Consistent with data for prostate cancer and leukemia cells, our results indicate that blockage of PI3K by LY294002 selleck chem overcomes resistance towards TRAIL in HCC cells[22,23]. The mTOR, a protein with growing clinical relevance in oncology, is located downstream of PI3K[41]. The significant sensitization towards TRAIL in Hep-G2 cells by mTOR inhibition underlines a pivotal role of PI3K/Akt signaling for the resistance of HCC towards TRAIL. In addition, the MAPK/ERK pathway exerts antiapoptotic effects in cancer cells. The MEK is a key component downstream of Raf serine/threonine kinases[42,43]. MEK inhibitors have been described as sensitizing human cancer cells to apoptosis, e.g. after treatment with chemotherapeutic agents[44,45].
In this study, we observed no apoptosis induction and only a slight decrease of cell viability after MEK inhibition in HCC cells. However, the combination of MEK inhibition and TRAIL caused a significant increase of TRAIL-induced apoptosis. This observation suggests that an abberantly activated Raf/MAPK/ERK pathway plays a crucial role for TRAIL resistance in HCC. Furthermore, we focused on the EGFR, which is an upstream receptor in Ras-Raf-MEK-ERK signaling[46]. It has been shown that overexpression of EGFR represents a protective factor against apoptosis stimuli in HCC[47,48]. The combined treatment of TRAIL with the specific EGFR kinase inhibitor AG1478 caused a significant increase of TRAIL-induced apoptosis in HCC cells. Thus, EGFR blockage is another promising approach for TRAIL sensitization of HCC cells.
Recently, it has been shown that JNK inhibition sensitizes HCC cells, but not healthy hepatocytes, towards TRAIL-induced apoptosis[49]. In contrast, other results indicate that JNK activation is not relevant for TRAIL-induced apoptosis[50]. We found a significantly increased proapoptotic effect of TRAIL if combined with the JNK inhibitor SP600125. Aberrant activity of survival signaling pathways exerts antiapoptotic effects at least in part via triggering of the expression of antiapoptotic proteins, such as antiapoptotic BCL-2 proteins. Importantly, antiapoptotic BCL-2 proteins, such as MCL-1 and BCL-xL, have been described as contributing to TRAIL resistance in cancer cells[51]. MCL-1 and BCL-xL mainly act by directly inhibiting their proapoptotic relatives BAX and BAK, thereby guarding the cell from various death stimuli.
In addition, expression of antiapoptotic BCL-2 proteins is a prognostic factor for various tumor entities, e.g. expression of MCL-1 in breast and gastric cancer[52,53]. In the liver, MCL-1 has been found to be a key factor for apoptosis regulation[13,54,55]. A lack of MCL-1 causes increased rates of apoptosis and a significantly higher susceptibility AV-951 towards chemotherapeutic treatment in HCC[54]. In addition, it has been shown that MCL-1 acts as a key factor for resistance towards TRAIL in leukemia cells[56].