GLUCOCORTICOID ACTIVATION OF CHROMOGRANIN-A GENE-EXPRESSION - IDENTIFICATION AND CHARACTERIZATION OF A NOVEL GLUCOCORTICOID RESPONSE ELEMENT

Glucocorticoids regulate catecholamine biosynthesis and storage at several sites. Chromogranin A, an abundant protein complexed with catecholamines in secretory vesicles of chromaffin cells and sympathetic axons, is also augmented by glucocorticoids. This study reports isolation of the rat chromogranin A promoter to elucidate transcriptional regulation of chromogranin A biosynthesis by glucocorticoids in neuroendocrine cells. Endogenous chromogranin A gene expression was activated up to 3.5-fold in chromaffin cells by glucocorticoid, in time-dependent fashion. Inhibition of new protein synthesis by cycloheximide did not alter the rise in chromogranin A mRNA, suggesting that glucocorticoids directly activate the chromogranin A promoter; nuclear runoff assays confirmed a 3.3-fold increased rate of initiation of new chromogranin A transcripts after glucocorticoid. Transfected rat chromogranin A promoter/luciferase reporter constructs were activated 2.6-3.1-fold by glucocorticoid, and selective agonist/antagonist studies determined that dexamethasone effects were mediated by glucocorticoid receptors. Both rat and mouse chromogranin A promoter/luciferase reporter constructs were activated by glucocorticoid. A series of promoter deletions narrowed the region of glucocorticoid action to a 93-bp section of the promoter, from position -526 to -619 bp upstream of the cap site. A 15-bp sequence ([-583 bp] 5'-ACATGAGTGTGTCCT-3' [-597 bp]) within this region showed partial homology to a glucocorticoid response element (GRE; half-site in italics) consensus sequence, and several lines of experimental evidence confirmed its function as a GRE: (a) site-directed mutation of this GRE prevented glucocorticoid activation of a chromogranin A promoter/reporter; (b) transfer of this GRE to a heterologous (thymidine kinase) promoter/reporter conferred activation by glucocorticoid, in copy number-dependent and orientation-independent fashion; and (c) electrophoretic gel mobility shifts demonstrated binding of this GRE by ligand-activated glucocorticoid receptor, though at 2.75-fold lower affinity than the glucocorticoid receptor interaction with a consensus GRE. The rat chromogranin A GRE showed functional and structural similarities to GREs in other genes proportionally regulated by glucocorticoids. We conclude that a discrete domain of the chromogranin A promoter is both necessary and sufficient to confer glucocorticoid regulation onto the gene, and that the activity of this region also explains the degree of activation of the endogenous gene by glucocorticoid.