Acquisition of apoptotic resistance in arsenic-induced malignant transformation: Role of the JNK signal transduction pathway

Abstract

This study examined the role of signal transduction and apoptosis in malignant transformation induced by arsenic. Prior study showed that chronic arsenite exposure (500 nM, ≥ 18 weeks) induced malignant transformation in rat liver TRL 1215 cells. In the present work, these transformed cells were compared with passage-matched control cells. In addition, TRL 1215 cells were treated subchronically (up to 6 weeks) with arsenic (termed pre-transformed cells) to define events occurring prior to arsenic-induced transformation. Flow cytometry using annexin/FITC revealed that arsenic-induced apoptosis in transformed cells was markedly suppressed in comparison to control or pre-transformed cells. Ro318220, a strong activator of JNK, enhanced arsenite-induced apoptosis in transformed cells. Densitometric analysis of western blots revealed that the ratios of both Bcl-xL/Bax and Bcl-2/Bax were significantly increased (>2.5-fold) in arsenic-transformed cells. Transformed, pre-transformed and control cells were treated with arsenic and levels of phosphorylated extracellular signal-regulated kinases, ERK1/2, JNK1/2 and p38 were determined by western blot analysis. The three mitogen-activated protein kinases (MAPKs) were phosphorylated in a dose-dependent fashion in all cell types. However, the levels of phosphorylated JNK1/2 were markedly decreased in the arsenic-transformed cells, whereas in pre-transformed cells the levels of phosphorylated MAPKs remained the same as in control cells. JNK kinase activity was suppressed in transformed cells whereas Ro318220 enhanced this activity. Thus, during arsenic-induced malignant transformation resistance to apoptosis develops, possibly due to perturbation of the JNK pathway.