Potential IL6R-ADAR1-STAT3 Interplay Promotes Oncogenicity in 1q21(amp) Multiple Myeloma

1q21 amplification (1q21(amp)) is an important prognostic marker in multiple myeloma (MM) and the critical genes within the minimally amplified region has not been fully characterized. Survival of MM cells is highly dependent on the bone marrow (BM) microenvironment, which is enriched with an array of growth factors secreted by the BM stromal cells. Amongst these growth factors, interleukin-6 (IL6) is a potent driver for MM disease progression, mechanistically through the activation of IL6R and the downstream STAT3 signaling. ADAR1, an RNA editing enzyme, exists in two distinct isoforms, namely the P110 and P150, of which its aberrant expression and activity have been reported in various cancer types, including MM. However, the knowledge on the upstream regulators of ADAR1 and its mechanism of actions have remained elusive. In view of the close proximity of IL6R and ADAR1 on chromosome 1q21, we postulated that concomitant gain of both genes could lead to a more aggressive disease nature in the 1q21(amp) patients and seek to elucidate the potential driving mechanisms.Herein, we analyzed the publicly available myeloma clinical datasets and identified that both IL6R and ADAR1 are overexpressed in the 1q21(amp) patients. With respect to ADAR1, we observed that among the 1q21(amp) MM cell lines, the IL6-responsive and -dependent cells lines demonstrated more intense expression of P150 isoform but the P110 did not differ much, indicating that P150 expression is associated with IL6 regulation. Besides potently activating the IL6R-STAT3 signaling, IL6 stimulation could indeed result in an increased expression of ADAR1 P150 isoform, at both the protein and mRNA level, with minimal effects seen on the P110 isoform, suggesting a transcriptional regulation of P150 by STAT3. Downstream validations with ChIP-qPCR and Luciferase Reporter Assay revealed that ADAR1 P150 is a direct target of STAT3 transcription factor. Specific STAT3 inhibition caused a downregulation of P150 and its oncogenic properties. Likewise, ectopic introduction of P150 confers growth advantage to the cells. Importantly, we also observed that IL6-induced-P150 expression could in turn mediate the activity of STAT3 signaling, in which high P150 cells had a more rapid IL6-induced-STAT3 activation. In summary, we identified that 1q21(amp) and IL6R-STAT3 hyperactivity render a double blow to the regulation of ADAR1-P150, leading to its overexpression, which in turn formed a regulatory feedback loop in further stimulating the STAT3 pathway, conferring disease aggressiveness in the 1q21(amp) patients. The potential interplay between IL6R, ADAR1 and STAT3 may represent a novel mechanism by which IL6 confers oncogenicity in 1q21(amp) MM.