Expression of heat shock protein 25,000 in rat uterus during pregnancy and pseudopregnancy

In previous studies, we found that the human estrogen-regulated heat shock protein (hsp) 27 (human homologue of rat hsp25) is modulated in the endometrium during the different phases of the menstrual cycle and that it is present in endometrial predecidual cells and in decidual cells attached to the placenta. In the present report, we describe the cell type-specific pattern of hsp25 expression in the rat uterus during the periimplantation period as well as during early and late decidualization and placentation. The hsp25 expression pattern was also analyzed in pseudopregnant rats with deciduomas. Immunocytochemistry was performed with an antibody generated against a chimeric hybrid protein containing the N-terminal of the murine hsp25 and the C-terminal of the human hsp27. During pregnancy at the time of implantation, hsp25 was expressed in the endothelial cells of the endometrial vessels and in the luminal epithelium of the antimesometrial region. As pregnancy advanced, hsp25 appeared in predecidual/decidual cells close to the implantation region and then expanded to the mesometrial region. This expression pattern was very similar during pseudopregnancy. Hsp25 was strongly expressed in trophoblastic giant cells beginning on Day 11 of gestation; less expression was noted in the junctional and labyrinth zones of the chorioallantoic placenta (in some cells lining the vascular spaces). In all the disparate cell types that expressed hsp25, the presence of the protein did not correlate with cell proliferation or with apoptosis but with the state of differentiation. Some placental PRL-family members with molecular weights similar to that of hsp25 are also present in antimesometrial decidua and in differentiated trophoblast giant cells; therefore, in this study we eliminated the possibility that our antibody was recognizing prolactin. We also determined that the hybrid hsp25/27 protein did not bind prolactin receptors, and noted that the hsp25 immunostaining pattern was not identical to that of decidual prolactin. In conclusion, the striking cell type-specific timing of hsp25 expression points to hsp25 as a molecule that is important during the implantation, decidualization, and placentation processes.