Abstract Introduction: Myxoid liposarcoma (MLS) is the second most common type of liposarcoma and an aggressive disease with particular propensity to develop hematogenic metastases. Ninety percent of MLS are characterized by a reciprocal translocation t(12;16) (q13;p11), leading to the pathogenic gene fusion of FUS and DDIT3. The resulting chimeric FUS-DDIT3 fusion protein is suggested to play a crucial role in MLS pathogenesis, although the specific mechanism of action remains to be substantiated. Aiming at the preclinical identification of novel therapeutic options, we here investigate the functional relevance of FUS-DDIT3 in IGF-IR/PI3K/Akt signal transduction. Experimental Procedures: Immunohistochemical analyses of IGF-IR/PI3K/Akt signaling effectors and modulators were performed in a comprehensive cohort of clinical MLS specimens. FUS-DDIT3-dependent activation of the IGF-IR/PI3K/Akt signaling cascade was analyzed by siRNA and immunoblotting in vitro. Cell proliferation and FACS assays were performed in multiple tumor-derived MLS cell lines. An established MLS chorioallantoic membrane model (CAM) was employed for in vivo confirmation of the preclinical in vitro data. Results: In a significant subset of MLS specimens, immunohistochemical staining revealed elevated phosphorylation levels of various signaling components, representing a strong indication of activated IGF-IR/PI3K/Akt signaling to be a frequent feature in MLS. IGF-IR inhibition significantly suppressed the IGF-IR/PI3K/Akt signaling cascade, associated with impairment of MLS cell viability and induction of apoptosis in vitro and in vivo. Furthermore, siRNA-mediated knockdown of FUS-DDIT3 led to dephosphorylation of IGF-IR/PI3K/Akt signaling components, implying that the FUS-DDIT3 fusion protein is involved in the IGF-IR regulated signaling cascade. Conclusions: Our preclinical study emphasizes the pivotal role of the IGF-IR/PI3K/Akt signaling pathway in MLS pathogenesis and indicates its functional dependence on the MLS-specific FUS-DDIT3 fusion protein. Furthermore, our in vitro and in vivo results demonstrate that targeting the IGF-IR/PI3K/Akt signaling pathway provides a rational, molecularly founded therapeutic strategy in the treatment of MLS. Citation Format: Marcel Trautmann, Magdalene Alice Cyra, Christian Bertling, Ilka Isfort, Jasmin Menzel, Konrad Steinestel, Inga Grünewald, Bianca Altvater, Claudia Rossig, Pierre Åman, Eva Wardelmann, Sebastian Huss, Wolfgang Hartmann. Functional characterization of IGF-IR/PI3K/Akt signaling in myxoid liposarcoma [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B04.