Rapamycin selectively inhibits the growth of childhood rhabdomyosarcoma cells through inhibition of signaling via the type I insulin-like growth factor receptor.

We show that cell lines derived from childhood alveolar rhabdomyosarcoma (RMS) are very sensitive to the growth-inhibitory effects of the immunosuppressive agent rapamycin (RAP), compared to other human cell lines (50% inhibitory concentration range of 0.1-8 ng/ml, compared to 1280 to > 10,000 ng/ml). Our data suggest that the sensitivity of RMS lines is due to RAP inhibition of insulin-like growth factor 1 receptor-mediated signaling, which is essential for continued proliferation of RMS cells. The embryonal RMS line Rh1, which was resistant to RAP in serum-containing medium (50% inhibitory concentration, 4180 ng/ml), was highly sensitive under autocrine conditions of growth, indicating that resistance was due to paracrine signaling pathways insensitive to RAP action. FK506 reversed RAP action in all cell lines, indicating a dependence on complexing with the cytosolic FK506-binding protein for activity.