Depletion of COPI in cancer cells: the role of reactive oxygen species in the induction of lipid accumulation, noncanonical lipophagy and apoptosis.

The coatomer protein complex 1 (COPI) is a multisubunit complex that coats intracellular vesicles and is involved in intracellular protein trafficking. Recently we and others found that depletion of COPI complex subunits zeta (COPZ1) and delta (ARCN1) preferentially kills tumor cells relative to normal cells. Here we delineate the specific cellular effects and sequence of events of COPI complex depletion in tumor cells. We find that this depletion leads to the inhibition of mitochondrial oxidative phosphorylation and the elevation of reactive oxygen species (ROS) production, followed by accumulation of lipid droplets (LDs) and autophagy-associated proteins LC3-II and SQSTM1/p62 and, finally, apoptosis of the tumor cells. Inactivation of ROS in COPI-depleted cells with the mitochondrial-specific quencher, mitoquinone mesylate, attenuated apoptosis and markedly decreased both the size and the number of LDs. COPI depletion caused ROS-dependent accumulation of LC3-II and SQSTM1 which colocalizes with LDs. Lack of double-membrane autophagosomes and insensitivity to Atg5 deletion suggested an accumulation of a microlipophagy complex on the surface of LDs induced by depletion of the COPI complex. Our findings suggest a sequence of cellular events triggered by COPI depletion, starting with inhibition of oxidative phosphorylation, followed by ROS activation and accumulation of LDs and apoptosis.

Increased tumour dihydroceramide production after Photofrin-PDT alone and improved tumour response after the combination with the ceramide analogue LCL29. Evidence from mouse squamous cell carcinomas.

Photodynamic therapy (PDT) has been proven effective for treatment of several types of cancer. Photodynamic therapy alone, however, attains limited cures with some tumours and there is need for its improved efficacy in such cases. Sphingolipid (SL) analogues can promote tumour response in combination with anticancer drugs. In this study, we used mouse SCCVII squamous cell carcinoma tumours to determine the impact of Photofrin-PDT on the in vivo SL profile and the effect of LCL29, a C6-pyridinium ceramide, on PDT tumour response. Following PDT, the levels of dihydroceramides (DHceramides), in particular C20-DHceramide, were elevated in tumours. Similarly, increases in DHceramides, in addition to C20:1-ceramide, were found in PDT-treated SCCVII cells. These findings indicate the importance of the de novo ceramide pathway in Photofrin-PDT response not only in cells but also in vivo. Notably, co-exposure of SCCVII tumours to Photofrin-PDT and LCL29 led to enhanced tumour response compared with PDT alone. Thus, we show for the first time that Photofrin-PDT has a distinct signature effect on the SL profile in vitro and in vivo, and that the combined treatment advances PDT therapeutic gain, implying translational significance of the combination.

Expression of the mutated p53 tumor suppressor protein and its molecular and biochemical characterization in multidrug resistant MCF-7/Adr human breast cancer cells.

Multidrug resistance in MCF-7/Adr human breast cancer cells is mediated by several mechanisms including overexpression of the MDR1 gene product, P-glycoprotein and glutathione-related detoxifying enzymes. Mutations in the p53 tumor suppressor protein have been reported to play a role in the development of resistance to DNA damaging agents in several human cancer cells. In the present study we have assessed the mutational status of the p53 protein and its expression levels, degree of stability and cellular localization to investigate whether it is involved in modulating multidrug resistance in MCF-7/Adr cells compared to sensitive MCF-7 cells. As revealed by immunofluorescence microscopy using the anti-p53 mouse monoclonal antibody DO-1, wild-type p53 is sequestered in the cytoplasm of MCF-7 cells, whereas in MCF-7/Adr cells, the protein is localized in the nucleus. The sequencing of full-length p53 cDNA revealed a 21 bp deletion in its one of the four conserved regions within the conformational domain, spanning codons 126-133 at exon five, in MCF-7/Adr cells. Moreover, detection of ThaI polymorphism of codon 72 showed that MCF-7 cells predominantly express wild-type p53 with proline, while mutated p53 in MCF-7/Adr cells contains an arginine residue at codon 72. In addition, we demonstrate that the half-life of p53 in MCF-7 cells is less than 30 min while the mutated protein is more stable; its half-life is about 4 h in MCF-7/Adr cells. Thus, this study demonstrates that the deletion of codons 126-133 in p53 causes increased stability, overexpression and nuclear localization of the protein in multidrug resistant MCF-7/Adr cells, and further suggests that mutated p53 might be involved in the development of multidrug resistance in this cell line.

Effects of staining of RNA with ethidium bromide before electrophoresis on performance of northern blots.

Staining of RNA with ethidium bromide (EtdBr) prior to running agarose gels has been reported to afford certain advantages over staining gels after electrophoresis. We have examined prior staining of RNA with a wide range of EtdBr concentrations, particularly with respect to its effects on Northern blot hybridizations using antisense RNA probes. Prior staining with EtdBr at concentrations of 100-1000 micrograms/ml caused significant alterations in RNA mobilities and significantly decreased hybridization with antisense RNA probes compared with unstained RNA. Prior staining with EtdBr at 10-50 micrograms/ml resulted in the best combination of staining sensitivity, absence of alterations in RNA mobility and efficiency of hybridization. Conventional staining of gels after electrophoresis also resulted in decreased hybridization efficiency with RNA probes compared with unstained RNA.

Effects of sodium saccharin and linoleic acid on mRNA levels of Her2/neu and p53 in a human breast epithelial cell line.

The effects of two food-related chemicals (sodium saccharin and linoleic acid) on the levels of Her2/neu and p53 mRNA in a non-cancerous human breast epithelial cell line (HBL-100) were tested in comparison with the effects of the known tumor promoter phorbol 12-myristate 13-acetate (TPA). Treatments were made both with and without prior treatment with two well-known tumor initiators, N-nitroso-N-methylurea (NMU) or 7,12-dimethylbenz[a]anthracene (DMBA). The effects in general were small, the greatest being increases of 46-67% in Her2/neu mRNA levels in response to treatments with TPA or sodium saccharin following NMU treatments. These results demonstrate that sodium saccharin following NMU treatments might be involved in transcriptional regulation of Her2/neu in HBL-100 cells and suggest that its effects may not be limited to urinary bladder.

Lack of correlation of MRP and gamma-glutamylcysteine synthetase overexpression with doxorubicin resistance due to increased apoptosis in SV40 large T-antigen-transformed human mesothelial cells.

PURPOSE: Evidence suggests that viral proteins such as simian virus large T-antigen (SV40 TAg) play a role in the response of cancer cells to chemotherapeutic agents. In this study, we investigated whether SV40 TAg-immortalized human mesothelial cells express drug resistance-related proteins and display resistance to chemotherapy, and whether SV40 TAg transformation affects apoptosis. METHODS: We determined the mRNA and protein levels of the multidrug resistance-associated protein (MRP), gamma-glutamylcysteine synthetase heavy subunit (gamma-GCSh), and P-glycoprotein (product of the MDR1 gene) by RT-PCR and Western blotting, respectively, in normal human mesothelial (NHM) cell and SV40 TAg-transformed human mesothelial (Met-5A) cells. The effect of increasing concentrations of doxorubicin (DOX) on these cells was investigated using an MTT cytotoxicity assay, and the glutathione (GSH) content was measured spectrophotometrically. DOX accumulation in these cells was measured by treating the cells with [14C]DOX followed by scintillation counting. Cytoplasmic bNA fragmentation due to apoptosis following DOX treatment of the cells was quantitated by ELISA. RESULTS: We showed that the MRP and gamma-GCSh genes, but not MDR1, are coordinately overexpressed in Met-5A cells compared with NHM cells. Expression of MRP protein as detected by an anti-MRP antibody correlated with increased GSH levels and decreased accumulation of [14C]DOX in Met-5A cells compared with NHM cells. However, Met-5A cells were twofold more sensitive to DOX than NHM cells. In addition, quantitative measurement of apoptosis when cells were treated with 0.05 and 0.5 microM DOX revealed that drug-induced apoptotic cell death was increased about 1.4- and 3.0-fold, respectively, in Met-5A cells compared with NHM cells. CONCLUSIONS: These results suggest that increased levels of apoptosis might help overcome the DOX resistance effects of MRP/gamma-GCSh overexpression in SV40 TAg-transformed Met-5A cells.

Enhanced tumor cures after Foscan photodynamic therapy combined with the ceramide analog LCL29. Evidence from mouse squamous cell carcinomas for sphingolipids as biomarkers of treatment response.

To improve anticancer therapeutic success of photodynamic therapy (PDT), combination treatments represent a viable strategy. Sphingolipid analogs combined with anticancer drugs can enhance tumor response. We have shown that LCL29, a C6-pyridinium ceramide, promotes therapeutic efficacy of Photofrin-PDT in mouse SCCVII squamous cell carcinoma tumors. The long-term effect of the combination PDT + LCL29 is unknown. In this study we used the same model to test the long-term curative potential of Foscan-PDT + LCL29. We show that treatment of SCCVII tumors with the combination led to enhanced long-term tumor cure compared to PDT alone. LCL29 itself did not prevent tumor growth. All treatments triggered early increases in tumor-associated C16-ceramide, C18-ceramide, dihydrosphingosine, and global levels of dihydroceramides. PDT-evoked increases in tumor-associated sphingosine-1-phosphate and dihydrosphingosine-1-phosphate remained elevated or were attenuated after the combination, respectively; in contrast, LCL29 had no effect on these two sphingolipids. Our data demonstrate that adjuvant LCL29 improves PDT long-term therapeutic efficacy, implying translational potential of the combination. Furthermore, our findings indicate that changes in the sphingolipid profile might serve as predictive biomarkers of tumor response to treatments.

Ceramide synthase 6 modulates TRAIL sensitivity and nuclear translocation of active caspase-3 in colon cancer cells.

We have previously shown that the death receptor ligand TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) induces an increase of intracellular C(16)-ceramide in sensitive SW480 but not in resistant SW620 cells. Resistance in SW620 cells was overcome by exogenous ceramide, leading us to propose that defective ceramide signaling contributes to TRAIL resistance. In this study we found that the increase in C(16)-ceramide in SW480 cells was inhibited by fumonisin B1, an inhibitor of ceramide synthases (CerS). Protein analysis revealed that TRAIL-resistant SW620 cells expressed lower levels of ceramide synthase 6 (CerS6, also known as longevity assurance homologue 6), which prompted us to investigate the effect of CerS6 modulation on TRAIL phenotype. RNAi against CerS6 resulted in a specific and significant decrease of the C(16)-ceramide species, which was sufficient to inhibit TRAIL-induced apoptosis. In cells with decreased levels of CerS6, caspase-3 was activated but failed to translocate into the nucleus. CerS6 localized primarily to the perinuclear region, suggesting this enzyme may be important in regulation of nuclear permeability. Moderate elevation in CerS6 expression was sufficient to reverse TRAIL resistance in SW620 cells. These results suggest that modulation of CerS6 expression may constitute a new therapeutic strategy to alter apoptotic susceptibility.

Updates on functions of ceramide in chemotherapy-induced cell death and in multidrug resistance.

The sphingolipid ceramide, a bioeffector lipid, is known to regulate anti-proliferative responses, such as apoptosis, growth arrest, differentiation and senescence in various human cancer cell lines. Previous studies have demonstrated that many anti-cancer agents cause elevation of endogenous ceramide levels generated via the de novo pathway and/or the hydrolysis of sphingomyelin, accompanied by apoptotic cell death in human cancer cells. It has also been shown that decreased levels of endogenous ceramide by over-expression of glucosylceramide synthase, which clears ceramide levels by incorporating it into glucosylceramide, results in the development of a multidrug resistant phenotype in cancer cells. These studies demonstrate that ceramide plays important roles in the response of cancer cells to chemotherapeutic drugs. The goal of this review is to provide an update on recent studies which shed new light into the roles of ceramide in chemotherapy-induced apoptosis and in multidrug resistance (MDR) in human cancer cells.

Down-regulation of apoptosis-related bcl-2 but not bcl-xL or bax proteins in multidrug-resistant MCF-7/Adr human breast cancer cells.

Recent studies have shown that high levels of the apoptosis-related proteins bcl-2 and bcl-xL increase, while over-expression of bcl-xs or bax decreases, resistance to drugs that induce apoptosis in some human cancer cells. In the present report, we investigated whether expression of these apoptosis-related proteins correlates with changes in the degree of resistance to apoptosis induced by doxorubucin, taxol, vincristine and VP-16 and contributes to the development of acquired resistance in multidrug-resistant MCF-7/Adr breast cancer cells. In this study, high levels of bcl-xL and bax proteins are detected in both MCF-7 and MCF-7/Adr cells. In contrast, bcl-2 protein is down-regulated about 10-fold in MCF-7/Adr cells compared with MCF-7 cells. RT-PCR analysis showed that MCF-7/Adr cells express approximately 2-fold less bcl-2 mRNA than MCF-7 cells. Moreover, 4-24 hr cycloheximide treatment of MCF-7 and MCF-7/Adr cells did not affect the expression of bcl-2 protein, indicating that this protein is very stable in both cell lines. Our results suggest that bcl-2 expression is modulated partly by transcriptional, but mainly by post-transcriptional, mechanisms. Despite the down-regulation of bcl-2 in MCF-7/Adr cells and equal levels of bcl-x, and bax proteins in both cell lines, cytoplasmic DNA-histone complexes induced by doxorubucin, taxol, vincristine and VP-16 indicate that MCF-7/Adr cells are highly resistant to apoptosis. Moreover, treatments of MCF-7/Adr cells with P-glycoprotein (P-gp) modulators, cyclosporin A and verapamil increased doxorubicin and vincristine-induced DNA fragmentation about 1.4- and 2.5-fold, indicating that P-gp is involved in the development of resistance to chemotherapy-induced apoptosis in this cell line.

Identification and characterization of the MDR1 promoter-enhancing factor 1 (MEF1) in the multidrug resistant HL60/VCR human acute myeloid leukemia cell line.

In this report, the molecular mechanisms involved in the overexpression of MDR1 mRNA in the multidrug resistant variant of the HL60 human acute myeloid leukemia cell line, HL60/VCR, were investigated. RT-PCR and nuclear run-on assays revealed that the expression of MDR1 mRNA is regulated by increased transcriptional initiation in HL60/VCR cells. Transient transfections with a 241 bp MDR1 promoter (spanning the -198 to +43 region) DNA fragment/pGL3-basic plasmid construct resulted in about 6-fold increased luciferase activity in HL60/VCR but not in HL60 cells. Moreover, ds CAAT-oligomer from the MDR1 promoter cloned upstream of the SV-40 promoter in the pGL3-promoter plasmid caused about a 7-fold increase in luciferase activity compared with plasmid constructs containing CAAT-deleted, GC-box, and nonspecific oligomers in HL60/VCR transfectants. These results were confirmed by transfecting HL60/VCR cells with the pGL3-basic plasmid containing a 237 bp mutated MDR1 proximal promoter lacking the CAAT sequence in which no change in luciferase activity was observed. However, a 5-6-fold increase in luciferase activity was measured in these cells when transfected with the wt MDR1 promoter DNA/pGL3-basic plasmid constructs. These results show that the CAAT-region is involved in upregulating the MDR1 promoter in HL60/VCR cells. A nuclear factor binding to the CAAT-region of the MDR1 promoter specifically was detected in electrophoretic mobility shift assays (EMSAs) in HL60/VCR but not in HL60 extracts. Two MDR1 promoter-associated polypeptides with molecular masses of about 130 and 162 kDa were identified in HL60/VCR cells by electroelution, specific DNA-affinity chromatography, and silver staining. Interestingly, cross-linking and Southwestern analysis indicate that only the 130 kDa protein, which we refer to as MDR1-promoter enhancing factor 1 (MEF1), has a strong DNA-binding ability, interacting with the 5'-GTCAATCC-3' element of the MDR1 promoter, as determined by DNase I protection assay. These data reveal that MEF1 upregulates the MDR1 promoter activity.

Negative regulation of MDR1 promoter activity in MCF-7, but not in multidrug resistant MCF-7/Adr, cells by cross-coupled NF-kappa B/p65 and c-Fos transcription factors and their interaction with the CAAT region.

In this study, the possible involvement of repressor protein(s) in suppressing MDR1 promoter activity in the sensitive MCF-7 human breast cancer cell line and its drug resistant variant MCF-7/Adr was investigated. RT-PCR revealed that MDR1 mRNA is under detectable levels in MCF-7, while it is highly expressed in MCF-7/Adr cells. After treatment of MCF-7 cells with cycloheximide (CHX), MDR1 mRNA reached detectable levels, suggesting that MDR1 mRNA expression might be controlled by a labile negative regulatory protein(s) in MCF-7 cells. In electrophoretic mobility shift assays (EMSA) using a 5'-end-labeled 241 bp MDR1 promoter DNA fragment (residues -198 to +43) as a probe, one protein complex that specifically binds to the CAAT region of the MDR1 promoter was detected in MCF-7, but not MCF-7/Adr. In addition, following transient transfections of MCF-7 and MCF-7/Adr cells with a pGL3-Basic plasmid construct containing a CAAT-deleted MDR1 promoter DNA fragment, a significant increase in luciferase activity was observed compared to the 241 bp MDR1 promoter in MCF-7 but not MCF-7/Adr cells. Moreover, a ds CAAT oligomer, cloned upstream of the SV-40 promoter in the pGL3-Promoter vector, resulted in a 70-80% decrease in luciferase activity in MCF-7 cells. To identify the CAAT binding protein complex, EMSA and SDS-PAGE were performed. Two proteins with molecular masses of about 65 and 60 kDa were detected by silver staining. Western blot analysis revealed that this complex consists of NF-kappa B/p65 and c-Fos transcription factors. Moreover, incubating MCF-7 nuclear extracts with antibodies specific for NF-kappa B/p65 or c-Fos in EMSAs almost completely inhibited formation of the complex, supporting the association of NF-kappa B/p65 and c-Fos. Therefore, this study provides evidence that molecular interplay between the NF-kappa B/p65 and c-Fos transcription factors exhibits a negative regulatory function on MDR1 promoter by interacting with the CAAT region in MCF-7.

Increased expression of lung resistance-related protein and multidrug resistance-associated protein messenger RNA in childhood acute lymphoblastic leukemia.

Immunophenotype might be an important indicator for multidrug resistance (MDR) profiles in childhood acute lymphoblastic leukemia (ALL). The authors analyzed the messenger RNA (mRNA) levels of MDR1, multidrug resistance-associated protein (MRP), and lung resistance-related protein (LRP) by reverse transcriptase-polymerase chain reaction (RT-PCR) in childhood pre-B ALL, T-cell ALL, and acute nonlymphoblastic leukemia (ANLL). Results showed that MRP and LRP, but not MDR1, mRNAs are overexpressed, particularly in children with pre-B ALL compared with T-cell ALL and ANLL tested. In addition, the MRP and LRP mRNA expression levels in initial diagnosis and first relapse samples of one patient with pre-B ALL were similar. Consequently, these preliminary results suggest that the expression of these MDR-related genes in childhood ALL might be regulated differently in a lineage dependent manner.

Molecular mechanisms of loss of beta 2-microglobulin expression in drug-resistant breast cancer sublines and its involvement in drug resistance.

In this study, we investigated the mechanism of the loss or decreased expression of beta 2-microglobulin (beta 2m) in several drug-resistant sublines of MCF-7 and in a doxorubicin (DOX)-resistant variant of the T-47D breast cancer cell line. beta 2m protein and RNA are not expressed in highly metastatic, multidrug-resistant MCF-7/Adr cells with high resistance to DOX. Nuclear run-on transcription and RNA stability assays demonstrate that while beta 2m in MCF-7/Adr cells is transcribed, its mRNA is rapidly degraded after synthesis in these cells, indicating that it is controlled by post-transcriptional mechanisms. We also show that an MCF-7 subline (MCF-7/Adr-5) expressing a very low level of resistance to DOX has a decreased level of beta 2m expression. Treatment with actinomycin D revealed that the half-life of beta 2m mRNA in MCF-7 and MCF-7/Adr-5 cell lines was comparable. Nuclear run-on transcription analysis revealed a decreased rate of beta2m transcription in MCF-7/Adr-5 cells compared to that in MCF-7 cells. Moreover, beta 2m mRNA remained undetectable in MCF-7/Adr cells following cycloheximide treatment. However, in MCF-7 cells, increased beta 2m mRNA was observed after 12 h, and a similar level of increased mRNA expression was observed after 36 h of cycloheximide treatment in MCF-7/Adr-5 cells; these results suggest that one of the mechanisms controlling beta 2m mRNA expression might be a negative regulatory protein in MCF-7/Adr-5 cells. Analysis of the beta 2m status of other drug-resistant MCF-7 sublines revealed that deregulation of beta 2m is not limited to DOX resistance, but can also be detected in cells selected for resistance to mAMSA and DOX-verapamil. In addition, our data show that reduced beta 2m expression correlates with the decreased levels of estrogen receptor (ER) expression in the DOX-resistant MCF-7/Adr and T-47D/Adr-4 human breast cell lines. Furthermore, we provide evidence that the partial inhibition of beta 2m by antisense RNA results in 2-3-fold decreased sensitivity of MCF-7 cells to DOX and mAMSA. Moreover, the addition of exogenous beta 2m protein near its physiological human serum concentration can modulate the DOX sensitivity of the MCF-7 antisense beta 2m and control transfectants. Therefore, these results indicate that lost or decreased beta 2m expression is involved in the development of the drug-resistant phenotype and correlates with the loss of ER in human breast cancer cell lines.

Selection and characterization of alpha-amylase-overproducing recombinant Escherichia coli containing the bacterial hemoglobin gene.

We previously reported that the presence of the bacterial (Vitreoscilla) hemoglobin gene enhances alpha-amylase production in recombinant Escherichia coli strain MK79. Using the growth of MK79 on starch as a selective method we have produced a mutant strain (BSC9) that produces up to four times as much alpha-amylase as MK79. Both MK79 and BSC9 produce the most alpha-amylase (per cell and per milliliter) in the stationary phase; almost all of the enzyme is intracellular in both strains. Modification of the standard alpha-amylase assay increases the amount of amylase detected about sixfold. BSC9 has about five to nine times as many copies per cell as MK79 of the recombinant plasmid, which carries both the amylase and hemoglobin genes, but both strains produce about the same amount of hemoglobin. While MK79 respiration decreases upon going from log to stationary phase, BSC9 respiration increases during the same period. The two latter results may be of particular importance in determining the way in which hemoglobin enhances the production of cloned protein products in recombinant bacteria.

FAS activation induces dephosphorylation of SR proteins; dependence on the de novo generation of ceramide and activation of protein phosphatase 1.

The search for potential targets for ceramide action led to the identification of ceramide-activated protein phosphatases (CAPP). To date, two serine/threonine protein phosphatases, protein phosphatase 2A (PP2A) and protein phosphatase 1 (PP1), have been demonstrated to function as ceramide-activated protein phosphatases. In this study, we show that treatment with either anti-FAS IgM (CH-11) (150 ng/ml) or exogenous d-(e)-C(6-)ceramide (20 microm) induces the dephosphorylation of the PP1 substrates, serine/arginine-rich (SR) proteins, in Jurkat acute leukemia T-cells. The serine/threonine protein phosphatase inhibitor, calyculin A, but not the PP2A-specific inhibitor, okadaic acid, inhibited both FAS- and ceramide-induced dephosphorylation of SR proteins. Anti-FAS IgM treatment of Jurkat cells led to a significant increase in levels of endogenous ceramide beginning at 2 h with a maximal increase of 10-fold after 7 h. A 2-h pretreatment of Jurkat cells with fumonisin B(1) (100 microm), a specific inhibitor of CoA-dependent ceramide synthase, blocked 80% of the ceramide generated and completely inhibited the dephosphorylation of SR proteins in response to anti-FAS IgM. Moreover, pretreatment of Jurkat cells with myriocin, a specific inhibitor of serine-palmitoyl transferase (the first step in de novo synthesis of ceramide), also blocked FAS-induced SR protein dephosphorylation, thus demonstrating a role for de novo ceramide. These results were further supported using A549 lung adenocarcinoma cells treated with d-(e)-C(6-)ceramide. Dephosphorylation of SR proteins was inhibited by fumonisin B(1) and by overexpression of glucosylceramide synthase; again implicating endogenous ceramide generated de novo in regulating the dephosphorylation of SR proteins in response to FAS activation. These results establish a specific intracellular pathway involving both de novo ceramide generation and activation of PP1 to mediate the effects of FAS activation on SR proteins.

Molecular mechanisms of ceramide-mediated telomerase inhibition in the A549 human lung adenocarcinoma cell line.

This study was aimed at identifying the molecular mechanisms by which ceramide inhibits telomerase activity in the A549 human lung adenocarcinoma cell line. C(6)-ceramide (20 microm) caused a significant reduction of telomerase activity at 24 h as detected using the telomeric repeat amplification protocol, and this inhibition correlated with decreased telomerase reverse transcriptase (hTERT) protein. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot analyses showed that C(6)-ceramide significantly decreased hTERT mRNA in a time-dependent manner. Electrophoretic mobility shift and supershift assays demonstrated that the binding activity of c-Myc transcription factor to the E-box sequence on the hTERT promoter was inhibited in response to C(6)-ceramide at 24 h. These results were also confirmed by transient transfections of A549 cells with pGL3-Basic plasmid constructs containing the functional hTERT promoter and its E-box deleted sequences cloned upstream of a luciferase reporter gene. Further analysis using RT-PCR and Western blotting showed that c-Myc protein but not its mRNA levels were decreased in response to C(6)-ceramide at 24 h. The effects of ceramide on the c-Myc protein were shown to be due to a reduction in half-life via increased ubiquitination. Similar results were obtained by increased endogenous ceramide levels in response to nontoxic concentrations of daunorubicin, resulting in the inhibition of telomerase and c-Myc activities. Furthermore, the elevation of endogenous ceramide by overexpression of bacterial sphingomyelinase after transient transfections also induced the inhibition of telomerase activity with concomitant decreased hTERT and c-Myc protein levels. Taken together, these results show for the first time that both exogenous and endogenous ceramides mediate the modulation of telomerase activity via decreased hTERT promoter activity caused by rapid proteolysis of the ubiquitin-conjugated c-Myc transcription factor.

Cystic fibrosis transmembrane regulator regulates uptake of sphingoid base phosphates and lysophosphatidic acid: modulation of cellular activity of sphingosine 1-phosphate.

Sphingolipids have been implicated in the regulation of cell growth, differentiation, and programmed cell death. Sphingosine 1-phosphate (SPP) has recently emerged as an important lipid messenger and a ligand for the endothelial differentiation gene receptor family of proteins through which it mediates its biologic effects. Recent studies in Saccharomyces cerevisiae in our laboratory implicated the yeast oligomycin resistance gene (YOR1), a member of the ATP binding cassette family of proteins, in the transport of SPP. The cystic fibrosis transmembrane regulator is a unique member of the ATP binding cassette transporter family and has high homology with YOR1. We therefore set out to investigate if this member of the family can regulate SPP transport. We demonstrate that C127/cystic fibrosis transmembrane regulator (CFTR) cells, expressing wild type CFTR, exhibited significantly higher uptake of sphingosine 1-phosphate than either cells expressing a mutant CFTR C127/DeltaF508 or C127/mock-transfected cells. This effect was specific, dose-dependent, and competed off by dihydrosphingosine 1-phosphate and lysophosphatidic acid. There was no difference in uptake of sphingosine, C(16)-ceramide, sphingomyelin, lysophingomyelin, phosphatidylcholine, lysophosphatidylcholine, or phosphatidic acid among the different cell lines. Pretreatment with forskolin or isobutylmethylxanthine to stimulate cAMP did not affect the uptake in any of the cell lines. Moreover, we found that mitogen-activated protein kinase activation by SPP was less responsive in C127/CFTR as compared with C127/mock-transfected cells, suggesting that uptake of SPP by CFTR may divert it from interacting with its cell surface receptors and attenuate signaling functions. Taken together, these data implicate CFTR in uptake of SPP and the related phosphorylated lipids dihydrosphingosine 1-phosphate and lysophosphatidic acid. This uptake influences the availability of SPP to modulate biologic activity via endothelial differentiation gene receptors. These studies may have important implications to cystic fibrosis.