Kaempferol: a new immunosuppressant of calcineurin.

Calcineurin (CN), the Ca(2+)/calmodulin (CaM)-dependant protein phosphatase, is the target for immunosuppressive drugs cyclosporine A (CsA) and FK506. These immunosuppressants can inhibit CN activity after binding with respective immunophilins. Based on the model of screening by using p-nitrophenyl phosphate as a substrate for preliminary screening and (32)P-labeled 19-residue phosphopeptide as a specific substrate for final determination, we found Kaempferol, a natural flavonol, could inhibit CN activity in purified enzyme and Jurkat T-cells. Unlike CsA and FK506, CN inhibition by kaempferol is independent of matchmaker protein and the inhibitory manner is noncompetitive. Through investigation of inhibitions for CNA and a series of its truncated mutants, we suggested that Kaempferol could directly act on the catalytic domain. Data also indicated that the CN inhibition by kaempferol could be enhanced when the enzyme was activated in the presence of CaM and CNB. CNB is necessary for mediating inhibition of enzyme by kaempferol. The result of RT-PCR also indicated that kaempferol had an inhibitory activity against IL-2 gene expression in activated Jurkat cells. All data suggested that kaempferol could be a new immunosuppressant of CN.