Presence of the Acute Phase Protein, Bikunin, in the Endometrium of Gilts During Estrous Cycle and Early Pregnancy1

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Abstract
Noninvasive, epitheliochorial placental attachment in the pig is regulated through endometrial production of protease inhibitors. The objective of the present study was to determine if the light-chain serine protease inhibitor of the inter-α-trypsin inhibitor family, bikunin, is produced by the porcine endometrium during the estrous cycle and early pregnancy. Western blot analysis revealed the presence of bikunin in uterine flushings of gilts collected during the luteal phase of the estrous cycle and early pregnancy (Days 12–18). However, bikunin unbound to the inter-α-trypsin heavy chains was detected only in endometrial explant culture medium obtained from estrus and pregnant (Days 12, 15, and 18) gilts. Endometrial bikunin gene expression was lowest on Day 10 of the estrous cycle and pregnancy, followed by a 30- to 77-fold increase on Day 15 of the estrous cycle and pregnancy. Bikunin gene expression decreased on Day 18 of the estrous cycle, whereas endometrial bikunin gene expression continued to increase in pregnant gilts. Bikunin mRNA was localized to the uterine glands between Days 15 and 18 of the estrous cycle and pregnancy. In addition to its role as a protease inhibitor, bikunin functions in stabilization of the extracellular matrix, which suggests that bikunin could be involved with facilitating placental attachment to the uterine epithelial surface in the pig.

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