FKBP-RAPAMYCIN INHIBITS A CYCLIN-DEPENDENT KINASE-ACTIVITY AND A CYCLIN D1-CDK ASSOCIATION IN EARLY G1 OF AN OSTEOSARCOMA CELL-LINE

Upon entering a cell the natural product rapamycin, like the structura lly related immunosuppressant FK506, associates with members of the FK BP family of proteins. One or more of the resulting FKBP-rapamycin com plexes blocks signaling pathways emanating from some growth factor rec eptors. Recently, the addition of rapamycin was shown to inhibit the p hosphorylation and activation of a 70-kDa ribosomal S6 protein kinase, which normally occurs minutes after the activation of certain cytokin e and growth factor receptors. We now report that rapamycin can be add ed 4 to 6 h after the addition of serum growth factors to quiescent hu man osteosarcoma cells and still arrest these cells in G1. This window of action correlates with the inducible appearance of a cyclin-depend ent kinase (cdk) activity, and the induction of this activity is inhib ited by the addition of rapamycin. Furthermore, p36cyclin D1 associate s with this cdk protein complex in lysates of untreated cells, but doe s not associate with this cdk protein complex in lysates of rapamycin- treated cells. Together, these studies demonstrate that FKBP-rapamycin can modulate a cyclin-dependent kinase activity and a cyclin D1-cdk a ssociation during early G1 in MG-63 human osteosarcoma cells.