Targeting the IRE1α/XBP1 Signaling Axis Suppresses Rhabdomyosarcoma Growth and Promotes Myogenic Differentiation

Rhabdomyosarcoma (RMS) is the most common and aggressive pediatric soft tissue sarcoma arising from skeletal muscle lineage. Despite multimodal therapy, treatment options for relapsed or refractory RMS remain limited, highlighting the need for new therapeutic targets. The unfolded protein response (UPR) maintains cellular proteostasis under endoplasmic reticulum (ER) stress, with the inositol-requiring enzyme 1 (IRE1)-X-box binding protein 1 (XBP1) axis serving as a key regulatory branch. While aberrant IRE1/XBP1 signaling has been implicated in several cancers, its role in RMS is unknown. Here, we show that the IRE1/XBP1 pathway is highly activated in both embryonal and alveolar RMS cell lines, as well as in patient tumor specimens. Genetic or pharmacological inhibition of IRE1/XBP1pathway suppresses cell proliferation, enhances chemosensitivity, and promotes terminal differentiation in RMS cells. Moreover, IRE1/XBP1 inhibition reduces the cancer stem cell population and suppresses the epithelial-to-mesenchymal transition phenotype. The IRE1/XBP1 signaling drives RMS progression through XBP1-mediated upregulation of BMPR1A, leading to the activation of BMP-Smad1 signaling. In addition, inducible knockdown of XBP1 significantly attenuates RMS tumor growth in xenograft mouse models. Similarly, inhibition of IRE1 activity using a small molecule reduces RMS xenograft growth in mice. Collectively, our study demonstrates that the IRE1/XBP1 axis is a key regulator of RMS growth and differentiation and highlights that this pathway can be a promising therapeutic target for treatment of patients with RMS.