The nucleoside antagonist cordycepin causes DNA double strand breaks in breast cancer cells.

The fungal drug cordycepin (3-deoxyadenosine) is known to exert anti-tumor activities, preferentially by interfering with RNA synthesis. We have investigated the effect of cordycepin on human breast epithelial cell lines, ranging from non-malignant MCF10A cells to highly de-differentiated MDA-MB-435 cancer cells. Treatment of human breast cancer cells with cordycepin caused either apoptosis or persistent cell cycle arrest that was associated with reduced clonal growth of cordycepin-treated breast cancer cells. Highly de-differentiated breast cancer cell lines, such as MDA-MB-231 and MDA-MB-435, reacted more sensitive to cordycepin than less aggressive breast cancer cell lines (MCF7, T47D) or non-malignant breast epithelial cells (MCF10A), which poorly reacted to cordycepin. In cordycepin-sensitive breast cancer cells, a marked induction of the DNA damage response (DDR), including the phosphorylation of ATM, ATR, and histone γH2AX could be observed. These data indicate that cordycepin, which was believed to cause cancer cell death by inhibition of RNA synthesis, induces DNA double strand breaks in breast cancer cells. The genotoxic effect of cordycepin on breast cancer cells indicates a new mechanism of cordycepin-induced cancer cell death, and its activity against highly undifferentiated breast cancer cells provides a new perspective of how cordycepin may be used in the treatment of advanced breast cancer.

The HIV reverse transcriptase inhibitor tenofovir induces cell cycle arrest in human cancer cells.

Selected HIV drugs, either of the protease inhibitor type or the nucleoside antagonist type, have been shown to exert tumoricidal effects. Here, we show that the HIV reverse transcriptase inhibitor Truvada, a combination drug of the cytidine analogue emtricitabine and the adenosine analogue tenofovir, induces DNA damage and cell cycle arrest in human cancer cells. Phosphorylation of the DNA repair enzyme H2AX by emtricitabine/tenofovir indicated that it interfered with the integrity of the DNA and replication machinery in human cancer cells. Long term incubation of cancer cells with emtricitabine/tenofovir caused the formation of multi-nuclear giant cells, further indicating DNA replication problems. When tested as single agents, the anti-tumoral activity of emtricitabine/tenofovir was predominantly caused by tenofovir, although the combination with emtricitabine enhanced its effect on cancer cells. Combined with established anti-cancer drugs, emtricitabine/tenofovir was preferentially found to enhance the cytotoxic effect of doxorubicin, a promising drug for the treatment of relapsed, chemoresistant cancer. These results show that especially the adenosine analogue tenofovir could be used to interfere with the proliferation machinery of human cancer cells and to be applied for chemosensitization of cancer cells to already established DNA-interacting drugs.

Nelfinavir induces mitochondria protection by ERK1/2-mediated mcl-1 stabilization that can be overcome by sorafenib.

The HIV protease inhibitor nelfinavir is an investigational drug for cancer treatment. We have previously demonstrated induction of apoptosis by nelfinavir even in chemo-resistant ovarian cancer cells. In contrast to the pro-apoptotic effect of nelfinavir on human cancer cells, we noticed a significant upregulation of the anti-apoptotic mitochondrial membrane protein mcl-1 by nelfinavir, resulting in a mitochondria-independent induction of apoptosis. Upregulation of mcl-1 was associated with enhanced phosphorylation of both mcl-1 and of ERK1/2 (extracellular signal-regulated kinases 1/2). ERK1/2 enhanced stability of mcl-1 protein expression by serine-163 phosphorylation. The combination of nelfinavir with sorafenib, a clinically applied inhibitor of the RAS/RAF/ERK1/2 pathway, inhibited nelfinavir-induced ERK1/2 activation and mcl-1 protein upregulation. Further, the combination of nelfinavir with sorafenib induced mitochondrial membrane potential disruption and resulted in an improved activity of nelfinavir on ovarian cancer cells. Thus, a combination of these two investigational anti-cancer drugs could be of interest especially because of their unique mechanism of apoptosis induction even in otherwise chemo-resistant human cancer cells.

Bortezomib treatment of ovarian cancer cells mediates endoplasmic reticulum stress, cell cycle arrest, and apoptosis.

Bortezomib, an approved drug for the treatment of certain haematological neoplasms, is currently being tested in clinical trials as a potential therapeutic agent against several types of solid cancer, including ovarian cancer. We have analyzed the effect of bortezomib on ovarian cancer cells and tissue explants either as a single agent or in combination with carboplatin, taxol, or TRAIL (tumor necrosis factor-related apoptosis-inducing ligand). Bortezomib alone efficiently induced apoptosis in ovarian cancer cells. Apoptosis was preceded by an upregulation of the endoplasmic reticulum stress sensor ATF3, and increased the expression of cytoplasmic heat shock proteins. Bortezomib enhanced the sensitivity of ovarian cancer cells and tissue explants to an apoptosis-inducing TRAIL receptor antibody by upregulating the TRAIL receptor DR5. In contrast to the synergistic effect observed for TRAIL, the efficacy of the taxol treatment was reduced by bortezomib, and bortezomib inhibited the G2/M phase accumulation of ovarian cancer cells treated with taxol. Bortezomib alone or in combination with taxol induced a cell cycle arrest within the S phase, and downregulation of cdk1, a cyclin-dependent kinase that is necessary for the entry into the M phase. Thus, bortezomib can be regarded as a promising agent for the treatment of ovarian cancer and could either be administered as a single agent or in combination with TRAIL. However, a combination treatment with taxanes may not be beneficial and may even be less effective.

Nelfinavir induces the unfolded protein response in ovarian cancer cells, resulting in ER vacuolization, cell cycle retardation and apoptosis.

Proteasome inhibitors and protease inhibitors are currently being discussed to be useful to sensitize drug-resistant cancer cells to chemotherapeutic agents or to act independently as single agents on drug-resistant cancer cells. We tested the effect of the clinically applied HIV protease inhibitor nelfinavir on ovarian cancer cells. Nelfinavir efficiently induced cell death in carboplatin-sensitive (SKOV3, OV-GH-5) and carboplatin-resistant (OVCAR3, OV-GH-1) ovarian cancer cell lines as well as in cancer biopsies and ascites samples from patients with recurrent ovarian cancer. Nelfinavir significantly changed the morphology of ovarian cancer cells, resulting in formation of large ER-derived vacuoles and induced upregulation of the hsp70 heat shock family member BiP (GRP78) which accumulated within swollen ER membranes. Upregulation of BiP and phosphorylation of eIF2alpha indicated induction of the unfolded protein response, which can cause cell cycle arrest and apoptosis. Correspondingly, we observed downregulation of cell cycle regulatory proteins after nelfinavir treatment, especially that of cyclin D3, and induction of apoptosis as confirmed by annexin binding. Because nelfinavir represents an already approved drug for use in humans with HIV infection, it could rapidly be tested in clinical studies as a potential treatment strategy against drug-resistant ovarian cancer.

Nelfinavir induces TRAIL receptor upregulation in ovarian cancer cells.

HIV protease inhibitors are currently being discussed to be useful as new and alternative anti-cancer agents, especially as second line treatments for chemo-resistant human cancer types. Among three clinically applied HIV protease inhibitors tested, we found a high efficacy of nelfinavir on ovarian cancer cells, accompanied by apoptosis (annexin binding) and necrosis (propidium iodide permeability). In vitro, at concentrations used to induce cell death in ovarian cancer cells, nelfinavir had no effect on the cellular viability of fibroblasts or peripheral blood mononuclear leukocytes. Nelfinavir sensitized ovarian cancer cells to treatment with an apoptosis-inducing TRAIL receptor antibody due to upregulation of the TRAIL receptor DR5 as shown by RT-PCR and FACScan analysis. We conclude that nelfinavir, an already approved drug, is a highly efficient agent against ovarian cancer cells and could sensitize ovarian cancer cells to TRAIL treatment, either therapeutically applied or endogenously produced by cells of the immune system.