No abstract

Human MHC class I molecules are encoded by three different loci (HLA-A, -B, and -C), which are regulated at the transcriptional level through several conserved cis-acting promoter elements. The presence of locus-specific residues throughout the entire promoter region strongly suggests that the various HLA class I loci are differentially regulated. To identify regulatory sequences involved in locus-specific HLA class I gene transcription, a series of truncated HLA-A2 and HLA-B7 promoter-reporter constructs were transfected into melanoma cell lines expressing high and low levels of endogenous HLA-B, but comparable levels of HLA-A. These experiments showed that differential regulation of HLA-B expression in melanoma cell lines is mediated by a previously unidentified co-operative action of enhancer A, located 175 bp upstream of the transcription initiation site (+1), and a specific region of 20 nucleotides located at +13 to +33 bp downstream of the transcription initiation site. Furthermore, we demonstrated binding of transcription factor Yin Yang 1 to the HLA-A +13/+33 bp region, but not to the equivalent HLA-B region. Based on these results, we present a model suggesting that YY1 displaces either activating or repressing transcription factors, thereby making the HLA-A gene resistant to differential regulation.

Tumor cells are thought to escape immune surveillance from T cells by suppressing expression of major histocompatibility complex (MHC) class I molecules at their cell surface. Human MHC class I molecules are encoded by three different loci (HLA-A, -B, and -C). In primary human melanomas as well as melanoma cell lines, HLA class I expression is frequently downregulated in a B locus-specific manner. To study the involvement of promoter elements in HLA-B locus-specific downregulation, a series of reporter constructs containing 5'-flanking sequences of the HLA-A2 and -B7 genes were transfected into melanoma cell lines expressing high and low levels of HLA-B antigens. It is shown that enhancer A, which is generally believed to be a potent enhancer in HLA class I gene transcription, only weakly activates transcription in melanoma cell lines. In contrast, the interferon-stimulated response element (ISRE), known to induce MHC class I expression in response to IFNs, as well as a region comprising site alpha/enhancer B significantly stimulate constitutive transcription of HLA class I genes. Although none of the promoter elements tested could be demonstrated to mediate HLA-B locus-specific downregulation, high and low HLA-B melanoma cell lines do differ in ISRE activity as well as in ISRE-binding nuclear factors. The finding that high and low HLA-B melanoma cell lines contain different transcription factors binding to elements not actively involved in the process of HLA-B locus abrogation suggests that these cell lines originate from distinct types of melanocyte precursor cells expressing a different set of transcription factors.

At the cell surface, polymorphic transmembrane glycoproteins encoded by major histocompatibility complex (MHC, HLA in humans) class I genes present viral, tumor-associated or minor histocompatibility antigen-derived peptides to T cells. Although MHC class I molecules are expressed on virtually all nucleated cells, expression levels are regulated in a cell type-and developmental stage-specific manner. In addition, a variety of different compounds, viruses, cytokines and hormones have been shown to modulate MHC class I expression.Regulation of MHC class I expression occurs mainly at the transcriptional level through distinct conserved cis-acting regulatory elements (Girdlestone, 1996). Enhancer A (enh A), located 175 bp upstream of the transcription-initiation site (ϩ1), consists of a perfect palindromic sequence, to which transcription factors belonging to the NF-B/Rel family bind. Enh A-binding factors are either constitutively expressed, like the p50-p50 homodimer KBF1, or inducible, like the p50-p65 heterodimer NF-B. An interferon (IFN)-stimulated response element (ISRE) is located immediately downstream of enh A. A number of IFN-inducible ISRE-binding factors, including IRF-1 and -2, have been identified. IFN-induced ISRE activity is dependent on the presence of site ␣, a 6-bp sequence located at -100 bp (Gobin et al., 1997). Site ␣ has been shown to regulate constitutive binding of transcription factors to the enh A and ISRE elements in vivo (Kushida et al., 1997). At position -75 bp, an element, termed enhancer B (enh B), is present. The inverted CCAAT (ATTGG) sequence of enh B has been suggested to be an essential element for HLA class I gene transcription (Schoneich et al., 1997).HLA class I molecules are encoded by 3 different loci: A, B and C. The presence of locus-specific sequences at cis-acting regulatory elements suggests that the HLA class I promoter is regulated in a locus-specific manner. Enh A, for example, contains in addition to a perfect palindromic sequence, which is completely conserved between HLA-A and -B genes, an imperfect copy of the palindromic sequence comprising 2 locus-specific residues. Presumably, this additional copy contributes to different levels of transactivation of HLA-A and -B enh A by NF-B (Girdlestone et al., 1993). Furthermore, HLA class I loci differ in response to treatment with IFNs: HLA-B expression is stronger augmented by IFNs than HLA-A expression, probably mediated by locus-specific ISRE sequences (Girdlestone, 1995).Complete loss as well as allele-and locus-specific downregulation of MHC class I expression have been observed in human tumors of different origins, probably leading to decreased recognition by T cells. MHC class I down-regulation in tumor cells is frequently mediated at the transcriptional level. Both cis-acting mechanisms, like alterations in chromatin structure and DNA methylation, and trans-acting mechanisms have been described. Trans-acting mechanisms frequently involve repression of enh A-driven transcription (Blanchet et al., 1992;van't Veer ...