Human papillomavirus 16 immortalization of normal human ectocervical epithelial cells alters retinoic acid regulation of cell growth and epidermal growth factor receptor expression.

Retinoids are potent regulators of epithelial cell growth and differentiation. Recently, they have been demonstrated to be effective in the treatment of preneoplastic cervical lesions in which human papillomavirus (HPV) is expressed. To better understand the mechanism of the antineoplastic effect of retinoic acid on HPV-positive cells, the effects of retinoic acid on both normal and HPV-immortalized human ectocervical epithelial cell growth, epidermal growth factor (EGF) receptor level, and EGF receptor function were investigated. Both HPV-immortalized cells (ECE16-1) and normal ectocervical cells (ECE cells) are growth stimulated by EGF. ECE16-1 but not normal ectocervical epithelial cells are growth inhibited by trans-retinoic acid which attenuates the stimulatory effect of EGF on ECE16-1 cell growth. Retinoic acid reduces both EGF binding and EGF receptor protein levels in ECE16-1 cells but not in normal ectocervical cells. The reduction in EGF receptor binding and receptor protein levels in ECE16-1 cells is not associated with the induced secretion of a soluble EGF receptor ligand, altered EGF receptor affinity, receptor internalization, or decreased receptor stability. Interestingly, the level of EGF receptors is consistently elevated in the ECE16-1 cell line as compared to normal ectocervical epithelial cells. Investigation of a second HPV-immortalized cell line (ECE16-D1) and two other HPV-positive cervical carcinoma cell lines revealed similar elevated EGF-binding capacity and regulation by retinoic acid. In contrast, two HPV-negative cervical carcinoma cell lines demonstrated various EGF-binding levels but demonstrated no significant loss of EGF binding following retinoic acid treatment. Other normal cells and an SV40 large T-antigen-immortalized foreskin keratinocyte cell line, KER-1, had EGF receptor levels similar to the normal ectocervical epithelial cells, and no regulation by retinoic acid was observed. These data indicate that HPV immortalization may increase EGF receptor levels in ectocervical cells, elevating their sensitivity to growth stimulation by EGF, and that retinoic acid can possibly attenuate this increased responsiveness to EGF.