Akt2 mediates glucocorticoid resistance in lymphoid malignancies through FoxO3a/Bim axis and serves as a direct target for resistance reversal

Glucocorticoids (GCs) are widely used drugs in the treatment of lymphoid malignancies; resistance of GCs in lymphocytes confers poor prognosis and the mechanisms are poorly understood. Here, we found T-acute lymphoblastic leukemia (T-ALL) cells acquire resistance to dexamethasone (DEX)-mediated killing through abnormal activation of Akt, resulting in inhibition of the FoxO3a/Bim pathway. The resistant state was reported to be associated with increased glycolysis, NOTCH1 activating mutations and activated PI3K/ serum GS regulated kinases (SGK) pathway. Use of aforementioned pathway inhibitors blocked FoxO3a-phosphorylation and partially improved DEX-mediated killing of GC-resistant T-ALL cells, further revealing the essential role of the FoxO3a/Bim pathway in the development of GC resistance. Inhibition of Akt is most effective at restoring sensitivity to DEX of GC-resistant lymphocytes in vitro and in vivo, but shows significant hepatotoxicity in vivo. A significantly elevated expression of Akt2 not Akt1 in intrinsically, secondarily GC-resistant lymphocytes and relapsed/refractory ALL patients implicates a more specific target for GC resistance. Mechanistically, Akt2 has a stronger binding capacity with FoxO3a compared to Akt1, and acts as a direct and major negative regulator of FoxO3a activity driving GC resistance. Pharmacologic inhibition of Akt2 more effectively restores sensitivity to GCs than inhibition of Akt1 in vitro, shows higher synergistic effect acting with DEX, and reverses GC resistance in GC-resistant T- or B- lymphoid tumors in vivo with reduced liver toxicity. In summary, these results suggest that Akt2 might serve as a more direct and specific kinase mediating GC resistance through FoxO3a/Bim signaling pathway, and Akt2 inhibition may be explored as a promising target for treating GC-resistant hematopoietic malignancies.