Relationship between myc oncogene activation and MHC class I expression

Tumor Growth Remains Refractory to Myc Ablation in Host Macrophages

Aberrant expression of the oncoprotein c-Myc (Myc) is frequently observed in solid tumors and is associated with reduced overall survival. In addition to well-recognized cancer cell-intrinsic roles of Myc, studies have also suggested tumor-promoting roles for Myc in cells of the tumor microenvironment, including macrophages and other myeloid cells. Here, we benchmark Myc inactivation in tumor cells against the contribution of its expression in myeloid cells of murine hosts that harbor endogenous or allograft tumors. Surprisingly, we observe that LysM^Cre-mediated Myc ablation in host macrophages does not attenuate tumor growth regardless of immunogenicity, the cellular origin of the tumor, the site it develops, or the stage along the tumor progression cascade. Likewise, we find no evidence for Myc ablation to revert or antagonize the polarization of alternatively activated immunosuppressive macrophages. Thus, we surmise that systemic targeting of Myc activity may confer therapeutic benefits primarily through limiting Myc activity in tumor cells rather than reinvigorating the anti-tumor activity of macrophages.

Therapeutic targeting of "undruggable" MYC

c-MYC controls global gene expression and regulates cell proliferation, cell differentiation, cell cycle, metabolism and apoptosis. According to some estimates, MYC is dysregulated in ≈70% of human cancers and strong evidence implicates aberrantly expressed MYC in both tumor initiation and maintenance. In vivo studies show that MYC inhibition elicits a prominent anti-proliferative effect and sustained tumor regression while any alteration on healthy tissue remains reversible. This opens an exploitable window for treatment that makes MYC one of the most appealing therapeutic targets for cancer drug development. This review describes the main functional and structural features of the protein structure of MYC and provides a general overview of the most relevant or recently identified interactors that modulate MYC oncogenic activity. This review also summarizes the different approaches aiming to abrogate MYC oncogenic function, with a particular focus on the prototype inhibitors designed for the direct and indirect targeting of MYC.

MYC Ran Up the Clock: The Complex Interplay between MYC and the Molecular Circadian Clock in Cancer

The MYC oncoprotein and its family members N-MYC and L-MYC are known to drive a wide variety of human cancers. Emerging evidence suggests that MYC has a bi-directional relationship with the molecular clock in cancer. The molecular clock is responsible for circadian (~24 h) rhythms in most eukaryotic cells and organisms, as a mechanism to adapt to light/dark cycles. Disruption of human circadian rhythms, such as through shift work, may serve as a risk factor for cancer, but connections with oncogenic drivers such as MYC were previously not well understood. In this review, we examine recent evidence that MYC in cancer cells can disrupt the molecular clock; and conversely, that molecular clock disruption in cancer can deregulate and elevate MYC. Since MYC and the molecular clock control many of the same processes, we then consider competition between MYC and the molecular clock in several select aspects of tumor biology, including chromatin state, global transcriptional profile, metabolic rewiring, and immune infiltrate in the tumor. Finally, we discuss how the molecular clock can be monitored or diagnosed in human tumors, and how MYC inhibition could potentially restore molecular clock function. Further study of the relationship between the molecular clock and MYC in cancer may reveal previously unsuspected vulnerabilities which could lead to new treatment strategies.

Induction of intercellular adhesion molecule 1 and class II histocompatibility antigens in colorectal tumour cells expressing activated ras oncogene

Aims-To determine whether there is a correlation between activation of the ras oncogene and the induction of MHC class II antigens and intercellular adhesion molecule 1 (ICAM-1) by interferon-gamma (IFN-gamma).Methods-Expression of class II antigens, ICAM-1 and intracellular ras oncoprotein (p21) in established colorectal cell lines and short term cultures of primary colorectal tumour cells was determined by flow cytometry and mutation in the ras gene by sequencing of amplified segments of the gene.Results-The cell lines showed a variation in their modulation of MHC class II antigens and ICAM-1, ranging from no induction to a 98-fold increase in class II antigen expression in the HT29 cell line. Previous work indicated that most tumours could not be induced to express class II antigens. Four of the five least inducible lines either contained mutant ras or highly expressed the oncoprotein. The four highly inducible cell lines all contained non-mutant ras. Of the 21 tumours studied in primary culture, 10 were inducible, one of which contained mutant ras. Of the remaining non-inducible tumours, four were mutant.Conclusions-Correlations between ras activation and failure to respond to IFN-gamma could not be shown to be significant. Therefore, ras activation, and concomitant subversion of intracellular signalling pathways, is probably not the major determinant in failure to activate class II antigens and ICAM-1.

c-MYC impairs immunogenicity of human B cells

Deregulation of c-myc expression through chromosomal translocation is essential in the pathogenesis of Burkitt's lymphoma (BL). A characteristic feature of BL cells, compared to Epstein-Barr Virus (EBV)-immortalized B cells, is their lack of immunogenicity. To study the contribution of EBV genes and of the c-MYC protein to this phenotype, we have generated a conditional B cell system in which the viral proliferation program and expression of c-myc can be regulated independently of each other. In cells proliferating due to exogenous c-myc overexpression, the cell surface phenotype, the pattern of proliferation in single cell suspension, and the immunological characteristics of BL cells could be completely recapitulated. Yet, it had remained open whether nonimmunogenicity is the default phenotype when EBNA2 and LMP1 are switched off, or whether c-MYC actively contributes to immunosuppression. We provide evidence also for the latter by showing that c-MYC down-regulates genes of the NF-kappaB and interferon pathway in a dose-dependent fashion. c-MYC acts at at least two different levels, the level of interferon induction as well as at the level of action of type I and type II interferons on their respective target promoters. c-MYC does not block the interferon pathway completely, it shifts the balance and increases the threshold of interferon induction and action.

Characteristic alteration of monocytes with increased intracellular IL-10 and IL-12 in patients with advanced-stage gastric cancer

BACKGROUND

It was previously reported that monocytes/macrophages play an important role in mediating T cell dysfunction in tumor-bearing hosts, in which monocytes/macrophages were found to induce the loss of T cell functions concomitantly with induction of defects in T cell signaling molecules. These observations encouraged us to investigate monocytes status in cancer-bearing hosts.

MATERIALS AND METHODS

We characterized peripheral blood monocytes in gastric cancer patients with advanced disease (n = 14), in those with early disease (n = 17), and in healthy individuals (n = 14), based on surface marker, oxygen-burst capacity, and intracellular cytokine status (IL-10 and IL-12).

RESULTS

Intracellular IL-10 and IL-12 status on monocytes in advanced disease was significantly increased in comparison with those in early disease or healthy individuals, while there were no differences in the surface marker or oxygen-burst capacity of monocytes. To clarify which mediators induced the characteristic differences of monocytes in cancer-bearing hosts, healthy donor-derived monocytes were coincubated with the patient's plasma. The plasma from the patients with advanced disease could induce healthy monocytes to increased intracellular IL-10 and IL-12 status. The phenomenon was significantly inhibited with neutralizing mAbs specific for VEGF. Furthermore, the contents of VEGF in the patient's plasma correlated with their capacity to induce healthy monocytes to increased intracellular IL-10. In addition, the treatment of healthy monocytes with exogenous VEGF resulted in increased intracellular IL-10.

CONCLUSIONS

Monocytes in gastric cancer patients with advanced disease showed different characteristics in comparison with those with early disease or healthy individuals, which might be potentially induced by circulating VEGF in the patients.

Macrophages in tumor-draining lymph node with different characteristics induce T-cell apoptosis in patients with advanced stage-gastric cancer

A host's immune-defense system is suppressive by many factors in patients with cancer. We have previously shown one possible mechanism behind the T-cell dysfunction, whereby H(2)O(2) secreted from macrophages in tumor-draining lymph node (MTDL) induced T-cell dysfunction with down-regulation of TCR zeta molecules. In the present study, we analyzed how MTDL affect T cells, with a particular focus on T-cell apoptosis, by co-culturing MTDL with autologous peripheral blood T cells in gastric cancer. Moreover, we characterized the MTDL according to surface marker, oxygen-burst capacity and intracellular cytokine status. T-cell apoptosis was significantly induced in comparison to T-cell alone control in patients with advanced disease, concomitant to the elevated caspase activity and following impaired T-cell function. In patients with early disease, no significant difference was seen in the proportions of T cells that underwent apoptosis between T cells plus MTDL and T cells alone. Moreover, the addition of a selective scavenger of H(2)O(2), catalase inhibited the apoptosis of T cells co-cultured with MTDL in patients with advanced disease. In the characterization of MTDL, the production of H(2)O(2) in MTDL from advanced disease was significantly higher than that in early disease. The amounts of intracellular IL-10 and IL-12 in MTDL in advanced disease were significantly higher than those in early disease. These results indicated that MTDL induced apoptosis of autologous T cells and this T-cell dysfunction was mediated by H(2)O(2) derived from MTDL. Furthermore, the characteristics of MTDL including the capacity of oxygen-burst and intracellular cytokine production were different depending on the disease progression.

MYC overexpression imposes a nonimmunogenic phenotype on Epstein-Barr virus-infected B cells

Lymphoblastoid cell lines, generated by immortalization of normal B cells by Epstein-Barr virus (EBV) in vitro, have strong antigen-presenting capacity, are sensitive to EBV-specific cytotoxic T cells, and are highly allostimulatory in mixed lymphocyte culture. By contrast, EBV-positive Burkitt lymphoma (BL) cells are poor antigen presenters, are not recognized by EBV-specific cytotoxic T cells, and are poorly allostimulatory, which raises the question of whether immunological pressure exerted during BL pathogenesis in vivo has selected for a 'nonimmunogenic' tumor phenotype. The present work addresses this question by examining the immunogenicity/antigenicity of cell lines, generated by conversion of a conditionally immortalized lymphoblastoid cell line to permanent growth independent of EBV-latent proteins by introduction of a constitutively active or tetracycline-regulated c-myc gene (A1 and P493-6 cells, respectively). Compared with its parental lymphoblastoid cell line, A1 cells showed many of the features of the nonimmunogenic BL phenotype, namely poor allostimulatory activity, poor antigen-presenting function associated with impaired proteasomal activity, down-regulation of peptide transporter, reduced HLA class I expression, and an inability to present endogenously expressed EBV-latent proteins to cytotoxic T cells. P493-6 cells, when grown in the presence of estrogen with the exogenous c-myc gene switched off, were strongly immunogenic. The cells had lost their immunogenic potential, however, when grown on a c-myc-driven proliferation program in the absence of estrogen. Deregulation of c-myc, a step central to the development of uncontrolled BL cell growth in vivo, can thus impose a nonimmunogenic phenotype on proliferating human B cells in the absence of any immune pressure.

Telomerase activity of the Lugol-stained and -unstained squamous epithelia in the process of oesophageal carcinogenesis

Up-regulation of telomerase has been reported in many cancers. Our aim was to characterize telomerase activity in various states of the oesophagus to facilitate better understanding of carcinogenesis of oesophageal squamous cell carcinoma. During endoscopic examinations, we obtained 45 Lugol-stained normal epithelia, 31 Lugol-unstained epithelia (14 oesophagitis, 7 mild dysplasia, 5 severe dysplasia and 5 intramucosal cancer) and 9 advanced cancer. Telomerase activity was semi-quantified by a telomeric repeat amplification protocol using enzyme-linked immunosorbent assay, and expression of human telomerase reverse transcriptase mRNA was examined by in situ hybridization. In the Lugol-stained normal epithelia, telomerase activity increased in proportion to the increase of severity of the accompanying lesions, with a rank order of advanced cancer, intramucosal cancer, mild dysplasia and oesophagitis. In the Lugol-unstained lesions and advanced cancer, telomerase activity was highest in advanced cancer. Up-regulation of telomerase in normal squamous epithelium may be a marker of progression of oesophageal squamous cell carcinoma.

T-cell-directed cancer vaccines: mechanisms of immune escape and immune tolerance

Recent clinical trials using vaccines directed toward tumour-associated antigens (TA) have shown the increasing capacity of vaccines to cause immunologic responses. In fact, strongly reactive TA-specific cytolytic T-lymphocytes and tumour-infiltrating lymphocytes (TIL) can be identified and expanded ex vivo from patients with metastatic melanoma vaccinated with melanoma-associated antigens. Paradoxically, this strong immunological response does not correlate with clinical tumour regression. Proposed mechanisms responsible for this glaring inconsistency are numerous and varied; systemic immunosuppressive as well as local mechanistic factors are implicated. In this review we will critically evaluate the possible mechanisms that allow tumours to escape immune destruction and be tolerated by the immune system. In addition, strategies that may allow further insight into the biology of tumour rejection are discussed, in the hope of deepening the understanding of this phenomenon and enhancing its therapeutic potential.

Heterogeneity in expression of human leukocyte antigens and melanoma-associated antigens in advanced melanoma

The study of tumor immunology has led to many innovative therapeutic strategies for the treatment of melanoma. The strategies are primarily dependent on melanoma-associated antigen peptide vaccination or T-cell-based therapy. These immunotherapies are totally reliant on proper copresentation of human leukocyte antigen class I molecules in sufficient quantity and the presence and availability of melanoma-associated antigenic peptides. Altered expression of either HLA class I molecules or melanoma antigens is known to occur. These defects lead to altered manufacture and copresentation of HLA class I molecules with melanoma-associated antigens to T-cells. Defects in any one combination can lead to loss of recognition of melanoma cells and their subsequent destruction by cytotoxic T-lymphocytes. Thus, these immunotherapy strategies can be thwarted by defects or heterogeneity of expression of human leukocyte antigen class I or of melanoma-associated antigens.

Monomorphic HLA class I-(non-A, non-B) expression on Ewing's tumor cell lines, modulation by TNF-alpha and IFN-gamma

In this study, the expression of polymorphic and non-polymorphic MHC antigens in Ewing's tumor (ET) cells was examined by surface staining, Western blots and transcriptional analysis. Cell lines derived from Ewing's tumors largely lack polymorphic HLA class Ia antigens of both the HLA-A and the HLA-B loci but binding of monomorphic HLA antibodies indicates significant expression of HLA-C locus antigens and/or HLA class Ib molecules. HLA Ib molecules encoded by the HLA-E, -F or -G loci with a molecular mass of less than 44 kDa were not detected in lysates of either constitutive or TNF-alpha plus IFN-gamma treated ET cells. Two representative ET cell lines with either detectable HLA-A, -B antigens (A673) or absolutely non-detectable HLA-A, -B antigens (SK-ES-1) were further subjected to transcriptional analysis. A673 mRNA hybridized with HLA-A, -B, -C and HLA-E-specific probes in Northern blots. By contrast, mRNA specific for HLA-A, -B, -C was negative in SK-ES-1 but TNF-alpha plus IFN-gamma reconstituted HLA-A, -B, -C transcription in this cell line. HLA-E was transcribed in A673 but not in SK-ES-1. Combining mRNA and surface expression of HLA class Ia molecules results in a highly variable pattern of defective HLA class I expression in this type of neuroectodermal tumor. The involvement of the ET-specific fusion transcript EWS/Fli-1 in modulating the HLA-A and -B locus antigens is likely to occur by the upregulation of c-myc in these tumors. The exceptionally constant expression of HLA-C or some other non-A, non-B antigens (reactive with defined monoclonal antibodies) implies important consequences on tumor-cell resistance against specific CTL and NK activity in vivo.

Affinity, specificity and T-cell-receptor diversity of melanoma-specific CTL generated in vitro against a single tyrosinase epitope

MHC-class-I-restricted cytotoxic T lymphocytes (CTL) specific for tumor-associated antigens expressed by malignant cells are important components of the immune response against cancer. Recently, tumor-specific CTL could be generated in vitro, with responding lymphocytes from the blood of healthy blood donors. In the present study, we confirm that peptide-specific stimulation in vitro can induce high-affinity CTL capable of recognizing tumor cells expressing the appropriate tumor antigen. These tyrosinase-specific CTL display a restricted usage of TCRAV and TCRBV gene segments but of diverse CDR3 regions, resulting in a distinct fine-specificity for each CTL clone. This suggests that, similar to in vivo priming, peptide-pulsed APC are capable of stimulating a T-cell response in vitro expressing a limited TCR repertoire against autologous tumors. The generated CTL can recognize their target structure with high affinity, and this correlates in part with tumor-cell lysis. This methodology may be used to treat melanoma patients with infusion of ex vivo-induced and -expanded CTL.

Identification of a cis-acting element in the class I major histocompatibility complex gene promoter responsive to activation by retroviral sequences

The infection of cells with Moloney murine leukemia virus (M-MuLV) causes an increase in specific cellular gene products, including the major histocompatibility complex (MHC) class I antigens. This upregulation occurs through a transactivation process mediated by the long terminal repeat (LTR) of M-MuLV, and we show here that the gene activation response to the LTR requires at least one specific cis element within the MHC proximal promoter region. Nested deletions of MHC class I H-2Kb gene promoter sequence were subcloned into a chloramphenicol acetyltransferase (CAT) reporter vector and then transiently introduced into BALB/c-3T3 cells expressing M-MuLV or cotransfected into BALB/c-3T3 cells with a vector containing subgenomic portions of the virus, including the LTR. CAT activity assays demonstrated that a minimal H-2Kb gene promoter (-64 to +12) contained elements sufficient for this transactivation. DNase I footprinting assays located a protein-binding site in the region of -64 to -34 bp from the transcriptional start site, and point mutation analysis confirmed the location of this cis-acting element, designated the let response element (LRE), and defined a binding motif. This LRE is distinct from binding sites for currently known transcription factors in the class I MHC gene promoter and is conserved in the promoters of human and murine MHC class I genes. Mutation of the LRE resulted in dramatic reduction in both DNA-protein binding activity in electrophoretic mobility shift assay and in the ability of the mutated promoter to respond to retroviral transactivation. Addition of the LRE to a heterologous promoter conferred the ability to respond to retroviral transactivation.

Inverse correlation between expression of HLA-B and c-myc in uveal melanoma

HLA class I is expressed in 75-85 per cent of uveal melanoma and cytoplasmic c-myc expression has been reported in 78 per cent of uveal melanoma. In skin melanoma, an inverse relationship has been observed between HLA class I expression and c-myc. The purpose of this study was to determine whether a similar correlation occurred between high expression of c-myc and low expression of HLA class I in uveal melanoma. The expression of c-myc, HLA-A, and HLA-B was determined by immunohistochemistry on formalin-fixed and paraffin-embedded sections of 30 uveal melanomas. Cell cultures from four primary uveal melanomas (lines 92-1, MEL 202, OCM-1, and EOM-3) and one uveal melanoma metastasis (line OMM-1) were tested for mRNA levels of c-myc and HLA-A and HLA-B in Northern blot assays. The high level of expression of cytoplasmic c-myc was significantly correlated with low expression of HLA-B (P = 0.03) and vice versa. High expression of HLA-B was significantly correlated with the presence of epithelioid cells (P = 0.004). The inverse correlation observed between c-myc and HLA-B expression is similar to previous observations in cutaneous melanoma. By downregulating HLA-B expression, c-myc may influence the immune response in uveal melanoma. Tumours containing epithelioid cells showed a significantly higher expression of HLA-B than tumours of the spindle cell type.

c-myc activation renders proliferation of Epstein-Barr virus (EBV)-transformed cells independent of EBV nuclear antigen 2 and latent membrane protein 1

Two genetic events contribute to the development of endemic Burkitt lymphoma (BL) infection of B lymphocytes with Epstein-Barr virus (EBV) and the activation of the protooncogene c-myc through chromosomal translocation. The viral genes EBV nuclear antigen 2 (EBNA2) and latent membrane protein 1 (LMP1) are essential for transformation of primary human B cells by EBV in vitro; however, these genes are not expressed in BL cells in vivo. To address the question whether c-myc activation might abrogate the requirement of the EBNA2 and LMP1 function, we have introduced an activated c-myc gene into an EBV-transformed cell line in which EBNA2 was rendered estrogen-dependent through fusion with the hormone binding domain of the estrogen receptor. The c-myc gene was placed under the control of regulatory elements of the immunoglobulin kappa locus composed a matrix attachment region, the intron enhancer, and the 3' enhancer. We show here that transfection of a c-myc expression plasmid followed by selection for high MYC expression is capable of inducing continuous proliferation of these cells in the absence of functional EBNA2 and LMP1. c-myc-induced hormone-independent proliferation was associated with a dramatic change in the growth behavior as well as cell surface marker expression of these cells. The typical lymphoblastoid morphology and phenotype of EBV-transformed cells completely changed into that of BL cells in vivo. We conclude that the phenotype of BL cells reflects the expression pattern of viral and cellular genes rather than its germinal center origin.

Genetic alterations in esophageal cancer and their relevance to etiology and pathogenesis: a review

Cancer of the esophagus exists in 2 main forms with different etiological and pathological characteristics-squamous cell carcinoma (SCC) and adenocarcinoma (ADC). This review focuses on the occurrence of genetic alterations in SSC and ADC of the esophagus and on their possible implications for the elucidation of the etiology and pathogenesis of these cancers. The most common alterations found in esophageal cancers include allelic losses at chromosomes 3p, 5q, 9p, 9q, 13q, 17p, 17q and 18q, as well as mutations of p53 (mostly missense), Rb (deletions), cyclin DI (amplifications) and c-myc (amplifications). The sequence of occurrence of these alterations with respect to histopathological tumor progression is discussed. Our findings underscore the different etiology and pathogenesis of SCC vs. ADC and suggest that the genetic alterations observed may represent molecular fingerprints of critical risk involved in the development of these 2 cancers.

C-myc represses transiently transfected HLA class I promoter sequences not locus-specifically

Overexpression of the c-myc oncogene is frequently accompanied by downregulation of Major Histocompatibility Complex (MHC, HLA in humans) class I antigens. In human melanoma c-myc overexpression downmodulates HLA-B expression, whereas HLA-A is hardly affected. Repression of HLA-B is mediated through the core promoter, containing a CAAT-box and a non-conventional TATA-box. We show evidence that in transient transfection assays the HLA-A2 and HLA-B7 promoters are repressed by c-myc to the same extent. Therefore, other sequences of the HLA-A and HLA-B genes, possibly intron/exon sequences, should contribute to the locus B-specificity of the downregulation. Furthermore, c-myc does not seem to alter binding of protein complexes to the CAAT- or TATA-box of HLA-B7 or HLA-A2 in gel retardation assays. Comparison of promoters repressed by c-myc reveals a weak consensus sequence of the initiator (Inr) element: TCA(+1)YYYNY. The presence of a TCA sequence in the initiator region of the MHC class I promoter makes downregulation by c-myc through the Inr likely. We speculate that the Inr contributes to MHC class I promoter activity by stimulating recruitment of TFIID to the weak, non-conventional TATA-box, thereby making it susceptible to repression by c-myc through the Inr.

Heritable formation of neuroectodermal tumor in transgenic mice carrying the combined E1 region gene of adenovirus type 12 with the deregulated human renin promoter

Adenovirus early 1 (E1) region gene products, including E1A and E1B, are required for transcriptional regulation of viral and cellular promoters in infected and transfected culture cells and for transformation of primary rodent cells. Here, we established a line of transgenic mice carrying the E1 region gene of human adenovirus type 12 under the control of the human renin promoter, in which a neuroectodermal tumor derived from retroperitoneal, olfactory, and/or pelvic regions was heritably developed with varying degrees of incidence and the phenotype was successfully passed through six generations. The transgenes were located in the region E2-E3 bands of chromosome 7 with which no genetic linkage to neuroectodermal tumors was previously demonstrated, and expressed only in the tumors but not in another tissue examined. Notably, in addition to the expression of a neural marker gene N-CAM, the three nuclear oncogenes, c-, L-, and N-myc, were coexpressed in the tumors. These results suggest that E1A and E1B are cooperatively involved in the heritable formation of neuroectodermal tumors associated with co-expression of the three sets of myc family genes.

Sequence polymorphism in the HLA-B promoter region

Transcription of major histocompatibility complex class I genes is controlled by the class I regulatory complex in the 5' flanking region. To investigate the molecular basis of this region, we studied the polymorphism of the promoter of the HLA-B locus extending from the ATG transcription initiation signal to -284 base pairs (bp) which includes a number of cis-acting elements: interferon response sequence (IRS), enhancer A and enhancer B. Genomic DNA from 35 homozygous cell lines from the 10th International Histocompatibility Workshop and from eight heterozygous panel members was amplified using two primers designed to specifically amplify the HLA-B locus. The double-stranded polymerase chain reaction products were sequenced using the cycle sequencing technique and an ABI 373A automatic sequencer. Promoter sequences of thirty-one different HLA-B alleles were determined in this study. Within the 284 bp upstream of the ATG signal, base substitutions were observed in 23 different nucleotide positions. Our study shows a high degree of polymorphism of the HLA-B promoter region, but conserved sequences of the known cis-acting elements with the exception of enhancer B in which there are two base substitutions for B7 and B42 (position -93 and position -95). The 23 polymorphic sites can be grouped into 12 different HLA-B promoter types (groups A to M) for 31 HLA-B locus alleles. Some of the groups of alleles sharing the same promoter sequence such as, for example, group A with B51, B52, B53, and B35, might have been predicted on the basis of serological similarity and/or exon 2, 3 sequence. In other groups, such as G (B18, B37, B27), it could not have been anticipated from serological experience that B18 and B27 carry the same promoter. Several sequencing errors were detected in the HLA-B promoter sequences published previously.

Oxidative stress as a mediator of apoptosis

Many agents which induce apoptosis are either oxidants or stimulators of cellular oxidative metabolism. Conversely, many inhibitors of apoptosis have antioxidant activities or enhance cellular antioxidant defenses. Mammalian cells exist in a state of oxidative siege in which survival requires an appropriate balance of oxidants and antioxidants. Thomas Buttke and Paul Sandstrom suggest that eukaryotic cells may benefit from this perilous existence by invoking oxidative stress as a common mediator of apoptosis.

Transcriptional suppression of HLA-B expression by c-Myc is mediated through the core promoter elements

In melanoma, HLA class I expression is suppressed by overexpression of the c-myc oncogene. This suppression has severe consequences for the recognition of these tumor cells by the immune system of the organism. We show here that transcription of the HLA-B locus, which is mainly affected by c-Myc, is downmodulated at the level of initiation of transcription. The transcriptional activity of various HLA-B reporter constructs was tested in a melanoma cell line with low endogenous c-myc expression and in transfectants with high stable and transient c-myc expression. We demonstrated that the responsive region can be mapped to the core promoter region of HLA class I, ruling out any effects of c-myc overexpression on the enhancer A or enhancer B regions. The region subject to downregulation is confined to a 43 base pair fragment encompassing the CCAAT and TATA elements. By coupling this region to a heterologous viral enhancer, we showed that the downmodulation by c-Myc is independent of the presence and nature of an enhancer. These results suggest a mechanism in which c-Myc downregulates the expression of HLA class I genes by interfering with the basal level of transcription.

Potential immunogenicity of oncogene and tumor suppressor gene products

The immunogenicity of viral oncoproteins has been established beyond doubt. Cytotoxic T lymphocytes directed against viral oncogene products can eradicate large established tumor masses. This stage has not yet been reached for cellular oncogene and tumor suppressor gene products, but T cells have been raised against MHC-binding peptides encoded by both mutant and wild-type alleles of the ras oncogene and the p53 tumor suppressor gene. In addition, T cells specific for joining region peptides of abnormal fusion proteins resulting from chromosome translocation in tumor cells have been generated. Some of these peptides are processed in cells infected with, for example, vaccinia-ras, but direct anti-tumor effects of peptide specific T lymphocytes remain to be demonstrated.

Downregulation of HLA class I expression by c-myc in human melanoma is independent of enhancer A

High constitutive expression of the c-myc oncogene in human melanoma leads to downregulation of expression of HLA Class I genes. The genes at the HLA-B locus are preferentially affected. To investigate the mechanism of downregulation, the activity of the main HLA Class I enhancer, enhancer A-region I, was compared in a panel of c-myc transfectants with increasing myc expression. Gel retardation experiments demonstrated in all tested cell lines binding of the transcription factors KBF1 and NF-kappa B to the enhancer. However, no correlation between the levels of HLA Class I expression and binding to the enhancer could be established. Strikingly, the cell line with the highest c-myc expression showed more binding of KBF1 and NF-kappa B than the parental cell line. By using CAT reporter plasmids in transient transfection assays we investigated the in vivo function of enhancer A-region I in the c-myc transfectant panel. Again, c-myc expression had no effect at all on the activity of enhancer A. This study shows that HLA Class I expression is regulated by the c-myc oncogene at the level of transcription, but that the main HLA Class I enhancer is not involved in this process.