The progressive rise in the expression of alpha crystallin B chain in human endometrium is initiated during the implantation window: modulation of gene expression by steroid hormones.

Human endometrium undergoes sequential changes during the menstrual cycle and becomes receptive to implantation during a defined period in the secretory phase. We attempted to identify the genes expressed during this period by representational difference analysis (RDA). When the cDNAs of a proliferative endometrium were used as the driver and the cDNAs of a post-ovulatory day 5 endometrium were used as the tester, a number of bands were identified by RDA. DNA of the cloned RDA products revealed that the majority of the clones contained a fragment of a cDNA identical to that of a crystallin B chain. Northern blot analysis showed that the expression of the alpha crystallin B chain mRNA was absent during the proliferative phase. The expression of the mRNA of alpha crystallin B chain first appeared in the secretory phase, progressively increased during this phase and peaked in the late secretory endometria. The pattern of expression of alpha crystallin B chain mRNA in the endometrium of mature cycling baboons (Papio anubis) was similar to that seen in human endometrium. As revealed by Western blot analysis, the expression of the alpha crystallin B chain protein in human endometrium followed a pattern of expression similar to its mRNA. At the cellular level, the immunoreactive protein first appeared in the surface epithelial cells of human endometrium within the implantation window without significant immunoreactivity in the underlying glandular cells. During the mid- and late secretory phases, the intensity of staining in the epithelial cells was enhanced and an intense immunoreactivity was developed in the glandular epithelium, alpha crystallin B chain was virtually an epithelial product and no immunoreactivity for this protein was detectable in the stromal cells, endothelial cells or lymphoid cells. The expression of alpha crystallin B chain could be regulated, by medroxy progesterone acetate as well as by oestrogen withdrawal, in human endometrial carcinoma cells (EnCa-101), transplanted to nude mice. Based on the data presented here, the known function of alpha crystallin B chain and its distinct pattern of expression in human endometrium, we suggest that this protein is an important factor within the molecular repertoire that makes endometrium receptive to implantation.