Microenvironment Influences Expression of TOSO – a Novel NF-Kappa B Target Gene In Chronic Lymphocytic Leukemia

We recently identified the transmembrane protein TOSO to be significantly over-expressed in chronic lymphocytic leukemia (CLL) compared to other B-cell lymphomas or healthy B-cells and T-cells. TOSO was initially characterized as inhibitor of Fas-mediator of apoptosis; however, it could be demonstrated to be the receptor for the IgM-specific Fc-domain in immune cells. TOSO is the only Fcμ receptor expressed on B-cells and is solely expressed in the lymphoid compartment. However, little is known on its regulation and the molecular background of over-expression in CLL. We investigated TOSO expression on mRNA and protein level in freshly isolated primary CLL cells (n=10) and healthy B-cells (n=4) after single treatment for 24 hours with a comprehensive panel of different cytokines or stimuli (interleukin (IL)-1, IL-2, IL-4, IL-6, IL-7, IL-10, IL-12, IL-15, interferon (IFN)-γ, transforming growth factor (TGF)-ß, tumor necrosis factor (TNF)-α, lipopolysaccharide, CpG, CD40-ligand (CD40L) and B-cell receptor (BCR)) being involved in B- and T-cell interplay, by qRT-PCR, western blotting and flow cytometry. Furthermore, we determined the impact of nurse-like cells (NLC) to TOSO expression and co-incubated primary CLL cells for up to 14 days with NLCs. To better understand the intracellular regulation of TOSO, we inhibited BCR and/or CD40L pathways, which were shown by us to be either stimulatory (BCR) or inhibitory (CD40L) in regard to TOSO expression. Since expression might be finally also controlled on epigenetic level, we determined the methylation status of the putative TOSO promoter in 64 CLL samples and 10 healthy B-cells samples. Quantitative DNA methylation analysis was conducted using the EpiTyper application by Sequenom (San Diego, CA, USA). Our experiments reveal novel extra- and intracellular stimuli regulating TOSO expression. We identified CD40L, IL-4 and CpGs to have strong inhibitory effects on TOSO expression (P<0.001) in primary CLL cells and healthy B-cells. In contrast, we identified NLCs (MFIR 15,8 vs. 25,8; P=0.049; n=4) and BCR cross-linking to induce TOSO expression on the cell surface of CLL cells. Based on extracellular stimuli, we were able to hypothesize on shared downstream pathways in order to identify the key regulatory factors and transcription factors controlling TOSO expression. By using a panel of inhibitors in BCR and CD40L downstream signaling, NF-kappa B was shown to have the strongest effect on TOSO expression (P=0,0294). Applying the I-kappa B kinase (IKK) inhibitor Wedelolactone at non-toxic concentrations (10μM), TOSO expression was profoundly suppressed after 24 hours. Regarding epigenetic alterations, our analysis from genome-wide screening experiments in CLL patients compared to healthy B-cells did reveal significant aberrant DNA de-methylation events in the TOSO promoter-associated CpG island (P<0.001). In conclusion, we revealed IL-4, CpG and CD40L as BCR stimulus and NLCs as the key components in regulation of TOSO in the CLL cell microenvironment. Furthermore, over-expression of TOSO in CLL cells compared to normal B-cells could be demonstrated being associated with epigenetic changes at its promoter. We identified TOSO as a novel NF-kappa B regulated target gene. In ongoing studies we elucidate whether NF-kappa B acts directly or in-directly on TOSO expression.

No relevant conflicts of interest to declare.