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

Int J Mol Med. 2010 Sep;26(3):341-9.

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.

MeSH terms

  • 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
  • NF-kappa B / metabolism
  • Phosphatidylinositol 3-Kinases / metabolism
  • Protein Precursors / metabolism
  • Protein Serine-Threonine Kinases / metabolism*
  • Saponins / chemistry
  • Saponins / pharmacology*
  • Signal Transduction / drug effects*
  • Signal Transduction / physiology
  • TOR Serine-Threonine Kinases
  • Tumor Necrosis Factor-alpha / metabolism

Substances

  • 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