Karen E. Chapman

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.

Postnatal Handling Increases the Expression of cAMP-Inducible Transcription Factors in the Rat Hippocampus: The Effects of Thyroid Hormones and Serotonin

Postnatal handling increases glucocorticoid receptor expression in the rat hippocampus, thus altering the regulation of hypothalamic synthesis of corticotropin-releasing hormone and the hypothalamic–pituitary–adrenal response to stress. The effect on glucocorticoid receptor gene expression represents one mechanism by which the early environment can exert a long-term effect on neural development. The handling effect on hippocampal glucocorticoid receptor expression is dependent on peripheral thyroid hormone release and the activation of ascending serotonergic pathways. In primary hippocampal cell cultures, serotonin (5-HT) increases glucocorticoid receptor expression, and this effect appears to be mediated by increased cAMP levels. In the current studies we examined the in vivoeffects of handling on hippocampal cAMP–protein kinase A (PKA) activity. In 7-d-old rat pups, we found that (1) postnatal handling increased adenylyl cyclase activity and hippocampal cAMP levels, (2) the effect of handling on cAMP levels was completely blocked by treatment with either propylthiouracil (PTU), a thyroid hormone synthesis inhibitor, or the 5-HT receptor antagonist, ketanserin, and (3) handling also increased hippocampal PKA activity. We then examined the effects of handling on cAMP-inducible transcription factors. Handling rapidly increased levels of the mRNAs for nerve growth factor-inducible factor A (NGFI-A) (zif268,krox24) and activator protein-2 (AP-2) as well as for NGFI-A and AP-2 immunoreactivity throughout the hippocampus. Finally, we found that the effects of handling on NGFI-A and AP-2 expression were significantly reduced by concurrent treatment with either PTU or ketanserin, effects that paralleled those on cAMP formation. NGFI-A and AP-2 have been implicated in the regulation of glucocorticoid receptor expression during development. Thus, these findings suggest that postnatal handling might alter glucocorticoid receptor gene expression via cAMP–PKA pathways involving the activation of NGFI-A and AP-2.

High Titers of Transmissible Spongiform Encephalopathy Infectivity Associated with Extremely Low Levels of PrPSc in Vivo

Diagnosis of transmissible spongiform encephalopathy (TSE) disease in humans and ruminants relies on the detection in post-mortem brain tissue of the protease-resistant form of the host glycoprotein PrP. The presence of this abnormal isoform (PrPSc) in tissues is taken as indicative of the presence of TSE infectivity. Here we demonstrate conclusively that high titers of TSE infectivity can be present in brain tissue of animals that show clinical and vacuolar signs of TSE disease but contain low or undetectable levels of PrPSc. This work questions the correlation between PrPSc level and the titer of infectivity and shows that tissues containing little or no proteinase K-resistant PrP can be infectious and harbor high titers of TSE infectivity. Reliance on protease-resistant PrPSc as a sole measure of infectivity may therefore in some instances significantly underestimate biological properties of diagnostic samples, thereby undermining efforts to contain and eradicate TSEs.

Local amplification of glucocorticoids by 11 beta-hydroxysteroid dehydrogenase type 1 promotes macrophage phagocytosis of apoptotic leukocytes.

Glucocorticoids promote macrophage phagocytosis of leukocytes undergoing apoptosis. Prereceptor metabolism of glucocorticoids by 11β-hydroxysteroid dehydrogenases (11β-HSDs) modulates cellular steroid action. 11β-HSD type 1 amplifies intracellular levels of active glucocorticoids in mice by reactivating corticosterone from inert 11-dehydrocorticosterone in cells expressing the enzyme. In this study we describe the rapid (within 3 h) induction of 11β-HSD activity in cells elicited in the peritoneum by a single thioglycolate injection in mice. Levels remained high in peritoneal cells until resolution. In vitro experiments on mouse macrophages demonstrated that treatment with inert 11-dehydrocorticosterone for 24 h increased phagocytosis of apoptotic neutrophils to the same extent as corticosterone. This effect was dependent upon 11β-HSD1, as 11β-HSD1 mRNA, but not 11β-HSD2 mRNA, was expressed in these cells; 11-dehydrocorticosterone was ineffective in promoting phagocytosis by Hsd11b1−/− macrophages, and carbenoxolone, an 11β-HSD inhibitor, prevented the increase in phagocytosis elicited in wild-type macrophages by 11-dehydrocorticosterone. Importantly, as experimental peritonitis progressed, clearance of apoptotic neutrophils was delayed in Hsd11b1−/− mice. These data point to an early role for 11β-HSD1 in promoting the rapid clearance of apoptotic cells during the resolution of inflammation and indicate a novel target for therapy.

cDNA sequence of human β-preprotachykinin, the common precursor to substance P and neurokinin A

The nucleotide sequence of cDNA encoding the human substance P precursor, β‐preprotachykinin (β‐PPT), has been determined. The source of mRNA was a human laryngeal carcinoid tumour that contained a high concentration of immunoreactive substance P. The human β‐PPT polypeptide is 129 amino acids long and contains regions encoding substance P and neurokinin A, each flanked by basic amino acid residues. Residues 72–107 of the human β‐PPT polypeptide encode the sequence of neuropeptide K, an N‐terminally extended form of neurokinin A recently isolated from porcine brain.

Early environmental regulation of hippocampal glucocorticoid receptor gene expression: characterization of intracellular mediators and potential genomic target sites.

Environmental conditions in early life permanently alter the development of glucocorticoid receptor gene expression in the hippocampus and hypothalamic-pituitary-adrenal responses to acute or chronic stress. In part, these effects can involve an activation of ascending serotonergic pathways and subsequent changes in the expression of transcription factors that might drive glucocorticoid receptor expression in the hippocampus. This paper summarizes the evidence in favor of these pathways as well as recent studies describing regulatory targets on the promoter region of the rat hippocampal glucocorticoid receptor gene.

Omental 11β‐hydroxysteroid Dehydrogenase 1 Correlates with Fat Cell Size Independently of Obesity

Objectives: In ideopathic obesity, there is evidence that enhanced cortisol regeneration within abdominal subcutaneous adipose tissue may contribute to adiposity and metabolic disease. Whether the cortisol regenerating enzyme, 11β‐hydroxysteroid dehydrogenase type 1 (11βHSD1), or glucocorticoid receptor (GRα) levels are altered in other adipose depots remains uncertain. Our objective was to determine the association between 11βHSD1 and GRα mRNA levels in four distinct adipose depots and measures of obesity and the metabolic syndrome.

Identification and cDNA sequence of δ‐preprotachykinin, a fourth splicing variant of the rat substance P precursor

The neuropeptides substance P and neurokinin A are synthesised from a family of precursor polypeptides encoded by the preprotachykinin A (PFT) gene. In addition to mRNA (β‐PPT) containing all 7 exons of the gene, alternatively spliced mRNAs lacking either exon 4 (γ‐PPT) or exon 6 (α‐PPT) have been identified. We have determined the sequences of cDNA clones encoding four variants of PPT mRNA from rat dorsal root ganglion (DRG), including a novel mRNA species (δ‐PPT) in which both exons 4 and 6 are absent. The sequence of δ‐PPT predicts the existence of a novel tachykinin precursor polypeptide.

MEDICAL AND PHYSIOLOGICAL ASPECTS OF THE 11BETA -HYDROXYSTEROID DEHYDROGENASE SYSTEM

11Beta-hydroxysteroid dehydrogenases (11beta-HSD) catalyse the interconversion of active glucocorticoids (cortisol, corticosterone) and their inert 11-keto derivatives (cortisone, 11-dehydrocorticosterone). The type-2 isozyme (11beta-HSD-2) is a high-affinity dehydrogenase that catalyses the rapid inactivation of glucocorticoids, thus ensuring selective access of aldosterone to otherwise non-selective mineralocorticoid receptors in the distal nephron. Mutations of the gene encoding 11beta-HSD-2 are responsible for the syndrome of apparent mineralocorticoid excess, in which cortisol illicitly occupies mineralocorticoid receptors, causing hypertension and hypokalaemia. 11Beta-HSD-2 is also highly expressed in the placenta and mid-gestation fetus, where it may protect developing tissues from the often deleterious actions of glucocorticoids upon fetal growth and organ maturation. 11Beta-HSD-1 is probably an 11beta-reductase in vivo. Its function is obscure, but may amplify glucocorticoid action during the diurnal nadir, drawing upon the substantial circulating levels of 11-keto steroids. Both isozymes are regulated during ontogeny and by a series of hormonal and other factors. 11Beta-HSD provide an important control of glucocorticoid action at a cellular level, and may represent new targets for therapeutic intervention.

The role and regulation of 11β-hydroxysteroid dehydrogenase type 1 in the inflammatory response

Cortisone, a glucocorticoid hormone, was first used to treat rheumatoid arthritis in humans in the late 1940s, for which Hench, Reichstein and Kendall were awarded a Nobel Prize in 1950 and which led to the discovery of the anti-inflammatory effects of glucocorticoids. To be effective, the intrinsically inert cortisone must be converted to the active glucocorticoid, cortisol, by the intracellular action of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1). Whilst orally administered cortisone is rapidly converted to the active hormone, cortisol, by first pass metabolism in the liver, recent work has highlighted an anti-inflammatory role for 11beta-HSD1 within specific tissues, including in leukocytes. Here, we review recent evidence pertaining to the anti-inflammatory role of 11beta-HSD1 and describe how inhibition of 11beta-HSD1, as widely proposed for treatment of metabolic disease, may impact upon inflammation. Finally, the mechanisms that regulate 11beta-HSD1 transcription will be discussed.