Enhanced NRF1-mediated proteasome bounce-back and nelfinavir-mediated bortezomib sensitization in gemcitabine/cisplatin-resistant bladder cancer cells.

Proteasome is an established therapeutic target in cancer, and proteasome inhibitors (PIs) such as bortezomib (BTZ) are clinically used for hematologic malignancies, including multiple myeloma. Although PIs have also been evaluated in solid tumors, their clinical efficacy has been limited. We therefore sought to develop a strategy to enhance PI efficacy using gemcitabine/cisplatin (GC)-resistant bladder cancer cells as a model of refractory solid cancer. One mechanism that limits PI efficacy is the nuclear factor erythroid 2-related factor 1 (NRF1, NFE2L1)-mediated proteasome bounce-back response, which restores proteasome capacity after inhibition. Because DNA damage-inducible 1 homolog 2 (DDI2)-mediated processing is required for NRF1 activation, we examined whether nelfinavir (NFV), a reported DDI2 inhibitor, could enhance BTZ sensitivity. GC-resistant T24GC cells exhibited increased basal proteasome activity, cross-resistance to PIs, and a stronger NRF1-mediated bounce-back response than parental T24 cells. NFV significantly enhanced BTZ-induced cytotoxicity and apoptosis in T24GC cells. Unexpectedly, NFV did not suppress the bounce-back response but instead further enhanced NRF1 activation in T24GC cells, possibly reflecting pharmacological actions of NFV beyond DDI2 inhibition. These findings identify enhanced NRF1-mediated proteasome adaptation as a feature of GC resistance and highlight the need for highly selective DDI2 inhibitors to evaluate DDI2-NRF1 targeting as a strategy to improve PI efficacy in drug-resistant solid cancers.