Expression of 11β-hydroxysteroid dehydrogenase type 2 in an ACTH-producing small cell lung cancer

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 11β-hydroxysteroid dehydrogenase (11β-HSD), the principal ‘gatekeeper’ enzyme known to metabolize glucocorticoids. 11β-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 11β-HSD2 but not 11β-HSD1 in DMS-79 cells. Control human fibroblasts expressed predominantly 11β-HSD1 but also had detectable 11β-HSD2 mRNA, while HepG2 hepatoma cells also expressed only 11β-HSD2 mRNA. Whole cell assays in DMS-79 cells revealed 11β-HSD activity with a K m for cortisol of 26.1 ± 9.0 nM and V max of 57.0 ± 5.9 pmol/h/mg protein. HepG2 cells expressed a similar high affinity enzyme activity, while control fibroblasts expressed 11β-HSD activity with a K m for cortisol of 652 nM. Conversion of cortisol to cortisone in DMS-79 cells was inhibited to 7% of baseline by addition of 10 μM 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 11β-HSD2. While DMS-79 cells harbor additional defects in glucocorticoid signaling, these data suggest that expression of 11β-HSD2 might contribute to the development of the glucocorticoid-resistant phenotype of some ACTH-producing tumors.