Astragalus saponins modulate mTOR and ERK signaling to promote apoptosis through the extrinsic pathway in HT-29 colon cancer cells.

Abstract

We have previously demonstrated that the total saponins of Astragalus membranaceus (AST) possess potential anti-tumorigenic effects in human colon cancer cells and tumor xenografts. In the present study, the proapoptotic effects of AST were investigated in native and cytokine-induced HT-29 cells to further unveil its mechanism of action. Growth-inhibitory action of AST (60 microg/ml) was demonstrated in native HT-29 cells, which was exaggerated in tumor necrosis factor (TNF) (5 ng/ml)-induced cells. These were accompanied by caspase 3 activation, cleavage of poly(ADP-ribose) polymerase and a subsequent increase in apoptotic cell numbers. Furthermore, activation of procaspase 8 indicates that the extrinsic apoptotic pathway was involved, while cleavage of Bid into t-Bid implicates cross-talk with the intrinsic apoptotic pathway. Alternatively, AST caused S and G2/M phase arrest, while in cytokine-induced cells S phase arrest was predominant. Further adding to our recent suggestion on its correlation with phosphatidylinositol 3-kinase (PI3K)-Akt signaling, we have now revealed that AST caused overexpression of PTEN and down-regulation of mammalian target of rapamycin (mTOR) expression. Nevertheless, these events were preceded by a decrease in nuclear factor-kappaB (NF-kappaB)/DNA binding activity with continuous ERK 1/2 activation. Some of these effects became more intense in cytokine-induced cells. Our findings in this study suggest that AST induces the extrinsic apoptotic cascade and causes cell cycle arrest in HT-29 cells by modulation of both mTOR and ERK signaling pathways, of which inhibition of NF-kappaB is important in the latter mechanism. Most of the above processes are more pronounced in cytokine-induced cells.

• Authors: Kathy Ka-Wai Auyeung, Nga-Lai Mok, Ching-Man Wong, Chi-Hin Cho, Joshua Ka-Shun Ko
• Journal: International journal of molecular medicine (Int J Mol Med) Vol. 26 Issue 3 Pg. 341-9 (Sep 2010) ISSN: 1791-244X [Electronic] Greece
• PMID: 20664949 (Publication Type: Journal Article)

Chemical References

• Intracellular Signaling Peptides and Proteins
• NF-kappa B
• Protein Precursors
• Saponins
• Tumor Necrosis Factor-alpha
• MTOR protein, human
• Protein Serine-Threonine Kinases
• TOR Serine-Threonine Kinases
• MAPK1 protein, human
• Mitogen-Activated Protein Kinase 1
• Mitogen-Activated Protein Kinase 3
• Caspase 8

Topics

• Apoptosis (drug effects)
• Astragalus Plant (chemistry)
• Caspase 8 (metabolism)
• Enzyme Activation
• HT29 Cells
• Humans
• Intracellular Signaling Peptides and Proteins (metabolism)
• Mitogen-Activated Protein Kinase 1 (metabolism)
• Mitogen-Activated Protein Kinase 3 (metabolism)
• NF-kappa B (antagonists & inhibitors, metabolism)
• Phosphatidylinositol 3-Kinases (metabolism)
• Protein Precursors (metabolism)
• Protein Serine-Threonine Kinases (metabolism)
• Saponins (chemistry, pharmacology)
• Signal Transduction (drug effects, physiology)
• TOR Serine-Threonine Kinases
• Tumor Necrosis Factor-alpha (metabolism)