Kazuhiro Washio

PTEN activity could be a predictive marker of trastuzumab efficacy in the treatment of ErbB2-overexpressing breast cancer

Trastuzumab is the only HER2/neu-directed therapy to have received Food and Drug Administration approval for the treatment of patients with metastatic breast cancer. The efficacy of trastuzumab depends on the HER2/neu status of the tumour and the patients prior treatment, but even when patients are selected on the basis of HER2/neu gene amplification, the single-agent response rate ranges from 12 to 30% and few patients respond to trastuzumab monotherapy. Here, we propose PTEN as a predictive biomarker for trastuzumab efficacy. Human breast cancer SKBR3 and drug-resistant SKBR3/R cells were investigated. We also examined clinical samples from patients who had been treated with trastuzumab and analysed the relationship between trastuzumab efficacy and PTEN level. The PI3K/Akt signalling pathway was observed to be highly active in the drug-resistant cells, and their level of PTEN was low. Delivery of antisense PTEN duplex siRNA significantly decreased the trastuzumab chemosensitivity of parental SKBR3 cells, and marked activation of Akt signalling pathway was also recognised. Moreover, immunohistochemical investigation revealed that trastuzumab treatment was remarkably successful in cells with elevated PTEN expression. Along with the immune-system-associated cytotoxic mechanism, several mechanisms have been proposed for the effect of trastuzumab. PTEN activity might play an important and major role in its HER2/PI3K/Akt-mediated antitumour effect, and could be a useful biomarker for predicting the efficacy of trastuzumab in the treatment of breast cancer.

Proteasome inhibitors can alter the signaling pathways and attenuate the P‐glycoprotein‐mediated multidrug resistance

Numerous signaling pathways were reported to be involved in the resistance for conventional cytotoxic drugs, although one of the main reasons is the overexpression of P‐glycoprotein (P‐gp) in multidrug resistant cancer cells. The overexpression of P‐gp has been associated with the resistance to a wide range of anticancer drugs. Doxorubicin and paclitaxel are substrates of this transporter system and have an important role for the various human malignancies. In the present study, drug‐sensitive MCF7 and multidrug resistant MCF7/ADR (characterized by overexpression of P‐gp) human breast cancer cell lines were used as an experimental model. We have found that PS341 and MG132, proteasome inhibitors, reduced the degree of the multidrug resistance (MDR) in MCF7/ADR cells. This phenomenon was accompanied by a decrease in the IC50 value of doxorubicin and paclitaxel from 55.9 ± 3.46 to 0.60 ± 0.08 μM, and from 17.61 ± 1.77 to 0.59 ± 0.12 μM, respectively. The IC50 values of sensitive cells for doxorubicin and paclitaxel were about 0.42 and 0.83 μM, respectively. The effect of PS341 and MG132 on MCF7/ADR cells was associated with a significant decrease in both protein and gene levels of P‐gp expression. Moreover, with regard to the expression of possible signal transduction pathways of mitogen‐activated protein kinase (MAPK) related to the activation of mdr1, proteasome inhibitors did significantly influence the activation of these proteins. Western blot analysis revealed that 24 hr exposure of multidrug resistant MCF7/ADR cells with proteasome inhibitors did change the levels of DNA binding activity of nuclear factor‐kappaB (NF‐kappaB), pERK1/2, c‐Jun, and p‐c‐Jun. In conclusion, we could remark that proteasome inhibitors (especially PS341) attenuate the resistance of MCF7/ADR cells for P‐gp substrate drugs of doxorubicin and paclitaxel. Several proteins are supposed to be associated with the resensitization of the cells to conventional cytotoxic drugs, although decreased activity of P‐gp is at least involved in the proteasome inhibitor‐related resensitization. And influence with MAPK pathways, which have been reported to be associated with the regulation of P‐gp, might be contributed to the resensitization brought by proteasome inhibitors.

Antitumor effects and drug interactions of the proteasome inhibitor bortezomib (PS341) in gastric cancer cells

The proteasome inhibitor bortezomib (PS341) inhibits the function of the 26S proteasome and has been extensively investigated in the clinical setting of hematologic malignancies. Remarkable efficacy has been reported in the treatment of multiple myeloma, but there have been few studies of its use in the treatment of gastrointestinal malignancy, especially gastric cancer. Here, we demonstrate its efficacy, both alone and in combination with other cytotoxic agents, in gastric cancer cell lines. The human gastric cancer cell lines AZ521, MKN45 and NUGC3 were used as experimental models. Bortezomib produced significant growth inhibition in these cells (mean IC50 values: 1.26, 9.44 and 8.63 μmol/l, respectively) and was also observed to decrease the activity of the extracellular signal-regulated kinase 1/2 and Akt signal pathways, increasing the accumulation of p21. Cell-cycle analysis revealed that a low concentration of bortezomib (10–100 nmol/l) increased accumulation in the G1 phase. Moreover, bortezomib showed synergistic growth inhibition in combination with the conventional cytotoxic agents 5-fluorouracil, paclitaxel, doxorubicin and SN-38, and also downregulates the activity of nuclear factor -κB, which is induced by these agents. Our results demonstrate that bortezomib could be an effective antitumor agent in the treatment of gastric cancer, both as single-agent therapy and in combination with conventional chemotherapeutic agents.