Translational studies of drug-induced tumor cell death

Abstract

It is essential to understand the mechanisms of action of anticancer drugs in order to obtain good treatment results. I have in the present thesis studied clinically used anticancer agents as well as other agents that induce tumor cell death. Cisplatin, an important agent used to treat various forms of malignancies, was found to have a more complex mode of action than previously appreciated. Different doses of cisplatin were found to induce two distinct cellular outcomes: low doses induce senescence and high doses apoptosis via a mechanism involving reactive oxygen species and calcium. Interestingly, these outcomes appeared to be triggered by different signaling mechanisms: apoptosis by a cytoplasmic mechanism and senescence by DNA damage.

The plant alkaloid ellipticine was identified as an agent that synergizes with cisplatin. Detailed studies showed that ellipticine induces an endoplasmic reticulum stress response characterized by induction of GRP78/BiP expression and splicing of the XBP1 transcription factor. Our studies of cisplatin and ellipticine emphasize the complex modes of action of many anticancer drugs.

Calcium signaling has been implicated in apoptosis induced by many agents, including cisplatin. In order to characterize the role of calcium signaling in apoptosis, we examined apoptosis by 40 different compounds that were identified by screening a chemical library. Our study suggested that calpain and calmodulin kinase II are important mediators of calcium signaling in apoptosis. The requirement for calcium was generally late during the apoptotic process, and calcium signaling was implied in activation of Jun N-terminal kinase.

Another aim of the work was to develop a method to monitor the activity of anticancer drugs in vivo. We measured caspase-cleaved cytokeratin 18 fragments in patient serum during cancer therapy. The method generated interesting clinical information. Docetaxel was found to be more effective in inducing tumor apoptosis in prostate cancer patients compared to two other agents. Of particular interest was that apoptosis was induced during a number of treatment cycles. By measuring the fraction of caspase-cleaved to total cytokeratin 18 in patient serum we obtained evidence suggesting that the CEF combination used to treat breast cancer induces a predominantly necrotic response. Improved methods to assess drug effects in vivo are expected to aid clinical development of new drugs and to be quite useful in clinical work.