Activation of apoptosis pathways by different classes of anticancer drugs

Abstract

It is known that in empirical medicine, different anticancer drugs have different characteristic concerning specific anti-tumor or anti-leukemic efficacy and side effects on normal tissue. While solid tumors are often treated with cisplatin, treatment of leukemia is based on the use of anthracyclines and antimetabolites. Many drugs also have unique toxicities affecting normal tissues, such as the cardiotoxicity associated with the anthracyclines, the hemorrhagic cystitis associated with the cyclophosphamide and ifosfamide. The peripheral neuropathy from vincristine, and the coagulopathy from L-asparaginase. We therefore hypothesized that the different clinical used anticancer drugs might induce apoptosis in a drug specific manner. Thus the clinical observed difference could be reflected by different activation of apoptosis signaling pathways. The molecular signaling pathways which are initiated in response to anticancer drug-induced cellular damage, and lead to the eventual apoptosis death of the cell, are largely undefined. Previous studies show the caspase family are the critical executioners of apoptosis and anticancer drug-induced apoptosis by activating two major cell-intrinsic pathways, one that begins with ligation of cell surface death receptors, such as CD95, and another that involves mitochondrial release of cytochrome c. We therefore investigated induction of apoptosis, activation of caspases and involvement of mitochondrial signaling in the well defined Jurkat human leukemic T cells by five conventional used anticancer drugs: etoposide, cytarabine, 4-hydroxy-cyclophosphamide, doxorubicin, and methotrexate, in order to identify drug specific activation of distinct apoptosis pathways. In present studies, we found some difference of apoptosis induced by these five anticancer drugs: (1) The anticancer drug-induced apoptosis appeared in different time kinetics, etoposide and cytarabine were early acting drugs, while 4-hydroxy-cyclophosphamide, doxorubicin, and methotrexate were late acting drugs. (2) Interestingly, higher doses of cytarabine induced less apoptosis, whereas lower doses of cytarabine induced more apoptosis. (3) Similar to CD95, early drug-induced PS externalization was earlier than drug-induced membrane integrity loss, whereas late drug-induced PS externalization and membrane integrity loss occurred in the same time, similar to g-radiation-induced apoptosis. (4) Etoposide strongly induced caspases activation, compared to cytarabine, 4-hydroxy-cyclophosphamide, doxorubicin and methotrexate. Besides these differences, we found that all drugs induced apoptosis in a similar manner by all anticancer drugs. All tested drugs can activate caspase-8, caspase-3 and cleave PARP. Furthermore, these five drug-induced apoptosis could be inhibited by the pan-caspase inhibitor zVAD-fmk, suggesting that drugs induce apoptosis in a caspase dependent way. All drugs induce mitochondrial transmembrane potential reduction and all drug-induced DYm loss cannot be blocked by caspase inhibitor zVAD-fmk, indicating that caspases are not required for early mitochondrial changes. All drugs induced cytochrome c translocation from mitochondria into cytosol. This suggests that mitochondrial signaling and caspase activation are commonly activated in programmed cell death induced by cytostatic drugs used for anti-cancer treatment. Since all tested drugs induced caspase activation and mitochondrial apoptosis signaling, measurement of these parameters can be used for assessment of drug efficacy in primary leukemia cells. The different characteristics of anticancer drugs, e.g. time of apoptosis induction, are probably due to events upstream of the common mitochondrial and caspase signaling pathways.