Morphometric and histochemical study of involution in the rat uterus after parturition

Expression of SIP24 in the peripartum and postpartum rat uterus

SIP24 is, a 24 kDa superinducible protein, is an acute phase protein that is expressed in several tissues and organs and is identified in the human, mouse, and rat genome as lipocalin 2 (lcn2). This study investigated SIP24 expression by the rat uterus just before delivery and during postpartum involution. The distribution and levels of expression of SIP24 and myeloperoxidase (MPO) were compared by Western blot analysis and immunocytochemistry. The SIP24 level was high on day 22 of pregnancy and on days 1 and 2 postpartum, decreased on day 3 postpartum, and declined to low levels similar to those in nonpregnant rats by day 5 postpartum. SIP24 positive cells were identified by immunohistochemistry in the luminal and glandular epithelium on day 22 of pregnancy and on days 1, 2, 3, and 5 postpartum in addition to nonpregnant rats. Immunostaining was intense on day 22 of pregnancy and on days 1 and 2 postpartum. MPO also was identified in uterine tissues by immunohistochemistry. An evaluation of the SIP24 and MPO distribution and levels of expression in the rat uterus is consistent with the hypothesis that SIP24 modulates the neutrophil population by activities such as inducing their apoptosis.