Crosstalk between the growth hormone/insulin-like growth factor-1 axis and the gut microbiome: a new frontier for microbial endocrinology

Abstract

Both the GH/IGF-1 axis and the gut microbiota independently play an important role in host growth, metabolism, and intestinal homeostasis. Inversely, abnormalities in GH action and microbial dysbiosis (or a lack of diversity) in the gut have been implicated in restricted growth, metabolic disorders (such as chronic undernutrition, anorexia nervosa, obesity, and diabetes), and intestinal dysfunction (such as pediatric Crohn’s disease, colonic polyps, and colon cancer). Over the last decade, studies have demonstrated that the microbial impact on growth may be mediated through the GH/IGF-1 axis, pointing toward a potential relationship between GH and the gut microbiota. This review covers current research on the GH/IGF-1 axis and the gut microbiome and its influence on overall host growth, metabolism, and intestinal health, proposing a bidirectional relationship between GH and the gut microbiome.

1. Introduction

The GH/IGF-1 axis plays a vital role in regulating growth, metabolism, and intestinal homeostasis in an individual. That is, GH is well known to promote overall bone growth with anabolic effects on other organs (like intestines), has catabolic effects on adipose tissue, and acts as a potent diabetogenic agent [1]. Studies on GH as early as 1953 have reported an impact on intestinal function (including mitotic rate, absorption, and secretin production) [2–4]. Since then, numerous studies on both clinical and animal models have demonstrated that physiological levels of GH maintain gut integrity (e.g. decreasing intestinal permeability and bacterial translocation) and improve gut function (e.g. macronutrient absorption and immune function) [5–7]. This pleiotropic effect of GH happens both synergistically with and independent of IGF-1, as exemplified in bone growth, metabolism, and intestinal homeostasis [1, 8–10].

Research on the gut microbiome has expanded the scope of host-microbe interactions to include an impact on host physiological processes, including growth, endocrine function, and metabolism. The microbial community, also known as the gut microbiota, is comprised of trillions of microbes (bacteria, viruses, and fungi) that reside throughout the gastrointestinal (GI) tract, with the majority concentrated in the terminal ileum and colon [11, 12]. This review shall focus on the bacteria and viruses of the gut microbiome. In particular, bacteria that dominate the gut are mostly categorized under Firmicutes (a gram-positive phylum) or Bacteroidetes (a gram-negative phylum) with the remaining majority from the Actinobacteria, Proteobacteria, or Campylobacterota phyla [13]. The collective microbial community and their genome, which is estimated to exceed the potential of the human genome by 500 times, is known as the gut microbiome [14, 15]. Due to the vast diversity in both the microbial community and associated genomic material, the gut microbiome has been implicated in the host immune system, endocrine system, nervous system, metabolism, growth, and intestinal homeostasis [12, 14, 16, 17]. Furthermore, microbial by-products, including short chain fatty acids (SCFAs), branched chain amino acids, dopamine, serotonin, vitamins (mostly K and Bs), and other bacterial components influence the host and its endocrine system [14, 18].

Several recent reviews have explored how the microbial community and its metabolites influence host metabolism, the endocrine system, and growth [14, 17–22]. This review will focus on the specific relationship between the GH/IGF-1 axis and the gut microbiome.

2. Gut microbiota in growth and growth restriction

2.1. Gut microbiome and normal growth

The connection between bacteria and host growth has been long-established. Several papers in the 1940s – a mere twenty years after the discovery of penicillin – reported a pro-growth effect of antibiotics in rats and chickens [23–26]. In the decades that followed, evidence expanded on the relationship between antibiotics and weight gain and growth in not only animals [26–29] but also in humans, including children with delayed growth due to premature birth, malnutrition, or diarrhea [26, 27, 30–32]. More recently, antibiotics have been associated with increased risk of obesity, especially with early exposure in infants [33]. Interestingly, with the discovery of bacteria in the intestines in 1902 [34], the scientific community hypothesized that the pro-growth effect of antibiotics depends upon depleting certain microbes and their metabolites in the intestines [26].

Over the past two decades, research on the relationship between bacteria and growth has refocused attention to the gut microbial community. Several studies have demonstrated that commensal microbes in the gut may improve both linear growth and weight gain of the host (Figure 1). For instance, Lactobacillus acidophilus, Enterococcus faecium, and resident Escherichia coli strain have all independently been shown to promote growth, increase weight gain, prevent colonization of pathogens, and inhibit muscular wasting [35–38]. Moreover, certain gut microbiome profiles have been associated with obesity in mice and humans [39, 40]. Increasing the microbial richness (number of bacterial populations) and evenness (proportionality of the bacterial populations) in the infant gut microbiome due to formula feeding has also been associated with increased risk for weight gain and obesity [41–43].

Figure 1. Crosstalk between microbes and host in states of healthy and restricted growth as mediated through the proposed gut-liver-bone axis.

In states of healthy growth, there is increased microbial richness and evenness and increased SCFA production, which promotes energy harvest, bile production, hormonal production, and vitamin D production and decreases inflammation. Inversely, restricted growth is associated with decreased microbial maturity, SCFAs, ghrelin, and leptin and increased inflammation and consequent GH resistance.

Multiple studies show that the gut microbiome impacts cortical bone growth (formation and resorption) (Figure 1) [44, 45]. Gut microbes and their by-products, such as SCFAs, regulate the production of hormones associated with bone health, including sex-steroids, vitamin D, and serotonin [17, 46]. Another mechanism by which microbes may alter bone formation and resorption is through the immune system [44–46]. Compared to that of conventionally raised mice (normal microbiome), young germ-free mice (absent microbiome) have increased bone mass yet impaired bone remodeling with a reduced number of osteoclasts, helper T-cells, and inflammatory cytokines [45]. Inversely, colonization of germ-free mice with gut microbiota normalizes these findings [45, 47]. Novince and colleagues further propose a gut-liver-bone axis, suggesting that certain microbes induce an inflammatory phenotype in the gut, which suppresses bone formation and enhances bone resorption via changes to the liver [44]. Yet, other studies have reported decreased bone mass in germ-free mice [48, 49], and these effects appear to be both sex- and age-dependent [19, 50]. Finally, microbes and SCFAs may regulate bone growth by altering serum and bone IGF-1 levels [44, 49], which will be discussed later in this review. Collectively, these studies highlight a complex microbial effect through many different mechanisms on bone remodeling, development, growth, and mechanics [19].

2.2. Gut microbiome and restricted growth

Microbial dysbiosis, or the lack of diversity in the gut microbial community, is implicated in several intestinal and metabolic diseases that cause secondary growth failure [51–53]. Microbial immaturity, or a delay in the expected development of the microbial community, has also been associated with metabolic diseases that result in restricted growth [51, 54–56]. Children with chronic undernutrition present with environmental enteric dysfunction, short stature, failure to gain weight even after nutrient restoration, and an altered gut microbiome (e.g. dysbiosis, immaturity, and altered SCFA levels) [51, 55, 57, 58]. Inflammatory bowel diseases (IBD) (e.g. pediatric Crohn’s disease) are associated with reduced growth and a unique microbial signature, including reduced microbial richness, evenness, and diversity [52, 59, 60]. Anorexia nervosa also is associated with microbial immaturity and alterations in Firmicutes, Bacteroidetes, and Proteobacteria [53, 54]. Furthermore, altered development of microbial richness, evenness, diversity, and SCFA production is associated with obesity, type 1 diabetes, and type 2 diabetes [61–63].

The aforementioned diseases associated with microbial dysbiosis and immaturity also result in decreased GH/IGF-1 signaling. For chronic undernutrition, the failure in childhood growth has been partially attributed to malabsorption. Yet, in nutrient depleted germ-free mice, acting as a model for chronic undernutrition, both GHR and IGF-1 expression in the liver and serum IGF-1 levels are decreased, suggesting growth failure may also be due to GH resistance [48]. Anorexia nervosa, IBD, and type 1 diabetes are other examples of GH resistant states due to nutrient restriction or an inflammatory phenotype in the liver [53, 64, 65]. In particular, Han et al. [66] demonstrate that gut inflammation increases TNF-α in the liver, which inhibits STAT5b downstream the GHR. Moreover, Soendergaard et al. reveal that intestinal inflammation (as observed in patients with ulcerative colitis and IL-10 knockout mice treated with piroxicam) not only results in GH resistance in the liver but also locally in the intestines [67]. Meanwhile, obesity has been associated with decreased GH and IGF-1 levels [1, 68]. Collectively, these diseases imply a link between the gut microbiome and the GH/IGF-1 axis.

3. Link between the gut microbiome and GH-modulating hormones

GH directly influences the growth of various organs, including intestines and bone [1]. GH also stimulates the production of IGF-1, a potent growth factor that works in a synergistic manner with GH to maintain overall growth and metabolism [1, 8]. Many factors regulate GH, including hormones and external conditions (such as fasting, free-fatty acids, exercise, and sleep) [1]. In terms of endocrine regulation, GH secretion is promoted by GH releasing hormone (GHRH), ghrelin, and leptin while inhibited by IGF-1 and hypothalamic secretion of somatostatin [69–72].

Ghrelin and somatostatin are secreted by both the hypothalamus and GI tract (stomach and intestines/pancreas, respectively) [69, 71, 73, 74]. Like somatostatin and ghrelin, leptin also exerts an effect on the hypothalamus; however, leptin is mainly produced in adipose tissue with a small quantity produced in the stomach [75, 76]. Moreover, all three hormones not only regulate GH secretion but also are known for their roles in appetite regulation, gut function, and bone growth [77–81] and have been linked with the gut microbiome. The following section describes the connection between each hormone and the gut microbiome, offering a potential mechanism as to how the gut microbiota may affect the GH/IGF-1 axis (Figure 2).

Figure 2. Crosstalk between the gut microbes and GH modulating hormones (ghrelin [gray], somatostatin [blue], and leptin [salmon]).

A. Gut microbes and their microbial by-products (SCFAs; yellow) alter somatostatin, leptin, and ghrelin. Several microbes have been associated with decreasing leptin. One example with Lactobacillus has been shown to decrease adipocyte size and thus, decrease leptin. Bacillus subtilis (green) and Escherichia coli (brown) have been shown to produce somatostatin, and microbes have been shown to metabolize somatostatin. *SCFA, lactate, and bacterial supernatants inhibit GHSR-1α. B. Ghrelin, somatostatin, and leptin have been associated with altering microbiome through various mechanisms. **In addition to its more direct effect on bacteria, somatostatin alters bile acid levels, decreases gut motility and bacterial translocation, and increases bacterial overgrowth. ***Ghrelin regulates gut motility and influences the intestines through its effect on GH and IGF-1 (covered in a later section and figure), which thus may affect the gut microbial community. ****Leptin binds to its receptor on Paneth cells, which would increase production of antimicrobial peptides and thus influence the microbial community.

3.1. Ghrelin

Ghrelin, a 28 amino acid acylated peptide hormone, is well-known for its role in appetite stimulation, metabolism, and intestinal function [69, 73, 80]. That is, ghrelin increases adipogenesis, inhibits insulin secretion, and regulates gluconeogenesis, gut motility, and bone formation [80, 82]. Ghrelin also stimulates GH production [69]. The GH secretagogue receptor (GHS-R), a G-protein coupled receptor to which ghrelin binds, was discovered first in 1997 for its role in promoting GH secretion [73]. Mutations in GHS-R in humans result in stunted growth and metabolic dysfunction; thus, both GHS-R and ghrelin have been viewed as attractive therapeutic targets for restricted growth, metabolic dysfunction, and eating disorders [73, 82].

Both ghrelin and the GHS-R are associated with the gut microbiome. For instance, germ-free mice have lower levels of ghrelin [83, 84], whereas colonization of commensal microbes in the gut increases levels of the hormone [84]. To that end, recent studies show that certain microbes and microbial metabolites, such as SCFAs, may alter ghrelin levels. In rats, ghrelin has been positively correlated with Bacteroides and Prevotella and negatively correlated with Bifidobacterium, Lactobacillus, and Bacteroides coccoides [17, 85]. Further, administration of an oligofructose prebiotic increases both Bifidobacterium and Lactobacillus in the human gut, resulting in decreased ghrelin levels in obese individuals [17, 86]. Lactobacillus and Bifidobacterium are both well-known commensals in the gut and have been associated with production of SCFAs [49, 62]. SCFAs also decrease ghrelin levels in both animal models and humans [87, 88]. Another potential target for microbes and their metabolites is the GHS-R. Torres-Fuentes and colleagues [89] show that SCFAs, lactate, and bacterial supernatants attenuate GHSR-1α. Collectively, these studies offer potential mechanisms as to how certain gut microbes influence ghrelin and GHS-R and consequentially, GH and IGF-1; all of which could contribute to intestinal homeostasis, metabolism, and growth of the host.

3.2. Leptin

Leptin is another peptide involved in appetite regulation, adipogenesis, metabolism, and bone formation and has been shown to increase GH production [70, 78, 79, 90]. Inhibiting leptin via somatostatin secretion in the hypothalamus through neuropeptide Y leads to a suppression of GH secretion [70]. Moreover, both GH deficient individuals and individuals with anorexia nervosa have a decrease in circulating leptin levels [91, 92].

The gut microbiome alters leptin levels in the host. That is, rats treated with the antibiotic vancomycin have a 38% decrease in leptin levels [17, 93]. This finding suggests that certain microbes (Gram-positive bacteria susceptible to vancomycin) alter leptin levels. To that end, leptin has been positively associated with the presence of Bifidobacterium and Lactobacillus in the gut microbiome and negatively correlated with that of Clostridium, Bacteroides, and Prevotella, in opposition to the effects of ghrelin [85]. In leptin deficient mice, the presence of Mucispirillium, Lactococcus, and Lachnospiraceae in the gut microbiome are positively correlated with circulating leptin, whereas the presence of Allobaculum is negatively correlated with leptin [94]. Several studies on a mixture of probiotics highlight a more complex relationship between leptin and different strains of Lactobacillus [95–97]. For instance, administration of Lactobacillus helveticus and Bifidobacterium longum [95] and a cocktail of L. rhamnosus, L. acidophilus, and B. bifidum increase leptin signaling [96]. Meanwhile, L. plantarum consistently decreases leptin levels in several studies [93, 97–99]. The cause of these correlations – i.e. whether this is a microbial effect on leptin or diet-based leptin effect on the presence of these microbes – is still unclear [17, 85, 94]. One study demonstrates that oral administration of L. plantarum reduces mean adipocyte cell size and thus, suppresses leptin levels in mice fed a high fat diet; however, additional studies need to discern the mechanism mediating this effect [98]. Another potential mediator between the gut microbiota and leptin appears to be SCFAs, which show contradictory findings between in vitro studies (which points to a direct relationship) and in vivo studies (which points to an inverse relationship with increased SCFA levels and decreased leptin in high fat diets) [100]. Another study suggests that leptin may also influence the gut microbiota independent of diet [101]. In that study, Rajala and colleagues show that in leptin deficient male mice, the leptin receptor is expressed in Paneth cells in the intestines and ablation of the receptor on the intestinal epithelium normalized the microbial composition seen with leptin deficiency [101]. Although potentially a more complicated connection with the gut microbiome and GH/IGF-1 axis compared to ghrelin, these studies further suggest a possible bidirectional relationship between the gut microbiota and hormones upstream of the GH/IGF-1 axis.

3.3. Somatostatin

Originally discovered in 1973 for its role in reducing GH secretion, somatostatin has since become known for its complex function in both the gut and hypothalamus [71, 74, 81]. Due to alternative splicing of prosomatostatin, there are two isoforms of somatostatin, a 14 amino acid peptide and a 28 amino acid peptide [74, 102]. Somatostatin 14 is mainly secreted by the hypothalamus and inhibits GH and thyroid stimulating hormone (TSH) in the pituitary [71, 74, 103]. Meanwhile, somatostatin 28 is secreted by delta cells in the pancreas and D cells in the small intestine and has been implicated in inhibiting insulin, secretin, glucagon, gastrin, and other hormones in the GI tract [74, 102]. This intestine-derived somatostatin prevents gastric acid secretion, slows gut motility, and alters bile acid levels [81, 102]. Due to its diverse physiological actions, somatostatin analogues (such as octreotide) have been used to treat acromegaly, short bowel syndrome, and idiopathic diarrhea, to name a few [104, 105].

Despite its major influence in the GI tract, somatostatin has only been loosely connected with the gut microbiome. For instance, individuals with acromegaly treated with octreotide can exhibit side effects including slower gut motility and increased bacterial overgrowth in the small intestine [106]. Likewise, somatostatin’s ability to slow gut motility is the rationale for its use in diarrhea (idiopathic or secondary to a GI condition) [81]. Somatostatin and octreotide have also been implicated in inhibiting translocation of bacterial pathogens like Salmonella from the intestinal lumen into the blood circulation and reducing bacterial induced TNF-α, IL-8, and IL-1ß in intestinal epithelial cells [107]. Research has further suggested an association between somatostatin and the gut microbiome. For instance, a mouse model for congenital diarrheal disorders with increased somatostatin and decreased gut hormones (cholecystokinin, secretin, neurotensin, glucose-dependent insulinotropic peptide, and glucagon-like peptide-1 and -2) alters development of the gut microbiome [108]. In another study, protein restriction in pigs is associated with lower gut hormones (including ghrelin, leptin, and somatostatin), decreased levels of Lactobacillus, Streptococcus, and SCFAs, and increased levels of Bacteroides and Prevotella [109]. Unfortunately, as both experiments study somatostatin along with the other gut hormones, it is difficult to discern a specific relationship.

Relatively few studies have focused on the direct relationship between somatostatin and the gut microbiota. In 1985 – approximately a decade after being discovered -, somatostatin production was detected in both Bacillus subtilis and E. coli [110, 111]. LeRoith and colleagues observed that both bacteria in culture produced somatostatin-related material SRIF-14 and SRIF-28, closely resembling both somatostatin isoforms [110, 111]. Moreover, when an antibody to somatostatin-14 is added to the media of the B. subtilis culture, the biological activity of the somatostatin-like peptides is neutralized [111]. From these results, the authors propose shared evolutionary origin between these conserved bacterial peptides and vertebrate-type peptide hormones/neurotransmitters [110, 111]. Not only does this pose an interesting commonality in evolutionary origin but also offers a potential mechanism of crosstalk between bacteria and the host GH/IGF-1 axis and endocrine system. Conversely, the gut microbial community in an in vitro model of the colon metabolizes somatostatin and its analogues, such as octreotide [112]. In fact, somatostatin is rapidly metabolized in the gut by the microbial community with complete degradation within five minutes, whereas octreotide is more stable and metabolized at a much slower pace [112]. More research is needed to confirm these results in animal models and clinical studies; however, these findings point to a relationship between the gut microbiota and somatostatin. Similar to those on ghrelin and leptin, the studies on somatostatin highlight potential mediators between the gut microbiome and GH/IGF-1 axis.

4. Influence of gut microbiome on GH and IGF-1

The aforementioned studies highlight a potential association between the gut microbiome and GH/IGF-1 axis or hormones associated with this axis. The following section describes our current understanding of how the microbial population influences GH and IGF-1 directly (Figure 3).

Figure 3. Gut microbiota (bacteria and viruses) and their by-products (SCFAs and VILPs) alter GH and IGF-1 levels in the host.

Microbes (such as Lactobacillus plantarum) have been shown to increase IGF-1. *SCFAs have been shown to inhibit GH secretion through the cAMP pathway, but this has only been shown in vitro on bovine anterior pituitary cells. **VILPs have been found in viruses in the gut and have a similar proposed action as IGF-1.

4.1. Absence and full colonization of the microbial community in the gut

As mentioned previously, germ-free mice have been associated with impaired growth, delayed development of the immune system, and reduced ghrelin levels. Consistent with these findings, several studies have demonstrated that germ-free mice have lower IGF-1 levels and/or GHR expression [44, 48, 49]. More specifically, Schwarzer and colleagues [48] show that young germ-free mice have reduced IGF-1 and IGFBP-3 levels and reduced expression of hepatic Igf1 and Igfbp3 compared to wild-types [48]. Moreover, when these mice are placed on a nutrient depleted diet (low in protein, fat, and vitamins) to mirror a state of undernutrition, germ-free mice exhibit more extensive weight loss with significantly decreased IGF-1 levels and gene expression of Igf1 and Ghr in the muscle and liver. These findings suggest that germ-free mice with chronic undernutrition represent a GH resistant state, consistent with findings seen in humans [51, 58]. In the second study, Yan et al. [49] note decreased levels of serum and bone IGF-1 in germ-free mice and mice treated with vancomycin compared to control mice. Although the authors in both studies did not find any significant difference in GH levels between germ-free mice and their controls, it is important to note that serum GH levels are difficult to accurately determine due to GH’s pulsatile secretion. Conversely, GH pulsatility in young germ-free mice is altered compared to conventional mice, with significant decreases in relative GH levels, peak height, and peak number, supporting an observed decrease in serum GH [84]. Regardless, these studies all suggest decreased GH action and IGF-1 levels in germ-free mice.

Conversely, colonization of the gut microbial community in germ-free mice and conventional wild-type mice has been associated with increased growth and an increase in IGF-1 levels [48, 49]. Recombinant IGF-1 administered to germ-free mice improves growth parameters (i.e. normalized weight and body and femur length to conventionally raised mice), suggesting that gut microbiota impact IGF-1 levels and thus growth [48]. Yan et al. [49] also observe that mice with full colonization of the gut microbiota have increased IGF-1 levels compared to those in germ-free mice at one and eight months of age. Interestingly, colonization of the gut microbial community has an age-dependent effect on bone health; that is, after one month, the gut microbiota promotes both bone resorption and formation, whereas long-term colonization (i.e. eight months) results in overall increased bone formation and growth [49]. Similarly, suckling pigs have increased growth after fecal microbiota transplant (FMT). After 14 and 21 days, FMT increases counts of Lactobacillus and Faecalibacterium, SCFAs (total, acetate, and butyrate), and growth parameters, including total body weight and plasma GH and IGF-1 levels [113]. In a humanized gnotobiotic mouse model, a high growth-associated microbiome as compared to a low-growth associated microbiome (i.e. microbiome from preterm infant donors with poor growth) alters early neuron and oligodendrocyte development and increases circulating and brain IGF-1 levels [114]. Together, these studies demonstrate that the presence of gut microbiota is positively correlated with GH and IGF-1 levels to influence overall host growth and development.

4.2. Addition of one commensal microbe or probiotics

The above studies not only indicate that the gut microbiota influence the GH/IGF-1 axis, but also hint at the possibility that this impact is mediated specifically through certain microbes, such as Lactobacillus. Several studies have confirmed the potential of a single microbe to impact the GH/IGF-1 axis and host growth; a finding conserved across several species, including mice, chickens, and sheep [48, 115–119]. In 2011, both Storelli et al. [115] and Shin et al. [116] observed that the addition of one commensal microbe (Lactobacillus plantarum or Acetobacter pomorum, respectively) in juvenile Drosophila increased growth factors comparable to GH/IGF-1 signaling in mammals. Likewise, administration of L. plantarum (WJL strain) to germ-free mice on a nutrient depleted diet improves sensitivity to the GH/IGF-1 axis, yielding increased IGF-1 levels, IGFBP3 levels, and overall growth [48]. Likewise, in chickens under heat stress, administration of L. plantarum results in higher body weight, improved intestinal villus height, and increased Ghr and Igf1 transcript levels in the liver [118]. Similar findings are seen with L. plantarum treatment in post-weaning lambs, including increased hepatic Igf1 transcript levels, as well as improved growth performance, nutrient intake, and nutrient digestibility [119]. A cocktail of probiotics (Lactobacillus, Pediococcus, Bifidobacterium, and Enterococcus) also result in increased hepatic Igf1 mRNA levels and overall growth in broiler chickens [117].

4.4. Microbial by-products (SCFAs) and GH/IGF-1 axis

Other potential mediators of the microbial impact on the GH/IGF-1 axis are microbial by-products. Since Lactobacillus appears to be one of the key microbes altering IGF-1 levels and members of this genus are well-known producers of SCFAs, it is unsurprising that acetate, propionate, and butyrate have been shown to alter GH and IGF-1 levels. Wang et al. [120] evaluated the effect of SCFAs acetate, propionate, and butyrate on GH, PRL, and Pit-1 gene transcription in bovine anterior pituitary cells in vitro. The authors observe that SCFAs directly inhibit GH production through the cAMP/PKA/CREB pathway [120]. Notably, propionate appears to have a more potent inhibitory effect than acetate or butyrate on GH production [120]. Although this relationship has only been tested in vitro, this finding substantiates the inhibitory relationship between SCFAs and ghrelin. In addition, SCFAs co-administered with antibiotics to deplete the gut microbiome increases serum, adipose tissue, and liver IGF-1 levels in mice [49]. Although more research needs to be conducted to understand the relationship between SCFAs and the GH/IGF-1 axis, both studies provide a potential mechanism as to how microbes may alter GH and/or IGF-1 action.

4.5. Microbial mimetics and the GH/IGF-1 axis

Another exciting frontier in understanding the intersection between the gut microbiome and GH/IGF-1 axis is microbial mimetics. Similar to the somatostatin-like material seen in E. coli and Bacillus subtilis (described in the previous section), insulin and IGF-1-like peptides have been discovered in viruses. A total of 14 viral hormone analogs have been found, including those to FGF-21, FGF-19, irisin, TGFß-1 and TGFß-2, insulin, IGF-1, and IGF-2 [121]. Specifically, Altindis and colleagues discovered viral insulin/IGF-1-like peptides (VILPs) in four different viruses in the Iridoviradae family, which exist in the human gut virome [122]. Not only is there significant similarity in primary sequences between VILPs and mammalian IGF-1 and insulin, but also VILPs have conservation of cysteine residues and spacing to allow for disulfide bond formation [122]. Importantly, VILPs bind to the human IGF-1 receptor (with higher affinity than human insulin) and to the insulin receptor [121, 122]. VILPs exert similar downstream actions to those of insulin and IGF-1, such as increasing glucose uptake and activating AKT and ERK phosphorylation [122]. This novel finding of VILPs and their homology to host hormones demonstrates direct crosstalk between the microbiome and insulin and IGF-1 signaling. Research is still needed to understand how VILPs may mimic or block insulin signaling, which may be manipulated in the future for potential diabetes therapeutics. Moreover, in vivo and in vitro studies show that VILPs can stimulate cell proliferation and may act as mitogens, which is important to investigate further for cancer therapeutics.

5. Potential impact of GH/IGF-1 on the gut microbiome: evidence of a bidirectional relationship

As discussed, increasing research shows microbial influence on the GH/IGF-1 axis. Yet, only five studies to date have examined the specific impact of the GH/IGF-1 axis on the gut microbiome.

5.1. IGF-1 on the gut microbiome

One of the first studies to explore the role of GH/IGF-1 on the microbial composition focused on the administration of IGF-1 in female BALB/c mice with nutrient restriction (as a model for anorexia nervosa) [54]. Compared to controls fed ad libitum, nutrient-restricted mice have microbial dysbiosis and immaturity [54]. In the study, a subset of diet-restricted BALB/c mice received food restoration or a combined treatment of food restoration and subcutaneous injections of IGF-1. IGF-1 restores body weight, alleviates microbial dysbiosis and immaturity, and normalizes the microbiome to that seen in the ad libitum group, independent of diet [54]. Combined treatment of diet and IGF-1 increases Bacteroidetes and Proteobacteria phyla and Sutterella genus and decreases Firmicutes phylum and several genera (such as Ruminococcus, Oscillospira, Coprococcus, and Adlercreutzia) [54].

Two 2018 studies examine the association between IGF-1 and the gut microbiome in humans [123] and mice with a targeted IGF-1 deletion in intestinal epithelial cells [124]. Buford et al. [123] compares the human gut microbiome and inflammatory parameters (including IGF-1) between healthy young and older adults. In this study, IGF-1 is significantly different between young and older adults and positively correlated with Bacteroidetes, TM7, and Tenericutes phyla and Leptospirae family [123]. The specific deletion of IGF-1 in intestinal epithelial cells in mice (cKO mice) decreases proliferation of intestinal stem and secretory cells and maturation of gut immune cells, increases bacterial translocation, and alters the cecal microbial composition at eight weeks of age [124]. In particular, cKO mice have increased Bacteroides and Odoribacter and decreased Lactobacillus, Oscillospira, and Helicobacter [124]. Collectively, these studies suggest that IGF-1 influences the presence of certain microbes at the phylum level (such as Bacteroidetes) and genus level (including Oscillospira, Sutterella, and Lactobacillus).

5.2. GH on the gut microbiome

In the last year, two studies have evaluated the role of GH on the gut microbiome in mice [125, 126]. The first study characterized the gut microbial abundance and diversity in the ileum, cecum, colon, and feces of Ames mice, a model for hypopituitarism with deficiencies in GH, prolactin (PRL), and TSH due to a mutation in the Prop1 gene [125]. Specifically, Wiesenborn et al. [125] show the microbial profile in Ames mice at two months of age compared to littermate controls (both fed ad libitum). Two-month-old Ames mice exhibit a significantly different microbiome compared to that in the controls with an increased Bacteroidetes/Firmicutes ratio. The Ames mice also have increased abundance of colonic Enterococcaceae, Enterobacteriaceae, Bacteroidales S24-7 group, Klebsiella, and Lachnospiraceae NK4A136 group and decreased colonic Lactobacillus, Rikenellaceae, and Planococcaceae as compared to control animals. Interestingly, the authors observe distinct shifts in the same bacterial genera between the ileum and cecum (e.g. increased Lactobacillus and decreased Lachnospiraceae and Ruminococcaceae in the ileum). In the second phase of the study, after six months of either ad libitum feeding or caloric restriction, Ames mice have relatively minor differences between the two diets, whereas caloric restriction promotes Lachnospiraceae and Ruminococcaceae in the ileum and decreased Turicibacter in the cecum and colon of control mice. The authors conclude that GH deficiency in Ames mice results in a distinct microbial composition throughout the GI tract compared to controls. However, Ames mice offer a rather complicated model for the study of the GH/IGF-1 axis due to their deficiencies in multiple pituitary hormones, including PRL and TSH. Thus, this study discounts the potential role of TSH or PRL on the microbiome. Still, their results point to a potential role of GH on gut microbial composition.

The second study, performed in our laboratory, confirms that GH is associated with an altered gut microbiome [126]. In this study, the gut microbiome is characterized in two GH altered mouse lines: GH gene disrupted (GH−/−) mice with absent GH and bovine GH transgenic (bGH) mice with chronically increased GH. Both GH−/− and bGH mice exhibit an altered microbial profile as compared to that of littermate controls at six months of age. Importantly, the two investigated mutations result in opposite alterations in several bacterial phyla and genera, microbial maturity, and predictive metabolic function. These data suggest an association between the gut microbiome and GH. For instance, although both bGH and GH−/− mice exhibit a similar shift in the Bacteroidetes/Firmicutes ratio, GH−/− and bGH mice have differences in abundance in Proteobacteria, Campylobacterota, and to a smaller degree, Actinobacteria. Likewise, several common bacterial genera differentiate the GH altered mouse lines from their controls. Several microbes, such as Parasutterella, Ruminococcaceae NK4A214, Lachnospiraceae NK4A214, Rikenellaceae, and Lactobacillus are abundant in both mouse lines yet in opposite directions (e.g. decreased in GH−/− mice and increased in bGH mice). Several metabolic functions are also predicted to be associated with GH, including SCFA acetate and butyrate production, folate biosynthesis, and heme B biosynthesis. Findings on the intestinal environment of these two mouse lines also demonstrate that GH is associated with altered intestinal length and weight, morphology, intestinal inflammation, and fecal output [126]. Based upon these results, the study concludes that GH influences the gut microbiome in adult male mice.

Both studies propose that GH has a role in the development of the gut microbiome. Although there are observed microbial differences, Lactobacillus is decreased and Lachnospiraceae is increased in both studies, suggesting GH action may play an important role in these genera. Moreover, the observations in microbial maturity, SCFA production, Proteobacteria, and several genera like Lactobacillus, Lachnospiraceae, and Parasutterella follow expected trends from previous studies mentioned above concerning IGF-1, ghrelin, leptin, and somatostatin.

6. Concluding remarks

Overall, increasing evidence suggests a link between the GH/IGF-1 axis and the gut microbiome. Several studies point to a role of the gut microbiota on GH and IGF-1 levels. This role may be mediated directly through its production of SCFAs and microbial mimetics like somatostatin and VILPs or indirectly through its impact on GH-modulating hormones, the intestinal environment, and the immune system. Emerging evidence also points to a role of the GH/IGF-1 axis on the gut microbiota, potentially mediated by its impact on the immune system, metabolism, and the intestinal environment (Figure 4). For instance, GH has been shown to inhibit bacterial translocation, decrease intestinal permeability, stimulate proliferation of enterocytes, colonocytes, and goblet cells, and promote differentiation of intestinal stem cells into Paneth and enteroendocrine cells [127–131]. All of these changes, in turn, influence the anaerobic and pH environment of the intestinal lumen, potentially creating a niche for certain bacteria (e.g. Lactobacillus and other SCFA producers) [62, 132]. Additionally, studies highlight a liver-gut microbiome axis, a muscle-gut microbiome axis, and adipose tissue-gut microbiome axis [44, 133–136]; all of these tissues are influenced by the GH/IGF-1 axis. More research is needed to fully understand the potential bidirectional relationship between the GH/IGF-1 axis and the gut microbiota, particularly in understanding the mechanisms of how the GH/IGF-1 axis alters the microbial community. To date, all studies looking at IGF-1 and GH on the gut microbiome demonstrate correlation but not causation.

Figure 4. Proposed role of GH and IGF-1 in impacting the gut microbiome.

Specifically, GH and IGF-1 have been associated with the presence of certain microbes, which thus could alter the maturity and predictive metabolic function of the microbial community. Currently, it is unknown whether GH or IGF-1 could directly impact the gut microbiota, as denoted by the “+?” symbol. GH and IGF-1, however, do have known roles in the intestinal environment. GH promotes proliferation of epithelial and goblet cells, and differentiation of intestinal stem cells into Paneth cells and enteroendocrine cells. GH has been associated with aiding in the immune response and muscle thickness in the gut. Endocrine and local IGF-1 has a promotive effect on epithelial cells, goblet cells, enteroendocrine cells, and intestinal stem cells. IGF-1 also has been associated with immune cell maturation and differentiation in the gut. Both hormones decrease permeability and bacterial translocation and increase mucin production. All of these effects could potentially influence the luminal and mucosal environment to cultivate a niche of gut microbes.

Additional confounding factors, such as geography, biological sex, ethnicity, and nutrition, may modulate the crosstalk between the GH/IGF-1 axis and the human gut microbiota. Large-scale studies have pointed toward variability in certain bacterial phyla (e.g. Actinobacteria) and genera (e.g. Lactobacillus and Bifidobacterium) explained by geography, diet, and ethnicity and have established a conserved “core” human gut microbiota with bacteria including Ruminococcaceae, Faecalibaculum, Lachnospiraceae, and Bacteroides [137–140]. All of the aforementioned microbes have been associated with the GH/IGF-1 axis as outlined in this review. Biological sex has also been demonstrated to alter both the gut microbiome and GH physiology [84, 141–144]. Diet is an additional consideration in the crosstalk between the GH/IGF-1 axis and the gut microbiota. Nutrition not only alters the GH/IGF-1 axis but also correlates with marked changes in microbial abundance (both phylum and genus shifts), richness, evenness, maturity, and levels of metabolites (e.g. SCFAs, branched chain amino acids, ammonia, and neurotransmitters) [12, 18, 22, 39, 57, 145]. Several different hypotheses as to how diet may affect this bidirectional relationship include: 1) diet changes the composition of the gut microbiota and levels of microbial metabolites (e.g. SCFAs), which in turn modulates hormones associated with the GH/IGF-1 axis, 2) diet (e.g. low protein and free-fatty acids) affects IGF-1, GH, ghrelin, and leptin levels, which also in turn influences the gut microbial composition, or 3) nutrition independently alters both the gut microbiota and the GH/IGF-1 axis, potentiating the impact on both. Future mechanistic studies are necessary to examine these hypotheses. Regardless, the majority of the studies presented in this review characterize the relationship of the GH/IGF-1 axis and the gut microbiome in animal models (in which many of these factors can be tightly controlled) and have not delved into these additional factors. Thus, future studies are needed to track the differences in these confounding factors in relation to the interaction between the GH/IGF-1 axis and human gut microbiome.

Other exciting areas of research are the microbial mimetics and exploring the specific role of somatostatin on the gut microbiome. Of note, many of these studies have been conducted in animal models, and thus, there is a need to translate this research to humans and observe the relationship between the GH/IGF-1 axis and the human gut microbiome. For instance, it would be valuable to characterize the microbiome in human populations with altered GH action, such as patients with acromegaly, Laron syndrome, or GH deficiency (before and after GH treatment). Additional studies are needed to test the efficacy of ghrelin mimetics, recombinant IGF-1, somatropin, or GH antagonists (e.g. pegvisomant) on altering the human gut microbiome. Regardless, these studies have broadened our perspective of the GH/IGF-1 axis and its impact on host metabolism, immune system, intestinal homeostasis, and growth.

Highlights.

• Both the GH/IGF-axis and gut microbes are important for host growth and metabolism

• Gut microbes impact growth partially through altering the GH/IGF-1 axis

• IGF-1 normalizes the diversity of the microbial community in the gut

• GH may alter microbes, microbial maturation, and short chain fatty acid production

Abbreviations:

bGH

bovine growth hormone transgenic mice

GI

gastrointestinal

GH−/−

growth hormone gene disrupted mice

GHS-R

growth hormone secretagogue receptor

IBD

inflammatory bowel diseases

PRL

prolactin

SCFA

short chain fatty acids

TSH

thyroid-stimulating hormone

VILPs

viral insulin/IGF-1 like peptides