Monoclonal antibodies to the reactive centre loop (RCL) of human corticosteroid-binding globulin (CBG) can protect against proteolytic cleavage

Corticosteroid-binding globulin (CBG) binds most of the cortisol in circulation and is a non-functional member of the family of serine protease inhibitors (serpins) with an exposed elastase sensitive reactive centre loop (RCL). The RCL can be cleaved by human neutrophil elastase, released from activated neutrophils, and can also be cleaved at nearby site(s) by elastase released by Pseudomonas aeruginosa, and at two further sites, also within the RCL, by bovine chymotrypsin. Cleavage of the RCL results in a conformational change accompanied by a marked decrease in affinity for cortisol and hence its release at the site of proteolysis. These cleavages are irreversible and the similar half-lives of cleaved and intact CBG could mean that there may be some advantage in slowing the rate of CBG cleavage in acute inflammation thereby increasing the proportion of intact CBG in circulation. Here we show, for the first time, that pre-incubation of tethered human CBG with two monoclonal antibodies to the RCL of CBG protects against cleavage by all three enzymes. Furthermore, in plasma, pre-incubation with both RCL monoclonal antibodies delays neutrophil elastase cleavage of the RCL and one of these RCL monoclonal antibodies also delays bovine chymotrypsin cleavage of the RCL. These findings may provide a basis and rationale for the concept of the use of RCL antibodies as therapeutic agents to effectively increase the proportion of intact CBG in circulation which may be of benefit in acute inflammation.

Inhibition of corticosteroid-binding globulin gene expression by glucocorticoids involves C/EBPβ

Corticosteroid-binding globulin (CBG), a negative acute phase protein produced primarily in the liver, is responsible for the transport of glucocorticoids (GCs). It also modulates the bioavailability of GCs, as only free or unbound steroids are biologically active. Fluctuations in CBG levels therefore can directly affect GC bioavailability. This study investigates the molecular mechanism whereby GCs inhibit the expression of CBG. GCs regulate gene expression via the glucocorticoid receptor (GR), which either directly binds to DNA or acts indirectly via tethering to other DNA-bound transcription factors. Although no GC-response elements (GRE) are present in the Cbg promoter, putative binding sites for C/EBPβ, able to tether to the GR, as well as HNF3α involved in GR signaling, are present. C/EBPβ, but not HNF3α, was identified as an important mediator of DEX-mediated inhibition of Cbg promoter activity by using specific deletion and mutant promoter reporter constructs of Cbg. Furthermore, knockdown of C/EBPβ protein expression reduced DEX-induced repression of CBG mRNA, confirming C/EBPβ’s involvement in GC-mediated CBG repression. Chromatin immunoprecipitation (ChIP) after DEX treatment indicated increased co-recruitment of C/EBPβ and GR to the Cbg promoter, while C/EBPβ knockdown prevented GR recruitment. Together, the results suggest that DEX repression of CBG involves tethering of the GR to C/EBPβ.

Pseudomonas aeruginosa elastase disrupts the cortisol-binding activity of corticosteroid-binding globulin

The serine protease inhibitor (SERPIN) family member corticosteroid-binding globulin (CBG) is the main carrier of glucocorticoids in plasma. Human CBG mediates the targeted release of cortisol at sites of inflammation through cleavage of its reactive center loop (RCL) by neutrophil elastase. The RCLs of SERPIN family members are targeted by diverse endogenous and exogenous proteases, including several bacterial proteases. We tested different bacteria for their ability to secrete proteases that disrupt CBG cortisol-binding activity, and characterized the responsible protease and site of CBG cleavage. Serum CBG integrity was assessed by Western blotting and cortisol-binding capacity assay. Effects of time, pH, temperature, and protease inhibitors were tested. Proteolytically active proteins from bacterial media were purified by fast protein liquid chromatography, and the active protease and CBG cleavage sites were identified by mass spectrometry. Among the bacteria tested, medium from Pseudomonas aeruginosa actively disrupted the cortisol-binding activity of CBG. This proteolytic activity was inhibited by zinc chelators and occurred most efficiently at pH 7 and elevated physiological temperature (ie, 41°C). Mass spectrometric analysis of a semi-purified fraction of P. aeruginosa media identified the virulence factor LasB as the responsible protease, and this was confirmed by assaying media from LasB-deficient P. aeruginosa. This metalloprotease cleaves the CBG RCL at a major site, distinct from that targeted by neutrophil elastase. Our results suggest that humoral responses to P. aeruginosa infection are influenced by this pathogen's ability to secrete a protease that promotes the release of the anti-inflammatory steroid, cortisol, from its plasma transport protein.

Corticosteroid-binding globulin receptor of the rat hepatic membrane: solubilization, partial characterization, and the effect of steroids on binding

The corticosteroid-binding globulin (CBG) receptor of rat hepatic membranes was solubilized using 1.5% Triton X-100. An assay for its activity was developed that was dependent upon the fact that the [125I] CBG-receptor complex adsorbs to hydroxylapatite, whereas [125I]CBG does not. Scatchard analysis of the soluble receptor at 37 C showed a single set of high affinity binding sites, with a Kd of 44 nM and a binding capacity of 7.3 pmol/mg protein. The association rate constant (k1) was 0.92 x 10(5) M-1 min-1 at 37 C, and the dissociation rate constant (k2) was 1.0 x 10(-3) min-1. Only unliganded CBG could bind to the receptor. Steroids that bound to CBG, e.g. corticosterone and cortisol, noncompetitively inhibited CBG's binding to the receptor. Steroids that did not bind to CBG, e.g, dexamethasone, were without effect on the interaction of CBG with its receptor.

Modification of human transcortin by tetranitromethane. Evidence for the implication of a tyrosine residue in cortisol binding

The effect of tetranitromethane on the cortisol binding activity of human transcortin has been investigated. This reagent induced a decrease of activity concomitant with nitration of tyrosine residues. An oxidation of sulphydryl groups was also observed but had no implication on cortisol binding. The nitration was specifically oriented in the site at pH6 and with low concentrations of reagent; under these conditions, a single essential tyrosine per molecule of transcortin seems implicated in cortisol binding. The absence of denaturation in modified transcortin was checked by circular dichroism spectra and polyacrylamide gel electrophoresis. Site specificity was demonstrated by full protection with cortisol against inactivation.