B-Chronic lymphocytic leukemia (B-CLL) is characterized by accumulation of apoptotic resistant CD5+ B lymphocytes. There is an increased secretion of Wnt ligands indicating an autocrine loop leading to the extended survival of B-CLL cells. Lymphoid enhancer factor 1 (LEF-1) is a potent transcription factor regulating the expression of several Wnt induced target genes. A comprehensive gene expression profiling from two independent studies revealed that LEF-1 mRNA was ~3000 fold overexpressed in B-CLL when compared to its healthy counterpart. The objective of this present study is to demonstrate the therapeutic benefit of inhibiting LEF-1 expression in B-CLL cells using novel small molecule inhibitors CGP049090 and PKF115-584 in vivo and in vitro. In order to explore the anti-leukemic potential of CGP049090 and PKF115-584 we tested its effects on freshly isolated B-CLL cells, prolymphocytic cell line (JVM-3 & MEC-1) and in a subcutaneous mouse xenograft model. The present study shows that, in freshly isolated B-CLL cells there was high protein expression and nuclear localization of LEF-1 and β-catenin indicating active LEF-1 mediated transcription whereas LEF-1 remained undetectable in healthy B cells. Preliminary experiments of LEF-1 inhibition using siRNAs resulted in increased apoptosis indicating LEF-1 plays an important role in the survival of B-CLL cells. This observation was extended using CGP049090 and PKF-115584 as they induce dose dependent cytotoxicity in B-CLL, whereas the healthy B cells are not significantly affected. The half maximal inhibitory concentration (IC50) was less than 1 µM in primary B-CLL cells and cell lines whereas it was more than 5 µM in healthy B cells. CGP049090 and PKF-115584 induced apoptotic cell death in primary B-CLL cells and cell lines by cleavage of caspases 8, 9, 3 and 7 and subsequent cleavage of Poly (adenosine diphospate-ribose) polymerase (PARP). Both inhibitors also altered the expression of several anti-apoptotic proteins like X-linked Inhibitor of Apoptosis Protein (XIAP), Mantle cell lymphoma-1 (Mcl-1) and B cell lymphoma-2 (Bcl-2). Co-Immunoprecipitation experiments revealed that both the inhibitors effectively disrupt the β-catenin/LEF-1 interaction, resulting in the down regulation of LEF-1 target genes such c-myc, cyclin D1 and LEF-1. Furthermore, when the inhibitors were tested in an in vivo JVM-3 subcutaneous xenograft nude mouse model, more than 70% inhibition of tumor growth and an increase in the median survival of the treated group without leading to systemic toxicity was observed. Immunohistochemistry analysis of the tumor sections revealed LEF-1 down regulation and subsequent inhibition of proliferation by down regulation of Proliferating Cell Nuclear Antigen (PCNA) and increase in apoptosis (cleaved PARP). In summary, the data showed that LEF-1 is a potential therapeutic target in the treatment of B-CLL. Both CGP049090 and PKF115-584 showed potent inhibitory effects on the survival of CLL cells in vitro and in vivo without affecting the healthy cells. Both CGP049090 and PKF115-584 are hence, potential anti-cancer agents in B-CLL and other neoplastic malignancies with aberrant LEF-1/ T cell factor (TCF) transcriptional activity. Further investigations are warranted to determine the feasibility of these small molecules for therapeutic approach in humans
