Characterization of Insulin-Like Growth Factor-Binding Proteins in the Uterus and Conceptus during Early Conceptus Elongation in Cattle1

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Abstract
Temporal and cell-type specific distribution of transforming growth factor alpha (TGF alpha) precursor (proTGF alpha) was examined in the mouse uterus during the periimplantation period, and after steroid hormone treatments of ovariectomized adult mice by immunohistochemistry using antibodies that recognize the precursor forms of the growth factor. These studies were complemented by immunoblot analysis of proTGF alpha in separated uterine cell-type preparations. The specificity of the antibodies used in these studies was confirmed by use of pancreas or lactating mammary glands from transgenic mice in which mutated proTGF alpha, lacking recognition sites for proteolytic cleavages, was targeted for expression under a tissue-specific enhancer/promoter. Analysis of histochemical studies revealed accumulation of immunoreactive proTGF alpha primarily in luminal and glandular epithelial cells on Day 1 of pregnancy or pseudopregnancy followed by little or no accumulation on Days 2 and 3. However, immunoreactive proTGF alpha started to reappear in the luminal epithelium on the morning of Day 4 and became more prominent in the afternoon. In pregnant mice, immunostaining persisted in these cells at the implantation sites during the time of attachment reaction (2130 h on Day 4), but disappeared by morning of Day 5. Immunostaining appeared to be situated at the apical border of the luminal epithelium. No positive immunostaining could be detected in the nonreceptive uterus on Day 5 or 6 of pseudopregnancy. Consistent with the immunohistochemistry results, Western blot analysis detected two species of precursor proteins (14.5 and 17 kDa) in isolated luminal epithelial cell-enriched preparations on Day 4, but not on Day 5, of pseudopregnancy. The results suggest that proTGF alpha accumulates in the luminal epithelium of the receptive uterus prior to implantation. The effects of ovarian steroids on uterine accumulation of proTGF alpha were examined in ovariectomized adult mice by immunohistochemistry and immunoblotting. Whereas an injection of estradiol-17 beta (E2) or progesterone (P4) had little or a modest effect on epithelial accumulation of proTGF alpha, P4 priming for several days resulted in distinct accumulation of proTGF alpha in epithelial cells. The superimposition of an E2 treatment on P4 priming showed a biphasic response, with an initial gradual loss of immunostaining through 12 h followed by a return by 24 h of E2 treatment. The combined hormone treatment schedule employed here is similar to the situation of inducing implantation with E2 in P4-primed delayed implanting mice. The results suggest a paracrine/"juxtacrine" role for this growth factor in implantation.