Cisplatin-induced apoptosis in human malignant testicular germ cell lines depends on MEK/ERK activation

Testicular germ cell tumours (TGCT) represent the most common malignancies in young males. Whereas in 1970s, the survival rate in patients with metastatic testicular tumours was only 5%, these days, 80% of the patients treated by modern chemotherapy will survive their disease. The drug that revolutionised the cure rate for patients with metastatic testicular tumours was cisdiamminedichloroplatinum (cisplatin, CDDP). In vitro experiments on neoplastic germ cell lines showed that their exquisite sensitivity to CDDP could be attributed to p53-dependent and -independent pathways. Applying cDNA macroarray, semiquantitative RT–PCR and Western blot analyses, blocking experiments, caspase activity assays, and morphological methods, we sought here to define the p53-independent pathway(s) involved in the CDDP-induced apoptosis. For this purpose, we used the human TGCT cell line NCCIT, the mutated p53 of which is known to remain inactive during the course of CDDP-induced apoptosis. Our experiments showed that within hours of CDDP application, two prototype members of the ‘mitogen-activated protein kinase’ (MAPK) family, designated ‘MAPK ERK kinase’ (MEK) and ‘extracellular signal-regulated kinase’ (ERK), were dually phosphorylated and caspase-3 became active. Functional assays using MEK inhibitors demonstrated that the phosphorylation of MEK and ERK was required for the activation of caspase-3 as the executing caspase. Interestingly, experiments with the human malignant germ cell line NTERA, which is known to possess wild-type p53, revealed the same results. Thus, our data suggest that CDDP mediates its p53-independent apoptosis-inducing effect on the malignant human testicular germ cells – at least partially – through activation of the MEK–ERK signalling pathway.