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Background: Breast cancer is a complex heterogeneous disease where many distinct subtypes are found. Younger African American (AA) women often present themselves with aggressive form of breast cancer with unique biology which is very difficult to treat. Better understanding the biology of AA breast tumors could lead to development of effective treatment strategies. Our previous studies indicate that AA but not Caucasian (CA) triple negative (TN) breast cancer cells were sensitive to nitrosative stress-induced cell death. In this study, we elucidate possible mechanisms that contribute to nitric oxide (NO)-induced apoptosis in AA TN breast cancer cells., Methods: Breast cancer cells were treated with various concentrations of long-acting NO donor, DETA-NONOate and cell viability was determined by trypan blue exclusion assay. Apoptosis was determined by TUNEL and caspase 3 activity as well as changes in mitochondrial membrane potential. Caspase 3 and Bax cleavage, levels of Cu/Zn superoxide dismutase (SOD) and Mn SOD was assessed by immunoblot analysis. Inhibition of Bax cleavage by Calpain inhibitor, and levels of reactive oxygen species (ROS) as well as SOD activity was measured in NO-induced apoptosis. In vitro and in vivo effect of NO treatment on mammary cancer stem cells (MCSCs) was assessed., Results and Discussion: NO induced mitocondria-mediated apoptosis in all AA but not in CA TN breast cancer cells. We found significant TUNEL-positive cells, cleavage of Bax and caspase-3 activation as well as depolarization mitochondrial membrane potential only in AA TN breast cancer cells exposed to NO. Inhibition of Bax cleavage and quenching of ROS partially inhibited NO-induced apoptosis in AA TN cells. Increase in ROS coincided with reduction in SOD activity in AA TN breast cancer cells. Furthermore, NO treatment of AA TN breast cancer cells dramatically reduced aldehyde dehydrogenase1 (ALDH1) expressing MCSCs and xenograft formation but not in breast cancer cells from CA origin., Conclusions: Ethnic differences in breast tumors dictate a need for tailoring treatment options more suited to the unique biology of the disease.

Apoptosis is regulated by a series of biochemical events that commits a cell to death. We are interested in understanding and have been investigating the mechanisms by which nitric oxide (NO) induces apoptosis in human breast cancer cell lines. In this study, we investigated the possible interplay of extracellular signal-regulated kinase (ERK) and Akt pathways in NO-induced apoptosis. MKP-1 transcripts were induced in these cells as early as 4 h, peaking at 8 h leading to inactivation of ERK1/2 at 16-24 h after exposure to NO. We also found 50% decrease in the levels pAkt at 24 h of DETA-NONOate treatment. The inactivation of ERK1/2 preceded the dephosphorylation of Akt and apoptosis. NO was not able to inactivate ERK1/2 or Akt or to induce apoptosis in the presence of a phosphatase inhibitor, sodium orthovanadate, or antisense oligonucleotides, suggesting a cross-talk between the two pathways. NO also up-regulated MKP-1 in another breast cancer cell line, ZR 75-30, which led to inactivation of ERK1/2 and induced apoptosis. In MDA-MB-231, NO did not induce MKP-1, and there was no ERK inactivation or apoptosis. Our results indicate that expression of MKP-1 by NO leading to dephosphorylation of ERK1/2 is the initial essential event that commits the cells to the apoptotic pathway in breast cancer cells.

We have previously reported that nitric oxide (NO) induces apoptosis in MDA-MB-468 cells through its action on the mitochondria and the release of cytochrome c. In this study, we investigated the critical events that must occur after which these cells are committed to apoptosis. We used the long-acting NO donor DETA-NONOate, which, at a concentration of 1 mM, releases NO in the range produced by activated macrophages. Depolarization of mitochondrial membrane potential (MMP) occurred at 4 h of DETA-NONOate treatment, which returned to control values and which was followed by another wave of depolarization at 24 h. There was a 2-fold increase of cytochrome c in the cytosol at 6 h, but it was not until 36 h that the level of cytochrome c was increased by 15-fold. Although the initial release of cytochrome c from the mitochondria could be inhibited by cyclosporin A or by bongkrekic acid, the later release continued even in its presence. We observed that the later release of cytochrome c at 36 h was independent of MMP depolarization but was dependent on Bax integration into the mitochondrial membrane, which committed the cells to apoptosis. We also observed a decline in the levels of cytosolic phospho-Akt at 16-24 h of DETA-NONOate treatment. We also conclude that decrease in phospho-Akt is an essential event upstream from Bax integration in MDA-MB-468 cells.

We have previously reported that N(omega)-hydroxy-l-arginine (NOHA), a stable intermediate product formed during the conversion of l-arginine to nitric oxide, induced apoptosis in MDA-MB-468 cells, and this action was antagonized in the presence of l-ornithine. We also reported that apoptosis induced by NOHA in this cell line could not be explained on the basis of a reduction of intracellular polyamines. In the current study, we investigated other potential mechanism(s) by which NOHA may have induced apoptosis in this cell line. We observed that NOHA initially activated caspase-8 and induced cleavage of BH(3) interacting domain. This was followed by release of cytochrome c and subsequently, activation of downstream caspases-9 and -3 to cleave poly(ADP-ribose) polymerase. We also observed that NOHA induced a rapid and persistent hyperpolarization of the mitochondrial membrane potential rather than depolarization indicating that the release of cytochrome c by NOHA was by a mechanism independent of the mitochondrial transition pore. Exogenous l-ornithine did not inhibit NOHA-induced caspase-8 activation and cleavage of BH(3) interacting domain but acted at the mitochondrial level and inhibited the NOHA-induced cytochrome c release and apoptosis.

We have shown previously that (NOHA) an intermediate in the nitric oxide (NO) synthetic pathway and an inhibitor of arginase significantly reduced intracellular polyamines, activated caspase-3 and induced apoptosis in the human breast cancer cell line MDA-MB-468. These actions of NOHA were abolished in the presence of exogenous L-ornithine suggesting that a reduction in the intracellular polyamine content might be responsible for the activation of caspase-3 and apoptotic actions of NOHA. In order to further explore this possibility, we used SAM-486A and alpha-difluoromethylornithine (DFMO), which are inhibitors of S-adenosylmethionine decarboxylase (SAMDC), and ornithine decarboxylase (ODC), respectively, either alone or in combination to reduce the intracellular polyamine levels. We then assessed whether a reduction in polyamine levels by these two compounds to a similar degree to that produced by NOHA activated caspase-3 which occurs prior to the onset of apoptosis. We observed that both SAM-486A and DFMO, either alone or in combination, inhibited cell proliferation, induced p21 and arrested cells in the G(0)-G(1) phase of the cell cycle but failed to activate caspase-3 as assessed by enzymatic assay of caspase-3, western blot analysis of the proteolytic cleavage of caspase-3 protein as well as TUNEL assay. Furthermore, pre-incubation of the cells with SAM-486A and DFMO for 4 days, either alone or in combination significantly inhibited the activation of caspase-3 and apoptosis by NOHA when compared with that observed with cells treated with NOHA alone. Our results, therefore, indicate that the activation of caspase-3 and apoptosis observed with NOHA cannot be solely explained by a reduction in intracellular polyamine levels and that other mechanisms need to be also considered.

High amounts of nitric oxide (NO) produced by activated macrophages or NO donors are required to induce cytotoxicity and apoptosis in pathogens and tumor cells. High concentrations of NO may lead to nonspecific toxicity thereby limiting the use of NO donors in the treatment of cancer. In this study, we tested the possibility of potentiating the apoptotic action of NO in a human breast cancer cell line, MDA-MB-468, by combining it with a farnesyltransferase inhibitor (FTI), which has been shown to induce apoptosis in some other cancer cell lines with minimal toxicity to normal cells. DETA-NONOate, a long acting NO donor which has a half-life of 20 h at 37 degrees C, was used in this study. DETA-NONOate (1 mM), which releases NO in the range produced by activated macrophages, induced apoptosis after 36 h in MDA-MB-468 cells via cytochrome c release and caspase-9 and -3 activation. FTI (25 microM) potentiated the action of lower concentrations of DETA-NONOate (25-100 microM) by inducing apoptosis in these cells within 24 h by increasing cytochrome c release and caspase-9 and -3 activation. This effect was observed preferentially in the cancer cell lines studied with no apoptosis induction in normal breast epithelial cells. This novel combination of FTI and NO may emerge as a promising approach for the treatment of breast cancer.

L-Arginine is the common substrate for two enzymes, arginase and nitric oxide synthase (NOS). Arginase converts L-arginine to L-ornithine, which is the precursor of polyamines, which are essential components of cell proliferation. NOS converts L-arginine to produce NO, which inhibits proliferation of many cell lines. Various human breast cancer cell lines were initially screened for the presence of arginase and NOS. Two cell lines, BT-474 and MDA-MB-468, were found to have relatively high arginase activity and very low NOS activity. Another cell line, ZR-75-30, had the highest NOS activity and comparatively low arginase activity. The basal proliferation rates of MDA-MB-468 and BT-474 were found to be higher than the ZR-75-30 cell line. N-Hydroxy-L-arginine (NOHA), a stable intermediate product formed during conversion of L-arginine to NO, inhibited proliferation of the high arginase-expressing MDA-MB-468 cells and induced apoptosis after 48 h. NOHA arrested these cells in the S phase, increased the expression of p21, and reduced spermine content. These effects of NOHA were not observed in the ZR-75-30 cell line, which expresses high NOS and relatively low arginase. The effects of NOHA were antagonized in the presence of L-ornithine (500 microM), which suggests that in MDA-MB-468 cell line, the arginase pathway is very important for cell proliferation. Inhibition of the arginase pathway led to depletion of intracellular spermine and apoptosis as observed by terminal deoxynucleotidyl transferase (Tdt)-mediated nick end labeling assay and induction of caspase 3. In contrast, the ZR-75-30 cell line maintained its viability and its L-ornithine and spermine levels in the presence of NOHA. We conclude that NOHA has antiproliferative and apoptotic actions on arginase-expressing human breast cancer cells that are independent of NO.

BackgroundMechanisms that increase resistance to apoptosis help promote cellular transformation. Cancer cells have deregulated apoptotic pathways, where increased expression and stability of anti-apoptotic proteins Mcl-1 and Bcl-2 increases resistance to apoptosis. Pathways that increase the stability of proteins in cancer cells remain poorly understood.MethodsUsing human mammary epithelial and established breast cancer cell lines, we assessed the mechanisms that increase the stability of anti-apoptotic proteins in breast cancer cells by caspase assay, western blot, small-inhibitory RNA treatment and immunoprecipitation.ResultsWhile breast cancer cells were resistant to de novo inhibition of protein synthesis, a rapid proteosome-mediated degradation of Mcl-1 and Bcl-2 induced apoptosis in mammary epithelial cells. Although Mule, an E3 ligase that targets Mcl-1 for degradation was expressed in mammary epithelial and breast cancer cell lines, rapid increase of polyubiquitinated Mcl-1 and Bcl-2 was detected only in mammary epithelial cells. Only transient formation of the Mule-Mcl-1 complex was detected in breast cancer cells. Downregulation of pERK1/2 in breast cancer cells reduced Mcl-1 levels and increased Mcl-1/Mule complex.ConclusionOur findings suggest that reduced Mule/Mcl-1 complex has a significant role in increasing the stability of Mcl-1 in breast cancer cells and increased resistance to apoptosis.