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

University of British Columbia

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.

Text

2011-02-18

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http://hdl.handle.net/2429/31495

Zoology

University of British Columbia

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