Abstract 3448: Fibroblast growth factor receptor is expressed as a constitutively active receptor tyrosine kinase in chronic lymphocytic leukemia B cells and exists in an active complex with Axl: Dual targeting in CLL
Authors: Sinha Sutapa , Boyson Justin , Secreto Charla , Warner Steven L. , Kay Neil E. , Ghosh Asish K. 
Published: May 2015

Abstract
B cell chronic lymphocytic leukemia (CLL) is an incurable disease and represents a significant health problem in the western world. We and others have reported that primary CLL B cells spontaneously produce increased levels of proangiogenic basic fibroblast growth factor (bFGF) in vitro and that CLL plasma contained elevated levels of bFGF. However, the precise role of bFGF in CLL pathobiology is not clearly understood. In this study we investigated functional implication of the FGF/FGFR signaling axis in CLL B cell biology.
We have detected expression of FGFR1 and FGFR3 with comparatively higher levels of the latter receptor, but not FGFR2/FGFR4, in primary CLL B cells. Interestingly, constitutively phosphorylated FGFR and its downstream signaling adaptor protein fibroblast receptor substrate 2-α were also detected in CLL B cells. However, in vitro stimulation of FGFR with recombinant bFGF was unable to increase endogenous phosphorylation levels of FGFR in CLL B cells from most CLL patients tested, suggesting existence of an autocrine FGF/FGFR signaling loop in the leukemic B cells. We know that Axl a known predominant RTK in CLL B cells seems to act as a docking site for multiple cellular kinases/lipase. Therefore, in an effort to determine whether Axl is functionally associated with FGFR in CLL B cells, we first examined if these two RTKs exist in the same molecular complex. Indeed we found that Axl forms a complex with FGFR3 in CLL B cells and is therefore likely to regulate FGFR signaling. Our hypothesis in this regard was further supported when we inhibited Axl phosphorylation in CLL B cells using a high-affinity Axl inhibitor (TP-0903; Tolero). Axl inhibition resulted in significant reduction of FGFR phosphorylation in CLL B cells. Finally, inhibition of FGFR signaling using a high-affinity FGFR inhibitor induced significant levels of apoptosis in CLL B cells (n=20) with a mean LD50 dose of ∼2.5 μM. Of interest, LD50 dose for CLL B cell apoptosis was much lower for the Axl inhibitor (mean LD50=140nM; n=20) compared to the FGFR inhibitor, which we believe occurs partly because TP-0903 seems to exert its inhibitory effect on P-FGFR likely via P-Axl inhibition in CLL B cells.
In conclusion, we have detected expression of constitutively active FGFR in primary CLL B cells. Interestingly, our preliminary findings suggest that Axl forms an active RTK complex with FGFR and thus likely regulates FGFR signaling in these leukemic B cells. This is evidenced by the finding that Axl inhibition significantly reduced FGFR phosphorylation accompanied by massive induction of CLL B cell apoptosis, suggesting that targeting dual RTK signaling pathways (Axl and FGFR) in CLL B cells warrants further investigation as a way to develop a more effective and efficient therapeutic intervention for CLL patients.
Citation Format: Sutapa Sinha, Justin Boyson, Charla Secreto, Steven L. Warner, Neil E. Kay, Asish K. Ghosh. Fibroblast growth factor receptor is expressed as a constitutively active receptor tyrosine kinase in chronic lymphocytic leukemia B cells and exists in an active complex with Axl: Dual targeting in CLL. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3448. doi:10.1158/1538-7445.AM2014-3448