EVIDENCE FOR THE INVOLVEMENT OF A NUCLEAR NF-KAPPA-B INHIBITOR IN GLOBAL DOWN-REGULATION OF THE MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I ENHANCER IN ADENOVIRUS TYPE 12-TRANSFORMED CELLS

Diminished expression of major histocompatibility complex class I anti gens on the surface of adenovirus type 12 (Ad12)-transformed cells con tributes to their high tumorigenic potential by enabling them to escap e immune recognition by cytotoxic T lymphocytes. This low class I anti gen expression is due to a block in class I transcription, which is me diated by Ad12 E1A. Genetic analysis has shown that the class I enhanc er is the target for transcriptional down-regulation. In this study, w e show that the ability of the R1 element of the class I enhancer to s timulate transcription is greatly reduced in Ad12-transformed cells. T he loss of functional activity by the R1 element was attributed to los s of binding by the NF-kappa B p50-p65 heterodimer, NF-kappa B binding appears to be blocked within the nucleus rather than at the level of nuclear translocation, Significantly, NF-kappa B binding activity coul d be recovered from the nuclear extracts of Ad12-transformed cells fol lowing detergent treatment, suggesting that the block is mediated thro ugh a nuclear inhibitor present in the Ad12-transformed cells. These r esults, taken together with the fact that the R2 element of the class I enhancer exhibits strong binding to the transcriptional repressor CO UP-TF, suggest that the class I enhancer is globally down-regulated in Ad12-transformed cells.