Mitochondria and cellular energy metabolism in platinum chemotherapy

Abstract

Chemotherapy is a major strategy in the treatment of cancer. Unfortunately, treatment of advanced cancers often fails due to tumor resistance to chemotherapeutic drugs. Studies of cell death signaling induced by chemotherapeutic agents are of importance to develop strategies for improved therapy, either by development of new drugs or by potentiation of existing drugs. Discovery of markers for prediction of therapeutic outcome is also of great importance for identification of responsive patients and thereby improved treatment.

This thesis presents results on apoptotic stress-signaling induced by the platinum chemotherapeutic drug cisplatin, and on combining chemotherapeutic drugs with inhibitors of cell metabolism as a potentiation strategy.

MEKK1, a kinase in the MAPK-signaling pathway, was shown to be responsible for cisplatin induced activation of the mitochondrial proapoptotic protein Bak, as seen by conformational change of this protein. However, activation of Bak was not sufficient to induce apoptosis unless signaling from another protein of the MAPK pathway, JNK, was present. Gel filtration experiments revealed Bak complexes of sizes between 80 to 170 kDa in cells with cisplatin-induced onset of apoptosis. By chemical inhibition and gene knock-out, JNK was shown to be crucial for the formation of these complexes.

p53 is involved in responses to platinum drugs. Here, a novel mechanism for negative regulation of p53 translocation to mitochondria is presented. Induction of iNOS after cisplatin treatment was shown to inhibit mitochondrial translocation of p53 as evaluated by chemical inhibition of iNOS. There was no upregulation of iNOS after treatment with oxaliplatin; however, addition of exogenous nitric oxide abrogated mitochondrial translocation of p53 after treatment with this drug.

Seventeen clinical and experimental drugs were screened for potentiation of apoptosis together with glycolysis inhibitor 2-deoxyglucose (DG) and inhibitor of fatty acid beta-oxidation etomoxir in HCT116 colon carcinoma cells. DG was more potent than etomoxir in this respect. Cytotoxic responses to combination treatment varied and included apoptosis, necrosis and growth arrest. The combination of cisplatin and DG showed substantial increase in apoptotic response compared to cisplatin alone.

Because treatment of ovarian cancer involves platinum drugs, this combination was further studied in two ovarian carcinoma cell lines and primary ovarian carcinoma cells purified from ascites. In the cell lines, higher use of glucose was coupled to increased resistance to cisplatin and carboplatin. In the primary tumor cells, low expression of mitochondrial beta-F1-ATPase correlated with reduced cisplatin IC50 in the presence of DG.