Thermoprotective molecules: Effect of insulin-like growth factor type I (IGF-1) in cattle oocytes exposed to high temperatures

Abstract

The adverse effects of heat stress (HS) on the welfare and productivity of cattle are the result of the associated hyperthermia and the physiological and behavioral mechanisms performed by the animal to regulate body temperature. The negative effects of HS on in vitro oocyte maturation and in vitro bovine embryo production have been reported; being one of the major concerns due to economic and productive losses, and several mechanisms have been implemented to reduce its impact. These mechanisms include supplementation of the medium with hormones, adjuvants, identification of protective genes, among others. This review aims to explore the cellular and molecular mechanisms of insulin-like growth factor-1 (IGF-1) during in vitro and in vivo maturation of bovine oocytes and its thermoprotective effect under HS. Although the supplementation of the culture medium during oocyte maturation with IGF-1 has been implemented during the last years, there are still controversial results, however, supplementation with low concentration showed a positive effect on maturation and thermoprotection of oocytes exposed to higher temperatures. Additionally, IGF-1 is involved in multiple cellular pathways, and it may regulate cell apoptosis in cases of HS and protect oocyte competence under in vitro conditions.

1. Introduction

Heat stress (HS) is defined as a physiological and behavioral response caused by the sum of internal (production) and external (environment) conditions that act on an animal [1,2], leading to an increase in core body temperature [3]. Main environmental factors that influence body temperature in livestock include air temperature, relative humidity, solar radiation, atmospheric pressure, and wind speed [4]. This increase in the body temperature, which exceeds the body's normal physiological range is known as hyperthermia [5,6], and occurs when body physiological mechanisms as increase in respiratory rate, sweating, and peripheral blood flow [7], cannot regulate the temperature to homeostatic levels [8].

The adverse effects of HS on bovine welfare and productivity depend on the physiological and behavioral mechanisms performed by the animal to regulate body temperature [5]. However, some physiological mechanisms such as estrus are compromised under high temperature conditions [9], leading to a decrease in pregnancy rates between 20% and 30% [10], together with an increase of the number of abortions and inflammation of the uterine mucosa [1]. In the same way, HS decreases the levels of luteinizing hormone (LH) and estradiol (E₂) produced by the ovary, impairing the growth, maturation, and function of dominant and pre-ovulatory follicles leading to ovarian inactivity [11,12].

In pregnant cattle, maternal hyperthermia increases morbidity, mortality and early embryonic death [6]. Similarly, it has been associated with a decrease in fetal and placental weight [13], as well as a direct negative effect on immune status, fertility and ovarian reserve of the progeny evidencing female offspring with lower anti-Mullerian hormone (AMH) concentration [9,14]. On the other hand, HS can also affect male fertility through the increase of reactive oxygen species (ROS) on spermatogenesis, interrupting the production of antioxidants that protect sperm from oxidative damage [8,15], compromising sperm motility and oocyte fertilization [16].

Additionally, HS has negative effects on the cellular and molecular dynamics that occur during in vitro oocyte maturation (IVM) and in vitro embryo production [17]. In particular, the maturing oocyte is highly sensitive to HS, which causes late activation of the transcriptional response [18] and high concentrations of prostaglandins E and F2α, which in turn modify the expression of about 637 genes related to reproductive function [19]. Regarding to this, in vitro reproductive techniques, which improve oocyte/embryo maturation, development, quality, and viability can be performed to reduce deleterious effect of HS [20]. They include the supplementation with adjuvants, such as growth factors, during the processes of IVM and in vitro production of bovine embryos to improve pregnancy rates, survival, and reduction of apoptosis [21].

Among growth factors, insulin-like growth factor type 1 (IGF-1) have been widely used, due to its role in follicular, embryonic and implantation development [22], playing an important function in bovine reproduction [23,24]. Supplementation with IGF-1 alone or in combination with other growth factors during IVM has shown an increase in the expansion of cell clusters [23,25,26], promoting steroid synthesis in granulosa and theca cells [[27], [28], [29]], reduction in oocyte apoptosis rates [30], and improve of embryo thermotolerance [31,32]. This review aims to explore the cellular and molecular mechanisms of IGF-1 during bovine oocyte IVM, and its thermoprotective effect under elevated temperatures.

2. Thermoprotective molecules

Thermoprotective molecules, such as antioxidants and growth factors, act by reducing cellular damage in bovine oocytes induced by high temperature, restoring mitochondrial activity [18,33], improving the response to oxidative stress in oocytes and granulosa cells, reducing the presence of saturated fatty acids achieving cell membrane stabilization [12], and inhibiting the caspase-mediated apoptosis [34].

Astaxanthin (ASTX), a fat-soluble carotenoid found in fishery products [35,36], is an antioxidant and thermoprotective molecule with a protective effect demonstrated in bovine [[37], [38], [39]] pig [40] and rodent [41] oocytes. ASTX promotes the transcription of antioxidant genes, inhibiting of lipid peroxidation, regulating of the mitochondrial membrane potential, leading to a reduction of DNA damage by ROS and the inhibition of apoptotic genes [42,43] (Table 1). In addition, ASTX has been shown to induce the expression of superoxide dismutase (CuZnSOD), Manganese superoxide dismutase (MnSOD) and catalase (CAT) genes and reduce the expression of apoptosis-related genes such as B-cell lymphoma 2 (BCL-2), caspase-3 (CASP3) and BCL2-associated X protein (BAX) in oocytes and embryos in vitro, subjected to oxidative stress by nitric oxide and sodium nitroprusside [38]. Furthermore, Ispada et al. [36] showed that the addition of 12,5 and 25 nM of ASTX recovered the competition of oocytes after HS for 14 h at 41 °C, counteracting the production of ROS.

Table 1.

Main thermoprotective molecules used in bovine oocytes culture.

Molecule	Effect on heat-stress	References
Astaxanthin	Increases the expression of superoxide dismutase two (SOD2) and nuclear erythroid factor 2-related factor 2 (NRF2), thereby enhancing lipid peroxidation, mitochondrial activity and lysosomal function in oocytes and embryos during ROS.	[42,43]
Resveratrol	It is a specific activator of SIRT1 that mediates cell survival and mitochondrial biogenesis through FOXO, and by reducing the expression of BAX, BAK, especially the Caspase-3/BAX pathway and increasing the synthesis of antioxidant genes.	[18,25]
Derivatives of vitamin A	Neutralizes the production of ROS at the intra-oocyte level by increasing the synthesis of CAT, Glutathione peroxidase 4 (GPX4) and superoxide dismutase 1 (SOD1) and promotes oocyte cytoplasmic maturation by regulating gonadotropin receptors.	[57,61,62]
Coenzyme Q10	Reduces the concentration of ROS and prevents oocyte apoptosis, increasing mitochondrial function by increasing ATP production, which improves the membrane potential of the mitochondria and the development capacity of the oocytes.	[64]
Cysteine	Increases CC expansion, maintains cellular redox homeostasis through the direct use of its sulfhydryl group, stimulates GSH synthesis and inhibits ROS production, and regulates intracellular MAPK signaling pathways responsible for cell proliferation, differentiation and apoptosis.	[18,68]
Insulin-like growth factor type I	Inhibits apoptosis using the phosphatidylinositol 3-kinase (PI3K/AKT) and MAPK3 pathway leading to activation of AKT by PI3K which reduces ROS generation at the mitochondrial level in oocytes subjected to HS.	[94,97]

Another protective molecule widely studied is Resveratrol (3,4,5-trihydroxy-trans-stilbene), a polyphenolic stress response molecule produced by plants such as red grapes and blackberries [44,45], with antioxidant activity and the ability to control the expression of genes related to DNA synthesis, cell cycle, proliferation, stress response and apoptosis [44]. Antioxidant activity is based on interaction with superoxide ions and peroxide radicals of lipids within the cell membrane [46], elimination of iron ions during the formation of hydroxyl radicals [47] and decreasing of ROS inside the oocyte by increasing the concentration of glutathione peroxidase, and keeping superoxide dismutase and catalase levels [18,[48], [49], [50]]. Supplementation of culture media with Resveratrol has shown an increase in mitochondrial function during oxidative stress by the up regulation of the gene Sirtuin 1 (SIRT1) [49], which forms a complex with the transcription factor (FOXO) increasing the transcription of antioxidant enzymes [51,52], improving oocyte competence and subsequent development in various species [18] as rodents [53] cattle [54], goats [55], and pigs [49,56]. Additionally, it has been observed that Resveratrol as dietary supply improves growth performance and reduces ROS in organs of the immune system undergoing HS by increasing serum growth hormone (GH) concentrations and modulating the expression of heat shock genes [53].

Vitamin A and its natural derivatives such as retinol have also been considered as thermoprotective molecules, since they act as cell growth regulators and support redox balance [57,58]. Retinol can have favorable or detrimental effects on follicular development, oocyte maturation and early embryonic development depending on the supplemented concentration [59,60]. Livingston et al. [58] suggest that administration of retinol during IVM improves oocyte competition. This was supported by Duque et al. [61] who found that retinoids act through cumulus and granulosa cells promoting cytoplasmic maturation, in oocytes exposed to HS during IVM. Abdelnour et al. [62] suggest that the addition of up to 50 nM of retinoic acid, a metabolite of vitamin A, has the potential to improve the IVM rates of oocytes in several mammalian species including bovines due to its antioxidant properties and the increase in the expression of anti-apoptotic genes such as BCL-2 and the reduction in the expression of the proapoptotic gene CASP8, especially in cumulus cells.

Coenzyme Q10 (CoQ10), another thermoprotective molecule, is an essential electron transporter in the mitochondrial electron transport chain (ETC) [63] This has been reported to enhance ATP production and restore the developmental capacity of oocytes based on their ability to prevent apoptosis in somatic cells [64]. CoQ10 suppresses aging-induced oxidative stress by reducing superoxide levels, DNA damage, and inhibiting apoptosis [65]. Abdulhasan et al. [66] suggested that CoQ10 improves mitochondrial function in vitro in bovine oocytes and embryos, where supplementation of the medium with 40 μM CoQ10 increased the percentage of oocytes in Metaphase II and decreased the cell death by 4,3 times. Ruiz-Conca et al. [67] obtained similar results when they test the addition of CoQ10 during IVM of oocytes, finding that it improved survival rates and reduced premature exocytosis of cortical granules, thus reducing the negative effects of vitrification.

Some growth factors have been demonstrated to induce a higher tolerance to heating in addition to promote and improve oocyte maturation and embryonic development in many mammalian species; these include cysteine (Cys) and IGF-1 [68,69]. Cys is a low molecular weight amino acid that activates glutathione (GSH) synthesis, thus inhibiting ROS production [18,68], decreasing the effects of HS on oocyte viability and development [69].

The IGF-1 is a small peptide of 70 amino acids with a molecular weight of 7649 kDa and its structure is like insulin with a 50% of sequence homology [70]. IGF-1 is actively involved in the regulation of follicular development, oocyte maturation, fertilization rate, early embryonic development, luteal body function, maternal recognition of gestation [71], and it is an indicator of optimal body condition and reproductive fitness [72]. IGF-1 reduces DNA fragmentation of the oocytes on the IVM or exposed to HS [32], suggesting a thermoprotective effect in oocyte and embryonic development in early stages under HS conditions [73].

3. Insulin-like growth factor I (IGF-1)

IGF-1 has been detected in several species of domestic animals [71,[74], [75], [76]], wild animals [77], and humans [78]. IGF-1 is a member of the family of insulin growth factors (IGFs), which stimulate cell growth, proliferation, and differentiation [[79], [80], [81]], and its structure and mitogenic activity is similar to insulin [82]. This family is made up of two insulin-like growth factors (IGF-1 and IGF-2), six binding or transporter proteins (IGFBPs), and two receptors, IGF-1R and IGF-2R [83]. The IGF-1 and IGF-2 mediate their effects through the IGF-1R, while the IGFBPs modulate the interaction between them [84].

IGF-1 is composed of two extracellular α chains and two β transmembrane subunits, belonging to the receptor tyrosine kinase superfamily [85]. It is secreted into the bloodstream as an endocrine hormone to a greater extent by the liver under GH control [79], but also locally by the oviduct [86,87] and bovine endometrium [31,84], acting in a paracrine and autocrine signaling [88]. Secreted IGF-1 binds to the IGF-1R and causing its autophosphorylation, triggering the formation of transcription complexes, leading to the modification of the activity of chromatin repair proteins, participating in the mechanisms of tolerance to DNA damage [70].

Some of the main functions of IGF-1 include ovarian follicular development, oogenesis, oocyte maturation, ovulation, luteal function, follicular atresia, and testicular function [89]. In the ovary, IGF-1 regulates the action of follicle stimulating hormone (FSH) in the granulosa cells of the antral follicles [74] and showed a synergistic action with LH, increasing androgen biosynthesis in theca cells [89]. In oocytes, both cluster cells and oocytes express mRNA for IGF-1 and IGF-2, their IGF-1R and IGF-2R receptors, and the IGFBP-2 and IGFBP-4 [73]. IGFs act autocrine or paracrine in granulosa and cumulus cells, thus regulating the mechanisms of proliferation, differentiation, and even steroidogenesis in the oocyte itself [25,90,91].

3.1. Regulation

IGF-1 is synthesized by several types of mesenchymal cells [92], and two mechanisms are involved in its production: First, hepatic synthesis by hepatocytes via pituitary-derived GH and activation of the GH receptor, which increases transcription of the IGF-1 gene, IGFBPs and an acid labile subunit (ALS), and its subsequent release into the bloodstream [93,94], establishing a negative feedback mechanism [95] in which IGF-1 inhibits GH secretion by acting on the hypothalamus and GH in conjunction with its receptor stimulates IGF-1 synthesis [96].

Second mechanism involves synthesis in peripheral tissues such as the oviduct and endometrium, and then passes into the systemic circulation [79]; in this case, synthesis is controlled by GH and sex steroids (e.g., FSH and E2) that are secreted locally by surrounding cells [97,98]. IGF-1 acts synergistically with gonadotropins modulating fundamental cellular functions in the maturation and regulation of granulosa and theca cells, including mitogenesis, cell survival, and steroidogenesis [97]. IGF-1 and GH promotes steroidogenesis in granulosa and theca cells and may also influence luteal function indirectly by increasing local expression of IGF-1 [27].

The availability, transport and function of IGF-1 is modulated by the interaction between the transmembrane IGF-1R receptors [23,99], and the conformation of binary or ternary complexes together with a family of six binding proteins (IGFBP-1 to -6) and ALS [79,82,100]. In the case of ternaries complexes constituted from the IGFBP-3 protein (protein with 95% affinity for circulating IGF-1) and ALS, they increase the half-life of IGF-1 around 15–20 h in the vascular compartments, when the complex dissociates by a protease, the IGF-1 is released, and it reaches the target tissues supported by others IGFBPs [95].

3.2. Signaling pathways

IGF-1R, is a heterotetramer receptor composed by two covalently linked polypeptide chains [101]. IGF-1 interacts with the α and β subunits of the receptor, causing conformational changes and leading to IGF-1R autophosphorylation at tyrosine residues [102]. Once the receptor is activated, phosphotyrosines are transformed into docking sites for adapter proteins of the family of insulin receptor substrates (IRS-1 and IRS-2), proto-oncogene tyrosine-protein kinase Src (Src) and the adapter protein SHC (SHC) [101,103,104], leading to the activation of signaling pathways such as Phosphatidylinositol-4,5-Bisphosphate 3-Kinase (PI3K) and Mitogen-Activated Protein Kinase (MAPK) pathways [94].

One of the main downstream PI3K products mediated by phosphorylation of membrane phospholipids is the protein phosphatidylinositol-3,4,5-triphosphate (PIP3), a signal pathway for cell growth and inhibition of cell apoptosis [105]. PIP3 binds to the PH domains of the PIP3-dependent kinase activating the activation of phosphoinositide-dependent protein kinase (PDK) [106], which in turn attracts protein kinase B (PKB or AKT) to the plasma membrane where AKT is phosphorylated at the Threonine 308 (Thr308) and Serine 473 (Ser473) residues [103,107,108].

AKT (PKB) is a Ser/Thr-specific protein kinase that regulates multiple biological processes including glucose metabolism, apoptosis, gene expression, and cell proliferation [107]. Stimulation of the AKT/PKB signaling cascade via IGF/IRS/PI3K activates several anabolic targets downstream including mTOR [109], in addition, inhibits catabolic pathways that include glycogen synthase kinase 3β (GSK3β) and FOXO and their atrophic genes atrogin-1 and MuRF1 [110]. Activation of the PI3K signaling pathway is associated with glucose transport and glycogen synthesis [103], participating in different cellular processes mediated by IGF-1 such as cell cycle, proliferation, differentiation, and cell survival [94,104,[111], [112], [113]].

The second activation pathway is the phosphorylation of SHC, leading to the activation of RAS-MAP [92] IGF-1R activation triggers the RAS-RAF-MEK-ERK-MAPK signaling cascade by a series of IRS-dependent protein phosphorylation and complex formations [111]. MAPK activation is necessary to promote steroidogenesis and steroidogenic gene expression in granulosa cells [ 114, 115], activation is a key regulator of cellular energy homeostasis, reducing progesterone secretion by inhibiting the MAPK ERK1/2 signaling pathway in bovine granulosa cells [114,117]. Steroidogenic events are mediated by aromatase expression in granulosa cells, and depends on AMPc response element binding protein (CREB) and StAR expression in all cell types [118]. The union of CREB with cAMP response elements causes the transcription of several genes including steroidogenic enzymes, cholesterol transport and the precursor substrate of the synthesis of sex steroids [97]. StAR allows cholesterol to enter the mitochondria where it can be converted to pregnenolone (PREG), and subsequently to testosterone/androgens, estrogens, and progesterone [115].

IGF-IR binds to the SH2 domain of growth factor receptor-bound protein 2 (Grb2), which, in turn, complexes with a guanine nucleotide exchange factor (GEF), which mediates GDP/GTP exchange in Ras GTPase, and leads to activation of Ras [116]. Activated Ras stimulates RAF kinase to phosphorylate and activate MAPK, which in turn activates extracellular signal-regulated kinase 1/2 (ERK1/2) through serine/threonine phosphorylation of the latter proteins [117]. Activated ERK1/2 enters the nucleus to phosphorylate and activate various transcription factors, leading to increased expression of cyclin D1 and reduced expression of P21 Cip and P27 [119,120]. This suggests that the RAS-ERK1/2 signaling pathway is related to the mitogen induced signaling of P27, which acts as a primary negative regulator of cell proliferation [121].

3.3. Heat stress and thermoprotection in cattle oocytes

The alteration caused by HS in the components of the female reproductive system (i.e. ovarian follicles, granulosa cells and oocytes) affects the regulation of the MAPK cascade and cellular redox homeostasis by the accumulation of intracellular ROS, leading to an inhibition of cell growth, impairment of ovarian follicle development, reduction of ovarian steroidogenesis, cholesterol transport, and levels of GSH, finally inducing apoptosis [7,8,12,[122], [123], [124], [125]]. In vitro exposure of bovine oocytes to severe HS reduces their ability to develop to the blastocyst stage once they have been fertilized [7,32,126].

Although the cellular mechanisms triggered by HS are not yet fully elucidated, exposure of bovine oocytes to HS during IVM may affect the organization of the cytoskeleton [127], causing alterations in the microtubular structure and the microfilaments [128,129], altering the separation of chromosomes during fertilization and cleavage [127], decreasing the proportion of oocytes that reach the MII stage after and increasing apoptosis [126]. On the other hand, HS blocks the nuclear maturation of oocytes [128], leading to a decrease in the rate of polar bodies, stimulating apoptosis in the surrounding cluster cells and decreasing the maturation rates of the oocytes during the degradation stage of the germinal vesicle [75,126], generating the impairment of the development capacity of the oocytes both in vivo and in vitro environments [5,11].

Additionally, HS also decreases the expression of member 1 of subfamily A of family 11 of cytochrome P450 (CYP11A1) and the steroidogenic acute regulatory protein (StAR) responsible for the transport of cholesterol through mitochondrial membranes. HS also reduces StAR promoter activity [128,130], producing a decrease in the secretion of E₂ and P₄ [[131], [132], [133]] which increases the accumulation of ROS in the mitochondria [134]. HS increases the gene upregulation of apoptotic regulator such as BCL-2, BAX, heat shock proteins (HSP), and apoptotic genes such as CASP-3 [135], apoptosis-related cysteine peptidase (CP), and CASP-6, which disturb the transmembrane potential of the mitochondria, resulting in the release of cytochrome C that leads to the induction of apoptosis [123].

Under stress, cells can activate FOXO3 and the Kelch-like ECH-associated protein 1 (KEAP1), which positively regulate antioxidant genes such as the enzymes SOD1 or SOD2, glutathione-disulfide reductase (GSR) and glutathione S-transferase, alpha 3 (GSTA3) and CAT to protect cells against oxidative stress [123]. Another mechanism is the use of HSPs as molecular chaperones, which are responsible for the synthesis, folding, assembly and transport of stress-denatured proteins [136] and in some cases their induction mediated through the MAPK pathway, generate CASP-3 inhibition and BAX activation [129].

Some studies suggest IGF-1 as a survival factor for oocytes and embryos with previous exposure to HS or high temperatures [18,137,138]. Both IGF-1 and its receptors are expressed in ovarian follicles and oocytes [32], where the cumulus cells may play a regulating role [97].

Considering the expression of the IGF-1 in different reproductive tissues, there are several mechanisms of this molecule to blocks the effects of HS [137]. One of this process, begins when the IGFBPs binding proteins who modulate the bioavailability of IGF-1, transport IGF-1 to the cell membrane, allowing the interaction with the IGF-1R, leading to a conformational change in the receptor by an autophosphorylation (Fig. 1), giving rise to docking sites for adapter proteins IRS-1, IRS-2 and SHC [97]. The interaction between IRS-1 and IRS-2 with IGF-1R induces the activation of PI3K, then PI3K transforms PIP2 into PIP3 and PDK1 activates the AKT family of kinases resulting in phosphorylation of Thr308 and Ser473 respectively [107,108].

Fig. 1.

Thermoprotective effect of insulin-like growth factor 1 in bovine oocyte exposed to heat stress. Inhibition of IGF-1-mediated cellular apoptosis produced by activation of phosphatidylinositol 3-kinase/protein kinase A (PI3K/AKT) pathways, triggering to the protein phosphorylation that regulates transcription and activation of cell survival related proteins. PI3K signaling activates AKT, which is related downstream to the positive regulation of anti-apoptotic proteins of the BCL-2 family, regulating the phosphorylation and inactivation of pro-apoptotic molecules (Bax, Bad and BAK) responsible for promoting the exit of cytochrome c (Cyt C) during alterations in the mitochondrial membrane caused by stress (reduction of membrane potential, increase in ROS and H₂O₂), thus preventing the activation of the apoptogenic factor and subsequent formation of the apoptosome (complex between protease activating factor (Apaf-1), caspase 9, and free Cyt C in the cytosol), which subsequently lead to the activation of the caspase pathway starting with procaspase 3 and caspase-3, which in turn drives Caspase-activated DNase (CAD) to the nucleus causing DNA fragmentation. IGF-1 regulates the levels of the BCL-2 family proteins, stabilizing the functionality of the mitochondrial membrane, preventing the continuation of apoptosis in the oocyte.

AKT regulates downstream signaling molecules by activating the BCL-2 family of anti-apoptotic proteins and initiating the cell survival pathway [96,112]. Within the PI3K/AKT pathway, transcription, and activation of the BCL-2 promoter by IGF-1 is initiated by phosphorylation of the cAMP response element binding and up regulation of the protein transcription factor (CREB) through the p38 stress-activated protein kinase [139,140]. The PI3K/AKT signaling pathway can negatively regulate proapoptotic gene expression by phosphorylation and inactivation of AKT to CASP-3, CASP-9, and BAD [96,140].

Underlying, IGF-1 increases the potential of the mitochondrial membrane by regulating the expression of the Cytochrome C Oxidase Subunit 1 (COX-1) gene, which in turn acts indirectly on the metabolism and activity of the respiratory chain in oocytes [138]. Additionally, they prevent cytochrome C release and reduce ROS production [94] which in turn reduces apoptosis in endothelial cells [141,142].

Despite the effects of IGF-1 during IVM of bovine oocytes under HS remain controversial, Lima et al. [75] determined the direct effects of various concentrations of IGF-1 (0, 12,5, 25, 50 and 100 ng/mL) in the germinal vesicle of oocytes cultured at normal temperature and at HS (38.5 °C and 41 °C respectively), for 14 h, finding that IGF-1 concentrations of 12.5 and 25 ng/mL tended to minimize the negative effects of HS such as decreased mitochondrial activity and developmental competence, which were determined by measuring of cleavage rate, the percentage of TUNEL-positive blastomeres and the total number of blastocyst cells. IGF-1 can stimulate a cascade of events, including protein synthesis, that generates positive regulation and promotes reactivation of meiosis in oocytes undergoing HS, thereby promoting germinal cells proliferation and differentiation [143].

Rodrigues et al. [73] reported that the addition of 100 ng/mL of IGF-1 partially minimizes cell damage induced by heat shock in bovine oocytes, however, a low concentration of IGF-1 (25 ng/mL) had a thermoprotective effect on the development capacity of the oocytes, which suggests that the beneficial effects of IGF-1 against HS depend on the concentration added to the medium and the ability of IGF-1 to activate the PI3K/AKT pathway which is critical for cellular metabolism and regulation of apoptosis especially under stress conditions, where oocyte mitochondrial function and integrity are involved [94].

Meiyu et al. [32] examined the effects of 100 ng/mL of IGF-1 on the IVM of oocytes exposed to HS (41.5 °C) for 22 h, although the results were not significant, the addition of IGF-1 increased to some extent the proportion of stage MII oocytes and partially reduced the deleterious effects of HS in terms of DNA fragmentation. In addition, Wasielak and Bogacki [30]. demonstrated that 100 ng/mL of IGF-1 served as an antiapoptotic factor during IVM of oocytes at 39 °C, inhibiting apoptosis in oocytes in the caspase activation stage compared to control treatment (oocytes without IGF-1 supplementation).

3.3.1. Final comments and prospects

With the gradual increase in temperature due to global warming, the effects of HS on gametes, mainly oocytes, have aroused great interest from scientific community and the pursue of additives and thermoprotective molecules that give the ability of oocytes to compete during IVM have increased in recent years, representing a great challenge for the reproduction of cattle, especially the female bovine and its gametes. IGF-1 modulates multiple cellular pathways that can reduce the cellular apoptosis caused by HS and protect the competition of oocytes under in vitro conditions, becoming a good candidate for culture supplementation. Nevertheless, the controversial results suggest that more conclusive studies are necessary to dilucidate the protective role of this growth factor on oocyte cell survival.

Author contribution statement

All authors listed have significantly contributed to the development and the writing of this article.

Funding statement

This work was supported by Sistema General de Regalías de Colombia [BPIN 2016000100026].

Data availability statement

No data was used for the research described in the article.

Declaration of interest's statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.