Caffeine modulates immunoproteasome activity and content in colorectal adenocarcinoma cells

Proteasomes hydrolyze most intracellular proteins. Immunoproteasome is a specific form of proteasome implicated in inflammation, cancer and autoimmune diseases. Modulation of immunoproteasome activity and content is a promising direction for the management of socially important pathologies. Using previously obtained reporter colorectal cancer cell lines, we tested how various commonly used compounds including ibuprofen, acetylsalicylic acid, vitamin C, caffeine and others, affect immunoproteasome expression. Methods: Immunoproteasome subunit expression was assessed by flow cytometry, qPCR and Western blot. Proteasome activity was tested using fluorogenic substrates and the activity probe. Transcriptome analysis was performed to identify patterns of gene expression changes. Results: Among the tested compounds caffeine was the only drug that revealed modest reduction in quantity of immunoproteasomes. The effect of caffeine on immunoproteasomes varied between cell lines and was stronger as a result of prolonged treatment. The reduction of immunoproteasome content in cells treated with caffeine coincided with decreased expression of immunoproteasome subunits and downregulation of Nrf3 transcription factor and a PAC4 proteasome assembly chaperone. Moreover, decreased expression of immunoproteasome subunits in cells treated with caffeine coincided with the reduction of oxidative stress levels. Caffeine did not affect the degradation of immunoproteasomes by autophagy. Conclusions: Modulation of immunoproteasome expression by caffeine might help to optimize existing and develop novel strategies for the treatment of colorectal cancer and several autoimmune diseases.