Bone Marrow Microenvironment Regulates Alternative Splicing Events in Myeloma Cells through Downregulation of RNA Binding Protein Fox2

Alternative splicing is a crucial mechanism for gene regulation, which enhances the diversity of transcriptome and proteome. Misregulation of alternative splicing has been implicated in number of disease processes including cancer. Our data utilizing exon array profile from 170 uniformly treated newly diagnosed patients with MM confirms clinical relevance of splicing as demonstrated by impact of level and extent of alternate splicing on both progression free and overall survival. Fox2, a RNA splicing factor, is one of the most important genes predicting clinical outcome in these patients. We confirmed Fox2 expression in 10 MM cell lines at both RNA and protein levels. Immunohistochemistry staining showed a predominant nuclear localization of Fox2. Importantly, we also observed that MM cell - bone marrow stromal cells (BMSC) interaction led to significant inhibition of Fox2 expression in MM cells. Similar response was also observed using BMSC supernatants. While IL6 treatment significantly downregulated the expression of Fox2 in MM1S and RPMI8226 cells in a dose-dependent manner, IGF-1 treatment had no significant impact on Fox2 expression in MM cell lines. Since Fox2 has been described to plays a role in the maintenance of cell cytoskeleton, we therefore evaluated whether Fox2 might influence the migration and adhesion in MM cells. Transwell migration assay showed enhanced migration rate of Fox2-knocking down- MM1S and RPMI8226 cells versus controls. We also observed the increased cell adhesion to fibronetin in both cell lines upon Fox2 knockdown. Actin polymerization evaluated by Alexa488-conjugated phalloidin staining and confocal microscope analysis showed Fox2 knocking down cells with increased actin polymerization in both MM1S and RPMI8226 cell lines. Interstingly, we observed that Fox2 knockdown in MM cell lines did not affect the cell proliferation and survival. As Fox-2 is a splicing factor, we further evaluated the molecular impact of Fox2 expression in multiple myeloma by RNA-seq analysis. Our data revealed that Fox2 functions in regulating both protein-coding and non-coding RNA alternative splicing. Knockdown of Fox2 resulted in significant isoform up-regulation (60 in MM1S and 151 in RPMI8226) and down-regulation (70 in MM1S and 69 in RPMI8226). Gene enrichment analysis showed these genes are clustered in cell cytoskeleton regulation, microtubule-based movement, ATP binding, amongst others. Our study then focused on Fox2 knockdown-induced significant isoform switch in MM cell lines. We designed the primers testing the spliced exons and confirm the isoform switch in MM cells by PCR analysis (e.g. Pyk2 and PFDN6). Importantly, our RNA seq data showed that Fox2 regulates the expression of a series of microRNAs and long-noncoding RNAs (e.g. MALAT1 RPMI8226 CNT (58) vs Fox2 knockdown (108)), which provides us a new insight into impact of Fox2 on non-protein coding RNAs. We have also validated the RNA-seq data by Q-PCR analysis.

In summary, our results identify Fox2 as a biologically important RNA binding protein that is regulated by bone marrow microenvironment interaction and with essential function and potential clinical implications in multiple myeloma.