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Mechanisms underlying the downregulation of the transporter associated with antigen processing (TAP)-1 in carcinomas Setiadi, Alvernia Francesca

Abstract

Expression of Transporter associated with Antigen Processing (TAP)-1 is often lost in metastatic carcinomas, resulting in defective antigen processing and presentation, and escape of the cancer cell from immune surveillance. In this study, the nature of TAP-1 deficiencies in tumors was investigated. By chromatin immunoprecipitation assay, it was shown that the recruitment of RNA polymerase II to the Tap-1 gene was impaired in TAP-deficient, metastatic cells derived from murine melanoma, prostate and lung carcinomas. Levels of TAP-1 promoter activity, as assessed by stable transfections with a reporter construct containing the TAP-1 promoter, were also relatively low in TAP-deficient cells. These suggest that the deficiency in TAM expression resulted--at least partially--from a reduced level of transcriptional activity of the Tap-1 gene. In order to examine genetic heritability of regulators of TAP-1 promoter activity, TAP- and MHC class I-deficient cells of H-2[sup b] origin were fused with wild-type fibroblasts of H-2[sup k] origin. Fusion with TAP-expressing cells complemented the low levels of TAP-1 promoter activity in TAP-deficient cells. However, these fused cells exhibited lower levels of TAP-1 mRNA and H-2[sup k] than unfused fibroblasts. Further analysis showed that TAP-1 mRNA stability was lower in fused carcinoma-fibroblasts than in unfused fibroblasts. Taken together, TAP deficiency in many carcinomas appears to be caused by a decrease in activity/expression of trans-acting factors regulating TAP-1 promoter activity, as well as a decrease in TAP-1 mRNA stability. The hypothesis that epigenetic regulation plays a fundamental role in controlling TAP-1 transcription was also tested. Chromatin immunoprecipitation analyses showed that the lack of TAP-1 transcription correlated with low levels of recruitment of a histone acetyltransferase, CBP, and of histone H3 acetylation at the TAP-1 promoter, leading to a decrease in accessibility of the RNA polymerase II complex to the TAP-1 promoter. These findings lie at the heart of understanding immune escape mechanisms in tumors and suggest that the reversal of epigenetic codes may provide novel immunotherapeutic paradigms for intervention in cancer.

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