Ligand-Independent Phosphorylation of the Glucocorticoid Receptor Integrates Cellular Stress Pathways with Nuclear Receptor Signaling

Abstract

Glucocorticoids are stress hormones that maintain homeostasis through gene regulation mediated by nuclear receptors. We have discovered that other cellular stressors are integrated with glucocorticoid signaling through a new hormone-independent phosphorylation site, Ser134, on the human glucocorticoid receptor (GR). Ser134 phosphorylation is induced by a variety of stress-activating stimuli in a p38 mitogen-activated protein kinase (MAPK)-dependent manner. Cells expressing a mutant glucocorticoid receptor incapable of phosphorylation at Ser134 (S134A-GR) had significantly altered hormone-dependent genome-wide transcriptional responses and associated hormone-mediated cellular functions. The phosphorylation of Ser134 significantly increased the association of the GR with the zeta isoform of the 14-3-3 class of signaling proteins (14-3-3zeta) on chromatin promoter regions, resulting in a blunted hormone-dependent transcriptional response of select genes. These data argue that the phosphorylation of Ser134 acts as a molecular sensor on the GR, monitoring the level of cellular stress to redirect glucocorticoid-regulated signaling through altered 14-3-3zeta cofactor binding and promoter recruitment. This posttranslational modification allows prior cellular stress signals to dictate the transcriptional response to glucocorticoids.