Tropomyosin in Normal and Malignant Cells and the Action of Picropodophyllin on the Microfilament and Microtubule Systems

Abstract: Cell motility is a fundamental process, enabling cells to migrate, for instance during embryogenesis, tissue repair and defense. Force is generated by two protein systems, which also participate in cell proliferation, control macromolecular and organelle distribution and determine the fine structure of the cell interior. The major components of these are actin and tubulin, respectively, and they are referred to as the microfilament and the microtubule systems. This thesis focuses on tropomyosin, one of many microfilament associated proteins coupled to actin dynamics and organization and expressed in several isoform variants. Altered distribution and isoform expression of tropomyosin are signatures of malignant cells and are dealt with in the current thesis. The presence of tropomyosin isoforms in protruding lamellipodia of migrating cells is demonstrated, and a method to fractionate tropomyosin depending on its organization in an easily extractable, and a more tightly bound cytoplasmic form is presented. Analysis of the loosely associated tropomyosin fraction by gel filtration chromatography revealed that most of the tropomyosins in this fraction exist in a multimeric form. It was also observed that the distribution of tropomyosin varied between non-transformed and transformed cells with most of the isoforms enriched in the loosely bound fraction in the latter category of cells. Possibly this reflects the extensive reorganization of the microfilament system observed in cancer cells and which, depending on the context, can be normalized by introduction of certain tropomyosin isoforms.Many anti-cancer drugs target the microtubule system, inhibit cell division and promote apoptosis. Here it is shown that picropodophyllin, which has promising anticancer properties has a destabilizing effect on microtubules and via the microfilament system causes cells to detach from their substratum. Furthermore, picropodophyllin interferes with stimulation of the insulin-like growth factor receptor, which is involved in growth stimulation, differentiation and survival and whose expression is up-regulated in cancer cells.