Zinc as a potential therapy for Burkitt’s lymphoma

Abstract

Burkitts lymphoma (BL) is a form of non-Hodgkin lymphoma (NHL) that arises from germinal center B cells. BL is characterized by translocations of the C-MYC oncogene to immunoglobulin light and heavy chain loci resulting in its constitutive deregulated expression. BL shows a rapid and aggressive growth pattern. There are three different forms of BL; sporadic BL (sBL), immunodeficiency-associated BL and endemic BL (eBL) which accounts for ~50% of all paediatric cancers in Sub-Saharan Africa. Due to financial restrictions, treatment and supportive care options are limited resulting in poorer outcomes in low - middle income countries (LMICs). Thus, there is a need to develop new affordable effective low toxicity treatments for eBL. Prior to this study, a panel of BL cell lines were tested against an in-house custom drug repurposing library developed in our lab (FMC Library) that contains ~100 approved and commonly used drug. This screen identified the nutritional supplement zinc acetate as an effective anti-BL candidate. Dose response studies showed that all BL cell lines tested had little/no response to zinc at 50 μM whereas 100 μM zinc killed all BL cell lines. In contrast, 100 μM zinc acetate induced no killing against a panel of non-BL cell lines including acute myeloid leukaemia (AML) which is a non BL cell tumour, diffuse large B cell lymphoma (DLBCL) which represent a B cell lymphoma that arise from germinal centre B cells and EBV infected lymphoblastoid cell lines (LCL) as a control cells. The latter were used as karyotypically normal B cell controls. Cell death in BL cells was associated with positive flow cytometry staining for propidium iodide and annexin V and activation of caspase 3 and 9 (western blotting) indicating cell death by apoptosis. The proto-oncogene C-MYC is mutated or deregulated in >50% of cancers. In BL, deregulated expression occurs as a consequence of translocation of C-MYC on chromosome 8q24 to either the immunoglobulin heavy chain enhancer region on 14q32 (85% of cases) or the immunoglobulin kappa light chain or lambda loci on 2p12 or 22q11, respectively (15% of cases). Thus, the effect of zinc on C-MYC protein levels were studied. Western blot analysis showed that 100 μM zinc was able to reduce C MYC protein levels rapidly and sustainably in BL cell lines whereas no change in C MYC protein levels was observed in non-BL cell lines. Zinc-induced reduction of C-MYC protein levels was time-dependent, reducing by approximately 20% after 6 hours with little/no protein detectable after 24 hours. C-MYC protein levels were not reduced following treatment with 50 μM zinc. Quantitative real time PCR (qRT-PCR) also showed a rapid reduction in C-MYC mRNA levels in BL cell lines after 6 hours exposure to 100 μM but not upon exposure to 50 μM. Again, no reduction in C-MYC mRNA levels was seen in non-BL cell lines. Translocations of other genes to the immunoglobulin loci occur in other forms of NHL. The DLBCL cell line SU-DHL-4 has a t(14;18) translocation resulting in deregulated expression of the protooncogene BCL2. Western blotting showed no decrease in BCL 2 protein levels in SU-DHL-4 in response to either 100 μM or 50 μM zinc acetate after 6 or 24 hours indicating a selectivity of zinc action against C-MYC protein in BL cells. To further investigate the role of altered C-MYC expression in zinc-mediated killing of BL cells, the eBL cell lines Raji and Namalwa were stably transfected with C-MYC using a piggyBac transposon system that allows gene expression under a constitutively active promoter. However, overexpression of C-MYC from an alternative promoter did not rescue BL cells from killing by 100µM zinc. Although western blotting showed that C MYC protein levels were protected after 6 hours, protein reduction and loss of viability was again observed after 24 hours indicating that loss of C-MYC is important in zinc mediated killing of BL cells. In a second approach to rescue C-MYC expression, the proteasome inhibitor Bortezomib was used to inhibit C-MYC protein degradation via the ubiquitin-proteasome system (UPS). Whilst increases were observed in overall ubiquitinated proteins indicating bortezomib was working, western blotting and flow cytometry showed no rescue of C-MYC protein levels. Furthermore, bortezomib did not rescue cells from zinc-mediated killing after 24 hours. In conclusion, findings from this study have identified that 100 mM zinc is effective at killing BL cell lines selectively, and that this killing is associated with activation of apoptotic markers. Treatment with zinc resulted in a rapid and sustained reduction in C-MYC mRNA and protein levels that could not be rescued through constitutive overexpression or the use of proteasome inhibitors. Given that zinc deficiency is common in sub-Saharan Africa and that zinc supplementation is safely used to treat diarrhoeal episodes in children, the studies proposed here indicate that zinc may safely be used as an adjunctive therapy to target C-MYC in BL.