Abstract 3383: JunB/AP-1 controls MM cell proliferation, survival and drug resistance in the bone marrow microenvironment
Authors: Fan Fengjuan , Vallet Sonia , Sattler Martin , Tonon Giovanni , Bashari Muhammad Hasan , Bakiri Latifa , Goldschmidt Hartmut , Wagner Erwin F. , Jaeger Dirk , Podar Klaus 
Published: May 2015

The activator protein-1 (AP-1) transcription factor is a dimeric complex that comprises members of the JUN, FOS, activating transcription factor (ATF) and musculoaponeurotic fibrosarcoma (MAF) protein families. Resultant heterodimers and homodimers determine the genes that are regulated by AP-1. Functionally, AP-1 has been implicated in a multitude of biological processes but also tumorigenesis, dependent on the cell type and the pathophysiologic context. In multiple myeloma (MM), the role of AP-1 is largely unknown. The present study focuses on the impact of JunB, a central member of the AP-1 family, in MM growth, survival and drug resistance.

First, our data demonstrate rapid and strong induction of AP-1/JunB expression in MM cell lines and primary tumor cells upon co-culture with bone marrow stromal cells (BMSCs), i.e. isotypic primary BMSCs as well as BMSC lines KM-105 and HS-27A. Importantly, utilizing a non-contact co-culture system, we show that this effect is predominantly mediated by soluble factors secreted by BMSCs rather than direct MM-BMSC contact. Consistent with gene expression profiling data, JunB mRNA levels detected by quantitative real-time PCR are increased in MM cells when co-cultured with BMSCs. Specifically, we found that upregulation of JunB occurs predominantly at the translational level, since cycloheximide, but not actinomycin D or MG-132 significantly blocked BMSC- induced JunB expression.

Pharmacologic inhibition was used next in order to identify upstream signaling pathways, which mediate BMSC- induced JunB upregulation in MM cells. Our data demonstrate that activation of MEK/MAPK or NF-kB, but not PI3K/Akt is required for induction of JunB expression and AP-1 transcriptional activity.

To delineate the specific functional role of JunB in MM pathogenesis, we subsequently generated stable MM cell lines carrying the pLKO.1-puro-JunB shRNA vector or the pLKO.1-puro control vector, respectively. Our preliminary data show significantly decreased proliferation and resistance to chemotherapy in MM/ pLKO.1-puro-JunB shRNA but not MM/ pLKO.1-puro control cells when co-cultured with BMSCs. Consistently, 4-hydroxytamoxifen (4-HT) treatment of MM cells stably transduced with pBabe-puro-JunB-ER-IRES-GFP but not of MM cells transduced with pBabe-puro-IRES-GFP promotes tumor cell proliferation, survival and drug resistance, independent of BMSC co-culture. Ongoing studies aim to identify downstream effectors of JunB, using gene array profiling of MM/ pLKO.1-puro-JunB shRNA cells versus MM/ pLKO.1-puro control cells. Furthermore, we are conducting a survey of published and unpublished datasets to uncover the clinical and prognostic relevance of JunB expression in MM patient samples.

In summary, our data demonstrate for the first time an important role of AP-1/JunB in MM tumorigenesis and strongly propose it as a novel therapeutic target in MM.

Citation Format: Fengjuan Fan, Sonia Vallet, Martin Sattler, Giovanni Tonon, Muhammad Hasan Bashari, Latifa Bakiri, Hartmut Goldschmidt, Erwin F. Wagner, Dirk Jaeger, Klaus Podar. JunB/AP-1 controls MM cell proliferation, survival and drug resistance in the bone marrow microenvironment. [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 3383. doi:10.1158/1538-7445.AM2014-3383