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.

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 occurrence of chemoresistance is a serious problem in the treatment of cancer, urging the need for second and third-line treatment options that rely on different cell death pathways to overcome previously acquired resistance mechanisms. The inhibition of proteasomal activity by specific proteasome inhibitors or cross-reactivity of certain protease inhibitors with proteasomal enzymes recently became of interest because of the anti-tumoral properties of these agents. We tested the proteasome inhibitor bortezomib and the HIV protease inhibitor nelfinavir on human cervical cancer cells. Both drugs induced cell cycle arrest in cervical cancer cells, as reflected by marked changes in the expression of cell cycle-regulatory cyclins and ensuing mitochondrial-independent apoptosis. Upregulation of the molecular chaperone BiP and the cell stress marker ATF3 indicated induction of the unfolded protein response (UPR) as the main cause of apoptosis induced by these drugs in cervical cancer cells. Unlike in leukemia cells, bortezomib mainly inhibited the caspase-like activity of the proteasome in cervical cancer cells. Nelfinavir exhibited no effects on proteasomal activity in cervical cancer cells and leukemia cells. Although both bortezomib and nelfinavir acted on cisplatin-resistant cervical cancer cells (SiHa), neither of the drugs induced a sensitization to cisplatin treatment. Instead, both drugs could effectively be combined with each other, and enhanced the efficacy of an apoptosis-inducing TRAIL receptor antibody. These results suggest that both bortezomib and nelfinavir are effective agents against chemoresistant cervical cancer cells and might be of interest for clinical studies on cervical cancer patients with recurrent or metastatic cancer.

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.

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