Inefficient proteasomal-degradation pathway stabilizes AP-2alpha and activates HER-2/neu gene in breast cancer.
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.
Li
Min
M
MetroHealth Medical Center, Case Western Reserve University, Cleveland, OH, USA.
Wang
Yingqun
Y
Hung
Mien-Chie
MC
Kannan
Perry
P
D000818:Animals; D001943:Breast Neoplasms / Q000235:genetics*; D002471:Cell Transformation, Neoplastic / Q000235:genetics; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D005260:Female; D005784:Gene Amplification; D005786:Gene Expression Regulation; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D015674:Mammary Neoplasms, Animal / Q000235:genetics; D051379:Mice; D011401:Promoter Regions, Genetic; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D000090063:Proto-Oncogene Mas; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000096:biosynthesis*; D050656:Transcription Factor AP-2 / Q000096:biosynthesis*; D014407:Tumor Cells, Cultured; D025801:Ubiquitin / Q000378:metabolism; D015854:Up-Regulation
D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D046988:Proteasome Endopeptidase Complex; D000090063:Proto-Oncogene Mas; D018719:Erb-b2 Receptor Tyrosine Kinases; D050656:Transcription Factor AP-2; D025801:Ubiquitin; C000715470:MAS1 protein, human; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde