Increased TCF-4 Expression Correlates with Reduced Caspase-3 Induction and Confers Resistance to Bortezomib.

Bortezomib (PS-341 / Velcade^®) has achieved responses in the setting of relapsed refractory multiple myeloma, yet some patients either fail to respond or become refractory after an initial response. Moreover, despite the essential role of proteasomes in cell proliferation, not all tumor cell types respond to Bortezomib treatment. To define mechanisms for differential sensitivity to Bortezomib, we used gene expression profiling to compare global gene expression patterns in SUDHL-4 and SUDHL-6 B-cell lymphoma cell lines, which respond differentially to Bortezomib treatment. Of genes that were differentially expressed in the two cell lines, we chose to further examine differential expression of the transcription factor ‘T-cell factor 4′ (TCF-4), a downstream effector in Wnt signaling. TCF-4 expression was 15 fold higher in resistant SUDHL-4 cells compared to sensitive SUDHL-6 cells by microarray analysis, which was confirmed by real-time PCR. TCF-4 was constitutively associated with ß-catenin, its transcriptional co-activator, only in resistant SUDHL-4 cells, but not in sensitive SUDHL-6 cells. TCF-4 overexpression was associated with increased transcription of TCF-4 target genes cyclin D1 (3.48 fold) and c-myc (1.7 fold). Since all TCF-4 target genes are not defined, we also confirmed increased transcription by the TCF-4 / ß-catenin complex using TCF-4 / ß-catenin driven reporter genes. Increased TCF-4 expression in resistant SUDHL-4 cells was associated with lower induction of caspase-3 expression and activity in response to Bortezomib. Importantly, siRNA-mediated downregulation of TCF-4 sensitized cells to Bortezomib treatment, suggesting that blocking TCF-4 can restore sensitivity to Bortezomib-induced apoptosis. Our studies provide the rationale to use small molecule antagonists of TCF-4 / ß-catenin interaction to overcome Bortezomib resistance and improve patient outcome.