Tip30-induced apoptosis requires translocation of Bax and involves mitochondrial release of cytochrome c and Smac/DIABLO in hepatocellular carcinoma cells

TIP30 (Tat-interacting protein 30), a newly found proapoptotic factor, appears to be involved in multiple functions including metabolic suppression, apoptosis induction, and diminishing angiogenic properties. In the present study, we reported that mitochondrial events were required for apoptosis induced by TIP30 in hepatocellular carcinoma cells (HCC cells). Translocation of Bax was essential for TIP30-induced apoptosis, whereas overexpression of the anti-apoptotic protein Bcl-xL delayed both second mitochondria-derived activator of caspases (Smac/DIABLO) release and onset of apoptosis. Furthermore, TIP30-induced apoptosis was dependent on caspase activity because the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (OMe)-fluoromethyl ketone (z-VAD-fmk) blocked DNA fragmentation. Release of Smac/DIABLO from the mitochondria through the TIP30-P53-Bax cascade was required to remove the inhibitory effect of XIAP (X-linked Inhibitor of Apoptosis) and allowed apoptosis to proceed. Our results showed for the first time that Bax-dependent release of Smac/DIABLO, cytochrome c and AIF from the mitochondria mediated the contribution of the mitochondrial pathway to TIP30-mediated apoptosis. Our data suggested that adenovirus-mediated overexpression of TIP30 was capable of inducing therapeutic programmed cell death in vitro by activating the mitochondrial pathway of apoptosis. On the basis of these studies, elucidating the mechanism by which TIP30 induces cell death might establish it as an anticancer approach.