Loss of Tsc1/Tsc2 activates mTOR and disrupts PI3K-Akt signaling through downregulation of PDGFR
Hongbing Zhang, Gregor Cicchetti, Hiroaki Onda, Henry B. Koon, Kirsten Asrican, Natalia Bajraszewski, Francisca Vazquez, Christopher L. Carpenter, David J. Kwiatkowski
Hongbing Zhang, Gregor Cicchetti, Hiroaki Onda, Henry B. Koon, Kirsten Asrican, Natalia Bajraszewski, Francisca Vazquez, Christopher L. Carpenter, David J. Kwiatkowski
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Article Oncology

Loss of Tsc1/Tsc2 activates mTOR and disrupts PI3K-Akt signaling through downregulation of PDGFR

Abstract

Tuberous 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 p21CIP1/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.

Authors

Hongbing Zhang, Gregor Cicchetti, Hiroaki Onda, Henry B. Koon, Kirsten Asrican, Natalia Bajraszewski, Francisca Vazquez, Christopher L. Carpenter, David J. Kwiatkowski

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Figure 7

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Correction of Akt signaling in Tsc2–/–TP53–/– cells by PDGFRβ transfecti...
Correction of Akt signaling in Tsc2–/–TP53–/– cells by PDGFRβ transfection. (a) YPH-Akt translocation. Tsc2–/–TP53–/– cells were cotransfected with YPH-Akt and PDGFRβ or an empty vector and were serum starved overnight before PDGF addition. The left and right panels are fluorescent images of the cells before and 4 minutes after PDGF addition, respectively. PDGF stimulation leads to uniform YFP staining of the plasma membrane in cells transfected with the PDGFRβ, but not in cells transfected with empty vector. The staining intensity in cross-sections of the same cells is shown in the graphs at right. (b) Recovery of Akt activation. Immunoblot analysis of Tsc2–/–TP53–/– cells transfected with empty vector (left) or PDGFRβ (right) that were serum starved and then stimulated with 50 ng/ml PDGF-BB (P), 100 ng/ml EGF (E), 10% serum (S), or 0.1 μM insulin (I). Note the increase in the Akt phosphorylation in the PDGFR-transfected cells in response to all stimuli.