THE UP-REGULATION OF P27(KIP1) BY RAPAMYCIN RESULTS IN G1 ARREST IN EXPONENTIALLY GROWING T-CELL LINES

An immunosuppressant Rapamycin (Rap) has been reported to cause G1 arr est by inhibiting p70 S6 kinase and G1 cyclin/cdks kinase activities w hen added to quiescent cells with mitogens. However, antiproliferative effects of Rap on exponentially growing cells have been poorly invest igated. We examined the intracellular events after the treatment of Ra p in exponentially growing T cells and found that Rap upregulated a cd ks inhibitor, p27(Kip1) at both mRNA and protein levels in Rap-sensiti ve cells. Antiproliferative effect of Rap was mainly ascribed to the i nhibition of cyclin E/cdk2 kinase activity through the formation of cy clin E/cdk2-p27(Kip1) complex rather than inhibition of p70 S6 kinase activity. Furthermore, we showed that Rap-sensitive cells with elevate d p27(Kip1) expression lost sensitivity to Rap when antisense p27(Kip1 ) was introduced, which indicates that the basal level of p27(Kip1) is of the limiting factors that determine the sensitivity to Rap in alre ady cycling cells. These data suggest the presence of a putative thres hold level of p27(Kip1) at late G1 phase in already cycling cells. Rap may cause G1 arrest by upregulating the amount of p27(Kip1) beyond th e threshold in some Rap-sensitive cells that are exponentially growing . (C) 1998 by The American Society of Hematology.