mTOR as a positive regulator of tumor cell responses to hypoxia.

Rapamycin is a clinically approved immunosuppressive agent that has recently shown promising antitumor activities in human patients. In contrast to many conventional chemotherapeutic agents, rapamycin displays a remarkably high level of selectivity for certain types of tumors. The pharmacological activities of rapamycin are attributable to the functional inhibition of a single target protein, termed the mammalian target of rapamycin (mTOR). Because mTOR is widely expressed in both normal and transformed cells, variations in mTOR expression levels are likely not a primary determinant of tumor sensitivity to rapamycin. However, recent studies highlighted an intriguing link between cancer cell sensitivity to rapamycin and deregulated signaling through the phosphoinositide (PI) 3-kinase pathway. These findings have prompted a search for cancer-related responses that are jointly regulated by the PI 3-kinase signaling cascade and mTOR. The oxygen-regulated transcription factor, hypoxia-induced factor (HIF)-1, has emerged as a candidate target for both of these two highly interactive signaling proteins. Here we review evidence that mTOR functions as a positive regulator of HIF-1-dependent responses to hypoxic stress in human cancer cells.