Salinosporamide A, a Marine-Derived Proteasome Inhibitor, Inhibits T Cell Activation through Regulating Proliferation and the Cell Cycle

The appropriate regulation of T cell activity under inflammatory conditions is crucial for maintaining immune homeostasis. Salinosporamide A discovered as a self-resistance product from the marine bacterium Salinospora tropica, has been used as a potent proteasome inhibitor (PI). Although PIs have been developed as novel therapeutics for autoimmune diseases, due to their immunosuppressive effect, whether salinosporamide A inhibits T cell activation remains unknown. The current study finds that salinosporamide A is not cytotoxic, but controls T cell proliferation. Results from our cell cycle arrest analysis revealed that salinosporamide A leads to cell cycle arrest and regulates the expression of cyclin-dependent kinases. Under activated conditions, salinosporamide A abrogated T cell activation by T cell receptor-mediated stimulation, in which the production of cytokines was inhibited by pretreatment with salinosporamide A. Furthermore, we demonstrated that the regulation of T cell activation by salinosporamide A is mediated by suppressing the MAPK pathway. Therefore, our results suggest that salinosporamide A effectively suppresses T cell activation through regulating T cell proliferation and the cell cycle and provides great insight into the development of novel therapeutics for autoimmune diseases or graft-versus-host disease.

(−)-Homosalinosporamide A and Its Mode of Proteasome Inhibition: An X-ray Crystallographic Study

Upon acylation of the proteasome by the β-lactone inhibitor salinosporamide A (SalA), tetrahydrofuran formation occurs by intramolecular alkylation of the incipient alkoxide onto the choroethyl sidechain and irreversibly blocks the active site. Our previously described synthetic approach to SalA, utilizing a bioinspired, late-stage, aldol-β-lactonization strategy to construct the bicyclic β-lactone core, enabled synthesis of (–)-homosalinosporamide A (homoSalA). This homolog was targeted to determine whether an intramolecular tetrahydropyran is formed in a similar manner to SalA. Herein, we report the X-ray structure of the yeast 20S proteasome:homoSalA-complex which reveals that tetrahydropyran ring formation does not occur despite comparable potency at the chymotrypsin-like active site in a luminogenic enzyme assay. Thus, the natural product derivative homoSalA blocks the proteasome by a covalent reversible mode of action, opening the door for further fine-tuning of proteasome inhibition.