Glucocorticoid receptors and physiology: a personal history

Review: Minimally invasive sampling media and the measurement of corticosteroids as biomarkers of stress in animals

Cook, N. J. 2012. Review: Minimally invasive sampling media and the measurement of corticosteroids as biomarkers of stress in animals. Can. J. Anim. Sci. 92: 227–259. The measurement of corticosteroid hormones is commonly used as a biomarker of an animal's response to stress. The difficulties in obtaining blood samples and the recognition of the stressor effect of blood sampling are primary drivers for the use of minimally invasive sample media. In mammals these include saliva, feces, urine, hair, and milk. In birds, samples include excreta, feathers, egg yolk and albumin. In fish, corticosteroids have been measured in excreta and swim-water. Each of these sample media incorporate corticosteroids in accordance with the processes by which they are formed, and this in turn dictates the periods of adrenocortical activity that each sample type represents. Cortisol in saliva represents a time-frame of minutes, whereas the production of feces may be hours to days depending on the species. The longest time-integrations are for hair and feathers which could be over a period of many weeks. The sample media also determines the structural changes that may occur via processes of conjugation to glucuronides and sulfides, metabolic conversion via enzymatic action, and bacterial breakdown. Structural changes determine the optimum methodologies used to measure corticosteroid hormones. In most sample media, measurement of a specific corticosteroid is a requirement depending on the species, e.g., cortisol in most mammals, or corticosterone in birds. However, in samples involving products of excretion, methodologies that measure a broad range of structurally related compounds are probably optimal. The utility of minimally invasive sample media as biomarkers of stress responses depends on the degree to which the corticosteroid content of the sample represents adrenocortical activity. Commonly, this involves comparisons between corticosteroid concentrations in blood plasma with concentrations in the alternative sample media. This review focuses on the methodological and biological validation of corticosteroid measurements in minimally invasive samples as biomarkers of adrenocortical responses to stress.

Col3.6-HSD2 transgenic mice: a glucocorticoid loss-of-function model spanning early and late osteoblast differentiation

The goal of this study was to characterize the bone phenotype and molecular alterations in Col3.6-HSD2 mice in which a 3.6-kb Col1a1 promoter fragment drives 11beta-HSD2 expression broadly in the osteoblast lineage to reduce glucocorticoid signaling. Serum corticosterone was unchanged in transgenic females excluding a systemic effect of the transgene. Adult transgenic mice showed reduced vertebral trabecular bone volume and reduced femoral and tibial sub-periosteal and sub-endosteal areas as assessed by microCT. In adult female transgenic mice, histomorphometry showed that vertebral bone mass and trabecular number were reduced but that osteoblast and osteoclast numbers and the mineral apposition and bone formation rates were not changed, suggesting a possible developmental defect in the formation of trabeculae. In a small sample of male mice, osteoblast number and percent osteoid surface were increased but the mineral apposition bone formation rates were not changed, indicating subtle sex-specific phenotypic differences in Col3.6-HSD2 bone. Serum from transgenic mice had decreased levels of the C-terminal telopeptide of alpha1(I) collagen but increased levels of osteocalcin. Transgenic calvarial osteoblast and bone marrow stromal cultures showed decreased alkaline phosphatase and mineral staining, reduced levels of Col1a1, bone sialoprotein and osteocalcin mRNA expression, and decreased cell growth and proliferation. Transgenic bone marrow cultures treated with RANKL and M-CSF showed greater osteoclast formation; however, osteoclast activity as assessed by resorption of a calcium phosphate substrate was decreased in transgenic cultures. Gene profiling of cultured calvarial osteoblasts enriched in the Col3.6-HSD2 transgene showed modest but significant changes in gene expression, particularly in cell cycle and integrin genes. In summary, Col3.6-HSD2 mice showed a low bone mass phenotype, with decreased ex vivo osteogenesis. These data further strengthen the concept that endogenous glucocorticoid signaling is required for optimal bone mass acquisition and highlight the complexities of glucocorticoid signaling in bone cell lineages.

Epigenetics and the biological definition of gene x environment interactions

Variations in phenotype reflect the influence of environmental conditions during development on cellular functions, including that of the genome. The recent integration of epigenetics into developmental psychobiology illustrates the processes by which environmental conditions in early life structurally alter DNA, providing a physical basis for the influence of the perinatal environmental signals on phenotype over the life of the individual. This review focuses on the enduring effects of naturally occurring variations in maternal care on gene expression and phenotype to provide an example of environmentally driven plasticity at the level of the DNA, revealing the interdependence of gene and environmental in the regulation of phenotype.

The adrenal cortex and life

The template for our understanding of the physiological role of the adrenal cortex was set by Hans Selye, who demonstrated its key involvement in the response to stress, of whatever origin, and who also introduced the terms glucocorticoid and mineralocorticoid. Despite this, from the late 1940s on there was certainly general awareness of the multiple actions of glucocorticoids, including effects on the thymus and immune system, cardiovascular system, water balance, and the CNS. For these reasons, and perhaps because in the early studies of the actions of individual steroids there was less clear-cut difference between them, there was some initial resistance to the use of these terms. Today they are universal and unchallenged. It can be argued that, with respect to the glucocorticoids, this term colours our perception of their physiological importance, and may be misleading. By taking evidence from disease states, emphasis is placed on extreme conditions that do not necessarily reveal normal physiology. In particular, evidence for the role of glucocorticoid regulation of gluconeogenesis and blood glucose in the normal subject or animal is inconclusive. Similarly, while highly plausible theories explaining glucocorticoid actions on inflammation or the immune system as part of normal physiology have been presented, direct evidence to support them is hard to find. Under extreme conditions of chronic stress, the cumulative actions of glucocorticoids on insulin resistance or immunocompromise may indeed seem to be actually damaging. Two well-documented and long recognized situations create huge variation in glucocorticoid secretion. These are the circadian rhythm, and the acute response to mild stress, such as handling, in the rat. Neither of these can be adequately explained by the need for glucocorticoid action, as we currently understand it, particularly on carbohydrate metabolism or on the immune system. Perhaps we should re-examine other targets at the physiological level. At the present time, some of these seem to be out of fashion.

Stepping stones in the path of glucocorticoid-driven apoptosis of lymphoid cells

Cumulative work on glucocorticoid (GC) regulation of genes in lymphoid cell cultures has revealed that apoptotic sensitivity to GCs depends on sufficient active GC receptors in the cells. The actions of the ligand-driven GC receptor that lead to apoptosis depend on interactions with other major cell-signaling systems, including the MAPK pathways, the cAMP/PKA pathway, the hedgehog pathway, the mTOR system and the c-myc system. The balance between these systems determines whether a given cell responds to GCs by undergoing apoptosis. A central core of networked genes may be found under GC control in many types of malignant, GC-sensitive cells. The partial core list identified should be tested in clinical cell samples from hematologic malignancies.

Cross talk of signaling pathways in the regulation of the glucocorticoid receptor function

Several posttranslational modifications including phosphorylation have been detected on the glucocorticoid receptor (GR). However, the interdependence and combinatorial regulation of these modifications and their role in GR functions are poorly understood. We studied the effects of c-Jun N-terminal kinase (JNK)-dependent phosphorylation of GR on its sumoylation status and the impact that these modifications have on GR transcriptional activity. GR is targeted for phosphorylation at serine 246 (S246) by the JNK protein family in a rapid and transient manner. The levels of S246 phosphorylation of endogenous GR increased significantly in cells treated with UV radiation that activates JNK. S246 GR phosphorylation by JNK facilitated subsequent GR sumoylation at lysines 297 and 313. GR sumoylation increased with JNK activation and was inhibited in cells treated with JNK inhibitor. GR sumoylation in cells with activated JNK was mediated preferentially by small ubiquitin-like modifier (SUMO)2 rather than SUMO1. An increase in GR transcriptional activity was observed after inhibition of JNK or SUMO pathways and suppression of GR transcriptional activity after activation of both pathways in cells transfected with GR-responsive reporter genes. Endogenous GR transcriptional activity was inhibited on endogenous target genes IGF binding protein (IGFBP) and glucocorticoid-induced leucine zipper (GILZ) when JNK and SUMO pathways were induced individually or simultaneously. Activation of both of these signals inhibited GR-mediated regulation of human inhibitor of apoptosis gene (hIAP), whereas simultaneous activation had no effect. We conclude that phosphorylation aids GR sumoylation and that cross talk of JNK and SUMO pathways fine tune GR transcriptional activity in a target gene-specific manner, thereby modulating the hormonal response of cells exposed to stress.