Characterization of the priming effect by pituitary canine growth hormone on canine polymorphonuclear neutrophil granulocyte function

Intrathymic somatotropic circuitry: consequences upon thymus involution

Growth hormone (GH) is a classic pituitary-derived hormone crucial to body growth and metabolism. In the pituitary gland, GH production is stimulated by GH-releasing hormone and inhibited by somatostatin. GH secretion can also be induced by other peptides, such as ghrelin, which interacts with receptors present in somatotropic cells. It is well established that GH acts directly on target cells or indirectly by stimulating the production of insulin-like growth factors (IGFs), particularly IGF-1. Notably, such somatotropic circuitry is also involved in the development and function of immune cells and organs, including the thymus. Interestingly, GH, IGF-1, ghrelin, and somatostatin are expressed in the thymus in the lymphoid and microenvironmental compartments, where they stimulate the secretion of soluble factors and extracellular matrix molecules involved in the general process of intrathymic T-cell development. Clinical trials in which GH was used to treat immunocompromised patients successfully recovered thymic function. Additionally, there is evidence that the reduction in the function of the somatotropic axis is associated with age-related thymus atrophy. Treatment with GH, IGF-1 or ghrelin can restore thymopoiesis of old animals, thus in keeping with a clinical study showing that treatment with GH, associated with metformin and dehydroepiandrosterone, could induce thymus regeneration in healthy aged individuals. In conclusion, the molecules of the somatotrophic axis can be envisioned as potential therapeutic targets for thymus regeneration in age-related or pathological thymus involution.

Expression, regulation and biological actions of growth hormone (GH) and ghrelin in the immune system

Immune and neuroendocrine systems have bidirectional communications. Growth hormone (GH) and an orexigenic hormone ghrelin are expressed in various immune cells such as T lymphocytes, B lymphocytes, monocytes and neutrophils. These immune cells also bear receptors for hormones: growth hormone receptor (GHR) for GH and growth hormone secretagogue receptor (GHS-R) for ghrelin. The expression of GH in immune cells is stimulated by ghrelin as in anterior pituitary cells, whereas the regulation of GH secretion in the immune system by other peptides seems to be different from that in the anterior pituitary gland. Cytokines and mitogens enhance GH secretion from immune cells. GH has several biological actions in the immune system: enhancing thymopoiesis and T cell development, modulating cytokine production, enhancing B cell development and antibody production, priming neutrophils and monocytes for superoxide anion secretion, enhancing neutrophil adhesion and monocyte migration and anti-apoptotic action. Biological actions of ghrelin include attenuation of septic shock and anti-inflammatory actions, modulating phagocytosis, and enhancing thymopoiesis. The effect of ghrelin may be direct or through GH production, and that of GH may be direct or through insulin like growth factor-I (IGF-I) production. Elucidation of the roles of GH and ghrelin in the immune system may shed light on the treatment and prevention of immunological disorders such as AIDS and organ damages due to obesity/ageing-related chronic inflammation.

Growth hormone increases circulating neutrophil activation and provokes lung microvascular injury in septic peritonitis rats

BACKGROUND

Growth hormone (GH) has been shown to increase mortality in critical illness, illustrating the need for better understanding of GH treatment. The neutrophil is a key mediator in producing organ injury following shock and trauma and is regulated by GH. Therefore, the purpose of the present study was to examine the effects of GH on circulating neutrophil activation and subsequent lung injury induced by sepsis in rats.

MATERIALS AND METHODS

Sepsis was induced in male Wistar rats via cecal ligation and puncture (CLP). Recombinant human growth hormone (1 IU/kg) was given subcutaneously right after CLP with an additional injection at 12 h after CLP. CD11b expression and an oxidative burst of neutrophils were detected using flow cytometric analysis. Lung myeloperoxidase activity was determined as an index of tissue neutrophil accumulation. Lung microvascular injury was assessed by quantitating extravasation of Evan's blue dye into lung parenchyma.

RESULTS

Growth hormone significantly increased sepsis-induced expression of CD11b and sepsis-induced circulating neutrophil activation. Also growth hormone increased neutrophil accumulation in lungs induced by sepsis. Lung microvascular injury was aggravated by growth hormone treatment in septic rats.

CONCLUSIONS

It is worthwhile to rethink GH administration in critical illness and further studies are required to determine the safety and clinical benefits of GH administration in critical illness.