Mol Cancer Res. 2013 May;11(5):474-81.

Suppression of AKT phosphorylation restores rapamycin-based synthetic lethality in SMAD4-defective pancreatic cancer cells.

Le Gendre O, Sookdeo A, Duliepre SA, Utter M, Frias M, Foster DA.

Department of Biological Sciences, Hunter College, City University of New York, New York, NY 10065, USA.

 

Abstract

mTOR has been implicated in survival signals for many human cancers. Rapamycin and TGF-β synergistically induce G1 cell-cycle arrest in several cell lines with intact TGF-β signaling pathway, which protects cells from the apoptotic effects of rapamycin during S-phase of the cell cycle. Thus, rapamycin is cytostatic in the presence of serum/TGF-β and cytotoxic in the absence of serum. However, if TGF-β signaling is defective, rapamycin induced apoptosis in both the presence and absence of serum/TGF-β in colon and breast cancer cell lines. Because genetic dysregulation of TGF-β signaling is commonly observed in pancreatic cancers-with defects in the Smad4 gene being most prevalent, we hypothesized that pancreatic cancers would display a synthetic lethality to rapamycin in the presence of serum/TGF-β. We report here that Smad4-deficient pancreatic cancer cells are killed by rapamycin in the absence of serum; however, in the presence of serum, we did not observe the predicted synthetic lethality with rapamycin. Rapamycin also induced elevated phosphorylation of the survival kinase Akt at Ser473. Suppression of rapamycin-induced Akt phosphorylation restored rapamycin sensitivity in Smad4-null, but not Smad4 wild-type pancreatic cancer cells. This study shows that the synthetic lethality to rapamycin in pancreatic cancers with defective TGF-β signaling is masked by rapamycin-induced increases in Akt phosphorylation. The implication is that a combination of approaches that suppress both Akt phosphorylation and mTOR could be effective in targeting pancreatic cancers with defective TGF-β signaling.

PMID: 23443316

 

SUPPLEMENTARY

Human cancers activate survival signals through the phosphatidylinositol-3-kinase (PI3K) and phospholipase D (PLD) pathways to suppress apoptotic programs in early stages of tumorigenesis. PI3K and PLD signaling pathways regulate the activation of both mTORC1 and mTORC2 complexes, which mediate cell growth and cell cycle progression. The TGF-β signaling pathway induces cell cycle arrest and is regulated by mTORC1 signaling. Our lab previously reported that in the absence of TGF-β signaling, mTORC1 inhibition leads to apoptotic cell death (Gadir et al., Oncogene 27: 1055, 2008). The simultaneous inhibition of two interacting signaling pathways that lead to cell death is described as a synthetic lethal phenotype.

This study shows that under conditions of serum deprivation, pancreatic cancer cells with defective TGF-β signaling (found in ~50% of pancreatic cancers) undergo apoptotic cell death upon inhibition of mTORC1 signaling. However, in the presence of serum, mTORC1 inhibition stimulates mTORC2-mediated phosphorylation of Akt at S473 via activation of PI3K signals, which prevents the synthetic lethal phenotype.

mTORC2-mediated Akt phosphorylation at S473 can be inhibited via inactivation of PI3K signaling or prevented by down-regulating transcription factor eIF4E, which avoids the feedback loop that activates PI3K signaling. Therefore, inhibiting both mTORC1 and mTORC2 signaling is required for the synthetic lethal phenotype in TGF- defective pancreatic cancer cells in the presence of serum.

Taken together, these data indicate that tumors with either innate or created defective TGF- signaling can be selectively killed by inhibition of both mTOR complexes.

David Foster and Onica Legendre-fig1

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