Ritonavir interacts with bortezomib to enhance protein ubiquitination and histone acetylation synergistically in renal cancer cells.

OBJECTIVE: To investigate the combined effects of the HIV protease inhibitor ritonavir and proteasome inhibitor bortezomib on renal cancer cells. Ritonavir induces endoplasmic reticulum (ER) stress and we hypothesized that inhibiting proteasome activity under ER stress would further inhibit cancer cell growth by enhancing protein ubiquitination.
METHODS: The effectiveness of the combination of ritonavir and bortezomib on renal cancer cells (Caki-1, ACHN, 786-O, 769-P) was assessed by MTS assay, colony formation assay, cell cycle analysis, and annexin-V assay. In vivo efficacy was evaluated using mice subcutaneous tumor models. Induction of ER stress, protein ubiquitination, histone acetylation, and changes in the expression of histone deacetylase (HDAC) were evaluated by Western blotting.
RESULTS: Ritonavir in combination with bortezomib induced apoptosis and inhibited renal cancer growth synergistically at clinically feasible concentrations. In subcutaneous tumor models using Caki-1 cells, 10-day treatment with the combination was well tolerated and inhibited tumor growth significantly. Ritonavir induced ER stress and the combination enhanced protein ubiquitination synergistically. The combination was also found to induce histone acetylation by suppressing the HDAC expression.
CONCLUSION: The combination of ritonavir and bortezomib inhibits renal cancer growth synergistically. The effectiveness of the combination is caused by protein ubiquitination and histone acetylation. Our results provide a rationale for investigating the combination in patients with renal cancer.