Growth hormone in fertility and infertility: Mechanisms of action and clinical applications.

In humans, the locus of the growth hormone ( GH ) gene is located in the long arm of chromosome 17, encompasses approximately 50 kb and harbors five genes: GH normal ( GH-N or GH1 ), GH variant ( GH-V or GH2 ), chorionic somatomammotropin hormone 1 ( CSH1 ), chorionic somatomammotropin hormone 2 ( CSH2 ), and chorionic somatomammotropin hormone-like 1 ( CSL ) ( 1 ). GH is an essential hormone for optimal female reproduction, as decreased fertility has been reported in women with GH deficiency. Additionally, GH replacement can enable successful pregnancy in these infertile women ( 2 , 3 ). Targeted depletion of GH receptor ( GHR ) in mice resulted in a decrease in various fertility parameters similarly seen in GH- deficient women ( 4 ). In the agriculture industry, injection with GH associated with ovulation induction or at the time of insemination enhanced the pregnancy rate in cattle ( 5 , 6 ). In the past three decades, the functional role of GH in regulating female reproduction has attracted increasing attention and has become a focus of reproductive and endocrine research. Several publications have revealed that locally produced GH and its downstream signaling are implicated in the function of mammalian ovaries and that they participate in the regulation of various follicular functions ( 7 ). In clinical practice, GH has widely been applied as an adjuvant reagent to promote various clinical outcomes in different scenarios during assisted reproductive technology (ART), aiming to increase collected oocyte numbers, improve oocyte/embryo quality, and improve pregnancy and live birth rates ( 8 – 10 ). A comprehensive understanding of the expression, actions and underlying molecular mechanisms of the GH/GHR system in the human ovary is critical to the development of diagnostic and/or therapeutic strategies for women suffering from infertility and ovulation disorders. In this review, we also summarize the scientific literature related to the clinical applications of recombinant human GH in ART.

Compared to the improvement of ovarian/follicular functions, less emphasis was paid to the role of GH in the implantation site. Emerging evidence provided by recent studies indicates that GH and its functional receptors are also expressed in the endometrium, and this locally produced hormone plays a critical role in regulating endometrial receptivity and embryo implantation. During the implantation window, the endometrial tissue undergoes structural and functional changes through an integrated interaction between various endometrial cell types modulated by many growth factors and steroid hormones ( 112 ). In clinical practice, several techniques, including ultrasonography (thickness and uterine perfusion), histology, and molecular biomarkers, have been used to evaluate endometrial receptivity and predict implantation potential in infertile women undergoing embryo transfer ( 112 , 113 ). The mechanisms underlying the effects of GH on endometrial thickness and uterine perfusion remain unclear. Animal models and in vitro cell cultures have been used to address this question, from basic science to the development and assessment of GH in clinical application ( 114 ). Targeted depletion of Ghr in mice leads to a detrimental impact on reproduction during early pregnancy because of fewer uterine implantation sites ( 115 ). After GH administration, increased concentrations of cytosolic estrogen receptors have been detected in the uteri of several species, including rabbits, guinea pigs and cows, indicating a potential estrogen-mediated function ( 116 – 119 ). For instance, injection of GH during an ovulation induction protocol associated with timed artificial insemination increased the pregnancy rate in lactating cows ( 119 ). The GH-induced upregulation of IGF-1 in the uterus may mediate the beneficial effect of GH on endometrial thickness ( 115 ). Similarly, the addition of porcine GH increased the mRNA levels of IGF-1, IGF-2, and insulin-like growth factor binding protein 2 (IGFBP2) in the pig uterus, suggesting the functional role of the GH/IGF axis in the uterine endometrium ( 27 , 115 ). Further in vitro experiments demonstrated differential roles of IGF-1 and IGF-2 in endometrial stromal cells, as IGF-1 mediates estrogen-induced mitogenic effects, while IGF-2 promotes endometrial differentiation ( 27 , 115 , 120 ). Studies using mouse models have identified several molecular biomarkers of endometrial receptivity in response to GH ( 114 ). Administration of GH in mice induced the upregulation of a variety of molecules in the endometrium, including matrix metalloproteinase 9 (Mmp-9), leukemia inhibitory factors (Lif) and integrin alpha v beta 3 (Itgavb3), which are critical regulators of embryo implantation ( 114 ). An animal study using a rat model also showed that supplementation with GH increased the expression of Itgavb3 and osteopontin, which further improved endometrial receptivity (see reviews ( 114 )). In vitro studies using cell-cultured endometrial and decidual cells have demonstrated that the addition of GH into the culture medium enhanced cell proliferation ( 121 ). Similarly, overexpression of GH in endometrial carcinoma cell lines (RL95-2 and AN3 cells) led to an increase in cell number because of the enhancement of cell proliferation and suppression of apoptotic cell death via the upregulation of IGF-1, VEGF, and Itgb3 ( 77 , 122 ). However, pretreatment with the Janus kinase (JAK) 2 inhibitor AG490 completely abolished the GH-induced upregulation of IGF-1, VEGF, and Itgb3, indicating that the JAK2 signaling pathway mediates GH action in these cells ( 122 ). During the periovulatory phase (on the day of human chorionic gonadotropin (hCG) administration or oocyte retrieval), women with an increased endometrial thickness and uterine perfusion have been reported higher pregnancy rates following IVF ( 123 – 126 ). The results obtained from clinical studies indicate that supplementation with GH may enhance endometrial receptivity and improve various pregnancy outcomes in women undergoing IVF/embryo transfer (ET) ( 113 , 127 – 129 ). Using ultrasound as a tool, changes in endometrial thickness and uterine perfusion can be evaluated among women undergoing IVF/ET ( 127 , 129 , 130 ). For instance, two infertile cases with panhypopituitarism have been reported to illustrate the functional role for GH in the promotion of endometrial receptivity ( 130 , 131 ). Specifically, the administration of GH followed by gonadotropin treatment increased the endometrial thickness in an infertile woman with panhypopituitarism, leading to a successful pregnancy ( 131 ). However, there are controversial results showing the clinical evidence of GH on endometrial receptivity to improve pregnancy outcomes in women undergoing IVF (see reviews ( 114 )). Despite no consistent results showing significant increases in implantation rate and pregnancy rate, there seems to be beneficial effects of GH on the number of retrieved oocytes and endometrial thickness in poor responders ( 114 , 132 , 133 ). A retrospective study including 1114 normal responders revealed that adjuvant GH treatment increased endometrial thickness in older patients (more than 35 years old) and improved the implantation rate and pregnancy rate among all ages ( 134 ). Similarly, the beneficial effects of GH on endometrial thickness, implantation rate and pregnancy rate have been shown in infertile women who are overweight or obese ( 114 , 134 ). Studies using animal models or in vitro cell cultures have detected changes in several biological markers related to endometrial receptivity after GH treatment ( 135 , 136 ).

GH has been considered an adjuvant treatment in human ART for three decades. In addition to the anterior pituitary gland, GH and its functional receptor are also expressed in the female reproductive system, including the ovaries and uterus. Emerging evidence supported by animal studies and clinical trials in humans has suggested that GH and IGF-1 play essential roles in the regulation of multiple ovarian functions, ranging from activation of the primordial follicles, folliculogenesis, ovarian steroidogenesis, oocyte maturation, and embryo implantation. The follicular concentrations of GH and IGF-1 are positively correlated with oocyte quality and the subsequent embryo morphology and cleavage rate. Clinical trials have demonstrated that GH intervention is associated with improved IVF reproductive outcomes, including the number of mature oocytes retrieved, fertilization rate, embryo quality, implantation rate, pregnancy rate, and live birth rate, especially in patients with poor ovarian response. Additionally, women with thin endometrium and recurrent implantation failure might benefit from GH administration. However, discrepancies exist between studies because of methodological weaknesses and limited sample sizes, leading to a lack of clinical recommendations for the application of optimal GH regimens during IVF treatment. More RCTs with rigorous methodology are needed to confirm the beneficial effects of GH on ART outcomes.

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Although GH has long been applied for the treatment of female infertility during ovulation induction ( 137 ), it is still controversial to address the beneficial effects of GH as an adjuvant in IVF/ET treatment ( 138 ). The debate has occurred because of the issues inherent to the relatively underpowered studies that included infertile women with poor prognosis in terms of pregnancy potential. These women have experienced several unsuccessful IVF treatment cycles due to either a poor response to ovulation induction or poor oocyte quality. The other issues raised in evaluating the role of GH in the treatment of female infertility are as follows: the optimal dosage to use; when GH treatment should be commenced; the duration of administration; and which subgroup of patients should be recruited ( 138 ). Furthermore, each study has different inclusion criteria for the subjects studied, which makes it difficult to compare studies performed in similar subsets of women undergoing fertility treatment, especially in women with poor response ( 139 ). At present, GH supplementation has been employed during assisted reproductive technology (ART) treatment in infertile women with normal response, women with poor response, women with poor embryonic development, women with oocyte donation, women with older age, women with polycystic ovarian syndrome, and even women with a thin endometrium resistant to any therapy ( Figure 1 ) ( 8 , 9 , 110 , 122 , 137 , 138 , 140 – 143 ). Overview of clinical application of growth hormone in women undergoing assisted reproductive technology. Clinical studies have shown that the follicular IGF-1 concentrations of the retrieved oocytes are positively associated with the number of developing follicles and are negatively associated with the amount and duration of ovarian stimulation ( 144 ). Additionally, the follicular GH concentrations are higher in the retrieved oocytes, resulting in successful fertilization, embryo development and implantation, and clinical pregnancy ( 105 , 106 ). Given its functional role in regulating follicular development and recruitment, GH has been used as an adjuvant treatment during ovulation induction for ART for more than three decades ( 145 ). For women with GH deficiency-associated infertility, GH supplementation has been used to manage their ovulation disorders ( 146 , 147 ). Initially, the first two randomized controlled trials (RCTs) regarding the application of GH as an adjunct for women undergoing IVF treatment were performed three decades ago ( 140 , 141 ). However, the results of these studies did not identify any better clinical outcomes associated with GH supplementation ( 140 , 141 ). Therefore, subsequent studies have focused more on the addition of GH restricted to patients undergoing IVF treatment who respond poorly to ovulation induction ( Table 1 ) ( 109 , 110 , 138 , 148 – 150 ). Although experimental research has demonstrated apparent benefits of GH in regulating follicular functions, there is a lack of clarity regarding the potential benefit of GH supplementation in some of the clinical parameters during ART treatment because of substantial differences in the inclusion criteria among clinical studies. A previous RCT including 16 women with amenorrhea and anovulatory infertility showed that patients receiving GH treatment (24 IU on alternate days with a total dose of 144 IU) had significant decreases in the dosage of human menopausal gonadotrophin (hMG), duration of treatment, and the daily effective dosage of gonadotropins ( 137 ). Additionally, this study also showed that the serum levels of IGF-1 rose during treatment with GH but not with the control, indicating that GH may augment the response to gonadotropin stimulation in human ovaries ( 137 ). Several meta-analyses have been used to assess the value of GH supplementation in women with poor response during IVF treatment. A meta-analysis performed by Kolibianakis et al. showed that GH supplementation increased the clinical pregnancy rate and the live birth rate in women with poor response ( 148 ). However, the total number of included cases was too small (6 RCTs, including 169 patients) ( 148 ). Kyrou et al. conducted a meta-analysis including 22 RCTs and evaluated various medical interventions for IVF/ET outcomes in poor responders ( 149 ). Among 15 interventions, GH addition to ovarian stimulation and embryo transfer on Day 2 (compared with Day 3) were the only two interventions that increased the probability of pregnancy ( 149 ). Similarly, a meta-analysis (11 RCTs) by Li et al. also showed that GH addition significantly improved pregnancy outcomes by increasing the clinical pregnancy rate and live birth rate as well as the number of collected oocytes and MII oocytes ( 150 ). Another meta-analysis conducted by Yu et al. showed that GH addition increased the serum levels of estradiol (on the day of hCG administration), number of MII oocytes, number of 2PN embryos, and number of transferred embryos ( 151 ). Using a network meta-analysis, our previous study evaluated the role of different adjuvant treatment strategies on IVF treatment outcomes in poor responders ( 152 ). Our results showed that adjuvant treatment with GH had the highest number of oocytes retrieved, the highest number of embryos transferred, and the highest estradiol level on the hCG day, although there was no significant increase in the clinical pregnancy rate found compared to the control group ( 152 ). A Cochrane review performed in 2003 concluded that GH addition during IVF treatment has no certain effect on the number of retrieved oocytes and live birth rates in normal responders ( 153 ). However, GH slightly increased the number of retrieved oocytes and clinical pregnancy rates in women with poor response ( 153 ). Nonetheless, there is an uncertain effect of GH addition on live birth rates in poor responders because the dosage and regimen of GH therapy used in trial studies were variable ( 153 ). Although the results obtained from the meta-analysis suggested that GH addition can improve early clinical outcomes, the beneficial effect of GH on live birth is still widely debated. A large-scale retrospective study including 3,080 poor responders receiving IVF treatment demonstrated that GH treatment may not improve the live birth rate in expected poor responders ( 154 ). These inconsistent results obtained from different studies need to be interpreted with caution. Some of the included trials were not RCTs and may have significant bias and imprecision. Additionally, the dosage and regimen of GH supplementation used in every study were quite variable. Future comprehensive research including more high-quality RCTs is required to determine the role of GH as an adjuvant reagent for poor responders during their IVF treatment. Studies of randomized control trial using growth hormone in a population of poor ovarian responders. * Statistical significant after using growth hormone in a population of poor ovary responders (P-value <0.05). Poor oocyte quality and subsequent poor embryonic development are the major causes of treatment failure after IVF/ET, especially in women with a normal response, as there is a close association between oocyte/embryo quality and pregnancy rate ( 155 , 156 ). Embryo development at the early stage is highly dependent on oocyte-derived substances, including mRNAs, proteins, and mitochondria, which are essential for successful implantation ( 157 , 158 ). Experimental results obtained from animal models and clinical samples have shown that GH can improve mitochondrial function, cytoplasmic maturation, and nuclear maturation of oocytes in mice, women with poor response, and women with advanced ages; thus, GH addition increases the number of matured oocytes retrieved and subsequent embryo quality ( 41 , 148 , 159 ). An RCT conducted by Li et al. recruited 158 patients with at least one previous IVF cycle failure due to a lack of top-quality embryos ( 160 ). Using a daily dose of 3 IU human GH (initial day of downregulation for the long protocol or stimulation day for the antagonist protocol until the hCG day), the results showed that GH supplementation increased the number of retrieved oocytes and cleaved embryos, leading to a higher implantation rate, clinical pregnancy rate, and live birth rate ( 160 ). Additionally, the cumulus granulosa cells obtained from the GH addition group had higher mitochondrial DNA copy numbers than those obtained from the control group, indicating that GH may affect the oocyte quality and developmental competence ( 160 ). Similarly, a previous RCT conducted by Dakhly et al. demonstrated that GH adjuvant therapy improved the number of retrieved oocytes, mature (MII) oocytes, fertilized oocytes, and embryos reaching the transfer stage ( 109 ), indicating a beneficial effect of GH on oocyte quality and corresponding embryo formation. Indeed, a Cochrane review by Duffy et al. summarized that adjuvant treatment using GH enhanced ovarian response to ovulation stimulation, increased the number of retrieved oocytes and embryos, and thus improved pregnancy rate and live birth rate ( 110 ). Successful implantation is a complex process that principally depends on the synchronization of good-quality embryos with the receptive endometrium ( 161 ). Inadequate uterine function accounts for approximately two-thirds of implantation failures, and poor embryo quality accounts for one-third of these failures ( 162 ). In addition to the ovary, the uterus is the other target site of GH action ( 7 ). Animal studies have demonstrated that GH not only enhances embryonic development but also increases the post-transferred pregnancy rate in embryo-recipient cows ( 163 ). The first RCT study (including 240 patients) in humans showing the beneficial effect of GH on the endometrium, indicating that GH addition associated with hormone replacement therapy can improve clinical outcomes in patients with frozen embryo transfer ( 129 ). Specifically, the endometrial thickness, uterine perfusion, and cytokine expression profile related to endometrial receptivity were greater in the GH group than in the control group ( 129 ). Oocyte donation has expanded the scope of ART for women with poor oocyte quantity and quality. Despite the relatively high success rate of oocyte donation programs in recipients, some patients still encounter implantation failure repeatedly ( 164 ). An RCT (including 70 women with repeated implantation failure with donated oocytes) was designed to evaluate the GH effect on uterine receptivity and found that administration of GH increased endometrial thickness, leading to a higher pregnancy rate and live birth rate ( 128 ). Similarly, a recent meta-analysis including 25 RCTs with 2,424 women evaluated whether GH administration can improve endometrial function and reproductive outcomes during IVF treatment ( 10 ). Administration of GH significantly increased endometrial thickness, which accounted for an enhanced clinical pregnancy rate and live birth rate ( 10 ). Additionally, there existed a dose- and time-dependent effect of GH on IVF outcomes, indicating that GH might improve the fertility outcome in the recipients through its effect on the endometrium rather than on oocytes or embryos ( 10 ). Moreover, further GH regimen analysis showed differential effects on the endometrium and oocytes, as < 5 IU/day GH administration from the follicular phase of the previous cycle until the hCG day resulted in a thicker endometrium, while 5–10 IU/day or administration from the luteal phase of the previous cycle until the hCG day led to better quality of oocytes and embryos ( 10 ). Data obtained from four RCTs on women with a thin endometrium suggested that improving endometrial function for these patients is essential for their ART outcomes ( 10 ). In this regard, GH administration might improve endometrial function by affecting several parameters, including endometrial thickness, endometrial morphology, and uterine perfusion ( 10 ). However, further studies of GH effects on human uterine receptivity will be required before providing any clinical recommendations in ART protocols. Currently short-term use of GH in ART is not expected to have any problems or safety concerns; however, GH administration may potentially have side effects on active cancers and some metabolic diseases, such as diabetes mellitus ( 165 , 166 ). Indeed, GH administration has been shown to lead to significant metabolic changes, including insulin resistance, glucose intolerance, elevated cholesterol, and disturbance of the renin-angiotensin system ( 166 – 168 ). However, van Bunderen et al. conducted a systematic review and found that long-term administration of GH had a beneficial effect on decreasing the risk of fracture, cardiovascular disease, and stroke without increasing the risk of malignancy ( 169 ). Nonetheless, clinical practitioners should consider the potential risks and comprehensively individualized assessment of the dosage and regimen for the application of GH during ART treatment.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

C-WC collected the information and designed the figure and table, and wrote the manuscript. Y-WS and Y-WH designed the figure. Y-YC, MH, H-CH, T-CY, and W-CL contributed to the critical discussion. H-MC critically and revised the manuscript and contributed to the conception of design. All authors contributed to the article and approved the submitted version.