Proopiomelanocortin gene promoter elements required for constitutive and glucocorticoid-repressed transcription

The POMC gene is expressed predominantly in the anterior pituitary. The high level of POMC transcription in this tissue is modulated by peptide hormones and repressed by glucocorticoids. In this present study we have investigated promoter elements required for the high basal transcription and glucocorticoid repression using transient transfection and in vitro transcription assays. We first determined that the region between -77 to -51 of the promoter, which has previously been shown to harbor a glucocorticoid receptor-binding site, is required for high basal expression both in vivo and in vitro. This promoter domain is also required for glucocorticoid repression of transcription in vivo. Two site-directed mutants within this area both decreased basal transcription, but were fully repressed by glucocorticoids, implying that the -77 to -51 region is a complex regulatory region harboring separable basal and glucocorticoid-repressible elements. Electrophoretic mobility shift and exonuclease III footprinting analysis revealed the existence of two factors that bind in this region. We also examined the effect of broad promoter deletions on basal expression and glucocorticoid repression. These experiments revealed that the region between -480 and -320 is also required for glucocorticoid repression. Taken together, the data suggest a model in which high basal transcription is generated by direct interaction of factors binding between -480 to -320 and -77 to -51. Glucocorticoid repression could occur by direct receptor disruption of these interactions.