Function of a truncated glucocorticoid receptor form at a negative glucocorticoid response element in the proopiomelanocortin gene

ACTH-producing tumors of nonpituitary origin characteristically exhibit ins ensitivity to the negative feedback effects of glucocorticoids. In the DMS- 79 cell line derived from an ACTH-producing small cell lung cancer we have previously identified an aberrantly spliced glucocorticoid receptor (GR Del ta) that lacks a ligand-binding domain. We examined the interactions of thi s truncated form of GR with the proximal human proopiomelanocortin (POMC) p romoter. In electrophoretic mobility shift assays GR Delta bound to the neg ative glucocorticoid response element (nGRE) at position -78 to -50 in the human POMC promoter. Nur77, an orphan nuclear receptor that exerts positive regulatory effects on the POMC gene is also known to bind to this DNA elem ent. The functional properties of GR and GR Delta binding to this DNA eleme nt were examined in transient transfection experiments in murine AtT-20 cor ticotroph tumor cells. Reporter gene expression under the control of proxim al POMC promoter elements was stimulated by addition of forskolin to the cu lture medium or by transfection with expression constructs for human Nak1, the human homologue of Nur77. Treatment of transfected cells with dexametha sone resulted in suppression of forskolin- or Nak1-stimulated POMC-reporter gene expression in the presence of co-transfected GR but not with GR Delta . The experiments indicate that in the human POMC promoter GR Delta is capa ble of binding to the nGRE but cannot effect trans-repression of POMC-repor ter gene expression.