Regulation of glucocorticoid sensitivity in thymocytes from burn-injured mice

Expression of 11beta-hydroxysteroid-dehydrogenase type 2 in human thymus

11beta-hydroxysteroid-dehydrogenase type 2 (11β-HSD2) is a high affinity dehydrogenase which rapidly inactivates physiologically-active glucocorticoids to protect key tissues. 11β-HSD2 expression has been described in peripheral cells of the innate and the adaptive immune system as well as in murine thymus. In absence of knowledge of 11β-HSD2 expression in human thymus, the study aimed to localize 11β-HSD2 in human thymic tissue. Thymic tissue was taken of six healthy, non-immunologically impaired male infants below 12months of age with congenital heart defects who had to undergo correction surgery. 11β-HSD2 protein expression was analyzed by immunohistochemistry and Western blot. Kidney tissue, peripheral blood mononuclear cells (PBMCs) and human umbilical vein endothelial cells (HUVEC) were taken as positive controls. Significant expression of 11β-HSD2 protein was found at single cell level in thymus parenchyma, at perivascular sites of capillaries and small vessels penetrating the thymus lobuli and within Hassall's bodies. The present study demonstrates that 11β-HSD2 is expressed in human thymus with predominant perivascular expression and also within Hassall's bodies. To our knowledge, this is the first report confirming 11β-HSD2 expression at the protein level in human thymic tissue underlining a potential role of this enzyme in regulating glucocorticoid function at the thymic level.

Regulation of 11β-hydroxysteroid dehydrogenase type 1 and 7α-hydroxylase CYP7B1 during social stress

11β-hydroxysteroid dehydrogenase type 1 (11HSD1) is an enzyme that amplifies intracellular glucocorticoid concentration by the conversion of inert glucocorticoids to active forms and is involved in the interconversion of 7-oxo- and 7-hydroxy-steroids, which can interfere with the activation of glucocorticoids. The presence of 11HSD1 in the structures of the hypothalamic-pituitary-adrenal (HPA) axis suggests that this enzyme might play a role in the regulation of HPA output. Here we show that the exposure of Fisher 344 rats to mild social stress based on the resident-intruder paradigm increased the expression of 11HSD1 and CYP7B1, an enzyme that catalyzes 7-hydroxylation of steroids. We found that social behavioral profile of intruders was significantly decreased whereas their plasma levels of corticosterone were increased more than in residents. The stress did not modulate 11HSD1 in the HPA axis (paraventricular nucleus, pituitary, adrenal cortex) but selectively upregulated 11HSD1 in some regions of the hippocampus, amygdala and prelimbic cortex. In contrast, CYP7B1 was upregulated not only in the hippocampus and amygdala but also in paraventricular nucleus and pituitary. Furthermore, the stress downregulated 11HSD1 in the thymus and upregulated it in the spleen and mesenteric lymphatic nodes whereas CYP7B1 was upregulated in all of these lymphoid organs. The response of 11HSD1 to stress was more obvious in intruders than in residents and the response of CYP7B1 to stress predominated in residents. We conclude that social stress induces changes in enzymes of local metabolism of glucocorticoids in lymphoid organs and in brain structures associated with the regulation of the HPA axis. In addition, the presented data clearly suggest a role of 11HSD1 in modulation of glucocorticoid feedback of the HPA axis during stress.

The anti-inflammatory and immunosuppressive effects of glucocorticoids, recent developments and mechanistic insights

Since the discovery of glucocorticoids in the 1940s and the recognition of their anti-inflammatory effects, they have been amongst the most widely used and effective treatments to control inflammatory and autoimmune diseases. However, their clinical efficacy is compromised by the metabolic effects of long-term treatment, which include osteoporosis, hypertension, dyslipidaemia and insulin resistance/type 2 diabetes mellitus. In recent years, a great deal of effort has been invested in identifying compounds that separate the beneficial anti-inflammatory effects from the adverse metabolic effects of glucocorticoids, with limited effect. It is clear that for these efforts to be effective, a greater understanding is required of the mechanisms by which glucocorticoids exert their anti-inflammatory and immunosuppressive actions. Recent research is shedding new light on some of these mechanisms and has produced some surprising new findings. Some of these recent developments are reviewed here.

T cells from burn-injured mice demonstrate a loss of sensitivity to glucocorticoids

Glucocorticoids (GC) are steroid hormones that modulate T cell functions and restrain their hyperresponsiveness following stimulation. Naive T lymphocytes are sensitive to GC but become more resistant when they are activated. A balance between activation and inhibition signals is important for a targeted and effective T cell response. Thermal injury is characterized by an immune dysfunction and hyperactive T cells visible at day 10 postburn. In this study, our objective was to evaluate T cell sensitivity to GC following thermal injury and to identify mechanisms that could modulate their sensitivity. One mechanism that we hypothesized was increased p38 mitogen-activated protein kinase (MAPK) activity that could lead to GC resistance. Male C57BL/6 mice underwent a full-thickness 20% total body surface area. At 10 days postinjury, splenic T cells were isolated. Glucocorticoid receptor (GR) expression was higher in T cells from burn-injured mice. Interestingly, these cells were also less sensitive to GC-induced apoptosis prior to and poststimulation. Furthermore, anti-CD3-activated T cells from burn-injured mice showed increased proliferation and CD25 expression, which resisted corticosterone's (CORT) suppressive effect. Anti-CD3-activated CD4(+)CD44(+) memory cells from burn-injured mice expressed the highest level of CD25 and were resistant to CORT. Increased phosphorylation of p38 MAPK was also noted in activated T cells from burn-injured mice. Pharmacological inhibition of p38 MAPK decreased cell proliferation and normalized interferon-gamma (IFNgamma) production. In conclusion, we demonstrate that a unique event like burn injury induces a loss of sensitivity to GC in splenic T cells and have identified p38 MAPK as a key modulator for this resistance.

Co-species housing in mice and rats: effects on physiological and behavioral stress responsivity

Co-species housing of mice and rats is common practice at most breeding facilities and research laboratories, neglecting the possible effects on the animals. We investigated physiological as well as behavioral stress-reactivity in mice and rats which were either derived from a co-species or species-separated housing condition at the breeding facilities. The animals were kept under the housing condition they were used to or assigned to the opposite one. Co-species housing had a significant impact on acute stress reactivity in mice and rats but only if they were used to this housing condition throughout their lives. Moreover, the stress-effects appeared to be long lasting. Assigning animals, derived from a species-separated housing condition, to co-species housing led to chronic stress in mice and affected experimental behavior of rats. Our findings led to the conclusion that co-species housing in mice and rats should be avoided, supporting the recommendations by the U.S. National Institutes of Health (NIH) and the Dutch Ministry of Health, Welfare and Sport (VWS). In order to support the interpretation, facilitate the reproducibility and comparability and subsequently the generalizability of experimental results, breeding facilities should at least provide detailed information about their housing conditions.

11Beta-hydroxysteroid dehydrogenase type 1 and its role in the hypothalamus-pituitary-adrenal axis, metabolic syndrome, and inflammation

CONTEXT

11Beta-hydroxysteroid dehydrogenase (11beta-HSD) enzymes are now appreciated to be important regulators of hormone action at a tissue level. 11Beta-HSD1 is widely expressed and increases glucocorticoid action through its unique ability to convert inactive glucocorticoids (cortisone in man, 11-dehydrocorticosterone in rodents) to their active forms (cortisol and corticosterone, respectively). The enzyme has roles in the normal hypothalamus-pituitary-adrenal (HPA) axis, has been implicated in metabolic syndrome, and may modulate various aspects of the immune response.

EVIDENCE ACQUISITION

A review of published, peer-reviewed medical literature (1990 to June 2009) on the physiology and pathophysiology of 11beta-HSD1 was performed with an emphasis on HPA axis consequences, the metabolic syndrome, and the inflammatory response.

EVIDENCE SYNTHESIS

Studies of patients with genetic defects in 11beta-HSD1 action show abnormal HPA axis responses with hyperandrogenism being a major consequence. The mechanisms underlying these abnormalities have been explored in mouse models with targeted deletion of components of the 11beta-HSD1 system. A range of experimental studies emphasize the role of 11beta-HSD1 in the metabolic syndrome and the potential for treatment with chemical inhibitors. An emerging area is the role of 11beta-HSD1 in the inflammatory response.

CONCLUSIONS

11Beta-HSD1 activity is an important component of the HPA axis and contributes to the metabolic syndrome and the normal immune response. Ongoing clinical observations and the development of selective inhibitors will further clarify the role of 11beta-HSD1 in these areas.

Thymocyte-synthesized glucocorticoids play a role in thymocyte homeostasis and are down-regulated by adrenocorticotropic hormone

Thymocytes from adult mice synthesize glucocorticoids (GCs), and some data indicate a role for this hormone production in thymic homeostasis. Here we present further support for this view by showing that the dramatic increase in thymocyte number seen after adrenalectomy (ADX) does not correlate with the decrease in systemic GCs but rather with an ACTH-mediated down-regulation of GC synthesis in thymocytes. High ACTH concentrations caused by ADX in wild-type mice down-regulated CYP11B1 mRNA expression, encoding the last enzyme required for corticosterone synthesis and as a consequence reduced GC synthesis in thymocytes. This was not seen in IL-1beta/IL-18 double-knockout mice unable to respond to ADX with high ACTH levels. However, if ADX IL-1beta/IL-18 double-knockout mice were treated with ACTH, this led to a down-regulation of CYP11B1 and GC synthesis in thymocytes. In addition, in vivo treatment of mice with the CYP11B1 antagonist metyrapone, without affecting the systemic corticosterone level, increased thymocyte numbers and in vitro treatment of isolated thymocytes prevented thymocyte loss. Furthermore, in vitro experiments showed that both ACTH and its receptor-induced second-messenger molecule cAMP down-regulated mRNA expression of critical enzymes in GC steroidogenesis and GC synthesis in thymocytes. We conclude that thymocyte-produced GCs are important for the homeostasis of adult mouse thymocytes and that high ACTH level, in contrast to stimulating GC synthesis in the adrenal glands, has the opposite effect in thymocytes.