Enhanced ceramide generation and induction of apoptosis in human leukemia cells exposed to DT(388)-granulocyte-macrophage colony-stimulating factor (GM-CSF), a truncated diphtheria toxin fused to human GM-CSF

DT(388)-GM-CSF, a targeted fusion toxin constructed by conjugation of human granulocyte-macrophage colony-stimulating factor (GM-CSF) with the catalytic and translocation domains of diphtheria toxin, is presently in phase I trials for patients with resistant acute myeloid leukemia. HL-60/VCR, a multidrug-resistant human myeloid leukemia cell line, and wild-type HL-60 cells were used to study the impact of DT(388)-GM-CSF on metabolism of ceramide, a modulator of apoptosis. After 48 hours with DT(388)-GM-CSF (10 nM), ceramide levels in HL-60/VCR cells rose 6-fold and viability fell to 10%, whereas GM-CSF alone was without influence. Similar results were obtained in HL-60 cells. Examination of the time course revealed that protein synthesis decreased by about 50% and cellular ceramide levels increased by about 80% between 4 and 6 hours after addition of DT(388)-GM-CSF. By 6 hours this was accompanied by activation of caspase-9, followed by activation of caspase-3, cleavage of caspase substrates, and chromatin fragmentation. Hygromycin B and emetine failed to elevate ceramide levels or induce apoptosis at concentrations that inhibited protein synthesis by 50%. Exposure to C(6)-ceramide inhibited protein synthesis (EC(50) approximately 5 microM) and decreased viability (EC(50) approximately 6 microM). Sphingomyelinase treatment depleted sphingomyelin by about 10%, while increasing ceramide levels and inhibiting protein synthesis. Diphtheria toxin increased ceramide and decreased sphingomyelin in U-937 cells, a cell line extremely sensitive to diphtheria toxin; exposure to DT(388)-GM-CSF showed sensitivity at less than 1.0 pM. Diphtheria toxin and conjugate trigger ceramide formation that contributes to apoptosis in human leukemia cells through caspase activation and inhibition of protein synthesis.

N-(4-hydroxyphenyl)retinamide elevates ceramide in neuroblastoma cell lines by coordinate activation of serine palmitoyltransferase and ceramide synthase

The retinoid N-(4-hydroxyphenyl)retinamide (4-HPR; fenretinide) is cytotoxic to a variety of cancer cell lines, and we previously showed an association between ceramide generation and 4-HPR cytotoxicity for neuroblastoma cell lines (B. J. Maurer et al., J. Natl. Cancer Inst. (Bethesda), 91: 1138-1146, 1999). Here we determine whether the increased ceramide mediated by 4-HPR in the CHLA-90 human neuroblastoma cell line results from de novo ceramide synthesis. Treatment of CHLA-90 with 4-HPR for 2 h, in the presence of [(3)H]palmitic acid, caused sequential formation of [(3)H]sphinganine (220% over control) and [(3)H]ceramide (160% over control), with sphinganine returning to baseline at 4 h, and ceramide continuing to increase (215% over control). 4-HPR treatment did not accelerate cellular decay of sphingomyelin. Preincubation of cells with either L-cycloserine, an inhibitor of serine palmitoyltransferase (SPT), or fumonisin B(1), an inhibitor of ceramide synthase, retarded ceramide formation in response to 4-HPR treatment, although sphinganine was still generated when 4-HPR and FB(1) were present. Data from in vitro enzyme assays using microsomes showed that preexposure of intact cells to 4-HPR resulted in a time (175% over control; 6 h)- and dose-dependent increase (173% over control; 10 microM) in SPT activity as well as a time (265% over control)- and dose-dependent increase (215% above control; 10 microM) in ceramide synthase activity. Our results show that 4-HPR-mediated ceramide generation is derived from the de novo synthetic pathway by coordinate activation of SPT and ceramide synthase. Knowledge of these biochemical events is of utility when downstream modulators of ceramide metabolism are used to heighten the cytotoxic response to chemotherapy.

Taxol-induced ceramide generation and apoptosis in human breast cancer cells

PURPOSE

Taxol has emerged as a valuable antimitotic chemotherapeutic agent, particularly in advanced breast and ovarian cancers. Although much is known about cytotoxic mechanisms, the effectiveness of Taxol cannot be solely explained by microtubular interaction. This study was undertaken to determine whether ceramide generation plays a role in Taxol-induced apoptosis.

METHODS

Hormone-independent MDA-MB-468 and hormone-dependent MCF-7 breast cancer cell lines were employed, and ceramide metabolism was characterized using [3H]palmitic acid as lipid precursor.

RESULTS

Exposure of cells to Taxol resulted in enhanced formation of [3H]ceramide. Ceramide increased nearly 2-fold in MDA-MB-468 cells exposed to 50 nM Taxol, and more than 2.5-fold in MCF-7 cells exposed to 1.0 microM Taxol. These concentrations mirrored the EC50 (amount of drug eliciting 50% cell kill) for Taxol in the two cell lines. Use of cell-permeable C6-ceramide as a medium supplement revealed that MDA-MB-468 cells were 20-fold more sensitive to ceramide than MCF-7 cells (P < 0.001). Ceramide was generated as early as 6 h after exposure to Taxol in MDA-MB-468 cells, whereas the earliest signs of apoptosis were detected 12 h after treatment, and by 24 h the apoptotic index was six times that of untreated cells. Both fumonisin B1, a ceramide synthase inhibitor, and L-cycloserine, a serine palmitoyltransferase inhibitor, blocked Taxol-induced ceramide generation, whilst sphingomyelin levels remained unchanged, indicating a de novo pathway of ceramide formation. L-Cycloserine reduced Taxol-induced apoptosis by 30% in MDA-MB-468 cells and totally blocked Taxol-induced apoptosis in MCF-7 cells.

CONCLUSIONS

These results suggest that Taxol-induced apoptosis is, in part, attributable to ceramide and sphingoid bases. This is of relevance to drug mechanism studies, as ceramide is a known messenger of apoptosis. Clinical use of Taxol with ceramide-enhancing agents may maximize cytotoxic potential.

Targeting ceramide metabolism--a strategy for overcoming drug resistance

Inherent or acquired drug resistance, which frequently characterizes cancer cells, is caused by multiple mechanisms, including dysfunctional metabolism of the lipid second messenger ceramide. Ceramide, the basic structural unit of the sphingolipids, plays a role in activating cell death signals initiated by cytokines, chemotherapeutic agents, and ionizing radiation. Recent discoveries about the metabolism of ceramide suggest that this agent may have an important influence on the effectiveness of various cancer therapeutics. In particular, the cytotoxic effect of chemotherapy is decreased when generation of ceramide is impaired but is increased when the degradation of ceramide is blocked. Herein, we review the mechanisms of resistance to chemotherapeutic agents in terms of ceramide metabolism.

Ceramide glycosylation potentiates cellular multidrug resistance

Ceramide glycosylation, through glucosylceramide synthase (GCS), allows cellular escape from ceramide-induced programmed cell death. This glycosylation event confers cancer cell resistance to cytotoxic anticancer agents [Liu, Y. Y., Han, T. Y., Giuliano, A. E., and M. C. Cabot. (1999) J. Biol. Chem. 274, 1140-1146]. We previously found that glucosylceramide, the glycosylated form of ceramide, accumulates in adriamycin-resistant breast carcinoma cells, in vinblastine-resistant epithelioid carcinoma cells, and in tumor specimens from patients showing poor response to chemotherapy. Here we show that multidrug resistance can be increased over baseline and then totally reversed in human breast cancer cells by GCS gene targeting. In adriamycin-resistant MCF-7-AdrR cells, transfection of GCS upgraded multidrug resistance, whereas transfection of GCS antisense markedly restored cellular sensitivity to anthracyclines, Vinca alkaloids, taxanes, and other anticancer drugs. Sensitivity to the various drugs by GCS antisense transfection increased 7- to 240-fold and was consistent with the resumption of ceramide-caspase-apoptotic signaling. GCS targeting had little influence on cellular sensitivity to either 5-FU or cisplatin, nor did it modify P-glycoprotein expression or rhodamine-123 efflux. GCS antisense transfection did enhance rhodamine-123 uptake compared with parent MCF-7-AdrR cells. This study reveals that GCS is a novel mechanism of multidrug resistance and positions GCS antisense as an innovative force to overcome multidrug resistance in cancer chemotherapy.

Synergistic cytotoxicity in solid tumor cell lines between N-(4-hydroxyphenyl)retinamide and modulators of ceramide metabolism

BACKGROUND

We previously reported that N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide) treatment caused large increases of ceramide levels in neuroblastoma cell lines and induced cell death by a combination of apoptosis and necrosis through p53 (also known as TP53)-independent and caspase-independent pathways. Our goal was to determine if several molecules that inhibit enzymes involved in ceramide metabolism-L-threo-dihydrosphingosine (safingol), d, l-threo-1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP), and tamoxifen-enhanced 4-HPR-mediated cytotoxicity and/or affected ceramide levels.

METHODS

Cellular lipids were quantified by radiolabeling and thin-layer chromatography. Cytotoxicity and cytotoxic synergy (expressed as combination index, where combination index <1 indicates synergy and >1 indicates antagonism) were measured in cultured cancer cell lines with the use of a fluorescence-based assay of cell viability employing digital imaging microscopy. Statistical tests were two-sided.

RESULTS

4-HPR increased ceramide levels by de novo synthesis. Safingol (1-4 microM) was incorporated into a stereochemical variant of ceramide and synergized with a 3:1 molar ratio of 4-HPR (3-12 microM), to produce a 100-fold to 10 000-fold (2 to 4 logs) increase in cytotoxicity relative to 4-HPR alone in neuroblastoma (combination index <0.1), lung (combination index <0.1-0.2), melanoma (combination index <0.1-0.2), prostate (combination index <0.1-1.0), colon (combination index 0.1-0.3), breast (combination index = 0.1-0.5), and pancreas (combination index = 0.2) cell lines, including p53 mutant and alkylator-resistant cell lines. The 4-HPR and safingol combination was cytotoxic in low-oxygen conditions and was minimally toxic to normal fibroblasts and bone marrow myeloid progenitor cells. Addition of agents that retard ceramide glucosylation and/or acylation, such as PPMP or tamoxifen, to 4-HPR or to the combination of 4-HPR and safingol further increased cytotoxicity to tumor cells.

CONCLUSIONS

Combinations of 4-HPR and modulators of ceramide metabolism may form the basis for a novel chemotherapy that is functional under hypoxic conditions (e.g., such as those within tumors) and is p53 independent and caspase independent.

Uncoupling ceramide glycosylation by transfection of glucosylceramide synthase antisense reverses adriamycin resistance

Previous work from our laboratory demonstrated that increased competence to glycosylate ceramide conferred adriamycin resistance in MCF-7 breast cancer cells (Liu, Y. Y., Han, T. Y., Giuliano, A. E. , and M. C. Cabot. (1999) J. Biol. Chem. 274, 1140-1146). This was achieved by cellular transfection with glucosylceramide synthase (GCS), the enzyme that converts ceramide to glucosylceramide. With this, we hypothesized that a decrease in cellular ceramide glycosylation would result in heightened drug sensitivity and reverse adriamycin resistance. To down-regulate ceramide glycosylation potential, we transfected adriamycin-resistant breast cancer cells (MCF-7-AdrR) with GCS antisense (asGCS), using a pcDNA 3.1/his A vector and developed a new cell line, MCF-7-AdrR/asGCS. Reverse transcription-polymerase chain reaction assay and Western blot analysis revealed marked decreases in both GCS mRNA and protein in MCF-7-AdrR/asGCS cells compared with the MCF-7-AdrR parental cells. MCF-7-AdrR/asGCS cells exhibited 30% less GCS activity by in vitro enzyme assay (19.7 +/- 1.1 versus 27.4 +/- 2.3 pmol GC/h/microg protein, p < 0.001) and were 28-fold more sensitive to adriamycin (EC(50), 0.44 +/- 0.01 versus 12.4 +/- 0.7 microM, p < 0. 0001). GCS antisense transfected cells were also 2.4-fold more sensitive to C(6)-ceramide compared with parental cells (EC(50) = 4. 0 +/- 0.03 versus 9.6 +/- 0.5 microM, p < 0.0005). Under adriamycin stress, GCS antisense transfected cells compared with parental cells displayed time- and dose-dependent increases in endogenous ceramide and dramatically higher levels of apoptotic effector, caspase-3. Western blotting showed that adriamycin sensitivity, introduced by asGCS gene transfection, was independent of P-glycoprotein and Bcl-2 expression. In summary, this work shows that transfection of GCS antisense tempers the expression of native GCS and restores cell sensitivity to adriamycin. Therefore, limiting the potential to glycosylate ceramide, which is an apoptotic signal in chemotherapy and radiotherapy, provides a promising approach to combat drug resistance.

SDZ PSC 833 the drug resistance modulator activates cellular ceramide formation by a pathway independent of P-glycoprotein

SDZ PSC 833 (PSC 833) is a new multidrug resistance modulator. Recent studies have shown that the principal mechanism of action of PSC 833 is to bind P-glycoprotein (P-gp) and prevent cellular efflux of chemotherapeutic drugs. We previously reported that PSC 833 increases cellular ceramide levels. The present study was conducted to determine whether the impact of PSC 833 on ceramide generation is dependent on P-gp. Work was carried out using the drug-sensitive P-gp-deficient human breast adenocarcinoma cell line, MCF-7, and drug resistant MCF-7/MDR1 clone 10.3 cells (MCF-7/MDR1), which show a stable MDR1 P-gp phenotype. Overexpression of P-gp in MCF-7/MDR1 cells did not increase the levels of glucosylceramide, a characteristic which has been associated with multidrug resistant cells. Treatment of MCF-7 and MCF-7/MDR1 cells with PSC 833 caused similar ceramide elevation, in a dose-responsive manner. At 5.0 microM, PSC 833 increased ceramide levels 4- to 5-fold. The increase in ceramide levels correlated with a decrease in survival in both cell lines. The EC50 (concentration of drug that kills 50% of cells) for PSC 833 in MCF-7 and MCF-7/MDR1 cells was 7.2 +/- 0.6 and 11.0 +/- 1.0 microM, respectively. C6-Ceramide exposure diminished survival of MCF-7 cells; whereas, MCF-7/MDR1 cells were resistant to this short chain ceramide analog. Preincubation of cells with cyclosporine A, which has high affinity for P-gp, did not diminish the levels of ceramide generated upon exposure to PSC 833. These results demonstrate that PSC 833-induced cellular ceramide formation occurs independently of P-gp. As such, these data indicate that reversal of drug resistance by classical P-gp blockers may be modulated by factors unrelated to drug efflux parameters.

Glycosylation of ceramide potentiates cellular resistance to tumor necrosis factor-alpha-induced apoptosis

Ceramide, as a second messenger, initiates one of the major signal transduction pathways in tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis. Glucosylceramide synthase (GCS) catalyzes glycosylation of ceramide and produces glucosylceramide. By introduction of the GCS gene, cytotoxic resistance to TNF-alpha has been conferred in human breast cancer cells. MCF-7/GCS-transfected cells expressed 4.1-fold higher levels of GCS activity and exhibited a 15-fold (P < 0.0005) greater EC(50) for TNF-alpha, compared with the parental MCF-7 cell line. DNA fragmentation and DNA synthesis studies showed that TNF-alpha had little influence on the induction of apoptosis or on growth arrest in MCF-7/GCS cells, compared to MCF-7 cells. These studies reveal that TNF-alpha resistance in MCF-7/GCS cells is closely related to ceramide hyperglycosylation, a hallmark of this transfected cell line, and resistance was not aligned with changes in TNF receptor 1 expression. This work demonstrates that GCS, which catalyzes ceramide glycosylation, potentiates cytotoxic resistance to TNF-alpha.

Ceramide toxicity and metabolism differ in wild-type and multidrug-resistant cancer cells

Previously we demonstrated that multidrug-resistant (MDR) cancer cells have elevated levels of a glycosylated form of ceramide, glucosylceramide. Here we compared ceramide metabolism and ceramide toxicity in MCF-7 and in adriamycin-resistant (MCF-7-AdrR) human breast cancer cells. MCF-7-AdrR cells were resistant to C6-ceramide (1-10 microM); however, in MCF-7 cells treated with C6-ceramide, viability dropped sharply. Ceramide, when supplemented, was not metabolized by MCF-7 cells. In contrast, ceramide was efficiently converted to glucosylceramide by MCF-7-AdrR cells. Analysis of extracellular [3H]ceramide in radiolabeled cells showed that MCF-7-AdrR cells do not have an enhanced capacity to efflux ceramide compared with MCF-7 cells. Triphenylethylene anti-estrogens, known modulators of drug resistance, were effective inhibitors of ceramide conversion to glucosylceramide, suggesting that blocking ceramide metabolism plays a role in chemosensitization. The anti-progestine, RU486, also blocked glucosylceramide synthesis in cells; however, LY117018, a raloxifene analog, was without influence. We propose that an enhanced capacity to glycosylate ceramide as evidenced in MCF-7-AdrR cells, is a molecular determinant of drug resistance, particularly as regards resistance to ceramide-enhancing agents such as anthracyclines, ionizing radiation, and tumor necrosis factor-alpha.

Modification of ceramide metabolism increases cancer cell sensitivity to cytotoxics

In the preceding report we demonstrated that MCF-7-AdrR cells (adriamycin resistant) were insensitive to ceramide, whereas MCF-7 wild-type cells were sensitive. It was also shown that the drug resistant cells had an increased capacity to convert ceramide to glucosylceramide. Here we demonstrate that blocking the conversion of ceramide to glucosylceramide increases MCF-7-AdrR cell sensitivity to ceramide as well as to antitumor agents. Treatment of MCF-7 cells with adriamycin elicited a 5-fold increase in ceramide, and caused oligonucleosomal fragmentation, characteristic to apoptosis. Under similar treatment conditions, ceramide was not generated in MCF-7-AdrR cells. In MCF-7-AdrR cells neither C6-ceramide nor tamoxifen was cytotoxic; however, the addition of tamoxifen to the C6-ceramide treatment regimen reduced cell viability to 42% and elicited apoptosis. Treatment of MCF-7-AdrR cells with Adriamycin promoted an increase in ceramide only if tamoxifen was present, in which case ceramide increased 7-fold, and cell viability decreased to 50%. The employment of another agent, RU486 (Mifepristone), which blocks ceramide glycosylation, increased MCF-7-AdrR cell sensitivity to adriamycin in a dose-dependent manner. Our data show that agents that block ceramide glycosylation potentiate cellular sensitivity to ceramide and to chemotherapeutic drugs, and suggest that the ceramide metabolic pathway is an important target for anticancer drug development.

Multidrug resistance modulators and doxorubicin synergize to elevate ceramide levels and elicit apoptosis in drug-resistant cancer cells

BACKGROUND

To provide insight for the development of more effective clinical agents, the authors attempted to elucidate the mechanisms of action of multidrug resistance (MDR) modulators. Previously, the authors found that MDR modulators blocked the conversion of ceramide to glucosylceramide in MDR cells, thereby enhancing cytotoxicity. Because ceramide is a critical component of the apoptosis signaling cascade, the current study examined the impact of therapy using agents that elicit ceramide formation combined with agents that block ceramide glycosylation.

METHODS

Doxorubicin-resistant human breast carcinoma cells (MCF-7-AdrR) were treated with either doxorubicin, tamoxifen, cyclosporine A, or the cyclosporine A analog SDZ PSC 833 (PSC 833) or with combinations thereof, and ceramide and glucosylceramide metabolisms were measured by cell radiolabeling. Cell viability was quantitated spectrophotometrically and apoptosis was evaluated analyzing DNA integrity by gel electrophoresis.

RESULTS

Whereas cyclosporine A blocked the generation of glucosylceramide in MCF-7-AdrR cells, a chemical cousin, PSC 833, elicited a 3-fold increase in glucosylceramide and a 5-fold increase in ceramide levels at 24 hours. The PSC 833 response was time-dependent(as early as 30 minutes) and dose-dependent (as low as 0.1 microM). The appearance of ceramide foreran the generation of glucosylceramide. Sphingomyelin levels were not decreased in response to PSC 833; however, Fumonisin B1, a ceramide synthase inhibitor, blocked PSC 833-induced ceramide generation. Adding tamoxifen, which blocks ceramide glycosylation, to the PSC 833 regimen boosted ceramide levels 11-fold over controls and caused DNA fragmentation. A 3-component regimen comprised of tamoxifen, doxorubicin, and PSC 833 increased ceramide levels 26-fold and brought cell viability to zero.

CONCLUSIONS

These results demonstrate that MDR modulators can be used separately, in combination, or in conjunction with chemotherapy at clinically relevant concentrations to manipulate cellular ceramide levels and restore sensitivity in the drug resistant setting. As such, this represents a new direction in the treatment of cancer.

Increase of ceramide and induction of mixed apoptosis/necrosis by N-(4-hydroxyphenyl)- retinamide in neuroblastoma cell lines

BACKGROUND

The synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR or fenretinide) is toxic to myeloid leukemia and cervical carcinoma cell lines, probably in part due to its ability to increase levels of reactive oxygen species (ROS). We have studied the effects of 4-HPR on neuroblastoma cell lines. Since neuroblastomas commonly relapse in bone marrow, a hypoxic tissue compartment, and many chemotherapeutic agents are antagonized by hypoxia, our purpose was to study in these cell lines several factors influencing 4-HPR-induced cytotoxicity, including induced levels of ROS, effects of physiologic hypoxia and antioxidants, levels of ceramide, and the mechanism of cell death.

METHODS

ROS generation was measured by carboxydichlorofluorescein diacetate fluoresence. Ceramide was quantified by radiolabeling and thin-layer chromatography. Immunoblotting was used to assess p53 protein levels. Apoptosis (programmed cell death) and necrosis were analyzed by nuclear morphology and internucleosomal DNA fragmentation patterns. Cytotoxicity was measured by a fluorescence-based assay employing digital imaging microscopy in the presence or absence of the pancaspase enzyme inhibitor BOC-d-fmk. Statistical tests were two-sided.

RESULTS/CONCLUSIONS

In addition to increasing ROS, 4-HPR (2.5-10 microM) statistically significantly increased the level of intracellular ceramide (up to approximately 10-fold; P<.001) in a dose-dependent manner in two neuroblastoma cell lines, one of which is highly resistant to alkylating agents and to etoposide. Cell death induced by 4-HPR was reduced but not abrogated by hypoxia in the presence or absence of an antioxidant, N-acetyl-L-cysteine. Expression of p53 protein was not affected by 4-HPR. Furthermore, the pan-caspase enzyme inhibitor BOC-d-fmk prevented apoptosis, but not necrosis, and only partially decreased cytotoxicity induced by 4-HPR, indicating that 4-HPR induced both apoptosis and necrosis in neuroblastoma cells.

IMPLICATIONS

4-HPR may form the basis for a novel, p53-independent chemotherapy that operates through increased intracellular levels of ceramide and that retains cytotoxicity under reduced oxygen conditions.

SDZ PSC 833, the cyclosporine A analogue and multidrug resistance modulator, activates ceramide synthesis and increases vinblastine sensitivity in drug-sensitive and drug-resistant cancer cells

Resistance to chemotherapy is the major cause of cancer treatment failure. Insight into the mechanism of action of agents that modulate multidrug resistance (MDR) is instrumental for the design of more effective treatment modalities. Here we show, using KB-V-1 MDR human epidermoid carcinoma cells and [3H]palmitic acid as metabolic tracer, that the MDR modulator SDZ PSC 833 (PSC 833) activates ceramide synthesis. In a short time course experiment, ceramide was generated as early as 15 min (40% increase) after the addition of PSC 833 (5.0 microM), and by 3 h, [3H]ceramide was >3-fold that of control cells. A 24-h dose-response experiment showed that at 1.0 and 10 microM PSC 833, ceramide levels were 2.5- and 13.6-fold higher, respectively, than in untreated cells. Concomitant with the increase in cellular ceramide was a progressive decrease in cell survival, suggesting that ceramide elicited a cytotoxic response. Analysis of DNA in cells treated with PSC 833 showed oligonucleosomal DNA fragmentation, characteristic of apoptosis. The inclusion of fumonisin B1, a ceramide synthase inhibitor, blocked PSC 833-induced ceramide generation. Assessment of ceramide mass by TLC lipid charring confirmed that PSC 833 markedly enhanced ceramide synthesis, not only in KB-V-1 cells but also in wild-type KB-3-1 cells. The capacity of PSC 833 to reverse drug resistance was demonstrated with vinblastine. Whereas each agent at a concentration of 1.0 microM reduced cell survival by approximately 20%, when PSC 833 and vinblastine were coadministered, cell viability fell to zero. In parallel experiments measuring ceramide metabolism, it was shown that the PSC 833/vinblastine combination synergistically increased cellular ceramide levels. Vinblastine toxicity, also intensified by PSC 833 in wild-type KB-3-1 cells, was as well accompanied by enhanced ceramide formation. These data demonstrate that PSC 833 has mechanisms of action in addition to P-glycoprotein chemotherapy efflux pumping.

Expression of glucosylceramide synthase, converting ceramide to glucosylceramide, confers adriamycin resistance in human breast cancer cells

Multidrug-resistant cancer cells display elevated levels of glucosylceramide (Lavie, Y., Cao, H. T., Volner, A., Lucci, A., Han, T. Y., Geffen, V., Giuliano, A. E., and Cabot, M. C. (1997) J. Biol. Chem. 272, 1682-1687). In this study, we have introduced glucosylceramide synthase (GCS) into wild type MCF-7 breast cancer cells using a retroviral tetracycline-on expression system, and we developed a cell line, MCF-7/GCS. MCF-7/GCS cells expressed an 11-fold higher level of GCS activity compared with the parental cell line. Interestingly, the transfected cells demonstrated strong resistance to adriamycin and to ceramide, whereas both agents were highly cytotoxic to MCF-7 cells. The EC50 values of adriamycin and ceramide were 11-fold (p < 0.0005) and 5-fold (p < 0.005) higher, respectively, in MCF-7/GCS cells compared with MCF-7 cells. Ceramide resistance displayed by MCF-7/GCS cells closely paralleled the activity of expressed GCS with a correlation coefficient of 0.99. In turn, cellular resistance and GCS activity were dependent upon the concentration of the expression mediator doxycycline. Adriamycin resistance in MCF-7/GCS cells was related to the hyperglycosylation of ceramide and was not related to shifts in the levels of either P-glycoprotein or Bcl-2. This work demonstrates that overexpression of GCS, which catalyzes ceramide glycosylation, induces resistance to adriamycin and ceramide in MCF-7 breast cancer cells.

Tamoxifen induces selective membrane association of protein kinase C epsilon in MCF-7 human breast cancer cells

Tamoxifen, a synthetic antiestrogen, is known for its antitumoral action in vivo; however, it is well accepted that many tamoxifen effects are elicited via estrogen receptor-independent routes. Previously, we reported that tamoxifen induces PKC translocation in fibroblasts. In the present study, we investigated the influence of tamoxifen, and several triphenylethylene derivatives, on protein kinase C (PKC) in MCF-7 human breast cancer cells. As measured by Western blot analysis, tamoxifen elicited isozyme-specific membrane association of PKC-epsilon, which was time-dependent (as early as 5 min post-treatment) and dose-dependent (5.0-20 microM). Tamoxifen did not influence translocation of alpha, beta, gamma, delta or zeta PKC isoforms. Structure-activity relationship studies demonstrated chemical requirements for PKC-epsilon translocation, with tamoxifen, 3-OH-tamoxifen and clomiphene being active. Compounds without the basic amino side chain, such as triphenylethylene, or minus a phenyl group, such as N,N-dimethyl-2-[(4-phenylmethyl)phenoxy]ethanamine, were not active. In vitro cell growth assays showed a correlation between agent-induced PKC-epsilon translocation and inhibition of cell growth. Exposure of cells to clomiphene resulted in apoptosis. Since PKC-epsilon has been associated with cell differentiation and cellular growth-related processes, the antiproliferative influence of tamoxifen on MCF-7 cells may be related to the interaction with PKC-epsilon.

The multidrug resistance modulator SDZ PSC 833 is a potent activator of cellular ceramide formation

In this study we demonstrate that the multidrug resistance (MDR) modulator PSC 833 is a potent agonist of ceramide metabolism. When added with [3H]serine or [3H]palmitic acid to the culture medium of MCF-7 cells, PSC 833, in a dose-responsive fashion (1-10 microM), increased the levels of [3H]ceramide as much as 16-fold over control. The actual increase in ceramide mass was verified by thin-layer chromatographic chars. Cellular sphingomyelin radioactivity did not decrease during treatment, indicating that PSC 833 does not elicit ceramide formation through a sphingomyelinase pathway. Inclusion of fumonisin B1, an inhibitor of ceramide synthase, blocked formation of ceramide by PSC 833. The results of cell proliferation assays demonstrated a clear correlation between PSC 833 elicitation of ceramide formation and increased cytotoxicity. The MDR modulator and chemical cousin of PSC 833, cyclosporin A, had little impact on cellular ceramide formation. At a concentration of 2.5 microM, cyclosporin A and PSC 833 treatment increased ceramide formation by 20% and 7.5-fold, respectively. These results reveal a new action of PSC 833 which may contribute to its potency as a drug resistance modulator.

Glucosylceramide: a marker for multiple-drug resistant cancers

BACKGROUND

Multiple-drug resistance (MDR) is a major reason for chemotherapy failure. Herein we describe glucosylceramide, a new marker for MDR.

METHODS

Cellular lipids were analyzed in three human MDR cancer cell lines and their drug-sensitive counterparts. Analysis of glucosylceramide was also performed in six melanoma specimens and one breast tumor specimen obtained from patients who had undergone chemotherapy. Glucosylceramide, analyzed by mass and by cellular utilization of radiolabeled precursor ([3Hpalmitic acid), was isolated by lipid extraction techniques and resolved from other components by thin-layer chromatography.

RESULTS

Glucosylceramide was present consistently in all MDR cell lines and was absent, or present only at very low levels, in the corresponding drug-sensitive cells. Examination of human tumor specimens documented presence of the marker in all patients who had failed chemotherapy, and absence of the marker in each of the patients with known clinical response to chemotherapy. The response to chemotherapy was followed for a median of 8 months in melanoma patients and for 22 months in the breast cancer patient.

CONCLUSION

These findings suggest that glucosylceramide may hold clinical significance for the early identification of drug-resistant tumors.

Tamoxifen activates cellular phospholipase C and D and elicits protein kinase C translocation

The antiestrogen tamoxifen is widely used for endocrine therapy of breast cancer; however, the mechanisms of estrogen receptor-independent interactions of tamoxifen remain ill defined. Here we examine the effect of tamoxifen on the initial steps of cell signal transduction. To this end, phospholipid metabolism and protein kinase C (PKC) translocation were assessed in CCD986SK human mammary fibroblasts treated with tamoxifen. The addition of tamoxifen resulted in dose-dependent and time-dependent increases in the cellular second messengers phosphatidate (PA) and diacylglycerol (DG). On addition of ethanol to the medium, tamoxifen induced the formation of phosphatidylethanol, demonstrating that tamoxifen activates phospholipase D (PLD). Cellular DG also increased in the presence of ethanol, showing that tamoxifen also activates phospholipase C (PLC). In cells prelabeled with choline and ethanolamine, tamoxifen caused increases in choline, phosphorylcholine, ethanolamine and phosphorylethanolamine. Structure-activity relationship studies for activation of PLD revealed that tamoxifen was the most effective, whereas 4-hydroxy tamoxifen was nearly devoid of activity. Phorbol diesters also activated PLD, but estrogen had no influence. Pretreatment of cells with phorbol dibutyrate (PKC down-regulation protocol) blocked phorbol diester- and tamoxifen-induced PLD activity. Exposure of cells to the PKC inhibitor GF 109203X diminished tamoxifen-induced PLD activity. Addition of tamoxifen to cultures elicited selective membrane association of PKC epsilon. We conclude that tamoxifen exerts considerable extra-nuclear influence at the transmembrane signaling level. These events may contribute to effects beyond the scope of estrogen receptor-dependent actions.

Agents that reverse multidrug resistance, tamoxifen, verapamil, and cyclosporin A, block glycosphingolipid metabolism by inhibiting ceramide glycosylation in human cancer cells

We have previously shown that multidrug-resistant cancer cells display elevated levels of glucosylceramide (Lavie, Y., Cao, H., Bursten, S. L., Giuliano, A. E., and Cabot, M. C. (1996) J. Biol. Chem. 271, 19530-19536). In this study we used the multidrug-resistant human breast cancer cell line MCF-7-Adriamycin-resistant (AdrR), which exhibits marked accumulation of glucosylceramide compared with the parental MCF-7 wild type (drug-sensitive) cell line, to define the relationship between glycolipids and multidrug resistance (MDR). Herein it is shown that clinically relevant concentrations of tamoxifen, verapamil, and cyclosporin A, all circumventors of MDR, markedly decrease glucosylceramide levels in MCF-7-AdrR cells (IC50 values, 1. 0, 0.8, and 2.3 microM, respectively). In intact cells, tamoxifen inhibited glycosphingolipid synthesis at the step of ceramide glycosylation. In cell-free assays for glucosylceramide synthase, tamoxifen (1:10 molar ratio with ceramide) inhibited glucosylceramide formation by nearly 50%. In cell cultures, inhibition of glucosylceramide synthesis by tamoxifen is correlated with its ability to sensitize MCF-7-AdrR cells to Adriamycin toxicity. Moreover, treatment of cells with 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, an inhibitor of glucosylceramide synthesis, likewise sensitized MCF-7-AdrR cells to Adriamycin. It is concluded that high cellular levels of glucosylceramide are correlated with MDR, and that glycolipids are a target for the action of MDR-reversing agents such as tamoxifen. The data entertain the notion that drug resistance phenomena are aligned with cell capacity to metabolize ceramide.

Tamoxifen retards glycosphingolipid metabolism in human cancer cells

In this study we provide evidence that tamoxifen, the widely used breast cancer drug, is a potent antagonist of glycolipid metabolism. When added to the medium of cultured multidrug resistant (MDR) KB-V-1 carcinoma cells, tamoxifen, at 5.0 microM, drastically lowered the levels of glucosylceramide (glc-cer), as evidenced by a reduction in glc-cer mass. In a similar fashion, in cultured human melanoma cells grown with [3H]galactose, tamoxifen inhibited formation of glc-cer by 44%, and retarded lactosylceramide and ganglioside formation by 50 and 35%, respectively. When glc-cer synthase of melanoma was assayed in cell-free incubations, the inclusion of tamoxifen, at a 1:10 molar ratio with ceramide, inhibited glc-cer synthesis by 50%. These results clearly reveal a new action of tamoxifen and thereby pose intriguing questions regarding mechanisms of action in the realm of estrogen receptor-independent modalities, including effects on MDR.

Accumulation of glucosylceramides in multidrug-resistant cancer cells

Multidrug-resistant (MDR) tumors and cancer cell lines demonstrate a wide variety of biochemical changes. In this study we used drug-sensitive wild-type (wt) cancer cell lines and respective MDR subclones, and we demonstrate the accumulation of distinct lipids in MDR cells. These lipids were either absent or present at very low levels in drug-sensitive cells. The compounds, termed lipid-1 and lipid-2, migrated on thin-layer chromatography as a doublet. They could be radiolabeled by incubating MCF-7-AdrR (Adriamycin-resistant) breast cancer cells with [3H]serine, [3H]palmitic acid, or [3H]galactose. Utilization of these precursors by MCF-7-wt cells for synthesis of lipid-1 and -2 was minimal. Two inhibitors of sphingolipid biosynthesis, L-cycloserine and fumonisin B1, prevented the observed accumulation of the lipid compounds. An inhibitor of glucosylceramide synthesis, 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, completely abolished the formation of lipid-1 and -2 in MCF-7-AdrR cells and, to a lesser extent, inhibited the formation of lactosylceramides and gangliosides. Utilizing mass spectrometry, the multidrug resistance-associated lipids were further characterized as monoglycosylceramides of two major species that contained either 16-carbon (palmitic) or 24-carbon (lignoceric and nervonic) fatty acids. The carbohydrate head group of glycosylceramides was identified as glucose, not galactose, thus designating the accumulated lipids as glucosylceramides. Incorporation of [3H]palmitic acid into glucosylceramide was strikingly higher (8-10 times) in MCF-7-AdrR cells compared with MCF-7-wt cells. Since the rate of glucosylceramide degradation in MCF-7-AdrR cells was not attenuated, accelerated glycosphingolipid synthesis in MDR cells is suggested. Glucosylceramide also accumulated in KB-V-1, a vinblastine-resistant epidermoid carcinoma but not in KB-3-1, drug-sensitive wt cells. MDR ovarian adenocarcinoma cells (NIH:OVCAR-3) also contained elevated levels of glucosylceramide. Our results demonstrate a correlation between cellular drug resistance and alterations in glucosylceramide metabolism.

The antitumor phospholipid analog, hexadecylphosphocholine, activates cellular phospholipase D

Hexadecylphosphocholine (HePC), a glycerol-free phospholipid analog, belongs to a new class of drugs that demonstrate selective anticancer activity. The mechanisms underlying the anticancer activity are unclear. To investigate possible signal transduction relationships we examined the influence of HePC on cellular phospholipid metabolism. When HePC was added to cultured human breast fibroblasts (CCD-986-SK cells) that had been radiolabeled with fatty acid, phosphatidylethanol (PEt, the transphosphatidylation product of phospholipase D (PLD)) formation was stimulated as early as 5 min after addition. In cells labeled with [3H]choline, HePC treatment caused release of choline-containing metabolites to the culture medium, concurrent with PEt formation. HePC also elicited formation of diacylglycerol (DG) which, after 30 min increased 3.5-fold over control. As little is known regarding HePC and PLD, attention was directed towards studies on PC metabolism by PLD. PEt formation was shown to be optimal at 20-50 microM HePC, and structure-activity studies showed HePC to be more potent than either lyso-phosphatidylcholine or 1-hexadecyl-2-O-methyl-rac-glycero-3-phosphocholine for PLD activation. PLD activity induced by HePC was totally inhibited by cellular pretreatment with phorbol dibutyrate, and 59% diminished by pretreatment of cells with staurosporine, a protein kinase C (PKC) inhibitor. Our results demonstrate for the first time that HePC activates PLD, and suggest that PKC participates in this response. The relationship of PLD to the anticancer properties of HePC may be clinically relevant to drug actions.

Lipid signal transduction pathways in angiotensin II type 1 receptor-transfected fibroblasts

A stable Chinese hamster ovary fibroblast line expressing the rat vascular type 1a angiotensin II (ANG II) receptor was used to study the lipid-derived signal transduction pathways elicited by type 1a ANG II receptor activation. ANG II caused a biphasic and dose-dependent increase in diacylglycerol (DAG) accumulation with an initial peak at 15 s (181 +/- 11% of control, P < 0.02) and a second sustained peak at 5-10 min (214 +/- 10% of control, P < 0.02). The late DAG peak was derived from phosphatidylcholine (PC), and the formation was blocked by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. ANG II also increased phosphatidic acid (PA) production nearly fourfold by 7.5 min. In the presence of ethanol, ANG II markedly increased phosphatidylethanol (PEt) formation, indicating activation of phospholipase D (PLD). ANG II was shown to increase the mass of three separate PA species, one of which apparently originated from DAG kinase action on PC-phospholipase C (PLC)-produced DAG, providing evidence for PC-PLC activity. ANG II also formed a third PA species, which originated neither from PLD nor from DAG kinase. These results demonstrate that multiple lipid signals propagated via collateral stimulation of PLC and PLD are generated by specific activation of the vascular type 1a ANG II receptor.

Tamoxifen elicits rapid transmembrane lipid signal responses in human breast cancer cells

The antiestrogen tamoxifen competes with estrogen for receptor occupancy, although reports indicate that not all effects of tamoxifen are mediated via this specific interaction. In the present study we sought to determine whether tamoxifen can initiate transmembrane lipid signals. Lipid signaling is a prominent mode by which hormones, growth factors, and phorbol diesters transduce messages. Using the human mammary carcinoma cell line MDA-MB-231, phospholipid metabolism was analyzed in cells prelabeled with 3H-fatty acid. After short-term (10 min) exposure to tamoxifen (10 microM), cellular phosphatidic acid (PA) increased by approximately 50%. Dose-response kinetics for PA formation were obtained over a tamoxifen range of 2.5-20 microM. Treatment of MDA-MB-231 cells with phorbol diester (12-O-tetradecanoylphorbol-13-acetate, TPA) also elicited PA generation (60% above control). Interestingly, addition of tamoxifen, a purported protein kinase C inhibitor, to TPA-treated cells, caused further increase in PA (approximately 100% above control). PA, a second messenger lipid produced upon effector-receptor coupling, shares a prominent role in signal transduction events that govern cellular proliferation. It is therefore suggested that some actions of tamoxifen are mediated by promoting production of second messenger lipids that elicit transmembrane signal transduction cascades. This view is in line with ideas on non-estrogen receptor associated actions of tamoxifen by way of alternate binding sites.

Comparison of phospholipase D activity in vasopressin- and phorbol ester-stimulated fibroblasts

Phospholipase D (PLD) activation by vasopressin (VP) was compared to activation by TPA in REF52 cells prelabeled with [3H]glycerol and [14C]myristic acid. Upon VP-treatment, the formation of [3H] and [14C]phosphatidic acid (PA) and phosphatidylethanol (PEt) was accompanied by the loss of radioactivity from PC and PI. However, upon TPA-treatment, radioactivity was lost from PC only. No significant changes of phosphatidylethanolamine and phosphatidylserine were detected in the same samples. The inclusion of 5 microM staurosporine for 10 min diminished the production of [3H]PEt and [14C]PEt by 27% and 53% in VP-treated cells, and by 100% and 75% in TPA-treated cells, respectively. Adding 1 mM EGTA to chelate extracellular Ca2+ inhibited [3H]PEt by approximately 31% and [14C]PEt by 17% after VP-stimulation. In contrast, EGTA had no effect on TPA-stimulation. The data suggest that REF52 cells contain dual PLD activities. The first is stimulated only by VP, requires Ca2+ and hydrolyzes PI. The second is stimulated by both TPA and VP, activated by protein kinase C and hydrolyzes PC.

Identification of phosphatidylcholine-selective and phosphatidylinositol-selective phospholipases D in Madin-Darby canine kidney cells

Intact cells and cell-free systems were employed to characterize phospholipase D (PLD) activity in Madin-Darby canine kidney (MDCK) cells. In cells prelabeled with [3H]glycerol, 12-O-tetradecanoylphorbol-13-acetate (TPA) elicited phosphatidylcholine (PC) hydrolysis by PLD, as shown by the prolonged formation of [3H]phosphatidylethanol (PEt) and an accompanying decrease in [3H]PC. In contrast, bradykinin elicited rapid formation of [3H]PEt (approximately 1 min) accompanied by a decrease in [3H]phosphatidylinositol (PI). When the agonists were administered simultaneously, [3H]PEt formation was biphasic. In cells prelabeled with [3H] choline, at times less than 1 min, bradykinin failed to induce significant change in [3H]choline release. Bradykinin-induced formation of [3H]PEt in the [3H]glycerol-labeled cells was strictly dependent on extracellular Ca2+, whereas TPA-induced formation of [3H]PEt did not require extracellular Ca2+. Cell-free assays for PLD were used to assess the enzyme location, substrate specificity, and cofactor requirements. The PC-PLD activity (PEt formation) against [3H]stearoyl-PC was primarily localized in the 440 x g pellet (membrane- and nuclear-associated), preferred PC as a substrate, required detergent, and was not influenced by Ca2+ at low concentrations but was inhibited by Ca2+ in excess of 0.5 mM. The PI-PLD activity against [3H]stearoyl-PI was found largely in the 100,000 x g supernatant (cytosol), was strictly Ca(2+)-dependent, and did not require detergent. From these data, we conclude that MDCK cells contain two PLD subtypes: 1) a membrane-associated, PC-selective enzyme that responds to TPA resulting in prolonged hydrolysis of PC (the PC-PLD is Ca(2+)-independent, but requires detergent); 2) a cytosolic, PI-selective enzyme that responds rapidly but transiently to bradykinin (the PI-PLD requires Ca2+ but not detergent).

Phospholipase D activity in nontransformed and transformed fibroblasts

Rat embryo fibroblasts (REF52 cells) and the simian virus 40 transformed derivative (WT6 Ag6) were employed to characterize phospholipase D (PLD) activity in normal and transformed cells. In cells prelabeled with [3H]myristic acid or [3H]glycerol and treated with 12-O-tetradecanoylphorbol-13-acetate (TPA, 50 ng/ml medium) or vasopressin (VP, 100 ng/ml medium) in the presence of ethanol, the formation of labeled phosphatidylethanol (PEt) was 3- to 5-fold higher in REF52 cells than in the transformed cells. The transphosphatidylation of phosphatidylcholine (PC) to PEt was further examined in cell-free assay systems. Results demonstrated that the formation of PEt in the cell-free assays was dependent on the mode of substrate presentation and the source of the PC. With endogenous membrane-bound substrate, the formation of [3H]myristoyl-PEt was 5-fold higher in homogenates derived from normal cells as compared to transformed cell homogenates. In experiments using exogenous labeled PC isolated from either REF52 or transformed cells as substrate, cell-free PLD activity differed greatly with regard to the source of the PC. The formation of PEt from REF52-derived PC was approx. 4-fold higher as compared to PEt formed with PC derived from the transformed cells, irrespective of enzyme source. The results demonstrate that PLD in intact nontransformed fibroblasts is activatable by TPA and VP to a greater extent than in the transformed counterpart. The results from cell-free assays suggest that PLD activity is more dependent on the type of PC substrate than on the source of the enzyme.

Vasopressin-induced polyphosphoinositide and phosphatidylcholine degradation in fibroblasts. Temporal relationship for formation of phospholipase C and phospholipase D hydrolysis products

Cultured fibroblasts (REF52 cells) were employed to investigate phospholipid degradation in response to vasopressin (VP) treatment. There have been few studies in fibroblasts which characterize the pattern and relationship of phosphatidylinositol 4,5-bisphosphate (PIP2) and non-phosphoinositide hydrolysis elicited by VP. Here we demonstrate that VP-induced PIP2 hydrolysis is closely accompanied by phosphatidylcholine (PC) degradation by phospholipase D. Cells prelabeled with [3H]arachidonic acid showed rapid formation and diminution of [3H]diacylglycerol (DG) (5-15s) when treated with VP; this was accompanied by a reduction in polyphosphoinositide radioactivity. Radiolabeled inositol trisphosphate was generated with a similar time frame. In cells prelabeled with [3H]myristic acid, which is predominantly incorporated into cellular PC, VP elicited the generation of [3H]myristoyl phosphatidate (PA) as early as 15 s, in the absence of an increase in labeled DG. In the presence of ethanol the pattern of [3H]myristoyl phosphatidylethanol (PEt) formation coincided with [3H]myristoyl-PA formation in the absence of ethanol. PEt was similarly formed, in response to VP treatment, in cells prelabeled with 1-O-[3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine. The formation of PC-derived [3H]myristoyl-DG was characterized by a lag period of approximately 1 min, after which DG increased steadily over a 10-min period. Biphasic formation of DG was observed in cells prelabeled with [3H]arachidonic acid, and the formation of [3H]PA occurred in an uninterrupted fashion. Two protein kinase C agonists, phorbol diester and dioctanoylglycerol, elicited the formation of [3H]myristoyl-PEt. The inclusion of staurosporine, a protein kinase C inhibitor, blocked VP-induced [3H]myristoyl-PEt formation by 88%. These data demonstrate that VP elicits the coordinated hydrolysis of PIP2 by phospholipase C and PC hydrolysis by phospholipase D. This event results in the prolonged generation of PA and biphasic formation of DG. From the time courses shown, we hypothesize that the early generation of PA, heretofore ascribed to products of the polyphosphoinositide cycle, are in part derived from PC by phospholipase D.

Phorbol diesters stimulate the accumulation of phosphatidate, phosphatidylethanol, and diacylglycerol in three cell types. Evidence for the indirect formation of phosphatidylcholine-derived diacylglycerol by a phospholipase D pathway and direct formation of diacylglycerol by a phospholipase C pathway

The effect of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), on phospholipid degradation was investigated in three cell lines of dissimilar origin, Madin-Darby canine kidney cells (MDCK), rat aorta smooth muscle cells (RASM), and bovine pulmonary artery endothelial cells (BPAE). In cells prelabeled with [3H]myristic acid, which is predominantly incorporated into phosphatidylcholine (PC), TPA treatment (80 nM) in the absence or presence of ethanol (2%) in the culture medium resulted in either the rapid generation of [3H]phosphatidate (PA) or the sustained accumulation of [3H]phosphatidylethanol (PEt), respectively. Increases in [3H]PA and [3H]PEt were paralleled by quantitative decreases in cellular [3H]PC radioactivity. TPA-induced [3H]PEt formation occurred in a similar fashion, irrespective of the presence of Ca2+ in the culture medium. The experiments demonstrate that TPA elicits PC degradation by phospholipase D (PLD) in cells of diverse origin. Data from further experiments revealed a complex relationship between TPA-induced [3H]PA and [3H]diacylglycerol (DG) generation in the three cell lines that was suggestive of dual pathways for the generation of [3H]DG. Experiments to discern the pathways for TPA-induced, PC-derived DG were conducted by comparing the variation of [3H]PA and [3H]DG formation in the absence and in the presence of increasing ethanol concentrations in the culture medium. With increasing amounts of ethanol, the formation of [3H]PA decreased at the expense of [3H]PEt formation, and depending upon the pathway operable, the amount of [3H]DG formed was either decreased, indicative of indirect formation of DG via PA phosphohydrolase, or not modified, indicative of DG formation by a direct phospholipase C (PLC) pathway. Increasing the concentration of ethanol in the medium blocked TPA-induced [3H]DG generation in MDCK cells in a concentration-dependent manner, while the formation of [3H]PEt increased at the expense of [3H]PA formation. In BPAE cells the presence of ethanol likewise reduced TPA-elicited formation of DG. Conversely, in two smooth muscle cell lines, RASM and A-10, ethanol was without influence on TPA-induced formation of [3H]DG, although [3H]PEt was generated at the expense of [3H]PA. In RASM cells prelabeled with [3H]choline, TPA induced the release to the medium of [3H]choline and [3H]phosphocholine, indicative of both PLD and PLC activation. These results show that TPA elicits DG formation from PC in MDCK cells predominantly by an indirect pathway, whereas in arterial smooth muscle cells DG is formed in part by the direct action of PLC.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a protein kinase C-directed mechanism in the phorbol diester-induced phospholipase D pathway of diacylglycerol generation from phosphatidylcholine

In this study we provide evidence for the involvement of protein kinase C (PKC) in phorbol diester-induced phosphatidylcholine (PC) hydrolysis by the phospholipase D pathway. Rat embryo fibroblasts (REF52) were prelabeled with either tritiated choline or myristic acid; these compounds are preferentially incorporated into cellular PC. Phorbol diester-induced PC degradation was determined by measuring the release of [3H]choline, and the formation of [3H]myristoyl-containing phosphatidate (PA), diacylglycerol (DG), and phosphatidylethanol (PE). Staurosporine, a PKC inhibitor, blocked from 73 to 90% of the phorbol diester-induced PC hydrolysis. The inhibition of phorbol diester-induced choline release by staurosporine was dose dependent with an approximate ED50 of 150 nM. Pretreatment of cells with phorbol diester inhibited subsequent phorbol diester-induced PC degradation by 78-92%. A close correlation between the ED50 for phorbol diester-stimulated choline release and the Kd for phorbol diester binding was demonstrated. Neither forskolin nor dibutyryl cAMP elicited cellular PC degradation. In vitro experiments using phospholipase D from Streptomyces chromofuscus showed that staurosporine did not inhibit and TPA did not stimulate enzyme activity.

Effects of proximate cholesterol precursors and steroid hormones on mouse myeloma growth in serum-free medium

The proximate cholesterol precursors lathosterol, 7-dehydrocholesterol and desmosterol supported the growth of NS-1 and X63 mouse myeloma cells. These cells and X63.653 cells are cholesterol auxotrophs, yet each was able to convert [3H]lathosterol to [3H]cholesterol. These results are consistent with the conclusion that cholesterol auxotrophy in these myeloma cells is due to a deficiency in 3-ketosteroid reductase activity. The steroid hormones testosterone, progesterone and hydrocortisone could not replace cholesterol as a medium supplement. These results provide a greater understanding of the cholesterol auxotrophy characteristic of cell lines clonally-derived from the MOPC 21 myeloma tumor, and they provide a rational basis for the use of sterols in defined culture medium for mouse myeloma cells.

The phosphatidylcholine pathway of diacylglycerol formation stimulated by phorbol diesters occurs via phospholipase D activation

Agonist-induced degradation of phosphatidylcholine (PC) is of interest as this pathway of diacylglycerol (DG) generation may provide added opportunities for the regulation of protein kinase C (PKC). In REF52 cells [3H]myristic acid is preferentially incorporated into PC; this, coupled with the use of [3H]choline, allows for quantitation of both the water-soluble and the lipid products generated when PC is degraded. In cells prelabeled with [3H]choline, TPA stimulated a time-dependent release, into the medium, of choline and not phosphocholine or glycerophosphocholine. Treatment of [3H]myristic acid-labeled cells with either phorbol diesters, sn-1,2-dioctanoylglycerol, or vasopressin elicited the formation of labeled phosphatidate (PA) and DG. The temporal pattern of PC hydrolysis in cells treated with TPA is indicative of a precursor (PA)-product (DG) relationship for an enzymatic sequence initiated by phospholipase D. Adding propranolol, a phosphatidate phosphohydrolase inhibitor, eliminated TPA-induced DG formation, whereas PA generation was unaffected. From these data we conclude that TPA elicits DG formation from PC by the sequential actions of phospholipase D and phosphatidate phosphohydrolase.

Vasopressin is the only component of serum-free medium that stimulates phosphatidylcholine hydrolysis and accumulation of diacylglycerol in cultured REF52 cells

Vasopressin stimulates phosphatidylcholine hydrolysis in REF52 cells, and this phosphatidylcholine hydrolysis results in increases in choline containing metabolites in the culture medium (2.3 x control levels) and accumulation of cellular diacylglycerol (6.5 x control levels). Vasopressin is the only component of a 6-component mixture of the serum-free medium for REF52 cells that induces the phosphatidylcholine hydrolysis response. The effect of vasopressin is both time- and concentration-dependent. Maximal levels of both phosphatidyl-choline hydrolysis and accumulation of diacylglycerol are observed between 10 and 20 min after treatment with vasopressin. Effects are maximal at vasopressin concentrations of 100 ng/ml; the ED50 for vasopressin-stimulated phosphatidyl-choline hydrolysis is approximately 0.7 ng/ml. The evolution of diacylglycerol occurs in a time frame that is consistent with the diacylglycerol activating protein kinase C in a "second phase" agonist response.

Vasopressin, phorbol diesters and serum elicit choline glycerophospholipid hydrolysis and diacylglycerol formation in nontransformed cells: transformed derivatives do not respond

REF52, a rat embryo cell line, and several transformed derivatives were used to examine the lipid-related events associated with agonist treatment (phorbol diesters, vasopressin, fetal bovine serum). Exposure of cells, prelabeled with [3H]glycerol, to TPA (12-O-tetradecanoylphorbol 13-acetate) resulted in 3-4-fold increase in the amount of intracellular diacyl[3H]glycerols as early as 10 min after treatment. Continued incubation (up to 60 min) revealed that the diacyl[3H]glycerol formed was under dynamic metabolic regulation as shown by the production of triacyl[3H]glycerols and free [3H]glycerol. Serum and vasopressin likewise induced the generation of intracellular diacyl[3H]glycerol, thereby illustrating that physiological agents provoke a similar reaction. In the three SV-40-transformed variants examined, the diacylglycerol generative-response to TPA, serum and vasopressin, was greatly diminished or totally absent. Experiments employing REF52 cells prelabeled with [3H]choline demonstrated that both TPA and vasopressin induce the hydrolysis of cellular choline-containing glycerophospholipids; this was measured by both a decrease in cell-associated phosphatidylcholine radioactivity and an increase in the production of water-soluble [3H]choline-containing metabolites in the culture medium. 92-97% of the tritium released to the medium was identified as [3H]choline. Vasopressin treatment of REF52 cells prelabeled with [3H]arachidonic acid elicited an increase of more than 11-fold in the amount of cellular diacyl[3H]glycerol and a concomitant release of arachidonic acid to the culture medium that was 12-fold higher than controls. These data demonstrate that tumor-promoting phorbol esters (agonists of protein kinase C), serum and vasopressin, increase the levels of cellular diacylglycerol by stimulating the hydrolysis of choline-containing glycerophospholipids. This agonist-directed mechanism is inoperable in transformed cells. Further, collateral with vasopressin-induced phosphatidylcholine hydrolysis, the cellular release of arachidonic acid occurs. The participation of these lipid-related responses in the signaling of agonist-directed events and their relation to cellular homeostasis is currently being explored.

sn-1,2-diacylglycerols and phorbol diesters: uptake, metabolism, and subsequent assimilation of the diacylglycerol metabolites into complex lipids of cultured cells

The cell-permeable diacylglycerol mediators have been shown to mimic partially the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on cultured cells. In order to evaluate the metabolic stability of the lipid mediators, several radiolabeled diacylglycerols were synthesized and their uptake and intracellular fate in cultured HL-60 (human promyelocytic leukemia) cells was compared with TPA. In addition to whole cell assessment, the stability of diacyl lipids and TPA was evaluated in a buffer/water system and in the presence of serum and subcellular fractions. The compounds studied include 1,2-dioleoyl-sn-glycerol (DiOG), 1-oleoyl-2-acetyl-sn-glycerol (OaG), 1-palmitoyl-2-acetyl-sn-glycerol (PaG), the ether-linked analog 1-palmityl-2-acetyl-sn-glycerol (ePaG), and TPA. TPA was comparatively stable to lipase hydrolysis in all systems examined. First, the data show that within 5 min at pH 7.9, nearly 50% of the PaG (originally greater than 92% 1,2-isomer) had isomerized, and rapid formation of the 1,3-isomer also occurred with OaG and ePaG. The metabolism of OaG and PaG by serum hydrolases, using a reaction medium containing 10% serum, was chiefly by acetate hydrolysis; however, fatty acid was also liberated. After a 60-min incubation 68% of the [14C]OaG was converted, by serum enzymes, to monooleoylglycerol plus oleic acid. Heat-inactivation of serum reduced the enzymatic formation of fatty acid by 60-70%. ePaG was also metabolized by serum enzymes, but the ether-linked alkylglycerol product was stable. The results of cell-free studies (postmitochondrial supernatant) showed that cellular enzymes were present that could, like serum, convert the diacylglycerols to monoacylglycerols and free fatty acids. Studies using cultured cells showed that radiolabeled OaG, PaG, and ePaG were rapidly taken up by the cells and metabolized. Labeled metabolic products from the diacylglycerols appeared, in a time-dependent manner, in cellular phospholipids and triacylglycerols. The results from experiments employing 1-acyl-2-acetyl-sn-[3H]glycerol and [3H]acyl-2-acetyl-sn-glycerol indicate that the intracellular mode of mediator metabolism is via complete hydrolysis with subsequent incorporation of 3H-acyl groups into complex lipids. Data are also presented which show that a substantial amount of cellular lipid acyl group modification occurs and large amounts of glycerol are produced when cells are cultured with OaG. Collectively, these results demonstrate that the diacylglycerol mediators, when compared with TPA, are not stable and are metabolized by both serum and cellular enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of serum, pancreatic, and microbial lipase activities by phorbol diesters

The influence of phorbol diesters on the in vitro hydrolysis of diacylglycerols was examined using enzymes from rat serum, porcine pancreas, and Rhizopus delemar. Two main phenomena were observed: 12-O-tetradecanoylphorbol-13-acetate (TPA), when added to the enzyme assay system, stimulated 2- to 3-fold the hydrolysis of [9,10-3H]dioleoylglycerol by serum lipase. The hydrolysis of dioleoylglycerol by either pancreatic or R. delemar lipase was, on the other hand, inhibited by TPA. A 50% inhibition of the pancreatic and R. delemar enzymes was attained with 10 and 2.0 microM TPA, respectively. The pattern of enzyme stimulation (rat serum), with regard to increasing TPA concentrations, was hyperbolic. Stimulation was not influenced by Triton X-100, but it was highly dependent on the structure of the phorbol ester: TPA greater than phorbol didecanoate greater than tetradecanoylphorbol. Phorbol dibutyrate, phorbol acetate, myristic acid, and mezerein were without influence. Lipase activity was inhibited most strongly by TPA and the nonpromoter 4-O-methyl-TPA; the weaker promoter, phorbol dibutyrate, was relatively inactive. The inhibition of R. delemar lipase by TPA was reversible. Collectively, these data show that phorbol diesters can interact with enzymes other than protein kinase C. It is believed, by virtue of their structural similarity to diacylglycerols, that phorbol diesters may serve directly as intracellular regulators of lipid metabolism. In such a manner phorbol esters could sustain or attenuate the second messenger signal by modifying diacylglycerol metabolism, a manifestation of the pleiotropic action.

Phorbol diesters inhibit enzymatic hydrolysis of diacylglycerols in vitro

The effect of phorbol 12-myristate 13-acetate (PMA) on diacylglycerol lipase activity was examined in rat serum, tissue, and cellular preparations by using di[14C]oleoylglycerol, [3H]palmitoylacetylglycerol, and membrane-resident phospholipase C-generated diacylglycerols as substrates. These experiments were conducted to address whether phorbol esters can mimic diacylglycerols in interacting with enzymes other than protein kinase C. Serum hydrolysis of palmitoylacetylglycerol, assayed by the formation of [3H]palmitic acid, was inhibited by PMA, 4-O-methyl-PMA, or phorbol 12,13-dibutyrate (in order of decreasing potency). The hydrolysis of palmitoylacetylglycerol was inhibited more than 40% by the addition of PMA at a 1:1 molar ratio with substrate. The inhibition resembled the competitive type, with a Ki of approximately 2.7 microM. PMA in the 10-60 microM range also inhibited hydrolysis of palmitoylacetylglycerol by lipases from rat brain microsomes and by homogenates of C3H/10T1/2 mouse fibroblasts. PMA was likewise inhibitory when assayed in an intramembrane enzyme-substrate milieu in which diacylglycerols were generated, in situ, by treatment of [3H]palmitate-labeled cell homogenates with phospholipase C. Collectively, these data demonstrate that PMA, which is now thought to act by mimicry of diacylglycerols, can inhibit the action of diacylglycerol lipase. It is possible that such a mechanism is linked to the multiplicity of responses elicited by phorbol diesters and that other agents may function by means of enzyme interactions (post-phospholipase C) to influence the levels of the cellular diacylglycerol mediators.

Hydrolysis of novel diacylglycerol analogs and phorbol diesters by serum lipase

Rat serum, active in the hydrolysis of the tumor-promoting phorbol diester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was examined with regard to lipid interferences of [3H]TPA hydrolysis and enzyme substrate specificity. The enzymatic hydrolysis of TPA could be enhanced 8-fold, over crude serum, by using a lipid-free acetone powder of rat serum. Addition of lipid to the lipid-free acetone powder produced potent inhibition of TPA hydrolysis. The inclusion of multilamallar liposomes resulted in similar inhibition, and isolation of liposomes by high-speed centrifugation showed that 95% of the radiolabeled TPA was associated with the fatty pellet. Substrate specificity studies demonstrated that the serum activity hydrolyzes the long-chain ester of TPA and the long-chain primary acyl group of diacylglycerols. TPA was hydrolyzed at approximately twice the rate of dioleoylglycerol; however, the most reactive substrates were those synthetic analogs of diacylglycerol containing a short-chain ester group at the sn-2 position. Palmitic acid was liberated from [1-14C]palmitoyl-2-acetyl-sn-glycerol and [1-14C]palmitoyl-2-butyryl-sn-glycerol at 120- and 33-times the rate of TPA hydrolysis, respectively. Lipase resistant 1-hexadecyl-2-[3H]acetylglycerol was also used as substrate, but the sn-2 ester moiety showed poor lability. The diacylglycerol analogs are new lipase substrates and, in view of their similarities to the fatty acyl portion of TPA, it is thought that these compounds could serve as protein kinase C activators.

Structural and chemical specificity of diacylglycerols for protein kinase C activation

The structural and chemical specificity of diacylglycerols, lipid components of the quaternary complex for protein kinase C activation, have been evaluated. The ether-linked analogs of the diacyl lipids, either dialkyl or alkyl acyl, were not effective activators of protein kinase C and thus had little influence on reducing the Ca++ requirement of the enzyme. Ester-linked compounds such as 1-palmitoyl-sn-2-butyrylglycerol were as effective as dioleoylglycerol in stimulating protein phosphorylation. Increasing the carbon number at the sn-2 position from two to four resulted in enhanced enzymatic activity, suggesting that the chain length at the secondary hydroxyl is also of importance. These data clearly establish the necessity of the sn-1 carbonyl group of ester-linked glycerolipids in the protein kinase C activation complex.

Tumor promoting phorbol diesters: substrates for diacylglycerol lipase

Enzyme activity in rat serum was examined utilizing the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and various glycerolipids as substrates. The serum activity was specific for hydrolysis of the long chain tetradecanoate moiety of TPA, hydrolyzed mono- and diacylglycerols, but was not effective against triacylglycerols, cholesterylesters, or phospholipids. Heating the enzyme preparation at 56 degrees C for 1 min was dually effective in reducing the hydrolysis of both TPA and dioleoylglycerol by 83-86% of control levels. The potent diacylglycerol lipase inhibitor, RHC 80267, inhibited the hydrolysis of TPA in the 0.2-1.0 microM range and was also a potent blocker of monoacyl- and diacylglycerol hydrolysis. In substrate competition studies, exogenous unlabeled TPA was added to the [14C]dioleoylglycerol-containing reaction mixture, however, this produced an approximate 3-fold stimulation of [14]dioleoylglycerol hydrolysis. Although we have not established whether the hydrolysis of TPA and diacylglycerol is the work of one enzyme, the effectiveness of the specific lipase inhibitor, RHC 80267, demonstrates that diacylglycerol lipase can utilize TPA as substrate, a finding never before documented. This point is of interest in light of the theory that phorbol esters act by mimicry of the natural lipid mediator, diacylglycerols.

Vertebrate class distribution of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetylhydrolase in serum

Serum from numerous mammals and lower vertebrates contains an enzyme activity that is specific for the hydrolysis of the acetate moiety of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF, platelet activating factor). Acetylhydrolase (EC 3.1.1.47, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine acetylhydrolase) was found in all mammalian sera with activity ranging from 11 (fetal calf) to 178 (rabbit) pmol acetate liberated/microliter serum/min. The enzyme is not present in avian serum but is a constituent of reptiles and bony fishes.

Acetate utilization and fatty acid metabolism in phorbol ester and dimethyl sulfoxide-differentiated human leukemia cells

Cultured human promyelocytic leukemia cells were incubated with [1-14C]acetate to assess the predominant mode of fatty acid synthesis (de novo vs. chain elongation) in these cells and in chemically differentiated populations. 12-O-Tetradecanoylphorbol-13-acetate (TPA) and dimethyl sulfoxide (DMSO) will induce differentiation of this leukemic cell line to macrophage- and granulocyte-like cells, respectively. Human peripheral and rat peritoneal macrophages were employed as controls for the TPA-induced counterpart. Labeling of the carbonyl carbon (C-1) was determined by Schmidt degradation and used to distinguish between chain elongation of pre-existing cellular fatty acids and de novo synthesis. Cultured leukemia cells and the TPA-derived macrophage only incorporated from 16% to 23% of the total radioactivity into the C-1 position, indicating an operable de novo pathway. Cells differentiated by exposure to DMSO displayed a preference for the chain elongation mechanism (89% 14C in C-1 position) of fatty acid synthesis. Both rat peritoneal and human peripheral macrophages likewise incorporated greater than 80% of the radioactivity in the C-1 position of the fatty acyl chains. Thus, DMSO-treated leukemia cells resemble normal differentiated cells, whereas phorbol ester-induced cells, in contrast, retain biochemical features of the undifferentiated cancer cell.

Effect of cellular phospholipid modification on phorbol diester binding

The influence of cellular lipid composition on the specific binding of [20-3H]phorbol-12,13-dibutyrate to intact human promyelocytic leukemia cells was investigated. Cellular phospholipid composition could be manipulated by culturing cells in serum-free, chemically defined media containing base analogues of phospholipid polar head groups. Human promyelocytic leukemia cells grown in the presence of dimethylethanolamine, monomethylethanolamine, 3-aminopropanol, or isopropylethanolamine assimilated these natural and unnatural base moieties into endogenous phospholipids to the extent that 22 to 52% of the cell glycerophospholipids contained the base analogue. The formation of the phospholipid analogues was accompanied by a pronounced reduction in the levels of intracellular choline and ethanolamine glycerophospholipids. Analogue-supplemented cultures exhibited a reduced growth rate compared to control cells maintained in choline-containing medium. Specific [20-3H ]phorbol-12,13-dibutyrate binding was examined in lipid-altered cells and shown to be markedly higher (approximately 200% of control) in cells grown with dimethyl- or monomethylethanolamine. In contrast, exposure of cells to 3-aminopropanol or isopropylethanolamine resulted in a major reduction in [20-3H]phorbol-12,13-dibutyrate binding. Only minimal changes in nonspecific binding occurred between control and experimental cells. Because phorbol esters are highly membrane targeted, it is possible that phospholipid modification or the resulting changes in membrane organization influence receptor dynamics.

Serum lipase active in the hydrolysis of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate

Sera from numerous animal sources contain enzymes that degrade the potent tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). The activity was present in rat, mouse, guinea pig, rabbit and goat, and absent in fetal bovine, bovine, porcine, horse, chicken and human sera. The enzyme shows a marked specificity for hydrolysis of the long-chain 12-O acyl group, which results in the formation of phorbol-13-monoacetate as the major product. Enzymatic catalysis displayed two pH optima (5.5 and 8.0) and was completely destroyed by heat inactivation (56 degrees C, 30 min) of the serum. N-Ethylmaleimide and p-chloromercuribenzoate were relatively weak inhibitors, whereas NaF (5.0 mM) produced an approximate 80% reduction of hydrolysis. Esterase-lipase substrates, such as cholesterol oleate, alpha-naphthylacetate, alpha-naphthylpalmitate, and monotetradecanoylglycerol, were not inhibitory when added to the reaction mixture. Porcine pancreatic lipase does not catalyze the removal of the acyl or the acetyl groups from TPA.