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Expression of 11beta-hydroxysteroid dehydrogenase type 2 in an ACTH-producing small cell lung cancer
L L Parks1, M K Turney, D Gaitan
1Division of Endocrinology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
Abstract:
Non-pituitary tumors that produce adrenocorticotropic hormone (ACTH) exhibit resistance to the normal feedback effects of glucocorticoids on proopiomelanocortin (POMC) gene expression. This glucocorticoid resistance is typically complete, although some tumors show only relative glucocorticoid resistance in the clinical setting. The molecular mechanisms responsible for these clinical pathophysiologic observations are unknown, but might include glucocorticoid receptor defects or aberrant expression of enzymes or transporters that exclude glucocorticoids from access to their intracellular receptors. We examined whether ACTH-producing non-pituitary tumor cells might express 11beta-hydroxysteroid dehydrogenase (11beta-HSD), the principal 'gatekeeper' enzyme known to metabolize glucocorticoids. 11Beta-HSD mRNA and enzyme activity were assessed in DMS-79 cells, a line derived from an ACTH-producing small cell lung cancer. RT-PCR studies showed expression of mRNA encoding 11beta-HSD2 but not 11beta-HSD1 in DMS-79 cells. Control human fibroblasts expressed predominantly 11beta-HSD1 but also had detectable 11beta-HSD2 mRNA, while HepG2 hepatoma cells also expressed only 11beta-HSD2 mRNA. Whole cell assays in DMS-79 cells revealed 11beta-HSD activity with a Km for cortisol of 26.1 +/- 9.0 nM and Vmax of 57.0 +/- 5.9 pmol/h/mg protein. HepG2 cells expressed a similar high affinity enzyme activity, while control fibroblasts expressed 11beta-HSD activity with a Km for cortisol of 652 nM. Conversion of cortisol to cortisone in DMS-79 cells was inhibited to 7% of baseline by addition of 10 microM glycyrrhetinic acid. Dexamethasone (20 nM) was converted to a single product in DMS-79 cells at a rate of 17.2 pmol/h/mg protein; this activity was also inhibited by glycyrrhetinic acid. We conclude that DMS-79 cells express 11beta-HSD2. While DMS-79 cells harbor additional defects in glucocorticoid signaling, these data suggest that expression of 11beta-HSD2 might contribute to the development of the glucocorticoid-resistant phenotype of some ACTH-producing tumors.
Insights
Non-pituitary tumors producing ACTH show glucocorticoid resistance. This study found that these tumors express 11beta-HSD2, an enzyme that metabolizes glucocorticoids, potentially explaining their resistance.
Area of Science:
- Endocrinology
- Molecular Biology
- Oncology
Background:
- Non-pituitary tumors producing adrenocorticotropic hormone (ACTH) often exhibit resistance to glucocorticoid feedback, impacting proopiomelanocortin (POMC) gene expression.
- The molecular basis for this glucocorticoid resistance in tumors is not fully understood.
- Potential mechanisms include defects in the glucocorticoid receptor or altered expression of enzymes affecting glucocorticoid access.
Purpose of the Study:
- To investigate the potential role of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) in the glucocorticoid resistance of ACTH-producing non-pituitary tumors.
- To assess the expression and activity of 11beta-HSD isoforms in a human ACTH-producing small cell lung cancer cell line (DMS-79).
Main Methods:
- Reverse transcription-polymerase chain reaction (RT-PCR) was used to detect 11beta-HSD1 and 11beta-HSD2 mRNA expression in DMS-79 cells.
- Enzyme activity assays were performed on whole DMS-79 cells to quantify 11beta-HSD activity and determine kinetic parameters (Km, Vmax) for cortisol.
- The effect of glycyrrhetinic acid, an 11beta-HSD inhibitor, on cortisol and dexamethasone metabolism was evaluated.
Main Results:
- DMS-79 cells expressed mRNA encoding 11beta-HSD2 but not 11beta-HSD1.
- Significant 11beta-HSD enzyme activity was detected in DMS-79 cells, with high affinity for cortisol (Km = 26.1 nM).
- Metabolism of cortisol and dexamethasone by DMS-79 cells was significantly inhibited by glycyrrhetinic acid, confirming 11beta-HSD2 activity.
Conclusions:
- ACTH-producing non-pituitary tumor cells (DMS-79) express functional 11beta-HSD2.
- The expression of 11beta-HSD2 may contribute to the glucocorticoid-resistant phenotype observed in some ACTH-producing tumors.
- Further investigation into glucocorticoid signaling defects is warranted, but 11beta-HSD2 is a potential factor in tumor-induced Cushing's syndrome.