Tumor suppressor tropomyosin Tpm2.1 regulates sensitivity to apoptosis beyond anoikis characterized by changes in the levels of intrinsic apoptosis proteins

Abstract

The actin cytoskeleton is a polymer system that acts both as a sensor and mediator of apoptosis. Tropomyosins (Tpm) are a family of actin binding proteins that form co-polymers with actin and diversify actin filament function. Previous studies have shown that elevated expression of the tropomyosin isoform Tpm2.1 sensitized cells to apoptosis induced by cell detachment (anoikis) via an unknown mechanism. It is not yet known whether Tpm2.1 or other tropomyosin isoforms regulate sensitivity to apoptosis beyond anoikis. In this study, rat neuroepithelial cells overexpressing specific tropomyosin isoforms (Tpm1.7, Tpm2.1, Tpm3.1, and Tpm4.2) were screened for sensitivity to different classes of apoptotic stimuli, including both cytoskeletal and non-cytoskeletal targeting compounds. Results showed that elevated expression of tropomyosins in general inhibited apoptosis sensitivity to different stimuli. However, Tpm2.1 overexpression consistently enhanced sensitivity to anoikis as well as apoptosis induced by the actin targeting drug jasplakinolide (JASP). In contrast the cancer-associated isoform Tpm3.1 inhibited the induction of apoptosis by a range of agents. Treatment of Tpm2.1 overexpressing cells with JASP was accompanied by enhanced sensitivity to mitochondrial depolarization, a hallmark of intrinsic apoptosis. Moreover, Tpm2.1 overexpressing cells showed elevated levels of the apoptosis proteins Bak (proapoptotic), Mcl-1 (prosurvival), Bcl-2 (prosurvival) and phosphorylated p53 (Ser392). Finally, JASP treatment of Tpm2.1 cells caused significantly reduced Mcl-1, Bcl-2 and p53 (Ser392) levels relative to control cells. We therefore propose that Tpm2.1 regulates sensitivity to apoptosis beyond the scope of anoikis by modulating the expression of key intrinsic apoptosis proteins which primes the cell for death.

Keywords: Bcl-2 proteins; actin; intrinsic apoptosis; mitochondria; tropomyosin.