Caffeine modulates immunoproteasome activity and content in colorectal adenocarcinoma cells

Proteasomes hydrolyze most intracellular proteins. Immunoproteasome is a form of proteasome implicated in inflammation, cancer and autoimmune diseases. Modulation of immunoproteasome activity is a promising approach against several pathologies. Using previously obtained reporter colorectal cancer cell lines, we tested how commonly used compounds including ibuprofen, acetylsalicylic acid, vitamin C, caffeine and others, affect immunoproteasome expression. Flow cytometry, qPCR and Western blot were used to evaluate immunoproteasome subunit expression. Proteasome activity was tested using fluorogenic substrates and the activity-based probe. Transcriptome analysis was performed to identify patterns of gene expression changes. Interestingly, caffeine was the only drug that stimulated modest reduction in quantity of immunoproteasomes. The effect of caffeine varied between cell lines and was stronger as a result of prolonged treatment. The reduction of immunoproteasome content in cells coincided with decreased expression of immunoproteasome subunits, genes encoding the Nrf3 transcription factor and a PAC4 proteasome assembly chaperone, as well as the reduced levels of oxidative stress. Caffeine did not affect the degradation of immunoproteasomes by autophagy. Obtained results uncover novel biological effects of caffeine, our data might help to optimize existing and develop new strategies for the treatment of colorectal cancer and several autoimmune diseases.