In our study, we evaluated the intensity and prevalence of ADAMTS-1 immunoreactivity in uterine tissues of rats at different stages of the estrous cycle and of OVX rats treated with E2 and P4.

The reproductive cycle often leads to changes in the levels of E2 and P4, which are produced by the ovaries (Foster and Gray 2010). E2 levels are low at the beginning of the diestrus phase but start to increase toward the end of the diestrus phase, reaching the highest level in the proestrus phase, thus triggering ovulation. The proestrus phase is followed by the estrus phase with a sudden decrease in E2 levels, with the levels continuing to decrease. On the other hand, P4 levels remain high until ovulation and start to decrease toward the end of the estrus phase, and continue to decrease at the beginning of the diestrus phase (Foster and Gray 2010). Our results revealed that the serum level of E2 was the highest in the proestrus phase and gradually decreased in the estrus and diestrus phases. The serum level of P4 increased until the estrus phase and started to decrease after the diestrus phase. Davidge et al. reported that E2 levels decreased in the diestrus phase and that E2 treatment did not cause a significant increase in this level in their study comparing sham and OVX groups (Davidge et al. 2001). Conversely, another study reported that P4 levels in the sham group were on average five times higher than E2 levels, and E2 and P4 levels, which decreased with OVX administration, increased with hormone replacement (Marks et al. 2013). Campbell et al. also stated that the serum levels of E2 and P4 decreased in the OVX group compared with the levels in the sham group; a significant increase was noted in these levels in the E2 + P4 group (Campbell et al. 2003). In the current study, it was determined that the E2 and P4 hormone profile data of the sham group were similar to the findings of Marks et al. (Marks et al. 2013), and that this situation may be due to a complex interaction between ovarian hormones. In addition, this study revealed similar results to the changes in E2 and P4 levels determined in studies where OVX and hormone replacement were applied.

The estrous cycle is a recurrent process under the influence of gonadotropin hormones. FSH and LH are gonadotropins; FSH ensures the growth and development of follicles, whereas LH stimulates ovulation and corpus luteum formation (Aritonang et al. 2017). FSH and LH levels increase and are the highest in the proestrus phase but are the lowest in the estrus phase. In the diestrus phase, the FSH level is the lowest but the LH level increases (Marcondes et al. 2002; Ajayi and Akhigbe 2020). Our findings suggest that FSH and LH levels in the phases of the estrous cycle show a trend similar to that reported in the literature. In our study, we determined that these hormone levels, which increased with OVX administration, decreased with E2 and P4 administration and reached the lowest level in the E2 + P4 combination. In similar studies with OVX, it was reported that serum FSH and LH levels increase (Koebele et al. 2019), and these levels were partially regulated with hormone replacement (Rouach et al. 2011).

The menstrual cycle is a process characterized by various structural changes in endometrial ECM (Demircan et al. 2014). During this cycle, complete tissue repair occurs in the proliferative phase with increasing estradiol levels, early stromal decidualization of the endometrium, menstrual disruption, and mucosal thickening. As estradiol and progesterone levels decrease in the late secretory phase, they lead to induction of menstruation, proteolytic ECM degradation, and shedding of the functional layer, thus ensuring the continuation of the cycle (McLaughlin 2022). This process represents a perfect paradigm for ECM remodeling, and changes in the expression of ECM components are also associated with hormonal fluctuations during the cycle phases (Gaide-Chevronnay et al. 2008). Conversely, it was noted that some proteins belonging to ECM components were not observed in the metestrus stage (Wood et al. 2007). The expression of ADAMTS-1, which is responsible for the degradation of proteoglycans such as syndecan and perlecan in the ECM, is promoted by gonadal steroids such as P4, E2, and androgens (Pelufo et al. 2011; Russell et al. 2015).

ADAMTS-1 expression has been detected in the glandular and stromal cells of the human endometrium in vivo and in vitro (Ng et al. 2006). Its localization has been reported in human endometrium at different stages of the menstrual cycle and pregnancy (Namli Kalem et al. 2018). An experimental study found that ADAMTS-1 mRNA expression was intense in luminal and glandular epithelial cells in the mouse uterus, especially in the E2-dependent proestrus and estrus stages (Kim et al. 2005). Similarly, in another study, it was stated that ADAMTS-1 was also intense in endometrial stromal cells, in addition to its expression in these cells, and that ADAMTS-1 was regulated by the E2-dependent EGR1 transcription factor in the uterus (Park et al. 2021). In our study, we determined that ADAMTS-1 immunolocalization is more intense in luminal and glandular epithelial and endometrial stromal cells in the proestrus phase, where E2 is dominant, compared with the estrus and diestrus phases. ADAMTS-1 localization in the uterus according to the phases of the estrous cycle may be associated with changes in E2 levels.

It has been reported that the presence of ADAMTS-1 in periovulatory follicles of mice is dependent on PRs in response to LH (Robker et al. 2000) and acts on ECM molecules and/or growth factors in the uterus, which is the main target of P4 (Kim et al. 2006). In PR gene knockout mice, it has been shown that ovulation was absent and ADAMTS-1 mRNA expression was significantly reduced (Namli Kalem et al. 2017). It has also been reported that ADAMTS-1 mRNA expression is stimulated by P4 (Mishra et al. 2013), which is effective in implantation, fetal development processes, and endometrial functions (McLaughlin 2022). On the contrary, another study stated that ADAMTS-1 expression was regulated by the E2-dependent EGR1 transcription factor in the uterus, and that ADAMTS-1 was absent in EGR1 knockout mice (Park et al. 2021). In addition, it was determined that ADAMTS-1 expression increased after E2 application and immunofluorescence activity was observed especially in glandular, luminal epithelial, and endometrial stromal cells (Park et al. 2021). Kim et al. determined that ADAMTS-1 gene expression was significantly increased by the combination of E2 + P4 compared with P4 administration alone in ovariectomized mice receiving hormone treatment (Kim et al. 2004). Similarly, in this study, E2 application increased the decreased ADAMTS-1 immunoreactivity compared with P4 in OVX rats receiving hormone treatment, and its localization was observed in luminal and endometrial stromal cells. It was therefore determined that the E2 + P4 combination was more effective. It can be said that the presence of ADAMTS-1, which mediates dynamic tissue regeneration in the uterus, and is affected by hormonal changes during the reproductive cycle, is regulated endocrinically.

This study has some limitations. The study findings could not be adequately compared due to the lack of information on the immunolocalization of ADAMTS-1 in the OVX and hormone replacement models, and in the estrous cycle stages. Even with this lack of information, our study is still considered to have important and current aspects. In our study, ADAMTS-1 localization in the uterus was examined only by immunohistochemical techniques. It is thought that the mechanisms by which ADAMTS-1 acts as a regulator in uterine remodeling and the determination of other related ECM components with different methods may be the focus of our future research.

In conclusion, we examined the uterine endometrium of rats in proestrus, estrus, and diestrus stages, as well as ovariectomized rats receiving hormone replacement therapy for ADAMTS-1 immunoreactivity. We found that the distribution of ADAMTS-1 in the uterus is highly sensitive to estrous stage and hormone replacement therapy, and that this immunolocalization is influenced by tissue-specific hormonal changes. ADAMTS-1, which is thought to contribute to tissue remodeling owing to this hormonal regulation, should be considered for its potential to guide future research.