The Flavonoid Wogonin Reduces CLL Cell Survival in Vitro and Leukemia Development in Eµ-TCL1 Mice By Targeting Aberrant TNF Receptor Signaling

The importance of the tumor microenvironment in the development of B-cell chronic lymphocytic lekuemia (CLL) is now widely accepted. Previous studies within our and other groups revealed the establishment of an inflammatory milieu in CLL characterized by enhanced expression and secretion of several cytokines and their receptors. Using 250 CLL serum samples and 50 age-matched controls, we found significantly increased levels of the TNFα receptors TNFR1 and TNFR2 in the patients that positively correlated with an adverse clinical outcome. Based on these findings we aimed to investigate the functional role of TNFR signaling in CLL development.

In contrast to healthy B cells that do not express TNFα receptors, we detected TNFR1 expression in CLL cells upon survival-inducing culture conditions and in lymph node sections of 80 CLL patients, where TNFR1 signals co-localized to the B cell marker CD20 and were mainly present within proliferation centers. These findings were confirmed in Eµ-TCL1 mice, a well-established mouse model for CLL. Here, CLL cells in the peripheral blood were negative for TNFR1. However, the cells upregulated the receptor upon entering the spleen where they get into contact with accessory cells, receive survival stimuli and undergo proliferation. In addition, increased levels of soluble TNFR1 in the serum were confirmed in the mice.

After ligand binding, TNFR1 can activate two different signaling pathways: (1) the extrinsic apoptosis cascade through its death receptor domain, or (2) survival-inducing NFκB signaling. The latter pathway can be blocked by wogonin, a naturally occurring monoflavonoid. In a multitude of in vitro and in vivo studies, wogonin has been shown to exert anti-oxidant, anti-inflammatory and anti-tumor activities. By ex vivo treatment of CLL cells with TNFα we observed NFκB activation which was reversed by TNFα-blocking antibody, suggesting TNFR1-mediated survival signaling in CLL. Therefore, we aimed to test whether wogonin can prevent this signaling in CLL cells and might therefore represent a novel potential drug for CLL.

In CLL cocultures, wogonin treatment resulted in a concentration-dependent apoptosis induction, which was significantly increased by the addition of TNFα. To test the effects of wogonin in vivo, we transplanted isogenic, immunocompetent wild-type mice with CLL cells from leukemic TCL1 animals and treated them daily with 40 mg/kg wogonin. When treatment was started 2 days after transplantation, wogonin significantly reduced spleen weights and lead to a reduced CLL content in the spleen, the bone marrow and the peritoneal cavity. If treatment was started in advanced disease stage, wogonin slightly lowered spleen weight and the CLL content in the spleen, whereas the percentage of CLL cells in the peripheral blood was increased. Interestingly, here wogonin treatment resulted in a loss of cell surface TNFR1 expression in splenic CLL cells and increased TNFR1 levels in the serum. These data suggest that wogonin induces a redistribution of CLL cells in vivo, preventing their homing to lymphoid organs and that loss of TNFR1 expression might be involved in this process.

In summary, our results show that TNFR1 signaling is involved in CLL cell activation and survival. Targeting this pathway with wogonin reduces CLL cell viability in vitro and leukemia development in TCL1 mice.