Ovarian follicular growth and atresia: the relationship between cell proliferation and survival.

Growth factors and steroids play an important role in the regulation of ovarian follicular development. In cattle, two of the earliest detectable differences between the healthy dominant follicle selected for development to the ovulatory stage and subordinate follicles destined to undergo atresia are the greater availability of IGF and the greater capacity to produce estradiol in the dominant follicle. We have shown that IGF-I and estradiol stimulate the proliferation of bovine granulosa cells in vitro and promote granulosa cell survival by increasing resistance to apoptosis. Furthermore, the ability of IGF-I and estradiol to increase resistance to apoptosis is tied to their ability to promote progression through the cell cycle. Blocking the cell cycle at the transition between the first gap phase and the DNA synthesis phase using a specific inhibitor prevented the protective effects of IGF-I and estradiol against apoptosis. Further experiments showed that the protective effect of IGF-I against apoptosis is mediated by the stimulation of phosphatidylinositol 3-kinase and its downstream target, protein kinase B/Akt. Constitutive activation of Akt by the infection of granulosa cells with a recombinant Akt adenovirus protected against apoptosis, and this effect also depended on cell cycle progression. These experiments show that the protective effect of estradiol and IGF-I against apoptosis depends on unperturbed progression through the cell cycle. Once follicles have developed to the preovulatory stage, the LH surge induces terminal differentiation of granulosa cells and withdrawal from the cell cycle. Bovine granulosa cells withdraw from the cell cycle by 12 h after the LH surge and become resistant to apoptosis, even in the absence of growth factors. Treatment with a progesterone receptor antagonist in vitro caused reentry of granulosa cells into the cell cycle and susceptibility to apoptosis, suggesting that induction of progesterone receptor expression by the LH surge is required for cell cycle withdrawal and resistance to apoptosis. In summary, the susceptibility of granulosa cells to apoptosis depends on the cell cycle. Proliferating granulosa cells in growing follicles depend on growth factors for survival, whereas cells that have terminally differentiated in response to the LH surge are resistant to apoptosis and relatively independent of growth factors for survival.

Susceptibility of ovarian granulosa cells to apoptosis differs in cells isolated before or after the preovulatory LH surge.

The luteinizing hormone (LH) surge initiates the final stages of ovarian follicle development, and induces ovulation and luteinization of preovulatory follicles. To investigate whether exposure to the LH surge alters follicle cell susceptibility to apoptosis, granulosa and theca cells were isolated from bovine preovulatory follicles before and 14 h after injection of GnRH to induce an LH surge. Granulosa cells isolated before the LH surge were susceptible to apoptosis induced by soluble Fas ligand or serum withdrawal, while cells isolated after the LH surge were resistant to apoptosis. Resistance to Fas-mediated apoptosis was not associated with decreased Fas mRNA or protein levels. Pretreatment of granulosa cells isolated after the LH surge with the protein synthesis inhibitor cycloheximide rendered the cells susceptible to Fas-mediated apoptosis, indicating that inhibition of apoptosis was mediated by expression of labile survival factors. Theca cells were sensitive to Fas-mediated apoptosis before and after exposure to the LH surge. Resistance to apoptosis of granulosa cells from preovulatory follicles after the LH surge may be important for normal ovulation and luteinization.

Relationship of Fas ligand expression and atresia during bovine follicle development.

The Fas antigen (Fas) is a cell surface receptor that may be involved in the initiation and progression of follicle cell apoptosis during atresia. Fas initiates apoptosis in sensitive cells after binding Fas ligand (FasL). Other experiments have shown that expression of Fas mRNA and responsiveness to Fas-mediated apoptosis vary in bovine granulosa and theca cells during follicle development. In the present study, FasL mRNA content was measured and Fas and FasL protein expression was examined in bovine granulosa and theca cells of healthy dominant follicles and the two largest atretic subordinate follicles on day 5 of the oestrous cycle (day 0 = oestrus), and of dominant follicles from the first wave of follicle development after they had become atretic and showed no growth for 4 days. FasL mRNA content was higher in granulosa cells from atretic compared with healthy follicles. FasL mRNA content was also higher in theca cells from atretic subordinate compared with healthy dominant follicles on day 5, but did not differ between theca cells from healthy and atretic dominant follicles. Immunohistochemical staining for FasL was more intense in theca compared with granulosa cells and in atretic compared with healthy follicles. Immunohistochemical staining for Fas was more intense in granulosa compared with theca cells and in atretic subordinate compared with healthy dominant follicles on day 5. Immune cells, known to express Fas and FasL, were localized in the theca, but not the granulosa, cell layer of all follicles. Higher concentrations of Fas and FasL in cells from atretic follicles, together with the previous demonstration of increased responsiveness of granulosa cells from subordinate follicles to FasL-induced apoptosis, support a potential role for FasL-mediated apoptosis during ovarian follicle atresia.

Apoptosis of bovine granulosa cells after serum withdrawal is mediated by Fas antigen (CD95) and Fas ligand.

Ovarian follicular atresia occurs by apoptosis of granulosa and theca cells. The Fas antigen (Fas), a cell surface receptor that triggers apoptosis when activated by Fas ligand (FasL), may be involved in this process. A possible role of the Fas pathway in mediating serum withdrawal-induced apoptosis of granulosa cells was examined. Granulosa cells collected from 5- to 10-mm bovine follicles were cultured in DMEM-F12 containing serum for 3 days, deprived of serum, and live cells were counted at various times after serum withdrawal. Cell death increased significantly 6 h after serum withdrawal (21% +/- 7%; P: < 0.05 vs. 0 h) and continued to increase until 24 h (43% +/- 6%). No further increases in cell death were observed through 72 h. Detection of the translocation of phosphatidylserine to the outer surface of the cell membrane by annexin V binding indicated that cells died by apoptosis. Quantitative reverse transcriptase-polymerase chain reaction assays showed no changes in Fas mRNA levels but a 4.7-fold increase in FasL mRNA 3 h after serum withdrawal (P: < 0.05 vs. 0 h). FasL mRNA remained elevated through 24 h and returned to basal levels at 48 h. Immunohistochemical staining showed that both Fas and FasL protein increased on the cell surface within 3 h and remained elevated through 12 h (the last time point tested). Binding of FasL to Fas was blocked with two reagents that bind to the extracellular domain of FasL: an anti-FasL antibody and Fas:Fc, a chimeric protein consisting of the Fc portion of human immunoglobulin G and the extracellular domain of human Fas. Cell death 24 h after serum withdrawal was reduced 55% +/- 10% and 34% +/- 12% by anti-FasL antibody and Fas:Fc, respectively (P: < 0.05 vs. no blocking protein). In conclusion, serum withdrawal-induced apoptosis of bovine granulosa cells is mediated at least partially by Fas/FasL interactions. These results are consistent with a potential role of Fas in an autocrine or paracrine pathway to trigger ovarian follicular atresia.

Apoptosis : a technical overview.

Physiological or programmed cell death plays an important role during a variety of physiological and pathological processes (1). It is an active form of cell death under tight regulation. Physiological cell death in mammals occurs during embryonic development of organs and tissues; for example, the loss of excess tissue between digits and the removal of excess neurons during development of the nervous system. Physiological cell death also maintains homeostasis of cell numbers during cyclic remodeling of many adult tissues including the intestinal epithelium, the uterine endometrium, and ovarian follicles, and is critical for the regulation of the immune response. Aberrant amplification of cell death occurs in AIDS and some neurodegenerative disorders. Conversely, cancers and autoimmune disease may arise when cells inappropriately fail to die or undergo mutations that inhibit normal cell death pathways (2).

Regulation of Fas antigen (Fas, CD95)-mediated apoptosis of bovine granulosa cells by serum and growth factors.

Our previous studies have shown that bovine granulosa cells cultured in basal media supplemented with 5% fetal bovine serum (BM-FBS) are resistant to apoptosis induced by recombinant Fas ligand (FasL) unless pretreated with interferon-gamma (IFN). Experiments were conducted to test the hypothesis that serum and growth factors alter the susceptibility of granulosa cells to FasL-induced apoptosis. Granulosa cells were cultured in BM-FBS, BM containing insulin, transferrin, selenium, and BSA (BM-ITS), and in BM-ITS supplemented with insulin-like growth factor-I (IGF). Cells were susceptible to FasL-induced killing in BM-ITS (27% killing) but were resistant in BM-FBS and in BM-ITS containing IGF (P < 0.05 vs. killing in BM-ITS). Exposure of phosphatidylserine residues on the outer cell membrane, an early marker of apoptosis, was stimulated by FasL and prevented in the presence of IGF. Neutralization of IGF activity in serum with IGF binding protein 3 reduced the protective effect of FBS on FasL-induced killing (P < 0.05), suggesting that IGF is an inhibitory component in FBS. Cotreatment with IFN overcame the inhibitory effects of serum and IGF on FasL-induced killing (31% and 29% killing, respectively, P > 0.05), but IFN did not potentiate killing of cells cultured in BM-ITS. IFN increased expression of Fas antigen (Fas, the receptor for FasL) mRNA five- to sevenfold (P: < 0. 05) and increased immunostaining for Fas protein similarly in all types of media. Addition of the growth factors epidermal growth factor or basic fibroblast growth factor to BM-ITS also inhibited FasL-induced killing (P < 0.05), whereas keratinocyte growth factor, transforming growth factor, platelet-derived growth factor, FSH, and LH had no effect. In summary, FasL-induced killing is inhibited by FBS and certain growth factors. IFN increased expression of Fas similarly in all types of media but was required for FasL-induced killing only in BM containing FBS or IGF. Therefore, modulation of responsiveness to FasL-induced apoptosis by growth factors and IFN is not directly related to the level of Fas expression.

Responsiveness of mouse corpora luteal cells to Fas antigen (CD95)-mediated apoptosis.

Regression of the corpus luteum (CL) occurs by apoptosis. The Fas antigen (Fas) is a cell surface receptor that induces apoptosis in sensitive cells when bound to Fas ligand or agonistic anti-Fas monoclonal antibodies (Fas mAb). A potential role for Fas to induce apoptosis in dispersed CL cell preparations was tested in cells isolated from mice on Days 2-4 of pseudopregnancy. Total CL dispersates, containing steroidogenic luteal cells, fibroblasts, and endothelial cells, were cultured. The effect of pretreatment of cultures with cytokines interferon gamma (IFN) and tumor necrosis factor alpha (TNF) was examined because these cytokines demonstrated effects on Fas-mediated apoptosis in other cell types. Fas mAb had no effect on viability of CL cells cultured in 5% fetal bovine serum (FBS) and pretreated with or without IFN or TNF, but Fas mAb did kill 23% of the cells in cultures pretreated with IFN + TNF. Fas mRNA was detectable in cultured CL cells and was increased 2.1-, 2. 0-, and 11.8-fold by treatment with TNF, IFN, or IFN + TNF, respectively. CL cells treated with the protein synthesis inhibitor cycloheximide (CX) were killed by Fas mAb in the absence of cytokine pretreatment (34%); pretreatment with IFN or IFN + TNF further potentiated killing (62% and 96%, respectively), whereas pretreatment with TNF had no effect (42%). Cells cultured in medium supplemented with insulin, transferrin, and selenium instead of FBS were killed by Fas mAb in the presence of IFN (23%) or IFN + TNF (29%) but not in the presence of TNF. Cells derived from the mouse CL have a functional Fas pathway that is inhibited by FBS and activated by treatment with CX, IFN, and IFN + TNF.

Expression and activity of the Fas antigen in bovine ovarian follicle cells.

The Fas antigen is a cell surface receptor that triggers apoptosis when bound to Fas ligand (FasL). Studies were undertaken to determine whether the cow provides a suitable model to study the role of the Fas pathway in inducing apoptosis of ovarian cells during follicular atresia. Expression of Fas antigen mRNA and responsiveness to FasL-induced killing in vitro were measured. Effects of the cytokines tumor necrosis factor (TNF)-alpha and interferon-gamma (IFN) were studied because of previous demonstrations of their role in Fas-mediated apoptosis in other cell types. Fas antigen mRNA was detectable in cultured granulosa and theca cells, and expression was increased by treatment with IFN but not TNF. Granulosa and theca cells were resistant to FasL-induced killing unless pretreated with IFN. TNF had no effect on FasL-induced killing. Granulosa and theca cell cultures in which killing occurred in response to FasL stained positively for annexin V, an early marker for cells undergoing apoptosis. These results provide a basis for further studies using the bovine ovary to examine the role of the Fas antigen in follicular atresia.

Expression and function of Fas antigen vary in bovine granulosa and theca cells during ovarian follicular development and atresia.

Fas antigen is a receptor that triggers apoptosis when bound by Fas ligand (FasL). A role for Fas antigen in follicular atresia was studied in follicles obtained during the first wave of follicular development during the bovine estrous cycle (estrus is Day 0). Granulosa and theca cells were isolated from healthy dominant follicles and the two largest atretic subordinate follicles on Day 5, atretic dominant follicles on Days 10-12, and preovulatory follicles on Day 1. Fas antigen mRNA levels were highest in granulosa cells from subordinate as compared to other follicles, and lowest in theca cells from healthy Day 5 dominant as compared to other follicles. FasL alone had no effect on viability of granulosa or theca cells but became cytotoxic in the presence of interferon-gamma (IFN). IFN has been shown to induce responsiveness to Fas antigen-mediated apoptosis in other cell types. In the presence of IFN, killing of granulosa cells by FasL was greater in subordinate compared to healthy dominant follicles on Day 5, did not differ between healthy and atretic dominant follicles, and was similar in theca among all follicles. Granulosa cells from preovulatory follicles, which had been exposed to the LH surge in vivo, were completely resistant to FasL-induced killing. In summary, Fas antigen expression, and responsiveness to Fas antigen-mediated apoptosis, vary during follicular development.

Potentiation of Fas-mediated apoptosis of murine granulosa cells by interferon-gamma, tumor necrosis factor-alpha, and cycloheximide.

The Fas antigen is a transmembrane receptor belonging to the tumor necrosis factor-alpha (TNF) receptor family that, when activated by Fas ligand or agonistic antibodies, induces death by apoptosis. Although the presence of Fas antigen in ovarian tissues has been demonstrated, little is known about whether Fas antigen is functional in the ovary. This report shows that murine granulosa cells are initially resistant to antibody-induced Fas-mediated apoptosis, but will undergo apoptosis when cotreated with TNF and interferon-gamma (IFN) or cycloheximide (CX). Granulosa cells were obtained from follicles of 23-day-old mice 2 days after injection of PMSG. Twenty-four hours after plating, cells were pretreated with either 0 or 200 U/ml IFN, which has been shown to induce Fas antigen expression and is required for Fas-mediated killing in many cell types. At 48 h, cells were treated with 2 microg/ml control IgG, 2 microg/ml anti-Fas antigen antibody (Fas mAb), 10 ng/ml TNF, or Fas mAb and TNF. Cytotoxicity (percent killing) relative to control IgG was determined at 72 h by counting granulosa cells after trypsinization. In the absence of IFN, no cytotoxicity was observed. In the presence of IFN, neither TNF or Fas mAb alone was cytotoxic, but the combination of Fas mAb and TNF resulted in 25% killing (P < 0.05). Fas antigen messenger RNA (mRNA) was detectable in cultures not treated with cytokines and was increased 5-fold by TNF, 2-fold by IFN, and 17-fold by the combination of IFN and TNF. To test whether the presence of a labile inhibitor(s) of Fas-mediated killing in granulosa cells is the cause of resistance to Fas mAb, the protein synthesis inhibitor CX was used. Experiments were performed as described above, except that cells were treated with 0.5 microg/ml CX in conjunction with other treatments at 48 h. Fas mAb treatment in the presence of CX induced 25% cell death without IFN pretreatment and 38% with IFN (P < 0.05). TNF treatment in the presence of CX had no effect alone, but potentiated the effects of Fas mAb, resulting in 56% killing in the absence of IFN and 86% killing in the presence of IFN (P < 0.05). Cells stained positively for DNA fragmentation and annexin V binding, features characteristic of apoptosis. Because initial experiments showed that treatment with TNF alone increased Fas mRNA levels, the effect of pretreating cells for 24 h with TNF before treatment with Fas mAb was tested. Pretreatment with TNF or IFN alone did not promote Fas mAb-mediated killing, but combined pretreatment with TNF and IFN resulted in 25% killing in response to Fas mAb. Treatment of cells with the combination of IFN and TNF induced a 19-fold increase in Fas antigen mRNA levels. Corresponding increases in Fas antigen protein expression on the surface of cells in response to cytokine treatments were detected by immunocytochemistry. Human TNF did not duplicate the effects of mouse TNF in inducing Fas antigen mRNA expression and Fas mAb-induced killing. As human TNF interacts exclusively with the type I, but not the type II, TNF receptor in the mouse, potentiating effects of mouse TNF on the Fas pathway are probably mediated via the type II TNF receptor. The effects of cytokine treatments on levels of mRNA for FAP-1, an inhibitor of Fas-mediated apoptosis, were determined. FAP-1 mRNA was detectable in untreated granulosa cells, and levels were not altered by treatment with TNF and/or IFN. In summary, the Fas-mediated pathway of apoptosis is functional in mouse granulosa cells that are stimulated with IFN and TNF. These cytokines may function at least partially by increasing Fas antigen expression. Granulosa cells appear to have inhibitors of the Fas antigen pathway, as treatment with CX potentiates Fas-mediated death. TNF promotes Fas-mediated killing in the presence and absence of CX. Therefore, TNF is not likely to act simply by increasing Fas antigen expression or decreasing protein inhibitors of the Fas pathway, because TNF remains effec

Fas antigen-mediated apoptosis of ovarian surface epithelial cells.

The Fas antigen is a cell surface receptor that, when engaged by Fas ligand or specific agonistic antibodies, triggers apoptosis. The effect of an agonistic monoclonal antibody to mouse Fas antigen (Fas mAb, clone J02) on the viability of cells from dispersed mouse corpora lutea (CL cultures) was tested. Cultures were prepared by enzymatic digestion of CL from day 4-7 pseudopregnant mice. Cultures were pretreated with 0, 1, 10, 100, or 1000 U/ml murine interferon-gamma (IFN) at 72 h of culture. IFN has been shown to increase Fas antigen expression in a number of cell types. At 96 h (time zero), cultures were treated with Fas mAb or IgG. By 4 h after Fas mAb treatment, discrete homogeneous patches of cells within the cultures showed characteristic signs of apoptosis, including blebbing of cell membranes, detachment, and disappearance from the culture. CL cultures contain luteal, stromal, and endothelial cells; fibroblasts; and surface epithelial cells (OSE). Cells dying in response to Fas mAb were identified as OSE. Affected cells had the cobblestone appearance and distinct nuclei typical of epithelial cells. Unlike luteal cells, OSE did not stain with the lipophilic dye, Nile red. The cells did not stain with acetylated low density lipoprotein conjugated to the fluorescent marker octadecyl indocarbocyanine, a marker for endothelial cells and monocytes. Cells in patches stained positively for cytokeratin, a marker for epithelial cells. Fas-mediated cytotoxicity was quantified by counting the number of cells present in discrete patches of OSE 0 and 8 h after Fas mAb treatment. Fas mAb treatment had no effect in cultures pretreated with 0 or 1 U/ml IFN, but induced significant death of OSE in cultures pretreated with 10, 100, and 1000 U/ml IFN (37 +/- 11%, 54 +/- 18%, and 60 +/- 11%, respectively). There was no apparent effect of Fas mAb on other cell types within the CL cultures. To confirm that cells dying in response to Fas mAb were OSE, experiments were also performed on enriched cultures of OSE prepared by enzymatic digestion of the outer surface of the ovary. In enriched OSE cultures pretreated with 200 U/ml IFN, there was 44% killing in response to Fas mAb, whereas in cells not pretreated with IFN, there was no effect. In situ fluorescent end labeling of DNA in CL cultures indicated that treatment with IFN and Fas mAb induced DNA fragmentation in OSE typical of apoptosis. Immunocytochemistry of CL cultures indicated that Fas antigen was expressed in OSE pretreated with IFN. Quantitative reverse transcriptase-PCR showed that IFN pretreatment increased Fas antigen messenger RNA levels 2.3-fold in enriched cultures of OSE. In summary, OSE in CL cultures and enriched cultures of OSE undergo apoptosis in response to Fas mAb when pretreated with IFN. In vivo, OSE undergo programmed cell death before ovulation and rapidly proliferate to repair the surface of the ovulatory follicle after ovulation. Most ovarian cancers are derived from the OSE. The results have implications for both normal ovarian function and oncogenesis in the ovary.

Expression of rat thrombin receptor and mutants in the baculovirus-infected insect cell system.

The rat thrombin receptor (TR-1) has been expressed in Sf9 cells. Two mutant receptors, one lacking a thrombin cleavage site (RTRM), and the other containing only the extracellular domain of the receptor (ECD), have been expressed. Antibodies to the thrombin receptor activating peptide (TRAP) and to the fibrinogen-binding exosite have been prepared. On Western blots these antibodies bound to TR-1 bands of 65-70 kDa and 44 kDa, similar bands for RTRM, and a doublet of about 10 and 12 kDa for ECD. The free cytosolic Ca++ concentration, measured by Fura-2, increased after thrombin or TRAP stimulation in the cells expressing TR-1. The RTRM cells did not respond to thrombin and had an attenuated response to TRAP. The ECD protein was found in the medium; it was not glycosylated. Both the mutants and the antibodies should be useful for studies of TR-1 structure and function.

Rat uterine stromal cells: thrombin receptor and growth stimulation by thrombin.

The estrogen-stimulated maturation of the immature rat uterus is mediated by peptide growth factors whose expression is regulated by estradiol. We present evidence that thrombin is a uterine growth factor. When an immature rat is given a single injection of estradiol, the uterus increases 50% in wet weight within 3 h through the imbibition of water and plasma proteins, including prothrombin. Tissue factor, the initiator of coagulation, is induced 3- to 4-fold over the same time period. Thrombin is generated in situ from prothrombin through the coagulation cascade. It acts as a growth factor through the proteolytically activated thrombin receptor. Thrombin's role as a growth factor in uterine stromal cells is proven by two lines of evidence: demonstrations that the proteolytically activated thrombin receptor is present and that cultured cells are stimulated to grow by thrombin. Thrombin receptor in the uterus is demonstrated by reverse transcription-PCR for receptor messenger RNA by specific [125I]peptide labeling of a membrane-bound binding protein of about 60 kDa and by Western blot with a thrombin receptor antipeptide antibody. Thrombin's effectiveness as a growth factor is shown by thrombin-stimulated growth of primary stromal cell cultures, with maximum stimulation at 100 nM. That the effect is mediated by the proteolytically activated thrombin receptor is shown by the inhibition of growth by hirudin, a highly specific inhibitor of thrombin; the absence of enhanced growth with Pro-Phe-Arg-chloromethyl ketone-thrombin, an active site-inhibited thrombin derivative; and the stimulation of growth by the thrombin receptor-activating peptide.

Fas antigen-mediated apoptosis in human granulosa/luteal cells.

The Fas antigen is a transmembrane receptor that can trigger apoptosis in a variety of tumor and hematopoietic cells. Ovarian follicular atresia and luteolysis are thought to occur by apoptosis. Using reverse transcriptase-polymerase chain reaction (RT-PCR) and flow cytometry, we demonstrated that human granulosa/luteal cells express the Fas antigen. An anti-human Fas antigen monoclonal antibody (Fas mAb; clone CH-11), which induces apoptosis in other cell types by binding to the Fas antigen, induced significant cell death (30%) in cultures pretreated with interferon gamma (IFN gamma). This agrees with studies on tumor cell lines showing that IFN gamma enhances cytotoxic effects of Fas mAb. Granulosa/luteal cells exhibited morphological characteristics typical of apoptosis, including membrane blebbing and condensed chromatin. DNA fragmentation into oligonucleosomal units of approximately 180 bp, typical of apoptosis, was detected at elevated levels in Fas mAb-treated cultures via 3' end-labeling and gel electrophoresis. Examination of cultured cells in situ for apoptotic DNA cleavage by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-digoxigenin nick end-labeling (TUNEL) indicated that more apoptotic death occurred in Fas mAb-treated cultures than in control cultures. Effects of hCG-induced luteinization of cultures on Fas mAb-induced cytotoxicity was examined: combined pretreatment with IFN gamma and hCG induced a synergistic increase in Fas mAb-induced cytotoxicity (40%) over that obtained with IFN gamma-pretreatment alone (15%). In summary, granulosa/luteal cells express the Fas antigen and are sensitive to Fas mAb-induced apoptosis. Human CG synergized with IFN gamma to increase Fas mAb-induced death.(ABSTRACT TRUNCATED AT 250 WORDS)

The regulation of uterine tissue factor by estrogen.

Tissue factor (TF) is a transmembrane protein that initiates coagulation and indirectly catalyzes the conversion of prothrombin to thrombin. We previously showed that treatment of immature rats with estradiol (E(2)) stimulated a rapid increase in TF mRNA and protein in the uterus. Our current experiments usingin situ hybridization show that the increase in TF mRNA occurred primarily in the stromal cell layer. The effect of E(2) to increase TF mRNA occurred in uterine organ cultures but not in separated epithelial and stromal cellsin vitro. Thrombin and the phorbol ester, TPA, compounds which regulate TF expression in other cell types by activation of protein kinase C (PKC), increased TF mRNA in both uterine organ cultures and in separated uteriné cells. The 5' regulatory region of the TF gene was examined for the presence of an estrogen response element (ERE) using a plasmid, pTFCAT, containing -740 to + 15 bp of the mouse TF promoter upstream of the chloramphenicol acetyltransferase (CAT) reporter gene. There was no response to E(2) in HeLa cells cotransfected with pTFCAT and a human ER construct, pHEO. In contrast, E(2) increased CAT activity in cells cotransfected with a positive-control plasmid, containing the consensus ERE cloned upstream of the thymidine kinase promoter-driven CAT gene, and pHEO. CAT activity was also increased by TPA in cells transfected with pTFCAT. In summary, E(2) induces TF mRNA in uterine organ culture indicating that systemic factors are not absolutely required for the effect. However, E(2) injection induces transudation of plasma prothrombin into the uterus where it may be converted to thrombin. Thus thrombin may contribute to E(2)-induction of TF mRNAin vivo. An ERE was not identified in the 750 bp immediately 5' to the transcription start site of the TF gene although a TPA-responsive element was present. It is postulated that E(2) may induce TF mRNA by multiple indirect pathways including stimulation of PKC and Jun and Fos transcription factors, and by generation of thrombin in the uterus.

The td intron endonuclease I-TevI makes extensive sequence-tolerant contacts across the minor groove of its DNA target.

I-TevI, a double-strand DNA endonuclease encoded by the mobile td intron of phage T4, has specificity for the intronless td allele. Genetic and physical studies indicate that the enzyme makes extensive contacts with its DNA substrate over at least three helical turns and around the circumference of the helix. Remarkably, no single nucleotide within a 48 bp region encompassing this interaction domain is essential for cleavage. Although two subdomains (DI and DII) contain preferred sequences, a third domain (DIII), a primary region of contact with the enzyme, displays much lower sequence preference. While DII and DIII suffice for recognition and binding of I-TevI, all three domains are important for formation of a cleavage-competent complex. Mutational, footprinting and interference studies indicate predominant interactions of I-TevI across the minor groove and phosphate backbone of the DNA. Contacts appear not to be at the single nucleotide level; rather, redundant interactions and/or structural recognition are implied. These unusual properties provide a basis for understanding how I-TevI recognizes T-even phage DNA, which is heavily modified in the major groove. These recognition characteristics may increase the range of natural substrates available to the endonuclease, thereby extending the invasive potential of the mobile intron.

I-TevI, the endonuclease encoded by the mobile td intron, recognizes binding and cleavage domains on its DNA target.

Mobility of the phage T4 td intron depends on activity of an intron-encoded endonuclease (I-TevI), which cleaves a homologous intronless (delta In) target gene. The double-strand break initiates a recombination event that leads to intron transfer. We found previously that I-TevI cleaves td delta In target DNA 23-26 nucleotides upstream of the intron insertion site. DNase I-footprinting experiments and gel-shift assays indicate that I-TevI makes primary contacts around the intron insertion site. A synthetic DNA duplex spanning the insertion site but lacking the cleavage site was shown to bind I-TevI specifically, and when cloned, to direct cleavage into vector sequences. The behavior of the cloned duplex and that of deletion and insertion mutants support a primary role for sequences surrounding the insertion site in directing I-TevI binding, conferring cleavage ability, and determining cleavage polarity. On the other hand, sequences around the cleavage site were shown to influence cleavage efficiency and cut-site selection. The role of cleavage-site sequences in determining cleavage distance argues against a strict "ruler" mechanism for cleavage by I-TevI. The complex nature of the homing site recognized by this unusual type of endonuclease is considered in the context of intron spread.

A site-specific endonuclease and co-conversion of flanking exons associated with the mobile td intron of phage T4.

The product of the td intron open reading frame (ORF) of phage T4 is required for high-frequency transfer of the intervening sequence from intron-plus (In+) to intron-minus (In-) alleles. In vivo studies have demonstrated that the td ORF product targets cleavage of td In- DNA, and that cleavage is correlated with intron inheritance [Quirk et al., Cell 56 (1989) 455-465]. In the present study we show by in vitro synthesis of the td intron ORF product, that the protein possesses endonuclease activity and efficiently cleaves double-stranded DNA at or near the site of intron integration. In addition, we demonstrate that intron insertion is accompanied by co-conversion of the flanking exon sequences. Co-conversion of markers within 50 nt surrounding the site of intron insertion occurred at a high frequency (80-100%), and decreased at greater distance from the intervening sequence. Co-conversion may provide a mechanism for maintaining exon-intron RNA contacts required for accurate splicing of the relocated intron. Cleavage of target DNA by an intron endonuclease and co-conversion of flanking exon sequences are both features associated with mobile introns of eukaryotes, indicating a common mechanism for intron transfer in the eukaryotic and prokaryotic kingdoms.

Intron mobility in the T-even phages: high frequency inheritance of group I introns promoted by intron open reading frames.

Intron mobility in the T-even phages has been demonstrated. Efficient nonreciprocal conversion of intron minus (In-) alleles to intron plus (In+) occurred for the td and sunY genes, but not for nrdB. Conversion to In+ was absolutely dependent on expression of the respective intron open reading frame (ORF). Introns were inserted at their cognate sites in an intronless phage genome via an RNA-independent, DNA-based, duplicative recombination event that was stimulated by exon homology. The td intron ORF product directs the endonucleolytic cleavage of DNA, targeting the site of intron integration. A 21 nucleotide deletion of the integration site abolished high frequency intron inheritance. These experiments provide a novel example of gene conversion in prokaryotes, while suggesting a molecular rationale for the inconsistent distribution of introns within highly conserved exon contexts of the T-even phage genomes.