Regulation of phospholipase D isoenzymes by transforming Ras and atypical protein kinase C-iota

Protein Kinase C-Independent Inhibition of Organic Cation Transporter 1 Activity by the Bisindolylmaleimide Ro 31-8220

Ro 31–8220 is a potent protein kinase C (PKC) inhibitor belonging to the chemical class of bisindolylmaleimides (BIMs). Various PKC-independent effects of Ro 31–8220 have however been demonstrated, including inhibition of the ATP-binding cassette drug transporter breast cancer resistance protein. In the present study, we reported that the BIM also blocks activity of the solute carrier organic cation transporter (OCT) 1, involved in uptake of marketed drugs in the liver, in a PKC-independent manner. Ro 31–8220, in contrast to other pan-PKC inhibitors such as staurosporine and chelerythrine, was thus shown to cis-inhibit uptake of the reference OCT1 substrate tetraethylammonium in OCT1-transfected HEK293 cells in a concentration-dependent manner (IC₅₀ = 0.18 μM) and without altering membrane expression of OCT1. This blockage of OCT1 was also observed in human hepatic HepaRG cells that constitutionally express OCT1. It likely occurred through a mixed mechanism of inhibition. Ro 31–8220 additionally trans-inhibited TEA uptake in OCT1-transfected HEK293 cells, which likely discards a transport of Ro 31–8220 by OCT1. Besides Ro 31–8220, 7 additional BIMs, including the PKC inhibitor LY 333531, inhibited OCT1 activity, whereas 4 other BIMs were without effect. In silico analysis of structure-activity relationships next revealed that various molecular descriptors, especially 3D-WHIM descriptors related to total size, correspond to key physico-chemical parameters for inhibition of OCT1 activity by BIMs. In addition to activity of OCT1, Ro 31–8220 inhibited those of other organic cation transporters such as multidrug and toxin extrusion protein (MATE) 1 and MATE2-K, whereas, by contrast, it stimulated that of OCT2. Taken together, these data extend the nature of cellular off-targets of the BIM Ro 31–8220 to OCT1 and other organic cation transporters, which has likely to be kept in mind when using Ro 31–8220 and other BIMs as PKC inhibitors in experimental or clinical studies.

Dehydroepiandrosterone sulfate directly activates protein kinase C-beta to increase human neutrophil superoxide generation

Dehydroepiandrosterone sulfate (DHEAS) is the most abundant steroid in the human circulation and is secreted by the adrenals in an age-dependent fashion, with maximum levels during the third decade and very low levels in old age. DHEAS is considered an inactive metabolite, whereas cleavage of the sulfate group generates dehydroepiandrosterone (DHEA), a crucial sex steroid precursor. However, here we show that DHEAS, but not DHEA, increases superoxide generation in primed human neutrophils in a dose-dependent fashion, thereby impacting on a key bactericidal mechanism. This effect was not prevented by coincubation with androgen and estrogen receptor antagonists but was reversed by the protein kinase C inhibitor Bisindolylmaleimide 1. Moreover, we found that neutrophils are unique among leukocytes in expressing an organic anion-transporting polypeptide D, able to mediate active DHEAS influx transport whereas they did not express steroid sulfatase that activates DHEAS to DHEA. A specific receptor for DHEAS has not yet been identified, but we show that DHEAS directly activated recombinant protein kinase C-beta (PKC-beta) in a cell-free assay. Enhanced PKC-beta activation by DHEAS resulted in increased phosphorylation of p47(phox), a crucial component of the active reduced nicotinamide adenine dinucleotide phosphate complex responsible for neutrophil superoxide generation. Our results demonstrate that PKC-beta acts as an intracellular receptor for DHEAS in human neutrophils, a signaling mechanism entirely distinct from the role of DHEA as sex steroid precursor and with important implications for immunesenescence, which includes reduced neutrophil superoxide generation in response to pathogens.

Mutated ras induced PLD1 gene expression through increased Sp1 transcription factor

The underlying mechanisms of oncogene-induced phospholipase D (PLD) activation have not been fully elucidated. The effect of the mutated-ras on PLD mRNA was examined using colon cancer cell lines as well as mock- and mutated ras-transfected NIH3T3 cells. Ras-mutation and activation were correlated, and cells with enhanced ras-activation showed increased PLD1 mRNA and protein. Analysis of the 5' PLD1 promoter using a representative cell line, DLD-1 and also mutated ras-NIH3T3, showed one Sp1-site as the important ras-responsible motif. Spl inhibition with mithramycin A and Spl siRNA inhibited PLD1 protein expression and its promoter activity. Sp1 but not Sp3 protein level and increased Sp1-motif binding activity were correlated with ras activation. Furthermore, overexpression of Sp1 in drosophila SL2 cells lacking Sp family proteins increased PLD1 promoter activity. EMSA and chromatin immunoprecipitation assay confirmed the importance of Sp1 protein binding to the Sp1-motif in ras-induced PLD1 mRNA expression.

House dust mite allergen Der f 2-induced phospholipase D1 activation is critical for the production of interleukin-13 through activating transcription factor-2 activation in human bronchial epithelial cells

The purpose of this study was to identify the role of phospholipase D1 (PLD1) in Der f 2-induced interleukin (IL)-13 production. The major house dust mite allergen, Der f 2, increased PLD activity in human bronchial epithelial cells (BEAS-2B), and dominant negative PLD1 or PLD1 siRNA decreased Der f 2-induced IL-13 expression and production. Treatment of Der f 2 activated the phospholipase Cgamma (PLCgamma)/protein kinase Calpha (PKCalpha)/p38 MAPK pathway. Der f 2-induced PLD activation was attenuated by PLCgamma inhibitors (U73122 and PAO), PKCalpha inhibitors (RO320432 and GO6976), and p38 MAPK inhibitors (SB203580 and SB202190). These results indicate that PLCgamma, PKCalpha, and p38 MAPK act as upstream activators of PLD in Der f 2-treated BEAS-2B cells. Furthermore, expression and production of IL-13 increased by Der f 2 were also blocked by inhibition of PLCgamma, PKCalpha, or p38 MAPK, indicating that IL-13 expression and production are related to a PLCgamma/PKCalpha/p38 MAPK pathway. We found that activating transcription factor-2 (ATF-2) was activated by Der f 2 in BEAS-2B cells and activation of ATF-2 was controlled by PLD1. When ATF-2 activity was blocked with ATF-2 siRNA, Der f 2-induced IL-13 expression and production were decreased. Thus, ATF-2 might be one of the transcriptional factors for the expression of IL-13 in Der f 2-treated BEAS-2B cells. Taken together, PLD1 acts as an important regulator in Der f 2-induced expression and production of IL-13 through activation of ATF-2 in BEAS-2B cells.

Investigation of the marine compound spongistatin 1 links the inhibition of PKCalpha translocation to nonmitotic effects of tubulin antagonism in angiogenesis

The aims of the study were to meet the demand of new tubulin antagonists with fewer side effects by characterizing the antiangiogenic properties of the experimental compound spongistatin 1, and to elucidate nonmitotic mechanisms by which tubulin antagonists inhibit angiogenesis. Although tubulin-inhibiting drugs and their antiangiogenic properties have been investigated for a long time, surprisingly little is known about their underlying mechanisms of action. Antiangiogenic effects of spongistatin 1 were investigated in endothelial cells in vitro, including functional cell-based assays, live-cell imaging, and a kinome array, and in the mouse cornea pocket assay in vivo. Spongistatin 1 inhibited angiogenesis at nanomolar concentrations (IC(50): cytotoxicity>50 nM, proliferation 100 pM, migration 1.0 nM, tube formation 1.0 nM, chemotaxis 1.0 nM, aortic ring sprouting 500 pM, neovascularization in vivo 10 microg/kg). Further, a kinome array and validating data showed that spongistatin 1 inhibits the phosphorylation activity of protein kinase Calpha (PKCalpha), an essential kinase in angiogenesis, and its translocation to the membrane. Thus, we conclude that PKCalpha might be an important target for the antiangiogenic effects of tubulin antagonism. In addition, the data from the kinase array suggest that different tubulin antagonists might have individual intracellular actions.

Protein kinase C iota: human oncogene, prognostic marker and therapeutic target

The protein kinase C (PKC) family of serine/threonine kinases has been the subject of intensive study in the field of cancer since their initial discovery as major cellular receptors for the tumor promoting phorbol esters nearly 30 years ago. However, despite these efforts, the search for a direct genetic link between members of the PKC family and human cancer has yielded only circumstantial evidence that any PKC isozyme is a true cancer gene. This situation changed in the past year with the discovery that atypical protein kinase C iota (PKC iota) is a bonafide human oncogene. PKC iota is required for the transformed growth of human cancer cells and the PKC iota gene is the target of tumor-specific gene amplification in multiple forms of human cancer. PKC iota participates in multiple aspects of the transformed phenotype of human cancer cells including transformed growth, invasion and survival. Herein, we review pertinent aspects of atypical PKC structure, function and regulation that relate to the role of these enzymes in oncogenesis. We discuss the evidence that PKC iota is a human oncogene, review mechanisms controlling PKC iota expression in human cancers, and describe the molecular details of PKC iota-mediated oncogenic signaling. We conclude with a discussion of how oncogenic PKC iota signaling has been successfully targeted to identify a novel, mechanism-based therapeutic drug currently entering clinical trials for treatment of human lung cancer. Throughout, we identify key unanswered questions and exciting future avenues of investigation regarding this important oncogenic molecule.

Mechanisms of indomethacin-induced alterations in the choline phospholipid metabolism of breast cancer cells

Human mammary epithelial cells (HMECs) exhibit an increase in phosphocholine (PC) and total choline-containing compounds, as well as a switch from high glycerophosphocholine (GPC)/low PC to low GPC/high PC, with progression to malignant phenotype. The treatment of human breast cancer cells with a nonsteroidal anti-inflammatory agent, indomethacin, reverted the high PC/low GPC pattern to a low PC/high GPC pattern indicative of a less malignant phenotype, supported by decreased invasion. Here, we have characterized mechanisms underlying indomethacin-induced alterations in choline membrane metabolism in malignant breast cancer cells and nonmalignant HMECs labeled with [1,2-13C]choline using 1H and 13C magnetic resonance spectroscopy. Microarray gene expression analysis was performed to understand the molecular mechanisms underlying these changes. In breast cancer cells, indomethacin treatment activated phospholipases that, combined with an increased choline phospholipid biosynthesis, led to increased GPC and decreased PC levels. However, in nonmalignant HMECs, activation of the anabolic pathway alone was detected following indomethacin treatment. Following indomethacin treatment in breast cancer cells, several candidate genes, such as interleukin 8, NGFB, CSF2, RHOB, EDN1, and JUNB, were differentially expressed, which may have contributed to changes in choline metabolism through secondary effects or signaling cascades leading to changes in enzyme activity.

Ovarian cancer: Homeobox genes, autocrine/paracrine growth, and kinase signaling

Epithelial ovarian cancer, the fourth leading cause of cancer deaths in American women, is currently classified by surgical and histologic appearance. However, the predictive value of this classification is limited. The risk of epithelial ovarian cancer increases with the number of ovulatory events. It is now thought that different ovarian tumors are derived from a single ovarian surface epithelial precursor cell with the degree and pattern of differentiation determined by combinatorial expression of homeobox genes normally involved in differentiation of the female genital tract. This aberrant differentiation occurs in association with histology-specific genomic aberrations, genomic instability, and resultant chromosomal changes, and may be triggered by prolonged abnormal or excessive exposure of surface epithelial cells to autocrine/paracrine stimulation by sex steroids and other growth factors. As the disease progresses, activation of kinase pathways and continued abnormal autocrine/paracrine stimulation contribute to genomic instability but also identify potential targets for novel therapeutic intervention.

Novel functions of the phospholipase D2-Phox homology domain in protein kinase Czeta activation

It has been established that protein kinase Czeta (PKCzeta) participates in diverse signaling pathways and cellular functions in a wide variety of cells, exhibiting properties relevant to cellular survival and proliferation. Currently, however, the regulation mechanism of PKCzeta remains elusive. Here, for the first time, we determine that phospholipase D2 (PLD2) enhances PKCzeta activity through direct interaction in a lipase activity-independent manner. This interaction of the PLD2-Phox homology (PX) domain with the PKCzeta-kinase domain also induces the activation loop phosphorylation of PKCzeta and downstream signal stimulation, as measured by p70 S6 kinase phosphorylation. Furthermore, only the PLD2-PX domain directly stimulates PKCzeta activity in vitro, and it is necessary for the formation of the ternary complex with phosphoinositide-dependent kinase 1 and PKCzeta. The mutant that substitutes the triple lysine residues (Lys101, Lys102, and Lys103) within the PLD2-PX domain with alanine abolishes interaction with the PKCzeta-kinase domain and activation of PKCzeta. Moreover, breast cancer cell viability is significantly affected by PLD2 silencing. Taken together, these results suggest that the PLD2-mediated PKCzeta activation is induced by its PX domain performing both direct activation of PKCzeta and assistance of activation loop phosphorylation. Furthermore, we find it is an important factor in the survival of breast cancer cells.

Protein kinase Czeta regulates phospholipase D activity in rat-1 fibroblasts expressing the alpha1A adrenergic receptor

BACKGROUND

Phenylephrine (PHE), an alpha1 adrenergic receptor agonist, increases phospholipase D (PLD) activity, independent of classical and novel protein kinase C (PKC) isoforms, in rat-1 fibroblasts expressing alpha1A adrenergic receptors. The aim of this study was to determine the contribution of atypical PKCzeta to PLD activation in response to PHE in these cells.

RESULTS

PHE stimulated a PLD activity as demonstrated by phosphatidylethanol production. PHE increased PKCzeta translocation to the particulate cell fraction in parallel with a time-dependent decrease in its activity. PKCzeta activity was reduced at 2 and 5 min and returned to a sub-basal level within 10-15 min. Ectopic expression of kinase-dead PKCzeta, but not constitutively active PKCzeta, potentiated PLD activation elicited by PHE. A cell-permeable pseudosubstrate inhibitor of PKCzeta reduced basal PKCzeta activity and abolished PHE-induced PLD activation.

CONCLUSION

alpha1A adrenergic receptor stimulation promotes the activation of a PLD activity by a mechanism dependent on PKCzeta; Our data also suggest that catalytic activation of PKCzeta is not required for PLD stimulation.

Elevated phospholipase D activity in H-Ras- but not K-Ras-transformed cells by the synergistic action of RalA and ARF6

Phospholipase D (PLD) activity is elevated in response to the oncogenic stimulus of H-Ras but not K-Ras. H-Ras and K-Ras have been reported to localize to different membrane microdomains, with H-Ras localizing to caveolin-enriched light membrane fractions. We reported previously that PLD activity elevated in response to mitogenic stimulation is restricted to the caveolin-enriched light membrane fractions. PLD activity in H-Ras-transformed cells is dependent upon RalA, and consistent with a lack of elevated PLD activity in K-Ras-transformed cells, RalA was not activated in K-Ras-transformed cells. Although H-Ras-induced PLD activity is dependent upon RalA, an activated mutant of RalA is not sufficient to elevate PLD activity. We reported previously that RalA interacts with PLD activating ADP ribosylation factor (ARF) proteins. In cells transformed by H-Ras, we found increased coprecipitation of ARF6 with RalA. Moreover, ARF6 colocalized with RalA in light membrane fractions. Interestingly, ARF6 protein levels were elevated in H-Ras- but not K-Ras-transformed cells. A dominant-negative mutant of ARF6 inhibited PLD activity in H-Ras-transformed NIH 3T3 cells. Activated mutants of either ARF6 or RalA were not sufficient to elevate PLD activity in NIH 3T3 cells; however, expression of both activated RalA and activated ARF6 in NIH 3T3 cells led to increased PLD activity. These data suggest a model whereby H-Ras stimulates the activation of both RalA and ARF6, which together lead to the elevation of PLD activity.