Divergent effects of acute and chronic PPT1 inhibition in melanoma.

Macroautophagy/autophagy-lysosome function promotes growth and survival of cancer cells, making them attractive targets for cancer therapy. One intriguing lysosomal target is PPT1 (palmitoyl-protein thioesterase 1). PPT1 inhibitors derived from chloroquine block autophagy, have significant antitumor activity in preclinical models and are being developed for clinical trials. However, the role of PPT1 in tumorigenesis remains poorly understood. Here we report that in melanoma cells, acute siRNA or pharmacological PPT1 inhibition led to increased ferroptosis sensitivity and significant loss of viability, whereas chronic PPT1 knockout using CRISPR-Cas9 produced blunted ferroptosis that led to sustained viability and growth. Each mode of PPT1 inhibition produced lysosome-autophagy inhibition but distinct proteomic changes, demonstrating the complexity of cellular adaptation mechanisms. To determine whether total genetic loss of Ppt1 would affect tumorigenesis in vivo, we developed a Ppt1 conditional knockout mouse model. We then crossed it into the BrafCA, PtenloxP, Tyr:CreERT2 melanoma mouse model to investigate the impact of Ppt1 loss on tumorigenesis. Loss of Ppt1 had no impact on melanoma histology, time to tumor initiation, or survival of tumor-bearing mice. These results suggest that chemical PPT1 inhibitors produce different adaptations than genetic PPT1 inhibition, and additional studies are warranted to fully understand the mechanism of chloroquine derivatives that target PPT1 in cancer.