SYK Is Involved in the CD38 Signal Transduction Pathway in Chronic Lymphocytic Leukemia

B-cell chronic lymphocytic leukemia (CLL) is one of the most prevalent B cell malignancies in adults and characterized by expansion of monoclonal mature B cells. Survival and proliferation of CLL cells depends on microenvironmental contact in lymphoid organs. The transmembrane glycoprotein CD38 acts as an important mediator of survival, proliferation and migration signals for CLL cells, and its expression is associated with poor prognosis. Spleen tyrosine kinase (SYK) is a central element of the B-cell receptor signal transduction pathway and has additionally been shown to be involved in cytokine and integrine signaling. In this study we demonstrate direct involvement of SYK in the CD38 signaling pathway in primary CLL samples.

CD38-stimulation of primary CLL cells by its ligand CD31 induced a consistent phosphorylation of the tyrosine residue Y352, the first activation site that releases SYK from its autoinhibitory conformation (p<0.001). SYK downstream targets AKT (p<0.05) and ERK (p<0.05) were subsequently induced and prolonged CD38-stimulation increased MCL-1-expression (p<0.05). Concomitant inhibition of SYK with the SYK inhibitor R406 resulted in inhibition of AKT- and ERK-activation (p<0.05 and p<0.01) and prevented upregulation of MCL-1 (p<0.01). Moreover, we observed SYK-dependent enhancement of BCR-signaling after CD38 ligation. Short-term exposure of CLL cells to CD31 led to an increase of ERK-phosphorylation after BCR-engagement by 41.9%  (p<0.05). This effect was completely abolished by concomitant R406-treatment (p<0.05). Additionally, we observed a SYK-dependent increase of Ca²⁺-flux in response to BCR-stimulation after previous CD38 activation. Moreover, preliminary experiments show that prolonged CD38-stimulation led to a SYK-dependent increase of baseline Ca²⁺-flux in CLL cells, indicating a potential involvement of CD38 in autonomous BCR-signaling. CD38 acts as an enhancer of migratory stimuli in CLL cells. We therefore analysed, whether SYK is also involved in this interaction process. CXCL12-dependent migration was increased by CD38 stimulation with the agonistic CD38 antibody IB4 by 28.3% (p=0.12). Treatment of CLL cells with R406 completely inhibited IB4-mediated migration (p<0.01). The expression of CD38 is regulated by a variety of mechanisms, including CD40 ligation. SYK is involved in CD40 signaling. We therefore tested, whether SYK-inhibition affects CD38-expression. Stimulation of CLL cells with recombinant CD40L resulted in a significant increase of CD38-expression (p<0.05). This effect was reversed by concomitant SYK-inhibition (p<0.01). In addition, we observed marked down-regulation of CD38 surface-expression (p<0.05) and mRNA-expression (p<0.05) for CLL cells treated with SYK-inhibitors R406 or P505-15 compared to vehicle control. This effect is at least partly based on transcriptional inhibition of CD38-regulators NF-kB (p<0.05) and E2A (p<0.05). Finally, we observed a clear correlation between CD38 expression on CLL cells and SYK-inhibitor efficacy (p<0.01).

In conclusion, our data show that SYK acts as a central downstream effector of CD38 signaling regulating survival-, proliferation-, and migration pathways in CLL and also functions as a strong regulator of CD38 expression. The interaction of CD38 and SYK involves the BCR pathways, where CD38 enhances the response to BCR-stimulation and, moreover, may act as an enhancer of autonomous BCR-signaling in CLL. Additionally, the CD38-SYK interaction involves BCR-independent microenvironmental pathways as shown for CD40 and CXCL12. CD38 expression not only serves as a negative prognostic marker but has also been shown to possess biological significance in CLL. We therefore propose that disruption of the CD38-SYK axis may represent a promising therapeutic option in CLL.