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Growth Control and Anti-Cancer Drug Mechanisms

Xiaofeng Steven Zheng

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National Institutes of Health (NIH)
TOR is a highly conserved protein serine/threonine kinase and the specific target of the anticancer drug rapamycin and rapamycin analogs (rapalogs). It is a central controller of cell growth and metabolism. PI3K-TOR pathway is mutated in over 50% of human tumors, resulting in TOR hyper-activation and uncontrolled cancer growth. Because of the cancer addiction the pathway, TOR is recognized as a major target for anti-cancer drug therapy. TOR regulation of translational initiation is a well known mechanism of growth control. Because translation is predominantly a cytoplasmic event, TOR has been historically viewed as a classical cytoplasmic kinase. However, we have observed that TOR is localized in the nucleus and binds to the promoters of ribosomal genes to stimulate their expression. We further showed that blocking this process is important for rapamycin to inhibit cell growth. Emerging evidence indicates that control of gene expression, particularly genes involved in ribosome biogenesis and other genes involved in protein synthesis and metabolic biosynthesis, is an important mechanism for TOR to promote protein synthesis and metabolism. Dysregulation of these processes has been linked to tumorigenesis and other TOR-related diseases. In contrast to translational control, the understanding of transcriptional regulation and other nuclear functions by TOR signaling is very limited. In this application, we will test the hypothesis that TOR has a broad role in rapamycin- sensitive transcription and other genomic functions inside the nucleus. We will further investigate the molecular mechanisms by which rapamycin represses ribosomal and other protein biosynthetic genes through Maf1 and Rpd3. Accomplishment of this proposal should provide invaluable new insights into TOR-regulated nuclear process that is poorly understood but highly relevant to the biology and therapy of cancer and metabolic diseases.

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