Decreased cellular retinol-binding protein expression coincides with the loss of retinol responsiveness in rat cervical epithelial cells.

In response to estrogen the rat cervical epithelium undergoes squamous metaplastic changes, progressing from a resting state through a proliferating, secretory stage and finally to a cornified stage before sloughing or being reabsorbed. The transition from a secretory to a cornified epithelium is preceded by a dramatic reduction in the expression of the cellular retinol binding protein (CRBP). The associations among retinoids (retinol and retinoic acid), CRBP expression, and estrogen-induced keratinocyte differentiation were explored in cultured cervical epithelial cells. Retinoids supported proliferation of cervical epithelial cells expressing basal keratins. Alone, estrogen had no effect on proliferation and enhanced expression of keratins characteristic of stratified cervical epithelial cells. When added together, estrogen prevented retinoid effects on proliferation, whereas retinoids prevented the estrogen-enhanced expression of differentiation-associated cytokeratins. When CRBP expression was repressed by elevating intracellular cyclic AMP levels, the ability of retinol, but not retinoic acid, to block estrogen-induced changes in keratin expression was severely compromised. These results support a critical role for CRBP in cervical cell responsiveness to circulating retinoids (primarily retinol). We hypothesize that retinol inhibits estrogen-induced keratinization of the cervical epithelium, and the drop in CRBP level results in transient vitamin A deficiency within cervical epithelial cells, permitting the orderly transition from the secretory to the cornified stage.