c‐FOS is an integral component of the IKZF1 transactivator complex and mediates lenalidomide resistance in multiple myeloma

Background The immunomodulatory drug lenalidomide, which is now widely used for the treatment of multiple myeloma (MM), exerts pharmacological action through the ubiquitin‐dependent degradation of IKZF1 and subsequent down‐regulation of interferon regulatory factor 4 (IRF4), a critical factor for the survival of MM cells. IKZF1 acts principally as a tumour suppressor via transcriptional repression of oncogenes in normal lymphoid lineages. In contrast, IKZF1 activates IRF4 and other oncogenes in MM cells, suggesting the involvement of unknown co‐factors in switching the IKZF1 complex from a transcriptional repressor to an activator. The transactivating components of the IKZF1 complex might promote lenalidomide resistance by residing on regulatory regions of the IRF4 gene to maintain its transcription after IKZF1 degradation. Methods To identify unknown components of the IKZF1 complex, we analyzed the genome‐wide binding of IKZF1 in MM cells using chromatin immunoprecipitation‐sequencing (ChIP‐seq) and screened for the co‐occupancy of IKZF1 with other DNA‐binding factors on the myeloma genome using the ChIP‐Atlas platform. Results We found that c‐FOS, a member of the activator protein‐1 (AP‐1) family, is an integral component of the IKZF1 complex and is primarily responsible for the activator function of the complex in MM cells. The genome‐wide screening revealed the co‐occupancy of c‐FOS with IKZF1 on the regulatory regions of IKZF1‐target genes, including IRF4 and SLAMF7, in MM cells but not normal bone marrow progenitors, pre‐B cells or mature T‐lymphocytes. c‐FOS and IKZF1 bound to the same consensus sequence as the IKZF1 complex through direct protein‐protein interactions. The complex also includes c‐JUN and IKZF3 but not IRF4. Treatment of MM cells with short‐hairpin RNA against FOS or a selective AP‐1 inhibitor significantly enhanced the anti‐MM activity of lenalidomide in vitro and in two murine MM models. Furthermore, an AP‐1 inhibitor mitigated the lenalidomide resistance of MM cells. Conclusions C‐FOS determines lenalidomide sensitivity and mediates drug resistance in MM cells as a co‐factor of IKZF1 and thus, could be a novel therapeutic target for further improvement of the prognosis of MM patients. Left panel: In multiple myeloma (MM), IKZF1/3 binds to the canonical IKZF‐binding motif CTTCC in a complex with c‐FOS/c‐JUN to activate the transcription of genes involved in the growth and survival of MM cells such as interferon regulatory factor 4