Abstract 4120: Low expression of PP2A/B55α is associated with increased MiR-191 expression and relapse in acute myeloid leukemia.

Acute myeloid leukemia (AML) is highly fatal so a better understanding of chemoresistance mechanisms is essential. Protein Phosphatase 2A (PP2A) B55α subunit is under-expressed in AML blast cells compared to normal CD34+ cells and patients with lower levels of the B subunit exhibit shorter remission duration (Ruvolo et al Leukemia, 2011). As remission duration is linked to relapse in AML, we have now examined B55α expression by Reverse Phase Protein Analysis (RPPA) in 47 paired samples taken from AML patients at first diagnosis and at relapse. We find B55α levels are reduced in AML blast cells during relapse. These data suggest suppressing the B subunit may activate survival pathways in the malignant cells that would shorten remission duration and promote relapse. To determine the role of B55α in AML cell survival, expression of the B subunit was blocked in OCI-AML3 cells using shRNA. Cells with B55α shRNA were more resistant to killing by the conventional chemotherapy agent AraC and the BH3 mimetic ABT-737 compared to OCI-AML3 cells containing a non-specific control shRNA. Consistent with RPPA data from AML patients, phosphorylation of AKT and PKCα was increased as were MYC levels in B55α shRNA cells. Though the role for PP2A as a regulator of signal transduction is well established, there is little if any information on how the enzyme regulates survival processes that are mediated by microRNAs (miRs). MiRs are emerging as important regulators of leukemic cell survival and proliferation. We postulated a role for B55α in miR regulation because the B subunit induces MYC, which in turn regulates many miRs including those in the mir17∼92 polycistron. MiR expression profiles were obtained by mircroarray analysis using OCI-AML3 B55α shRNA cells and control shRNA cells. We found that reduction of the B subunit resulted in at least a 2 fold reduction in 30 miRs. MiR-1260 showed the greatest change with a 23 fold decrease in expression. Only three miRs were increased by 2 fold or more in the B55α shRNA cells (i.e. miR-4290; miR-4508; miR-191-5p). The identification of miR-191-5p is especially intriguing as we previously found that AML patients with increased levels of this miR had worse overall survival (Garzon et al Blood 2008). Increased miR-191 expression has been observed in solid tumors, therefore it is possible that this miR supports tumor survival in many cancers. Interestingly, none of the members of the 17∼92 polycistron were affected by loss of the B subunit suggesting that the B subunit regulates miR expression by mechanisms independent of MYC. Since the B subunit has pro-death effects in the AML cells and MYC supports survival it is not surprising that MYC function may be suppressed. While the mechanism how B55α suppresses miR-191-5p and what proteins the miR targets in AML are not clear, a possible AML therapeutic strategy could be to target miR-191 either by activating the B subunit or targeting the miR itself. Citation Format: Peter P. Ruvolo, Vivian Ruvolo, Rodrigo Jacamo, Yihua Qiu, Kevin Coombes, Nianxiang Zhang, Ramiro Garzon, Steven M. Kornblau, Carlo Croce, Michael Andreeff. Low expression of PP2A/B55α is associated with increased MiR-191 expression and relapse in acute myeloid leukemia. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4120. doi:10.1158/1538-7445.AM2013-4120