Impaired Protein Stability Of 11β-Hydroxysteroid Dehydrogenase Type 2: A Novel Mechanism Of Apparent

Drareg

Member
Joined
Feb 18, 2016
Messages
4,772
Abstract
Apparent mineralocorticoid excess (AME) is a severe form of hypertension that is caused by impaired activity of 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which converts biologically active cortisol into inactive cortisone. Mutations in HSD11B2 result in cortisol-induced activation of mineralocorticoid receptors and cause hypertension with hypokalemia, metabolic alkalosis, and suppressed circulating renin and aldosterone concentrations. This study uncovered the first patient with AME who was described in the literature, identified the genetic defect in HSD11B2, and provided evidence for a novel mechanism of reduced 11β-HSD2 activity. This study identified a cluster of amino acids (335 to 339) in the C-terminus of 11β-HSD2 that are essential for protein stability. The cluster includes Tyr338, which is mutated in the index patient, and Arg335 and Arg337, previously reported to be mutated in hypertensive patients. It was found that wild-type 11β-HSD2 is a relatively stable enzyme with a half-life of 21 h, whereas that of Tyr338His and Arg337His was 3 and 4 h, respectively. Enzymatic activity of Tyr338His was partially retained at 26°C or in the presence of the chemical chaperones glycerol and dexamethasone, indicating thermodynamic instability and misfolding. The results provide evidence that the degradation of both misfolded mutant Tyr338His and wild-type 11β-HSD2 occurs through the proteasome pathway. Therefore, impaired 11β-HSD2 protein stability rather than reduced gene expression or loss of catalytic activity seems to be responsible for the development of hypertension in some individuals with AME.

In the late 1960s, a severe form of hypertension that was characterized by low birth weight, failure to thrive, polyuria, and polydipsia was diagnosed in a 3-yr-old girl (1). The patient had severe hypokalemia, hypernatremia, and metabolic alkalosis despite undetectable plasma renin and reduced plasma and urinary aldosterone concentrations. Plasma cortisol was normal; however, the urinary cortisone metabolite tetrahydrocortisone (THE) was very low. Dexamethasone and spironolactone lowered BP efficiently, suggesting the presence of an unidentified mineralocorticoid. New et al.(2) identified another patient with similar features and coined the term apparent mineralocorticoid excess (AME). Later, genetic defects that cause a functional loss of 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which converts the active 11β-hydroxyglucocorticoid cortisol into inactive cortisone and protects mineralocorticoid receptors (MR) from promiscuous activation by cortisol, were identified as the primary defect in AME (35).

AME is a rare autosomal recessive inherited form of hypertension, with fewer than 100 cases reported (6). Despite the rarity of monogenic diseases, they represent “extreme situations” that often reveal the key underlying targets and mechanisms of pathophysiologic processes. Inhibition of 11β-HSD2 by endogenous and exogenous compounds can cause sodium retention and hypertension, resembling the features that are observed in AME (79). In addition, 11β-HSD2 modulates anti-inflammatory and antiproliferative actions of cortisol (10,11), and placental 11β-HSD2 protects the fetus from high maternal cortisol concentrations, whereby its decreased activity has been associated with reduced birth weight and an elevated risk for cardiovascular diseases in later life (12,13).

Despite its importance, most studies that have addressed 11β-HSD2 function focused on transcriptional regulation, and neither protein half-life nor its mechanism of degradation have been elucidated. Here, we investigated the underlying molecular mechanism of the 11β-HSD2 deficiency in the first patient with AME described in the literature (1) and identified compound heterozygosity with a premature stop in exon 1 and the substitution Tyr338His in exon 5. Moreover, we compared the catalytic activities of wild-type 11β-HSD2 and AME mutants Arg337Cys and Tyr338His and investigated their intracellular distribution pattern, protein stability, and half-life.

Impaired Protein Stability of 11β-Hydroxysteroid Dehydrogenase Type 2: A Novel Mechanism of Apparent Mineralocorticoid Excess
 

Similar threads

Back
Top Bottom