Retinol binding protein is produced by the bovine endometrium and accumulates in uterine secretions in a progesterone-dependent manner

Abstract

Studies were conducted to establish whether (a) uterine lumenal fluids contain an immunoreactive retinol binding protein (RBP), (b) such a protein is produced by the uterine endometrium, (c) retinol binding activity can be associated with immunoreactive RBP, and (d) presence of the immunoreactive RBP is regulated by progesterone. When using antiserum to human RBP (serum form) as a probe in immunoblotting, three molecular weight classes of protein species could be detected in uterine lavage fluids of pregnant and bred/nonpregnant cows at Day 17 after estrus and in culture supernatants of cyclic, bred/nonpregnant and pregnant cows at Day 17 after estrus. The predominant band was at a molecular weight (Mr) of 22 000 (RBP 22) but several other minor bands could be detected at molecular weights of 25 000–30 000 (RBP 25–30) and 55 000–65 000 (RBP 55–65). These immunoreactive forms of RBP could be detected in serum. Addition of cycloheximide to endometrial cultures inhibited amounts of RBP 22 in supernatants of endometrial cultures, confirming in vitro synthesis of RBP by endometrium. Biological activity was confirmed by incubation of immunoblots with 3H-retinol: specific binding of radiolabel was associated with RBP 22 and RBP 55–65. To investigate regulation of secretion of RBP, ovariectomized cows were treated with corn oil vehicle for 12 days or with 750 mg day −1 progesterone for 12 or 30 days. While immunoblots revealed RBP 22 in uterine lavage fluids of endometrial explants from all cows, progesterone-treated cows had more RBP 22 in uterine lavage fluids than did cows treated with vehicle alone. Staining intensities of RBP 25–35 and RBP 55–65 were not affected by treatment. It was concluded that the uterine lumen contains a retinol binding protein that is derived from both local endometrial synthesis and blood. Furthermore, amounts of RBP in uterine secretions are regulated by progesterone.