Abstract #2796: Ligation of the CD44 adhesion molecule by the antibody A3D8 induces and enhances drug-induced apoptosis in acute myeloid leukemia cells through down-regulation of Mcl-1.

Cell surface molecule CD44, an extracellular matrix receptor, is highly expressed on acute myeloid leukemia blasts and plays important roles in many aspects such as cell survival, proliferation, differentiation, and death. Some of these events can be changed by ligation of CD44 with monoclonal antibodies. It has been found that CD44 monoclonal antibody A3D8 inhibits proliferation and induces incomplete differentiation/apoptosis in acute promyelocytic leukemia (APL, M3 type of AML) derived NB4 cells. The apoptotic effects of A3D8 in six AML cell lines, HL-60, NB4, ME1-2f, SKNO-1, U937 and THP-1 cells were investigated. NB4, ME1-2f (M4 type of AML) and SKNO-1 (M2 type of AML) cells are responsive to A3D8-induced apoptosis. A3D8 at a concentration of 2.5 \#956;g/ml induces more than 30 % of these cells to undergo apoptosis. HL-60 (M2 type of AML) cells are less sensitive to A3D8-induced apoptosis with less 20 % of cells undergoing apoptosis after treatment with A3D8 at a concentration of 2.5 \#956;g/ml. U937 and THP-1 cells (both M5 types of AML) are not sensitive to A3D8 treatment even at a concentration of 5 \#956;g/ml. Currently Ara-C is used for the treatment of AML and As₂O₃ is used for the treatment of APL. The combined effects of A3D8 with Ara-C or As₂O₃ were investigated in HL-60 cells. Pretreatment with A3D8 at 2.5 \#956;g/ml for 16 hours increases apoptosis induction (about 50%) by Ara-C or As₂O₃ in HL-60 cells. Correlated with apoptosis induction, the levels of antiapoptotic protein Mcl-1, but not Bcl-2, were down-regulated in NB4, ME1-2f and SKNO-1 cells. Although HL-60 cells are less sensitive to A3D8-induced apoptosis, the level of Mcl-1 was decreased which contributes to the enhancement of Ara-C and As₂O₃-induced apoptosis. Hyaluronan, which also binds to CD44, does not induce nor block A3D8-induced apoptosis. Our data suggest that A3D8 targets CD44 signaling through a mechanism different from that of hyaluronan. A3D8 alone or in combination with Ara-C or As₂O₃ could be used to selectively target AML stem/progenitor cells which express high levels of CD44 and are thought to be resistant to current therapy.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2796.