Placental protein 12 (PP12) in the human endometrium: tissue concentration in relation to histology and serum levels of PP12, progesterone and oestradiol

Estradiol Elicits Proapoptotic and Antiproliferative Effects in Human Trophoblast Cells

During the first trimester of pregnancy, appropriate regulation of estradiol (E2) is essential for normal placental development. Previous studies demonstrate that premature elevation in E2 concentrations can lead to abnormal placentation, but have not fully elaborated the mechanism of this effect in the first-trimester trophoblast. Our aim was to determine whether E2 elicits trophoblast cell death or inhibits proliferation. The first-trimester human cytotrophoblast cell line HTR-8/SVneo was cultured in phenol red-free medium containing charcoal-stripped serum and treated with 17beta-E2 at concentrations between 0 and 100 nM. TUNEL and invasion assays indicated that E2 significantly increased cell death and reduced cell invasion at 10 nM, and nuclear Ki67 expression revealed that it decreased cell proliferation at 1 nM. A similar effect on cell death was observed in first-trimester placental explants. The E2 antagonist fulvestrant blocked all effects of E2. Immunohistochemistry showed that protein expression of proapoptotic caspases 3, 8, and 9 increased at E2 concentrations of 25 nM and greater, whereas expression of antiapoptotic BCL2-alpha decreased at E2 concentrations of 10 nM and greater. Additionally, treatments with estrogen receptor (ER) alpha-specific and ERbeta-specific agonists at concentrations between 0 and 1000 nM indicated that only ERalpha mediates E2's effects, although immunohistochemistry and Western immunoblotting showed that HTR-8/SVneo cells and placental explants express both ERalpha and ERbeta. Taken together, these findings reveal the interplay between elevated serum E2 and apoptosis in the first trimester of pregnancy. These factors could be associated with pregnancy complications including infertility and uteroplacental insufficiency.

Vessel remodelling, pregnancy hormones and extravillous trophoblast function

During early human pregnancy, extravillous trophoblast (EVT) cells from the placenta invade the uterine decidual spiral arterioles and mediate the remodelling of these vessels such that a low pressure, high blood flow can be supplied to the placenta. This is essential to facilitate normal growth and development of the foetus. Defects in remodelling can manifest as the serious pregnancy complication pre-eclampsia. During the period of vessel remodelling three key pregnancy-associated hormones, human chorionic gonadotrophin (hCG), progesterone (P(4)) and oestradiol (E(2)), are found in high concentrations at the maternal-foetal interface. Potentially these hormones may control EVT movement and thus act as regulators of vessel remodelling. This review will discuss what is known about how these hormones affect EVT proliferation, migration and invasion during vascular remodelling and the potential relationship between hCG, P(4), E(2) and the development of pre-eclampsia.

The effects of human chorionic gonadotrophin, progesterone and oestradiol on trophoblast function

Remodelling of the uterine vasculature during the first trimester of human pregnancy requires invasion of trophoblast from the placenta into decidual spiral arterioles. The pregnancy-associated hormones human chorionic gonadotropin (hCG), progesterone (P(4)) and oestradiol (E(2)) are present at high concentrations at the maternal-fetal interface during the remodelling period and thus may contribute to the regulation of trophoblast movement. This study examined the effects of these hormones on trophoblast functions. HTR8/SVneo cells were treated with hCG (5-100mIU/mL), P(4) (20nM-20μM) or E(2) (0.07-734nM). hCG significantly stimulated migration and MMP-9 activity but did not affect cell numbers. P(4) significantly inhibited migration, MMP-2 and -9 activity and reduced cell numbers. E(2) had no effect on migration, MMP activity or cell numbers. We conclude that hCG and P(4), but not E(2), play direct roles in controlling trophoblast invasion, acting as positive and negative stimuli respectively to regulate trophoblast movement during vascular remodelling in early pregnancy.

Peptides and proteins in the human endometrium

Human endometrium is composed of three layers: stratum compactum, stratum spongiosum and stratum basale. Stratum compactum is the superficial layer made up of openings of the uterine glands and some stromal cells. Stratum spongiosum is the middle layer containing mainly dilated glands and little stroma. Stratum basale is the deepest layer adjoining the muscularis. It consists of primordial glands and compact stroma. Stratum compactum and stratum spongiosum form stratum functionale which is subject to cyclic changes and is removed during menstruation.

Insulin-like growth factor and interleukin-1beta levels and subsequent fetal size in response to chronic Porphyromonas gingivalis exposure in the pregnant rabbit

OBJECTIVE

The purpose of this study was to describe maternal insulin-like growth factor, interleukin-1beta, and fetal size in a rabbit model of Porphyromonas gingivalis exposure.

STUDY DESIGN

With the use of a previously described model, 8 New Zealand White rabbits were exposed to either P gingivalis or media during pregnancy and killed at term. Kit and placenta weight were compared between groups. Doe serum insulin-like growth factor system protein and interleukin-1beta levels were compared by analysis of variance for repeated measures; a probability value of <.05 was considered to be significant.

RESULTS

No significant differences in kit and placental weights between exposed and unexposed groups were observed. Insulin-like growth factor system proteins increased significantly as pregnancy progressed, but there were no significant differences in insulin-like growth factor system proteins or interleukin-1beta between exposed and unexposed does.

CONCLUSION

Chronic P gingivalis exposure does not disrupt insulin-like growth factor system proteins or systemic inflammation and does not impair fetal growth in the pregnant rabbit. Gestational age changes in doe insulin-like growth factor system proteins occur, and the timing of exposure to oral pathogens may influence fetal growth.

IGFBP-1 and glycodelin serum levels before and after hysterectomy with and without bilateral oophorectomy in premenopausal women

OBJECTIVES

The aim of our study was to investigate the role of the intact endometrium and ovaries for serum levels of insulin-like growth factor binding protein-1 (IGFBP-1) and glycodelin.

STUDY DESIGN

In 35 premenopausal patients with a planned hysterectomy, serum measurements of IGFBP-1 and glycodelin were done before surgery and 1, 3, 5, and 10 days after surgery. Patients were divided into three groups according to the kind and time of operation: (1) hysterectomy with bilateral adnexectomy in the luteal phase and (2) hysterectomy without adnexectomy in the follicular phase or (3) the luteal phase.

RESULTS

IGFBP-1-we could not show any differences in IGFBP-1 serum levels before and after hysterectomy with or without bilateral oophorectomy. Glycodelin-hysterectomized and oophorectomized patients showed decreasing serum levels up to day 3. After day 5, circulating concentrations of glycodelin increased continuously but remained below pre-operative levels. In both non-adnexectomized groups we saw a reduction up to day 5 but a rise at day 10. None of the results reached statistical significance.

CONCLUSION

The results indicate that endometrium and ovary are not the only sources of IGFBP-1 and glycodelin.

Paracrine actions of insulin-like growth factors and IGF binding protein-1 in non-pregnant human endometrium and at the decidual-trophoblast interface

Insulin-like growth factors, IGF-I, IGF-II, and IGF binding protein (IGFBP-1) appear to play major roles in endometrial development during the menstrual cycle and in the process of implantation. The mitogenic, differentiative, and anti-apoptotic properties of these growth factors, as well as their spatial and temporal expression in cycling endometrium, suggest that they may participate in endometrial growth, differentiation, inhibition of apoptosis, and perhaps angiogenesis. IGFBP-1 is a major protein product of non-pregnant endometrium during the mid-late secretory phase and occurs in abundance in decidua. Its roles as an IGF-binding protein and as a trophoblast integrin ligand suggest that it may have multiple roles in endometrial development and in interactions between the decidua and the invading trophoblast. Precise elucidation of the mechanisms underlying IGF and IGFBP-1 action at the decidual-trophoblast interface in early pregnancy awaits further investigation. The future also awaits elucidation of the potential predictive utility of IGFBP-1 in serum and in decidua in, for example, pre-eclampsia and perhaps implantation failure.

Insulin-like growth factor binding protein-1 binds to placental cytotrophoblast alpha5beta1 integrin and inhibits cytotrophoblast invasion into decidualized endometrial stromal cultures

Insulin-like growth factor binding protein-1 (IGFBP-1) can bind to the alpha5beta1 integrin and stimulate cellular migration in Chinese hamster ovary (CHO) cells. IGFBP-1 is a major product of the endometrium of pregnancy (decidua), and may interact with invading cytotrophoblasts expressing alpha5beta1 integrin to modulate their invasion. The present study investigated IGFBP-1 interaction with cytotrophoblast alpha5beta1 integrin, and its effects on trophoblast attachment to fibronectin and invasion into decidualized endometrial stromal cell multilayers. IGFBP-1 incubated with cytotrophoblast extracts was co-precipitated by an antibody to the alpha5 integrin subunit. Up to 55% of radiolabeled IGFBP-1 bound to cytotrophoblasts was displaced by excess non-radioactive IGFBP-1, but not by IGFBP-3. Cytotrophoblast attachment to fibronectin was inhibited by an RGD-containing octapeptide, by antibodies to the alpha5 subunit or the alpha5beta1 heterodimer, and by IGFBP-1. Cytotrophoblasts showed limited invasion into endometrial stromal multilayers decidualized in vitro secreting abundant IGFBP-1, but invaded multilayers when IGFBP-1 production was inhibited by insulin. Invasion into insulin-treated multilayers was prevented by addition of exogenous IGFBP-1 but not by IGFBP-3. These findings suggest IGFBP-1 may modulate trophoblast invasiveness.

Multifaceted roles for IGFBP-1 in human endometrium during implantation and pregnancy

IGFBP-1 is a major protein product of nonpregnant endometrium during the mid-late secretory phase and occurs in abundance in decidua. Its roles as an IGF-binding protein and as a trophoblast integrin ligand suggest that it may have multiple roles in endometrial development and in interactions between the decidua and the invading trophoblast. IGFBP-1 in vaginal/cervical secretions has already had clinical application as a predictor of premature rupture of fetal membranes. The future awaits elucidation of the potential utility of IGFBP-1 in serum and in decidua in predicting fetal growth restriction and preeclampsia and perhaps implantation failure.

Progesterone receptor (PR) inhibits expression of insulin-like growth factor-binding protein-1 (IGFBP-1) in human endometrial cell line HEC-1B: characterization of the inhibitory effect of PR on the distal promoter region of the IGFBP-1 gene

Progestin has been shown to have both stimulatory and inhibitory effects on the expression of insulin-like growth factor binding protein-1 (IGFBP-1) in human endometrial cells. In this study, progestin was found to reduce levels of secreted IGFBP-1 and IGFBP-1 messenger RNA and IGFBP-1 promoter activity after stably transfecting a progesterone receptor (PR; B form) expression vector into HEC-1B cells. Deletion analysis of the IGFBP-1 promoter revealed that PR specifically inhibited promoter activity derived from a 59-bp distal BsaHI/RsaI fragment. It was concluded that PR inhibited the promoter activity through protein-protein interactions based on the facts that 1) no progesterone-responsive element was revealed by a series block mutation in the BsaHI/RsaI fragment; 2) PR bound by the antiprogesterone ZK98299 inhibited IGFBP-1 promoter activity; 3) a DNA-binding mutant of PR inhibited the IGFBP-1 promoter activity; and 4) in an in vivo competition assay, the DNA-binding domain of PR did not release the inhibitory effect of intact PR. Analysis of PR deletion mutants indicated that both transcriptional activation domains of PR (TAF-1 and TAF-2) were involved in the inhibition of IGFBP-1 expression. Thus, our data may explain the superinduction of IGFBP-1 in human endometrial cells after progestin withdrawal or progestin replacement with antiprogestin.

Insulin-like growth factor binding protein-1 at the maternal-fetal interface and insulin-like growth factor-I, insulin-like growth factor-II, and insulin-like growth factor binding protein-1 in the circulation of women with severe preeclampsia

OBJECTIVES

Preeclampsia is characterized by maternal hypertension, proteinuria, edema, and shallow placental invasion. Insulin-like growth factor binding protein-1, abundant in maternal decidua, is believed to play a role in limiting trophoblast invasiveness. In this study we addressed the hypothesis that this binding protein is aberrantly expressed in preeclampsia. We also investigated circulating levels of insulin-like growth factor-I and insulin-like growth factor-II in subjects with severe preeclampsia compared with controls.

STUDY DESIGN

Insulin-like growth factor binding protein-1 was investigated by immunohistochemistry at the maternal-fetal interface of eight pregnancies complicated by severe preeclampsia and six controls between 21 and 34 weeks of gestation. Cell types were identified with use of cell-specific markers. Circulating levels of insulin-like growth factor binding protein-1, insulin-like growth factor-I, and insulin-like growth factor-II in 16 patients with severe preeclampsia and 29 controls at the same gestational age were determined by an immunoradiometric assay and correlated with clinical parameters. Data were analyzed by t test and Pearson's method.

RESULTS

Insulin-like growth factor binding protein-1 was highly expressed on syncytiotrophoblasts, cytotrophoblasts, and decidual cells but not on placental fibroblasts. Immunostaining was greater at the maternal-fetal interface in severe preeclamptic patients compared with controls. Circulating insulin-like growth factor binding protein-1 levels in subjects with severe preeclampsia were 428.3 +/- 85.9 ng/ml compared with 76.6 +/- 11.8 in controls (p = 0.0007). Circulating insulin-like growth factor-I levels were 80.9 +/- 17.2 ng/ml compared with 179.4 +/- 28.2 ng/ml in controls (p = 0.0001). In contrast, insulin-like growth factor-II levels were not significantly different in the two groups. In subjects with severe preeclampsia insulin-like growth factor binding protein-1 levels correlated with diastolic blood pressure (r = 0.498, p 0.049) and aspartate transcarbamylase (0.621, p = 0.010).

CONCLUSIONS

The abundance of insulin-like growth factor binding protein-1 at the maternal-fetal interface in severely preeclamptic pregnancies suggests that the binding protein may participate in the pathogenesis of the shallow placental invasion observed in this disorder. Low circulating insulin-like growth factor-I and elevated insulin-like growth factor binding protein-1 levels may contribute to restricted placental and therefore fetal growth.

The effects of labor on maternal and fetal levels of insulin-like growth factor binding protein-1

OBJECTIVE

Our purpose was to determine the effects of labor and fetal hypoxia on the levels of insulin-like growth factor binding protein-1 in the maternal and fetal circulation.

STUDY DESIGN

Serum levels of insulin-like growth factor binding protein-1 were determined in maternal and umbilical blood at delivery in two groups. The first group included 43 vaginal deliveries and 23 elective cesarean sections. The second group consisted of 44 women; in 24 the liquor was meconium stained and in 20 it was clear.

RESULTS

Levels of insulin-like growth factor binding protein-1 in the neonate were lower in deliveries occurring before onset of labor (p < 0.001), Mann-Whitney U test) and higher in cases with severe meconium staining (p = 0.01). There were no differences in maternal levels of insulin-like growth factor binding protein-1 between subjects in labor and not in labor or those with or without meconium staining.

CONCLUSION

The process of labor leads to an increase in fetal levels of insulin-like growth factor binding protein-1. This increase may well be associated with the relative fetal stress that occurs during labor. This suggestion is supported by the finding of the highest levels in labors in which there was thick staining of the liquor.

Immunohistochemical localization of two endometrial proteins in the early days of human pregnancy

Insulin-like growth factor binding protein-1 (IGFBP-1) [also known as placental protein 12(PP12)] and placental protein 14 (PP14) have been identified by specific immunostaining in early pregnancy specimens obtained 13-35 days of gestation. PP12 was evident in a discrete number of stromal decidual cells at the deciduotrophoblastic interface and under the endometrial surface epithelium. These cells did not have the rounded appearance of classic decidual cells but most often displayed cytoplasmic expansions. Staining for PP14 was strictly localized to the glandular epithelium of the endometrium. Implantation of the conceptus may be an important mechanism in the early expression of PP12 but not PP14.

Growth factors and growth modulators in human uterine endometrium: their potential relevance to reproductive medicine

OBJECTIVE

To provide an up-to-date, comprehensive review on the presence and regulation of growth factors (GFs), GF receptors, and GF regulatory proteins in human endometrium in an effort to understand the potential roles of these proteins in endometrial cell mitosis and differentiation and in endometrial-trophoblast interactions.

DESIGN

Relevant studies were identified through a computerized bibliographic search (MEDLINE; BRS Information Technologies, a division of Maxwell Online, Inc., McLean, VA) and through manual scanning of recent relevant journals.

RESULTS

Several GFs, their receptors, and regulatory proteins have been identified in endometrium, and cellular localization and steroid-dependence of these proteins as well as action of several growth modulators on endometrial cell function have been studied. Epidermal growth factor, transforming growth factor (TGF)-alpha, platelet-derived growth factor, insulin-like growth factors (IGFs) and their binding proteins, fibroblast growth factor (FGF), TGF-beta, colony-stimulating factor (CSF)-1, and interferon-gamma regulate mitosis of endometrial cellular components in vitro. Endothelin-1 may participate in vasoconstriction and FGF may participate in angiogenesis in this tissue in vivo. Interleukins-1 and -6 are believed to be involved in endometrial T-cell activation, and TGF-beta, CSF-1, the interleukins, and the IGFs likely mediate endometrial-trophoblast interactions. The role of tumor necrosis factor in endometrium remains uncertain.

CONCLUSIONS

Current evidence supports the thesis that GFs play a central role in cyclic mitosis and differentiation of endometrial cellular components, recruitment of macrophages in decidualizing endometrium, endometrial-trophoblast interactions, early pregnancy maintenance, tissue shedding in the absence of implantation, and endometrial functionalis regeneration.

Insulin-like growth factor-II (IGF-II) and IGF binding proteins in human endometrium

The insulin-like growth factor autocrine/paracrine system is believed to play a role in steroid-mediated endometrial differentiation. It is constituted of the mitogenic peptides (IGF-I and IGF-II), membrane receptors, and a family of high-affinity binding proteins (IGFBPs) that regulate the actions of the IGFs at their target cells. We have investigated expression of the mRNAs encoding the three major IGFBPs (IGFBP-1, IGFBP-2, and IGFBP-3) in human endometrium and have found, by Northern analysis, differential expression of all three mRNAs in secretory compared to proliferative endometrium, different steroidal milieux. IGF-II mRNAs were also detected in secretory endometrium. Finally, we found that human endometrial stromal cells in culture synthesize and secrete IGFBP-2 and IGFBP-3, and that the synthesis of IGFBP-2 is regulated by steroid hormones.

Progesterone-associated proteins PP12 and PP14 in the human endometrium

Two proteins, designated as PP12 and PP14 were originally isolated from soluble extracts of the human placenta and its adjacent membranes. We have shown that they are synthesized by decidualized/secretory endometrium and not by placenta. Both proteins occur at high concentrations in human amniotic fluid, which is therefore an excellent source for purification. PP12 is a 34-kDa glycoprotein, which has an N-terminal amino acid sequence of Ala-Pro-Trp-Gln-Cys-Ala-Pro-Cys-Ser-Ala. This is identical with that of somatomedin-binding protein purified from the amniotic fluid. PP12 too binds somatomedin-C, or IGF-I (insulin-like growth factor-I). Human secretory endometrium synthesizes and secretes PP12, and progesterone stimulates its secretion. PP14 is a 28-kDa glycoprotein. Its N-terminal sequence shows homology to that of beta-lactoglobulins from various species. We have found PP14 in the human endometrium, serum and milk. Immunologically, PP14 is related to progestagen-associated endometrial protein (PEP), alpha-2 pregnancy-associated endometrial protein (alpha-2, PEG), endometrial protein 15 (EP15), alpha-uterine protein (AUP) and chorionic alpha-2 microglobulin (CAG-2). In ovulatory menstrual cycles, the concentration of PP14 increases in endometrial tissue as the secretory changes advance. In serum, the PP14 concentration begins to rise later than the progesterone levels, and high serum PP14 levels are maintained for the first days of the next cycle. By contrast, no elevation of serum PP14 level is seen in anovulatory cycles. Our results show that progesterone-associated proteins are synthesized by the human endometrium and appear in the peripheral circulation, where they can be quantitatively measured using immunochemical techniques.

Elevated levels of a somatomedin-binding protein PP12 in patients with ovarian cancer

The circulating levels of placental protein 12 (PP12), a somatomedin-binding protein, were measured by radioimmunoassay in 37 postmenopausal women with ovarian tumors. Elevated levels (above 47 micrograms/l) were observed in 16 of 25 patients (64%) with malignant and 2 of 12 patients (17%) with benign tumors. Elevated levels were more frequent and higher in patients with advanced disease. Within 1 week of surgery, the levels fell in 13 of 17 cases (77%). In addition to clinical interest, these results contribute to our knowledge of carrier proteins of growth factors in patients bearing malignant ovarian neoplasms.

New placental proteins: chemistry, physiology and clinical use

In the past decade, several new placental proteins have been isolated and studied. The 'pregnancy-specific' beta 1-glycoprotein (SP1) is a major placental product with unusual physicochemical properties that has been extensively investigated, but its biological function remains uncertain. Pregnancy-associated plasma protein A (PAPP-A), a glycoprotein of mol. wt 400,000, has effects in vitro on the coagulation and complement cascades, probably by its properties of protease inhibition. Placental protein 5 (PP5) may be involved in the coagulation and fibrinolytic systems, and in follicle maturation and semen liquefaction. 'Placental protein 12' (PP12) is not a product of the placenta at all; it appears to be produced in the female genital tract under the influence of progesterone and may also be produced by proliferating liver cells. Further study may reveal new roles for these placental proteins beyond their traditional roles as tumour markers.

The content and immunohistochemical localization of placental protein 10 (PP10) in the fallopian tube

Radioimmunoassay, gel filtration and immunoperoxidase staining were used to study the content and localization of placental protein 10 (PP10) in 15 fallopian tubes removed on medical grounds from patients aged between 35 and 53 years. PP10 was consistently present in all parts of the tube at all ages and in all phases of the menstrual cycle. The PP10 concentration in tissue ranged from 0.08 to 2.95 micrograms/g of tubal cytosol protein. Immunoperoxidase staining localized PP10 in monocytic and lymphoid cells that were unevenly scattered in the subepithelial layer of the mucosa. In gel filtration, PP10 from the fallopian tube and purified placental PP10 eluted in the same volume, and graded amounts of PP10-immunoreactive material from the tube and purified PP10 gave parallel dose-response curves in radioimmunoassay. We conclude that PP10 is another 'placental protein' that has been identified in the fallopian tube.

Placental protein 12 (PP12): a new test for the prediction of the small-for-gestational-age infant

The circulating levels of placental protein 12 (PP12) and placental lactogen (hPL) were measured in 501 women between 36 and 41 weeks gestation. There was a significant positive association between hPL levels and infant birthweight and a significant negative association in the case of PP12 levels. The clinical efficiency of elevated PP12 levels in the prediction of low-birthweight infant at term compared favourably with that of reduced hPL levels.

Protein synthesis and secretion by the human endometrium and decidua during early pregnancy

To investigate the role of the endometrium in sustaining early pregnancy, specimens of endometrium from early pregnancy were incubated in vitro with radiolabelled amino acids, and protein synthesis and secretion were studied by subsequent fluorographic analysis of one-and two-dimensional polyacrylamide gels. Seventeen secreted endometrial proteins (EP) were identified. Changes were demonstrated in the rate of synthesis and/or secretion of a number of these proteins during early pregnancy. Two proteins, EP 14 and EP 15, are the principal secretory proteins of pregnancy endometrium. EP 14 (subunit mol. wt 32 000) is associated with decidualized endometrium and its rate of synthesis/secretion increases during early pregnancy. EP 15 (subunit mol. wt 28 000) is also synthesized and secreted by the secretory endometrium during the menstrual cycle, and during early pregnancy, but its secretion declines to undetectable levels by week 15-16. These proteins may provide useful markers of endometrial function and differentiation during the menstrual cycle and pregnancy.

The content of placental protein 12 in decidua and fetal membranes is greater than in placenta

Radioimmunoassay, gel filtration and sodium dodecyl sulphate-polyacrylamide gel electrophoresis were used to study the content and properties of placental protein 12 (PP12) in the placenta, decidua and fetal membranes. The tissues were obtained from early pregnancy in 12 cases, and after normal term delivery in eight cases in seven of which chorion and amnion laeve were also studied. There was more PP12 in decidua and fetal membranes than in placenta. The decidua/placenta ratio of PP12 content ranged from 2 to 1154 (mean 193, SEM 66). These results suggest that PP12 is a decidual rather than placental protein.

Placental protein 12 (PP12) in menstrual fluid

In a previous study we found placental protein 12 (PP12) in the human endometrium. We now show that PP12-like immunoreactive material is released into menstrual fluid where it occurs at concentrations from 43 to 32 000 micrograms/1. The PP12 levels in menstrual fluid are 2- to 2000-fold higher than in serum of the same individuals thus suggesting that the protein either concentrates or is produced in the endometrium. The addition of protease inhibitors has no effect on the PP12 levels. In radioimmunoassay, the dose-response curves of menstrual fluid PP12 and placental PP12 are parallel. Experiments with fused rocket immuno-electrophoresis show that menstrual and placental PP12 are indistinguishable from each other. In gel filtration, menstrual PP12 has the same molecular weight as that of purified placental PP12. By immuno-peroxidase staining, PP12 is localized in the glandular epithelial cells of the menstrual endometrium. The glandular localization and high concentration of PP12 in menstrual fluid, as compared with PP12 levels in serum and endometrium, suggest that PP12 may be secreted by the endometrium.