Corticosteroid-Binding Globulin – a Targeted Delivery System for Cortisol. Physiology and Responses to Acute and Chronic Inflammation

Abstract

Corticosteroid-binding globulin (CBG) is the large glycoprotein principally responsible for transporting the life-sustaining hormone cortisol. Levels of biologically active free cortisol are regulated by the concentration and the cortisol-binding affinity of CBG. A member of the serine protease inhibitor superfamily, CBG has a single high-affinity cortisol-binding site per molecule and an exposed reactive centre loop that acts as a protease bait domain for neutrophil elastase, inducing permanent conformational change upon enzymatic cleavage. Allosteric modulation of the inherently plastic binding site reduces the cortisol-binding affinity of CBG by ten-fold (high cortisol-binding affinity CBG [haCBG] -> low cortisolbinding affinity CBG [laCBG]), favouring cortisol release in vitro. The targeted deposition of free cortisol at sites of inflammation facilitates down-stream genomic and non-genomic immunomodulatory, anti-inflammatory effects, neurocognitive and metabolic effects. Novel measurement of haCBG and total CBG by enzyme-linked immunosorbent assay is at the forefront of research into CBG cleavage in vivo. The present thesis utilises this recent development in seven clinical studies to translate existing evidence of the cleavage phenomenon to the clinical setting for the first time, with the aim of determining whether haCBG levels were associated with illness severity and clinical outcomes in inflammatory states. We hypothesised that depletion of anti-inflammatory CBG-cortisol is a key mechanism in the pathogenesis of uncontrolled inflammation in systemic inflammatory disorders. Studies of haCBG and laCBG levels in acute inflammatory conditions showed that increasing illness severity in sepsis and septic shock was associated with markedly reduced circulating haCBG concentrations in vivo, with illness severity correlating better with haCBG levels than either free or total cortisol levels. CBG cleavage also occurred in patients with severe infection, including Pseudomonal infection. Administration of the pro-inflammatory cytokine tumour necrosis factor-α did not induce CBG cleavage or the acute phase response immediately (< 6 hours) post-infusion. These studies suggest that in acute inflammation, depletion of haCBG may limit the availability of cortisol to inflammatory sites, perpetuating inflammation, however greater than six hours is required for CBG cleavage to become effectual. Additionally, the pathogen inciting a systemic inflammatory response may influence the propensity for CBG cleavage. We also present evidence of CBG as a thermocouple early in inflammation to enhance free cortisol levels. In contrast, chronic inflammatory states displayed reduced CBG cleavage, including the metabolic syndrome and rheumatoid arthritis, in association with worsening disease activity. Thus compromised cleavage may hinder CBG-mediated delivery of anti-inflammatory cortisol. Studies in patients with α1 antitrypsin deficiency showed that paradoxically, proteolytic cleavage of CBG was reduced despite increased neutrophil elastase activity. Our data suggest cleavage is mediated by alternate proteases in some circumstances. In pregnancy, CBG increases three-fold, and we show that this increase is due to haCBG alone which may provide an increased reservoir of CBG-bound cortisol for immunomodulatory purposes in puerperal infection. In comparison, the rise in CBG in women receiving exogenous oestrogens is due to elevated haCBG as well as laCBG, while neither haCBG nor laCBG levels change following menopause, challenging the mechanisms governing oestrogen-mediated CBG production. In summary, this pioneering research reveals perturbations in haCBG and laCBG levels in health and disease which had hitherto been unrecognised. The absolute pool of haCBGcortisol, and the accessibility of that pool to cleavage bears significant influence over inflammatory outcomes. Post-translational modification including glycosylation or genetic CBG variation may contribute to haCBG dysregulation. The role and relevance of CBG as a releasing agent and an anti-inflammatory molecule is more complex and adapted than currently appreciated, and holds great opportunity for diagnostic and therapeutic application.