Insulin-Like Growth Factor II Activates Phosphatidylinositol 3-Kinase-Protooncogenic Protein Kinase 1 and Mitogen-Activated Protein Kinase Cell Signaling Pathways, and Stimulates Migration of Ovine Trophectoderm Cells

IGF-II, a potent stimulator of cellular proliferation, differentiation, and development, regulates uterine function and conceptus growth in several species. In situ hybridization analyses found that IGF-II mRNA was most abundant in the caruncular endometrial stroma of both cyclical and pregnant ewes. In the intercaruncular endometrium, IGF-II mRNA transitioned from stroma to luminal epithelium between d 14 and 20 of pregnancy. IGF-II mRNA was present in all cells of the conceptus but was particularly abundant in the yolk sac. Immunohistochemical analyses revealed that phosphorylated (p)-protooncogenic protein kinase 1, p-ribosomal protein S6 kinase, p-ERK1/2, and p-P38 MAPK proteins were present at low levels in a majority of endometrial cells but were most abundant in the nuclei of endometrial luminal epithelium and conceptus trophectoderm of pregnant ewes. In mononuclear trophectoderm cells isolated from d-15 conceptuses, IGF-II increased the abundance of p-pyruvate dehydrogenase kinase 1, p-protooncogenic protein kinase 1, p-glycogen synthase kinase 3B, p-FK506 binding protein 12-rapamycin associated protein 1, and p-ribosomal protein S6 kinase protein within 15 min, and the increase was maintained for 90 min. IGF-II also elicited a rapid increase in p-ERK1/2 and p-P38 MAPK proteins that was maximal at 15 or 30 min posttreatment. Moreover, IGF-II increased migration of trophectoderm cells. Collectively, these results support the hypothesis that IGF-II coordinately activates multiple cell signaling pathways critical to survival, growth, and differentiation of the ovine conceptus during early pregnancy.

ESTABLISHMENT AND MAINTENANCE of pregnancy in eutherian mammals, including sheep, require reciprocal communication via endocrine and paracrine signals from the ovary, conceptus (embryo/fetus and associated extraembryonic membranes), and endometrium to support implantation and placentation. Implantation of the conceptus is an important and complex developmental event of early pregnancy and is a key evolutionary advance associated with viviparity (1, 2). During the peri-implantation period in the ovine uterus, the spherical blastocyst elongates to a tubular and then a filamentous form, and develops into a conceptus that then attaches to the uterine luminal wall. These events are supported by endometrial epithelial secretions and selective transport of molecules, including growth factors, cytokines, ions, glucose, and amino acids that are predominantly regulated by progesterone, and are required for peri-implantation conceptus survival, elongation, and development (1–9).

IGFs I and II are single chain polypeptides that are structurally similar to proinsulin (10, 11). The IGFs function as endocrine and paracrine/autocrine hormones that stimulate proliferation and differentiation in a variety of cell types, and mediate their effects by binding to specific cell membrane receptors, the type I and type II IGF receptors (IGF-IR and IGF-IIR, respectively) (12, 13). The IGFs are considered important regulators of pre-implantation conceptus development and placental development (14–19). During the pre-implantation period, both IGF-I and IGF-II are expressed in the uterus, oviduct, ovary, and conceptus of domestic mammals, including cattle (20–24), sheep (14, 25–29), and pigs (30). IGF-II is an imprinted and paternally expressed gene in the fetus and placenta of mice, humans, and sheep (31), and plays an important role in regulation of fetal-placental growth/differentiation, stimulation of extravillous trophoblast migration/invasion, and facilitation of nutrient transfer through the development of placental exchange mechanisms (17, 32–37). In mice, deletion of IGF-II in the labyrinthine trophoblast of the placenta reduced placental and fetal growth, and decreased passive permeability for placental exchange of nutrients (35). Furthermore, Smith et al. (38) reported that altered expression of IGF-II in human trophoblast during the first and early second trimester resulted in serious complications of pregnancy such as increased risk of intrauterine growth restriction, premature delivery, and preeclampsia.

Little is known of the biological roles of IGF-II in the peri-implantation ovine conceptus and endometrium. However, in the neonatal ovine uterus, expression of IGF-I, IGF-II, and IGF-IR regulates epithelial-stroma interactions and endometrial gland morphogenesis, suggesting that the ovine IGF system is important to support uterine development (39, 40). In sheep the IGF-IR is present in the pre-implantation blastocyst and in the luminal epithelium (LE), glandular epithelium (GE), and to a lesser extent, the caruncular stroma and myometrium of the uterus (26, 40, 42). Specific cell signaling pathways stimulated by IGF-II in other cell types include activation of: 1) phosphatidylinositol 3-kinase (PI3K)/protooncogenic protein kinase 1 (AKT1); 2) FK506 binding protein 12-rapamycin associated protein 1 (FRAP1), also known as mammalian target of rapamycin (mTOR); 3) ribosomal protein S6 kinase (RPS6K); 4) ERK1/2 (also known as p42/p44 or MAPK); and 5) P38 MAPK. However, these signaling pathways and their regulation by IGF-II in the ovine conceptus have not been investigated. Our working hypothesis is that IGF-II from the endometrium and within the conceptus regulates trophoblast growth and differentiation via the PI3K-AKT1 and MAPK signaling pathways. As a first step in testing this hypothesis, we conducted studies to determine the following: 1) the distribution of IGF-II mRNA in ovine endometria and conceptuses during early pregnancy, 2) effects of IGF-II on PI3K-AKT1 and MAPK signaling pathways in mononuclear ovine trophectoderm (oTr) cells, and 3) effects of IGF-II on trophectoderm migration.

Materials and Methods

Experimental design

Animals.

Mature crossbred Suffolk ewes (Ovis aries) were observed daily for estrus in the presence of vasectomized rams and used in experiments after they had exhibited at least two estrous cycles of normal duration (16–18 d). All experimental and surgical procedures were in compliance with the Guide for the Care and Use of Agriculture Animals and approved by the Institutional Animal Care and Use Committee of Texas A&M University.

Study 1

Tissue collection.

At estrus (d 0), ewes were mated to either an intact or vasectomized ram and then hysterectomized (n = 5 ewes per day) on either d 10, 12, 14, or 16 of the estrous cycle, or d 10, 12, 14, 16, 18, or 20 of pregnancy as described previously (43). At hysterectomy, the uterus was flushed with 20 ml 10 mm Tris buffer (pH 7.0). Pregnancy was confirmed on d 10–16 after mating by the presence of a morphologically normal conceptus(es) in the uterine flush. It was not possible to obtain uterine flushes on either d 18 or 20 of pregnancy because the conceptus is firmly adhered to the endometrial LE. At hysterectomy, several sections (∼0.5 cm) from the midportion of each uterine horn ipsilateral to the corpora lutea were fixed in fresh 4% paraformaldehyde in PBS (pH 7.2). After 24 h, fixed tissues were changed to 70% ethanol for 24 h, dehydrated through a graded series of alcohol to xylene, and then embedded in Paraplast-Plus (Oxford Labware, St. Louis, MO). The remaining endometrium was physically dissected from myometrium, frozen in liquid nitrogen, and stored at −80 C for subsequent protein extraction. In monovulatory pregnant ewes, uterine tissue samples were marked as either contralateral or ipsilateral to the ovary bearing the corpora lutea, and only tissues from the ipsilateral uterine horn were used in subsequent analyses.

Cloning of partial cDNA for ovine RPS6K

A partial cDNA for ovine RPS6K mRNA was amplified by RT-PCR using total RNA from d-18 pregnant ovine endometrial tissues by specific primers based on data for the bovine RPS6K mRNA (GenBank accession no. AY396564; forward, 5′-ATT TGC CTC CCT ACC TCA CG-3′; reverse, 5′-AAT TTG ACT GGG CTG ACA GG-3′). PCR amplification was conducted as follows for ovine RPS6K: 1) 95 C for 5 min; 2) 95 C for 45 sec, 56.5 C for 1 min, and 72 C for 1 min for 35 cycles; and 3) 72 C for 10 min. The partial cDNAs for RPS6K were cloned into pCRII using a T/A Cloning Kit (Invitrogen Corp., Carlsbad, CA), and the sequence was verified using an ABI PRISM Dye Terminator Cycle Sequencing Kit and ABI PRISM automated DNA sequencer (PerkinElmer Applied Biosystems, Foster City, CA).

In situ hybridization analyses

Location of mRNA expression in sections (5 μm) of ovine uterine endometria and conceptuses was determined by radioactive in situ hybridization analysis as described previously (44). Briefly, deparaffinized, rehydrated, and deproteinated uterine tissue sections were hybridized with radiolabeled antisense or sense cRNA probes generated from linearized ovine IGF-II (39) and RPS6K partial cDNA using in vitro transcription with [α-³⁵S]uridine 5′-triphosphate. After hybridization, washing, and ribonuclease A digestion, slides were dipped in NTB-2 liquid photographic emulsion (Kodak, Rochester, NY), and exposed at 4 C for 6 d for IGF-II and 4 wk for RPS6K. Slides were developed in Kodak D-19 developer, counterstained with Gill’s hematoxylin (Fisher Scientific, Fairlawn, NJ), dehydrated through a graded series of alcohol to xylene, and coverslips affixed with Permount (Fisher Scientific). Images of representative fields were recorded under bright-field or dark-field illumination using a Nikon Eclipse 1000 photomicroscope (Nikon Instruments Inc., Lewisville, TX) fitted with a Nikon DXM1200 digital camera.

Immunohistochemistry

Immunocytochemical localization of phosphorylated (p)-AKT1, p-P38 MAPK, p-RPS6K, and p-ERK1/2 protein in ovine uteri and conceptuses was performed using methods described previously (43). Rabbit antimouse phospho-AKT1 polyclonal IgG (Ser493/Thr308) (catalog no. 3061) at a 1:100 dilution, rabbit antihuman phospho-P38 MAPK monoclonal IgG (catalog no. 4631) at a 1:50 dilution, and rabbit antihuman phospho-RPS6K polyclonal IgG (Thr421/Ser424) (catalog no. 9204) at a 1:500 dilution were purchased from Cell Signaling Technology, Inc. (Danvers, MA), and rabbit antimouse phospho-ERK1/2 monoclonal IgG (catalog no. sc-7383) at 2.0 μg/ml was purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA). Antigen retrieval was performed using Pronase E digestion for p-AKT1, p-ERK1/2, p-RPS6K, and the boiling citrate method for p-P38 MAPK localization. Negative controls included substitution of the primary antibody with purified rabbit IgG at the same final concentration.

Study 2

Cell culture.

Mononuclear oTr cells from d-15 conceptuses were isolated and cultured as described previously (45). One cell line, referred to as oTr, was cultured in DMEM-F12 that included 10% fetal bovine serum, 50 U penicillin 50 μg streptomycin, 0.1 mm each nonessential amino acids, 1 mm sodium pyruvate, 2 mm glutamine, and 0.7 μm insulin. When the density of cells in dishes reached about 80% confluence, they were passaged at a ratio of 1:3, and frozen stocks of cells were prepared at each passage. For experiments, monolayer cultures of oTr cells (between passages 9 and 13) were grown in culture medium to 80% confluence on 100-mm tissue culture plates. Cells were serum and insulin starved for 24 h, and then treated with recombinant human IGF-II (50 ng/ml; R&D Systems, Inc., Minneapolis, MN) for 0, 15, 30, 60, and 90 min. Based on preliminary dose-response experiments, 50 ng/ml IGF-II was the dose selected for use in all experiments in the present study. This design was replicated in three independent experiments.

Western blot analyses.

Whole cell extracts and immunoblot assays were prepared and performed as described previously (46). To harvest total cellular protein for Western blot analyses, oTr cells were rinsed with cold PBS and lysed by incubation in lysis buffer (1% Triton X-100, 0.5% Nonidet P-40, 150 mm NaCl, 10 mm Tris, 1 mm EDTA, 1 mm EGTA, 0.2 mm Na₃VO₄, 0.2 mm phenylmethylsulfonylfluoride, 50 mm NaF, 30 mm Na₄P₂O₇, 1 μg/ml leupeptin, and 1 μg/ml pepstatin) for 30 min at 4 C. Cell lysates were passed through a 26-gauge needle and clarified by centrifugation (16,000 × g, 15 min, 4 C). The protein content was determined using the Bradford protein assay (Bio-Rad Laboratories, Inc., Hercules, CA) with BSA as the standard. Proteins were denatured, separated using SDS-PAGE, transferred to nitrocellulose, and Western blot analyses were performed as described previously (43) using enhanced chemiluminescence detection (SuperSignal West Pico; Pierce, Rockford, IL) and X-OMAT AR x-ray film (Kodak) according to the manufacturer’s recommendations. All antibodies used in these experiments were purchased from Cell Signaling Technology (Table 1). Multiple exposures of each Western blot were performed to ensure linearity of chemiluminescent signals. Western blots were quantified by measuring the intensity of light emitted from correctly sized bands under UV light using a ChemiDoc EQ system and Quantity One software (Bio-Rad Laboratories).

Table 1.

Antibodies, dilutions, and SDS-PAGE gels used for Western blot analyses

Antibody	Catalog no.	Dilution	SDS-PAGE gel (%)
Rabbit antihuman polyclonal phospho-PDK1 (Ser241)	3061	1:2000	10
Rabbit antimouse polyclonal phospho-AKT1 (Ser473/Thr308)	9271/9275	1:1500	10
Rabbit antihuman polyclonal phospho-GSK3B	9336	1:1500	10
Rabbit antihuman polyclonal phospho-FRAP1 (Ser2448)	2971	1:1000	8
Rabbit antihuman polyclonal phospho-RPS6K (Thr421/Ser424)	9204	1:1000	10
Rabbit antihuman polyclonal phospho-ERK1/2	9101	1:2000	12
Rabbit antihuman monoclonal phospho-P38 MAPK	9215	1:1000	12
Rabbit antihuman polyclonal PDK1	3062	1:5000	10
Rabbit antimouse polyclonal AKT1	9272	1:2000	10
Rabbit antihuman monoclonal GSK3B	9315	1:2000	10
Rabbit antihuman polyclonal FRAP1	2972	1:2000	8
Rabbit antihuman polyclonal RPS6K	9202	1:2000	10
Rabbit antihuman monoclonal ERK1/2	4695	1:5000	12
Rabbit antihuman polyclonal P38 MAPK	9212	1:2000	12

Sandwich ELISA.

Whole cell extracts were prepared as described previously (46, 47) and subjected to Sandwich ELISA according to the manufacturer’s recommendations. Endogenous amounts of phospho-AKT1, phospho-P38 MAPK, and phospho-ERK1/2 protein were measured by Sandwich ELISA (catalog nos. 7160, 7140, and 7315, respectively) from Cell Signaling Technology.

Migration assay

Migration assays were conducted with oTr cells as described previously (48) with minor modifications. Briefly, oTr cells (50,000 cells per 100 μl serum and insulin-free DMEM) were seeded on 8-μm pore Transwell inserts (Corning Costar no. 3422; Corning, Inc., Corning, NY). Treatments were then added to each well (n = 3 wells per treatment). After 12 h, cells on the upper side of the inserts were removed with a cotton swab. For evaluation of cells that migrated onto the lower surface, inserts were fixed in 50% ethanol for 5 min. The transwell membranes were then removed, placed on a glass slide with the side containing cells facing up, overlaid with Prolong antifade mounting reagent with 4′,6-diamidino-2-phenylindole, and overlaid with a coverslip (Invitrogen-Molecular Probes, Eugene, OR). Migrated cells were counted systematically in five nonoverlapping locations, which covered approximately 70% of the insert membrane growth area, using a Zeiss Axioplan 2 fluorescence microscope with Axiocam HR digital camera and Axiovision 4.3 software (Carl Zeiss Microimaging, Thornwood, NY). The entire experiment was repeated at least three times with different batches of oTr cells between passages 7 and 10.

Statistical analyses

All quantitative data were subjected to least squares ANOVAs using the General Linear Model procedures of the Statistical Analysis System (SAS Institute Inc., Cary, NC). Western blot data were corrected for differences in sample loading using the α tubulin data as a covariate. All tests of significance were performed using the appropriate error terms according to the expectation of the mean squares for error. A P value less than or equal to 0.05 was considered significant. Data are presented as least-square means (LSMs) with ses.

Results

IGF-II mRNA expression during the estrous cycle and pregnancy (Fig. 1)

Fig. 1.

In situ hybridization analyses of IGF-II mRNA in the uteri of cyclical and early pregnant ewes. Cross-sections of the uterine wall from cyclical (C) and pregnant (P) ewes were hybridized with radiolabeled antisense or sense ovine IGF-II cRNA probes. IGF-II mRNA was most abundant in compact stroma (S) of endometrial caruncles of both cyclical and pregnant ewes. Interestingly, IGF-II mRNA expression transitioned from the stroma to the luminal and superficial glandular epithelia in intercaruncular endometria between d 15 and 20 of pregnancy. IGF-II mRNA was also present in all cells of the conceptus and particularly abundant in the yolk sac (YS) as shown for d 18 of pregnancy. Scale bar represents 5 μm. En, Endoderm; Tr, trophectoderm.

IGF-II mRNA was abundant within caruncular stroma and, to a lesser extent, within the intercaruncular stratum compactum and stratum spongiosum stroma on d 10–12 of the estrous cycle and pregnancy. By d 14, for both cyclical and pregnant ewes, IGF-II mRNA was less abundant within the intercaruncular stroma, whereas IGF-II mRNA remained abundant within the endometrial caruncle. As ewes approached estrus on d 16 of the estrous cycle, IGF-II mRNA was also localized to the LE in addition to the caruncular and intercaruncular stroma. Furthermore, IGF-II mRNA was more abundant within the myometrium on d 16 of the cycle compared with any of the previous days of the estrous cycle and all days of pregnancy (data not shown). On d 16 of pregnancy, IGF-II mRNA also increased in LE as noted for cyclical ewes; however, abundance of IGF-II mRNA within the caruncles declined compared with d 16 of the cycle and previous days of pregnancy. IGF-II mRNA on d 18 of pregnancy was also localized to LE and caruncular stroma, similar to d 16 of pregnancy. IGF-II mRNA was particularly abundant in the yolk sac of the conceptus but less abundant in the conceptus trophectoderm and endoderm. By d 20 of pregnancy, IGF-II mRNA in the caruncular stroma was low, whereas expression in LE was abundant but variable in localization due to the loss of the LE from fusion with trophoblast giant binucleate giant cells (BNCs). The conceptus endoderm, but not trophectoderm, expressed abundant IGF-II mRNA on d 20 of pregnancy.

Localization of p-AKT1, p-ERK1/2, and p-P38 MAPK protein in ovine endometrium and conceptus (Fig. 2)

Fig. 2.

Immunohistochemical localization of p-AKT1, p-ERK1/2, and p-P38 MAPK protein in endometria from pregnant (P) ewes from study 1. A, C, and E, Immunoreactive p-AKT1, p-ERK1/2, and p-P38 MAPK proteins, respectively, were localized using rabbit antimouse phospho-AKT1 polyclonal antibody (Ser493/Thr308), rabbit antimouse phospho-ERK1/2 monoclonal antibody, and rabbit antihuman phospho-P38 MAPK monoclonal antibody. Normal rabbit IgG was substituted for the primary antibody as a negative control. Sections were not counterstained. Scale bar represents 5 μm. B, D, and F, Immunoreactive p-AKT1, p-ERK1/2, and p-P38 MAPK proteins were present at low levels in most endometrial cells, but p-AKT1 and p-P38 MAPK were particularly abundant in the nuclei of endometrial LE and conceptus trophectoderm (Tr) of pregnant ewes. Sections were not counterstained. Scale bar represents 2.5 μm. En, Endoderm; S, stroma.

Immunohistochemical analyses revealed that p-AKT1, p-ERK1/2, and p-P38 MAPK proteins were present at low levels in most endometrial cells, but p-AKT1 and p-P38 MAPK were particularly abundant in the nuclei of endometrial LE and conceptus trophectoderm in pregnant ewes. Localization of immunoreactive p-ERK1/2 and p-P38 MAPK proteins in conceptus trophectoderm, including BNCs, was based on cell morphology.

Localization of RPS6K mRNA and p-RPS6K protein in ovine endometrium and conceptus (Fig. 3)

Fig. 3.

In situ hybridization analysis of RPS6K mRNA and immunohistochemical analysis of RPS6K protein in the uteri of early pregnant ewes. A, Cross-sections of the uterine wall from pregnant (P) ewes were hybridized with radiolabeled antisense or sense ovine RPS6K cRNA probes. RPS6K mRNA is expressed only in endometrial LE and sGE, and trophectoderm (Tr) of the conceptuses. B, Immunoreactive p-RPS6K protein was localized using rabbit antihuman phospho-RPS6K (Thr421/Ser424) polyclonal antibody. Normal rabbit IgG was substituted for the primary antibody as a negative control. Sections were not counterstained. Scale bar represents 10 μm (A) and 2.5 μm (B). S, Stroma.

In situ hybridization analysis indicated that RPS6K mRNA was expressed mainly in endometrial LE and superficial ductal GE (sGE) (Fig. 3A). Furthermore, RPS6K mRNA was abundant in the conceptus trophectoderm. Immunohistochemical analysis revealed that p-RPS6K protein was present at low levels in a majority of endometrial cells but was particularly abundant in the nuclei of endometrial LE and conceptus trophectoderm of pregnant ewes (Fig. 3B).

IGF-II activates PI3K-AKT1 and FRAP1-RPS6K signal transduction in ovine trophoblast cells (Figs. 4 and 5)

Fig. 4.

IGF-II-induced phosphorylation of PDK1, AKT1, and GSK3B in oTr cells. Monolayers of 80% confluent oTr cells were serum starved for 24 h and then treated with IGF-II (50 ng/ml) for the indicated times. Blots were imaged to calculate the normalized values presented in graphs (bottom) by measurements of levels p-protein relative to total protein. The bars represent the relative abundance of p-protein. A, Phosphorylation of PDK1 by IGF-II. Representative Western blots for p-PDK1 (top) and total PDK1 (middle; visualized with total PDK1 antibody after stripping the same blot). IGF-II increased p-PDK1 level 2.5-fold (P < 0.01) over basal levels, within 15 min, and this activation was maintained to 90 min. B, Phosphorylation of AKT1 by IGF-II. Representative Western blots for p-AKT1 (top) and total AKT1 (middle; visualized with total AKT1 antibody after stripping the same blot). IGF-II increased p-AKT1 level 4-fold (P < 0.001) over basal levels within 15 min, and this activation was maintained to 90 min. C, Phosphorylation of GSK3B by IGF-II. Representative Western blots for p-GSK3B (top) and total GSK3B (middle; visualized with total GSK3B antibody after stripping the same blot). IGF-II increased p-GSK3B level 1.4-fold (P < 0.05) within 15 min, and this activation was sustained through 90 min. D, Measurement of endogenous levels of phospho-AKT1 protein in oTr cells using a Sandwich ELISA. Results of a Sandwich ELISA to determine activation of AKT1 (P < 0.001) were consistent with results from Western blot analyses. All quantitative data are presented as LSMs with overall ses.

Fig. 5.

IGF-II-induced phosphorylation of FRAP1 and RPS6K in oTr cells. Monolayers of 80% confluent oTr cells were serum starved for 24 h and then treated with IGF-II (50 ng/ml) for the times indicated. Blots were imaged to calculate the normalized values presented in the graph (bottom) by measurements of levels p-protein relative to total protein. The bars represent the relative abundance of p-protein. A, Phosphorylation of FRAP1 by IGF-II. Representative Western blots for p-FRAP1 (top) and total FRAP1 (middle; visualized with total FRAP1 antibody after stripping the same blot). IGF-II increased p-FRAP1 level 1.9-fold (P < 0.05) within 15 min, and this activation decreased about 1.7-fold by 90 min. B, Phosphorylation of RPS6K by IGF-II. Representative Western blots for p-RPS6K (top) and total RPS6K (middle; visualized with total RPS6K antibody after stripping the same blot). IGF-II increased p-RPS6K (70/85 kDa) level 1.4-fold (P < 0.001) within 15 min, and this activation was sustained through 90 min. C, Inhibition of AKT1 and RPS6K phosphorylation. Serum-starved oTr cells were pretreated with either 200 nm wortmannin, 25 μm LY294002, or 50 nm rapamycin for 30 min and then stimulated with IGF-II (50 ng/ml) for 15 min. Representative Western blots for p-AKT1 (top), RPS6K (middle), total AKT1, and total RPS6K (bottom; visualized with total AKT1 and total RPS6K antibody after stripping the same blot) are presented. Blots were imaged to calculate the normalized values presented in the graph by measurements of levels p-AKT1 (upper) and RPS6K (lower) relative to total AKT1 and total RPS6K. The bars represent the relative abundance of p-protein. Induction of p-AKT1 and RPS6K by IGF-II was inhibited by both PI3K inhibitors (P < 0.01 or P < 0.001), whereas rapamycin only inhibited RPS6K (P < 0.01). The asterisk (*) denotes an effect of treatment (*, P < 0.01; **, P < 0.001). All quantitative data are presented as LSMs with overall ses.

Western blot analyses of whole oTr cell extracts with antibodies to p-(activated) target proteins found that IGF-II increased levels of p-pyruvate dehydrogenase kinase (PDK) 1 and p-AKT1 by 2.5- (P < 0.01) and 4-fold (P < 0.001) over basal levels, respectively, within 15 min, and this activation was maintained to 90 min (Fig. 4, A and B). In addition, results of a Sandwich ELISA to determine activation of AKT1 (P < 0.001) were consistent with results from Western blot analyses (Fig. 4D). IGF-II also stimulated a rapid 1.4-fold increase in p-glycogen synthase kinase (GSK) 3B (P < 0.05) within 15 min (Fig. 4C), as well as increases in p-FRAP1 (P < 0.05) and p-RPS6K (70/85 kDa) (P < 0.001) protein abundance between 0 and 15 min posttreatment that were sustained through 90 min (Fig. 5, A and B). To determine the cell signaling pathways mediating effects of IGF-II on AKT1 and RPS6K, oTr cells were pretreated with pharmacological inhibitors of PI3K (200 nm wortmannin or 25 μm LY294002) and FRAP1/mTOR kinase activity (50 nm rapamycin), respectively. Induction of p-AKT1 and RPS6K by IGF-II was inhibited by both PI3K inhibitors (P < 0.01 or P < 0.001), whereas rapamycin only inhibited RPS6K (P < 0.01) (Fig. 5C). These results suggested that activation of the PI3K-AKT1 pathway by IGF-II is required for translational activation of both p-FRAP1 and p-RPS6K in the IGF-II-induced cell signaling cascade.

IGF-II activates ERK1/2 and P38 MAPK phosphorylation in ovine trophoblast cells (Fig. 6)

Fig. 6.

IGF-II-induced phosphorylation of P38 MAPK and ERK1/2 in oTr cells. Monolayers of 80% confluent oTr cells were serum starved for 24 h and then treated with IGF-II (50 ng/ml) for the times indicated. Blots were imaged to calculate the normalized values presented in the graph (bottom) by measurements of levels p-protein relative to total protein. A, Phosphorylation of P38 MAPK by IGF-II. Representative Western blots for p-P38 MAPK (top) and total P38 MAPK (middle; visualized with total P38 MAPK antibody after stripping the same blot). IGF-II increased p-P38 MAPK level 2.3-fold (P < 0.01) within 30 min, and values decreased slightly by 90 min. B, Phosphorylation of ERK1/2 (p44/p42) induced by IGF-II. Representative Western blots for p-ERK1/2 (top) and total ERK1/2 (middle; visualized with total ERK1/2 antibody after stripping the same blot). IGF-II stimulated a rapid 2.6-fold (P < 0.01) increase in p-p44 (ERK1) and a 1.8-fold (P < 0.05) change in p-p42 (ERK2) within 15 min that was attenuated by 90 min. C, Measurement of endogenous levels of phospho-P38 MAPK protein in oTr cells using a Sandwich ELISA. D, Measurement of endogenous levels of phospho-ERK1/2 protein in oTr cells using a Sandwich ELISA. Results from the Sandwich ELISA verified that IGF-II activated p-P38 MAPK and p-ERK1/2 proteins within 15 min (P < 0.05), and that these proteins returned to basal levels at 90 min. All quantitative data are presented as LSMs with overall se values.

Effects of IGF-II on MAPK signaling transduction cascades in oTr cells were evaluated on SDS-PAGE gel-separated and immunoblotted proteins probed with antibodies against p-P38 MAPK and p-ERK1/2 (p42/p44) proteins. In response to IGF-II, p-P38 MAPK increased 2.3-fold (P < 0.01) over basal levels within 30 min and decreased only slightly by 90 min (Fig. 6A). Meanwhile, IGF-II stimulated a rapid 2.6-fold (P < 0.01) increase in p-p44 (ERK1) and a 1.8-fold (P < 0.05) change in p-p42 (ERK2) within 15 min that was attenuated by 90 min (Fig. 6B). Results from the Sandwich ELISA verified that IGF-II activated p-P38 MAPK and p-ERK1/2 proteins within 15 min (P < 0.05) and that these proteins returned to basal levels at 90 min (Fig. 6, C and D).

IGF-II stimulates trophectoderm cell migration (Fig. 7)

Fig. 7.

IGF-II stimulates migration of oTr cells. The oTr cells were cultured in a transwell plate (n = 3 wells per treatment) and treated with recombinant human IGF-II, wortmannin, LY294002, rapamycin, or their combination. Cells grown in serum- and insulin-containing DMEM served as a negative control. Cell migration was determined after 12-h treatment and expressed as LSMs ± se. IGF-II stimulated (P < 0.01) and the inhibitors blocked (P < 0.01) the effect of IGF-II on cell migration. The asterisk (*) denotes an effect of treatment (P < 0.01).

To investigate functional effects of IGF-II in oTr cells, cell migration assays were conducted. After longer incubation periods with IGF-II (12–48 h), amounts of total AKT1, P38 MAPK, and ERK1/2 (p42/p44) in oTr cells did not change (results are shown in supplemental Fig. 1, which is published as supplemental data on The Endocrine Society’s Journals Online web site at http://endo.endojournals.org). In cell migration assays, treatment of oTr cells with IGF-II in serum and insulin-free DMEM increased their migration approximately 197%. Meanwhile, treatment of oTr cells with wortmannin, LY294002, and rapamycin along with IGF-II decreased oTr cell migration about 52, 73, and 46%, respectively. Of note, treatment of the oTr cells with the inhibitors alone for 12 h did not affect cell number (data not shown). Collectively, these results strongly support our hypothesis that IGF-II from endometrial LE acts in a paracrine manner on conceptus trophectoderm to stimulate cell migration via activation of PI3K-AKT1 and MAPK signaling cascades.

Discussion

Ovine conceptuses undergo growth and morphological differentiation (transition from spherical to tubular to filamentous forms) between d 13 and 14, immediately before attachment to the uterine epithelium (49). Factors affecting early placental development appear to have an important impact on the ability of the dam to supply sufficient nutrients for fetal growth because ovine placental growth occurs during the first half of pregnancy and reaches a plateau by d 90 in advance of the period of rapid fetal growth and development (14). Among these factors, IGF-II is a recognized potent stimulator of cell proliferation, differentiation, and development, and it is considered to play a central role during implantation and establishment of pregnancy in most mammalian species, including humans, mice, and domestic animals (14, 22, 24, 30, 35, 38). IGFs play pivotal roles in fetal-placental development and may act, in part, to affect transfer of maternal nutrients to fetal-placental tissues throughout gestation (14, 24, 42). All components of the IGF system are present in the reproductive tract or placenta of ruminants at some stage of pregnancy (14). However, regardless of their spatial and temporal expression, mechanisms of action of IGFs have not been defined clearly. Results of the present study are the first to provide detailed analyses of temporal and spatial expression of IGF-II mRNA in both the ovine uterus and conceptus throughout the estrous cycle and peri-implantation period of early pregnancy, as well as IGF-II-induced cell signaling pathways in oTr cells. The present results establish a direct link between IGF-II and cell signaling pathways (PI3K-AKT1-FRAP1-RPS6K, and/or ERK1/2 or P38 MAPK) clearly implicated in growth and differentiation of ovine conceptuses (Fig. 8).

Fig. 8.

Schematic illustrating the current working hypothesis on IGF-II-induced PI3K-AKT1-FRAP1-RPS6K and MAPK signaling cascades in oTr cells during the peri-implantation period. Evidence from the present study indicates that IGF-II stimulates the multiple cell-signaling pathways noted by the asterisks. LY294002 and wortmannin are selective PI3K inhibitors. Rapamycin inhibits FRAP1. EIF2, eukaryotic translation initiation factor 2; EIF2B, eukaryotic translation initiation factor 2B; EIF4E, eukaryotic translation initiation factor 4ε; EIF4EBP1, eukaryotic translation initiation factor 4E-binding protein 1; Ras, synaptic Ras-GTPase-activating protein.

In study 1 we found that IGF-II mRNA was most abundant in the compact stroma of endometrial caruncles, and was lower in the stroma of intercaruncular endometria of both cyclical and pregnant ewes (Fig. 1), as reported previously (26, 29). However, we also observed abundant IGF-II mRNA in LE of intercaruncular endometria of d-16 cyclical ewes. In human endometria, IGF-II mRNA is predominantly expressed in the mid to late secretory phase and is proposed to be a progestamedin (50–52). Notably, low levels of IGF-II mRNA are expressed in the rat and pig uterus, and do not exhibit distinctive temporal or spatial (cell specific) changes during the estrous cycle (52–54). Interestingly, in the present study, IGF-II mRNA expression transitioned from stroma to LE in intercaruncular endometria between d 15 and 20 of pregnancy, and was also expressed in all cells of the conceptus but particularly abundant in the yolk sac. Similarly, in human and mouse, abundant IGF-II transcripts are found in primitive endoderm and extraembryonic mesoderm during early implantation (52, 55). It is known that the IGF-IIR acts as a signaling antagonist that prevents IGF-II responses by targeting IGF-II to lysosomes for degradation. For example, targeted disruption of the IGF-IIR gene in mice and studies with cultured rat granulosa cells showed that IGF-IIR is likely functioning as a clearance receptor for IGF-II (19, 56). Therefore, effects of IGF-II are considered mediated through the IGF-IR. Reynolds et al. (29) reported that IGF-IR mRNA is expressed by epithelial cells of superficial and deep uterine glands in ewes during early pregnancy. The IGF-IR was also detected in the trophectoderm cells of cotyledons of placentae of pregnant ewes using affinity cross-linking and immunohistochemical analyses (57). The spatial distribution of IGF-II mRNA in compact caruncular stroma and in LE of intercaruncular endometria supports our hypothesis that IGF-II plays essential roles in conceptus development and implantation in the ovine uterus, and that IGF-II mediated autocrine-paracrine cell signaling is essential for conceptus development.

The activities of both IGF-I and IGF-II are modulated by their association with members of a family of IGF binding proteins (IGFBPs) (13). By binding IGF-I and IGF-II, IGFBPs have growth-inhibitory effects by restricting availability of these ligands for binding to IGF-IR (13, 58). However, IGFBPs can also regulate IGF bioavailability by maintaining a circulating reservoir of IGFs and prolonging their half-life (59). IGFBPs synthesized in the uterus are considered to have important roles in implantation and placental development in many species (18, 54, 60). In the ovine uterus, IGFBP-1 and IGFBP-3 mRNAs were detected in the LE during early pregnancy (29, 61), IGFBP-2, IGFBP-4, and IGFBP-6 mRNAs were expressed in dense caruncular stroma beneath overlying LE (29, 62), and IGFBP-5 (63) mRNA was found in endometrial LE, GE, and stroma. Available evidence indicates that expression of IGFBPs is regulated by steroid hormones during the estrous cycle and also by conceptus factors during pregnancy (14, 52). Therefore, results of the present study and others indicate that ovarian factors (e.g. estrogen and progesterone) and conceptus-derived factors (e.g. interferon τ) may act on ovine endometria to regulate IGFBP and IGF-II expression, as well as their interactions during implantation and placentation.

The PI3K-AKT1 cell signaling pathway has emerged as a critical component of insulin and IGF signaling cascades. Toyofuku et al. (64) demonstrated that IGF-I activates AKT1 via PI3K in human endometrium and decidua. However, to our knowledge, little is known of the signal transduction pathways activated by IGF-II in the conceptus during the peri-implantation period. In the present studies, we used oTr cells to demonstrate that IGF-II induces activation of PDK1-AKT1-GSK3B within 15 min and maintains activation to 90 min. Furthermore, IGF-II-induced phosphorylation of AKT1 was inhibited completely by wortmannin (PI3K-AKT1 inhibitor). In addition, immunoreactive p-AKT1 protein was present at low levels in most endometrial cells but was particularly abundant in the nuclei of endometrial LE and conceptus trophectoderm of early pregnant ewes. These results suggest that IGF-II plays an essential role in trophoblast survival, growth, and differentiation by activating the PI3K-AKT1 cell signaling pathway in the ovine uterus during the peri-implantation period.

Furthermore, we found that IGF-II induces phosphorylation of FRAP1-RPS6K in oTr cells. The FRAP1-RPS6K cell-signaling cascade also mediates cell signaling by nutrients such as amino acids and by mitogenic signals (65) to stimulate cell proliferation, differentiation, and gene expression. FRAP1, also known as mTOR, and PI3K-AKT1 pathways act in parallel to transduce growth factor signals and regulate common downstream targets such as RPS6K (52). In the present study, we determined that low levels of RPS6K mRNA and protein were expressed in endometrial epithelia of pregnant ewes, and that RPS6K mRNA and protein were most abundant in conceptus trophectoderm during the peri-implantation period. These results suggest that the FRAP1-RPS6K cell signaling cascade induced by IGF-II plays an important role in implantation events in the ovine uterus.

In the present study, we further demonstrated that IGF-II stimulates activation of ERK1/2 and P38 MAPK phosphorylation in oTr cells. As a family of protein kinases, MAPKs are highly conserved in most organisms, from yeast to humans (66). Among the three well-characterized subfamilies of MAPKs, the ERK1/2 and P38 MAPK pathways play important roles in differentiation processes, including embryonic and placental development (41, 67–69). However, little is known about either member of the MAPK subfamily in oTr development during early pregnancy. In the present study, immunoreactive ERK1/2 and P38 MAPK proteins were detected in ovine endometrial epithelia and conceptus trophectoderm, including BNCs. Furthermore, IGF-II induced rapid increases in phosphorylation of both ERK1/2 and P38 MAPK, reaching a maximum at 15 or 30 min for oTr cells, respectively, followed by a decline through 90 min. Involvement of ERK1/2 and P38 MAPK in IGF-II-induced cell signaling in oTr cells indicates a novel cell signaling pathway. Therefore, we suggest that IGF-II may influence ovine fetal/placental development by activating these MAPK pathways during the peri-implantation period.

In addition to activation of PI3K-AKT1 and MAPK cell signaling pathways, IGF-II increased migration of oTr cells, which was prevented by inhibitors of PI3K and FRAP1 (Fig. 7). Thus, results of the present studies strongly support the hypothesis that IGF-II from the endometrial LE and/or conceptus tissues acts in a paracrine or autocrine manner to stimulate migration of conceptus trophectoderm via activation of PI3K-AKT1 and MAPK signaling cascades.

In conclusion, IGF-II activates the PI3K-AKT1-FRAP1-RPS6K and stimulates the MAPK signal transduction cascades likely involved in expression of growth- and/or development-related genes affecting ovine fetal/placental cell migration during the peri-implantation period (Fig. 8). These results provide important insights into the mechanisms by which IGF-II regulates conceptus development during the peri-implantation period of sheep.

Acknowledgments

We thank all members of the Laboratory for Uterine Biology and Pregnancy for assistance and management of animals.

This research was funded by the United States Department of Agriculture Cooperative State Research, Education, and Extension Service National Research Initiative Grant 2006-35203-17283.

Disclosure Statement: The authors have nothing to declare.

Abbreviations

 
• AKT1

  Protooncogenic protein kinase 1

 
• BNC

  binucleate giant cell

 
• FRAP1

  FK506 binding protein 12-rapamycin associated protein 1

 
• GE

  glandular epithelium

 
• GSK

  glycogen synthase kinase

 
• IGFBP

  IGF binding protein

 
• IGF-IR

  type I IGF receptor

 
• IGF-IIR

  type II IGF receptor

 
• LE

  luminal epithelium

 
• LSM

  least-square mean

 
• mTOR

  mammalian target of rapamycin

 
• oTr

  ovine trophectoderm

 
• p

  phosphorylated

 
• PDK

  pyruvate dehydrogenase kinase

 
• PI3K

  phosphatidylinositol 3-kinase

 
• RPS6K

  ribosomal protein S6 kinase

 
• sGE

  superficial ductal glandular epithelium