Apoptotic effect of Vibrio vulnificus cytolysin on A549 human lung adenocarcinoma cells

Vibrio vulnificus cytolysin (VVC) has a very strong cytotoxic effect on various types of mammalian cells. However, the inhibitory effect of VVC on the proliferation of human lung cancer cells has scarcely been reported. This study aimed to analyze the effects of recombinant VVC (rVVC) on the A549 human lung adenocarcinoma cell line and to investigate the underlying molecular mechanisms governing these effects. This study showed that rVVC inhibited the proliferation of A549 cells in a concentration- and time-dependent manner (as measured by the MTT assay). rVVC failed to induce the release of intracellular lactate dehydrogenase (LDH) from the target cells suggesting that osmotic lysis may not contribute to its cytolysin-induced cytotoxicity. Treatment of A549 cells with an IC50 (concentration of drug that inhibits cell growth by 50%) of rVVC resulted in morphological changes and blebbing typical of apoptosis. Annexin-V FITC analysis by FCM indicated that rVVC-induced apoptosis in A549 cells occurs in a dose-dependent manner. A DNA fragmentation assay was utilized to investigate the apoptosis induced by rVVC. The pro-apoptotic activity of rVVC was attributed to its ability to modulate, in a concerted manner, the expression of Bcl-2 and Bax proteins, which were down- and up-regulated, respectively. Caspase-9 and -3 were subsequently activated, however caspase-8 was not. These results prove that rVVC effectively induces programmed cell death and suggests that rVVC-induced apoptosis in the A549 cell line is mediated by the regulation of Bcl-2 protein expression and the activation of caspase-9 and -3.