Novel Tyrosine Kinase Inhibitors Trigger Drug Sensitivity of Primary CLL Cells by Controlling Glucosylation of Ceramides

Apoptosis resistance of chronic lymphocytic leukemia (CLL) cells is mediated by several pro-survival stimuli. In particular, engagement of the B-cell receptor (BCR), CD40-CD40 ligand (CD40L) interaction or stimulation by interleukin-(IL)-4 were identified as major factors to regulate chemoresistance. Sphingolipids are known to be involved in several metabolic pathways involved in chemoresitance. Therefore, we focused on ceramide as pro-apoptotic molecule and its counterpart glucosylceramide, which rather contributes to proliferation and survival.

Applying liquid chromatography electrospray ionization tandem mass spectrometry, we revealed a significant decrease of pro-apoptotic ceramide in BCR/IL-4/CD40L-stimulated primary CLL cells compared to untreated controls (p=0.0258, p=0.0478, p=0.0114). Anti-apoptotic glucosylceramide levels were significantly increased after BCR cross-linking (p=0.0435) while other stimuli caused no relevant change in glucosylceramide expression. We identified BCR engagement to catalyze the crucial modification of ceramide to glucosylceramide via the enzyme UDP-glucose ceramide glucosyltransferase (UGCG) (p=0.0001). Besides specific UGCG inhibitors, we could show for the first time that IgM-mediated UGCG expression was significantly inhibited by the novel and highly effective PI3Kδ and BTK inhibitors CAL-101 and PCI-32765, which were able to revert IgM-induced apoptosis resistance of CLL cells. Recently published data revealed sphingolipids to be essential for mediation of apoptosis via mitochondria. Therefore, we chose ABT-737 – a well-known and also mitochondria-targeting drug – as candidate partner for PI3Kδ and BTK inhibition. When combining each tyrosine kinase inhibitor with ABT-737, a synergistic apoptotic effect could be documented, even under protection by BCR stimulation.

In summary, we could demonstrate that sphingolipids are critically involved in CLL pathogenesis. UGCG could be identified as drugable target by the novel kinase inhibitors CAL-101 and PCI-32765 resulting in even synergistic apoptosis following additional application of ABT-737. Sphingolipids seem to offer further targets providing novel treatment options in CLL.

C.M.W. and L.P.F. contributed equally to this work.

No relevant conflicts of interest to declare.