Tumor Microenvironment Regulates Survival of Mantle Cell Lymphoma Cells through Various Signaling Pathways

Introduction

An aggressive, incurable form of non-Hodgkin's lymphoma (NHL) is classifed as Mantle cell lymphoma (MCL). The health complications associated with advanced age of restrict treatment with intense chemotherapy. Overexpression of cyclin-D1 due to Translocation t(11;14) , is the hallmark of MCL. More detailed insight into MCL pathogenesis has been delayed until the recent development of a tissue culture system, using human mesenchymal stromal cells (hMSC), suitable for propagating primary MCL cells. We hypothesized that tumor-initiating cells are responsible for MCL relapse and chemoresistance and thus, identification of signals responsible for survival and maintenance of MCL-initiating cells (MCL-ICs) is essential for design of curative treatment strategies.

Methods

Isolates of primary MCL cells (n=24) were co-cultured with human mesenchymal stem cells (hMSCs) and the content of MCL-ICs was analyzed by flow-cytometry based on marker expression profile; CD34-CD3-CD45+CD19-. Cytokine array was used to identify the soluble factors enriched in the co-cultures and the expression of these factors was confirmed by RT-PCR analysis. The signaling pathways employed by the newly-identified factors were blocked in 3 MCL cell lines (JVM2, Mino, Z138) to confirm their essential role in survival of MCL cells and, more importantly, for MCL-ICs.

Results

Co-cultures of primary MCL isolates with hMSCs supported the growth of MCL cells for over 4 weeks with continued presence of MCL-ICs (CD34-CD3-CD45+CD19-) representing about 1% of MCL cells. We found that IL-6 produced by hMSCs triggered an FGF/FGFR autocrine loop in MCL-ICs. The extent of FGFR expression correlated tightly with expression of SOX11, a pathology related negative prognostic marker in MCL. MCL cell survival and growth was regulated via the FGFR/mir101/ EZH2/ NF-κB/XIAP axis. Blocking of this signaling pathway with FGFR1 inhibitors consistently induced early reduction in XIAP levels and subsequently MCL cell death.

Conclusion

We established that propagation of primary MCL in co-cultures with hMSCs depends on an FGF/FGFR autocrine loop that enhances XIAP protein expression and thus, supports survival of MCL cells. We identified the factors essential for survival of MCL and MCL-ICs that present new targets for improved MCL treatment strategies.