The upregulation of p27Kip1 by rapamycin results in G1 arrest in exponentially growing T-cell lines.

An immunosuppressant Rapamycin (Rap) has been reported to cause G1 arrest by inhibiting p70 S6 kinase and G1 cyclin/cdks kinase activities when added to quiescent cells with mitogens. However, antiproliferative effects of Rap on exponentially growing cells have been poorly investigated. We examined the intracellular events after the treatment of Rap in exponentially growing T cells and found that Rap upregulated a cdks inhibitor, p27Kip1 at both mRNA and protein levels in Rap-sensitive cells. Antiproliferative effect of Rap was mainly ascribed to the inhibition of cyclin E/cdk2 kinase activity through the formation of cyclin E/cdk2-p27Kip1 complex rather than inhibition of p70 S6 kinase activity. Furthermore, we showed that Rap-sensitive cells with elevated p27Kip1 expression lost sensitivity to Rap when antisense p27Kip1 was introduced, which indicates that the basal level of p27Kip1 is one of the limiting factors that determine the sensitivity to Rap in already cycling cells. These data suggest the presence of a putative threshold level of p27Kip1 at late G1 phase in already cycling cells. Rap may cause G1 arrest by upregulating the amount of p27Kip1 beyond the threshold in some Rap-sensitive cells that are exponentially growing.