Studies have focused on the interaction between human papillomaviruses (HPV) and normal human genital epithelial cells. Specifically, 1) high-titer retroviruses were developed to examine the role of specific HPV genes in regulation of cellular growth and differentiation, and 2) the molecular mechanism by which transforming growth factors beta (TGFbetas) 1 and 2 regulate HPV gene expression was investigated in genital epithelial cells at different stages of malignant progression. HPV E6 and E7 genes are implicated in maintenance of the malignant phenotype in cervical cancer. To examine whether E6 and E7 proteins directly regulate cell growth and differentiation genital keratinocytes were infected with recombinant retroviruses containing these genes alone or in combination. Retroviruses encoding the E6/E7 and E7 genes stimulated cell proliferation, reduced the requirement for bovine pituitary extract, delayed onset of terminal differentiation and induced immortality. However, these infected keratinocytes stratified and formed well-differentiated squamous epithelia when transplanted beneath a skin-muscle flap of nude mice. Thus, expression of HPV-18 E6 and E7 genes alters cell growth and commitment to differentiation but does not directly induce aberrant squamous differentiation in normal genital keratinocytes in vivo. TGFbetas 1 and 2, polypeptides that regulate cellular growth and differentiation, reversibly inhibited expression of the HPV-16 E6 and E7 genes in several immortal genital epithelial cell lines. Loss of E6 and E7 protein expression followed a dramatic time- and dose-dependent decrease in E6 and E7 RNA levels and was accompanied by cessation of cell proliferation. TGFbetas 1 and 2 also induced a six- to seven-fold increase in TGFbetas RNA. Cells became partially resistant to the inhibitory effects of TGFbeta1 on cell growth and HPV early gene expression after prolonged cultivation in vitro or after malignant transformation. Thus, TGFbeta1 may function as an autocrine regulator of HPV gene expression in infected genital epithelial cells.
