Retinoids suppress epidermal growth factor-associated cell proliferation by inhibiting epidermal growth factor receptor-dependent ERK1/2 activation.

Human papillomavirus (HPV) is an important etiological agent in the genesis of cervical cancer. HPV-positive cervical tumors and human papillomavirus-positive cell lines display increased epidermal growth factor receptor (EGFR) expression, which is associated with increased cell proliferation. ECE16-1 cells are an HPV-immortalized human ectocervical epithelial cell line that is a model of HPV-associated cervical neoplasia and displays elevated EGFR levels. In the present study, we evaluated the effects of receptor-selective retinoid ligands on EGFR-associated signal transduction. We show that retinoic acid receptor (RAR)-selective ligands reduce EGFR level and the magnitude and duration of EGFR activation in EGF-stimulated cells. These effects are reversed by cotreatment with an RAR antagonist. To identify the mechanism, we examined the effects of retinoid treatments on EGF-dependent signaling. Stimulation with EGF causes a biphasic activation of the ERK1/2 MAPK. The first peak of activation is present at 20 min, and the second is present at 36 h. This activation subsequently leads to an increase in the cyclin D1 level and increased cell proliferation. Simultaneous treatment with EGF and a RAR-selective retinoid inhibits both phases of ERK1/2 activation, completely eliminates the cyclin D1 induction, and suppresses EGF-dependent cell proliferation. This effect is specific as retinoid treatment does not alter the level or activity of other EGFR-regulated kinases, including AKT and the MAPKs p38 and JNK. Retinoid X receptor-selective ligands, in contrast, did not regulate these responses. These results suggest that RAR ligand-associated down-regulation of EGFR activity reduces cell proliferation by reducing the magnitude and duration of EGF-dependent ERK1/2 activation.