Autocrine motility factor and its receptor expression in musculoskeletal tumors

Management of aggressive malignant musculoskeletal tumors is clinically challenging and awaits the identification of regulator(s) that can be therapeutically used to improve patient outcome. Autocrine motility factor (AMF), a secreted cytokine, is known to alter the bone microenvironment by linking to its receptor AMFR (AMF Receptor), leading to tumor progression. It was noted that both the ligand and its receptor belong to the moonlighting family of proteins, as they contribute to intracellular metabolic function such as glycolysis and gluconeogenesis by expressing glucose-6-phosphate isomerase AMF/GPI and higher protein degradation by expressing AMFR/gp78 functioning as ubiquitin ligase activity. Thus, AMF/GPI and AMFR/gp78 contribute to higher metabolic turnover of protein and glucose. Recently, a large-scale cohort study including 23 different histological types of musculoskeletal tumors revealed that patients with osteosarcoma, multiple myeloma, rhabdomyosarcoma, and angiosarcoma tend to express higher levels of AMF, whereas multiple myeloma patients expressed high levels of AMFR. Consistently, the cellular data showed that a variety of musculoskeletal tumors express AMF and components of bone microenvironment express AMFR. Thus, a novel outlook suggests a cellular link and cross-talk between musculoskeletal tumors and the skeletal milieu are regulated by AMF-AMFR signaling. This review will highlight the pharmacological need for AMF and AMFR inhibitors as unmet medical needs for patients with malignant musculoskeletal tumors.