Computational and biochemical studies of isothiocyanates as inhibitors of proteasomal cysteine deubiquitinases in human cancer cells.

Isothiocyanates (ITCs) are natural chemoprotective products found abundantly in cruciferous vegetables. However, the cancer-relevant targets and molecular mechanisms of ITCs remain unclear. We hypothesize that ITCs, as electrophiles, can interact with the catalytic triads (CYS, HIS, and ASP) of the proteasomal cysteine deubiquitinases USP14 and UCHL5, ultimately inhibiting their activities. In the current study, we exploited this possibility by performing both computational docking and biochemical validation assays using human breast and prostate cancer cell models. Docking results suggest that benzyl isothiocyanate, phenethyl isothiocyanate, and DL-sulforaphane are more potent inhibitors of UCHL5 than USP14, and these ITCs could interact with the catalytic triads of UCHL5 and USP14. Indeed, ubiquitin vinyl sulfone assay confirmed the inhibitory activity of each ITC on the ubiquitin-binding activity of UCHL5 and USP14. We also found that inhibition of USP-14 and UCHL5 activities by the ITCs caused increased levels of USP14 and UCHL5 proteins, but not the third 19S-deubiquitinating enzyme (DUB), POH1/RPN11, suggesting feedback loop activation and further supporting that ITCs are inhibitors of proteasomal cysteine DUBs. Associated with DUB inhibition by ITCs, ubiquitinated proteins were significantly increased, accompanied with induction of apoptosis, inhibition of proliferation and suppression of cell invasion. Our findings of ITCs as proteasomal cysteine DUB inhibitors should provide insightful information for designing, discovering and developing potent, specific 19S-DUB inhibitors for cancer therapies.