The 20S proteasome is a 700-kDa protease, responsible for the degradation of most cytosolic proteins and in particular, short-lived proteins such as p53, c-myc, c-jun, various cyclins, NF-κB, and I-κB. These proteins are critical in cell proliferation, cell cycle regulation, transcriptional regulation and, under certain conditions, are involved in regulation of apoptosis.
26 There are several reports in the literature about proteasome involvement in the apoptotic process as both a pro- and an antiapoptotic agent, depending mainly on cell type or proliferating activity of the cells. Meriin et al.
53 reported both pro- and antiapoptotic effects of proteasome inhibitor with the same cell line, depending on the conditions of treatment. We have observed upregulation of immunoproteasome in the lenses of IFN-γ–expressing transgenic mice.
23 In the current study, we sought to verify the role of the proteasome in IFN-γ–induced changes in lens epithelial cells. We observed that MG132, a peptide aldehyde inhibitor of the catalytic subunit of the proteasome, at 10 μM concentration and 12 hours of incubation, did not cause lens epithelial cell apoptosis. When the cells were cotreated with IFN-γ and MG132, MG132 completely blocked IFN-γ–induced apoptosis, as observed by phase-contrast microscopy and flow cytometry. MG132 is highly potent, but also inhibits calpain and cathepsin B.
54 Therefore, we also tested lactacystin, considered to be a more proteasome-specific inhibitor,
39 although it has been shown to inhibit the human platelet cathepsin A-like protease.
55 Lactacystin also prevents IFN-γ–induced apoptosis. The results from the two inhibitors together indicate that proteasome inhibitors can provide protection against IFN-γ–induced apoptosis of lens epithelial cells, or at least delay the onset of the apoptosis cascade. Lin et al.
56 have reported that the concentration of a proteasome inhibitor is critical for its action as a pro- or antiapoptotic agent. The proapoptotic effect of lactacystin on bovine lens epithelial cells observed by Andersson et al.
49 may have been due to the longer incubation time of 25 hours. We expect that at higher concentrations or longer incubation periods, proteasome inhibitors would be proapoptotic in most cells. We are currently investigating the mechanism by which proteasome inhibition provides protection against IFN-γ–induced apoptosis.