Inhibition of vacuolar H+ ATPase enhances sensitivity to tamoxifen via up-regulation of CHOP in breast cancer cells

Antitumor and immunomodulatory activities of diphyllin and its derivatives

Immune surveillance plays a key role in controlling tumor formation and development, and immune cell-based therapies, such as chimeric antigen receptor (CAR)-T cells and CAR-natural killer (NK) cells, have become important for the treatment of cancer. The proton pump (PP), vacuolar H+-ATPase (V-ATPase), acidifies intracellular organelles, pumps protons across the cell plasma membranes, and regulates the activity of various signaling pathways, and thus has been regarded as a potential target for cancer treatment. In addition, V-ATPase plays an important role in cytotoxic T lymphocytes, extracellular vesicle (EV) endocytosis, innate immune responses (IIR), and phagocytosis and hence has the potential to function as a target for the enhancement of immunotherapy. As potent V-ATPase inhibitors, the arylnaphthalene lignans, diphyllin and its derivatives, have exhibited potent antitumor and immunomodulatory activities. The structurally related aryltetralin lignan, podophyllotoxin, has served as a lead compound for both etoposide and teniposide, which have been developed as effective anticancer agents. In the present review, the role of V-ATPase in cancer immunotherapy and the structure-activity relationships (SARs) of diphyllin and its cytotoxic and V-ATPase inhibitory activities and the mechanisms of action are discussed. Also, the promise of diphyllin and its derivatives in the development of new adjuvants for cancer immunotherapies has been proposed.

S6K1 inhibition enhances the apoptotic cell death of breast cancer cells in response to Bcl-2/Bcl-xL inhibition by the downregulation of survivin

Breast cancer cells possess a deregulated apoptotic pathway with increased expression levels of anti-apoptotic B-cell lymphoma-2 (Bcl-2) family proteins and ribosomal S6 kinase 1 (S6K1) protein activity. Therefore, combined interference of anti-apoptotic Bcl-2 family and S6K1 protein expression may be a reasonable therapeutic strategy for the treatment of patients with breast cancer. In the present study, it was identified that the administration of a combination of ABT263 [navitoclax; a Bcl-2/Bcl-extra large (Bcl-xL) inhibitor] and PF4708671 (an S6K1 inhibitor) markedly increased apoptotic cell death in the BT474 breast cancer cells compared with the administration of either agent alone. Furthermore, the downregulation of Bcl-2/Bcl-xL and S6K1 with small interfering RNA induced a significant increase in cell death compared with RNA interference of either agent alone. Notably, combination treatment with ABT263 and PF4708671 decreased the expression level of survivin protein, with this ectopic expression of survivin attenuating cell death. Thus, the present study determined that the combined inhibition of Bcl-2/Bcl-xL and S6K1 may be a good strategy for treating patients with breast cancer.

BRAF-induced tumorigenesis is IKKα-dependent but NF-κB-independent

KRAS mutations contribute to cell proliferation and survival in numerous cancers, including colorectal cancers (CRC). One pathway through which mutant KRAS acts is an inflammatory pathway that involves the kinase IKK and activates the transcription factor NF-κB. BRAF, a kinase that is downstream of KRAS, is mutated in a subset of CRC and is predictive of poor prognosis and therapeutic resistance. We found that, in contrast to mutant KRAS, mutant BRAF (BRAF(V600E)) did not trigger NF-κB activation but instead triggered the phosphorylation of a proteolytic fragment of IKKα (p45-IKKα) in CRC cells. BRAF(V600E) CRC cells had a high abundance of phosphorylated p45-IKKα, which was decreased by a RAF inhibitor. However, the abundance and DNA binding of NF-κB in these cells were unaffected by the RAF inhibitor, and expression of BRAF(V600E) in human embryonic kidney-293T cells did not activate an NF-κB reporter. Moreover, BRAF-induced transformation of NIH-3T3 cells and BRAF-dependent transcription required phosphorylation of p45-IKKα. The kinase TAK1, which was associated with the endosomal compartment, phosphorylated p45-IKKα. Inhibition of endosomal vacuolar adenosine triphosphatase (V-ATPase) with chloroquine or bafilomycin A1 blocked p45-IKKα phosphorylation and induced apoptosis in BRAF-mutant CRC cells independent of autophagy. Treating mice with V-ATPase inhibitors reduced the growth and metastasis of BRAF(V600E) xenograft tumors in the cecum of mice.

Controlling the unfolded protein response-mediated life and death decisions in cancer

Cancer cells are exposed to intrinsic (oncogene) or extrinsic (microenvironmental) challenges, leading to activation of stress response pathways. The unfolded protein response (UPR) is the cellular response to endoplasmic reticulum (ER) stress and plays a pivotal role in tumor development. Depending on ER stress intensity and duration, the UPR is either pro-survival to preserve ER homeostasis or pro-death if the stress cannot be resolved. On one hand, the adaptive arm of the UPR is essential for cancer cells to survive the harsh conditions they are facing, and on the other hand, cancer cells have evolved mechanisms to bypass ER stress-induced cell death, thereby conferring them with a selective advantage for malignant transformation. Therefore, the mechanisms involved in the balance between survival and death outcomes of the UPR may be exploited as therapeutic tools to treat cancer.

Piperlongumine induces cell death through ROS-mediated CHOP activation and potentiates TRAIL-induced cell death in breast cancer cells

PURPOSE

Piperlongumine (PL) has been shown to selectively induce apoptotic cell death in cancer cells via reactive oxygen species (ROS) accumulation. In this study, we characterized a molecular mechanism for PL-induced cell death.

METHODS

Cell viability and cell death were assessed by MTT assay and Annexin V-FITC/PI staining, respectively. ROS generation was measured using the H2DCFDA. Small interfering RNA (siRNA) was used for suppressing gene expression. The mRNA and protein expression were analyzed by RT-PCR and Western blot analysis, respectively.

RESULTS

We found that PL promotes C/EBP homologous protein (CHOP) induction, which leads to the up-regulation of its targets Bim and DR5. Pretreatment with the ROS scavenger N-acetyl-cysteine abolishes the PL-induced up-regulation of CHOP and its target genes, suggesting an essential role for ROS in PL-induced CHOP activation. The down-regulation of CHOP or Bim with siRNA efficiently attenuates PL-induced cell death, suggesting a critical role for CHOP in this cell death. Furthermore, PL potentiates TRAIL-induced cytotoxicity in breast cancer cells by upregulating DR5, as DR5 knockdown abolished the sensitizing effect of PL on TRAIL responses.

CONCLUSIONS

Overall, our data suggest a new mechanism for the PL-induced cell death in which ROS mediates CHOP activation, and combination treatment with PL and TRAIL could be a potential strategy for breast cancer therapy.