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The Journal of Clinical Endocrinology & Metabolism Vol. 82, No. 7 2122-2128
Copyright © 1997 by The Endocrine Society


Reproductive Endocrinology

Laminin Suppresses Progesterone Production by Human Luteinizing Granulosa Cells via Interaction with Integrin {alpha}6ß11

Hiroshi Fujiwara, Tetsuro Honda, Masamichi Ueda, Kimihiko Nakamura, Shigetoshi Yamada, Michiyuki Maeda and Takahide Mori

Department of Gynecology and Obstetrics, Faculty of Medicine (H.F., T.H., K.N., S.Y., T.M.), Institute for Virus Research (M.U.), and Chest Disease Research Institute (M.M.), Kyoto University, Sakyo-ku, Kyoto 606–01, Japan

Address all correspondence and requests for reprints to: Hiroshi Fujiwara, M.D., Department of Gynecology and Obstetrics, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto, 606–01, Japan.


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We previously raised a murine monoclonal antibody (mAb), OG-1, against human granulosa cells (GC) and reported that human GC express the OG-1 antigen with the highest immunoreactivity during the periovulatory phase. Later, we showed that the OG-1 antigen is identical to human integrin {alpha}6, and that human GC express integrin {alpha}6ß1, but not {alpha}6ß4.

In the present study, we examined the expression of laminin (LN), the ligand for integrin {alpha}6ß1. Flow cytometry showed that LN was bound to the cell surface of some GC obtained from preovulatory follicles of patients undergoing in vitro fertilization. Immunohistochemistry showed that LN was detected between luteinizing GC in the early corpora lutea.

To examine the effect of LN on steroidogenesis by human luteinizing GC, GC obtained from patients undergoing in vitro fertilization were cultured on mouse LN-coated or noncoated plastic dishes in medium containing 5% FCS for 24 h. In the absence or presence of hCG (1 IU/mL), GC cultured on LN-coated dishes produced 0.70- and 0.67-fold less progesterone than those on noncoated dishes, respectively (P < 0.05).

We examined the effect of the interaction of integrin {alpha}6ß1 and LN on steroidogenesis by human luteinizing GC. We cultured GC with 5 µg/mL of the anti-{alpha}6 mAb GoH3, which inhibits the interaction between human integrin {alpha}6ß1 and mouse LN, or with a control rat mAb (TER199) on mouse LN-coated dishes in serum-free medium for 24 h. In the absence or presence of hCG (1 IU/mL), GC cultured with GoH3 produced 1.97- and 1.94-fold more progesterone than the control cells (P < 0.01 and P < 0.05, respectively). In contrast, when GC were cultured on dishes coated with type IV collagen, progesterone production was not enhanced by GoH3. Furthermore, the anti-{alpha}6 mAb OG-1, which does not inhibit the interaction between integrin {alpha}6ß1 and LN, had no effect on the progesterone production by GC cultured on LN.

These results indicate that LN suppresses the luteinization of human luteinizing GC via integrin {alpha}6ß1 and that integrin {alpha}6ß1 regulates the luteinization of human GC during the periovulatory phase.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
TO INVESTIGATE the regulatory factors involved in ovarian physiology, we raised monoclonal antibodies (mAbs) against human or porcine ovarian cells and reported that ovarian cells express several molecules in a differentiation stage- and cell-specific manner, including aminopeptidase-N, dipeptidyl peptidase IV, human leukocyte antigen-DR, and leukocyte functional antigen-3 (1, 2, 3, 4). These antigens may regulate the differentiation and functions of the ovarian cells (5, 6).

We raised the mAb OG-1, which recognizes a cell surface molecule of human granulosa cells (GC) (7). N-Terminal amino acid sequencing of the purified OG-1 antigen showed that it is identical to human integrin {alpha}6 (8). We also raised the mAb POG-2 that recognizes a cell surface molecule of the porcine GC and demonstrated that POG-2 antigen is a porcine homolog of integrin {alpha}6 by N-terminal amino acid sequencing (9). The expression profiles of integrin {alpha}6 differ between species. In the human ovary, integrin {alpha}6 is expressed on GC of medium to large follicles and on luteinizing GC in the early luteal phase. The expression level was highest during the periovulatory stage. In contrast, in the porcine ovary, integrin {alpha}6 was expressed on GC in the small follicles (1–2 mm in diameter) with maximal immunoreactivity and was undetectable in corpora lutea (CL). These stage-specific expression profiles suggested the involvement of integrin {alpha}6 in ovarian physiology (10).

We reported that human GC express integrin ß1, but not integrin ß4, indicating that integrin {alpha}6 expressed on GC forms a heterodimer with ß1, but not with ß4 (8). As the ligand for integrin {alpha}6ß1 is laminin (LN) (11), we examined the expression of LN on GC and the physiological function of the interaction between integrin {alpha}6ß1 and LN.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Antibodies

The murine antihuman LN mAb, LAM89 (IgG1 isotype), was purchased from Sigma Chemical Co. (St. Louis, MO). The murine antihuman integrin {alpha}6 mAb OG-1 (IgG1) was raised in our laboratory (7). The murine antitrinitrophenyl (anti-TNP) mAb (IgG1) was used as a control (12). The rat antimouse integrin {alpha}6 mAb, GoH3 (IgG2a), which cross-reacts with human integrin {alpha}6 (13, 14), was purchased from Serotec (Oxford, UK). This mAb partially blocks the interaction of integrin {alpha}6 and LN (14). The rat antimouse erythroid cell mAb, TER199 (IgG2a), a gift from Dr. T. Kina (Chest Disease Research Institute, Kyoto University, Kyoto, Japan), was used as a control (15).

The mAb GoH3 inhibits the interaction between human integrin {alpha}6ß1 and murine LN, but OG-1 does not, as shown below.

Human ovaries

Preovulatory growing ovarian follicles (n = 4) and CL of the early luteal phase (n = 6) were obtained from 10 patients, aged 33–45 yr, with regular menstrual cycles, undergoing surgery for benign disease. Written informed consent was obtained from each patient.

Follicles were morphologically evaluated by staining cryosections with hematoxylin and eosin (HE). Follicles obtained in the follicular phase (18–20 mm in diameter) with GC having mitotic figures and regularly shaped nuclei, cytoplasm, and stratified layers were classified as preovulatory growing follicles (16).

The postovulatory date of CL was evaluated according to the histological dating described by Corner (17), using HE-stained tissue sections of 10% formalin-fixed and paraffin-embedded samples. In this work, the term CL day was used according to his definition. For example, CL day 2 is the next day of ovulation.

Indirect immunohistochemical staining of frozen sections

Indirect immunofluorescence histochemistry proceeded as previously described (7). Each specimen was embedded in OCT compound (Tissue-Tec, Miles Scientific, Naperville, IL), snap-frozen in liquid nitrogen, and stored at -80 C. Frozen tissues were sliced to 7-µm thickness using a cryostat microtome (Cryocut 1800, Reichert-Jung, Heidelberg, Germany), immediately air-dried on Neoplene (Nisshin EM, Tokyo, Japan)-coated glass slides, and fixed in acetone at -20 C for 5 min. The slides were incubated with the anti-LN mAb (ascites diluted 1:1000), OG-1 (5 µg/mL), or the anti-TNP mAb (5 µg/mL; negative control) for 40 min at room temperature. After washing in phosphate-buffered saline (PBS), they were incubated with the fluorescein isothiocyanate-conjugated rabbit antimouse Ig antibody (diluted 1:40; Dakopatts, Glostrup, Denmark) for 40 min at room temperature in the dark. The slides were washed, mounted with Perma Fluor Aqueous Mounting Medium (Immunon, Pittsburgh, PA), and examined under a fluorescence microscope (Nikon, Tokyo, Japan). Serial cryosections were also stained with HE after acetone fixation.

Isolation of human luteinizing GC

Human GC were isolated from 26 patients, aged 28–38 yr, undergoing in vitro fertilization (IVF), as previously reported (18). Briefly, patients receiving a GnRH analog (buserelin acetate, Hoechst Japan Co., Tokyo, Japan) beginning on the first day of the cycle were hyperstimulated with human menopausal gonadotropin (Organon Japan Co., Tokyo, Japan) until the follicles reached maturity. Follicles were aspirated 36 h after the administration of hCG (Mochida Pharmaceutical Co., Osaka, Japan). The follicular fluid was centrifuged, and the resuspended GC were overlaid on Ficoll-Hypaque and centrifuged at 400 x g for 30 min. Cells were collected from the interphase.

Flow cytometric analysis of LN bound to isolated human luteinizing GC

The isolated human GC were washed in Hanks’ balanced salt solution (HBSS) with 0.1% BSA and 0.1% NaN3, sedimented by centrifugation, and incubated with 5 µL anti-LN mAb (ascites diluted 1:20) or the anti-TNP mAb (100 µg/mL) for 30 min at 4 C. After washing in HBSS, the cell pellet was incubated with fluorescein isothiocyanate-conjugated rabbit antimouse Ig for 30 min at 4 C in the dark. After washing in HBSS, the cells were resuspended in the same solution, and viable cells were analyzed by flow cytometry (FACScan, Becton Dickinson Immunocytometry Systems Japan, Tokyo, Japan). The ratio of contaminating monocytes, identified by the anti-CD14 mAb (Becton Dickinson), was less than 3%.

Culture of human luteinizing GC on LN-coated or noncoated dishes in medium containing 5% FCS

Dishes were coated as previously described with minor modification (14). Briefly, mouse LN (Becton Dickinson Labware, Bedford, MA), composed of the whole LN fragments and with a molecular mass of 900,000 daltons, was dissolved at 40 µg/mL in PBS, and 100 µL were placed in each well (6.3 µg/cm2 LN) of 96-well noncoated polystyrene dishes (Corning Glass Works, Corning, NY). The dishes were stored at 4 C overnight and washed in PBS before use. The GC isolated as described above were suspended in DMEM-Ham’s F-12 medium (1:1, vol/vol; Life Technologies, Gaithersburg, MD) medium containing 5% FCS (Flow Laboratories, McLean, VA) and 10 mmol/L HEPES (Nacalai Tesque, Kyoto, Japan) without antibiotics at a density of 3 x 105 cells/mL. The cells were inoculated on 96-well noncoated polystyrene or on mouse LN-coated dishes at 100 µL/well and cultured in the absence or presence of hCG (1 IU/mL; Serono Japan, Tokyo, Japan). After 24 h, the supernatant was collected to assay steroid production. To assay estradiol, 10-7 mol/L testosterone was added to the medium. The cells were detached by incubation with PBS containing with 0.05% trypsin (Difco, Detroit, MI) and 0.05% ethylenediamine tetraacetate (Nacalai Tesque), then the number of viable GC per well were counted under microscopy by trypan blue exclusion.

Inhibition assay of BeWo cell attachment to LN by the mAbs OG-1 and GoH3

BeWo, a human choriocarcinoma cell line, was obtained from the Japanese Cancer Research Resources Bank (Tokyo, Japan). Single cells obtained with trypsin treatment were analyzed by flow cytometry as described above, using OG-1 mAb and the mouse antihuman integrin ß1 mAb (P4C10, Life Technologies). For assay of the inhibition of attachment, single cells were incubated with the mAb OG-1 (0, 10, and 20 µg/mL), the mAb GoH3 (0, 2.5, and 5 µg/mL), or control mouse (anti-TNP; 20 µg/mL) and rat (TER199; 5 µg/mL) mAbs at 4 C for 30 min. The cells were inoculated on commercially available dishes coated with mouse LN (Becton Dickinson Labware), in serum-free DMEM-Ham’s F-12 medium at a density of 2 x 105 cells/mL. After 6 h, the wells were gently rinsed three times with PBS to remove unattached cells, then the number of the attached viable cells was counted as described above.

Culture of human luteinizing GC with the antiintegrin {alpha}6 mAbs on LN- or type IV collagen (CLN)-coated dishes in serum-free medium

The isolated GC were washed twice in serum-free DMEM-Ham’s F-12 medium with 10 mmol/L HEPES without antibiotics. The cells were suspended at a density of 3 x 105 cells/mL in the same serum-free medium, then incubated with the antiintegrin {alpha}6 mAb (GoH3 or OG-1) or the control mAb (TER199 or anti-TNP, respectively) at a final concentration of 5 µg/mL at 4 C for 30 min. The cells were inoculated on 96-well mouse LN-coated or mouse type IV CLN-coated dishes (Becton Dickinson Labware; 100 µL/well) in the absence or presence of hCG (1 IU/mL). After 24 h, the supernatants were collected to assay steroid production, and the number of viable GC per well was counted as described above.

Assay of steroid production by human luteinizing GC

The concentrations of progesterone and estradiol were measured using RIA kits (Daiichi Radio Isotope Research, Tokyo, Japan). Inter- and intraassay coefficients of variation were 6.5% and 5.3% for the progesterone assay and 7.4% and 6.3% for the estradiol assay, respectively.

Statistics

Each experiment was performed in triplicate. Data are shown as the mean \ SEM. The inhibition assay of cell attachment was examined by one-way ANOVA, followed by Scheffe’s F test. Steroid production was analyzed by two-tailed paired t test.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Expression profiles of LN in the human ovary

In preovulatory follicles (18–20 mm in diameter; n = 4), LN was detected on the basal lamina, around vessels, and in stroma, including theca interna cells, but not between GC (Fig. 1BGo). In CL of the early luteal phase (CL day 2, n = 2; day 4, n = 2; day 5, n = 2), LN was expressed between luteinizing GC from the next day of ovulation (Fig. 2BGo).



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Figure 1. Immunohistochemistry of a preovulatory follicle (18 mm in diameter) using anti-LN mAb (LAM89) and anti-integrin {alpha}6 mAb (OG-1). A, HE stain. B, LN was detected on the basal lamina, around vessels, on theca interna cells (TI), and on stromal cells, but was undetectable between GC. C, Integrin {alpha}6 was expressed on GC. D, Negative control. Magnification, x120.

 


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Figure 2. Immunohistochemistry of a CL on the second day of ovulation using anti-LN mAb (LAM89) and antiintegrin {alpha}6 mAb (OG-1). A, HE stain. B, LN was detected between luteinizing GC, around vessels, and in stroma. C, Integrin {alpha}6 was expressed on luteinizing GC. D, Negative control. Magnification, x120.

 
Flow cytometry of LN bound to isolated human luteinizing GC

The percentage of LN-bound human luteinizing GC varied greatly between individuals, ranging from undetectable (0%) to 51.2% (n = 12; mean \ SD, 14.5 \ 14.4%). LN was detected on the cell surface region of GC (Fig. 3Go). A representative histogram is shown in Fig. 4Go.



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Figure 3. Indirect immunofluorescence staining of LN on the isolated viable GC obtained from a patient undergoing IVF. A, Phase contrast. B, LN was detected on the cell surface region of some GC (arrows), but not on other GC (arrowheads). Magnification, x240.

 


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Figure 4. Flow cytometry of LN expression on GC obtained from a patient undergoing IVF. A, Negative control. B, LN was expressed on 28% of the GC from this patient.

 
Effects of LN on progesterone and estradiol production by human luteinizing GC

After 24-h culture, there was no difference in cell morphology or in the number of viable human luteinizing GC between the groups cultured on LN-coated or noncoated dishes. In the absence or presence of hCG, progesterone production by cells cultured on LN-coated dishes was 0.70- and 0.67-fold less than that in cells on noncoated dishes (P = 0.0123 and P = 0.0233, respectively; Fig. 5AGo). There was no significant difference in estradiol production (Fig. 5BGo).



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Figure 5. Human luteinizing GC were cultured on LN-coated dishes or on noncoated plastic dishes in medium with 5% FCS for 24 h in the absence or presence of hCG (1 IU/mL). There was no difference in the number of viable human luteinizing GC between the groups. A, Progesterone production per viable GC was significantly lower in the group cultured on LN, either with or without hCG (n = 7). B, There was no significant difference in estradiol production between the groups (n = 7). Bars indicate mean ± SEM.

 
Inhibition assay of BeWo cell attachment to LN by the mAbs OG-1 and GoH3

Flow cytometry showed that integrins {alpha}6 and ß1 are expressed on 75 \ 5.3% and 79 \ 6.2% of BeWo cells, respectively (n = 3). A histogram is shown in Fig. 6Go.



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Figure 6. Flow cytometry of integrin {alpha}6 and ß1 expression on BeWo cells, a human choriocarcinoma cell line. A, Negative control. B, Integrin {alpha}6 was expressed on 77% of BeWo cells. C, Integrin ß1 was expressed on 78% of BeWo cells.

 
The mAb GoH3 inhibited the attachment of BeWo cells to LN in a dose-dependent manner, whereas the mAb OG-1 did not inhibit attachment at concentrations up to 20 µg/mL (Fig. 7Go).



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Figure 7. Inhibition assay of cell attachment by antiintegrin {alpha}6 mAbs using BeWo cells. A, Single cells were cultured with the mAb GoH3 (0, 2.5, and 5 µg/mL) or control mAb (TER199, 5 µg/mL) on mouse LN-coated dishes in serum-free medium for 6 h. GoH3 inhibited cell attachment to LN in a dose-dependent manner (*, P < 0.01 vs. control). B, Single cells were cultured with the mAb OG-1 (0, 10, and 20 µg/mL) or control mAb (anti-TNP; 20 µg/mL) as described above. OG-1 did not inhibit cell attachment at concentrations up to 20 µg/mL. Bars indicate the mean ± SEM.

 
Effects of antiintegrin {alpha}6 mAbs on progesterone and estradiol production by human luteinizing GC cultured on LN-coated or type IV CLN-coated dishes

Human luteinizing GC were cultured with the antiintegrin {alpha}6 mAb GoH3 or with the control mAb on mouse LN-coated dishes in serum-free medium. After 24 h, there was no difference in cell morphology or in the number of viable cells between the two groups. Progesterone production by GC cultured with GoH3 without hCG was 1.97-fold higher (P = 0.0031) than that by GC cultured with control mAb and was 1.94-fold higher (P = 0.0177) than the control value when cultured with 1 IU/mL of hCG (Fig. 8AGo). There was no difference in the level of estradiol production (data not shown). In contrast, GoH3 did not enhance progesterone production by GC cultured on type IV CLN-coated dishes (Fig. 8BGo) and had no effect on estradiol production (data not shown).



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Figure 8. Progesterone production of luteinizing GC cultured in serum-free medium for 24 h with 5 µg/mL antiintegrin {alpha}6 mAb (GoH3) or control mAb (TER199) in the absence or presence of hCG (1 IU/mL). A, GC cultured on LN-coated dishes. Progesterone production was higher in the group given GoH3 than in the controls, either with or without hCG (n = 7). B, GC cultured on type IV CLN-coated dishes. GoH3 had no effect on progesterone production (n = 7). Bars indicate the mean ± SEM.

 
Furthermore, there was no difference in progesterone production between the group treated with OG-1, that treated with nonblocking antiintegrin {alpha}6 mAb, and the control group when GC were cultured on either LN-coated dishes or type IV CLN-coated dishes (data not shown). There was no difference in estradiol production between the two groups (data not shown).


    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We reported that integrin {alpha}6ß1 is a differentiation-related cell surface molecule of human GC and is expressed at the highest levels during the periovulatory phase (7, 8). In this study, we examined the expression of LN, the ligand for integrin {alpha}6ß1, during the periovulatory phase. Although immunoreactive LN was undetectable between GC in preovulatory follicles, LN was detected between luteinizing GC by immunohistochemistry after ovulation. Moreover, LN was detected by flow cytometry on the cell surface of luteinizing GC obtained from the patients undergoing IVF, indicating that LN is actually bound to GC in vivo before ovulation. These findings demonstrated that the expression of LN on GC increases during ovulation and suggest a physiological interaction between integrin {alpha}6ß1 and LN, from the preovulatory to the early luteal phase.

We examined the effect of LN on steroidogenesis by human luteinizing GC. When cultured on mouse LN-coated dishes for 24 h, human luteinizing GC produced less progesterone than those on noncoated dishes. As there were no differences in estradiol production or cell numbers between the two groups, this result indicates that LN suppressed luteinization of GC without decreasing cell viability.

An inhibition assay of BeWo cell attachment showed that the interaction between human integrin {alpha}6ß1 and mouse LN was inhibited by the antihuman and antimouse integrin {alpha}6 mAb GoH3, but not by our antihuman integrin {alpha}6 mAb OG-1. Using the blocking mAb (GoH3) and the nonblocking mAb (OG-1), we examined the effect of the interaction between integrin {alpha}6ß1 and LN on the luteinization of human luteinizing GC. GoH3 enhanced progesterone production by human luteinizing GC cultured on LN-coated dishes without altering cell viability. In contrast, it did not enhance progesterone production by those cells cultured on dishes coated with type IV CLN, which is not the ligand for integrin {alpha}6ß1. These results demonstrate that progesterone production was enhanced by specific blocking between integrin {alpha}6ß1 and LN. Furthermore, the nonblocking antiintegrin {alpha}6 mAb, OG-1, had no effect on progesterone production by GC cultured on LN-coated dishes. This result indicates that binding to integrin {alpha}6ß1 by the mAb does not affect progesterone production by GC cultured on LN and supported the idea that the specific blocking of the interaction between integrin {alpha}6ß1 and LN enhances it. Therefore, we concluded that LN suppresses GC luteinization via interaction with integrin {alpha}6ß1.

GC luteinization is a process in which the capacity of progesterone production is augmented, coupled with the morphological changes in cytoplasmic organelles. Human GC gradually differentiate to large luteal cells within 5 days after ovulation (17). The serum level of progesterone gradually increases until it reaches a peak 5–6 days after LH surge (19). We reported that the expression of several differentiation-related cell surface molecules on luteinizing GC, such as dipeptidyl peptidase IV, leukocyte functional antigen-3, HLA-DR, and human CL antigen-1, increases during CL formation (2, 3, 4, 20). We consider that the culture of human luteinizing GC obtained from patients undergoing IVF is a model of the stage of CL formation. Using this culture system, we examined the effects of hCG, interleukin-1{alpha} (IL-1{alpha}), and tumor necrosis factor-{alpha} (TNF{alpha}) on the induction of differentiation-related molecules on cultured GC. The expression of dipeptidyl peptidase IV was enhanced by TNF{alpha} and IL-1{alpha}, but not by hCG, and that of leukocyte functional antigen-3 was enhanced by TNF-{alpha}, but not by hCG (4, 18). These findings showed that TNF{alpha} and IL-1{alpha} induce luteal cell differentiation. On the other hand, hCG suppressed the expression of human CL antigen-1 antigen, whereas its expression on luteinizing GC increased during CL formation in vivo and in vitro (20). These findings suggest that after ovulation, a local regulatory mechanism as well as an endocrine system operate in the differentiation process of human GC into luteal cells. In this study, we demonstrated that LN suppresses the luteinization of human GC via interaction with integrin {alpha}6ß1, indicating that the integrin {alpha}6ß1-LN interaction serves as a local inhibitory regulator of luteinization. This study is the first to show that integrins regulate ovarian cell differentiation. It is reasonable that the intensity of integrin {alpha}6ß1 expression on human luteinizing GC decreases from the early luteal to the midluteal phase (7, 8), as human GC become fully luteinize toward the midluteal phase. Amsterdam et al. reported that the luteinization of human luteinizing GC is enhanced when cultured on mixed bovine extracellular matrices (ECM) composed of CLN (types I, III, and IV), heparin sulfate proteoglycans, LN, entactin, elastin, fibronectin, dermatan sulfate, and chondroitin sulfate proteoglycans (21). Recently, they also reported that several ECM have differential effects on apoptosis and steroidogenesis of cultured rat mature GC, showing that LN suppressed progesterone production by rat mature GC (22). Taken together with our findings, it is strongly suggested that each ECM has a differential regulatory effect on luteinization of human GC.

As blocking of the interaction between integrin {alpha}6ß1 and LN enhanced progesterone production independently of hCG, a system(s) other than that of the LH and LH receptor may operate in the inhibitory effect of the integrin {alpha}6ß1-LN interaction on the luteinization of human GC. Several systems, including the focal adhesion kinase (pp125), are involved in the intracellular signaling via integrins (23). Therefore, approaches using integrins may help clarify the molecular mechanisms of luteinization of human GC.

Leardkamolkarn et al. showed by immunoelectron microscopy that rat GC produce LN (24), and Zhao Y. et al. proved by Northern blotting that bovine GC produce LN-B2 chain (25). Similarly, human GC may secrete LN in the follicular fluid, as flow cytometry detected LN on the cell surface of preovulatory GC. During the menstrual cycle of women, serum progesterone rapidly increases just before the LH surge (26). The increase in serum progesterone is considered to be important for the onset of the LH surge (27). In the IVF program, serum progesterone occasionally increases before hCG administration even when the endogenous LH surge is suppressed by a GnRH analog (28). These progesterone increases are considered to be due to the luteinization of GC before the LH surge, but the precise mechanism of its regulation is unknown. If integrin {alpha}6ß1 expressed on GC in large follicles interacts with LN and suppresses the luteinization of GC, this interaction may regulate the timing of the onset of the LH surge or prevent GC from premature luteinization, which interferes with oocyte maturation (29, 30).

In conclusion, this study suggests a physiological interaction between GC and LN from the preovulatory to the early luteal phase and that LN suppresses GC luteinization. We demonstrated that the integrin {alpha}6ß1-LN interaction suppresses the luteinization of human GC and proposed that integrin {alpha}6ß1 is a local regulator of GC luteinization during the periovulatory phase. These findings support the idea that a differentiation stage-specifically expressed molecule(s) on ovarian cells is involved in their differentiation and function.


    Acknowledgments
 
We are grateful to Mrs. Hisako Takahashi for technical assistance with the steroid assay, to Mrs. Yumiko Tomita for technical assistance with the flow cytometrical analysis, and to Mr. Daniel Mrozek for reading the manuscript.


    Footnotes
 
1 This work was supported in part by Grants-in-Aid for Scientific Research 08671884 and 07457385, the Shimizu Foundation Research Grant for 1994, and the Naito Foundation. Back

Received August 27, 1996.

Revised February 5, 1997.

Revised April 1, 1997.

Accepted April 11, 1997.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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