Inefficient proteasomal-degradation pathway stabilizes AP-2alpha and activates HER-2/neu gene in breast cancer

Int J Cancer. 2006 Feb 15;118(4):802-11. doi: 10.1002/ijc.21426.

Abstract

HER-2/neu proto-oncogene is overexpressed in about one fourth of human breast cancers. AP-2 transcription factors bind to the HER-2/neu gene promoter and activate its expression. In a striking concurrence, anomalous abundance of AP-2alpha protein or its homolog AP-2gamma is also detected with HER-2/neu protein in mammary tumor-derived cell lines. This suggests that the deregulation of AP-2 is the preceding pathogenic event and probably the pivotal one in this type of mammary carcinogenesis. We examined the process of AP-2alpha gene expression in mammary carcinoma cell lines to identify where the aberration had occurred. We found no amplification of the AP-2alpha gene. Its promoter was marginally upregulated; however, it did not significantly increase the mRNA levels. When the AP-2alpha protein was examined, a remarkable stability was seen in breast cancer cell lines MDA-MB-453 and SK-BR-3, with a half-life of over 30 hr. This is sharply higher than the approximate 1 hr observed in mammary epithelial cell line MCF-10A and murine cell line NIH 3T3. Treatment of MCF-10A and NIH 3T3 cells with the proteasome inhibitor MG-132 showed that AP-2alpha was ubiquitinated and its level significantly increased. Moreover, this increase was accompanied by elevated levels HER-2/neu protein. In contrast, weaker ubiquitination of AP-2alpha was seen in MDA-MB-453 and SK-BR-3 cancer cells, and MG-132 treatment did not raise the AP-2alpha level any further. These results uncover that unusual stability is the main mechanism that raises the levels of AP-2 proteins, and in addition, provide the first clue that defective ubiquitin-dependent proteasomal-degradation pathway is possibly the prime cause that affects the HER-2/neu gene and culminates in breast cancer.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Breast Neoplasms / genetics*
  • Cell Transformation, Neoplastic / genetics
  • Cysteine Proteinase Inhibitors / pharmacology
  • Female
  • Gene Amplification
  • Gene Expression Regulation
  • Gene Expression Regulation, Neoplastic*
  • Humans
  • Leupeptins / pharmacology
  • Mammary Neoplasms, Animal / genetics
  • Mice
  • Promoter Regions, Genetic
  • Proteasome Endopeptidase Complex / metabolism*
  • Receptor, ErbB-2 / biosynthesis*
  • Transcription Factor AP-2 / biosynthesis*
  • Tumor Cells, Cultured
  • Ubiquitin / metabolism
  • Up-Regulation

Substances

  • Cysteine Proteinase Inhibitors
  • Leupeptins
  • Transcription Factor AP-2
  • Ubiquitin
  • Receptor, ErbB-2
  • Proteasome Endopeptidase Complex
  • benzyloxycarbonylleucyl-leucyl-leucine aldehyde