Sexual dimorphic impact of adult-onset somatopause on life span and age-induced osteoarthritis

Growth hormone-receptor disruption in mice reduces osteoarthritis and chondrocyte hypertrophy

Excessive growth hormone (GH) has been shown to promote joint degeneration in both preclinical and clinical studies. Little is known about the effect of disrupted GH or GH receptor (GHR) on joint health. The goal of this study is to investigate joint pathology in mice with either germline (GHR-/-) or adult inducible (iGHR-/-) GHR deficiency. Knee joints from male and female GHR-/- and WT mice at 24 months of age were processed for histological analysis. Also, knee joints from male and female iGHR-/- and WT mice at 22 months of age were scanned by micro-CT (μCT) for subchondral bone changes and characterized via histology for cartilage degeneration. Joint sections were also stained for the chondrocyte hypertrophy marker, COLX, and the cartilage degeneration marker, ADAMTS-5, using immunohistochemistry. Compared to WT mice, GHR-/- mice had remarkably smooth articular joint surfaces and an even distribution of proteoglycan with no signs of degeneration. Quantitatively, GHR-/- mice had lower OARSI and Mankin scores compared to WT controls. By contrast, iGHR-/- mice were only moderately protected from developing aging-associated OA. iGHR-/- mice had a significantly lower Mankin score compared to WT. However, Mankin scores were not significantly different between iGHR-/- and WT when males and females were analyzed separately. OARSI scores did not differ significantly between WT and iGHR-/- in either individual or combined sex analyses. Both GHR-/- and iGHR-/- mice had fewer COLX+ hypertrophic chondrocytes compared to WT, while no significant difference was observed in ADAMTS-5 staining. Compared to WT, a significantly lower trabecular thickness in the subchondral bone was observed in the iGHR-/- male mice but not in the female mice. However, there were no significant differences between WT and iGHR-/- mice in the bone volume to total tissue volume (BV/TV), bone mineral density (BMD), and trabecular number in either sex. This study identified that both germline and adult-induced GHR deficiency protected mice from developing aging-associated OA with more effective protection in GHR-/- mice.

Deletion of absent in melanoma (AIM) 2 gene alters bone morphology

Absent in Melanoma (AIM) 2 is a gene that is induced by interferon and acts as a cytosolic sensor for double-stranded (ds) DNA. It forms the AIM2 inflammasome, leading to the production of interleukin (IL)-1β and IL-18. Our previous research demonstrated that mice lacking AIM2 exhibit spontaneous obesity, insulin resistance, and inflammation in adipose tissue. In this study, we aimed to explore the impact of AIM2 gene deletion on bone structure in adult and aged mice. Utilizing micro-computed tomography (micro-CT), we discovered that female mice lacking AIM2 showed an increase in the total cross-sectional area at 5 months of age, accompanied by an increase in cortical thickness in the mid-diaphysis of the femur at both 5 and 15 months of age. At 15 months of age, the cortical bone mineral density (BMD) significantly decreased in AIM2 null females compared to wild-type (WT) mice. In AIM2 null mice, both trabecular bone volume and BMD at the distal metaphysis of the femur significantly decreased at 5 and 15 months of age. Similarly, micro-CT analysis of the L4 vertebra revealed significant decreases in trabecular bone volume and BMD in aged AIM2 null females compared to WT mice. Histological examination of femurs from aged mice demonstrated increased bone marrow adiposity in AIM2 null mice, accompanied by a significant increase in CD45-/CD31-/Sca1+/Pdgfa+ adipose progenitor cells, and a decrease in the ratio of CD31-/CD31+ osteogenic progenitor cells, as determined by flow cytometry of bone marrow cells. Our findings suggest that AIM2 deficiency affects bone health by promoting adipogenesis in bone marrow cells and inducing a pro-inflammatory environment, potentially contributing to the decreased bone mineral density.

Promotion of Joint Degeneration and Chondrocyte Metabolic Dysfunction by Excessive Growth Hormone in Mice

OBJECTIVE

Many patients with acromegaly, a hormonal disorder with excessive growth hormone (GH) production, report pain in joints. We undertook this study to characterize the joint pathology of mice with overexpression of bovine GH (bGH) or a GH receptor antagonist (GHa) and to investigate the effect of GH on regulation of chondrocyte cellular metabolism.

METHODS

Knee joints from mice overexpressing bGH or GHa and wild-type (WT) control mice were examined using histology and micro-computed tomography for osteoarthritic (OA) pathologies. Additionally, cartilage from bGH mice was used for metabolomics analysis. Mouse primary chondrocytes from bGH and WT mice, with or without pegvisomant treatment, were used for quantitative polymerase chain reaction and Seahorse respirometry analyses.

RESULTS

Both male and female bGH mice at ~13 months of age had increased knee joint degeneration, which was characterized by loss of cartilage structure, expansion of hypertrophic chondrocytes, synovitis, and subchondral plate thinning. The joint pathologies were also demonstrated by significantly higher Osteoarthritis Research Society International and Mankin scores in bGH mice compared to WT control mice. Metabolomics analysis revealed changes in a wide range of metabolic pathways in bGH mice, including beta-alanine metabolism, tryptophan metabolism, lysine degradation, and ascorbate and aldarate metabolism. Also, bGH chondrocytes up-regulated fatty acid oxidation and increased expression of Col10a. Joints of GHa mice were remarkably protected from developing age-associated joint degeneration, with smooth articular joint surface.

CONCLUSION

This study showed that an excessive amount of GH promotes joint degeneration in mice, which was associated with chondrocyte metabolic dysfunction and hypertrophic changes, whereas antagonizing GH action through a GHa protects mice from OA development.

Disruption of Growth Hormone Receptor in Adipocytes Improves Insulin Sensitivity and Lifespan in Mice

Growth hormone receptor knockout (GHRKO) mice have been used for 25 years to uncover some of the many actions of growth hormone (GH). Since they are extremely long-lived with enhanced insulin sensitivity and protected from multiple age-related diseases, they are often used to study healthy aging. To determine the effect that adipose tissue has on the GHRKO phenotype, our laboratory recently created and characterized adipocyte-specific GHRKO (AdGHRKO) mice, which have increased adiposity but appear healthy with enhanced insulin sensitivity. To test the hypothesis that removal of GH action in adipocytes might partially replicate the increased lifespan and healthspan observed in global GHRKO mice, we assessed adiposity, cytokines/adipokines, glucose homeostasis, frailty, and lifespan in aging AdGHRKO mice of both sexes. Our results show that disrupting the GH receptor gene in adipocytes improved insulin sensitivity at advanced age and increased lifespan in male AdGHRKO mice. AdGHRKO mice also exhibited increased fat mass, reduced circulating levels of insulin, c-peptide, adiponectin, resistin, and improved frailty scores with increased grip strength at advanced ages. Comparison of published mean lifespan data from GHRKO mice to that from AdGHRKO and muscle-specific GHRKO mice suggests that approximately 23% of lifespan extension in male GHRKO is due to GHR disruption in adipocytes vs approximately 19% in muscle. Females benefited less from GHR disruption in these 2 tissues with approximately 19% and approximately 0%, respectively. These data indicate that removal of GH's action, even in a single tissue, is sufficient for observable health benefits that promote long-term health, reduce frailty, and increase longevity.

The Modulatory Role of Growth Hormone in Inflammation and Macrophage Activation

Inflammation is a body's response to remove harmful stimuli and heal tissue damage, which is involved in various physiology and pathophysiology conditions. If dysregulated, inflammation may lead to significant negative impacts. Growth hormone (GH) has been shown responsible for not only body growth but also critical in the modulation of inflammation. In this review, we summarize the current clinical and animal studies about the complex and critical role of GH in inflammation. Briefly, GH excess or deficiency may lead to pathological inflammatory status. In inflammatory diseases, GH may serve as an inflammatory modulator to control the disease progression and promote disease resolution. The detailed mechanisms and signaling pathways of GH on inflammation, with a focus on the modulation of macrophage polarization, are carefully discussed with potential direction for future investigations.

Musculoskeletal Effects of Altered GH Action

Growth hormone (GH) is a peptide hormone that can signal directly through its receptor or indirectly through insulin-like growth factor 1 (IGF-1) stimulation. GH draws its name from its anabolic effects on muscle and bone but also has distinct metabolic effects in multiple tissues. In addition to its metabolic and musculoskeletal effects, GH is closely associated with aging, with levels declining as individuals age but GH action negatively correlating with lifespan. GH’s effects have been studied in human conditions of GH alteration, such as acromegaly and Laron syndrome, and GH therapies have been suggested to combat aging-related musculoskeletal diseases, in part, because of the decline in GH levels with advanced age. While clinical data are inconclusive, animal models have been indispensable in understanding the underlying molecular mechanisms of GH action. This review will provide a brief overview of the musculoskeletal effects of GH, focusing on clinical and animal models.

Induction of somatopause in adult mice compromises bone morphology and exacerbates bone loss during aging

Somatopause refers to the gradual declines in growth hormone (GH) and insulin‐like growth factor‐1 throughout aging. To define how induced somatopause affects skeletal integrity, we used an inducible GH receptor knockout (iGHRKO) mouse model. Somatopause, induced globally at 6 months of age, resulted in significantly more slender bones in both male and female iGHRKO mice. In males, induced somatopause was associated with progressive expansion of the marrow cavity leading to significant thinning of the cortices, which compromised bone strength. We report progressive declines in osteocyte lacunar number, and increases in lacunar volume, in iGHRKO males, and reductions in lacunar number accompanied by ~20% loss of overall canalicular connectivity in iGHRKO females by 30 months of age. Induced somatopause did not affect mineral/matrix ratio assessed by Raman microspectroscopy. We found significant increases in bone marrow adiposity and high levels of sclerostin, a negative regulator of bone formation in iGHRKO mice. Surprisingly, however, despite compromised bone morphology, osteocyte senescence was reduced in the iGHRKO mice. In this study, we avoided the confounded effects of constitutive deficiency in the GH/IGF‐1 axis on the skeleton during growth, and specifically dissected its effects on the aging skeleton. We show here, for the first time, that induced somatopause compromises bone morphology and the bone marrow environment.

Sexual dimorphic impact of adult-onset somatopause on life span and age-induced osteoarthritis

Osteoarthritis (OA), the most prevalent joint disease, is a major cause of disability worldwide. Growth hormone (GH) has been suggested to play significant roles in maintaining articular chondrocyte function and ultimately articular cartilage (AC) homeostasis. In humans, the age-associated decline in GH levels was hypothesized to play a role in the etiology of OA. We studied the impact of adult-onset isolated GH deficiency (AOiGHD) on the life span and skeletal integrity including the AC, in 23- to 30-month-old male and female mice on C57/BL6 genetic background. Reductions in GH during adulthood were associated with extended life span and reductions in body temperature in female mice only. However, end-of-life pathology revealed high levels of lymphomas in both sexes, independent of GH status. Skeletal characterization revealed increases in OA severity in AOiGHD mice, evidenced by AC degradation in both femur and tibia, and significantly increased osteophyte formation in AOiGHD females. AOiGHD males showed significant increases in the thickness of the synovial lining cell layer that was associated with increased markers of inflammation (IL-6, iNOS). Furthermore, male AOiGHD showed significant increases in matrix metalloproteinase-13 (MMP-13), p16, and β-galactosidase immunoreactivity in the AC as compared to controls, indicating increased cell senescence. In conclusion, while the life span of AOiGHD females increased, their health span was compromised by high-grade lymphomas and the development of severe OA. In contrast, AOiGHD males, which did not show extended life span, showed an overall low grade of lymphomas but exhibited significantly decreased health span, evidenced by increased OA severity.