Up-Regulation of c-Jun contributes to the Induction of Apoptosis by Adaphostin in Human Multiple Myeloma Cells.

Adaphostin (NCI, Bethesda, MD) has been examined as an alternative to the 2-phenylaminopyrimidine derivate imatinib mesylate, with remarkable efficacy in the treatment of chronic myeloic leukemia (CML) including (1) Bcr/Abl+ cells; (2) imatinib mesylate resistant cells; and (3) Bcr/ Abl - cells. However exact functional mechanisms by which adaphostin induces cell apoptosis are unknown. Previously we have compared effects of adaphostin, a small molecule of the tyrophostin family versus imatinib mesylate in human MM cells: significant concentration - dependent antiproliferative the effects were observed after treatment with adaphostin, but not with imatinib mesylate, on MM cells alone and adherent to BMSCs. Here, we used gene expression microarray to identify signaling molecules modulated by adaphostin. As confirmed by protein expression, several proteins, most prominently c-Jun, were upregulated in a dose- and time- dependent manner. In contrast, c-Jun upregulation was not observed in MM cells treated with imatinib mesylate. Although PARP cleavage was induced, c-Jun upregulation was also not observed after treatment with dexamethasone (0.5 and 1mM), bortezomib (5 and 20nM) or melphalan (5 and 20mM). The functional relevance of adaphostin- induced c-Jun- upregulation was next investigated using small- interfering RNAs targeting c-Jun mRNA. Adaphostin- induced caspase- cleavage is abrogated in MM cells transfected with cJun- specific RNAi, but not with mock-RNAi. Consequently, a decrease of thymidine incorporation induced by adaphostin was only observed in MM cells transfected with mockRNAi, but not c-Jun- specific RNAi. Moreover, our results also demonstrate that adaphostin- induced caspase cleavage is dependent on the expression of c-Jun, but not visa versa. These studies for the first time provide evidence for a specific adaphostin- induced c-Jun upregulation and related induction of apoptosis in MM cells. Based on these findings, ongoing studies are investigating the role of c-Jun in MM pathogenesis.