T uberous sclerosis (TSC) is a familial tumor syndrome due to mutations in TSC1 or TSC2, in which progression to malignancy is rare. Primary Tsc2^–/– murine embryo fibroblast cultures display early senescence with overexpression of p21^CIP1/WAF1 that is rescued by loss of TP53. Tsc2^–/–TP53^–/– cells, as well as tumors from Tsc2^+/– mice, display an mTOR-activation signature with constitutive activation of S6K, which is reverted by treatment with rapamycin. Rapamycin also reverts a growth advantage of Tsc2^–/–TP53^–/– cells. Tsc1/Tsc2 does not bind directly to mTOR, however, nor does it directly influence mTOR kinase activity or cellular phosphatase activity. There is a marked reduction in Akt activation in Tsc2^–/–TP53^–/– and Tsc1^–/– cells in response to serum and PDGF, along with a reduction in cell ruffling. PDGFRα and PDGFRβ expression is markedly reduced in both the cell lines and Tsc mouse renal cystadenomas, and ectopic expression of PDGFRβ in Tsc2-null cells restores Akt phosphorylation in response to serum, PDGF, EGF, and insulin. This activation of mTOR along with downregulation of PDGFR PI3K-Akt signaling in cells lacking Tsc1 or Tsc2 may explain why these genes are rarely involved in human cancer. This is in contrast to PTEN, which is a negative upstream regulator of this pathway.