How Changes in Affinity of Corticosteroid-binding Globulin Modulate Free Cortisol Concentration

Abstract

Context:Recent studies of corticosteroid-binding globulin (CBG) indicate that it does not merely transport cortisol passively but also actively regulates its release in the circulation. We show how CBG binding affinity can vary to give changes in free cortisol concentration in a physiologically relevant range.Objective:The objective was to determine how the binding affinity of plasma CBG is affected by glycosylation, changes in body temperature, and the conformational change induced by proteases at sites of inflammation.Design:Binding assays were performed over a range of temperatures with plasma and recombinant CBG to determine the contribution of glycosylation. The role of conformational change was assessed by measuring binding affinities of plasma CBG before and after reactive loop cleavage by neutrophil elastase.Main Outcome Measures:Determination of binding constants allows calculation of clinically relevant changes in CBG saturation and free cortisol concentrations.Results:On reactive loop cleavage at inflammation sites, CBG can continue to act as a buffered source of cortisol, although with a much reduced affinity, to give a potential quadrupling of free cortisol. Predicted increases in systemic free cortisol resulting from elevated body temperatures, previously reported based on affinity measurements using nonglycosylated recombinant CBG, were shown here to be considerably increased using glycosylated plasma CBG, with a doubling for every 2°C rise in body temperature.Conclusions:The ability of CBG to modulate free cortisol levels in blood must be considered in the understanding and management of disease processes, as illustrated here with predictable changes in inflammation and fever.

The half-lives of intact and elastase cleaved human corticosteroid-binding globulin (CBG) are identical in the rabbit

“…On the other hand similar half-lives would likely compromise enzyme mediated cortisol available for release from CBG. Importantly however, in addition to its transport role, CBG also has a buffering role in stabilizing free cortisol concentrations which is dependent on the binding affinity of intact CBG as well as the much reduced binding affinity of cleaved CBG [11,12]. Hence in conditions where there is a relative high concentration of cleaved CBG knowledge of relative half-lives assumes importance.…”

MECHANISMS IN ENDOCRINOLOGY: New concepts to further unravel adrenal insufficiency during critical illness

“…Only free cortisol can cross the cell membrane to exert its function by binding to the GR and consequently binding as a complex in the nucleus. The acute response to critical illness comprises an immediate fall in the circulating levels of the binding proteins, CBG and albumin, together with an altered CBGbinding affinity, which occurs via the cleavage of CBG at the sites of inflammation or in the circulation evoked by fever (17,18,19,20,21). Plasma free cortisol has been suggested by other authors to be more appropriate than total plasma cortisol for the assessment of the HPA-axis functioning .…”

Asn347 Glycosylation of Corticosteroid-binding Globulin Fine-tunes the Host Immune Response by Modulating Proteolysis by Pseudomonas aeruginosa and Neutrophil Elastase

“…glycosylated CBG was found to bind cortisol with significantly higher affinity and was more temperature-sensitive relative to nonglycosylated CBG) (29). Moreover, upon NE-based RCL cleavage, the cortisol affinity of glycosylated CBG was reduced more than non-glycosylated forms, indicating that CBG glycosylation may yield a more rapid surge of free cortisol at target tissues to thereby facilitate a quicker resolution of inflammation (29). Intriguingly, the C-terminally located Asn 347 , which is 84.7% occupied by primarily triantennary sialoglycans, is located on the RCL in close proximity to the reported cleavage sites (36).…”

“…Readers are directed to a recent review as a source of more in depth coverage of the CBG biology (28 to form two complementary fragments: the large (50 -55 kDa) N-terminal fragment (CBG-Nt) and the small (5-10 kDa) C-terminal fragment (CBG-Ct) (22). The reduced cortisol affinity of cleaved CBG increases the local concentration of free cortisol, which is beneficial toward resolving inflammation in affected tissues by the anti-inflammatory effects of cortisol (17,29). It was recently suggested that in basal, low inflammatory conditions, proteases other than NE may be causing systemic CBG cleavage (30).…”

“…Other lines of evidence suggest that N-glycosylation is a key modulator of CBG functions, including cortisol binding and delivery (e.g. glycosylated CBG was found to bind cortisol with significantly higher affinity and was more temperature-sensitive relative to nonglycosylated CBG) (29). Moreover, upon NE-based RCL cleavage, the cortisol affinity of glycosylated CBG was reduced more than non-glycosylated forms, indicating that CBG glycosylation may yield a more rapid surge of free cortisol at target tissues to thereby facilitate a quicker resolution of inflammation (29).…”