Role of the p38 mitogen-activated protein kinase pathway in the generation of arsenic trioxide-dependent cellular responses

Arsenic trioxide (As₂O₃) induces differentiation and apoptosis of leukemic cells in vitro and in vivo, but the precise mechanisms that mediate such effects are not known. In the present study, we provide evidence that the kinases MAPK kinase 3 (Mkk3) and Mkk6 are activated during treatment of Ieukemic cell lines with As₂O₃ to regulate downstream engagement of the p38 mitogen-activated protein kinase. Using cells with targeted disruption of both the Mkk3 and Mkk6 genes, we show that As ₂O₃-dependent activation of p38 is defective in the absence of Mkk3 and Mkk6, establishing that these kinases are essential for As₂O₃-dependent engagement of the p38 pathway. Pharmacologic inhibition of p38 enhances As2O3-dependent activation of the c-jun NH₂-terminal kinase (JIVK) and subsequent induction of apoptosis of chronic myelogenous leukemia (CML)- or acute promyelocytic leukemia (APL)-derived cell lines. In addition, in APL blasts, inhibition of p38 enhances myeloid cell differentiation in response to As₂O₃, as well as suppression of Bcl-2 expression and loss of mitochondrial membrane potential. Similarly, induction of As₂O₃-dependent apoptosis is enhanced in mouse embryonic fibroblasts (MEF) with targeted disruption of both the Mkk3 and Mkk6 genes, establishing a key role for this pathway in the regulation of As₂O₃-induced apoptosis. In other studies, we show that the small-molecule p38 inhibitors SD-282 and SCIO-469 potentiate As₂O₃-mediated suppression of myeloid leukemic progenitor growth from CML patients, indicating a critical regulatory role for p38 in the induction of antileukemic responses. Altogether, our data indicate that the Mkk3/6-p38 signaling cascade is activated in a negative regulatory feedback manner to control induction of As₂O ₃-mediated antileukemic effects.