The importance of IκBNS and Ca²+ signaling in DLBCL development

Abstract

Diffuse large B-cell lymphoma (DLBCL) is an aggressive disorder of mature B-lymphocytes presenting with 40% of novel lymphoma cases worldwide the most common subtype of adult Non-Hodgkin lymphoma. DLBCL is characterised by a high degree of heterogeneity regarding clinical, pathologic and molecular genetic issues. Gene expression profiling revealed distinct molecular DLBCL subtypes resembling germinal centre B-cells (GCB DLBCL) and activated B-cells (ABC DLBCL). Despite advances in therapy, the 3-year progression free survival rates of GCB and ABC DLBCL upon immunochemotherapy are still at 74% and 40%, respectively. Consequently, there is an obvious need for identifying new molecular targets and biomarkers serving as starting points to develop new, more efficient therapeutic strategies especially of the adverse ABC DLBCL subtype.

In the present study we were able to demonstrate that IκBNS is constitutively expressed in ABC DLBCL cell lines and human biopsies, whereas IκBNS expression is absent in GCB DLBCL. We noticed that two IκBNS isoforms were detectable, the described p35 and a second larger IκBNS isoform exhibiting an additional N-terminal unstructured portion. Silencing of IκBNS led to a reduced growth of ABC DLBCL cell lines, suggesting an oncogenic function of IκBNS. Promoter studies suggested that IκBNS expression is under the control of NF-κB and NFAT signalling. Interestingly, we detected constitutive activation of NFAT in DLBCL cell lines, which was necessary to drive the expression of IκBNS in ABC DLBCL. Calcineurin inhibitors, which decrease the activity of NFAT proteins, did not only impair the expression of IκBNS, but also induced cell death in multiple ABC DLBCL cell lines. The expression of the pro-survival cytokines IL-6/-10 was markedly reduced by calcineurin inhibition, which might at least partially explain their toxicity in ABC DLBCL cell lines. With the help of phosphoproteomics, we identified several new calcineurin substrates in ABC DLBCL, including CD79, suggesting a more central role of calcineurin in tumorigenesis.

Taken together, these results provide first insights into the essential function of IκBNS in the development of ABC DLBCL. Furthermore, we suggest IκBNS as a promising biomarker to discriminate ABC from GCB DLBCL. Finally, we propose that calcineurin inhibitors could have therapeutic potential for the treatment of aggressive ABC DLBCL.