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

mTOR has been implicated in survival signals for many human cancers. Rapamycin and TGF-ß synergistically induce G1 cell-cycle arrest in several cell lines with intact TGF-ß 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-ß and cytotoxic in the absence of serum. However, if TGF-ß signaling is defective, rapamycin induced apoptosis in both the presence and absence of serum/TGF-ß in colon and breast cancer cell lines. Because genetic dysregulation of TGF-ß 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-ß. 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-ß 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-ß signaling.

Garcinia benzophenones inhibit the growth of human colon cancer cells and synergize with sulindac sulfide and turmeric.

Previous studies indicate that extracts and purified components from Garcinia species inhibit the growth of human colon cancer cells. Garcinia benzophenones activate the expression of genes in the endoplasmic reticulum and cellular energy stress (mTOR) pathways. This study examines the growth inhibitory and synergistic effects of Garcinia benzophenones, alone or combined with chemopreventive agents, on human colon cancer cells. To find optimal combination treatments, HT29 colon cancer cells were treated with benzophenones alone, or combined with chemopreventive agents, and cell growth measured using the MTT assay. To reveal effects on signaling pathways, we assessed effects of the MEK inhibitor U0126 and the ER IP3 receptor antagonist heparin, as well as effects on the phosphorylation of 4E-BP-1 (mTOR pathway), using Western blot analysis. New and known benzophenones from Garcinia intermedia inhibited the growth of human colon cancer cells; an alcohol extract of Garcinia xanthochymus, as well as purified guttiferones (guttiferone E and xanthochymol), preferentially inhibited the growth of colon cancer versus nonmalignant intestinal epithelial cells. Guttiferone E exhibited synergy with the NSAID sulindac sulfide and xanthochymol, with the spice turmeric. Guttiferone A did not alter phosphorylation of 4E-BP-1, indicating that the mTORC1 pathway is not involved in its action. The effects of xanthochymol were enhanced by U0126, at low doses, and were blocked by heparin, indicating that the MEK pathway is involved, while the ER IP3 receptor is critical for its action. These studies indicate the potential of benzophenones, alone or combined with sulindac sulfide or turmeric, to prevent and treat colon cancer.