In our last post on this topic, we concluded that imbalances of the fibrinolytic system are commonly found in women with no other obvious explanations for recurrent early pregnancy loss (RPL) and that one of the more consistent findings in this imbalance is elevated activity of plasminogen activator inhibitor-1 (PAI-1) and a genetic predisposition for the same. Today we are going to begin to review mechanisms by which increased PAI-1 activity could have a deleterious affect on early pregnancy success. In my search through the literature, I first came across several articles that addressed the role of PAI-1 and the fibrinolytic system in placentation. But, I also found several intriguing articles that addressed their roles in ovulation and in the development and regression of the corpus luteum. So let’s start our discussion at that point, since aberrations with ovulation and subsequent development of the corpus luteum might also increase the risk of RPL, and then we will move on to the observations that have been made related to placentation.
First, let me state that this is a very complex subject and in no way do I profess any specialized expertise in this area. Indeed, if any of our readers do have this expertise and want to set me straight or have another interpretation of the scientific literature, please feel free to comment. I am fully aware of my limitations and am here to learn too! Secondly, I am only going to focus on observations related to the fibrinolytic system. I am not going to address the plethora of factors that might affect the actual expression of the various fibrinolytic system components, plasminogen activators or activator inhibitors. So, I ask forgiveness for oversimplification at the outset…
During each normal menstrual cycle, ovarian follicles are ‘recruited’ to produce the ‘egg of the month’; these must undergo development and differentiation and usually one (and sometimes more) becomes the ‘dominant follicle’ and differentiates further. The follicle is comprised of an outer perimeter of extracellular matrix, theca, and granulosa cells that surround the egg and its accompanying (follicular) fluid. At the appropriate time in the cycle, under the influence of luteinizing hormone (LH), the extracellular matrix surrounding the follicle rapidly degrades, the cells at the leading edge of the follicle migrate to and penetrate the surface (capsule) of the ovary, and the egg and its immediate layers of surrounding cells detach from the inner layer of the follicular granulosa cells and is released from the ovary. Once free, the fallopian tube can embrace it, introduce it to friendly sperm along the way, and facilitate the ultimate journey of the fertilized egg to the uterus.
Following ovulation, what’s left of the follicle (the corpus luteum) begins to make the hormone progesterone that helps to prepare (decidualize) the lining of the uterus (the endometrium) to receive the fertilized egg, aiding attachment and implantation of the early embryo. There is a very narrow ‘implantation window.’ With implantation, the fetal cells begin to proliferate and invade the endometrium and they send hormonal messages back to the corpus luteum instructing it to remain healthy and to continue production of progesterone. If conception does not occur, or implantation fails too early, the corpus luteum degenerates, progesterone production drops, and the menstrual cycle starts all over again. So, where does the fibrinolytic system play a role in this sequence of events? Quite frankly, roles have been proposed for every step of this process, but let me focus on just a couple of areas of interest.
Remember from our first post on this subject, plasminogen activators (PA), such as tissue plasminogen activator (t-PA) and urokinase-like plasminogen activator (u-PA), convert plasminogen to its active form, plasmin, which can then direct the breakdown (fibrinolysis) of clots. Well, plasmin actually is in a class of compounds called serine proteases that can help breakdown and rearrange many protein compounds other than fibrin. With regard to the ovary (and other tissues), the extracellular matrix (ECM) is composed of a network of cross-linked protein compounds (mostly collagen) and ‘ground substance.’ In order for the follicle to develop, migrate to the surface of the ovary, and penetrate its surface, this ECM must be loosened up. Plasmin appears to play an important role in the remodeling of the ECM, thereby facilitating follicular cell migration, the actual rupture of the follicle and, perhaps, release of the egg from the follicle as well. PAI-1, which can inhibit both t-PA and u-PA (thereby limiting the production of plasmin), appears to play a key role in the regulation and balance of these events around and within the follicle and probably also plays a role in protecting less developed follicles from destruction as the result of plasmin and other proteolytic enzymes produced at the time of the LH surge.
In various animal studies and studies of tissue samples taken from humans in the periovulatory period, there is certainly evidence to support a role of the fibrinolytic system in the events culminating in ovulation and subsequent development (and regression) of the corpus luteum. Without belaboring this issue, I would simply like to make a few key points by citing a couple of articles on this complicated subject. Tsafriri and Reich (Exp Clin Endocrinol Diabetes 1999;107;1-11) reviewed the data to support the role of the fibrinolytic system in the degradation of the ECM. Following the LH surge, preeovulatory follicles are stimulated to make “a cascade of proteolytic enzymes, including plasminogen activator, plasmin, and matrix metalloproteinases (MMPs). These enzymes bring about the degradation of the perifollicular matrix and, most notably, the decomposition of the meshwork of collagen fibers which provides the strength to follicular wall. Pharmacological blockage of any of these enzymes resulted in the reduction of ovulation rate.” Instead of a “pharmacological blockade”, we could certainly imagine conditions wherein there is increased PAI-1 activity (reducing the activity of these proteolytic enzymes), inhibiting, or simply delaying ,ovulation and, thereby, disturbing the timing related to the ‘implantation window’ of endometrial receptivity and, perhaps, increasing the risk of implantation failure and early pregnancy loss.
Along the same lines, but dealing specifically with regard to the events surrounding follicular rupture and ovulation itself, several observations can be made. In granulosa cells from preovulatory follicles in humans, there appear to be very high concentrations of messenger RNA (mRNA) for both PAI-1 and PAI-2 and low concentrations for PA mRNA and this is reflected in a relative excess of PAI-1 over PA in the follicular fluid surrounding the egg (Jones, et al., J Clin Endocrinol Metab 1989;68:1039-45). In other words, until the egg is ready to hatch, inhibition of the proteolytic system predominates. Studies in the pig (Politis, et al., Biol Med 1990;43:636-42), however, indicate that immediately before ovulation occurs, PAI activity decreases and PA and plasmin activity increase, preceding ovulation. Thus we could imagine, just as we did with ECM degradation, under conditions of excess of PAI activity, ovulation might be prevented or delayed. To add support to this concept, excessive amounts of the pituitary hormone prolactin (PRL) are known to be associated with subfertility. Liu and colleagues (Hum Reprod 1997;12:2748-55) demonstrated in a rat model that PRL delays ovulation and this is accompanied by both a decrease in t-PA production and an increase in PAI-1 production.
Finally, let’s briefly mention the corpus luteum itself. The two primary plasminogen activators, t-PA and u-PA, appear to play divergent roles with regard to the CL. t-PA appears to be involved in the regression of the CL if pregnancy does not occur successfully, and its activity is directly correlated with a decrease in progesterone production. However, an “increase in u-PA mRNA and activity in the early stages of CL development is correlated with an increase in progesterone secretion” (Liu Biol Signals Recept 1999;8:160-77) and the development, growth, and support of the CL (Liu, et al., Endocrinology 2003;144:3611-17) that is essential for the implantation and survival of an early pregnancy. So, again we can use our imaginations. Since PAI-1 can inhibit both t-PA and u-PA, perhaps under circumstances of increased PAI-1 activity as seen in certain women with RPL, the balance shifts from u-PA predominance in the CL to t-PA predominance, causing the CL to regress rather than to grow to support an early pregnancy, thereby leading to pregnancy loss due to inadequate progesterone support. No data on this subject have I found…just a thought!
In our next post on this topic, we will (finally) look at the role of the fibrinolytic system in implantation and placentation….