ASAP1 and ARF1 Regulate Myogenic Differentiation in Rhabdomyosarcoma by Modulating TAZ Activity.

Despite aggressive, multimodal therapies, the prognosis of patients with refractory or recurrent rhabdomyosarcoma (RMS) has not improved in four decades. Because RMS resembles skeletal muscle precursor cells, differentiation-inducing therapy has been proposed for patients with advanced disease. In RAS-mutant PAX fusion-negative RMS (FN-RMS) preclinical models, MEK1/2 inhibition (MEKi) induces differentiation, slows tumor growth, and extends survival. However, the response is short-lived. A better understanding of the molecular mechanisms regulating FN-RMS differentiation could improve differentiation therapy. In this study, we identified a role in FN-RMS differentiation for ASAP1, an ADP ribosylation factor (ARF) GTPase-activating protein (GAP) with both proinvasive and tumor-suppressor functions. We found that ASAP1 knockdown inhibited differentiation in FN-RMS cells. Interestingly, knockdown of the GTPases ARF1 or ARF5, targets of ASAP1 GAP activity, also blocked differentiation of FN-RMS. We discovered that loss of ARF pathway components blocked myogenic transcription factor expression. Therefore, we examined the effects on transcriptional regulators. MEKi led to the phosphorylation and inactivation of WW domain-containing transcriptional regulator 1 (WWTR1; TAZ), a homolog of the pro-proliferative transcriptional co-activator YAP1, regulated by the Hippo pathway. However, loss of ASAP1 or ARF1 blocked this inactivation, which inhibits MEKi-induced differentiation. Finally, MEKi-induced differentiation was rescued by dual knockdown of ASAP1 and WWTR1. This study shows that ASAP1 and ARF1 are necessary for myogenic differentiation, providing a deeper understanding of differentiation in FN-RMS and illuminating an opportunity to advance differentiation therapy. Implications: ASAP1 and ARF1 regulate MEKi-induced differentiation of FN-RMS cells by modulating WWTR1 (TAZ) activity, supporting YAP1/TAZ inhibition as a FN-RMS differentiation therapy strategy.