Regulation of growth and gene expression in human papillomavirus‐transformed keratinocytes by transforming growth factor‐β: Implications for the control of papillomavirus infection

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Abstract
Cervical carcinogenesis is a multistep process that appears to be initiated by infection of squamous epithelial cells in the cervix with one of a limited number of human papillomavirus (HPV) types. However, the mechanisms involved in the evolution of benign, HPV-induced lesions to malignancy have not yet been fully elucidated. Transforming growth factor-β (TGF-β), a multifunctional growth factor produced by cells in the skin, inhibits the proliferation of foreskin and cervical keratinocytes in vitro. We examined the effects of TGF-β on growth and virus early-gene expression in cell lines immortalized by two HPV types associated with cervical carcinogenesis as well as the expression of TGF-β1 mRNA transcripts in normal and HPV-positive cells in vivo and in vitro. We found that normal and HPV-positive cells expressed similar levels of TGF-β1 mRNAs and exhibited similar patterns of responsiveness to three isoforms of TGF-β in both monolayer and modified organotypic cultures. Of particular interest is our finding that the expression of the E6 and E7 early viral transforming regions of both HPV16 and HPV18 was reversibly and rapidly inhibited by TGF-β. In one HPV16-positive cell line examined in detail, inhibition of HPV expression required protein synthesis and occurred at the level of transcription. HPV-immortalized cells selected for resistance to in vitro differentiation signals remained sensitive to TGF-β-mediated growth inhibition. These results, showing that both growth and virus gene expression in HPV-transformed cells were responsive to TGF-β, suggest that endogenous growth factors produced by different cell types in squamous epithelium may play a role in the progression of cervical neoplasia.