| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
ARTICLES |
Celso E. Gomez-Sanchez, Medical and Research Services, Harry S. Truman Memorial Veterans Hospital, Cosmopolitan International Diabetes Center and Department of Internal Medicine University of Missouri-Columbia Columbia, Missouri 65201
Address all correspondence and request for reprints to: Elise P. Gomez-Sanchez, D.V.M., Ph.D., Research Services, Harry S. Truman Memorial Veterans Hospital, 800 Hospital Drive, Columbia, Missouri 65201. E-mail: intmdepg{at}showme.missouri.edu
 |
Introduction |
|---|
 Top Introduction References |
|---|
The 11-HSD enzyme was again relegated to the side lines. Two major discoveries returned it to the limelight. The first was the observation that the affinities of the isolated and recombinant mineralocorticoid receptor (MR) were similar for aldosterone, corticosterone, or cortisol regardless of the tissue source (2). This left a conundrum: if the MR were so promiscuous, how could aldosterone act through the kidney MR and retain control of sodium retention and potassium excretion in competition with the glucocorticoids that circulate at 100-1000 times the levels of aldosterone?
A clue for the second discovery appeared in the form of a rare disorder in which the patients present with hypertension and hypokalemia, pathognomonic signs of excessive mineralocorticoids, but have low serum mineralocorticoids. Secretory rates for cortisol are also low, but plasma concentrations are normal. Short-term cortisol infusions raised the blood pressure in these patients, which does not normally occur. Searches for steroids responsible for this disorder, "apparent mineralocorticoid excess," were in vain. Due to a low cortisone to cortisol ratio, the deficiency of the 11-HSD enzyme was surmised in these patients, but the mechanism of hypertension was still puzzling (3).
The search for an answer to this question and that of MR ligand specificity led to the second discovery: the presence in aldosterone target tissues of an enzyme, 11-HSD, which inactivated corticosterone and cortisol by converting them to 11-dehydrocorticosterone and cortisone, allowing aldosterone, which is not metabolized, access to the receptor. The hypothesis of an MR guardian enzyme also explained the pathogenesis of the rare syndrome of apparent mineralocorticoid excess (4).
However appealing, the assignment of the title of MR "gate keeper" to the original 11-HSD enzyme isolated and cloned from liver microsomes (5, 6) did not answer the question of MR specificity. This enzyme (11-HSD-1) is NADP+-dependent and oxidizes corticosterone with a Km in the low micromolar range (1–5 µM), far greater than the physiological range of cortisol or corticosterone concentration in plasma. Moreover, under physiological conditions and in intact cells in vitro, 11-HSD-1 functions predominantly as a reductase (7, 8) and it does not colocalize with the MR in the kidney. The search for another and more relevant enzyme led to the description of 11-HSD-2, which is NAD+-dependent, has an affinity in the low nanomolar range (4–20 nM), is an oxidase for natural glucocorticoids, and colocalizes with the MR in transport epithelia, from whence it has been cloned (3). Mutations of the 11-HSD-2 that allow normal circulating levels of cortisol to gain access to the MR and behave as a mineralocorticoid have been demonstrated in patients with apparent mineralocorticoid excess syndrome (reviewed in Ref.3).
While the protection of the MR has been the focus of the 11-HSD story, the regulation of local levels of glucocorticoids for binding the glucocorticoid receptor (GR) in the sea of very high circulating levels of steroid is finally receiving deserved attention. Glucocorticoids have a variety of actions in a wide range of tissues. How can a single high serum concentration of ligand satisfy the particular needs for regulation of glucocorticoids at the tissue and cell level? While 11-HSD-2 is limited primarily to mineralocorticoid target tissues, 11-HSD-1 has a wide tissue distribution, perhaps reflecting the wide distribution of glucocorticoid receptors and diverse actions. The predominant reductase activity of 11-HSD-1 is crucial for the therapeutic use of the otherwise inactive prednisone and cortisone; however, due to a Km in excess of normal circulating levels of corticosteroids, the physiological role of the 11-HSD-1 reductase activity remains theoretical.
Deficient liver 11-HSD-1 activity is thought to occur in the syndrome of "apparent cortisone reductase deficiency," which is associated with hirsutism and infertility (9), but the pathogenesis of this disorder is unclear. No mutations in the 11-HSD-1 have been found (10), nor is there evidence of a deficiency in the conversion of cortisol to cortisone in these patients. The only reported abnormalities in the 11-HSD-1 gene knock-out mice are lower fasting glucose levels and mild adrenal hyperplasia (11).
The paper from M. P. Hardy’s laboratory in this issue of Endocrinology addresses the ontogeny of 11-HSD oxidase and reductase activity and modulation of glucocorticoid action upon testosterone synthesis in Leydig cells (12). Elevations in circulating levels of glucocorticoids depress testosterone production by mature Leydig cells, resulting in decreased serum testosterone levels (13) and the elimination of endogenous corticosterone levels in vivo increases the steroidogenic capacity of purified Leydig cells in vitro. The demonstration of 11-HSD-1 messenger RNA and protein in Leydig cells led to the hypothesis that this isozyme regulates steroidogenesis in the testes by inactivating glucocorticoids within the cell, allowing normal testosterone synthesis (14).
Adult Leydig cells have both oxidative and reductive 11-HSD activity with oxidation prevailing over reduction. Adrenalectomy decreases the oxidative, without changing the reductive activity (15), suggesting a regulatory function of the substrate steroid. Dexamethasone treatment of cultured Leydig cells increased oxidase activity. While there is both message and protein for the 11-HSD-1 enzyme, no low Km NAD+-dependent activity has been demonstrated in the Leydig cells (8). Kinetic analysis of the 11-HSD oxidative activity revealed two different NADP+-dependent components: a low (nM) Km activity effectively inhibited by 100 nM carbenoxolone, which had mild end-product activation (40%), and a high (µM) Km activity inhibited only by high (>1 µM) concentrations of carbenoxolone. The latter component was greatly activated (350%) by high concentrations of corticosterone (8). Such a low Km could be relevant to the physiologic concentrations of glucocorticoids in tissues.
The paper in this issue provides additional evidence of differential regulation of the oxidative and reductive activity of testicular 11-HSD enzymes paired with steroidogenesis data. Both steroidogenesis and 11-HSD oxidative and reductive activities in Leydig cells increase at puberty; however, the ratios of oxidative and reductive activity change with development, reduction predominating in prepubertal testes, and oxidation in the adult. Thus, the oxidative activity in these cells not only has a different Km than that of the 11-HSD-1, it is also regulated differently during development. Evidence for a third NADP+-dependent 11-HSD isozyme has been described for other tissues, including a human choriocarcinoma cell line and the sheep kidney (16, 17). In both of these tissues, the Km for this enzyme is significantly lower than that for 11-HSD-1 and activity predominates in the mitochondrial and large microsomal fraction, not in nuclei, where NAD+-dependent activity is found. Kinetic studies in rat vascular smooth muscle also suggest the existence of an NADP+-dependent isozyme with a higher affinity for oxidation of corticosterone than that of the liver 11-HSD-1 (18).
These studies, taken with the current paper, suggest the presence of a third high affinity, NADP+-dependent isozyme with a Km within the concentration range of corticosteroids in the circulation. One would predict this third enzyme to be frequently coexpressed with the 11-HSD-1, explaining the oxidation of physiologic levels of glucocorticoids now ascribed to the high Km 11-HSD-1 isozyme. The definitive demonstration of a third (and more?) 11-HSD isozyme will occur when it is cloned. Measurement of NADP+-dependent 11-HSD activity in Leydig cells of 11-HSD-1 knockout mice would provide biochemical evidence for the existence of a separate gene for the third isozyme. Homozygous mice for the inactivated 11-HSD-1 gene exhibit minor changes in fasting glucose levels and some adrenal hyperplasia but are fertile, suggesting that protection of the Leydig cells from circulating corticosterone is fully operative, thus not dependent upon 11-HSD-1 (11). Inactivating mutations of this hypothetical NADP+-dependent 11-HSD with predominant or exclusive oxidative properties in Leydig cells would affect testosterone production and may be among the causes of idiopathic hypogonadism and/or infertility. These patients do not have abnormal metabolite:cortisol urinary excretion ratios. However, if one postulates the existence of an enzyme, which fine tunes ligand levels at the cell level, its effect upon bulk steroid metabolism by the liver and kidney would be undetectable.
Received September 25, 1997.
|
References |
|---|
|
TopIntroductionReferences |
|---|
This article has been cited by other articles:
 |
|
 |
|
  E. P. Gomez-Sanchez, D. G. Romero, A. F. de Rodriguez, M. P. Warden, Z. Krozowski, and C. E. Gomez-Sanchez Hexose-6-Phosphate Dehydrogenase and 11{beta}-Hydroxysteroid Dehydrogenase-1 Tissue Distribution in the Rat Endocrinology, February 1, 2008; 149(2): 525 - 533. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. P. Gomez-Sanchez, V. Ganjam, Y. J. Chen, Y. Liu, M. Y. Zhou, C. Toroslu, D. G. Romero, M. D. Hughson, A. de Rodriguez, and C. E. Gomez-Sanchez Regulation of 11{beta}-hydroxysteroid dehydrogenase enzymes in the rat kidney by estradiol Am J Physiol Endocrinol Metab, August 1, 2003; 285(2): E272 - E279. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
