cell lung carcinoma growth and regulates the expression of apoptotic genes

Protein kinase C (PKC)ɛ, a member of the novel PKC family, has key roles in mitogenesis and survival in normal and cancer cells. PKCɛ is frequently overexpressed in epithelial cancers, particularly in lung cancer. Using a short-hairpin RNA approach, here we established that PKCɛ is required for non-small cell lung carcinoma (NSCLC) growth in vitro as well as tumor growth when inoculated into athymic mice. Moreover, sustained delivery of a PKCɛ-selective inhibitor peptide, ɛV1-2, reduced xenograft growth in mice. Both RNA interference depletion and pharmacological inhibition of PKCɛ caused a marked elevation in the number of apoptotic cells in NSCLC tumors. PKCɛ-depleted NSCLC cells show elevated expression of pro-apoptotic proteins of the Bcl-2 family, caspase recruitment domain-containing proteins and tumor necrosis factor ligands/receptor superfamily members. Moreover, a Gene Set Enrichment Analysis revealed that a vast majority of the genes changed in PKCɛ-depleted cells were also deregulated in human NSCLC. Our results strongly suggest that PKCɛ is required for NSCLC cell survival and maintenance of NSCLC tumor growth. Therefore, PKCɛ may represent an attractive therapeutic target for NSCLC.

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Flat revertants of EJ human bladder carcinoma cells show two different mechanisms of reversion

To investigate the way in which ras proteins cause transformation, we have isolated revertants from human tumour cell lines which contain transforming ras genes. Two types of revertant have been isolated from the human fibrosarcoma cell line, HT1080. One class has normal and mutant alleles in a ratio of 2:1, compared to 1:1 in the parental cells, showing that reversion can be a dosage phenomenon. The other class has lost the transforming allele. All the HT1080 revertants isolated can be re-transformed by transforming ras proteins. To test whether reversion is due to a change in the relative amounts of normal and mutant proteins, or to a reduction in the absolute amount of the transforming protein, mixtures of the purified proteins were microinjected into 208F (Rat-1) cells, chosen because they are less sensitive to transformation by p21ras. Normal H-ras p21 was unable to suppress the transforming effects of the mutant ras protein when co-injected at up to ninefold excess. Revertants of EJ human bladder carcinoma cells were of two types: one was sensitive to re-transformation by oncogenically activated ras proteins, the other was not. The EJ revertants that are resistant to re-transformation fall into two classes, since hybrids of one revertant with the parental EJ cells are non-transformed, whereas hybrids of another revertant with the parental cells are transformed.

Oncogene induction of metastases

A metastatic colony is the end result of a complex series of steps involving multiple gene products. In some cases, the augmented metastatic potential of certain tumour cells may be due to the increased expression of specific gene products which confer a selective advantage. Transfection of the c-Ha-ras oncogene into suitable recipient cells constitutes a powerful experimental model with which to identify putative gene products augmented in highly metastatic tumour cells compared to their non-metastatic counterparts. Transfection of the activated ras oncogene into 3T3 and 10T1/2 embryo fibroblasts, and adult rat fibroblasts, results in transformants which produce high numbers of spontaneous metastases in nude mice or syngeneic recipients. The ras oncogene will also increase the metastatic aggressiveness of murine tumours with low metastatic potential. However, the ras oncogene will not induce the metastatic phenotype in all recipient cells. Furthermore, specific genes such as adenovirus 2 E1A suppress the ability of ras to induce the metastatic phenotype. Natural 'suppressor' gene products may exist which render certain cells resistant to the induction of metastases by ras. Ras oncogene transfection induces the production of type IV collagenase, motility factors and growth factors. The ras oncogene therefore induces a cascade of gene functions leading to rapid progression to the metastatic phenotype. The mechanism of the induction probably involves complex interactions between the ras p21 product and multiple cellular gene products.

Metabolic control of glucose degradation in yeast and tumor cells

Regulation of glucose degradation in both yeasts and tumor cells is very similar in many respects. In both cases it leads to excretion of intermediary metabolites (e.g., ethanol, lactate) in those cell types where uptake of glucose is unrestricted (Saccharomyces cerevisiae, Bowes melanoma cells). The similarities between glucose metabolism observed in yeast and tumor cells is explained by the fact that cell transformation of animal cells leads to inadequate expression of (proto-)oncogenes, which force the cell to enter the cell cycle. These events are accompanied by alterations at the signal transduction level, a marked increase of glucose transporter synthesis, enhancement of glycolytic key enzyme activities, and slightly reduced respiration of the tumor cell. In relation to homologous glucose degradation found in yeast and tumor cells there exist strong similarities on the level of cell division cycle genes, signal transduction and regulation of glycolytic key enzymes. It has been demonstrated that ethanol and lactate excretion in yeast and tumor cells, respectively, result from an overflow reaction at the point of pyruvate that is due to a carbon flux exceeding the capacity of oxidative breakdown. Therefore, the respiratory capacity of a cell determines the amount of glycolytic breakdown products if ample glucose is available. This restricted flux is also referred to as the respiratory bottleneck. The expression "catabolite repression", which is often used in textbooks to explain ethanol and acid excretion, should be abandoned, unless specific mechanisms can be demonstrated. Furthermore, it was shown that maximum respiration and growth rates are only obtained under optimum culture conditions, where the carbon source is limiting.

Regulation of phospholipase C isozymes by ras superfamily GTPases

The physiological effects of many extracellular stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid-signaling pathways. These signaling responses include the classically described conversion of PtdIns(4,5)P(2) to the Ca(2+)-mobilizing second messenger Ins(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. Here we discuss how the family of PLCs elaborates a minimal catalytic core typified by PLC-delta to confer multiple modes of regulation on their phospholipase activities. Although PLC-dependent signaling is prominently regulated by direct interactions with heterotrimeric G proteins or tyrosine kinases, the existence of at least 13 divergent PLC isozymes promises a diverse repertoire of regulatory mechanisms for this class of important signaling proteins. We focus here on the recently realized and extensive regulation of inositol lipid signaling by Ras superfamily GTPases directly acting on PLC isozymes and conclude by considering the biological and pharmacological ramifications of this regulation.

Quantification of isozyme-specific activation of phospholipase C-beta2 by Rac GTPases and phospholipase C-epsilon by Rho GTPases in an intact cell assay system

Phospholipase C (PLC) catalyzes the hydrolysis of PtdIns(4,5)P2, which results in both formation of the second messengers Ins(1,4,5)P3 and diacylglycerol and alteration in the membrane association and/or activity of PtdIns(4,5)P2-binding proteins. The existence of 13 different PLC isozymes suggests multiple mechanisms of regulation of inositol lipid signaling, and the recent realization that Rho-family GTPases directly bind and activate certain PLC isozymes has added to this potential diversity of inositol lipid-related signal transduction. With the goal of delineating a less labor-intensive method for quantification of intracellular inositol phosphate production, we have applied a commercially available yttrium silicate RNA binding resin selective for inositol phosphates to develop a high-throughput inositol phosphate scintillation proximity assay (SPA). We highlight the utility of this assay using COS-7 cells robotically transfected in a 96-well format. This method is readily applied to quantify activation of PLC by receptors and G proteins, and we illustrate here the selective activation of PLC-beta2 by Rac but not by Rho GTPases and the selective activation of PLC-epsilon by Rho but not Rac GTPases.

Intestinal trefoil factor promotes invasion in non-tumorigenic Rat-2 fibroblast cell

Intestinal trefoil factor (TFF3) is essential in regulating cell migration and maintaining mucosal integrity in gastrointestinal tract. We previously showed that TFF3 was overexpressed in gastric carcinoma. Whether TFF3 possesses malignant potential is not fully elucidated. We sought to investigate the effects of inducting TFF3 expression in a non-malignant rat fibroblast cell line (Rat-2) on the cell proliferation, invasion and the genes regulating cell invasion. Invasiveness and proliferation of transfected Rat-2 cell line were assessed using in vitro invasion chamber assay and colorimetric MTS assay. Differential mRNA expressions of invasion-related genes, namely, metalloproteinases (MMP-9), tissue inhibitors of metalloproteinases (TIMP-1), beta-catenin and E-cadherin, were determined by quantitative real-time polymerase chain reaction (PCR). We showed that TFF3 did not inhibit the proliferation of Rat-2 cells. We also demonstrated that transfection of TFF3 significantly promoted invasion of Rat-2 cells by 1.4- to 2.2-folds. There was an upregulation of beta-catenin (13.1-23.0%) and MMP-9 (43.4-92.2%) mRNA expression levels, and downregulation of E-cadherin (25.6-33.8%) and TIMP-1 (31.5-37.8%) in TFF3-transfected cells compared to controls during 48-h incubation. Our results suggested that TFF3 possesses malignant potential through promotion of cell invasiveness and alteration of invasion-related genes.

Ras is involved in the negative control of autophagy through the class I PI3-kinase

Ras proteins exert a pivotal regulatory function in signal transduction involved in cell proliferation and their activation mutation leads to malignant cell transformation. However, the role of Ras proteins in autophagy, an intracellular protein degradation process in cell growth control is unknown. In the present study, we demonstrate that the degradation of long-lived proteins in NIH3T3 cells in response to nutrient starvation was significantly suppressed by oncogenic RasVal12 transformation in a rapamycin (mTOR inhibitor)-sensitive manner. Morphologic observations also show the decrease in the formation of autophagic vacuoles upon the Ras transformation. Furthermore, epidermal growth factor or serum downregulated the protein degradation induced by serum starvation and the dominant-negative RasAsn17 mutant counteracted this suppressive effect, indicating that Ras mediates the growth factor downregulation of autophagy. The suppression of protein degradation by the activated RasVal12 was mediated by the class I phosphatidyl inositol 3-kinase (PI3-kinase), but not either or Raf Ral GDS. Consistent with this, RasVal12 and class I PI3-kinase inhibited the rate of autophagic sequestration of LDH. These data suggest that Ras plays a critical role as a negative regulator for nutrient deprivation-induced autophagy through the class I PI3-kinase signaling pathway.

Enhanced cathepsin L expression is mediated by different Ras effector pathways in fibroblasts and epithelial cells

Ras expression induces increased expression and altered targeting of lysosomal proteases in multiple cell types, but the specific downstream cytoplasmic signaling pathways mediating these changes have not been identified. In this study, we compared the involvement of 3 major Ras effectors, Raf, phosphatidylinositol 3-kinase (PI3K) and Ral guanine nucleotide exchange factor (RalGEF) in the Ras-mediated alteration of lysosomal protease protein expression and targeting in rat 208F fibroblasts and rat ovarian surface epithelial (ROSE) cells. Effector domain mutants of Ras, constitutively activated variants of Raf, PI3K and RalGEF and pharmacologic inhibitors of MEK and PI3K were utilized to determine the role of these downstream pathways in mediating fibroblast transformation and lysosomal protease regulation in the fibroblasts and epithelial cells. We found that Raf activation of the ERK mitogen-activated protein kinase pathway alone was sufficient to cause morphologic and growth transformation of the fibroblasts and was necessary and sufficient to alter cathepsin L expression and targeting. In contrast, transformation and upregulation of cathepsin L expression in the epithelial cells required the activity of all 3 Ras effectors. Increased protease secretion from the epithelial cells was not observed on ectopic expression of Ras, as it was from the fibroblasts, consistent with the utilization of different signaling pathways in the 2 cell types. In neither cell type did Ras expression increase the expression, processing or secretion of 2 other major lysosomal proteases, cathepsin B and cathepsin D. Thus, Ras utilizes different effectors to mediate transformation and to deregulate cathepsin L expression and secretion in fibroblast and epithelial cells.

Oncogenic Ha-Ras transformation modulates the transcription of the CTP:phosphocholine cytidylyltransferase alpha gene via p42/44MAPK and transcription factor Sp3

We have shown previously that expression of the murine CTP:phosphocholine cytidylyltransferase (CT) alpha gene is regulated during cell proliferation (Golfman, L. S., Bakovic, M., and Vance, D. E. (2001) J. Biol. Chem. 276, 43688-43692). We have now characterized the role of Ha-Ras in the transcriptional regulation of the CTalpha gene. The expression of CTalpha and CTbeta2 proteins and mRNAs was stimulated in C3H10T1/2 murine fibroblasts expressing oncogenic Ha-Ras. Incubation of cells with the specific inhibitor (PD98059) of p42/44(MAPK) decreased the expression of both CT isoforms. Transfection of fibroblasts with CTalpha promoter-luciferase constructs resulted in an approximately 2-fold enhanced luciferase expression in Ha-Ras-transformed, compared with nontransformed, fibroblasts. Electromobility shift assays indicated enhanced binding of the Sp3 transcription factor to the CTalpha promoter in Ha-Ras-transformed cells. Expression of several forms of Sp3 was increased in nuclear extracts of Ha-Ras-transformed fibroblasts compared with nontransformed cells. Tyrosine phosphorylation of one Sp3 form was decreased, whereas phosphorylation of two other forms of Sp3 was increased in nuclear extracts of Ha-Ras-transformed cells. When control fibroblasts were transfected with a Sp3-expressing plasmid, an enhanced expression of CTalpha and CTbeta was observed. However, the expression of CTalpha or CTbeta was not increased in Ha-Ras-transformed cells transfected with a Sp3 plasmid presumably because expression was already maximally enhanced. The results suggest that Sp3 is a downstream effector of a Ras/p42/44(MAPK) signaling pathway which increases CTalpha gene transcription.

Growth suppression of a tumorigenic rat liver cell line by the anticancer agent, ET-18-O-CH(3), is mediated by inhibition of cytokinesis

PURPOSE

This study was undertaken to elucidate the potential mechanism of the antitumor activity of ET-18-O-CH(3), a synthetic analogue of lysophosphatidyl choline, and a known antitumor agent and specific inhibitor of phosphoinositide phospholipase C (PI-PLC).

METHODS

A normal rat liver epithelial "oval" cell line (WB-F344) was neoplastically transformed by the H-ras oncogene (WB-ras2) and treated with a series of ET-18-O-CH(3) concentrates for a number of days. Cell growth, morphological "differentiation", cell cycle regulation, karyotypic changes, growth in soft agar (anchorage-independent growth) and the expression of cdk2, cdc2 and ERK genes were studied to determine the effect of ET-18-O-CH(3) on these neoplastic cells.

RESULTS

ET-18-O-CH(3) at 5 and 10 microg/ml was found to cause an increase in cell size, suppress cell growth, reduce the colony-forming efficiency and inhibit the anchorage-independent growth of the WB-ras2 cells. Significantly, flow-cytometric analysis revealed that while control cells and cells treated with concentrations of ET-18-O-CH(3) below 5 microg/ml were diploid, cell populations treated with 5 and 10 microg/ml ET-18-O-CH(3) comprised 33-37% diploid cells and over 60% tetraploid cells (4n-8n cycle cells). ET-18-O-CH(3) was found to induce aberrant cytokinesis as evidenced by the presence of a high frequency of enlarged cells, which were binucleated or multinucleated and mitotic cells with 4n and 8n numbers of chromosomes. ET-18-O-CH(3) was also capable of inhibiting both the expression of cdk2 and cdc2 and the activation of ERK1/2, while no effect was found on the expression of p21 ras.

CONCLUSIONS

The effect of ET-18-O-CH(3) on neoplastically transformed H-ras rat liver cells has been interpreted as the result of an altered phenotype characterized by an enlarged and flattened cell morphology with ploidy changes caused by inhibition of cytokinesis.

Ras effectors: Buying shares in Ras plc

The debate over whether activated Ras can regulate phosphoinositide-specific phospholipase C (PLC) has been contentious and at times heated. The argument may be resolved by the recent identification of a novel Ras-regulated PLC, but some unexpected properties of this protein are sure to stimulate further controversy.

Regulation of a novel human phospholipase C, PLCepsilon, through membrane targeting by Ras

Phosphoinositide-specific phospholipase C (PI-PLC) plays a pivotal role in regulation of intracellular signal transduction from various receptor molecules. More than 10 members of human PI-PLC isoforms have been identified and classified into three classes beta, gamma, and delta, which are regulated by distinct mechanisms. Here we report identification of a novel class of human PI-PLC, named PLCepsilon, which is characterized by the presence of a Ras-associating domain at its C terminus and a CDC25-like domain at its N terminus. The Ras-associating domain of PLCepsilon specifically binds to the GTP-bound forms of Ha-Ras and Rap1A. The dissociation constant for Ha-Ras is estimated to be approximately 40 nm, comparable with those of other Ras effectors. Co-expression of an activated Ha-Ras mutant with PLCepsilon induces its translocation from the cytosol to the plasma membrane. Upon stimulation with epidermal growth factor, similar translocation of ectopically expressed PLCepsilon is observed, which is inhibited by co-expression of dominant-negative Ha-Ras. Furthermore, using a liposome-based reconstitution assay, it is shown that the phosphatidylinositol 4,5-bisphosphate-hydrolyzing activity of PLCepsilon is stimulated in vitro by Ha-Ras in a GTP-dependent manner. These results indicate that Ras directly regulates phosphoinositide breakdown through membrane targeting of PLCepsilon.

Ras inactivation of the retinoblastoma pathway by distinct mechanisms in NIH 3T3 fibroblast and RIE-1 epithelial cells

Although Ras and Raf cause transformation of NIH 3T3 fibroblasts, only Ras causes transformation of RIE-1 intestinal epithelial cells. To determine if the inability of Raf to transform RIE-1 cells is due to a failure to deregulate cell cycle progression, we evaluated the consequences of sustained Ras and Raf activation on steady state levels of cyclin D1, p21(CIP/WAF), and p27(KIP1). Both Ras- and Raf-transformed NIH 3T3 cells showed up-regulated expression of cyclin D1, p21, and p27 protein, increased retinoblastoma (Rb) hyperphosphorylation, and increased activation of E2F-mediated transcription. Similarly, Ras-transformed RIE-1 cells also showed up-regulation of cyclin D1, p21, and hyperphosphorylated Rb. In contrast, Ras-mediated down-regulation of p27 was seen in RIE-1 cells. Conversely, stable expression of activated Raf alone caused only a partial up-regulation of p21 and Rb hyperphosphorylation but no activation of E2F-responsive transcription or down-regulation of p27 in RIE-1 cells. Moreover, we found that the AP-1 site was dispensable for Ras-mediated stimulation of the cyclin-D1 promoter in NIH 3T3 cells but was essential for Ras-mediated stimulation in RIE-1 cells. Thus, up-regulation of p21, rather than the down-regulation seen in previous transient expression studies, is seen with sustained Ras activation. Additionally, p27 may serve a positive (NIH 3T3) or negative (RIE-1) regulatory role in Ras transformation that is cell type-dependent. The involvement of Raf and phosphatidylinositol 3-kinase in mediating Ras changes in cyclin D1, p21, and p27 was also very distinct in NIH 3T3 and RIE-1 cells. Taken together, these results demonstrate the importance of Raf-independent pathways in mediating oncogenic Ras deregulation of cell cycle progression in epithelial cells.

Inhibition of phospholipase cgamma1 and cancer cell proliferation by triterpene esters from Uncaria rhynchophylla

Investigation of the hooks of Uncaria rhynchophylla resulted in isolation of six phospholipase Cgamma1 (PLCgamma1) inhibitors (1-6). The structures of these compounds were elucidated as pentacyclic triterpene esters by spectroscopic and chemical analysis. Three of them, namely uncarinic acids C (1), D (2), and E (3), are newly reported as natural products. All the compounds showed dose-dependent inhibitory activities against PLCgamma1 in vitro with IC(50) values of 9.5-44.6 microM and inhibited the proliferation of human cancer cells with IC(50) values of 0.5-6.5 microg/mL.

Phosphatidylethanolamine and phosphatidylcholine are sources of diacylglycerol in ras-transformed NIH 3T3 fibroblasts

Ras-transformation of cells is accompanied by an increase of the level of diacylglycerol (DAG), which participates in the signal transduction pathways. DAG could be generated from phospholipids either by activation of phospholipase C or by a more complex pathway involving phospholipase D and phosphatidate phosphohydrolase. To clarify which phospholipids produce DAG and which pathways are involved, we examined the DAG generating enzyme activities, using phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) as substrates. The study showed that the breakdown of PC and more markedly of PE by phospholipases C and D was stimulated in membranes from ras-transformed cells. Phosphatidate phosphohydrolase activity was also elevated in oncogene-expressing cells. The increase in glycerol uptake was most pronounced in cells given PE, followed by PC. The fatty acid analysis revealed apparent similarities between the acyl chains of PE and DAG only in the transformed cells. These findings suggest that PE is a source of DAG in ras-fibroblasts but does not rule out the role of PC in DAG production, due to the activation of the PC-specific phospholipases C and D.

Suppression of diacylglycerol levels by antibodies reactive with the c-erbB-2 (HER-2/neu) gene product p185c-erbB-2 in breast and ovarian cancer cell lines

Seven of 10 murine monoclonal antibodies reactive with the extracellular domain of p185c-erbB-2 inhibited the anchorage independent growth of the SKBr3 breast cancer cell line that overexpressed p185c-erbB-2. Significant inhibition (56-72%) of diacylglycerol (DAG) levels (P < 0.0001) was observed with the 10 antibodies that inhibited SKBr3 growth (RC1, NB3, RC6, PB3, 741F8, DB5, ID5), whereas the 3 antibodies (TA1, 520C9, 454C11) that failed to inhibit SKBr3 growth also failed to affect DAG levels. Thus, DAG levels correlated with antibody-mediated growth regulation for each of the 10 monoclonal reagents. Antibody-induced inhibition of anchorage-independent growth of SKBr3 could be reversed by incubation with phorbol myristate acetate. The ID5 antibody inhibited growth of the SKBr3, SKOv3, and OVCA 432 tumor cell lines, but not of OVCA 420, OVCA 429, and OVCA 433. DAG levels were significantly decreased after ID5 treatment of the SKBr3 and SKOv3 cell lines, but not the OVCA 420, OVCA 429, and OVCA 433 lines. In the 432 line, there was a decrease which did not reach significance. Consequently, changes in DAG levels correlated with growth regulation in 5 of 6 breast and ovarian carcinoma cell lines tested with a trend toward correlation in the sixth. Decreases in DAG may be one mediator of the growth regulatory signals produced by anti-p185c-erbB-2 antibodies.

Cooperative effects of v-myc and c-Ha-ras oncogenes on gap junctional intercellular communication and tumorigenicity in rat liver epithelial cells

The objective of this study was to isolate and partially characterize several rat liver epithelial cell clones containing myc, ras and myc/ras oncogenes in order to study their roles in apoptosis and to test the hypothesis that gap junctional intercellular communication is necessary for apoptosis in solid tissues and that the loss of junctional communication leads to tumorigenesis. The co-transfection of the myc and ras oncogenes in the normal rat liver epithelial cell line (WB-F344) resulted in a loss of functional channels and normal growth regulation; cell-cell communication was significantly decreased and tumorigenicity determined in adult male F344 rats was induced. We examined cell growth properties, gap junctional intercellular communication (GJIC), using the scrape-loading-dye transfer and fluorescence-redistribution-after-photobleaching assays, and tumorigenicity in a series of normal and v-myc-, c-Ha-ras- and v-myc/c-Ha-ras-transfected WB-F344 cell lines. The c-Ha-ras- and the v-myc/c-Ha-ras-transduced cell lines appeared distinctly different from the other lines, having spindle-shaped morphology, shorter generation time and contact insensitivity. On the other hand, the normal WB-F344 cell line and the v-myc-transduced cell line showed excellent GJIC. Moreover, the c-Ha-ras-transduced cell lines displayed decreasing levels of GJIC associated with their increasing tumorigenicity. The v-myc/c-Ha-ras-transformed cell lines showed the lowest levels of GJIC and were also the most tumorigenic. These findings suggest that the reduction of GJIC in c-Ha-ras- and v-myc/c-Ha-ras-transformed WB-F344 cells is linked to their tumorigenic potential. These cell lines should provide valuable tools to study the role of GJIC in apoptosis during tumorigenesis.

Sphingomyelin synthase, a potential regulator of intracellular levels of ceramide and diacylglycerol during SV40 transformation. Does sphingomyelin synthase account for the putative phosphatidylcholine-specific phospholipase C?

Sphingomyelin synthase (SMS), an enzyme involved in sphingomyelin (SM) and ceramide metabolism, can potentially regulate, in opposite directions, the levels of ceramide and diacylglycerol. In this study SMS activity was investigated in normal and SV40-transformed human lung fibroblasts (WI38). The addition of [3H]C2-ceramide to cells resulted in a time-dependent formation of [3H]C2-SM. At 24 h after treatment, normal WI38 cells cleared 17% of [3H]C2-ceramide producing [3H]C2-SM, which accounted for 13% of total radioactivity. On the other hand, SV40-transformed cells cleared 45% of [3H]C2-ceramide and produced C2-SM, which accounted for 24% of total radioactivity. This enhanced production of C2-SM was also supported by an increase in the total SMS activity of cells (measured in vitro), such that SV40-transformed cells had SMS activity of 222 pmol/mg of protein/h, whereas wild type cells had 78 pmol/mg of protein/h of activity. Additional studies aimed at examining the SMS activity directed at ceramide produced in the plasma membrane. Treatment of cells with exogenous bacterial sphingomyelinase (SMase) for 25 min resulted in cleavage of 90-95% of total SM and the concomitant generation of ceramide. After bacterial SMase treatment, wild type WI38 cells cleared ceramide very slowly (19.2 pmol of ceramide/nmol of phosholipid Pi after 6 h of incubation) and hardly regenerated any SM. On the other hand, SV40-transformed cells cleared ceramide much faster (41.1 pmol/nmol of Pi after 6 h of incubation) and regenerated approximately 80% of the original SM. These results show that the enhanced SMS activity of transformed cells is particularly pronounced when ceramide is produced in the plasma membrane. Finally, several observations led us to consider the relationship of SMS to the "putative" phosphatidylcholine-specific phospholipase C (PC-PLC). We, therefore, tested the effects of D609, a purported PC-PLC-specific inhibitor on the activity of SMS. D609 inhibited SMS activity in vitro. In addition, cellular studies showed that SMS activity was dramatically inhibited by concentrations of D609 used previously to study PC-PLC (10-50 microg/ml). These results suggest SMS as an important biochemical target for D609, and they raise the distinct possibility that many of the roles of PC-PLC, especially in cell transformation, may be attributable to SMS.

Alterations of signal transduction pathways involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced malignant transformation of human cells in culture

Effects of signal transduction pathways in TCDD-induced neoplastic transformation of human cells were assessed with respect to PLC-coupled signaling pathways, adenylyl cyclase-mediated responses and PKC isozyme expressions. A lower stimulation of the intracellular free calcium levels with exposure to extracellular ATP or histamine was observed in the transformed cells, as compared to the parental cells. While the steady-state level of IP3 was higher in the transformed cells, the magnitude of stimulation of IP3 generation by ATP or histamine was significantly lower in the transformed cells than the parental cells. These results indicate that a downregulation PLC-coupled signaling pathways may be involved in the TCDD-induced transformation of human cells. While the steady-state levels of cAMP accumulation were similar between the two cell lines, treatment of PGE2, a potent differentiation inducer, stimulated a higher accumulation of cAMP in the parental cells but isoproterenol, a typical beta-adrenergic agonist, did not induce a significant difference between the two cell lines. These results suggest that desensitization of cAMP-mediated response to extracellular signals including differentiation signals may be associated with a possible mechanism of the carcinogenesis. Elevated expression of PKC-alpha, -gamma, -zeta, -epsilon, -lambda, and -tau were observed in TCDD-transformed cells, indicating a possible association of altered expression of PKC isozymes with TCDD-induced transformation of human cells. The present study demonstrates that alterations of signal transduction pathways are involved in the TCDD-induced transformation of human cells and provides a valuable basis to investigate effects of signaling pathway as a possible mechanism of TCDD-induced carcinogenesis in human cells.

Identification of PLC210, a Caenorhabditis elegans phospholipase C, as a putative effector of Ras

Mammalian Ras proteins regulate multiple effectors including Raf, Ral guanine nucleotide dissociation stimulator (RalGDS), and phosphoinositide 3-kinase. In the nematode Caenorhabditis elegans, LIN-45 Raf has been identified by genetic analyses as an effector of LET-60 Ras. To search for other effectors in C. elegans, we performed a yeast two-hybrid screening for LET-60-binding proteins. The screening identified two cDNA clones encoding a phosphoinositide-specific phospholipase C (PI-PLC) with a predicted molecular mass of 210 kDa, designated PLC210. PLC210 possesses two additional functional domains unseen in any known PI-PLCs. One is the C-terminal Ras-associating domain bearing a structural homology with those of RalGDS and AF-6. This domain, which could be narrowed down to 100 amino acid residues, associated in vitro with human Ha-Ras in a GTP-dependent manner and competed with yeast adenylyl cyclase for binding Ha-Ras. The binding was abolished by specific mutations within the effector region of Ha-Ras. The other functional domain is the N-terminal CDC25-like domain, which possesses a structural homology to guanine nucleotide exchange proteins for Ras. These results strongly suggest that PLC210 belongs to a novel class of PI-PLC, which is a putative effector of Ras.

The role of protein kinase C (PKC) in the evolution and proliferation of malignant gliomas, and the application of PKC inhibition as a novel approach to anti-glioma therapy

The present article reviews the role of the second messenger enzyme protein kinase C (PKC) in the growth regulation of high-grade gliomas, and evaluates the efficacy of therapeutic strategies directed against PKC for blocking the proliferation of these malignancies in in vitro and in vivo models. The translation of such strategies to the treatment of patients with malignant gliomas may provide a novel approach for improving the otherwise grim outlook associated with these neoplasms.

In vitro and in vivo inhibitory actions of morin on rat brain phosphatidylinositolphosphate kinase activity

Phosphatidylinositol-4,5-bisphosphate occupies a central role in signal transduction and in cellular transformation. Phosphatidylinositol-4,5-bisphosphate is produced by the enzymatic phosphorylation of phosphatidylinositol-4-phosphate by phosphatidylinositolphosphate kinase (EC 2.7.1.68). Inhibition of this enzyme might conceivably lowers the cellular pool of phosphatidylinositol-4,5-bisphosphate, thus constituting a feasible control point in regulating signal transduction and cellular transformation. Morin, a plant flavonoid, was demonstrated to exhibit in vitro inhibitory action on phosphatidylinositolphosphate kinase extracted from rat brain. This inhibition of enzymatic activity was found to be dose-dependent, with an IC50 value of approximately 10 microM morin. Lineweaver-Burk transformation of the inhibition data indicates that inhibition was competitive with respect to ATP. The Ki was calculated to be 5.15 x 10(-6) M. Inhibition was uncompetitive with respect to phosphatidylinositol-4-phosphate. The Ki was determined to be 0.94 x 10(-5) M. Administration of morin to rats led to a decrease in phosphatidylinositolphosphate kinase activity in brain extracts. This in vivo action of morin was found to be dose-dependent and time-dependent. These effects of morin on rat brain phosphatidylinositolphosphate kinase activity are discussed in relation to the other reported biological actions of this flavonoid.

Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, Galphas or Galphai1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Galpha conformation. To study the possibility of tubulin involvement in Galphaq-mediated phospholipase Cbeta1 (PLCbeta1) signaling, the m1 muscarinic receptor, Galphaq, and PLCbeta1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCbeta1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCbeta1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Galphaq and PLCbeta1, accompanied by guanine nucleotide transfer from tubulin to Galphaq, is suggested as a mechanism for the enzyme activation. The PLCbeta1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCbeta1 by tubulin, since high tubulin concentrations might prevent the access of PLCbeta1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Galphaq for tubulin. Involvement of tubulin in the articulation between PLCbeta1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.

Pholipeptin, a Novel Cyclic Lipoundecapeptide fromPseudomonas fluorescens

An inhibitor of phosphatidylinositol-specific phospholipase C (PI-PLC), pholipeptin (1), was purified from the culture broth of Pseudomonas sp. by solvent extraction and column chromatography. Acid hydrolysis of 1 gave Leu, Ile, Ser, Thr, and Asp moieties. Although 1 was a peptide compound, fragmentation by mild hydrolysis was not accomplished under any conditions. So, we performed the structure elucidation using various 2D NMR techniques. In the NMR studies, the addition of a small amount of trifluoroacetic acid gave relatively sharp and resolved signals, such that the structure of this novel cyclic lipodepsipeptide consisting of 11 amino acids and a 3-hydroxydecanoic acid moiety could be determined. Chirality of the constituent amino acids was analyzed by chiral HPLC, but two Asp residues could not be distinguished because they were contained as a racemic mixture. Finally, their chiralities were determined by NMR analysis of (13)C-labeled 1 into which [L-(13)C]Asp had been biosynthetically incorporated.

Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells

Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.

Phenylamino-pyrimidine (PAP) derivatives: a new class of potent and selective inhibitors of protein kinase C (PKC)

Phenylamino-pyrimidines represent a novel class of inhibitors of the protein kinase C with a high degree of selectivity versus other serine/threonine and tyrosine kinases. Steady state kinetic analysis of N-(3-[1-imidazolyl]-phenyl-4-(3-pyridyl)-2-pyrimidinamine (5), which showed potent inhibitory activity, revealed competitive kinetics relative to ATP. The adjacent H-bond acceptor of the pyrimidine moiety next to an H-bond donor of the phenylamine was found to be crucial for inhibitory activity. N-(3-Nitro-phenyl)-4-(3-pyridyl)-2-pyrimidinamine (7) preferentially inhibited PKC-alpha (IC50 = 0.79 microM) and not the other subtypes tested. The inhibition constants of PKC-alpha and the antiproliferative effect on T24 human bladder carcinoma cells showed a qualitative correlation, although with some exceptions.

Inhibition of PDGF-induced phospholipase C activation by herbimycin A

Herbimycin A, an inhibitor of protein tyrosine kinases, dose-dependently reduced PDGF-induced inositol phosphates (IPt) accumulation without effect on phosphatidylethanol (PEt) formation in PLC-gamma 1-overexpressing NIH 3T3 (NIH 3T3 gamma 1) cells. The compound also reduced tyrosine phosphorylations of some proteins including PLC-gamma 1 in response to PDGF. On the other hand, phorbol 12-myristate 13-acetate (PMA)-induced phospholipase D (PLD) activation was reduced by herbimycin A in the cells, indicating that the pathways for PLD activation by PDGF and PMA are different from each other. Also, these results suggest that PLC-gamma 1 activation is not always an upstream event for PLD activation and that tyrosine phosphorylation of one or more proteins not affected by herbimycin A should be indispensable for PLD activation in PDGF-stimulated NIH 3T3 gamma 1 cells.

Signal transduction for proliferation of glioma cells in vitro occurs predominantly through a protein kinase C-mediated pathway

Previous work has demonstrated that glioma cells have very high protein kinase C (PKC) enzyme activity when compared to non-malignant glia, and that their PKC activity correlates with their proliferation rate. The purpose of this study was to determine whether the elevated PKC activity in glioma is secondary to an autonomously active PKC isoform implying oncogenic transformation, or whether this activity is driven by upstream ligand-receptor tyrosine kinase interactions. We treated established human glioma cell lines A172, U563 or U251 with either the highly selective PKC inhibitor CGP 41 251, or with genistein, a tyrosine kinase inhibitor. The proliferation rate and PKC activity of all the glioma lines was reduced by CGP 41 251; the IC50 values for inhibiting cell proliferation corresponded to the IC50v values for inhibition of PKC activity. Genistein also inhibited cell proliferation, with IC50 proliferation values approximating those for inhibition of tyrosine kinase activity in cell free protein extracts. Importantly, in genistein-treated cells, downstream PKC enzyme activity was dose dependently reduced such that the correlation coefficient for effects of genistein on proliferation rate and PKC activity was 0.92. These findings suggest that upstream tyrosine kinase linked events, rather than an autonomously functioning PKC, result in the high PKC activity observed in glioma. Finally, fetal calf serum (FCS) evoked a strong mitogenic effect on glioma cell lines. This mitogenic activity was completely blocked by CGP 41 251, suggesting that although the many mitogens in FCS for glioma cells signal initially through genistein-inhibitable tyrosine kinases, they ultimately channel through a PKC-dependent pathway. We conclude that proliferative signal transduction in glioma cells occurs through a predominantly PKC-dependent pathway and that selectively targeting this enzyme provides an approach to glioma therapy.

Alteration of glycerolipid and sphingolipid-derived second messenger kinetics in ras transformed 3T3 cells

The effect of ras transformation (rasB fibroblasts) on basal and serum-stimulated diacylglycerol (DAG) composition and mass was examined over time with respect to changes in membrane phospholipid composition and ceramide mass. RasB cells vs. nontransformed control cells (rasD and NR6) had chronically elevated DAG levels (up to 240 min) following serum stimulation, indicating a defect in the recovery phase of the intracellular DAG pulse. Ras transformation also had a dramatic effect on DAG composition. Molecular species analysis revealed that DAG from unstimulated rasB cells was enriched in the delta 9 desaturase fatty acyl species (monoenoate 18:1(n - 7) and 18:1(n - 9)), and depleted in arachidonic acid (20:4(n - 6)). With the exception of glycerophosphoinositol (GPI), DAG remodeling paralleled the compositional alterations in individual phospholipid classes. Importantly, ras transformation altered the fatty acyl composition of sphingomyelin, a precursor to the ceramide second messenger. With the addition of serum, control cells (rasD) had a progressive increase in ceramide mass with levels approximately 5-fold higher by 240 min. In contrast, ceramide levels did not increase in rasB cells at either 4 or 240 min. These results demonstrate that ras-oncogene, in addition to its effects on DAG metabolism, can also abolish the cellular increase in ceramide mass in response to serum stimulation. Since DAG and ceramide may have opposing biological functions, the prolonged elevation of DAG and the suppression of ceramide levels would be consistent with an enhanced proliferative capacity.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction mechanism in response to aflatoxin B1 exposure: phosphatidylinositol metabolism

A single dose of aflatoxin B1 (7 mg/kg body weight) to male Wistar rats significantly stimulated the hepatic activity of phosphatidylinositol kinase, a key enzyme in the cell signalling mechanism, 1-7 h following its administration. Phosphatidylinositol 4-phosphate kinase activity showed only marginal increase, whereas activities of diacylglycerol kinase and phosphatidylinositol synthetase remained unchanged. The level of diacylglycerol, however, recorded a sharp increase at 1 and 2 h after carcinogen treatment. The stimulation of phosphatidylinositol cycle with faster turnover of active second messengers might be an early step in the manifestation of toxicity and/or carcinogenicity.

Protein kinase C and growth regulation of malignant gliomas

This article reviews the role of the signal transduction enzyme protein kinase C in the regulation of growth of malignant gliomas, and describes how targetting this enzyme clinically can provide a novel approach to glioma therapy.

On the reaction kinetics of the radioadaptive response in cultured mouse cells

The reaction kinetics of radioadaptive response of low doses of X-rays have been studied in quiescent cultured mouse cells. Mouse m5S cells pre-exposed in G1 to low doses of X-rays became insensitive to the induction of chromosome aberrations, mutation toward 6-thioguanine resistance, and cell killing. Adapted cells were, however, more susceptible to morphological transformation by subsequent high challenging doses of X-rays. The cytogenetic adaptation, which lasted about 20 h pertained to a narrow dose range. X-ray doses below and above 0.1 Gy appeared to be recognized as different signals; higher doses of X-rays were incapable of inducing adaptation and rapidly extinguished the adapted condition. Treatment with 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and hydrogen peroxide, but not the xanthine/xanthine oxidase superoxide-generating system, mimicked X-rays in inducing adaptation when applied at low doses. Over-exposure to TPA or inhibitors of protein kinase C (PKC) abrogated the adaptive response to X-rays, providing evidence for the involvement of a PKC-mediated signalling pathway. The lack of radioadaptive response in a tumorigenic variant, clone 6110, and its restoration in the morphological revertant obtained by introducing human chromosome 11 further suggested that interference of signalling pathways may alter radioadaptive responses in malignant cells.

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

Phosphoinositide turnover during hepatocarcinogenesis induced by N-nitrosodiethylamine

The metabolism of phosphatidylinositol (PI) has been examined in rat liver during administration of N-nitrosodiethylamine (NDEA). The activities of PI kinase and PIP kinase were observed to be decreased as early as 7 days from the onset of NDEA administration, and remained suppressed until 60 days. PI synthetase showed a transient increase in activity at 7 days and thereafter the activity declined. The level of diacylglycerol (DAG), a key second messenger, showed a steady rise during the period of NDEA administration. A parallel increase in DAG kinase activity was also apparent. The results suggest that alterations of enzymes central to second messenger system with resulting changes in phosphoinositide turnover are important events during hepatocarcinogenesis induced by NDEA.

Inhibition of ras p21 membrane localization and modulation of protein kinase C isozyme expression during regression of chemical carcinogen-induced murine skin tumors by lovastatin

We investigated the ras p21 membrane localization and the expression and activation of protein kinase C (PKC) isozymes in activated ras oncogene-containing tumors and assessed whether these events were related to tumor growth. We used 7,12-dimethylbenz[a]anthracene-initiated and 12-O-tetradecanoylphorbol-13-acetate-promoted SENCAR mouse skin tumors, which were shown to contain Ha-ras oncogene activated by point mutation at codon 61, as an in vivo model for these studies. Compared with levels in epidermis, highly elevated levels of membrane-bound Ha-ras p21 were observed in growing tumors, which also showed strong expression and membrane translocation of PKC zeta and beta II and weak expression of PCK alpha. However, when ras p21 membrane localization was blocked in vivo in growing tumors by lovastatin, opposite results were evident. Compared with saline-treated animals, in which tumor growth continued, lovastatin-treated animals had significantly inhibited tumor growth, which led to tumor regression with concomitant inhibition of Ha-ras p21 membrane localization. These regressing tumors from lovastatin-treated animals also showed a decrease in the expression and membrane translocation of PKC zeta and beta II but increased expression of PKC alpha. Taken together, our results indicate that ras p21 membrane localization and the expression and activation of PKC zeta, beta II, and alpha may be the critical events in the regulation of the growth of tumors that contain activated ras oncogenes.