Cancerous inhibitor of protein phosphatase 2A mediates bortezomib-induced autophagy in hepatocellular carcinoma independent of proteasome

PLoS One. 2013;8(2):e55705. doi: 10.1371/journal.pone.0055705. Epub 2013 Feb 1.

Abstract

Previously, we reported that cancerous inhibitor of protein phosphatase 2A (CIP2A) mediates the apoptotic effect of bortezomib in hepatocellular carcinoma (HCC). Here, we report a proteasome-independent mechanism by which bortezomib induces autophagy in HCC. Our data indicate that bortezomib activated autophagy in a dose- and time- dependent manner in HCC cell lines including Huh-7, Sk-Hep1, and Hep3B. Bortezomib downregulated CIP2A, phospho-Akt (P-Akt) and phospho-4EBP1 (P-4EBP1) in a dose- and time-dependent manner in all tested HCC cells. Ectopic expression of CIP2A abolished the effect of bortezomib on autophagy. Co-treatment of bortezomib and calyculin A, a PP2A inhibitor, reduced the effect of bortezomib on P-Akt, P-4EBP1, and autophagy. Increased phosphorylation of either Akt or 4EBP1 by ectopic overexpression protected cells from bortezomib-induced autophagy. Furthermore, we examined the effect of ΔBtz, a bortezomib derivative that closely resembles bortezomib structurally but has no proteasome activity, in HCC. Interestingly, ΔBtz demonstrated similar effects to bortezomib on autophagy, CIP2A, P-Akt and P-4EBP1, suggesting that the effect of bortezomib on autophagy is independent of proteasome inhibition. Moreover, our in vivo data showed that both bortezomib and ΔBtz inhibited tumor growth, downregulated CIP2A, P-Akt and induced autophagy in Huh-7 tumors. In conclusion, bortezomib induces autophagy in HCC through a CIP2A-PP2A-Akt-4EBP1 pathway.

MeSH terms

  • Acridine Orange
  • Autoantigens / metabolism*
  • Autophagy / drug effects*
  • Blotting, Western
  • Boronic Acids / metabolism
  • Boronic Acids / pharmacology*
  • Bortezomib
  • Carcinoma, Hepatocellular / metabolism*
  • Cell Line, Tumor
  • Dose-Response Relationship, Drug
  • Flow Cytometry
  • Fluorescent Antibody Technique
  • Gene Expression Regulation / drug effects*
  • Humans
  • Intracellular Signaling Peptides and Proteins
  • Liver Neoplasms / metabolism*
  • Membrane Proteins / metabolism*
  • Microscopy, Fluorescence
  • Phosphorylation
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Phosphatase 2 / antagonists & inhibitors
  • Protein Phosphatase 2 / metabolism
  • Pyrazines / metabolism
  • Pyrazines / pharmacology*
  • Time Factors

Substances

  • Autoantigens
  • Boronic Acids
  • CIP2A protein, human
  • Intracellular Signaling Peptides and Proteins
  • Membrane Proteins
  • Pyrazines
  • Bortezomib
  • Protein Phosphatase 2
  • Proteasome Endopeptidase Complex
  • Acridine Orange

Grants and funding

This study is supported by grants, NTUH 101P01 (KFC) from National Taiwan University Hospital; NSC99-2314-B-002-017-MY2 (KFC), and NSC100-2325-B-002-036 (KFC) from the National Science Council, Taiwan. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.