Rapamycin resistance tied to defective regulation of p27(Kip1)

The potent antiproliferative activity of the macrolide antibiotic rapamycin is known to involve binding of the drug to its cytosolic receptor, FKBP12, and subsequent interaction with targets of rapamycin, resulting in inhibition of p70 S6 kinase (p70(S6K)). However, the downstream events that lead to inhibition of cell cycle progression remain to be elucidated. The antiproliferative effects of rapamycin are associated with prevention of mitogen-induced downregulation of the cyclin-dependent kinase inhibitor p27(Kip1), suggesting that the latter may play an important role in the growth pathway targeted by rapamycin. Murine BC3H1 cells, selected for resistance to growth inhibition by rapamycin, exhibited an intact p70(S6K) pathway but had abnormally low p27 levels that were no longer responsive to nitrogens or rapamycin. Fibroblasts and T lymphocytes from mice with a targeted disruption of the p27(Kip1) gene had impaired growth-inhibitory responses to rapamycin. These results suggest that the ability to regulate p27(Kip1) levels is important for rapamycin to exert its antiproliferative effects.