Targeting BCR Activated STAT3 By Ibrutinib In Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is a clinically challenging, usually aggressive B-cell non-Hodgkin’s lymphoma (NHL-B) showing hyperactive, autonomously growing neoplastic B cells with extended tumor cell survival, demonstrate wide-spread chemokine-mediated lymphoma cell migration and homing into lymphoid tissue with poorly-characterized chemo-resistance. Recent introduction of molecularly targeted therapies, such as targeting the Bruton’s tyrosine kinase (BTK) in B-cell receptor (BCR) pathway with ibrutinib revealed significant advances in the treatment of this disease. Ibrutinib was shown to completely and irreversibly inhibit B-cell activation and block signaling downstream of BTK. In this study we have examined the pathophysiological significance of targeting BTK with ibrutinib in MCL cells. Our initial findings showed that STAT3 is constitutively activated in MCL cells and ibrutinib, even at low drug concentration, can effectively down-regulate both phospho-STAT3 (pSTAT3) and NF-κB activation, two key pathways involved in MCL growth/survival (G/S). To confirm the direct target effect of BTK, we employed siRNA that specifically targets BTK. Our results revealed that BTK knock-down in MCL cells lead to a significant reduction of pSTAT3 and inhibition of the NF-κB pathway.

Recent studies have indicated that transcription factor STAT3 has emerged as an important regulator of MicroRNAs (miRNAs) and STAT3 signaling pathways are also controlled by distinct miRNAs. microRNA-155 (miR-155) is an oncogenic microRNA that is frequently up-regulated in B-cell lymphoproliferative disorders including MCL. Activating the BCR pathway leads to miR-155 activation and down-regulation of SOCS1, a negative regulator of both STAT3 and NF-κB. To investigate the role of miR-155 in the activation of STAT3 and NF-kB in MCL, we inhibited miR-155 expression in MCL cells by transiently transfecting an anti-miR155 into MCL cells. Our results indicated that inhibition of miR-155 suppresses STAT3 and NF-kB activation, leading to cell growth inhibition as well as reducing MCL cellular migration. Our data also showed that the BTK inhibitor ibrutinib can moderately down regulate miR-155 expression, an effect similar to the direct knock-down of BTK using siRNA, suggested that miR-155 may, at least in part, play a role in the regulation of STAT3 by BCR. Next, we utilized an in vivo hu-SCID MCL mouse model to test the effect of ibrutinib on the BTK-miR-155 pathway. Consistent with clinical observation, ibrutinib compartmentally shifted MCL cells from the lymphoid tissue into the peripheral blood, leading to MCL cytotoxicity and extending SCID mouse survival, and down-regulated pSTAT3 activities. Our data indicated that the BTK- STAT3 pathway is a critical mechanism for growth and survival in MCL, A better understanding of the complex regulatory networks between BCR and STAT3 may lead to better therapeutic efficacy in MCL.