CHARACTERIZATION OF THE EXPRESSION AND STEROID-HORMONE CONTROL OF SIALOMUCIN COMPLEX IN THE RAT UTERUS - IMPLICATIONS FOR UTERINE RECEPTIVITY

The sialomucin complex (SMC), originally isolated as a heterodimeric g lycoprotein complex from membranes of ascites sublines of a highly met astatic mammary adenocarcinoma, consists of a high M-r mucin subunit ( ASGP-1, ascites sialoglycoprotein) and a transmembrane subunit (ASGP-2 ) with two epidermal growth factor-like domains. SMC has been characte rized in the mammary gland, where it is present in both membrane and n onmembrane (soluble) forms, the latter lacking its transmembrane domai n. SMC in the mammary gland is observed during pregnancy and lactation , but not in the virgin gland, and is regulated by a posttranscription al mechanism. Both membrane and nonmembrane forms of SMC are found in rat uterus, also as a complex of ASGP-1 and ASGP-2. Immunocytochemical analyses indicate that the primary site of expression is at the lumin al surface of the endometrium. Approximately 40% of the SMC, correspon ding to the nonmembrane fraction, is removed by rinsing uterine prepar ations with saline, indicating that the soluble form is adsorbed loose ly to the cell surface. In contrast to mammary gland, SMC is most high ly expressed in the virgin animal, and its expression varies during th e estrous cycle with the steady state level of transcript. The complex is present in a location consistent with steric inhibition of blastoc yst implantation and is abruptly lost at the beginning of the period o f receptivity for implantation. Expression of SMC in the uterus is reg ulated by estrogen and progesterone and is inversely correlated with r eceptivity. Both implantation and loss of SMC expression can be blocke d with RU486. We propose that the downregulation of SMC and its loss f rom the apical surface of the rat uterine lining contribute to the gen eration of the receptive state for uterine implantation. Furthermore, the presence of both membrane and soluble SMC at the luminal surface o f the endometrium may provide a model for its proposed protective func tion in other accessible and vulnerable epithelia. (C) 1998 Wiley-Liss , Inc.