Myeloid leukemic cells (M1) proliferate continuously in culture unless induced by Interleukin-6 (IL6) to undergo a terminal differentiation program into macrophages, followed by apoptosis. M1 cells lack the tumor suppressor p53, which is a critical determinant of the cellular decision to either growth arrest and repair DNA damage or to undergo apoptosis. Activation of a temperature sensitive p53 protein (p53 val) at the permissive temperature in M1 cells results in rapid apoptosis. IL6 treatment blocks this p53-mediated apoptosis. Towards understanding the basis for this p53-mediated apoptosis and its abrogation by IL6, we have shown that at the permissive temperature p53 activates the pro-apoptotic Fas/CD95 pathway by up regulating the Fas/CD95 receptor and cleaving antiapoptotic c-FLIP. On the other hand, antagonistic Fas antibody protects against apoptosis. IL6 decreases Fas/CD95 ligand expression and prevents cleavage of FLIP. Treatment of M1p53ts cells with the specific Akt inhibitor Ly294002 abrogated IL6 protection and resulted in Flip cleavage, suggesting that IL-6 blocks apoptosis by phosphorylating and activating P13kinase/Akt, which in turn promotes FLIP stability. Inhibition of the ERK pro-survival pathway did not abrogate IL6 protection against p53 apoptosis like Akt inhibition caused. Nevertheless M1p53ts-Flip cell lines fail to completely protect against p53 mediated apoptosis, suggesting that FLIP is itself insufficient to protect against apoptosis. Therefore we examined what other proteins may synergize with FLIP to protect from apoptosis, and found that that the pro-survival bcl2 protein family member MCL-1 is strongly up regulated by IL6 in our M1p53 cell line. Generation of M1p53-FLIP-MCL1, M1p53-MCL1, and M1p53 -uncleavable FLIP cell lines is underway to determine whether there is synergy between MCL-1 and FLIP in escape from p53 mediated apoptosis of M1p53 cells. Thus far taken together these data support a model for leukemic progression where cells that acquire the ability to produce an autocrine survival factor, such as IL6, can bypass normal p53 surveillance function by targeting downstream inhibitors of apoptosis such Akt, c-Flip and MCL-1.

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