How an inhibitor of the HIV-I protease modulates proteasome activity

The human immunodeficiency virus, type I protease inhibitor Ritonavir has b een used successfully in AIDS therapy for 4 years. Clinical observations su ggested that Ritonavir may exert a direct effect on the immune system unrel ated to inhibition of the human immunodeficiency virus, type I protease. In fact, Ritonavir inhibited the major histocompatibility complex class I res tricted presentation of several viral antigens at therapeutically relevant concentrations (5 mu M). In search of a molecular target we found that Rito navir inhibited the chymotrypsin-like activity of the proteasome whereas th e tryptic activity was enhanced. In this study we kinetically analyzed how Ritonavir modulates proteasome activity and what consequences this has on c ellular functions of the proteasome. Ritonavir is a reversible effector of proteasome activity that protected the subunits MB-1 (X) and/or LMP7 from c ovalent active site modification with the vinyl sulfone inhibitor(125)I-NLV S, suggesting that they are the prime targets for competitive inhibition by Ritonavir. At low concentrations of Ritonavir (5 mu M) cells were more sen sitive to canavanine but proliferated normally whereas at higher concentrat ions (50 mu M) protein degradation was affected, and the cell cycle was arr ested in the G(1)/S phase, Ritonavir thus modulates antigen processing at c oncentrations at which vital cellular functions of the proteasome are not y et severely impeded. Proteasome modulators may hence qualify as therapeutic s for the control of the cytotoxic immune response.