Apigenin manipulates the ubiquitin-proteasome system to rescue estrogen receptor-β from degradation and induce apoptosis in prostate cancer cells

Eur J Nutr. 2015 Dec;54(8):1255-67. doi: 10.1007/s00394-014-0803-z. Epub 2014 Nov 19.


Purpose: To investigate apigenin (5,7,4-trihydroxyflavone), a dietary flavonoid with proteasome-inhibitory activity (desired for the management of multiple types of cancers), against FDA-approved anticancer proteasome inhibitor bortezomib in context to its effects on the tumor suppressor estrogen receptor-beta (ER-β) in prostate cancer cells.

Methods: Prostate cancer (PC-3) cells were treated with either apigenin or bortezomib, and proliferation inhibition was correlated with proteasomal biochemistry, ER-degradation and cell apoptosis.

Results: Apigenin specifically inhibited only chymotrypsin-like activity of proteasome without affecting trypsin and caspase-like activities, which was in contrast to the non-specific inhibition of all the three activities by bortezomib. Apigenin selectively increased the protein levels of ER-β at 1.8 and 10.0 µM (without affecting mRNA levels) and preferentially accumulated ubiquitinated ER-β over ER-α in PC-3. Apigenin-treated cells exhibited increased ER-β interactions with ubiquitin-protein ligase E6AP, downregulated PSMA5 (α-5 subunit for assembly of 20S proteasome) without affecting PSMB1 (β-1 subunit), PSMB2 (β-2 subunit) and PSMB5 (β-5 subunit, whose overexpression by bortezomib causes drug resistance) of proteasome at mRNA levels. Caspase-3 activation in PC-3 by apigenin was dependent on caspase-8 activity but independent of mitochondrial membrane depolarization. The deubiquitinase USP14 activity, which antagonizes degradation of proteins via proteasome, was significantly increased by apigenin treatment.

Conclusions: Apigenin selectively inhibits proteasomal degradation of tumor suppressor ER-β by specifically inhibiting chymotrypsin-like activity of proteasome, preventing its assembly via PSMA5 and inhibiting USP14 enzyme activity in prostate cancer cells, resulting in cancer cell apoptosis. Unlike bortezomib, apigenin's actions are subtle, precise, mechanistically distinct and capable of abstaining drug resistance.

Keywords: Apigenin; Bortezomib; Estrogen receptor-beta; Prostate cancer; Proteasome; Ubiquitination.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apigenin / pharmacology*
  • Apoptosis / drug effects*
  • Bortezomib / pharmacology
  • Caspase 3 / genetics
  • Caspase 3 / metabolism
  • Caspase 8 / genetics
  • Caspase 8 / metabolism
  • Cell Line, Tumor / drug effects
  • Cell Proliferation / drug effects
  • Down-Regulation
  • Estrogen Receptor alpha / genetics
  • Estrogen Receptor alpha / metabolism
  • Estrogen Receptor beta / genetics
  • Estrogen Receptor beta / metabolism*
  • Humans
  • Male
  • Prostatic Neoplasms / pathology*
  • Proteasome Endopeptidase Complex / genetics
  • Proteasome Endopeptidase Complex / metabolism
  • Proteasome Inhibitors / pharmacology*
  • Ubiquitin / genetics
  • Ubiquitin / metabolism*
  • Ubiquitin Thiolesterase / genetics
  • Ubiquitin Thiolesterase / metabolism
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / metabolism


  • Estrogen Receptor alpha
  • Estrogen Receptor beta
  • Proteasome Inhibitors
  • USP14 protein, human
  • Ubiquitin
  • estrogen receptor alpha, human
  • Bortezomib
  • Apigenin
  • UBE3A protein, human
  • Ubiquitin-Protein Ligases
  • Ubiquitin Thiolesterase
  • CASP3 protein, human
  • CASP8 protein, human
  • Caspase 3
  • Caspase 8
  • PSMA5 protein, human
  • PSMB1 protein, human
  • PSMB2 protein, human
  • Proteasome Endopeptidase Complex