Astragalus saponins modulate mTOR and ERK signaling to promote apoptosis through the extrinsic pathway in HT-29 colon cancer cells.
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.
Auyeung
Kathy Ka-Wai
KK
Center for Cancer and Inflammation Research, School of Chinese Medicine, Hong Kong Baptist University, Hong Kong SAR, PR China.
Mok
Nga-Lai
NL
Wong
Ching-Man
CM
Cho
Chi-Hin
CH
Ko
Joshua Ka-Shun
JK
D017209:Apoptosis / Q000187:drug effects*; D027883:Astragalus Plant / Q000737:chemistry*; D053181:Caspase 8 / Q000378:metabolism; D004789:Enzyme Activation; D019073:HT29 Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism*; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism*; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D012503:Saponins / Q000737:chemistry / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects* / Q000502:physiology; D058570:TOR Serine-Threonine Kinases; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D053181:Caspase 8; D047908:Intracellular Signaling Peptides and Proteins; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D016328:NF-kappa B; D019869:Phosphatidylinositol 3-Kinases; D011498:Protein Precursors; D017346:Protein Serine-Threonine Kinases; D012503:Saponins; D058570:TOR Serine-Threonine Kinases; D014409:Tumor Necrosis Factor-alpha; C546842:MTOR protein, human; C535150:MAPK1 protein, human