Argininosuccinate Synthetase 1 Loss in Invasive Bladder Cancer Regulates Survival Through General Control Nonderepressible 2 Kinase-Mediated Eukaryotic Initiation Factor 2α Activity and Is Targetable By Pegylated Arginine Deiminase

Loss of argininosuccinate synthetase 1 (ASS1), a key enzyme for arginine synthesis, occurs in many cancers, making cells dependent on extracellular arginine and targetable by the arginine-degrading enzyme pegylated arginine deiminase (ADI-PEG 20). We evaluated ASS1 expression and effects of ASS1 loss in bladder cancer which, despite affecting >70,000 people in the United States annually, has limited therapies. ASS1 loss was identified in conventional and micropapillary urothelial carcinoma, small cell, and squamous cell carcinoma subtypes of invasive bladder cancer, as well as in T24, J82, and UM-UC-3 but not in 5637, RT112, and RT4 cell lines. ASS1-deficient cells showed preferential sensitivity to ADI-PEG 20, evidenced by decreased colony formation, reduced cell viability, and increased sub-G1 fractions. ADI-PEG 20 induced general control nonderepressible 2-dependent eukaryotic initiation factor 2α phosphorylation and activating transcription factor 4 and C/EBP homologous protein up-regulation, associated with caspase-independent apoptosis and autophagy. These effects were ablated with selective siRNA silencing of these proteins. ASS1 overexpression in UM-UC-3 or ASS1 silencing in RT112 cells reversed these effects. ADI-PEG 20 treatment of mice bearing contralateral flank UM-UC-3 and RT112 xenografts selectively arrested tumor growth in UM-UC-3 xenografts, which had reduced tumor size, reduced Ki-67, and increased terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. This suggests that ASS1 loss occurs in invasive bladder cancer and is targetable by ADI-PEG 20.