All-trans retinoic acid and RARβ agonists enhance the efficacy of IMiD-based treatments for t(4;14) multiple myeloma

Abstract The t(4;14) (p16;q32) translocation stands as a formidable high-risk cytogenetic aberration in patients with multiple myeloma (MM). This genetic anomaly triggers the overexpression of fibroblast growth factor receptor 3 (FGFR3), which subsequently hijacks critical cell signaling pathways such as RAS/MAPK, JAK/STAT. These pathways, essential for cell growth, become dysregulated, leading to a dismal prognosis in MM patients. Despite significant efforts, the effectiveness of current small molecule FGFR3 inhibitors and monoclonal antibodies remains elusive. Moreover, the emergence of resistance, often stemming from mutations in the targets of FGFR3, poses a substantial roadblock to successful treatment, underscoring an urgent need for innovative, safe, and cost-effective therapeutic strategies. Immunomodulatory drugs (IMiDs) have been long served as the backbone of MM treatment. However, the development of resistance over time and subsequent relapse are inevitable challenges faced by MM patients. To combat this, combination therapies leveraging agents with diverse mechanisms of action have emerged as a promising approach to overcome IMiD resistance. In our previous study, we embarked on a comprehensive high-throughput screening of 1,855 FDA-approved drugs, aiming to identify repurposed agents capable of augmenting the sensitivity of MM cells to chemotherapy. Intriguingly, we unearthed all-trans retinoic acid (ATRA) as a potential game-changer and demonstrated its ability to (re)sensitize human MM cells to carfilzomib treatment. Building on this discovery, our current research identified ATRA, which exhibits no inherent anti-MM activity alone, as a powerful agent that can enhance and (re)sensitize t(4;14) MM cells to IMiDs such as lenalidomide and pomalidomide. Mechanistically, ATRA binds to and activates RARβ (retinoic acid receptor β), which then targets the retinoic acid response elements (RAREs) on the FGFR3 promoter. Our findings are further bolstered by the in vitro efficacy of CD2314, a selective RARβ agonist, that also sensitizes and re-sensitizes t(4;14) MM cells to IMiDs. We generated RARβ knockout (KO) t(4;14)-positive MM cell lines using CRISPR/Cas9. RARβ-KO MM cells showed a marked decrease in apoptosis when treated with ATRA plus POM compared to that of control-KO MM cells. CRBN (a reported direct target of IMiDs) knockdown did not prevent the combination treatment from significantly reducing FGFR3 expression. In addition,we found that IMiDs target HDAC5 at the Ser498 site and reduce its phosphorylation level to enhance the nuclear translocation of HDAC5 and HDAC3. This orchestrates the formation of a co-repressor complex involving RARβ, HDAC5, and HDAC3, and effectively reduces chromatin accessibility (ATAC-seq) and H3K27 acetylation (ChIP-qPCR) at the FGFR3 promoter. As a result, FGFR3 expression is significantly downregulated, leading to suppression of the PI3K/AKT signaling pathway (RNA-seq) and more MM cell death. Importantly, ATAR and RARβ agonists sensitize t(4;14) MM cells to IMiD treatment in vivo. These results not only highlight the significant therapeutic potential of ATRA and RARβ agonists in enhancing the efficacy of IMiD-based treatments for t(4;14) MM but also offer a beacon of hope for overcoming IMiD resistance. To conclude, we present a novel and promising strategy to improve the clinical outcomes for this high-risk subgroup of MM patients, marking a significant step forward in the battle against this challenging disease.