MicroRNA 18 and 124a down-regulate the glucocorticoid receptor: implications for glucocorticoid responsiveness in the brain.

Glucocorticoids (GCs) exert profound effects on a variety of physiological processes, including adaptation to stress, metabolism, immunity, and neuronal development. Cellular responsiveness to GCs depends on numerous factors, including the amount of the glucocorticoid receptor (GR) protein. We tested the hypothesis that micro-RNAs (miRs), a recently discovered group of noncoding RNAs involved in mRNA translation, might control GR activity by reducing GR protein levels in neuronal tissues. We tested a panel of five miRs consisting of 124aa, 328, 524, 22, and 18. We found that miRs 18 and 124a reduced GR-mediated events in addition to decreasing GR protein levels. miR reporter assays revealed binding of miR-124a to the 3' untranslated region of GR. In correspondence, the activation of the GR-responsive gene glucocorticoid-induced leucine zipper was strongly impaired by miR-124a and -18 overexpression. Although miR-18 is expressed widely throughout the body, expression of miR-124a is restricted to the brain. Endogenous miR-124a up-regulation during neuronal differentiation of P19 cells was associated with a decreasing amount of GR protein levels and reduced activity of luciferase reporter constructs bearing GR 3' untranslated regions. Furthermore, we show that miR-124a expression varies over time during the stress hyporesponsive period, a neonatal period when GC signaling is modulated. Our findings demonstrate a potential role for miRs in the regulation of cell type-specific responsiveness to GCs, as may occur during critical periods of neuronal development. Ultimately, our results may provide a better understanding of the etiology of stress-related diseases as well as the efficacy of GC therapy.