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Tuberous sclerosis is an autosomal dominant genetic syndrome in which afflicted individuals succumb to benign hyperproliferative lesions termed hamartomas in a number of different tissues. The disease is caused by mutations in one of two genes termed TSC1 and TSC2, whose protein products form a stoichiometric complex that is thought to function by inhibiting the activity of the small GTPase Rheb, thereby regulating the activity of the protein kinase mTOR. mTOR is thought to regulate protein translation through phosphorylation of its downstream substrates 4EBP1, an inhibitor of protein translation, and p70S6K. It is currently unclear exactly which mRNAs are sensitive to translational regulation by mTOR activity. mTOR activity is regulated by many inputs, including growth factors and a variety of cellular nutrients including glucose, amino acids and ATP. To further investigate the regulation of mTOR activity by TSC1 and TSC2 and characterize their effects on mRNA translation, we used mouse embryonic fibroblasts (MEFs) lacking either TSC1 or TSC2 as model systems. Our results show that deletion of either TSC1 or TSC2 raises the basal level of mTOR activity (visualized by increased 4EBP1, p70S6K, and ribosomal S6 phosphorylation), such that fewer signals are required to maintain maximal mTOR activity. However, MEFs lacking TSC1or TSC2 are still subject to regulation by either growth factors or nutrients, therefore precluding a linear relationship between either of these signals and hamartin/tuberin function. Increased mTOR activity is accompanied by increased assembly of the eIF4F complex, and increased recruitment of mRNAs to polysomes. To analyze global translational regulation, we applied genome wide DNA microarrays using polysomal and subpolysomal mRNA fractions from TSC1/TSC2 deficient MEFs. We have identified some mRNAs that are translationally regulated by mTOR under differing conditions of nutrient and growth factor availability. These results, as well as additional data, are consistent with a model in which mTOR activity is defined by the additive sum of multiple signals, with specific mRNAs being translationally upregulated to respond to cellular environmental requirements.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]