Expression of insulin-like growth factor-binding protein-2 and -3 messenger ribonucleic acid in the porcine ovary: localization and physiological changes

Insulin-Like Growth Factor Binding Proteins and IGFBP Proteases: A Dynamic System Regulating the Ovarian Folliculogenesis

The aim of the present article is to update our understanding of the expression of the insulin-like growth factor binding proteins (IGFBPs), IGFBP proteases and their implication in the different processes of ovarian folliculogenesis in mammals. In the studied species, IGFs and several small-molecular weight IGFBPs (in particular IGFBP-2 and IGFBP-4) are considered, respectively, as stimulators and inhibitors of follicular growth and maturation. IGFs play a key role in sensitizing ovarian granulosa cells to FSH action during terminal follicular growth. Concentrations of IGFBP-2 and IGFBP-4 in follicular fluid strongly decrease during follicular growth, leading to an increase in IGF bioavailability. Inversely, atresia is characterized by an increase of IGFBP-2 and IGFBP-4 levels, leading to a decrease in IGF bioavailability. Changes in intrafollicular IGFBPs content are due to variations in mRNA expression and/or proteolytic degradation by the pregnancy-associated plasma protein-A (PAPP-A), and likely participates in the selection of dominant follicles. The identification of PAPP-A2, as an IGFBP-3 and -5 protease, and stanniocalcins (STCs) as inhibitors of PAPP-A activity extends the IGF system. Studies on their implication in folliculogenesis in mammals are still in the early stages.

Genetic variation and association of insulin-like growth factor binding protein-3 with performance in Swine

Genetic variation of insulin-like growth factor binding protein-3 was analyzed in 17 pig breeds (14 native Chinese and 3 European). Using PCR-single strand conformation polymorphism, we found a polymorphism in intron 2, and this SNP was the combined mutation of G897T-G903A-C911T. The Chinese breeds carried a higher TAT/TAT genotype frequency (over 50%), except for Bamei (22%), Yujiang Black (0.0%), and Erhualian (10.0%); the European breeds had a higher GGC/GGC genotype frequency (Large White 1.67%, Landrace 13.89%, Duroc 0.0%). The allelic frequency of TAT in Chinese breeds was over 50%, except for Yujiang Black (12.5%); the allelic frequency of GGC was over 50% in all European breeds. The effect of genotype on 43 performance traits was investigated in one population (Lantang x Landrace). Pigs with the TAT/TAT genotype had higher B-point and C-point back-fat thickness than pigs with the GGC/GGC genotype. The TAT/TAT pigs also scored higher in meat color than the GGC/GGC pigs. These results implied that IGFBP-3 may affect meat quality and carcass traits.

Growth characteristics, blood metabolites, and insulin-like growth factor system components in maternal tissues of gilts fed L-carnitine through day seventy of gestation1,2

A total of 59 gilts (BW = 137.7 kg) from 3 breeding groups were used to assess the effects of feeding l-carnitine during gestation on gilt growth characteristics, blood metabolites, and uterine and chorioallantoic expression of IGF axis components at d 40, 55, and 70 of gestation. Experimental treatments were arranged in a 2 x 3 factorial, with main effects of added l-carnitine (0 or 50 ppm) and day after initial breeding (d 40, 55, or 70 of gestation). All gilts received a constant feed allowance of 1.75 kg/d and a top-dress containing 0 or 50 ppm of l-carnitine beginning on the first day of breeding through the assigned day of gestation. No dietary treatment differences were observed for gilt BW, backfat, or estimated protein or fat mass at any day of gestation. No differences were observed in circulating total and free carnitine at breeding, but concentrations increased (P < 0.01) as day of gestation increased for gilts fed diets containing l-carnitine compared with those fed the control diet. Maternal IGF-I concentration decreased (P < 0.01) from d 0 to 70 for all gilts, with no differences between treatments. Insulin-like growth factor binding protein-3 mRNA (P = 0.05) and IGFBP-5 mRNA (P = 0.01) increased in the endometrium of gilts supplemented with l-carnitine. These data demonstrate that l-carnitine supplementation and day of gestation alter the expression of the IGF axis by changing the expression of IGFBP at the fetal-maternal interface in swine. These changes in the IGF axis at the fetal maternal interface may aid in determining the reasons for the effects of l-carnitine on reproductive traits.

Insulin-like growth factor-binding protein-3 in porcine ovarian granulosa cells: gene cloning, promoter mapping, and follicle-stimulating hormone regulation

The role and regulation of IGF-binding protein-3 (IGFBP-3) in the ovary is not fully understood. We cloned and determined the sequence of 12,257 bp of the pig IGFBP-3 gene that includes 4,296 bp of the flanking promoter sequence. The porcine IGFBP-3 promoter sequence shares two highly conserved regions with the human and bovine IGFBP-3 promoters and a mouse DNA clone. The first is a 38 bp region between -1095 and -1058, whereas the second is a 73-bp region between -63 and +10 of the pig sequence. Projected translation of the open reading frame of our sequence gave a peptide sequence identical to that determined by peptide sequencing, but with 27 additional amino acids upstream of this sequence and is highly similar to the human, bovine, rat, and mouse IGFBP-3 peptides. Using RT-PCR we demonstrated that FSH regulates IGFBP-3 mRNA expression in a biphasic manner, with an early induction (maximal at 3 h) and an inhibition at 24 h after FSH treatment. The inhibition at 24 h was not due to changes in IGFBP-3 mRNA stability. A similar pattern of FSH modulation of the IGFBP-3 gene transcription was demonstrated by the reporter activity of granulosa cells transiently transfected with IGFBP-3 promoter constructs. The site for FSH stimulation of the IGFBP-3 gene was localized to the sequence between -61 and -48 relative to the transcription start site. Regulation of IGFBP-3 transcription by FSH suggests a role for IGFBP-3 in follicular development that may be independent of IGF-I.

Insulin-like growth factor binding proteins in granulosa and thecal cells from bovine ovarian follicles at different stages of development1,2

Because IGFBP inhibit IGF-stimulated cellular proliferation and differentiation, it is hypothesized that variations among IGFBP in individual follicles might contribute to the regulation of recruitment, selection, dominance, and turnover of ovarian follicles. Sources of IGFBP in fluid of bovine follicles are not well established; thus, objectives of this study were to determine levels of IGFBP binding activities and messenger RNA (mRNA) in granulosa and theca interna cells at different stages of follicular development (small [< 6 mm], medium [6 to < 8 mm], and large [> or = 8 mm]) and to characterize associations of these levels measured in the cells with levels of IGFBP and steroids in follicular fluid. Thecal and granulosa cells from large healthy follicles contained two- to twentyfold less (P < 0.05) IGFBP-2, -3, and -5 than cells from small, medium, and large atretic follicles. Thecal cells from small, medium, and large atretic follicles contained more (P < 0.05) IGFBP-3 and -4 than granulosa cells from these follicles, whereas granulosa cells from these follicles contained more IGFBP-2 activity than thecal cells. Differences in IGF binding activity were paralleled by differences in levels of mRNA for the respective IGFBP. Developmental differences in IGFBP activity in follicular fluid were positively associated with activity in granulosa and/or thecal cells, with the exception of IGFBP-4, which was low in fluid from large healthy follicles but markedly increased (mRNA and binding activity) in granulosa cells from these follicles. It is concluded that developmental changes in follicular fluid IGFBP-2 and -5 binding activities seem to be controlled in part by alterations in synthesis of these IGFBP by granulosa and thecal cells, whereas diminished IGFBP-4 in fluid from large healthy follicles occurs concomitantly with increased levels of IGFBP-4 mRNA and activity in granulosa cells, implicating posttranslational regulation by specific proteases.

The insulin-like growth factor system: a key determinant role in the growth and selection of ovarian follicles? a comparative species study

The aim of the present paper is to make a comparative study of the expression of the elements of the insulin-like growth factor (IGF) system in different mammalian species and thus illuminate their potential role in the process of ovarian folliculogenesis in mammals. In most mammalian species, IGFs and IGFBPs (in particular IGFBP-2 and IGFBP-4) are considered, respectively, as stimulators and inhibitors of follicular growth and maturation. In mammalian species, IGFs might play a key role in sensitizing ovarian granulosa cells to FSH action during terminal follicular growth. Concentrations of IGFBP-2 and IGFBP-4 in follicular fluid strongly decrease and increase during follicular growth and atresia, respectively, leading to an increase and a decrease in IGF bioavailability, respectively. The decrease in these IGFBPs is because of a decrease in mRNA expression (IGFBP-2) and an increase in proteolytic degradation by PAPP-A in follicular fluid (IGFBP-2, IGFBP-4 and IGFBP-5), and likely participates in the selection of dominant follicles. In contrast, levels and/or sites of expression of IGF-I, IGF-II, IGFBP-4, IGFBP-5 and type II receptor in follicular cells strongly differ between mammalian species, suggesting that these phenomena might play species-specific or secondary roles in ovarian folliculogenesis.

Insulin-like growth factors and their binding proteins: Potential relevance to reproductive physiology

Cyclic ovarian follicular development is a complex process that involves proliferation, differentiation, and death of follicle cells. Gonadotropins produced by the pituitary gland have a central role in the regulation of these processes. In addition, a wide range of paracrine and autocrine factors produced in the reproductive organs have been proposed as regulators of reproductive functions. Components of the insulin-like growth factors (IGF) system are widely expressed in the female reproductive tract. The IGFs and their binding proteins play a significant role in several processes of reproductive physiology, including ovarian follicular development, oogenesis and oocyte maturation, ovulation, luteal function, follicular atresia, and testicular function. The majority of these physiological actions of the IGFs are believed to occur via activation of the IGF-I receptor, although the IGF-I effects are modulated by IGF binding proteins (IGFBPs). As much of the data obtained to date have been in the rodent reproductive organs, it may not be possible to directly extrapolate the results to the primate organs. There is a distinct species-difference in the gene expression and functional roles of the IGF-IGFBP system in reproductive organs. However, the disturbance of the IGF-IGFBP system in human reproductive physiology may lead to anovulation, disorders of androgen excess, infertility associated with implantation failure, and male infertility. Further research is needed in domestic animals to determine if manipulation of the IGF-IGFBP system may result in improved reproductive efficiency. As our understanding of the IGF-IGFBP system increases, the uses of human recombinant IGF peptides and IGFBPs as clinical therapy for disease states is becoming a reality. (Reprod Med Biol 2003; 2: 1-24).

Pregnancy-associated plasma protein-A is involved in insulin-like growth factor binding protein-2 (IGFBP-2) proteolytic degradation in bovine and porcine preovulatory follicles: identification of cleavage site and characterization of IGFBP-2 degradation

In mammalian ovaries, terminal follicular growth is accompanied by a decrease in levels of intrafollicular insulin-like growth factor binding protein-2 (IGFBP-2) and IGFBP-4. The decrease in IGFBP-4 is essentially due to an increase in proteolytic degradation by intrafollicular pregnancy-associated plasma protein-A (PAPP-A) in growing healthy follicles. In contrast, the decrease in IGFBP-2 is partly due to a decrease in mRNA expression by follicular cells and also to an increase in IGFBP-2 proteolytic degradation, as previously shown in ewes and sows. In the present work we show that bovine and porcine preovulatory follicular fluid contains a proteolytic activity that degrades IGFBP-2. Bovine and porcine preovulatory follicular fluids contain undetectable levels of native IGFBP-2 as assessed by Western ligand blotting in comparison with the corresponding serum. In contrast, much higher levels of 23- and 12-kDa proteolytic fragments were found by immunoblotting in bovine and porcine preovulatory follicular fluid than in the corresponding serum. Moreover, bovine and porcine preovulatory follicular fluids were able to induce proteolytic degradation of exogenous IGFBP-2, and this degradation was enhanced by insulin-like growth factors. Intrafollicular IGFBP-2 proteolytic activity was surprisingly immunoneutralized in both species by a polyclonal antibody raised against human PAPP-A. In addition, recombinant human PAPP-A (rhPAPP-A) was able to cleave IGFBP-2 between Gln165 and Met166 in vitro, generating 23- and 12-kDa proteolytic fragments. IGFBP-2 was shown to be less sensitive than IGFBP-4 to cleavage by rhPAPP-A in vitro. As in follicular fluid, cleavage of IGFBP-2 by rhPAPP-A was dose-dependently enhanced by IGFs and inhibited by a peptide derived from the heparin-binding domain of IGFBP-5 (P5). Finally, Biacore analysis showed that P5 peptide-induced inhibition of IGFBP-2 cleavage was due to a direct interaction of P5 with PAPP-A rather than with IGFBP-2. Overall, these data show that in bovine and porcine preovulatory follicles, PAPP-A is responsible for IGF-dependent IGFBP-2 degradation. During follicular growth, the increase in IGFBP-2 cleavage by PAPP-A, as well as the decrease in IGFBP-2 expression, are responsible for the decrease in intact IGFBP-2 levels and the increase in IGF bioavailability. In atretic follicles, the increase and decrease in IGFBP-2 and PAPP-A mRNA expression, respectively, as well as the inhibition of PAPP-A activity by heparin-binding domains present in IGFBP-5 or other proteins, might participate in higher IGFBP-2 levels and a decrease in IGF bioavailability.

Regulation of the insulin-like growth factor axis by increasing cell number in intestinal epithelial (IEC-6) cells

Insulin-like growth factor binding protein-2 (IGFBP-2) production as a function of cell number by intestinal epithelial cells (IEC-6) was regulated such that the IGFBP-2 concentration in 24-h conditioned medium reached a maximum, which was maintained despite increasing cell number. Northern blot analysis revealed that this effect could largely be attributed to decreasing IGFBP-2 mRNA. In contrast to IGFBP-2, secretion of IGF-II and accumulation of IGF-II mRNA by IEC-6 cells correlated positively with cell number. The highest level of IGF-II protein detected by immunoblotting of conditioned medium occurred in post-confluent cells. IGF-I stimulated the cells to grow to a high cellular density and inhibited IGFBP-2 secretion in a concentration-dependent fashion. We conclude that expression of IGF-II and IGFBP-2 are regulated in IEC-6 cells by cellular density, and IGF-II may act as a survival factor at high cell density.

Insulin-like growth factor (IGF)-I, IGF-I receptor, and IGF binding protein-3 messenger ribonucleic acids and protein in corpora lutea from prostaglandin F(2alpha)-treated gilts

Insulin-like growth factor-I (IGF-I) is produced within the porcine corpus luteum (CL) and is thought to play an autocrine/paracrine role in CL development/function during the early luteal phase. This study examines the hypotheses that the luteolytic actions of prostaglandin F(2alpha) (PGF(2alpha)) during the early luteal phase may involve either a decrease in IGF-I or IGF receptor (IGF-IR), or an increase in IGF binding protein (IGFBP)-3, expression, any of which could interfere with the luteotropic actions of IGF-I in this tissue. Cycling gilts were treated twice daily with PGF(2alpha) (or saline) on Days 5-9 of the cycle to induce premature luteolysis. CL were collected on Days 6-9, and RNA, protein, or progesterone was extracted. By slot blot analysis, steady-state levels of IGF-I and IGFBP-3 mRNA were not different in PGF(2alpha)-treated vs. control animals; however, IGF-IR mRNA was increased in treated animals on Day 9. No changes in IGF-I content (ng/CL measured by RIA) were observed with respect to treatment. According to ligand blot analysis, the levels of IGFBP-3 increased on Day 6 and decreased on Days 8-9, while IGFBP-2 was higher on Days 6-7 and decreased on Day 9 in treated animals. IGF-IR levels, determined from Western blots, were higher on Day 7 (P < 0.05) and lower on Day 9 in PGF(2alpha)-treated animals vs. control animals (P < 0.05). In conclusion, PGF(2alpha)-induced premature luteolysis was associated with an increase in steady-state levels of IGF-IR mRNA, but it did not appear to be linked to changes in mRNA levels for IGF-I or IGFBP-3. However, since IGFBP-2 and -3 protein levels increased early in the treatment period (Days 6-7), it is possible that they may mediate the luteolytic actions of PGF(2alpha) by sequestering IGF-I and preventing its interaction with the IGF-IR.

Effect of insulin-like growth factor (IGF)-I and Des (1-3) IGF-I on the level of IGF binding protein-3 and IGF binding protein-3 mRNA in cultured porcine embryonic muscle cells

Insulin-like growth factor binding protein (IGFBP)-3 effects proliferation and differentiation of numerous cell types by binding to insulin-like growth factors (IGF) and attenuating their activity or by directly affecting cells in an IGF-independent manner. Consequently, IGFBPs produced by specific cells may affect their differentiation and proliferation. In this study we show that embryonic porcine myogenic cells, unlike murine muscle cell lines, produce significant quantities of a binding protein immunologically identified as IGFBP-3. Nonfusing cells subcultured from highly fused porcine myogenic cell cultures do not produce detectable IGFBP-3 protein or mRNA, thus suggesting the IGFBP-3 is produced by muscle cells in the porcine myogenic cell cultures. Treatment of porcine myogenic cultures with 20 ng of IGF-I or 20 ng of Des (1-3) IGF-I/ml serum-free media for 24 h results in a threefold reduction in the level of IGFBP-3 in conditioned media. This reduction is not affected by cell density over a sixfold range. Additionally, treatment for 24 h with 20 ng of IGF-I/ml media results in a sevenfold decrease in the steady-state level of IGFBP-3 mRNA. This IGF-I-induced decrease in IGFBP-3 mRNA level appears to be relatively unique to myogenic cells. IGF-I treatment also causes a fourfold increase in the steady-state level of myogenin mRNA. This increase in myogenin mRNA suggests that, as expected, IGF-I treatment accelerates differentiation of myogenic cells. The simultaneous decrease in IGFBP-3 mRNA and protein that accompanies IGF-I-induced myogenin expression suggests that differentiation of myogenic cells may be preceded or accompanied by decreased production of IGFBP-3.

Insulin-like growth factors and ovarian physiology

OBJECTIVE

To review the available information regarding the roles of insulin-like growth factor (IGF)-IGF binding protein (IGFBP) system in ovarian physiology.

DESIGN

Studies that specifically relate to the roles of ovarian folliculogenesis, oocyte maturation, and ovulation were identified through the literature and Medline searches.

RESULTS

Numerous actions of the IGFs have been demonstrated in the ovary, including an enhancement of cell proliferation, aromatase activity, and progesterone biosynthesis. The ovarian IGF system, comprised of IGF-I and IGF-II peptides, IGFBPs and IGF receptors, plays a significant role in the process of follicular development. In addition, IGF-I stimulates the meiotic maturation of follicle-enclosed oocytes in vitro via the IGF-I receptors. IGFBP-3 significantly inhibit gonadotropin-induced ovulation and oocyte maturation by neutralizing endogenously produced IGF-I. Thus, the intraovarian IGF-IGFBP system play a significant role in the processes of follicular development, oocyte maturation, and ovulation.

CONCLUSION

IGF-IGFBP systems have autocrine/paracrine regulatory actions in ovarian physiology. The disturbance of the IGF-IGFBP system in human ovaries may lead to an ovulation, disorders of androgen excess, and infertility.