Modulation of Muc-1 mucin expression in the mouse uterus during the estrus cycle, early pregnancy and placentation

The Muc-1 protein is an integral membrane protein that is expressed apically by simple secretory epithelia in many different organs. We present in this paper a study on Muc-1 protein expression in the mouse uterus during early pregnancy, placentation and the estrus cycle. Muc-1 immunopositive reaction is found in the decidua by day 8 of pregnancy onwards. The observed pattern was unusual, since Muc-1 protein was present in spherical cytoplasmic granules in granular metrial gland cells. Both the intracellular pattern of expression and the lymphoid origin of these cells were striking results. Muc-1 is thought to be an epithelial differentiation marker, and this is the first report of its expression by non-epithelial cells. Our results on Muc-1 expression in the uterus of cycling mice showed that higher levels of Muc-1 mRNA and protein correlate with higher levels of plasma estrogen in the estrus and proestrus phases. However, in ovariectomized mice without hormone replacement, the endometrium expressed high levels of this protein. These levels could not be substantially changed by estrogen, although progesterone reduced the levels of Muc-1 protein associated with the epithelium. These data together with the normal expression in the cycling mice suggest that progesterone might repress Muc-1 expression during the metestrus and diestrus phases. In cycling mice, when plasma progesterone is at its nadir and the estrogen level is elevated in estrus and proestrus phases, Muc-1 concentration would increase to its basal level, not because of estrogen stimulation, but due to lack of progesterone repression. The low level of expression is also observed in the endometrium during early pregnancy, where reduced levels of Muc-1 protein are seen at the time of implantation.