J Obes Metab Syndr 2024; 33(4): 302-313
Published online December 30, 2024 https://doi.org/10.7570/jomes24032
Copyright © Korean Society for the Study of Obesity.
Licio A. Velloso1,2, Jose Donato Jr.3,*
1Laboratory of Cell Signalling‑Obesity and Comorbidities Research Center, University of Campinas, Campinas; 2National Institute of Science and Technology on Neuroimmunomodulation, Campinas; 3Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of Sao Paulo, Sao Paulo, Brazil
Correspondence to:
Jose Donato Jr.
https://orcid.org/0000-0002-4166-7608
Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of Sao Paulo, Av. Prof. Lineu Prestes, 1524, São Paulo 05508-000, Brazil
Tel: +55-11-3091-0929
Fax: +55-11-3091-7285
E-mail: jdonato@icb.usp.br
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
While inflammation is a crucial response in injury repair and tissue regeneration, chronic inflammation is a prevalent feature in various chronic, non-communicable diseases such as obesity, diabetes, and cancer and in cardiovascular and neurodegenerative diseases. Long-term inflammation considerably affects disease prevalence, quality of life, and longevity. Our research indicates that the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis is a pivotal regulator of inflammation in some tissues, including the hypothalamus, which is a key player in systemic metabolism regulation. Moreover, the GH/IGF-1 axis is strongly linked to longevity, as GH- or GH receptor-deficient mice live approximately twice as long as wild-type animals and exhibit protection against aging-induced inflammation. Conversely, GH excess leads to increased neuroinflammation and reduced longevity. Our review studies the associations between the GH/IGF-1 axis, inflammation, and aging, with a particular focus on evidence suggesting that GH receptor signaling directly induces hypothalamic inflammation. This finding underscores the significant impact of changes in the GH axis on metabolism and on the predisposition to chronic, non-communicable diseases.
Keywords: Chronic disease, Cytokines, Growth hormone, Insulin-like growth factor I, Neuroinflammatory diseases, Obesity
During the last century, humanity has experienced the historically most significant advancement in socioeconomic standards, science, medicine, lifestyle, technology, and many other areas directly impacting life. One of the consequences of this revolution is that worldwide human longevity has increased from 32 years in 1900 to 77 years in 2020 (www.cdc.gov). While living longer is regarded as a positive aspect of modernity, it is closely linked with the increased prevalence of several chronic, non-communicable diseases such as cancer, cardiovascular diseases, neurodegenerative diseases, diabetes, obesity, and hypertension (Fig. 1). These are all multifactorial conditions that can appear at any time during a lifetime; however, after the age of 60 years, their prevalence undergoes a sharp increase, which places them among the greatest causes of death in the world. Due to their multifactorial nature, no single factor explains or connects these health conditions; however, inflammation permeates all these diseases in different ways, generating great interest regarding its involvement in the pathophysiology of these diseases and its potential as a therapeutic target (Fig. 1).1
Inflammation is a response to distinct challenges to body homeostasis, such as tissue damage, cellular stress, and infections. Inflammatory signals, which include factors such as chemokines, cytokines, biogenic amines, and eicosanoids induce multiple changes in biological processes, resulting in the abnormal regulation of local vascular tonus and permeability, alterations of local or whole-body temperature, edema, and pain, the clinical hallmarks of inflammation.1 The biological purpose of inflammation is to restore cellular, organic, or systemic function when standard homeostatic systems fail. If inflammation cannot acutely restore function or if the mechanisms behind the damage are not controlled and become chronic, there is a risk that inflammation also becomes chronic, adding further complexity to the progression of related disease.2
Several factors play important and determining roles in the inflammatory process. Inflammation begins when immune cells detect potentially harmful signals, such as the presence of a pathogen or the occurrence of tissue injury. Immune cells have receptors that are activated in response to molecular patterns present in pathogens (pathogen-associated molecular patterns [PAMPs]) and in damaged cells (danger-associated molecular patterns [DAMPs]).3 Upon engagement of receptors with DAMPs and PAMPs, intracellular signaling pathways orchestrate a transcriptional program aimed at inducing the expression of key inflammatory mediators, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-6, and monocyte chemoattractant protein-1, which compose the canonical tetrad of pro-inflammatory cytokines, and chemokines that coordinate the recruitment of other immune cells to the anatomical site under threat.4
Both genetic and environmental factors are involved in the regulation of inflammatory responses. One such environmental factor that plays an essential role in this context is food (Fig. 1). Studies performed over the last 30 years have identified specific nutrients with potent proinflammatory actions; moreover, the mechanisms by which nutrients activate inflammation and how they impact chronic, non-communicable disease progression have been characterized.5 Due to the focus of this review, emphasis will be given to metabolic inflammation, which affects the pathophysiology of obesity and several of its comorbidities.
In 1993, Hotamisligil et al.6 provided one of the most significant advances in understanding the mechanisms linking obesity and insulin resistance. Studying the adipose tissue of rodents, they demonstrated that mice and rats with obesity and diabetes expressed TNF-α in the adipose tissue, which did not occur in lean mice without diabetes. In addition, they showed that systemically injected TNF-α promoted glucose intolerance due to insulin resistance.6 It was proposed that the hypertrophic adipose tissue was acting as a chronically damaged tissue, which triggered an inflammatory response in an attempt to restore physiological parameters.
Thirty years after the first description of metabolic-associated inflammation, it is known to be a multifactorial process that depends on environmental and genetic factors and can promote damage in distinct tissues and organs. Nutritional factors are important triggers of the process, particularly the consumption of large amounts of saturated fats, which can act by different mechanisms (Fig. 1).7
In animal models, consuming large amounts of saturated fats promotes the rapid activation of an inflammatory response in the hypothalamus, damaging neurons that play a central role in controlling food intake and energy expenditure.8 As neurons are rapidly affected, mice lose the capacity to properly balance food intake and energy expenditure, which accelerates body mass gain and deteriorates systemic metabolism.9
The gut microbiota is also rapidly affected by the consumption of energy-dense foods, particularly those that are poor in fiber and subproducts of fermentation and rich in fats and sugar.10 The major outcomes of this process are related to a reduction of microbial diversity, increased prevalence of microorganisms that promote greater energy harvesting from food, and increased prevalence of microorganisms that disrupt the gut barrier.11 Taken together, these abnormalities favor a positive energy balance and an increase of the transposition of gut lipopolysaccharide to the bloodstream, boosting systemic metabolic inflammation (Fig. 1).5
Dietary fats also affect adipocytes and adipose tissue-resident macrophages, inducing the expression of inflammatory cytokines and chemokines.12 At least in part, this process depends on the capacity of long-chain saturated fats to activate toll-like receptor 4 in bone marrow-derived cells and on the recruitment of systemic monocytes to the adipose tissue.13 These effects are not immediate and depend on long-term consumption of a high-fat diet plus an increase in the mass of adipose tissue, which is different from the rapid effects of saturated fats on the hypothalamus and the gut microbiota.14
The effects of surplus dietary fats on the hypothalamus, the gut microbiota, and adipose tissue are central to metabolic inflammation (Fig. 1).5,9,15 However, there are several other outcomes resulting from the chronicity of this phenomenon, including predisposing the liver to metabolic-associated fatty liver disease, atherosclerosis, Alzheimer’s disease, and some types of cancer. Details regarding the impact of metabolic inflammation on these conditions are out of the scope of this article, and readers interested in these themes are encouraged to read the following authoritative reviews.15-18 Here, we discuss the potential role of the growth hormone (GH) axis in regulating inflammation, metabolism, predisposition to chronic, non-communicable diseases, and longevity.
Numerous brain regions express the GH receptor (GHR).19-21 Although some studies suggest the presence of local GH production in some areas of the central nervous system (CNS),22,23 pituitary-derived GH is probably the primary source in the brain since GH readily crosses the blood-brain barrier.24,25 GHR signaling recruits signal transducer and activator of transcription 5 (STAT5) as its major intracellular signaling pathway.26 Thus, immunoreactivity against phosphorylated STAT5 (pSTAT5) after an acute GH injection has been frequently used as a marker for GHR-expressing cells in the CNS.19,27,28 Notably, a systemic GH injection induces pSTAT5 in several neuronal populations of the hypothalamus, amygdala, hippocampus, and brainstem.19 Despite the robust responsiveness to GH in the brain, many previous studies focused on the potential role of insulin-like growth factor 1 (IGF-1) to convey the effects induced by changes in GH secretion,29,30 since GH secretion controls hepatic IGF-1 expression and serum IGF-1 level (Fig. 2).31 Thus, separating the direct effects induced by GH on the CNS from the indirect effects mediated by IGF-1 has been challenging.
In recent years, several studies have established the critical role of GHR signaling in the CNS in regulating metabolism.32 A central GH infusion stimulates food intake in mice.33 Furthermore, GH overexpression causes hyperphagia in mice or fish and induces obesity.34,35 GHR ablation in neurons reduces pregnancy-induced hyperphagia.36 GHR signaling in glutamatergic or proopiomelanocortin-expressing neurons blunts the glucopenia-induced hyperphagia.37,38 GHR inactivation in neurons or blockade of ghrelin-induced GH secretion completely abrogates the orexigenic response to ghrelin.39,40 Thus, GH acts in the CNS to stimulate hunger, and this effect is required for the ghrelin orexigenic response. Considering that body and tissue growth are energy-demanding processes, the association between growth and the orexigenic effect of GH makes sense from a physiological perspective.
GHR expression is found in approximately 95% of agouti-related protein (AgRP)-expressing neurons.33 GHR ablation in AgRP neurons prevents key energy-saving adaptations during chronic food restriction, including suppression of the thyroid and reproductive axes and thermogenesis. This precludes the decrease in energy expenditure of food-deprived mice, enhancing weight loss.33,41,42 Central GH action also regulates glucose homeostasis. GHR inactivation in the ventromedial nucleus of the hypothalamus impairs the capacity to recover from hypoglycemia.43 Moreover, central GHR signaling modulates insulin sensitivity in pregnant mice.36
GHR signaling in the CNS affects not only metabolism, but also several cognitive and neuropsychiatric aspects. The GH/IGF-1 axis and the STAT5 signaling pathway regulate learning, memory, and hippocampal function.44-46 Children and adults with GH deficiency exhibit an increased prevalence of neuropsychiatric diseases, such as anxiety and depression.47 Dos Santos et al.48 demonstrated that GHR ablation in somatostatin-expressing cells increases anxiety-like behavior in mice. These effects are associated with changes in the expression of genes involved in synaptic plasticity and function in the central and basolateral amygdala complex. Somatostatin-specific GHR knockout mice also exhibited reduced fear memory during auditory Pavlovian fear conditioning.48 These findings are in accordance with previous studies showing that GH overexpression in the amygdala increases fear memory and enhances local neuronal activity and dendritic spine density.22,49 Thus, central GHR signaling modulates several neurological aspects, possibly affecting predisposition to neuropsychiatric diseases including anxiety and post-traumatic stress disorder.
A remarkable feature of whole-body GHR knockout mice is their increased longevity, reaching nearly 5 years.50,51 GHR knockout or GH-deficient mice are protected against aging-induced decline in insulin sensitivity, memory, and neurological functions.45,52-54 In this section, we discuss the role of GH in regulating inflammation, insulin sensitivity, and longevity either directly via GHR signaling or indirectly by modulating IGF-1 secretion.
Treatment of Wistar rats with GH for 1 week increases the hypothalamic and hippocampal expression of glial fibrillary acidic protein (GFAP),55 an astroglial marker frequently associated with neuroinflammation and brain damage.56,57 GH oversecretion triggers inflammation in the body58 and some brain regions.59 The GFAP expression in the brain of 3.5-month-old mice oversecreting GH is equivalent to that found in 12-month-old wild-type mice, indicating accelerated aging when the GH/IGF-1 axis is chronically overstimulated.59 In contrast, GHR-deficient mice are protected against age-related NLR family pyrin domain containing 3 inflammasome activation and immune senescence,60 and they present an increased neuron/glia ratio in the cerebral cortex.61 Eighteen-month-old GH-deficient mice exhibit reduced expression of neuroinflammation markers in the hypothalamus compared to control animals.62 GH or GHR deficiency also protects mice against obesity-induced inflammation in the hypothalamus and adipose tissue.63,64 Thus, suppressed activity of the GH axis appears to reduce systemic and brain inflammation. However, since GH or GHR deficiency leads to suppressed IGF-1 expression and secretion, it was unclear until recently whether the lower activity of GH or IGF-1 receptor signaling underlaid these effects.
A recent study confirmed the critical role of the GH/IGF-1 axis in regulating neuroinflammation, initially using mice presenting GH deficiency or oversecretion.30 While GH-deficient mice exhibit reduced hypothalamic expression of GFAP, F4/80, ionized calcium-binding adapter molecule 1 (Iba1), and TNF-α, indicating reduced neuroinflammation, transgenic mice overexpressing GH show the opposite phenotype.30 However, GH and IGF-1 levels are suppressed in GH-deficient mice, whereas these hormones are elevated in the blood of mice oversecreting GH.30 Tissue-specific GHR knockout mice were used to uncover the individual roles of GH and IGF-1 in inducing hypothalamic neuroinflammation. GHR ablation in hepatocytes generates mice with very low circulating IGF-1 level but increased GH secretion, probably due to the loss of IGF-1 negative feedback.30,65 In such cases, hepatocyte-specific GHR knockout mice show increased hypothalamic expression of GFAP, Iba1, and TNF-α, suggesting increased neuroinflammation in the hypothalamus.30 In contrast, GHR ablation in nestin-expressing cells, which comprise neurons and part of glial cells, decreases the expression of GFAP and F4/80 in the hypothalamus, indicating reduced neuroinflammation.30 These results suggest that the protection against neuroinflammation observed in GH- or GHR-deficient mice is not explained by reduced IGF-1 secretion but rather by decreased GHR signaling in the brain. Thus, GHR signaling has a direct pro-inflammatory effect on the hypothalamus, independent of changes in IGF-1 secretion (Fig. 2).
Several pieces of evidence indicate that the pro-inflammatory effect of GHR signaling involves activation of nuclear factor kappa B (NF-kB). GH-releasing hormone (GHRH)-deficient mice or administration of GHRH antagonists in wild-type mice, conditions that suppress GH secretion, decrease the expression of NF-kB in the prefrontal cortex.66 GH activates the NF-kB pathway in growth plate chondrogenesis to promote longitudinal bone growth.67 In vitro GH treatment in macrophages stimulates the production of pro-inflammatory cytokines, such as IL-1β, interferon-γ, IL-12, C-C motif chemokine ligand 5 (CCL5), and CCL3, via NF-kB-dependent mechanisms.68 GH also increases NF-kB activity in preadipocytes, leading to higher inflammatory response and stimulation of multiple pro-inflammatory cytokines, including TNF-α, IL-6, and CCL2.69
Although chronic inflammation produces negative consequences, acute inflammation is essential during tissue damage and recovery. In this regard, GH and IGF-1 can regulate inflammation and act as growth factors contributing to tissue regeneration.70 GH treatment improves neuron survival, reduces apoptosis, decreases the expression of inflammatory mediators, and enhances the expression of neurotrophic factors in chicken embryos exposed to hypoxia.71,72 GH also presents neuroprotective effects against excitotoxicity and increases the expression of neurotrophins and growth factors in chicken neuroretinal cells.73 Preliminary evidence suggests that GH treatment has a beneficial effect on spinal cord injury.74 GH administration improves motor function in rats with spinal cord injury.75 Topical application of GH in rats improves neuronal function and attenuates edema formation and cell injury after spinal cord injury.76 A recent study demonstrated that GH treatment enhances the recovery of spinal cord-lesioned animals and presents an anti-inflammatory effect.77 Thus, the pro-inflammatory effect of GHR signaling in normal physiological conditions or chronically elevated GH secretion is not observed during brain injury. Conversely, GH seems to have an anti-inflammatory effect in this situation and contributes to neuroregeneration.
Despite the evidence that GHR signaling directly influences inflammation and other physiological aspects of the brain, the IGF-1 receptor is widely expressed in neuronal and glial cells.78 IGF-1 robustly stimulates 2-deoxyglucose uptake in glial cells but not neurons, suggesting signaling via insulin receptors in glial cells.78 IGF-1 can also act as a neurotrophic growth factor in neurons and glial cells.78 Additionally, IGF-1 is critical in brain development, maturation, and neuroplasticity.79-82 IGF-1 stimulates neurogenesis and synaptic plasticity.83-86 For example, IGF-1 treatment increases the expression of N-methyl-D-aspartate receptor subunits in the rat hippocampus.84 IGF-1 can modulate synaptic plasticity in the prefrontal cortex and basolateral amygdala to affect fear memory.85,86 IGF-1 is also involved in wound repair after brain damage.87,88 Thus, IGF-1 is an essential modulator of various neurological functions, including those in development, and can have either GH-independent or -dependent effects on the CNS.
As GH prevents high lipolytic activity, GH or GHR deficiency leads to increased body fat percentage. Conversely, patients with acromegaly may exhibit decreased adiposity.89-91 Obese individuals frequently present reduced blood GH levels.92,93 The reasons for this reduction are unknown, but some authors suggest that GH secretion is suppressed by increased fatty acid flux89,93 and insulin levels,92 which are elevated in obese subjects. Thus, GH replacement has been considered a potential therapy for obesity treatment. Although GH treatment slightly reduces body adiposity, some limitations were observed, mainly because GH causes insulin resistance.93,94
Overnutrition increases endoplasmic reticulum stress in the hypothalamus, which activates NF-kB signaling.95 Activation of NF-kB in the hypothalamus decreases insulin sensitivity and favors the development of obesity.95,96 Patients with acromegaly exhibit decreased insulin sensitivity and a higher risk of developing type 2 diabetes mellitus.97,98 Conversely, the reduced activation of pro-inflammatory pathways in GH- or GHR-deficient animals likely contributes to their increased insulin sensitivity. Several studies also suggest that the GH-induced increase in fatty acid flux leads to lipotoxicity in several tissues, explaining the diabetogenic effect of GH.89 Thus, decreased GH secretion in obese individuals has a beneficial effect on glucose homeostasis, although a probable reduction in GH-induced lipolytic effect occurs in obesity.
The increased insulin sensitivity in GH- and GHR-deficient mice likely impacts longevity positively. Accordingly, centenarians present lower insulin resistance and more highly preserved β-cell function than non-centenarians.99 Decreased insulin/IGF-1 signaling increases lifespan more than two-fold in Caenorhabditis elegans.100 Since insulin and IGF-1 share the same intracellular signaling pathway, insulin and IGF-1 resistance can emerge simultaneously.101 Notably, Alzheimer’s disease patients exhibit insulin and IGF-1 resistance in the brain, which are associated with cognitive decline.101 In addition to insulin resistance, neuroinflammation is also associated with neurodegenerative and neuropsychiatric diseases.102,103 For example, neuroinflammation is a hallmark of Alzheimer’s and Parkinson’s diseases.104,105 Additionally, clinical and preclinical evidence indicates that neuroinflammation plays a significant role in the neurobiological dysfunctions associated with major depressive disorder.103 Since GHR signaling in different areas of the brain stimulates the NF-kB pathway, pro-inflammatory cytokine production, and insulin/IGF-1 resistance, these effects influence the predisposition to metabolic and neurological diseases and impact longevity.106,107
This review presents the current evidence for association among GH action in the brain and other tissues, inflammation, and longevity. As GH secretion declines during aging,26,108 it is tempting to speculate that reduced GH secretion during aging may protect against its negative effects since GHR signaling stimulates neuroinflammation and insulin resistance (Fig. 3). Although it was not in the scope of the present review to discuss the role of GH in stimulating cell proliferation, it is possible that aging-induced suppression of GH secretion may have a beneficial effect in preventing cancer.109 The remarkable increase in longevity in GH- and GHR-deficient animals and their observed protection against several age-related problems, including metabolic, neurodegenerative, and cancer diseases, have sparked discussions about the benefits of GH deficiency (either genetically or physiologically induced).109 On the other hand, considerable evidence demonstrates the physiological importance of GHR signaling in neuroprotective and neuroregenerative effects upon injury,70-77 as well as its critical role in homeostasis under metabolic stress108 and several other positive effects (Fig. 3). Therefore, like numerous other physiological systems, ideal activity of the GH/IGF-1 axis is likely required to promote health, and both excess and absence of GH action can cause undesired consequences that impact the quality of life, the predisposition to diseases, and longevity (Fig. 3).
The authors declare no conflict of interest.
The authors are supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP/Brazil; grant numbers 2013/07607-8 to Licio A. Velloso and 2020/01318-8 to Jose Donato Jr.) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil; grant number: 306024/2023-3 to Jose Donato Jr.).
Study concept and design: JD; drafting of the manuscript: LAV and JD; critical revision of the manuscript: LAV and JD; and study supervision: JD.
Online ISSN : 2508-7576Print ISSN : 2508-6235
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