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The intracellular pH (pHi) of cancer cells such as the KB-3-1 (human epidermoid carcinoma cell line) and K562 cells (human chronic myeloid leukemia cell line) cultured in a medium (pH 7.4) was found to be much higher (pH 7.65 and 7.8, respectively) than that of normal cells (pHi is usually < or =7.2). When a phenoxazine derivative, 2-aminophenoxazine-3-one (Phx-3) or 2-amino-4,4alpha-dihydro-4alpha-7-dimethyl-3H-phenoxazine-3-one (Phx-1) was added to these cells, pHi rapidly decreased within 20 min, dose-dependently, though the extent of the decrease of pHi was significantly larger for Phx-3 (a decrease of 0.9 units) than for Phx-1 (a decrease of 0.4 units). Phx-3 and Phx-1 caused the proliferative suppression of these cells 24 h after the addition, dose-dependently. The anti-proliferative effects of Phx-3 on KB-3-1 and K562 cells were far greater than those of Phx-1. It was proposed that the proliferative suppression of KB-3-1 and K562 cells caused by Phx-3 and Phx-1, may be preceded by a rapid and extensive decrease in pHi, which possibly influenced the intracellular homeostasis, finally causing the suppressed proliferation and apoptosis of these cancer cells. The present results suggest that the anti-cancer effects of Phx-3 and Phx-1 may be strengthened by the intracellular acidification of cancer cells by these compounds.

The aim of the present study was to investigate the mechanism of apoptosis in human multiple myeloma cell line, U266, caused by 2-aminophenoxazine-3-one (Phx-3). Flow-cytometrical and morphological analyses showed that Phx-3 increased the population of annexin V-positive cells including early stage apoptotic cells and late stage apoptotic cells and induced DNA fragmentation or apoptotic body formation in U266 cells, indicating that Phx-3 induced the apoptosis of U266 cells. Activity of caspase-3 was extensively increased in U266 cells treated with Phx-3 time-dependently within 24 h, but this Phx-3-stimulated activity of the enzyme in the cells was completely cancelled by the addition of N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), a pan-caspase inhibitor. The addition of z-VAD-fmk almost blocked the apoptotic effect of Phx-3 against U266 cells, indicating that Phx-3-induced apoptosis of U266 cells was dependent on a caspase signaling pathway. Moreover, the apoptosis of U266 cells occurred after the induction of cell cycle arrest of the cells in the S and G(2)/M phase, the loss of mitochondrial membrane potential, and activation of caspase-3 reached maximum, which were caused by Phx-3 within 24 h. These results support the views that the apoptosis of U266 cells caused by Phx-3 may be preceded by the cell cycle arrest, depolarization of mitochondria and activation of caspase-3. These results support the view that Phx-3 may be utilized in future as chemotherapeutic agent against multiple myeloma which is extremely refractory to chemotherapy.

The aim of this study was to investigate the anticancer effects of the phenoxazine derivatives, 2-amino-4,4alpha-dihydro-4alpha,7-dimethyl-3H-phenoxazine-3-one (Phx-1), 3-amino-1,4alpha-dihydro-4alpha,8-dimethyl-2H-phenoxazine-2-one (Phx-2), and 2-aminophenoxazine-3-one (Phx-3) on human pancreatic cancer cell lines, KLM-1 and MIA-PaCa-2, in combination with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a member of the tumor necrosis factor superfamily of cytokines. Of these three phenoxazines, Phx-1 and Phx-3 inhibited proliferation of KLM-1 dose-dependently, but Phx-2 did not. Phx-3 caused both apoptosis and necrosis in KLM-1 cells, as evidenced by the phosphatidylserine externalization and propidium iodide permeable cells detected by a flow cytometric method using annexin-V and propidium iodide. Down-regulation of Bcl-2 expression appeared to be involved in the Phx-3-induced cell death. TRAIL did not affect proliferation of KLM-1, and the inhibitory effects of Phx-1 and Phx-3 on the KLM-1 cell line were not augmented by the combination with TRAIL. On the other hand, proliferation of the MIA-PaCa-2 cell line was not affected by Phx-1, Phx-2 and Phx-3, although it was significantly inhibited by TRAIL in a dose-dependent manner. Inhibitory effects of TRAIL on MIA-PaCa-2 were synergistically augmented by the addition of Phx-1 and Phx-3, but not by Phx-2. These results suggest that both Phx-1 and Phx-3 exert anticancer effects against human pancreatic cancer cells, KLM-1 and MIA-PaCa-2, through distinct action modes. Phx-1 and Phx-3 may be effective for the treatment of pancreatic cancer.

STI571, an Abl-specific tyrosine kinase inhibitor, selectively kills Bcr-Abl-containing cells in vitro and in vivo. However, some chronic myelogenous leukemia (CML) cell lines are resistant to STI571. We evaluated whether STI571 interacts with P-glycoprotein (P-gp) and multidrug resistance protein 1 (MRP1), and examined the effect of agents that reverse multidrug resistance (MDR) on the resistance to SI571 in MDR cells. STI571 inhibited the [(125)I]azidoagosterol A-photolabeling of P-gp, but not that of MRP1. K562/MDR cells that overexpress P-gp were 3.67 times more resistant to STI571 than the parental Philadelphia-chromosome-positive (Ph +) CML K562 cells, and this resistance was most effectively reversed by cepharanthine among the tested reversing agents. The concentration of STI571 required to completely inhibit tyrosine phosphorylation in K562/MDR cells was about 3 times higher than that in K562 cells, and cepharanthine abolished the difference. In KB-G2 cells that overexpress P-gp, but not Bcr-Abl, 2.5 micro M STI571 partly reversed the resistance to vincristine (VCR), paclitaxel, etoposide (VP-16) and actinomycin D (ACD) but not to Adriamycin (ADM) or colchicine. STI571 increased the accumulation of VCR, but not that of ADM in KB-G2 cells. STI571 did not reverse resistance to any agent in KB/MRP cells that overexpress MRP1. These findings suggest that STI571 is a substrate for P-gp, but is less efficiently transported by P-gp than VCR, and STI571 is not a substrate for MRP1. Among the tested reversing agents that interact with P-gp, cepharanthine was the most effective agent for the reversal of the resistance to STI571 in K562/MDR cells. Furthermore, STI571 itself was a potent reversing agent for MDR in P-gp-expressing KB-G2 cells.

An angiogenic factor, platelet-derived endothelial cell growth factor/thymidine phosphorylase (PD-ECGF/TP), stimulates the chemotaxis of endothelial cells and confers resistance to apoptosis induced by hypoxia. 2-Deoxy-D-ribose, a degradation product of thymidine generated by TP, partially prevents hypoxia-induced apoptosis. TP is expressed at higher levels in tumor tissues compared to the adjacent non-neoplastic tissues in a variety of human carcinomas. High expression of TP is associated with an unfavorable prognosis. To investigate the effect of TP on cisplatin-induced apoptosis, human leukemia Jurkat cells were transfected with wild-type or mutant (L148R) TP cDNA. TP inhibited a number of steps in the cisplatin-induced apoptotic pathway, activation of caspases 3 and 9 and mitochondrial cytochrome c release. These findings suggest a mechanism by which TP confers resistance to apoptosis induced by cisplatin. Moreover, mutant TP that has no enzymatic activity also suppressed cisplatin-induced apoptosis. These findings indicate that TP has cytoprotective functions against cytotoxic agents which are independent of its enzymatic activity.