Selective suppression of human papillomavirus transcription in non-tumorigenic cells by 5-azacytidine.

The transcription of human papillomavirus type 18 (HPV 18) is selectively suppressed in non-tumorigenic HeLa x fibroblast or HeLa x keratinocyte cell hybrids by 5-azacytidine. In contrast, viral gene expression is not influenced by 5-azacytidine in both tumorigenic hybrid segregants and in the parental HeLa cells. The suppression mechanism seems to operate at the level of initiation of transcription since nuclear run-on experiments show the absence of elongated nascent viral RNA, whereas the transcription of cellular reference genes remains unaffected. Down-regulation of HPV 18 mRNA correlates directly with cessation of cellular growth and can be abolished using the protein synthesis inhibitor cycloheximide. Furthermore human keratinocytes immortalized by HPV 16 but still retaining the non-tumorigenic phenotype reveal the same inhibitory effect on viral transcription after treatment with 5-azacytidine. These results support a model of a postulated intracellular control mechanism, directed against papillomavirus transcription, which can be induced by 5-azacytidine and appears to correlate with the presence of specific chromosomes in non-tumorigenic cells.