Purpose: Arsenic trioxide (ATO) induces B-cell chronic lymphocytic leukemia (B-CLL) cell apoptosis in vitro. We sought to study the mechanism involved in this effect and whether ATO is suitable for combination therapies with protein kinase inhibitors.
Experimental design: B-CLL cells were isolated from the peripheral blood of 28 patients. Cell viability studies with ATO alone or in combination with kinase inhibitors were done by flow cytometry, Western blotting, and immunofluorescence analyses.
Results: After 48 hours, 3 mumol/L ATO induced apoptosis (average 75%) in all B-CLL samples studied and with minimal effect on normal peripheral blood lymphocytes. Apoptosis entailed Akt and NF-kappaB inactivation, XIAP downregulation, and PTEN upregulation, thus implying inhibition of the phosphoinositide 3-kinase (PI3K) survival pathway. Indeed, the combination of ATO and PI3K inhibitors increased the apoptotic effect of either agent alone. ATO also induced c-jun-NH(2) terminal kinase (JNK) activation, and this was crucial and required for subsequent apoptotic events, as inhibiting JNK activity by either gene silencing or specific inhibitors prevented Akt and NF-kappaB inactivation, caspase activation, and mitochondrial damage. Moreover, JNK activation was the earliest response to ATO, preceding and determining reactive oxygen species production.
Conclusions: We identified the mechanism involved in ATO action on B-CLL cells and show that the combination of low doses of ATO and PI3K inhibitors efficiently induces B-CLL cell death. ATO may therefore constitute an efficient treatment for B-CLL, particularly in combined therapies.