The chromosomal translocation t(14;18) in follicular lymphoma (FL) cells is a primary oncogenic event resulting in BCL-2 over-expression. BCL-2 is not directly responsible for growth of tumor cells, but it protects them against death by binding pro-apoptotic proteins. This study investigates in vitro and in vivo activity of the BH3 mimetic venetoclax (ABT-199) and mechanisms of acquiring resistance in FL.
Primary FL cells with similar expression of anti-apoptotic proteins, such as BCL-2 and MCL-1, showed variable sensitivity to treatment, which correlated with expression of BIM. To further study this correlation we used two t(14;18)+ cell lines, WSU-FSCCL and FC-TxFL2. As with primary cells, FC-TxFL2 cells with higher expression of BIM protein were more sensitive to venetoclax-induced apoptosis. The treatment caused dissociation of BCL-2/ BIM complexes, decreased mitochondrial potential, activation of caspase-3 and caspase-7, and PARP cleavage. Interestingly the population of cells that survived venetoclax treatment showed an increase of phospho-ERK1/2 and phospho-BIM (S69), as well as a decrease of total BIM levels.
Venetoclax treatment of murine xenografts of FC-TxFL2 cells resulted in significant delay in tumor growth. However, viable cells isolated from treated tumors showed a decrease of sensitivity to venetoclax exposure (IC50 increased almost 10 times) in comparison with the cells from the mice treated with vehicle only. These cells contained higher levels of MCL-1 and lower levels of BIM protein compared to control cells. BCL-2 levels remained unchanged.
Venetoclax resistant cells prepared in vitro by continuous treatment with increasing concentrations of venetoclax initially showed elevated levels of p-AKT, p-FOX01/3a, MCL-1, dissociation of BIM/BCL-2/BECLIN1 complex, decrease of p62 level (a sign of an increased autophagy) together with a slight decline of BIM expression. After a stable resistant cell line was established, a significant reduction of BCL-2 levels and almost total absence of BIMEL, BIML and BIMS isoforms was observed.
Detailed knowledge of venetoclax action and the mechanisms for acquired resistance in t(14;18)+ cells may help inform biomarker strategies and dosing schedules in future clinical trials. Inhibition of pathways activated in order to protect the FL cells (e.g. by down-regulating BIM (ERK or AKT pathway) or by autophagy) can significantly augment venetoclax potency and decrease incidence of acquired resistance.
Citation Format: Juraj Bodo, Xiaoxian Zhao, Andrew J. Souers, Darren C. Phillips, Mitchell R. Smith, Eric D. Hsi. BIM, a key player in apoptosis induction and acquired venetoclax resistance in follicular lymphoma cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B58.
- ©2015 American Association for Cancer Research.