Dysfunctional transforming growth factor-β signaling with constitutively active Notch signaling in Barrett's esophageal adenocarcinoma

BACKGROUND

Esophageal adenocarcinoma is often considered to arise from a clonal stem-like population of cells, which is potentially responsible for its poor prognosis. Transforming growth factor β (TGF-β) and Notch signaling pathways play important roles in regulating self-renewal of stem cells and cell-fate determination. Both pathways are frequently implicated in gastrointestinal carcinogenesis. However, their contributions to esophageal adenocarcinoma remain unclear.

METHODS

We evaluated TGF-β and Notch signaling components in normal esophagus, Barrett's esophagus, and adenocarcinoma tissues and cell lines via immunohistochemical analysis and immunoblotting; Hes-1 transcription was assayed using a Hes-1 luciferase reporter.

RESULTS

We observed loss of Smad4 (P<.05) and β2 spectrin (β2SP) (P<.01) in 5/10 Barrett's esophagus and 17/22 adenocarcinoma tissue sections. Concomitantly, dramatically raised levels of Notch signaling components Hes1 and Jagged1 occurred in adenocarcinoma tissues and cell lines compared with normal tissues. In normal esophagus, Oct3/4-positive cells are located in the basal layer (2-3 per cluster), representing a pool of progenitor cells. We observed an expansion of this pool of Oct3/4 positive cells in esophageal adenocarcinoma (15 per cluster). Furthermore, a panel of SOXs proteins documented for stem cell markers exhibit increased expression in tumor cells, indicating expansion of putative cancer stem cells. Finally, we observed growth inhibition in BE3 cells with a γ-secretase inhibitor, but not in SKGT-4 cells. Unlike SKGT-4 cells, BE3 cells have activated Notch signaling with disruption of TGF-β signaling.

CONCLUSIONS

Our findings demonstrated a potential therapeutic value for targeted therapy in esophageal adenocarcinoma in the setting of loss of β2SP/TGF-β with concomitant constitutively active Notch signaling.

Telomerase inhibition by siRNA causes senescence and apoptosis in Barrett's adenocarcinoma cells: mechanism and therapeutic potential

BACKGROUND

In cancer cells, telomerase induction helps maintain telomere length and thereby bypasses senescence and provides enhanced replicative potential. Chemical inhibitors of telomerase have been shown to reactivate telomere shortening and cause replicative senescence and apoptotic cell death of tumor cells while having little or no effect on normal diploid cells.

RESULTS

We designed siRNAs against two different regions of telomerase gene and evaluated their effect on telomere length, proliferative potential, and gene expression in Barrett's adenocarcinoma SEG-1 cells. The mixture of siRNAs in nanomolar concentrations caused a loss of telomerase activity that appeared as early as day 1 and was essentially complete at day 3. Inhibition of telomerase activity was associated with marked reduction in median telomere length and complete loss of detectable telomeres in more than 50% of the treated cells. Telomere loss caused senescence in 40% and apoptosis in 86% of the treated cells. These responses appeared to be associated with activation of DNA sensor HR23B and subsequent activation of p53 homolog p73 and p63 and E2F1. Changes in these gene regulators were probably the source of observed up-regulation of cell cycle inhibitors, p16 and GADD45. Elevated transcript levels of FasL, Fas and caspase 8 that activate death receptors and CARD 9 that interacts with Bcl10 and NFKB to enhance mitochondrial translocation and activation of caspase 9 were also observed.

CONCLUSION

These studies show that telomerase siRNAs can cause effective suppression of telomerase and telomere shortening leading to both cell cycle arrest and apoptosis via mechanisms that include up-regulation of several genes involved in cell cycle arrest and apoptosis. Telomerase siRNAs may therefore be strong candidates for highly selective therapy for chemoprevention and treatment of Barrett's adenocarcinoma.