Inhibition of cyclin A gene expression in human B cells by an immunosuppressant mizoribine.

Mizoribine has been shown to have beneficial effects in the treatment of rheumatoid arthritis and lupus nephritis, in which abnormal B cell functions are involved. Previous studies demonstrated that mizoribine directly suppresses the function of human B cells. The current study explored in detail the mechanism of the suppression of human B cell responses by mizoribine at the molecular level. Highly purified peripheral blood B cells obtained from normal healthy individuals were stimulated with Staphylococcus aureus Cowan I (SAC) plus IL-2 in the presence or absence of mizoribine or methotrexate for 48 h to 72 h. The expression of cyclin A mRNA was determined by semiquantitative reverse transcriptase-polymerase chain reaction followed by Southern hybridization. Although at pharmacologically attainable concentrations both mizoribine and methotrexate suppressed the production of IgM of SAC-activated B cells, mizoribine, but not methotrexate, decreased the expression of cyclin A protein as well as mRNA in B cells stimulated with SAC + IL-2. Of note, mizoribine facilitated the degradation of cyclin A mRNA in the presence of actinomycin D, indicating that mizoribine shortens the stability of cyclin A mRNA. The results indicate that mizoribine suppresses the expression of cyclin A mRNA in human B cells by down-regulating its stability, and thus down-regulates their responses.