STL-1, a new Akt inhibitor, synergizes with flavonoid quercetin in enhancing type-I and type-II cell death in a chronic lymphocytic leukemia-derived cell line

Chronic lymphocytic leukemia (CLL) is the most common form of leukemia in the adult population. A significant percentage of patients becomes resistant to traditional chemotherapy treatment. New therapeutic strategies, which may include the use of bioactive molecules from natural sources, can be effective in CLL treatment. Previous studies showed that, in CLL cells, quercetin, the major flavonoid present in the Western diet, is able to optimize the efficacy of new CLL drugs acting as BH3 mimetics (e.g., ABT-737) by inhibiting protein kinase CK2 and the PI3K/Akt pathway (1). The present communication aims to biochemically characterize the combined effect of quercetin and a new Akt inhibitor, namely STL-1, identified throughout an in silico screening. HG-3 cells, derived from a human CLL have been treated with 20-40 ?M STL-1 concentrations in the presence/absence of quercetin (5-10 ?M). Cell viability was measured by the CyQuant assay, while the additive or synergistic effect, following the combined treatment, was evaluated by the calculation of the Combination Index (C.I.). Single-treatment with STL-1 did not induce a significant cytotoxicity in HG-3 cells, while co-incubation with quercetin induced a synergistic effect (C.I. <1), reducing cell viability of 20% and 60% at 24 and 48 h, respectively. The antiproliferative effect of the combined treatment was attributed to type-I cell death (apoptosis) measured with caspase-3 activation (1.5 fold increase after 6 h compared to untreated controls) and apoptotic nuclei quantification (>30%). Moreover, quercetin and STL-1 in mono-treatment induced a protective form of autophagy, which was bypassed when the two agents were combined. In fact, the co-treatment stimulated type-II cell death (autophagy associated cell death) in HG-3 cells, as indicated by a 30% increase in Cyto-ID staining (specific for autophagosome quantification) and a 6-fold increase in LC3-II (a marker of autophagy pathway). These results confirm that the PI3K/Akt pathway represents the "Achilles' heel" in CLL and support the evidence that cocktails of synthetic and natural occurring compounds can be effective in chemotherapeutic protocols. This study also proposes quercetin as a potential adjuvant agent in clinical trial against CLL. (1) Russo M, Milito A, Spagnuolo C, Carbone V, Rosén A, Minasi P, Lauria F, Russo GL. CK2 and PI3K are direct molecular targets of quercetin in chronic lymphocytic leukaemia. Oncotarget. 2017;8(26):42571-42587