Expression of cellular retinol- and cellular retinoic acid-binding proteins in the rat cervical epithelium is regulated by endocrine stimuli during normal squamous metaplasia

To determine the potential roles of retinoids in the growth and differentiation of the reproductive tract epithelium, we have studied the expression of the cellular retinol- and retinoic acid-binding proteins, CRBP I and CRABP I, in the reproductive tract of female rats. CRBP I and CRABP I gene expression have been examined in the oviduct, ovary, uterus, and particularly in the cervix, which normally undergoes a cyclical squamous metaplasia during the estrus cycle. CRBP I was expressed in all four tissues examined, whereas CRABP I was expressed predominantly in cervix and uterus. In the cervix, CRBP I was detected in all epithelial layers including the columnar epithelium but was greatly reduced in the superficial, cornified layers of the stratified squamous epithelium. CRABP I was localized to the basement membrane region of the epithelium with the strongest expression in the basal layer of epithelial cells. While the expression of CRBP I and CRABP I in the keratinizing exocervix changed during the estrus cycle, it remained constant in the incompletely keratinized endocervix. The highest levels of CRBP I were seen during anestrus and proestrus, and for CRABP I during proestrus. Both CRBP I and CRABP I levels fell to barely detectable levels during estrus and metestrus. Using estrogen repletion of ovariectomized rats, we found that CRABP I levels transiently increased during the early proliferative response to estrogen, whereas CRBP I levels gradually declined, becoming barely detectable by 24 to 48 hours. These results suggest that CRBP I and CRABP I play different roles in the cyclical squamous metaplasia normally occurring in this tissue and that hormonal control of CRBP I and CRABP I expression might modulate the retinoid responsiveness of the epithelium during this process.