Poster Presentation 25th Lorne Cancer Conference 2013

Determining the Role of Tropomyosins in Actin-Mediated Apoptosis (#156)

Melissa Desouza 1 , Margaret Ngyuen 1 , Peter Gunning 1 , Justine Stehn 1
  1. University of New South Wales, Sydney, NSW, Australia

The actin cytoskeleton is vital for biological processes such as migration, proliferation and apoptosis. Changes in actin filament dynamics may enable a cancer cell to evade apoptosis thus contributing to cancer progression. The mechanism for this process currently remains unclear. Tropomyosins are actin-regulating proteins that define the functional properties of actin filaments in an isoform-specific manner. Cellular transformation induces a reduction in high-molecular weight isoforms, such as Tm1, and increases reliance on low-molecular weight isoforms. Tm1 has also been highlighted as a sensor of apoptosis induced by cell detachment (anoikis). We aim to determine the role of specific tropomyosin filament populations in apoptosis and to exploit points of vulnerability in the transformed cytoskeleton as possible therapeutic targets. Rat neuroblastoma cells over-expressing Tm1 showed an increase in actin filament bundles and larger focal adhesions. These cells also exhibited an increased sensitivity to anoikis when grown on the adhesion-free-substrate PolyHEMA. The pro-apoptotic BH3 protein Bmf has been postulated as a sensor of actin filament integrity and, once activated via cell detachment, translocates to the mitochondria initiating apoptosis. To determine if tropomyosins interact with BH3 proteins to induce an apoptotic signal, Bax and Bak expression in Tm1 over-expressing cells were measured revealing an increase in Bak but no change in Bax expression. Mouse embryonic fibroblasts that are null for Bmf and Bax/Bak, also show a trend in the up-regulation of Tm1. In conclusion Tm1 may have an isoform-specific role in the regulation of anoikis via interaction with specific BH3 proteins. An altered tropomyosin expression in tumour cells may disrupt the apoptotic mechanism via changes in the regulation and/or translocation of BH3 proteins. Future studies will investigate the molecular-interaction between the actin cytoskeleton and BH3 proteins as well as examine which tropomyosin isoforms are important for regulating apoptosis in transformed cells.