PIM2 Upregulation Leads to Physiological Buffering of Metformin-Induced Cell Death Mediated by ER Stress/UPR in Acute Lymphoblastic Leukemia,

Abstract 3496

Acute Lymphoblastic Leukemia (ALL) is the most common malignancy in children and adolescents. Despite significant overall improvements in cure rates, outcome remains dismal for patients with resistant phenotypes or after relapse. Therefore, novel treatment strategies are warranted. Recently, we identified the AMP activated protein kinase (AMPK), a regulator of energy homeostasis in eukaryotic cells, as a potential target for ALL therapy due to its effects on cell growth, proliferation, and cell cycle regulation, as well as its crosstalk with critical metabolic and oncogenic pathways. We showed that activation of AMPK using metformin (1-5 mM) induced significant cell growth inhibition and apoptosis in CCRF-CEM (T-ALL) and NALM6 (Bp-ALL) cell line models. Western blot analysis revealed that metformin led to activation of p-AMPK (Thr172) and its downstream target p-ACC (Ser79), the cell proliferation regulator p-Akt (Ser473), and the protein translation regulator p-4EBP1 (Thr70), suggesting that protein translation may be an important determinant in the mechanism of metformin-induced cell death. Indeed, we demonstrated that blocking protein translation with the mTOR inhibitor rapamycin (1 μg/ml) rescued ALL cells from metformin-induced cell death (p < 0.01). In addition, knockdown of AMPK1α expression using shRNAs (shAMPK) abrogated metformin-induced growth inhibition and apoptosis in ALL cells as compared to control cells expressing scramble shRNAs (shCTRL), indicating that AMPK mediates metformin's cytotoxicity in our models. Western blots demonstrated that ALL cells expressing shAMPK exhibit decreased expression of total AMPK, p-p38MAPK (Thr180), p-mTOR (Ser2448), and p-4EBP1 (Thr70) compared to shCTRL cells, implicating regulation of protein translation in the mechanism of cell death induced by metformin. In addition, metformin-induced p-Akt (Ser473) activation observed in shCTRL cells is blocked in shAMPK expressing cells, suggesting that the contextual crosstalk between AMPK and Akt is relevant for metformin's cytotoxicity. Indeed, experiments co-targeting Akt and AMPK using perifosine (6 μM) or the Akt inhibitor X (AIX, 5 μM) plus metformin (5 mM) for 72 h induced synergistic cell death in NALM6 cells (Combination Index (CI) values of 0.21 for perifosine + metformin, and 0.19 for AIX + metformin). Our studies uncovered that apoptotic death in NALM6 and CCRF-CEM cells treated with metformin correlated with metformin's induction of ER stress/UPR in ALL cells, as demonstrated by increased expression of the UPR markers IRE1α and CHOP. More important, rapamycin rescued metformin-treated ALL cells by relieving ER stress/UPR as demonstrated by decreased IRE1α and CHOP. These observations support our previous findings that ER stress/UPR mediates cell death in ALL cells under metabolic stress, and is tightly coupled to regulation of protein translation (Mol Cancer Ther 10:437, 2011). To further investigate the relationship between protein translation and ER stress/UPR, we examined the role of PIM1/2 kinases, particularly PIM2 known to regulate CAP protein translation, in metformin-induced ALL cell death. Our results indicate that expression of PIM2 is significantly increased in NALM6 cells treated with metformin (5–10 mM) for 72 h. We also observed concomitant decrease in the expression of the UPR markers IRE1α, ATF6, and CHOP, raising the possibility that PIM2 upregulation may be a compensatory survival mechanism to regulate protein translation and suppress metformin-induced ER stress/UPR. To test this hypothesis, we co-treated NALM6 cells with the small molecule PIM1/2 kinase inhibitor V (80 μM) and metformin (5 mM) and found that inhibition of PIM2 in metformin-treated NALM6 cells induced synergistic cell death (CI = 0.28). Taken together, our data indicate that PIM2 plays a role in buffering cell death in metformin treated cells, and that regulation of protein translation modulates ER stress/UPR induced apoptosis in ALL cells. Consequently, our data support strategies that exploit synthetic lethality by combining activators of AMPK such as metformin and compounds that target regulation of protein translation or protein degradation as suitable for clinical translation in patients with ALL.