Abstract #4624: MKK7-dependent activation of SAPK/JNK is critical for parthenolide-mediated NF-\#954;
B inhibition to potentiate histone deacetylase inhibitor lethality in human leukemia cells

NF-\#954;B activation represents a barrier limiting the antileukemic activity of histone deacetylase inhibitors (HDACIs) based upon evidence that interruption of this signaling pathway enhances HDACI lethality in human leukemia cells. Notably, the latter appears to involve activation of the stress-related SAPK/JNK. However, mechanisms underlying this phenomenon remain to be explored. To establish the link between NF-\#954;B inhibition and the functional role of SAPK/JNK activation in potentiation of HDACI lethality, genetic approaches targeting SAPK/JNK pathways were employed to elucidate interactions between the NF-\#954;B inhibitor parthenolide (PTL) and the pan-HDACIs vorinostat (SAHA) or LBH589 in human leukemia cells. Coadministration of PTL sharply blocked HDACI-induced IKK phosphorylation, p65/RelA phosphorylation and nuclear translocation, and NF-\#954;B activation, accompanied by induction of mitochondrial injury, caspase activation, and apoptosis in a highly synergistic manner in various human leukemia cell lines (e.g. U937, Jurkat, HL-60). Significantly, these regimens were highly active against primary human AML blasts, but minimally toxic toward normal human mononuclear cells. Notably, these events were associated with a striking increase in SAPK/JNK activation. Moreover, transfection of U937 cells with either DN-JNK1 (JNK1/APF) or DN-MKK7 dramatically diminished PTL/HDACI-mediated JNK activation and lethality, whereas transfection with DN-MKK4 failed to do so. As both PTL and its clinically-relevant analog LC-1 have shown activity against leukemia progenitor/stem cells, the question of whether similar mechanisms might be involved in interactions between PTL and HDACIs in these cells was examined. In vitro colony assays indicated that cotreatment with PTL and vorinostat strongly inhibited the colony-forming ability (CFU) of AML blasts but were relatively sparing toward normal hematopoietic cells, suggesting selective targeting of leukemic progenitor cells. Parallel studies were performed in hematopoietic cells transfected with the MLL-ENL fusion protein which exhibit certain leukemia-initiating cell characteristics. Co-exposure of MLL-ENL cells to low concentrations of vorinostat (e.g., 0.5uM) or LBH589 (e.g. 4nM) in conjunction with PTL resulted in a dramatic increase in apoptosis. This phenomenon was also associated with downregulation of p65/RelA as well as activation of the SAPK/JNK signaling cascade. Together, these findings indicate that NF-\#954;B inhibition by PTL markedly potentiates HDACI lethality through a process involving MKK7- rather than MKK4-dependent activation of SAPK/JNK pathway. They also raise the possibility that these regimens may preferentially kill leukemia cells compared to their normal counterparts, and may also target leukemia progenitor/stem cells.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4624.