Most of the data on glucocrticoid resistance in critical illness originates from experimental septic models. Endotoxin and lipopolysaccharide (LPS) injury models have shown decreased ligand affinity and down-regulation of GCR-α expression [34-39]. One group showed that impaired GCR-α dimerization resulted in worse lung barrier function during lipopolysaccharide (LPS)-induced inflammation and glucocorticoid treatment [
40]. A study by the same group demonstrated that impairment of GCR-α dimerization aggravated systemic hypotension and worsened lung function during LPS-induced endotoxic shock in mice [
41]. Hence, they concluded that the GCR-α dimer is an important mediator of hemodynamic stability and lung function, during LPS-induced systemic inflammation. Other sepsis animals models have demonstrated down-regulation of GCR-α, and/or decreased ligand affinity and up-regulation of GCR-β expression [39,42-45]. In a cecal ligation and puncture (CLP)-induced polymicrobial sepsis model, an intense initial activation of the GCR-α was noted prior to the induction of profound glucocorticoid resistance. Nuclear translocation of GCR and dexamethasone binding were not affected, however DNA binding was. Hence, the authors suggested that the initial augmented GCR-α activity caused the unresponsiveness towards exogenously administered glucocorticoids seen later in the disease, since this initial GCR-α activation could “exhaust” the receptor [
46]. Overall, it seems that mRNA expression of GCR-α is downregulated, while mRNA expression of GCR-β is up-regulated in the animal models of sepsis.
Most human clinical studies have investigated cortisol availability in critical illness, with only a few exploring the role of GCR. The data from these studies suggest the existence of glucocrticoid resistance, especially in sepsis. More specifically, in septic patients glucocorticoid treatment induced expression of miR-124, which in turn down-regulated GCR-α and limited the anti-inflammatory effects of glucocorticoids, prompting the authors to suggest that treatment with steroids might aggravate GC resistance in patients with increased GCR-β mRNA levels [47]. The expression of GCR-β in peripheral mononuclear cells of septic patients, and the effect of serum from septic patients on GCR expression and glucocorticoid sensitivity in cultured immune cells was evaluated in another study [48]. A transient increase in GCR-β mRNA expression was observed in sepsis, while serum from septic patients could induce glucocorticoid resistance in vitro [48]. Reduced GCR-α mRNA expression in peripheral blood mononuclear leucocytes in patients with sepsis or septic shock have been reported [49]. Another study examined GCR isoform abundance in tissues harvested from patients immediately after death from sepsis or non-septic critical illness, and found decreased GCR-α and increased GCR-β receptor numbers in the heart and liver [42]. In septic shock, GCR-α expression was increased in T-lymphocytes, regardless of glucocorticoid treatment, while the GCR binding capacity was reduced in neutrophils of glucocorticoid-treated patients, suggesting the hampered response to exogenous or endogenous glucocorticoids since neutrophils are the predominant circulating leucocyte in septic shock, [
50]. More recently, septic non-survivors showed lower GCR-α expression and higher cortisol levels than septic survivors. Moreover, the septic patients exhibited upregulated plasma cortisol levels along with downregulated GCR-α expression in peripheral blood mononuclear cells (PBMCs) compared to controls, whereas GCR-β showed the opposite trend [
51]. The reduced GCR-α expression and/or affinity in the blood of critically ill patients suggests that these patients become glucocorticoid-resistant requiring stress-doses of glucocorticoids. On the other hand, one study showed that despite variation, the GCR number and affinity in mononuclear cells from patients during the haemodynamic compensatory phase of sepsis did not differ from control subjects, suggesting that glucocorticoids could be effective in the haemodynamic compensatory phase of sepsis [
52]. Increased GCR-α mRNA and protein expression was shown in the acute phase of sepsis compared to systemic inflammatory response syndrome (SIRS) and healthy subjects, implying no need for exogenous steroids at this phase [
53]. Only one study, in ventilated critically ill patients within the first 24-hrs post-intubation and within three days of extubation, has demonstrated decreased cytosolic GCR protein levels, and subsequent downregulation of cortisol binding [
54]. Finally, our group showed that polymorphonuclear cells (PMNs), isolated from critically ill patients who had not received steroids, displayed a highly variable expression of GCR-α and GCR-β mRNA, with the expression levels of both receptors decreasing during ICU stay [
55]. In the follow-up study, and compared to healthy controls, the mRNA expression of both GCR-α and GCR-β was increased, while during the sub-acute phase, the expression of both isoforms was lower compared to controls, as was the expression of FKBP5 and GILZ [
56]. A recent report agrees with the results from these two studies. More specifically, Téblick and co-workers quantified the gene expression of key regulators of local glucocorticoid action, including 11β-HSD1, GCR-α, GCR-β, FKBP51, and GILZ in various immune cells and tissues[
57]. The expression profiles were compared in relation to duration of critical illness and glucocorticoid availability. In the patients’ neutrophils, GCR-α and GILZ were significantly suppressed throughout ICU stay, while low-to-normal GCR-α and increased GILZ expression was found in the patients’ monocytes. FKBP51 was increased in the monocytes, and transiently increased in the neutrophils, whereas GCR-β was undetectable. In the septic patients, increased systemic glucocorticoid availability in most tissues was associated with suppressed GCR-α, increased FKBP51 and unaltered GILZ. Only in the lung, and the adjacent diaphragm and adipose tissues, an increase in circulating glucocorticoids resulted in higher GCR-α activity [
57]. Thus, the authors proposed that these tissue-specific and time-independent adaptations to critical illness facilitated GCR-α action primarily to the lung, protecting against damage in other cells and tissues, including neutrophils. Throughout critical illness, GCR action was inhibited in neutrophils, possibly due to suppression of GCR-α expression, and hence glucocorticoid resistance could not be overcome by further increasing glucocorticoid availability. These findings do not seem to support the notion of a maladaptive generalised glucocorticoid resistance requiring treatment with glucocorticoids [
57]. Finally, a novel human GCR variant, G459V, exhibited a hyperactive response when treated with hydrocortisone. More specifically, its activity was more than 30 times greater than the reference GCR-α. Unexpectedly, G459V showed considerably increased activity when treated with the GCR antagonist RU486. Thus, the authors concluded that variants of GCR can potentially alter the response to stress and steroid treatment, which could explain the mitigated clinical response in sepsis [
58].
In respiratory syncytial virus (RSV) bronchiolitis-infected infants, the α:β GCR mRNA expression ratio was decreased significantly in severe disease compared to mild and normal controls. Furthermore, the expression of GCR-β positively correlated with the clinical score of severity, and might in part explain the insensitivity to glucocorticoid treatment in RSV-infection [
59]. Another study measured the cellular GCR-α activity in PBMCs from critically ill children, and found significantly lower total and cytoplasmic, but not nuclear, GCR-α protein levels in critically ill children compared to healthy controls [
60]. The authors suggested that the lower total and cytoplasmic receptor levels in critically ill children limit the GCR-mediated response to exogenous glucocorticoid therapy. The undisturbed translocation indicated that residual receptors retain their functionality and accessibility to therapeutic treatments. In another study in critically ill paediatric patients, those with shock and increased illness severity had lower GCR-α expression in CD4 and CD8 lymphocytes, while GCR-α expression did not correlate with cortisol levels [
61]. Finally, in a different paediatric septic shock cohort, GCR-α expression did not differ between SIRS, sepsis, and septic shock. Decreased expression of the GCR-α protein, however, correlated with poor outcomes in septic shock, especially in patients with high cortisol levels [
62].