MMPs play an important role in the remodeling of different structural and support components during ovulation [40, 41] decidualization [42, 43], and implantation [44–46]. The main findings of the present study can be summarized in five points in relation to the activity of proMMP-2 and proMMP-9 in the culture media of the embryos on day 5 of their development 1) the activity of proMMP-2 was found in 14 of 14 culture media corresponding to the patients who achieved a full-term pregnancy and in 3 of 25 culture media associated with women who did not have this outcome. It was 1.4-fold greater in the former group; 2) The activity of proMMP-2 was observed in 11 of 14 culture media corresponding to the patients who carried their pregnancy to term and in 11 of 25 culture media associated with women who did not have this outcome. It was 1.2-fold greater in the former group (Fig. 1); 3) In the three patients who did not carry their pregnancy to term, only proMMP-9 activity was detected; 4) the activity of both proMMP-2 and proMMP-9 was identified in three non-pregnant patients who had some pre-pregnancy complications (Fig. 1); and 5) There were no significant differences in the concentration of the hormones HCG, E2, or P4 between pregnant and non-pregnant patients (Table 1).
Gu et al. (2015) reported the concentration of the active form of MMP-9 at 0.698 ± 0.022 ng/mL in the culture medium of developing human embryos, which resulted in pregnancy for 77.0% of the participating patients [47–49]. According to the present study, the activity of proMMP-2 and proMMP-9 (Fig. 2) was significantly higher in the culture media of the embryos yielding a full-term pregnancy than in the culture media of the other cases (Table 1). The culture media exhibited a 1.4-fold greater proMMP-2 activity (p = 0.045) for the embryos generating a full term pregnancy compared the other cases (3/25) showing proMMP-2 activity. Similarly, the culture medium displayed a 1.2-fold greater proMMP-9 activity (p = 0.002) for the embryos engendering a full term pregnancy (11/14) compared to the other cases (11/28), considering the media with proMMP-9 activity.
A conceptual model is herein provided (Fig. 4) how MMPs are activated by epidermal growth factors , interleukin (IL) -1β, and tumor necrosis factor (TNF)-α [50, 51]. Sequeira et al. (2015) reported a significantly (15.4-fold) greater value for the level of IL-1β in the culture media of developing human embryos that became implanted in patients versus the culture media of non-implanted embryos (0.55 ± 0.25 pg/mL), finding implantation in 42.0% of the total participants .
In the first and second trimesters of pregnancy, syncytiotrophoblast cells have been described to secrete a 2.4- and 3.8-fold higher amount of IL-1β, respectively, than the pre-pregnancy level. The secretion of IL-1β is associated with an increase in MMP-9 expression . After interacting with its receptor, IL-1β regulates the signaling pathway that involves the activation of the mitogen-activated protein kinase (MAPK), p38 MAPK, c-Jun N-terminal kinase (JNK), and the extracellular regulatory kinase (ERK) [54, 55]. As a consequence, IL-1β promotes the activation of nuclear factor kappa-beta (NFkβ) and the expression of MMPs .
The collagenolytic activity of MMPs is regulated by the specific tissue inhibitors of these proteinases [49, 57]. Cytotrophoblast cells, treated with 50 nM of their tissue inhibitor, known as tissue inhibitor of metalloproteinase-2 (TIMP-2) exhibit an up to 40% reduction in invasiveness according to Librach (1991) [51, 58]. The present results showed a 1.4-fold and 1.2-fold decrease in the activity of proMMP-2 and proMMP-9 respectively (Fig. 2), in the culture media corresponding to the cases of patients who were pregnant but did not carry to term. However, the expression of TIMPs was not herein evaluated in the culture medium of developing embryos. It would be interesting to determine the MMP/TIMP relationship is involved in the mechanism responsible for regulating the progress of implantation and pregnancy.
Recently, localized polymorphisms identified in the promoter region of MMP-2 (-1306 C/T; rs 243865) and MMP-9 (-1562 C/T; rs 3918242) were found to induce changes in the levels of transcription and or expression of the protein. These mutations have been proposed as a risk factor for spontaneous abortion [50, 59]. Regarding the three patients of the present study with implanted embryos that spontaneously aborted (P52, P57, and P58), the corresponding culture media displayed proMMP-2 and proMMP-9 (Fig. 2) activity. Future research should explore the possible relation of such pregnancy complications to mutations.
MMP-2 has the ability to degrade fibronectin, elastin, and collagen type IV, V, VII, and. In contrast MMP-9 degrades proteoglycans, elastin, and collagen I, IV, V, and XI [51, 60–62] which allows the cytotrophoblast cells to invade the endometrium to prepare the way for implantation)  (Fig. 5). During the implantation window, according to in vivo (in animal models  and in vitro studies , the epithelial cadherin-like binding protein (E-cadherin) enables the embryo to adhere to the endometrial epithelium, which is degraded by MMP- 9 . Hence, previous reports evidence a key role by MMP-2 and MMP-9 in embryonic development. The present analysis based on the cutoff points of the ROC curve for both proteinases suggests that their evaluation in the culture medium of developing embryos has plausible predictive power in relation to the success of implantation.