Many cytotoxic chemotherapeutic drugs sensitize cancer cells to TRAIL by increasing its receptor expression [29]. In this respect, STI571 did not change caspase 8 activation caused by TRAIL, ruling out STI571′s action is related to death receptor expression or activation of upstream death www.selleckchem.com/products/Roscovitine.html signals. Moreover, we conducted immunoblotting with DR4 and DR5 antibodies and flow cytometry to detect surface DR5 expression. The lack of any effects in these experiments (data not shown) indicates that STI571 does not change expression of the death receptors. TRAIL-induced apoptosis has been shown to involve p38 and JNK followed by caspase 3 activation in HeLa and HCT116 cells [4,29]. Thus, sensitizing cancer cells to TRAIL through activating JNK and p38, which subsequently regulate pro-apoptotic and anti-apoptotic Bcl-2 family members and p53, becomes a promising approach to cancer therapy [41-44].

Using pharmacological inhibitors, we showed the involvement of JNK and p38 in TRAIL-induced cytotoxicity and in STI571-induced cell protection in HCT116 cells. Under conditions of p38 or JNK inhibition, TRAIL-elicited cell death was inhibited. Moreover, STI571 also inhibited activation of both stress kinases induced by TRAIL, but no longer exerted its cytoprotection when TRAIL-elicited MAPK activation was already abolished. We thus suggest that inhibition of JNK and p38 are involved in STI571-induced protection. Activation of c-Abl by certain DNA-damaging agents contributes to cell apoptosis via p53-dependent and -independent mechanisms.

First of all, Yuan and colleagues found that c-Abl is activated by infrared and in turn leads to G1 growth arrest via a p53-dependent mechanism. However, they also noted that transfecting p53-/- cells with wild-type c-Abl could still sensitize cell apoptosis in response to DNA damage, whereas expressing the kinase dead c-Abl could not [10]. Later, they identified p73, a homologue of p53, as a downstream mediator of c-Abl for inducing cell apoptosis [37,45]. c-Abl was shown to stabilize p73 through phosphorylation-dependent posttranslational regulation [17,18,45-47]. To determine if c-Abl and p73 are targets of STI571 in initiating cytoprotection, we silenced c-Abl and p73 using the siRNA approach. As the results seen in experiments using the kinase inhibitor, we found that downregulation of c-Abl or p73 rendered cells less sensitive to TRAIL for JNK and p38 activation as well as for cell apoptosis. We therefore conclude that c-Abl-dependent p73 Anacetrapib activation is involved in TRAIL-induced apoptosis in HCT116 cells. Moreover, in agreement with previous findings [29,48], we did not observe effects of TRAIL to increase protein expression of p53 and Bax in p53-proficient HCT116 cells (data not shown).