The mammalian Target of Rapamycin (mTOR) is a Ser/Thr kinase that regulates cell growth through its control of cell cycle progression, translation, ribosome biogenesis and metabolism. Dysregulation of the mTOR signaling pathway is associated with tumorigenesis. Rapamycin, an inhibitor of mTOR signaling, is undergoing clinical trials because it is well tolerated and suppresses the growth of numerous human tumors. Regrettably, resistance to the antiproliferative effects of rapamycin occurs across different cancer cell types. A key observation made in our lab is that hepatocyte proliferation in the late gestation fetal rat is resistant to the anti-proliferative effects of rapamycin; while adult hepatocytes are sensitive. The fact that both transformed cell lines and fetal rat hepatocytes share the phenotype of rapamycin resistance led us to study the mechanisms accounting for rapamycin resistance in hepatic cells. To address this issue, we examined the effect of rapamycin on cell proliferation, cell cycle progression, cell size, transcription and translation in a spectrum of hepatic cell lines that ranged from non-tumorigenic to highly tumorigenic. The data show that alterations of signaling downstream from mTOR may account for cell cycle dysregulation and differential gene expression in tumorigenic and non-tumorigenic cells; nevertheless, the mechanism for rapamycin resistance is heterogeneous in hepatic cells.

Microarray analysis revealed a set of genes whose expression was regulated by rapamycin independent of growth arrest, raising the possibility that transcriptional control downstream of mTOR could be attributed to critical transcription factor(s). Unexpectedly, we discovered an overlap in the regulation of gene expression by mTOR and signaling through the Myc/Max/Mad network. An important implication is that Myc and mTOR may work in concert to promote tumor cell growth by regulating gene expression and translation in cancer cells. The regulation of gene expression via mTOR appears to be independent of Myc, indicating that two distinct mechanisms may mediate transcription regulation via these pathways. Overall, our data provides distinctive insights into the role of mTOR in the regulation of gene expression and the use of rapamycin as a chemotherapeutic agent.