Submicromolar doses of alkyl-lysophospholipids induce rapid internalization, but not activation, of epidermal growth factor receptor and concomitant MAPK/ERK activation in A431 cells

Epidermal growth factor receptor cascade prioritizes the maximization of signal transduction

Many studies have been performed to quantify cell signaling. Cell signaling molecules are phosphorylated in response to extracellular stimuli, with the phosphorylation sequence forming a signal cascade. The information gain during a signal event is given by the logarithm of the phosphorylation molecule ratio. The average information gain can be regarded as the signal transduction quantity (ST), which is identical to the Kullback-Leibler divergence (KLD), a relative entropy. We previously reported that if the total ST value in a given signal cascade is maximized, the ST rate (STR) of each signaling molecule per signal duration (min) approaches a constant value. To experimentally verify this theoretical conclusion, we measured the STR of the epidermal growth factor (EGF)-related cascade in A431 skin cancer cells following stimulation with EGF using antibody microarrays against phosphorylated signal molecules. The results were consistent with those from the theoretical analysis. Thus, signaling transduction systems may adopt a strategy that prioritizes the maximization of ST. Furthermore, signal molecules with similar STRs may form a signal cascade. In conclusion, ST and STR are promising properties for quantitative analysis of signal transduction.

Anticancer mechanisms and clinical application of alkylphospholipids

Synthetic alkylphospholipids (ALPs), such as edelfosine, miltefosine, perifosine, erucylphosphocholine and erufosine, represent a relatively new class of structurally related antitumor agents that act on cell membranes rather than on DNA. They selectively target proliferating (tumor) cells, inducing growth arrest and apoptosis, and are potent sensitizers of conventional chemo- and radiotherapy. ALPs easily insert in the outer leaflet of the plasma membrane and cross the membrane via an ATP-dependent CDC50a-containing 'flippase' complex (in carcinoma cells), or are internalized by lipid raft-dependent endocytosis (in lymphoma/leukemic cells). ALPs resist catabolic degradation, therefore accumulate in the cell and interfere with lipid-dependent survival signaling pathways, notably PI3K-Akt and Raf-Erk1/2, and de novo phospholipid biosynthesis. At the same time, stress pathways (e.g. stress-activated protein kinase/JNK) are activated to promote apoptosis. In many preclinical and clinical studies, perifosine was the most effective ALP, mainly because it inhibits Akt activity potently and consistently, also in vivo. This property is successfully exploited clinically in highly malignant tumors, such as multiple myeloma and neuroblastoma, in which a tyrosine kinase receptor/Akt pathway is amplified. In such cases, perifosine therapy is most effective in combination with conventional anticancer regimens or with rapamycin-type mTOR inhibitors, and may overcome resistance to these agents. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Bee venom secretory phospholipase A2 and phosphatidylinositol-homologues cooperatively disrupt membrane integrity, abrogate signal transduction and inhibit proliferation of renal cancer cells

Bee venom secretory phospholipase A2 (bv-sPLA2) and phosphatidylinositol-(3,4)-bisphosphate (PtdIns(3,4)P2) act synergistically to induce cell death in tumour cells of various origins with concomitant stimulation of the immune system. Here, we investigated the mechanisms involved in such actions and examined structural requirements of PtdIns-homologues to inhibit tumour cells in combination with bv-sPLA2. Renal cancer cells were treated with bv-sPLA2 alone or in combination with PtdIns-homologues. Inhibitory effects on [(3)H] thymidine incorporation and intracellular signal transduction pathways were tested. Reaction products generated by bv-sPLA2 interaction with PtdIns(3,4)P2 were identified by mass spectrometry. Among the tested PtdIns-homologues those with a phosphate esterified to position 3 of the inositol head group, were most efficient in cooperating with bv-sPLA2 to block tumour cell proliferation. Growth inhibition induced by the combined action of bv-sPLA2 with either PtdIns(3,4)bisphosphate or PtdIns(3,4,5)trisphosphate were synergistic and accompanied by potent cell lysis. In contrast, PtdIns, which lacked the phosphate group at position 3, failed to promote synergistic growth inhibition. The combined administration of PtdIns(3,4)P2 and bv-sPLA2 abrogated signal transduction mediated by extracellular signal regulated kinase 1 and 2 and prevented transduction of survival signals mediated by protein kinase B. Surface expression of the epidermal growth factor (EGF)-receptor was reduced after PtdIns(3,4)P2-bv-sPLA2 administration and associated with a blockade of EGF-induced signalling. In addition, mass spectroscopy revealed that bv-sPLA2 cleaves PtdIns(3,4)P2 to generate lyso-PtdIns(3,4)P2. In conclusion, we suggest that the cytotoxic activity mediated by PtdIns(3,4)P2 and bv-sPLA2 is due to cell death that results from disruption of membrane integrity, abrogation of signal transduction and the generation of cytotoxic lyso-PtdIns(3,4)P2.

Synergistic induction of apoptosis in human leukemia T cells by the Akt inhibitor perifosine and etoposide through activation of intrinsic and Fas-mediated extrinsic cell death pathways

Perifosine is an Akt inhibitor displaying strong antineoplastic effects in human tumor cell lines and is currently being tested in phase II clinical trials for treatment of major human cancers. Several recent studies showed the apoptotic effect of perifosine alone or in combination with other anticancer agents. However, this is the first study describing the effects of combining perifosine with the commonly used chemotherapy drug etoposide in cultured human Jurkat T-leukemia cells. Low concentrations of perifosine (5 micromol/L) induced cell death in a synergistic fashion with etoposide if used simultaneously or immediately following exposure to etoposide (posttreatment). The increase in cell death seems to be due to an inactivation of the Akt survival pathway, where treated cells showed a complete dephosphorylation of Akt. Moreover, combined drug-induced Akt deactivation was associated with a parallel decrease in phosphorylation of FoxO1 transcription factor and in expression of antiapoptotic Bcl-xL. Furthermore, the increase in cell death was associated with a specific activation of the caspase-dependent Fas death receptor pathway. These findings might be useful when designing clinical trials where chemotherapy is combined with perifosine for a potential broad use against hematologic malignancies in which the Akt survival pathway is frequently activated.

Effect of anti-tumor ether lipids on ordered domains in model membranes

1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (OMPC, edelfosine) and 1-hexadecylphosphocholine (HePC, miltefosine) represent two groups of synthetic ether lipid analogues with anti-tumor activity. Because of their hydrophobic nature, they may become incorporated into plasma membranes of cells, and it has been argued that they may act via association with lipid rafts. With the quenching of steady-state fluorescence of probes preferentially partitioning into sterol-rich ordered domains (cholestatrienol and trans-parinaric acid), we showed that OMPC and HePC by themselves did not form sterol-rich domains in fluid model membranes, in contrast to the two chain ether lipid 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine. Nevertheless, all three ether lipids significantly stabilized palmitoyl-sphingomyelin/cholesterol-rich domains against temperature induced melting. In conclusion, this study shows that anti-tumor ether lipids are likely to affect the properties of cholesterol-sphingomyelin domains (i.e., lipid rafts) when incorporated into cell membranes.

Enhancement of antitumor activity of the anti-EGF receptor monoclonal antibody cetuximab/C225 by perifosine in PTEN-deficient cancer cells

Mutational inactivation or deletion of the phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/MMAC1/TEP gene in human cancer cells leads to a constitutively active status of the phosphatidylinositol 3-kinase/Akt pathway in the cells and has been linked to the lack of responses of the cells to the epidermal growth factor (EGF) receptor-targeted therapeutics. Akt is strongly inhibited by perifosine, an orally active alkyl-lysophospholipid currently being evaluated as an anti-cancer agent in phase 1 and 2 clinical trials. To determine whether perifosine may enhance the antitumor activity of the anti-EGF receptor monoclonal antibody cetuximab/C225 in PTEN-deficient cancer cells, we exposed MDA468 breast cancer cells (which contain mutated PTEN gene) and PC3 prostate cancer cells (in which the PTEN gene is deleted) to perifosine and cetuximab, alone and in combination. Treatment of the cells with perifosine reduced baseline levels of phosphorylated Akt, phosphorylated p44/42 mitogen-activated protein kinase (MAPK) and p38MAPK, and increased baseline levels of phosphorylated stress-activated protein kinase (SAPK)/c-jun NH(2)-terminal kinase (JNK). A 72-h exposure of the MDA468 and PC3 cells to perifosine alone resulted in cell death in a dose-dependent manner, which was enhanced by cetuximab. Addition of subtoxic doses of perifosine to cetuximab treatment also enhanced the cetuximab-induced growth inhibition. The combination treatment enhanced the inhibition of phosphorylation of Akt, p44/42MAPK and p38MAPK, but offset the phosphorylation of SAPK/JNK that was activated by perifosine treatment alone. Taken together, the data showed that perifosine enhances the antitumor activity of cetuximab in PTEN-deficient cancer cells. Further evaluation of the combination treatment in preclinical and clinical studies is warranted.

TGF-beta 1-induced PAI-1 expression is E box/USF-dependent and requires EGFR signaling

Transforming growth factor-beta1 (TGF-beta1) transcriptionally regulates the expression of genes that encode specific proteins (e.g., plasminogen activator inhibitor-1; PAI-1) important in stromal remodeling and cellular invasion. Definition of molecular events underlying TGF-beta1-initiated PAI-1 transcription, therefore, may lead to the identification of new therapeutic targets for diseases associated with elevated PAI-1 synthesis (e.g., tissue fibrosis, vascular disorders, tumor progression). An intact upstream stimulatory factor (USF)-binding E box motif (5'-(-165)CACGTG(-160)-3') at the HRE-2 site in the rat PAI-1 gene was required for PAI-1 transcription in TGF-beta1-treated cells. Mutation of the CA dinucleotide to TC at position -165/-164 in a reporter construct driven by 764 bp of PAI-1 promoter sequence decreased TGF-beta1-dependent CAT activity by >80% indicating the necessity for a consensus hexanucleotide E box motif in induced expression. The same CA --> TC substitution eliminated USF binding to an 18-bp HRE-2 DNA target highlighting the importance of site occupancy to transcriptional activation. Transfection of a dominant-negative USF construct, moreover, completely inhibited formation of USF/HRE-2 probe complexes, attenuated PAI-1 promoter-driven luciferase activity and reduced the response of the endogenous PAI-1 gene to TGF-beta1 (to that approximating quiescent controls). Maximal immediate-early PAI-1 induction upon exposure to TGF-beta1 required EGFR, p21ras, MEK and pp60(c-src) signaling as pharmacologic or dominant-negative inhibition of any of the four intermediates (EGFR, p21ras, MEK, pp60(c-src)) virtually eliminated TGF-beta1-augmented PAI-1 levels. U0126 titering experiments, furthermore, revealed that the same MEK inhibitor concentration that blocked the TGF-beta1 increase in ERK1/2 phosphorylation (20 microM) also effectively attenuated the PAI-1 inductive response suggesting a requirement for stimulated ERK signaling in TGF-beta1-mediated PAI-1 expression. These data suggest a model whereby TGF-beta1 activates a complex signaling cascade to affect PAI-1 gene control and involves USF occupancy of a critical E box motif at the HRE-2 site in the PAI-1 gene.

Structure-function relationships among human cathelicidin peptides: dissociation of antimicrobial properties from host immunostimulatory activities

Cathelicidins and other antimicrobial peptides are deployed at epithelial surfaces to defend against infection. These molecules have broad-spectrum killing activity against microbes and can have effects on specific mammalian cell types, potentially stimulating additional immune defense through direct chemotactic activity or induction of cytokine release. In humans, the cathelicidin hCAP18/LL-37 is processed to LL-37 in neutrophils, but on skin it can be further proteolytically processed to shorter forms. The influence of these cathelicidin peptides on keratinocyte function is not known. In the current study, DNA microarray analysis and confirmatory protein analysis showed that LL-37 affects the expression of several chemokines and cytokines by keratinocytes. Analysis of a synthetic peptide library derived from LL-37 showed that antimicrobial activity against bacterial, fungal, and viral skin pathogens resides within specific domains of the parent peptide, but antimicrobial activity does not directly correlate with the ability to stimulate IL-8 production in keratinocytes. IL-8 release was induced by d- and l-amino acid forms of cathelicidin and correlated with membrane permeability, suggesting that highly structure-specific binding to a cell surface receptor is not likely. However, this effect was inhibited by either pertussis toxin or AG1478, an epidermal growth factor receptor tyrosine kinase inhibitor, suggesting that cathelicidin may indirectly stimulate multiple signaling pathways associated with cell surface receptors. Taken together, these observations suggest that proteolytic processing may alter the balance between cathelicidin antimicrobial and host immunostimulatory functions.

Apoptosis-modulating agents in combination with radiotherapy-current status and outlook

PURPOSE

To increase the therapeutic efficacy of ionizing radiation or to reduce radiation-mediated side effects, diverse research centers for translational radiation oncology have headed for a specific modulation of defined cellular death pathways. In this regard, several signaling systems have proved to be of high potential value.

RESULTS

It has previously been shown that apoptotic pathways induced by ionizing radiation are distinct from death pathways triggered by death ligands such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). The combination of both radiation and TRAIL was highly efficient in vitro and in preclinical mouse models. However, several aspects of normal tissue toxicity have not been solved, and no Phase I data are available yet. A second approach tested in a Phase I trial is based on the observation that synthetic phospholipid derivatives (alkyllysophospholipids and alkylphosphocholines) strongly enhance apoptotic effects by modulating the balance among the mitogenic, anti-apoptotic MAPK, phosphatidylinositol 3'-kinase (PI3K)/Akt, and the pro-apoptotic SAPK/JNK signaling pathways. Furthermore, others have provided evidence that inhibition of anti-apoptotic signals generated by mitogenic stimuli may increase radiation responses. In this context, controversial data are available regarding the influence of a pharmacologic abrogation of MEK1, Erk1/2 signaling on apoptotic sensitivity but no Phase I trials of MEK inhibitors either alone or in combination with radiation have yet been published. However, inhibition of the PI3K/Akt survival pathway using compounds such as the protein kinase C (PKC) inhibitor PKC412 has been shown to induce apoptosis or to increase the apoptotic sensitivity of tumor cells. Therefore, these drugs may be used alone or in combination with radiation to increase tumor control; however, Phase I data are lacking. Several other drugs, including cyclooxygenase-2 inhibitors, betulinic acid, and proteasome inhibitors, have been shown to interact with apoptotic signal transduction. Again, most of the drugs have not been tested in combination with radiation in vivo or-in the case of cyclooxygenase-2 inhibitors-exert pleiotropic effects.

CONCLUSION

Although the examples do not reflect all available strategies, it is clear that several promising approaches targeting defined cell death pathways have been developed and entered into clinical trials. The use of synthetic phospholipid derivatives in a Phase I trial is an important example, proving that basic research in radiation biology finally guides the development of new treatment strategies. This, and other approaches, will hopefully increase tumor control rates and reduce side effects in the future.

Different modes of internalization of apoptotic alkyl-lysophospholipid and cell-rescuing lysophosphatidylcholine

The synthetic alkyl-lysophospholipid (ALP), Et-18-OCH3 (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine), can induce apoptosis in tumour cells. Unlike conventional chemotherapeutic drugs, ALP acts at the cell-membrane level. We have reported previously that ALP is internalized, and interferes with phosphatidylcholine (PC) biosynthesis de novo, which appeared to be essential for survival in lymphoma cells [Van der Luit, Budde, Ruurs, Verheij and Van Blitterswijk (2002) J. Biol. Chem. 277, 39541-39547]. Here, we report that, in HeLa cells, ALP accumulates in lipid rafts, and that internalization is inhibited by low temperature, monensin, disruption of lipid rafts and expression of a dominant-negative mutant of dynamin bearing a replacement of Lys44 with alanine (K44A). Thus ALP is internalized via raft- and dynamin-mediated endocytosis. Dynamin-K44A alleviated the ALP-induced inhibition of PC synthesis and rescued the cells from apoptosis induction. Additional cell rescue was attained by exogenous lysoPC, which after internalization serves as an alternative substrate for PC synthesis (through acylation). Unlike ALP, and despite the high structural similarity to ALP, lysoPC uptake did not occur via lipid rafts and did not depend on functional dynamin, indicating no involvement of endocytosis. Albumin back-extraction experiments suggested that (radiolabelled) lysoPC undergoes transbilayer movement (flipping). We conclude that ALP is internalized by endocytosis via lipid rafts to cause apoptosis, while exogenous cell-rescuing lysoPC traverses the plasma membrane outside rafts by flipping. Additionally, our data imply the importance of ether bonds in lyso-phospholipids, such as in ALP, for partitioning in lipid rafts.