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Sodium ascorbate (vitamin C) has a reputation for inconsistent effects upon malignant tumor cells, which vary from growth stimulation to apoptosis induction. Melanoma cells were found to be more susceptible to vitamin C toxicity than any other tumor cells. The present study has shown that sodium ascorbate decreases cellular iron uptake by melanoma cells in a dose- and time-dependent fashion, indicating that intracellular iron levels may be a critical factor in sodium ascorbate-induced apoptosis. Indeed, sodium ascorbate-induced apoptosis is enhanced by the iron chelator, desferrioxamine (DFO) while it is inhibited by the iron donor, ferric ammonium citrate (FAC). Moreover, the inhibitory effects of sodium ascorbate on intracellular iron levels are blocked by addition of transferrin, suggesting that transferrin receptor (TfR) dependent pathway of iron uptake may be regulated by sodium ascorbate. Cells exposed to sodium ascorbate demonstrated down-regulation of TfR expression and this precedes sodium ascorbate-induced apoptosis. Taken together, sodium ascorbate-mediated apoptosis appears to be initiated by a reduction of TfR expression, resulting in a down-regulation of iron uptake followed by an induction of apoptosis. This study demonstrates the specific mechanism of sodium ascorbate-induced apoptosis and these findings support future clinical trial of sodium ascorbate in the prevention of human melanoma relapse.

L-ascorbic acid (vitamin C) has been reported to play a role in the treatment and prevention of cancer. However, its specific mechanistic pathways remain obscure. This study was carried out to identify the sodium ascorbate-induced apoptotic pathway in B16F10 murine melanoma cells. Sodium ascorbate was found to induce the apoptosis of B16F10 murine melanoma in a time- and dose-dependent manner, and this was prevented by pretreatment with N-acetyl- L-cysteine (NAC), a well-known antioxidant. In fact, sodium ascorbate-treated B16F10 melanoma cells showed increased intracellular reactive oxygen species generation (ROS) levels. These results indicate that sodium ascorbate induced apoptosis in B16F10 murine melanoma cells by acting as a prooxidant. We examined the involvement of caspase-8 using a specific caspase-8 inhibitor (z-IETD-fmk) on the sodium ascorbate-induced apoptotic pathway. Cell death was found not to be inhibited by z-IETD-fmk treatment, indicating that sodium ascorbate-induced apoptosis is not mediated by caspase-8. In addition, we detected a reduction in the mitochondrial membrane potential during apoptosis and confirmed cytochrome-c release from mitochondria by immunoblotting. Taken together, it appears that the induction of a prooxidant state by sodium ascorbate and a subsequent reduction in mitochondrial membrane potential are involved in the apoptotic pathway of B16F10 murine melanoma cells, and that this occurs in a caspase-8-independent manner.

Summary Apoptosis has been implicated in compensatory proliferation signaling (CPS), whereby dying cells induce proliferation in neighboring cells as a means to restore homeostasis. The nature of signaling between apoptotic cells and their neighboring cells remains largely unknown. Here we show that a fraction of apoptotic cells produce and release CrkI-containing microvesicles (distinct from exosomes and apoptotic bodies), which induce proliferation in neighboring cells upon contact. We provide visual evidence of CPS by videomicroscopy. We show that purified vesicles in vitro and in vivo are sufficient to stimulate proliferation in other cells. Our data demonstrate that CrkI inactivation by ExoT bacterial toxin or by mutagenesis blocks vesicle formation in apoptotic cells and inhibits CPS, thus uncoupling apoptosis from CPS. We further show that c-Jun amino-terminal kinase (JNK) plays a pivotal role in mediating vesicle-induced CPS in recipient cells. CPS could have important ramifications in diseases that involve apoptotic cell death.

Vitamin C plays a crucial role in the suppression of proliferation of several types of cancer. Over-expression of cyclooxygenase (COX)-2 and type I insulin-like growth factor (IGF) receptor are important for proliferation and protection from apoptosis in malignancies. However, its specific mechanisms, especially the interaction between COX-2 expression and IGF-I axis mediated by vitamin C, remain yet to be clarified. Therefore, we investigated the effects of vitamin C on the proliferation of melanoma cells via the modulation of COX-2 expression and IGF-I axis. As a result, we found that 1.0 mM vitamin C inhibits the proliferation of SK-MEL-2 without induction of apoptosis. At that moment, IGF-II production was decreased, followed by the inhibition of COX-2 activity. IGF-IR expression was also down-regulated by vitamin C treatment. It coincided with the result from the inhibition of COX-2 by NS-398 and COX-2 siRNA. In addition, the decreased IGF-IR expression by vitamin C was restored by the treatment of recombinant prostaglandin E2. Finally, we determined whether the signal pathway would be involved in vitamin C-induced IGF-II and IGF-IR down-regulation. When the cells were exposed to SB203580, a specific inhibitor of p38 MAPK, COX-2 expression was dramatically recovered. In addition, phosphorylated p38 MAPK was increased after vitamin C treatment. Taken together, vitamin C suppresses proliferation of the human melanoma cell line SK-MEL2 via the down-regulation of IGF-II production and IGF-IR expression, which is followed by the activation of p38 MAPK and the inhibition of COX-2 expression.

Although ascorbate (Vitamin C) has been shown to inhibit cell growth and induce cell death in variety of cancer cells, results reported in other studies are inconsistent with this conclusion. It was previously reported that ascorbate induces apoptosis in human breast cancer cells. However, the molecular mechanism for this is not clear. In this study, we demonstrate that ascorbate induces cell death through the apoptosis-inducing factor (AIF) in the human breast cancer cell lines, SK-BR3 and Hs578T, but not in a normal breast cell line, Hs578. Ascorbate treatment caused the nuclear translocation of AIF, which is retained in the mitochondria in healthy cells, but caspase cleavage is not induced. Moreover, MG132, an inhibitor of AIF release from mitochondria, blocked the induction of cell death. Furthermore, cells that had been treated with human AIF-specific siRNA resisted cell death induced by ascorbate, implying that the translocation of AIF from mitochondria to the nucleus is responsible for ascorbate-mediated cell death. Therefore, these results suggest that ascorbate activates a caspase-independent and AIF-mediated cell death pathway in human breast cancer cells, SK-BR3, and Hs578T.

CD54 is a cell surface adhesive glycoprotein, which is expressed in most cells. Interaction between CD54 and its ligands is involved in several cellular events including activation, proliferation, and cell death and also cell-to-cell adhesion. In the present study, we found that cross-linking of CD54 on Burkitt lymphoma cell lines, Raji and Ramos, induced apoptosis. We investigated that cross-linking of CD54 on Raji and Ramos using immobilized anti-CD54 mAb (clone 6.5B5) leads to apoptosis. CD54-induced apoptosis took place in association with an increase of intracellular reactive oxygen species (ROS) and a loss of the mitochondrial membrane potential and also the activation of caspases 3 and 9, resulting in the degradation of the proteolytic poly (ADP-ribose) polymerase. Pretreatment of each N-acetyl cystein and N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (a broad caspase inhibitor) blocked apoptosis. Cross-linking of CD54 immediately induced expression of fasL, which was inhibited by pretreatment of N-acetyl cystein. NOK-1 (antagonistic anti-fasL), ZB4 (antagonistic anti-fas), and N-benzyloxycarbonyl-Ile-Glu-Thr-Asp-fluoromethylketon (caspase 8 inhibitor) effectively rescued cells from apoptosis via adjacent fas-fasL interaction but did not block ROS generation. Taken together, it is concluded that engagement of CD54 on B lymphoma cell lines by anti-CD54 mAb may trigger fasL expression through ROS generation and may subsequently induce apoptosis in adjacent fas-fasL interaction.

It is known that B lymphoma cells are sensitive to cytotoxic chemotherapy, but primary or secondary chemoresistance frequently occurs and is the major cause of death in these patients. However, the mechanisms by which lymphoma cells acquire resistance to cytotoxic drugs are not fully understood. Recently, it was reported that B cells secrete immunoglobulin and produce cytokines after interacting with NK cells, thus indicating the importance of NK/B interactions. In this study, we investigated the mechanism of resistance to cytotoxic chemotherapy induced in cocultures of NK cells and Raji cells. Normally, Raji cells are doxorubicin-sensitive, but Raji cells cocultured with NK cells become doxorubicin-resistant. In addition, we detected the upregulation of CD69 and CD70 on Raji cells cocultured with NK cells, suggesting that Raji cells are activated by NK cells. We also found that the resistance of Raji cells to doxorubicin increased when they had been treated with NK cell coculture supernatant. Furthermore, boiled culture supernatant did not inhibit doxorubicin-mediated cell death, indicating that soluble factors are involved. Finally, we confirmed that NK cells produce TNF alpha, and that doxorubicin-sensitive Raji cells become doxorubicin-resistant after TNF alpha treatment. Taken together, these results suggest that B lymphoma cell resistance to doxorubicin-mediated cell death is induced by coculture with NK cells, because of TNF alpha secretion.

Burkitt lymphoma (BL) is a tumor with the characteristics of germinal center B cells. We previously reported that the CM1 (centrocyte/-blast marker 1) molecule is expressed only in germinal center B cells, specifically, in a subpopulation of centroblasts and centrocytes. In the present study, we investigated the apoptosis induced by anti-CM1 in the Ramos and Raji human BL cell lines. The Ramos is protected from apoptosis by the crosslinking of sIgM and the calcium ionophore by the ligation of CD40 with anti-CD40 monoclonal antibodies (mAb) or soluble CD40 ligand (sCD40L). In this investigation on the effect of CM1 on apoptosis in BL cell lines, we found that cellular signaling by CM1 induces apoptosis and decreases cell viability, in BL cell lines cultured for 24 hours with protein-G agarose beads conjugated anti-CM1 mAb. Stimulation by CD40 ligated with sCD40L protected Raji cells from CM1-induced apoptosis, but did not protect Ramos cells. Furthermore, after anti-CM1 mAb stimulation, CD95 expression was upregulated and CD40 expression was unaltered or slightly decreased in Ramos cells, whereas CD95 was downregulated and CD40 was slightly upregulated in Raji cells. The engagement of CD40 by sCD40L enhanced CD95 expression, but the level of CM1 expression was unchanged in Ramos. However, sCD40L downregulated both CD95 and CM1 expression in Raji. In addition, the caspase-8 specific inhibitor blocked CM1-induced apoptosis in Ramos cells, but not in Raji cells. Increased mitochondrial membrane permeabilization was observed only in Raji cells. Moreover, the effector caspase inhibitor, z-DEVD, blocked CM1-mediated apoptosis in both cell lines. We found that CM1-induced apoptosis is achieved via different initiation pathways, which are cell-type dependent.

CM1 (Centrocyte/-blast Marker I) defined by a mAb developed against concanavalin-A activated PBMC, is expressed specifically on a subpopulation of centroblasts and centrocytes of human germinal center (GC) B cells. Burkitt lymphoma (BL) is a tumor consisting of tumor cells with the characteristics of GC B cell. Previously we reported that CM1 ligation with anti-CM1 mAb induced apoptosis in Ramos (IgM high ) and Raji IgM low cells. Methods & Results: In the present study, we observed that CM1 ligation with anti-CM1 mAb induced Fas ligand and Fas expression in Ramos cells, but not in Raji cells. Furthermore, anti-Fas blocking antibody, ZB4, blocked CM1-mediated apoptosis effectively in Ramos cells, but not in Raji cells. Increased mitochondrial membrane permeabilization, which was measured by DiOC 6 , was observed only in Raji cells. In contrast to no significant change of Bax known as pro-apoptotic protein, anti-apoptotic protein Bcl-2 was significantly decreased in Raji cells. In addition, we observed that CM1 ligation increased release of mitochondrial cytochrome c and upregulated caspase-9 activity in Raji cells. Conclusion: These results suggest that apoptosis induced by CM1-ligation is mediated by Fas-Fas ligand interaction in Ramos cells, whereas apoptosis is mediated by down-regulation of Bcl-2 and subsequent decrease of mitochondrial membrane potential in Raji cells.