Silibinin inhibits hypoxia-inducible factor-1alpha and mTOR/p70S6K/4E-BP1 signalling pathway in human cervical and hepatoma cancer cells: implications for anticancer therapy

Oncogene. 2009 Jan 22;28(3):313-24. doi: 10.1038/onc.2008.398. Epub 2008 Nov 3.

Abstract

The hypoxia-inducible factor 1 (HIF-1) plays a critical role for tumour adaptation to microenvironmental hypoxia, and represents an appealing chemotherapeutic target. Silibinin is a nontoxic flavonoid reported to exhibit anticancer properties. However, the mechanisms by which silibinin inhibits tumour growth are not fully understood. In this study, silibinin was found to inhibit hypoxia-induced HIF-1alpha accumulation and HIF-1 transcriptional activity in human cervical (HeLa) and hepatoma (Hep3B) cells. Neither HIF-1alpha protein degradation rate nor HIF-1alpha steady-state mRNA level was affected by silibinin. Rather, we found that suppression of HIF-1alpha accumulation by silibinin correlated with strong dephosphorylation of mammalian target of rapamycin (mTOR) and its effectors ribosomal protein S6 kinase (p70S6K) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1), a pathway known to regulate HIF-1alpha expression at the translational level. Silibinin also activated Akt, a mechanistic feature exhibited by established mTOR inhibitors in many tumour cells. Moreover, silibinin reduced hypoxia-induced vascular endothelial growth factor (VEGF) release by HeLa and Hep3B cells, and this effect was potentiated by the PI3K/Akt inhibitor LY294002. Finally, silibinin was found to be a potent inhibitor of cell proliferation. These results show that silibinin is an effective inhibitor of HIF-1 and provide new perspectives into the mechanism of its anticancer activity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing / metabolism*
  • Antioxidants / pharmacology*
  • Apoptosis / drug effects
  • Blotting, Western
  • Carcinoma, Hepatocellular / drug therapy
  • Cell Hypoxia
  • Cell Proliferation / drug effects
  • Female
  • HeLa Cells / drug effects
  • Humans
  • Hypoxia-Inducible Factor 1, alpha Subunit / antagonists & inhibitors*
  • Hypoxia-Inducible Factor 1, alpha Subunit / genetics
  • Immunoprecipitation
  • Luciferases / metabolism
  • Milk Thistle / chemistry
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoproteins / metabolism*
  • Protein Kinases / metabolism*
  • Proto-Oncogene Proteins c-akt / metabolism
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Reverse Transcriptase Polymerase Chain Reaction
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism*
  • Signal Transduction / drug effects*
  • Silybin
  • Silymarin / pharmacology
  • TOR Serine-Threonine Kinases
  • Vascular Endothelial Growth Factor A / metabolism

Substances

  • Adaptor Proteins, Signal Transducing
  • Antioxidants
  • EIF4EBP1 protein, human
  • HIF1A protein, human
  • Hypoxia-Inducible Factor 1, alpha Subunit
  • Phosphoproteins
  • RNA, Messenger
  • Silymarin
  • VEGFA protein, human
  • Vascular Endothelial Growth Factor A
  • Silybin
  • Luciferases
  • Protein Kinases
  • Phosphatidylinositol 3-Kinases
  • MTOR protein, human
  • TOR Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases, 70-kDa