Usp9x Promotes Survival in Human Pancreatic Cancer and Its Inhibition Suppresses Pancreatic Ductal Adenocarcinoma In Vivo Tumor Growth

Usp9x has emerged as a potential therapeutic target in some hematologic malignancies and a broad range of solid tumors including brain, breast, and prostate. To examine Usp9x tumorigenicity and consequence of Usp9x inhibition in human pancreatic tumor models, we carried out gain- and loss-of-function studies using established human pancreatic tumor cell lines (PANC1 and MIAPACA2) and four spontaneously immortalized human pancreatic patient-derived tumor (PDX) cell lines. The effect of Usp9x activity inhibition by small molecule deubiquitinase inhibitor G9 was assessed in 2D and 3D culture, and its efficacy was tested in human tumor xenografts. Overexpression of Usp9x increased 3D growth and invasion in PANC1 cells and up-regulated the expression of known Usp9x substrates Mcl-1 and ITCH. Usp9x inhibition by shRNA-knockdown or by G9 treatment reduced 3D colony formation in PANC1 and PDX cell lines, induced rapid apoptosis in MIAPACA2 cells, and associated with reduced Mcl-1 and ITCH protein levels. Although G9 treatment reduced human MIAPACA2 tumor burden in vivo, in mouse pancreatic cancer cell lines established from constitutive (8041) and doxycycline-inducible (4668) KrasG12D/Tp53R172H mouse pancreatic tumors, Usp9x inhibition increased and sustained the 3D colony growth and showed no significant effect on tumor growth in 8041-xenografts. Thus, Usp9x inhibition may be therapeutically active in human PDAC, but this activity was not predicted from studies of genetically engineered mouse pancreatic tumor models.