Identification of E2F1 as an Important Transcription Factor for the Regulation of Tapasin Expression

Abstract

HER-2/neu overexpression in tumor cells caused abnormalities of MHC class I surface expression due to impaired expression of components of the antigen-processing machinery (APM) including the low molecular weight proteins, the transporter associated with antigen processing (TAP), and the chaperone tapasin, whereas the expression of MHC class I heavy chain as well as ␤ 2 -microglobulin was only marginally affected. This oncogene-mediated deficient APM component expression could be reverted by interferon-␥ treatment, suggesting a deregulation rather than structural alterations as underlying molecular mechanisms. To determine the level of regulation, the transcriptional activity of APM components was analyzed in HER-2/neu ؊ and HER-2/neu ؉ cells. All major APM components were transcriptionally down-regulated in HER-2/neu ؉ when compared with HER-2/neu ؊ cells, which was accompanied by a reduced binding of RNA polymerase II to the APM promoters. Site-directed mutagenesis of the p300-and E2F-binding sites in the APM promoters did not reconstitute the oncogene-mediated decreased transcription rate with the exception of tapasin, which was restored in HER-2/neu ؉ cells to levels of wild type tapasin promoter activity in HER-2/neu ؊ fibroblasts. The E2F-directed control of tapasin expression was further confirmed by chromatin immunoprecipitation analyses showing that E2F1 and p300 bind to the tapasin and APM promoters in both cell lines. Moreover, siRNA-mediated silencing of E2F1 was associated with an increased tapasin expression, whereas transient overexpression of E2F1 launch a reduced tapasin transcription, suggesting that E2F1 is an essential transcription factor for tapasin.The expression of multiple components of the antigen-processing machinery (APM) 2 is a prerequisite for constitutive MHC class I surface expression and necessary for recognition of non-self antigens by CD8 ϩ cytotoxic T lymphocytes (1-3). Abnormalities in the MHC class I surface expression have been described in tumors of distinct origin that allow their escape from immune cell recognition (4 -8). These defects often result in decreased immunogenicity of tumors, disease progression, and reduced survival of patients (9, 10). Recently, the molecular mechanisms underlying altered MHC class I surface expression have been attributed to impaired expression of the low molecular weight proteins (LMP)-2, -7, and -10, the proteasome activator (PA)28, the transporter associated with antigen processing (TAP) 1 and 2, ␤ 2 -microglobulin (␤ 2 -m), and the MHC class I heavy chain (HC).Despite the identification of structural alterations in ␤ 2 -m, HC, and TAP, which are caused by deletions, mutations, loss of heterozygosity, and/or recombinations, their occurrence is rare (11)(12)(13)(14). Because in most cases MHC class I APM defects could be restored by treatment with proinflammatory and inflammatory cytokines such as tumor necrosis factor (TNF)-␣ and type I and type II interferons (IFN), their impaired expression appears to be mainly due to deregulation rat...

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The link between MHC class I abnormalities of tumors, oncogenes, tumor suppressor genes, and transcription factors

“…Even more interestingly, transcriptional down-regulation of TPN -as demonstrated in HER-2/neu over-expressing tumor cells -was directly associated with the over-expression of the transcription factor (TF) E2F1. TPN expression levels could be modulated via E2F1, which can be linked to altering cell proliferation ratesthereby suggesting that expression of MHC class I APM components might be (at least to some extent) regulated in a cell cycle-dependent manner (Bukur et al, 2010). Moreover, reduced MHC class I molecule expression levels could be due to a general, locus, haplotype, or allele-specific deletions.…”