Cross-Talk between Cytokine and NF-ĸb Signaling in the CLL Microenvironment Can Affect Sensitivity for Venetoclax
Authors: Haselager Marco , Thijssen Rachel , Kater Arnon P. , Eldering Eric 
Published: November 2019

INTRODUCTION. The Bcl-2 inhibitor Venetoclax provides profound reductions in circulating chronic lymphocytic leukemia (CLL) cells in the majority of patients. However, lymph node (LN) responses are less robust, which may be linked to an acquired resistance imposed by pro-survival signals. Prime among these is CD40 stimulation leading to activation of NF-kB, and induction of Bcl-XL expression1. Bcl-XL is a prime determinant of resistance to Venetoclax2 and regulatory mechanisms of its expression are of clinical significance. Cytokines IL-21 and IL-4 are secreted by T helper cells and abundant in the CLL lymph node microenvironment. Importantly, IL-21 and IL-4 play an important role in CLL survival and proliferation3. In the present study, we investigated how signals from T helper cytokines IL-21 or IL-4 affect Bcl-XL expression as a model for the CLL LN microenvironment, specifically in relation to Venetoclax resistance.
RESULTS. Following CD40 stimulation, IL-21 and IL-4 show opposing effects on Bcl-XL expression. Correspondingly, this was associated with CD40-induced resistance to Venetoclax which was augmented by IL-4 and reversed by IL-21. We subsequently investigated the rewiring between CD40 activation, differential cytokine signaling and Bcl-XL expression. IL-21 or IL-4 stimulation correspond with differential STAT3 or -6 phosphorylation and STAT3 and -6 have predicted binding sites near the known p65 and p52 binding sites in the Bcl-XL promoter region. Using reporter assays with Bcl-XL promotor constructs we demonstrate competition (through IL-21-induced STAT3) or synergy (through IL-4 induced-STAT6) with CD40-mediated activation of the NF-kB pathway. By applying in situ proximity ligation (isPLA) in primary CLL cells, we showed direct interaction of both (non-)canonical p65 and p52 with STAT3 and STAT6. Moreover, time-course analyses indicated that STAT3 drives NF-kB out of the nucleus, whereas STAT6 keeps NF-kB inside the nucleus, and this distinction controls Bcl-XL expression. These observations suggest that cross-talk between JAK/STAT signaling and NF-kB signaling happens by direct binding to the Bcl-XL promoter and by limiting NF-kB availability at the Bcl-XL promoter.
CONCLUSIONS. These data show that protective signals from the CLL microenvironment can be tipped towards apoptosis sensitivity by interfering with JAK/STAT and NF-kB signaling, providing novel therapeutic clues in case of emerging resistance to targeted drugs such as Venetoclax.
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Disclosures
Eldering: Celgene: Research Funding; Roche: Research Funding; Janssen Pharmaceutical Companies: Research Funding.