Antibodies to phosphotyrosine injected in Xenopus laevis oocytes modulate maturation induced by insulin/IGF-I

Influence of Insulin-Like Growth Factor-I on Maturation and Fertilization Rate of Immature Oocyte and Embryo Development in NMRI Mouse with TCM199 and α-MEM Medium

INTRODUCTION

In vitro maturation (IVM) of oocytes and subsequent, in vitro fertilization (IVF) for the generation of embryos in the laboratory has important values. Growth factors are a component of a complex system of autocrine and paracrine factors that have a regulatory role in ovarian function and affect oocyte maturation. Therefore, the aim of this study is to evaluate the effect of IGF-I on IVM and IVF of mice oocytes during culture with α-MEM and TCM199 medium.

MATERIALS AND METHODS

Cumulus oocyte complexes (COCs) and denuded oocyte were obtained from 4-6 week old NMRI mice and underwent in vitro maturation and in vitro fertilization in presence or absence of IGF-I with α-MEM and TCM199.

RESULT

Maturation rate (79.6%), fertilization rate (87.2%), two cells development rate (79.5%) and blastocyst rate(43.2%) was higher in COCs cultured in α-MEM with IGF-I, while lower maturation rate (50.6%) fertilization rate (61%), two cells development rate (48.8%) and blastocyst rate(14.6%) were seen in cultured denuded oocytes (DOs) in TCM199 without growth factor. As well as, maturation fertilization, two cells development and blastocyst rates in COCs were higher than DOs.

CONCLUSION

Our findings have shown that IGF-I is involved in the oocyte biology and improve the oocyte maturation, fertilization and embryo development to blastocyst competence in vitro. In addition, it has also shown that cumulus cells are vital for oocyte development when IGF-1 added to the mediums.

IGF-I slightly improves nuclear maturation and cleavage rate of bovine oocytes exposed to acute heat shock in vitro

An in vitro model of embryo production was used to examine the effects of insulin-like growth factor (IGF)-I on maturation and developmental competence of oocytes exposed to heat shock. Cumulus–oocyte complexes were matured at 38.5°C or exposed to acute heat shock (HS; 41.5°C), with or without 100 ng/ml IGF-I, for 22 h through in vitro maturation. The experimental groups were control (C), C + IGF-I, HS, and HS + IGF-I. Oocytes were fertilized at the end of maturation, and the proportion of cleaved embryos was recorded 44 h later. HS during maturation increased the proportion of TUNEL-positive oocytes (P < 0.05). HS did not have any effect on cortical granule translocation but impaired resumption of meiosis, expressed as a decreased proportion of oocytes with nuclei in metaphase I (P < 0.05) and metaphase II (MII; P < 0.05). HS decreased the proportion of oocytes that cleaved (P < 0.05), in particular those oocytes that further developed to 4-cell-stage embryos (P < 0.05). IGF-I alleviated, to some extent, the deleterious effects of HS on the oocytes as reflected by a reduced proportion of TUNEL-positive oocytes (P < 0.03). While not significant, IGF-I tended to increase the proportion of MII-stage oocytes (P < 0.08) and 4-cell-stage cleaved embryos (P < 0.06). Further examination is required to explore whether IGF-I also affects the developmental competence of oocytes exposed to HS.

The effects of hCG and growth factors on in vitro nuclear maturation of dog oocytes obtained during anoestrus

The effects of human chorionic gonadotrophin (hCG) and a combination of growth factors on the developmental competence of canine oocytes during in vitro maturation was examined. Oocytes recovered from domestic dog ovaries at routine ovariectomy were cultured in a basic tissue culture medium with 0.3% BSA, 7 μg mL–1 progesterone and antibiotics. After the appropriate culture periods (up to 96 h), they were fixed and labelled by double-antibody immunofluorescence for tubulin and with propidium iodide for chromatin. Human chorionic gonadotrophin increased the proportion of oocytes resuming meiosis and reduced the degeneration rate. Supplementing with hCG in declining concentrations was of no superior benefit but the presence of a combination of growth factors (growth hormone, insulin-like growth factor-1, transforming growth factor-α and fibroblast growth factor) improved both the resumption of meiosis and the degeneration rate. No particular synergisms between pairs of growth factors could be demonstrated. Human chorionic gonadotrophin and growth factors together gave poorer results, implying that hCG inhibited the beneficial effects of the growth factors. A growth factor combination is the present most successful treatment, with 49% of total oocytes (inclusive of degenerated) recovered from anoestrous bitches at MI or MII by 96 h of culture. This is the highest result so far demonstrated for cultured dog oocytes.

In vitro effects of insulin-like growth factors and insulin on oocyte maturation and maturation-inducing steroid production in ovarian follicles of common carp, Cyprinus carpio

In vitro germinal vesicle breakdown (GVBD) in Cyprinus carpio oocytes was induced by recombinant human insulin-like growth factor-I and -II (IGF-I and IGF-II) and bovine insulin (b-insulin). Treatment of postvitellogenic ovarian follicles with IGF-I and b-insulin increased concentration of maturation-inducing hormone (MIH), 17alpha,20beta-dihydroxy-4-pregnane-3-one (DHP) in the medium. IGF-I and IGF-II both and b-insulin induced GVBD in denuded oocytes. IGF-I analogue R3 IGF-I was more potent than IGF-I in inducing GVBD of postvitellogenic follicles suggesting that ovarian IGF binding proteins may inhibit IGF-I action. Vitellogenic follicles, which were immature for oocytes to complete GVBD in response to DHP or HCG, underwent GVBD by IGF-I, not by b-insulin. IGF-I was also able to stimulate DHP production in such follicles. Addition of DHP and HCG to the culture of vitellogenic follicles containing IGF-I or b-insulin did neither potentiate the stimulation of GVBD induced by IGF-I nor initiate the same in response to b-insulin. Incubation of postvitellogenic follicles with trilostane (3beta-HSD inhibitor) had no inhibitory effects on IGF-I- and b-insulin-induced GVBD but attenuated the same under HCG stimulation. Trilostane, however, strongly inhibited DHP production induced by all these effectors. Induction of GVBD by IGF-I and b-insulin was not altered in the presence of actinomycin D. However, it significantly blocked the HCG-induced GVBD. Cycloheximide was shown to inhibit the induction of GVBD and DHP production by IGF-I, b-insulin and HCG. Both actinomycin D and cycloheximide were found to inhibit DHP production stimulated by all the three effectors. Collectively, these observations indicate that IGF-I and b-insulin can induce GVBD via MIH- and transcription-independent pathway. Incubation of the follicles with gap junction uncouplers, 1-heptanol or 1-octanol, had no effect on IGF-I- and b-insulin-induced GVBD, but attenuated the same induced by HCG. These uncouplers, however, inhibited DHP production induced by IGF-I, b-insulin and HCG. This result suggests that both IGF-I and b-insulin can induce oocyte maturation without coupled gap junction between oocytes and granulosa cells, while homologous gap junctions are required for DHP production. Inhibitors of phosphatidylinositol-3 kinase (PI-3 kinase), wortmannin and LY294002 inhibited GVBD by IGF-I and b-insulin. These two inhibitors also attenuated HCG-induced GVBD. These data suggest that PI-3 kinase activity is required for IGF-I, b-insulin and HCG induction of GVBD in C. carpio.

Regulation of Src kinase activity during Xenopus oocyte maturation

Expression of constitutively active Src protein tyrosine kinase in Xenopus oocytes has been shown to accelerate oocyte maturation suggesting that Src may be involved in meiotic progression. However, meiotic regulation of endogenous Src kinase in oocytes has not been investigated in detail. To address this problem, we measured the activity, expression level, and phosphorylation state of the endogenous Xenopus Src (xSrc) and overexpressed xSrc mutants in the process of progesterone-induced oocyte maturation. We found that the enzyme is first transiently activated in the plasma membrane-containing fraction of oocytes within 3 min of progesterone administration. This event represents one of the earliest responses of oocytes to the hormone and should be related to triggering some early signaling pathways of maturation. Thereafter, xSrc activity increases again at the time of germinal vesicle breakdown (GVBD) and remains elevated till the completion of maturation. This elevation of xSrc activity is associated with a 2-fold increase of xSrc protein content in the absence of change in its specific activity and xSrc mRNA content. No significant changes in the phosphorylation state of C-terminal regulatory phosphotyrosine can be registered either in endogenous xSrc or in overexpressed kinase-negative and wild-type xSrc proteins during maturation. Altogether, these results indicate that upregulation of xSrc in the meiotic metaphase occurs at the translation level. We also demonstrate here that the expression of constitutively active xSrc in Xenopus oocytes is accompanied by the activation of mitogen-activated protein kinase (MAPK). Our data suggest that the Src kinase acts through the MAPK pathway to accelerate oocyte maturation.

Metformin (Glucophage) inhibits tyrosine phosphatase activity to stimulate the insulin receptor tyrosine kinase

Metformin is a commonly used anti-diabetic but whether its mechanism involves action on the insulin receptor or on downstream events is still controversial. With a time course that was slow compared with insulin action, metformin increased tyrosine phosphorylation of the regulatory domain of the insulin receptor (specifically, tyrosine residues 1150 and 1151). In a direct action, therapeutic levels of metformin stimulated the tyrosine kinase activity of the soluble intracellular portion of the beta subunit of the human insulin receptor toward a substrate derived from the insulin receptor regulatory domain. However, metformin did not alter the order of substrate phosphorylation by the insulin receptor kinase. Using a Xenopus oocyte preparation, we simultaneously recorded tyrosine kinase and phosphatase activities that regulate the insulin receptor by measuring the tyrosine phosphorylation and dephosphorylation of peptides derived from the regulatory domain of the human insulin receptor. In an indirect stimulation of the insulin receptor, metformin inhibited endogenous tyrosine phosphatases and purified human protein tyrosine phosphatase 1B that dephosphorylate and inhibit the insulin receptor kinase. Thus, there was evidence that metformin acted directly upon the insulin receptor and indirectly through inhibition of tyrosine phosphatases.

Quantification and distribution of equine oocyte cortical granules during meiotic maturation and after activation

In vitro fertilization (IVF) is being routinely used in humans and several domestic species, however, limited success has been achieved in the horse. Although immature equine oocytes are capable of completing meiosis in vitro, subsequent fertilization, and embryonic development of those oocytes are questionable. The lack of development of these oocytes could be attributed to an impaired cytoplasmic maturation. In the horse, the study of oocyte cytoplasmic maturation and post-fertilization development has been hindered by the lack of progress in IVF. In mammalian oocytes, migration of cortical granules (CG) has been used as an important criterion to evaluate cytoplasmic maturation. The aim of this study was to describe and quantify the CG distribution of equine oocytes during in vitro meiotic maturation and to assess activation of oocytes with calcium ionophore based upon fluorescein isothiocyanate (FITC)-labeled Lens culinaris agglutinin (LCA) and laser confocal microscopy. The results of this study indicate that CG are distributed throughout the cytoplasm of oocytes at the germinal vesicle (GV) stage (immature). As maturation proceeds, a progressive centripetal migration of CG to the oocyte cortex occurs with the formation of a monolayer adjacent to the plasma membrane starting by the end of a 30 hr incubation period and increasing significantly after 36 hr. After activation, significant reduction in the number of CG was observed (P < 0.001) suggesting that oocytes cultured under the present conditions possess the ability to release CG in response to the elevation of intracellular free calcium.

Effect of vascular endothelial growth factor on maturation, fertilization and developmental competence of bovine oocytes

To examine the effect of Vascular Endothelial Growth Factor (VEGF) on the maturation of bovine oocytes, human recombinant VEGF(165) was used in 3 experiments. In Exp. 1, bovine cumulus oocyte complexes (COCs) were matured for 22 hr in modified Synthetic Oviduct Fluid (m-SOF) supplemented with 0 (control) or 5 ng/ml of VEGF. Maturation rate increased (P<0.05) from 78.2% in the control to 90.5% in the VEGF treated group. In Exp. 2, bovine COCs were matured in m-SOF and co-incubated with sperm in modified BO medium, each supplemented with or without 5 ng/ml VEGF. Normal fertilization rate was improved (P<0.05) from 63.0% (control) to 79.8% or 82.3% with VEGF during maturation or both maturation and fertilization. In Exp. 3, bovine COCs were matured the same way as in Exp. 1, then co-incubated with sperm for 6 hr and cultured for 162 hr in m-SOF without VEGF. Cleavage rate and development rate to the 4- to 8-cell stage were examined at 42 hr post-co-incubation and development rate to blastocyst was examined at 162 hr post-co-incubation. Cleavage, the development to the 4- to 8-cell stage and blastocyst rates (82.0%, 70.3% and 45.1%, respectively) were significantly higher (P<0.05) in the VEGF group than those in the control (67.3%, 52.5% and 33.3%, respectively). These results indicate that VEGF has a beneficial effect on the maturation of bovine oocytes.

Influence of insulin-like growth factor-I and its interaction with gonadotropins, estradiol, and fetal calf serum on in vitro maturation and parthenogenic development in equine oocytes

The effects of insulin-like growth factor-I (IGF-I) and its interaction with gonadotropins, estradiol, and fetal calf serum (FCS) on in vitro maturation (IVM) of equine oocytes were investigated in this study. We also examined the role of IGF-I in the presence or absence of gonadotropins, estradiol, and FCS in parthenogenic cleavage after oocyte activation with calcium ionophore combined with 6-dimethylaminopurine (6-DMAP), using cleavage rate as a measure of cytoplasmic maturation. Only equine cumulus-oocyte complexes with compact cumulus and homogenous ooplasm (n = 817) were used. In experiment 1, oocytes were cultured in TCM-199 supplemented with BSA, antibiotics, and IGF-I at 0 (control), 50, 100, 200 ng/ml, at 39 degrees C in air with 5% CO(2), 95% humidity for 36 or 48 h. In experiment 2, oocytes were cultured with FSH, LH, estradiol, and FCS with IGF-I at the concentration that promoted the highest nuclear maturation rate in experiment 1. In experiment 3, oocytes from the three experimental groups (IGF-I; hormones; and IGF-I + hormones) were chemically activated by exposure to calcium ionophore followed by culture in 6-DMAP. In experiment 1, IGF-I stimulated equine oocyte maturation in a dose-dependent manner with the highest nuclear maturation rate at a concentration of 200 ng/ml. No significant effect of IGF-I on nuclear maturation was observed in experiment 2. In experiment 3, a significant difference in cleavage rate was observed between the hormone + IGF-I group (15 of 33; 45.4%) compared with IGF-I (10 of 36; 27.8%) and hormone (4 of 31; 12.9%) alone (P < 0.05). These results demonstrated that IGF-I has a positive effect on nuclear maturation rate of equine oocytes in vitro. The addition of IGF-I to an IVM medium containing hormones and FCS did not increase nuclear maturation, but resulted in a positive effect on cytoplasmic maturation of equine oocytes measured by parthenogenic cleavage.

Gonadotropic control of ovarian follicle maturation: the two-stage concept and its mechanisms

Most research on the control of oocyte maturation by luteinizing hormone (LH) in teleosts and amphibians has focused on the production and action of maturation-inducing hormone (MIH), the follicular hormone that directly triggers the resumption of oocyte meiosis. However, current information indicates that LH regulates maturation in two stages, and that 'oocyte maturation' can be appropriately described within the broader context of 'ovarian follicle maturation'. During the first stage of maturation the follicle (somatic) cells acquire the ability to produce MIH and the oocyte to respond to MIH (i.e. oocyte maturational competence, OMC), whereas in the second stage the follicle cells produce MIH and, consequently, the oocyte is released from meiotic arrest. A number of factors such as insulin-like growth factor-I, serotonin, and others may mediate or modulate the OMC-inducing action of LH. Like the acquisition of MIH-producing ability, the acquisition of OMC requires activation of the protein kinase A pathway. Two major cellular events associated with OMC acquisition are increases in homologous and heterologous gap junction contacts and in oocyte MIH receptor activity. The increased oocyte MIH receptor activity is presumably associated with OMC acquisition, but the significance of changes in gap junction contacts is at present uncertain. To eliminate inconsistency and ambiguity associated with current terminology we propose that the term, ovarian follicle (or oocyte) maturation be used for teleosts without qualifiers such as 'final' to define the first and second stages of follicular maturation.

Nongenomic action of progesterone: activation of Xenopus oocyte phospholipase C through a plasma membrane-associated tyrosine kinase

Using a plasma membrane-cortex preparation (wherein the nucleus and >90% of the total cell protein are removed), progesterone stimulated tyrosine kinase activity that stimulated phospholipase C. Although it has been known for over 20 yr that progesterone acts at the plasma membrane of Xenopus oocytes to induce oocyte maturation, this is the first report that progesterone stimulates this tyrosine kinase activity that is associated with the oocyte plasma membrane and cortex. A tyrosine kinase inhibitor (tyrphostin B46) inhibited steroid stimulation of tyrosine kinase and phospholipase C (PLC) activities, but did not block lipase C stimulation by G protein activators. A fusion protein that contains tandem N- and C-terminal SH2 domains of PLCgamma also blocked progesterone stimulation of PLC (a fusion protein with the SH2 domain from Shc was ineffective). Lowering the Ca2+ concentration in the medium inhibited progesterone, but not guanosine 5'-O-(3-thiotriphosphate), stimulation of PLC, and the effects of progesterone and a G protein agonist were additive. However, neither progesterone nor insulin increased phosphotyrosine on PLCgamma. To evaluate another tyrosine kinase path involving phosphatidylinositol 3-kinase, we added wortmannin to our membrane preparation, but wortmannin did not inhibit progesterone's ability to activate PLC.

Effect of insulin-like growth factor I and its interaction with gonadotropins on in vitro maturation and embryonic development, cell proliferation, and biosynthetic activity of cumulus-oocyte complexes and granulosa cells in buffalo

In this study we have examined the effect of insulin like growth factor I (IGF-I) and its interaction with gonadotropins in the presence or absence of granulosa cell coculture on in vitro oocyte maturation (IVM) and their subsequent embryonic development in buffalo. We also have examined the role of IGF-I alone or in combination with gonadotropins on DNA synthesis, steroidogenesis, and protein synthesis of cumulus-oocytes complexes (COCs) and granulosa cells. Results showed that IGF-I stimulates oocytes maturation in a dose-dependent manner, with maximal effect at a dose of 100 ng/ml (P < 0.05). IGF-I showed positive interaction with follicle-stimulating hormone (FSH) in the presence or absence of granulosa cells on meiotic maturation and synergistically enhanced DNA synthesis, protein synthesis, and steroidogenesis in the presence of granulosa cells. This synergistic effect is mainly caused by the increase of IGF-I receptors in granulosa cells by FSH, as evident by [125I]IGF-I binding study. Luteinizing hormone (LH), however, was found to suppress IGF-I and IGF-I + FSH stimulated oocyte maturation. Addition of LH to cultures containing IGF-I + FSH, on the contrary, caused a significant increase in oocyte maturation when cocultured with granulosa cells. Addition of IGF-I during IVM significantly improve cleavage and blastocyst development rate over the control group. However, there was no cumulative effect when IGF-I and gonadotropins were present together. Addition of granulosa cells during IVM, however, enhanced blastocyst development in the IGF-I + FSH and IGF-I + FSH + LH groups. Our results demonstrated that IGF-I is a major follicular factor responsible for stimulating oocyte maturation in the buffalo. Interaction between IGF-I and FSH suggests that they seem to act synergistically as an autocrine and paracrine regulator of granulosa cells and therefore together promote mitosis, steroidogenesis, and protein synthesis.

Steroid-level response to insulin-like growth factor-1 in oocytes matured in vitro

The objective was to establish the influence of insulin-like growth factor-1 (IGF-1) on steroid production and nuclear maturation during oocyte in vitro maturation (IVM). Immature-selected rabbit follicular oocytes, divided as cumulus-oocyte complexes (COC) and denuded oocytes (DO), were cultured in Brackett's medium with different concentrations of IGF-1 at 0, 50, 100 and 200 ng/ml. After 8 and 16 h of culture, the oocytes were assessed for nuclear maturation by acetic-orcein stain, and media were analyzed by enzyme-immunoassay (EIA) for 17 beta-estradiol (E), progesterone (P), androstenedione (A) and testosterone (T) content. After culture treatments with IGF-1 significantly increased (P < 0.01) the incidence of nuclear activation (germinal vesicle breakdown stage, GVBD) and nuclear maturation (metaphase II stage); maximum stimulation occurred at 100 ng IGF-1/ml (86.9 vs. 49.3% in control). Compared to controls, the presence of IGF-1 in cultures was associated with a significant increase of E and A production by COCs (P < 0.01). However, P and T levels were not significantly influenced by the IGF-1. In addition, positive correlations between E/T and E/A ratios and nuclear maturation rates were only found in the IGF-1 treatments. Regarding the DOs, neither positive effects in nuclear maturation rates nor increase of steroid levels in culture were observed for any treatment. These results suggest that: (1) IGF-1 had a significant effect on E and A production during oocyte maturation; (2) the addition of IGF-1 enhanced nuclear maturation significantly in rabbit oocytes; and (3) all these effects are only possible in oocytes surrounded by cumulus cells.

Ovarian receptors for insulin and insulin-like growth factor I (IGF-I) and effects of IGF-I on steroid production by isolated follicular layers of the preovulatory coho salmon ovarian follicle

In this study, receptors for insulin and insulin-like growth factor I (IGF-I) in isolated theca-interstitial layers and granulosa cells of the coho salmon preovulatory ovary were characterized, and the effects of IGF-I on ovarian steroidogenesis were examined. Specific receptors for insulin and IGF-I were found in granulosa and theca-interstitial layers. In both follicular layers, IGF-I receptors were greater in number and higher in affinity than insulin receptors. The effects of IGF-I on in vitro production of testosterone (T) and 17 alpha-hydroxyprogesterone (17OH-P) by theca-interstitial layers and of 17 beta-estradiol (E2) and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) by granulosa cell layers were evaluated during the preovulatory period. Both human and salmon recombinant IGF-I inhibited the basal and GTH II-stimulated T and 17OH-P production by theca-interstitial layers throughout the preovulatory period. In contrast, IGF-I stimulated the production of both E2 and 17,20 beta-P by granulosa cell layers prior to germinal vesicle breakdown (GVBD) but only stimulated the production of 17,20 beta-P by granulosa cell layers after GVBD. The inhibitory effects of IGF-I on steroid production by the theca-interstitial layer and the opposite stimulatory effects on steroid production by the granulosa cell layer, coupled by the presence of specific IGF-I receptors in both follicular layers, suggest that IGF-I may play a role in the regulation of steroidogenesis in the preovulatory coho salmon ovary.

Localization and function of tyrosine-phosphorylated protein in pig oocytes

Several proteins with phosphorylated tyrosine residues have been shown to be closely involved in the control meiotic nuclear division. We identified a 42-kD protein in pig oocytes, using a polyclonal antibody to a synthetic phosphotyrosine construct that increases significantly in amount after 12 hr of maturation culture, and is discretely localized to condensing and condensed chromosomes. However, since microinjection of the antibody into oocytes blocks spindle formation, the role of this protein appears to be at that stage rather than directly in chromosome condensation. Specificity of action of the 42-kD protein indicates that it may be a phosphorylation-dependent component necessary for successful spindle assembly.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Insulinlike growth factors and binding proteins in colon cancer

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

Insulin and IGF-I receptors and tyrosine kinase activity in carp ovaries: changes with reproductive cycle

Insulin and insulin-like growth factor I (IGF-I) receptors from carp ovaries were semipurified with wheat germ agglutinin at different moments of the reproductive cycle and their binding characteristics and tyrosine kinase activity were studied. Specific receptors for insulin and IGF-I were found. IGF-I receptors presented higher binding (23.8 ± 1.5%), number of receptors (965 ± 20fm/mg) and affinity (KD 0.24 ± 0.03nM) than those shown for insulin receptors (4.1 ± 1%, 530 ± 85fm/mg and 0.85 ± 0.1nM, respectively). Insulin and IGF-I receptors have a tyrosine kinase activity which is not different from that found in muscle of the same species. Seasonal changes were found in binding, with maximum values for insulin and IGF-I reached at the end of pre-spawning period (June). However, while IGF-I binding was observed in all stages, insulin binding decreased in autumn and disappeared in winter, which suggests a different role for the two peptides in ovarian physiology.

Insulin receptor substrate 1 mediates insulin and insulin-like growth factor I-stimulated maturation of Xenopus oocytes

Insulin and insulin-like growth factor I (IGF-I) initiate cellular functions by activating their homologous tyrosine kinase receptors. In most mammalian cell types, this results in rapid tyrosine phosphorylation of a high-molecular-weight substrate termed insulin receptor substrate 1 (IRS-1). Previous studies suggest that IRS-1 may act as a "docking" protein that noncovalently associates with certain signal-transducing molecules containing src homology 2 domains; however, direct evidence for the role of IRS-1 in the final biological actions of these hormones is still lacking. We have developed a reconstitution system to study the role of IRS-1 in insulin and IGF-I signaling, taking advantage of the fact that Xenopus oocytes possess endogenous IGF-I receptors but have little or no IRS-1, as determined by immunoblotting with anti-IRS-1 and antiphosphotyrosine antibodies. After microinjection of IRS-1 protein produced in a baculovirus expression system, tyrosyl phosphorylation of injected IRS-1 is stimulated by both insulin and IGF-I in a concentration-dependent manner, with IGF-I more potent than insulin. Furthermore, after IRS-1 injection, both hormones induce a maturation response that correlates well with the amount of injected IRS-1. By contrast, overexpression of human insulin receptors in the Xenopus oocytes does not enhance either IRS-1 phosphorylation or oocyte maturation response upon insulin stimulation. These results demonstrate that IRS-1 serves a critical role in linking IGF-I and insulin to their final cellular responses.