EXPRESSION OF GLUCOCORTICOID RECEPTOR GENE ISOFORMS IN CORTICOTROPIN-SECRETING TUMORS

The molecular basis of Gushing's disease is not known. One of the most characteristic features of such tumors is their resistance to cortico steroid feedback at the pituitary level. We have hypothesized that abn ormalities of the glucocorticoid receptor (GR) gene might play a role in the development of Gushing's disease via an increase in the relativ e production of the nonligand-binding splice variant of the GR, GR bet a, known to exert dominant negative effects over the ligand-binding is oform, GR alpha. Alternatively, a change in overall GR expression, or mutations of some functional domains of the GR gene, might be involved in the pathogenesis of corticotroph tumors. We studied 22 tumors (17 pituitary ACTH-secreting tumors, 2 ectopic ACTH-producing tumors, 2 pr olactinomas, and 1 nonfunctioning adenoma) and three normal pituitarie s. RT-PCR was performed with primers specific to GR alpha and GR beta complementary DNA, followed by Southern blotting using an internal pro be, and the ratio of the two bands quantitated by densitometry. We als o assessed the overall expression of GR relative to the message of bot h the POMC gene and a housekeeping gene. Single-strand conformation po lymorphism analysis of the DNA-binding domain and splice junction regi on of the gene was also performed. GR alpha messenger RNA was expresse d at 37.3-fold +/- 5.7 (range, 32 to 46) excess, as compared with the GR beta subform. This pattern was observed both in the tumor samples a nd in the normal pituitaries used as controls. A majority of the ACTH- secreting tumors (16/19), including the ectopic secretors, showed vari able but increased overall GR expression, whereas 3 tumors showed an e xpression approximately equivalent to the normal controls; however, no correlation was found between these two groups and the response to th e high-dose dexamethasone test, nor was there any correlation with tum or histology. No mutations were found in any of the tumors by PGR-sing le-strand conformation polymorphism analysis. In conclusion, although both pituitary and ectopic ACTH-secreting tumors are at least partiall y glucocorticoid-resistant, no significant abnormalities in the relati ve expression of the two main GR subforms were observed in a series of such tumors. Additionally, mutations of regions critical to normal fu nction of the receptor do not seem to be a frequent event in these tum ors.