Bafilomycin A1 Targets Both Autophagy and Apoptosis Pathways in Pediatric t(1;19) Pre-B Acute Lymphoblastic Leukemia Cells

The t (1; 19) subtype leukemia accounts for a quarter of pre-B acute lymphoblastic leukemia (ALL) and up to 5% of all ALL patients. Despite plausible remission rate, current treatment regimen on the pediatric pre-B ALL is associated with side effects and CNS relapse, which poses the need for more effective and safer drugs. Bafilomycin A1 (Baf-A1) is known as an inhibitor of late phase of autophagy by inhibiting fusion between autophagosomes and lysosomes as well as by inhibiting lysosomal degradation.

Here we show that Baf-A1 of low concentration (1 nM) effectively and specifically inhibits and kills the pre-B ALL cells. E2A/Pbx1 fusion gene positivepre-B ALL 697 cells were used for In vitro experiments. The results of flow cytometry analysis and western blotting showed that Baf-A1 induced cell cycle arrest and proliferation inhibition of ALL cells by upregualting cell cycle negative regulators and downregulating cell cycle positive regulators. In contrast, AML and CML cell lines were insensitive to Baf-A1 treatment. Western blotting and confocal observation on protein LC3 also showed that Baf-A1 at 1 nM blocked basal autophagic flux. Baf-A1 treatment activated mTOR signaling and induced the formation of Becn1–Bcl-2 complex to inhibit the induction of autophagy. Furthermore, apoptosis was induced in ALL cells treated with Baf-A1 for 72 h. However, procaspase-3 and poly-(ADP-ribose) polymerase (PARP) were not cleaved in these cells. We observed that AIF relocalized to the nucleus after 72h Baf-A1 treatment by confocal and immunoblotting. Knockdown of AIF significantly attenuated apoptosis induced by Baf-A1. These data suggest that Baf-A1 targets mitochondria membrane to trigger apoptosis via AIF pathway. In the in vivo experiment, Baf-A1 treatment extended survival and improved pathology of 697 xenograft mice, and significantly reduced the E2A/PBX1 positive leukemia cells in the bone marrow of mice. In vivomouse toxicity assay confirms Baf-A1 as a safe compound. The bone marrow cells of pre-B ALL leukemia patients were sorted against CD133+CD19+ markers, and treatment with Baf-A1 induced a clear inhibition on the CD133+CD19+ primary cells with a significant increased cell death in the sorted B-ALL patient samples. Conversely, Baf-A1 had no inhibitory effect on the bone marrow cells isolated from acute myeloid leukemia patients and healthy people.

In summary, Baf-A1 treatment at low concentration effectively and specifically inhibited autophagy by activating mTOR and inducing beclin1-Bcl-2 interaction and induced AIF-dependent apoptosis in t (1; 19) pre-B ALL 697 cells. In the pre-B ALL xenograft mouse model, Baf-A1 specifically targets the leukemia cells while sparing normal cells. More importantly, Baf-A1 potently inhibits and kills the primary cells from pediatric pre-B ALL patients both at initial diagnosis and relapse without compromising normal human hematopoietic cells, all proposing Baf-A1 as a promising drug candidate for this pre-B ALL.