PIN1 prolyl isomerase promotes initiation and progression of bladder cancer through the SREBP2-mediated cholesterol biosynthesis pathway

Identities of functional pSer/Thr.Pro protein substrates of the PIN1 prolyl isomerase and its effects on downstream signaling in bladder carcinogenesis remain largely unknown. Phenotypically, we found that PIN1 positively regulated bladder cancer cell proliferation, cell motility and urothelium clearance capacity in vitro and controlled tumor growth and potential metastasis in vivo. Mechanistically, we observed a negative enrichment of SREBP2-driven cholesterol metabolism pathways and a decrease in free/total cholesterol levels in PIN1-knockout bladder cancer cells. Moreover, we showed that PIN1 interacted with SREBP2 following its phosphorylation by the JNK MAP kinase at Ser455, which lies near the Site-2 cleavage site that generates the active, nuclear-form of SREBP2. Therapeutically, a combination of the sulfopin PIN1 covalent inhibitor and the simvastatin HMGCoA reductase inhibitor suppressed cell proliferation in vitro and tumor growth in vivo synergistically. Together, these findings emphasize that PIN1 can act as a driver and potential therapeutic target in bladder cancer.