RAPAMYCIN INHIBITS PROTEASOME ACTIVATOR EXPRESSION AND PROTEASOME ACTIVITY

Rapamycin (RAPA) is a potent immunosuppressive drug, and certain of it s direct or indirect targets might be of vital importance to the regul ation of an immune response. In this study, we used differential hybri dization to search for human genes whose expression was sensitive to R APA. Seven RAPA-sensitive genes were found and one of them encoded a p rotein with high homology to the ct subunit of a proteasome activator (PA28 beta). This gene was later found to code for the IJ subunit of t he proteasome activator (PA28 beta). Activated T and B cells had up-re gulated PA28 beta expression at the mRNA level. Such up-regulation cou ld be suppressed by RAPA, FK506, and cyclosporin A. RAPA and FK506 als o repressed the up-regulated PA28 alpha messages in phytohemagglutinin (PHA) stimulated T cells. At the protein level, RAPA inhibited PA28 a lpha and PA28 beta in the activated T cells according to immunoblottin g and confocal microscopy. Probably as a consequence, there was a four fold increase of proteasome activities in the peripheral blood mononuc lear cell lysate after the PHA activation. RAPA could inhibit the enha nced part of the proteasome activity. Considering the critical role pl ayed by the proteasome in degrading regulatory proteins, our data sugg est that the proteasome activator is a relevant and important downstre am target of rapamycin, and that the immune response could be modulate d through the activity of the proteasome.