Rapamycin inhibits didemnin B‐induced apoptosis in human HL‐60 cells: Evidence for the possible involvement of FK506‐binding protein 25

In the present paper we show that the immunosuppressant rapamycin inhibits the induction of apoptosis by didemnin B in human promyeloid HL-60 cells. The mechanism of this inhibition is investigated using FK506, which competes with rapamycin for binding to their common target FK506-binding protein (FKBP)12. The lack of competition for rapamycin-mediated inhibition of didemnin B-induced apoptosis by FK506 suggests that rapamycin inhibits apoptosis through some mechanism other than inhibition of p70 S6 kinase activation. The lack of inhibition of didemnin B-induced apoptosis by inhibitors of phosphatidylinositol 3-kinase and mitogen-activated protein (MAP) kinase kinase further supports the conclusion that rapamycin does not inhibit didemnin B-induced apoptosis through inhibition of the MAP kinase pathway. Furthermore, didemnin B-induced apoptosis is not inhibited by the inhibitors of cyclin-dependent kinase, roscovitine and olomoucine. This indicates that rapamycin does not act through inhibition of cyclin-dependent kinases. Together with the lack of competition for the effect of rapamycin by FK506, our data suggest the possible involvement of the FK506-binding protein, FKBP25, which is localized in the nucleus. This interpretation of our data gains support from the fact that didemnin B does not induce apoptosis in enucleated HL-60 cells, which supports the possible involvement of FKBP25 in the inhibition of apoptosis by rapamycin.