Synergistic induction of apoptosis of rheumatoid arthritis synovial cells by H2O2 and N-acetyl-leucyl-leucyl-norleucinal

Abstract

The effects of proteolysis inhibitors on hydrogen peroxide (H2O2)-induced apoptosis were examined in cultured human synovial cells of rheumatoid arthritis (RA) patients. RA synovial cells were resistant to apoptosis induced by H2O2. In the presence of 100 μM N-acetyl-leucyl-leucylnorleucinal (ALLN, known as calpain inhibitor 1 and also a proteasome inhibitor), but not N-acetyl-leucyl-leucylmethioninal (ALLM), apoptotic cell death was elicited by 400 μM H2O2 at a concentration that alone never induced cell death. ALLN induced the expression of tumor suppressor p53 protein and p21WAF-1 protein, probably through inhibition of proteasome. H2O2 further potentiated ALLN-induced p53 expression. H2O2 appeared to activate c-Jun N-terminal kinase (JNK) as well as extracellular signal-regulated kinase (ERK) and AKT. After administration of H2O2 and p53 induction by ALLN, we found that either one alone is insufficient to induce apoptosis of RA synovial cells but their combination synergistically does so. These results suggest that induction of p53 by ALLN may be potentially important for triggering H2O2-induced apoptosis processes in RA synovial cells.