Modulation of Glucocorticoid Action in vivo: Role of Lipid Rafts and Clocks

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Abstract

Glucocorticoids (Gcs) are a commonly used drug to target the glucocorticoid receptor (GR). The GR has a myriad of cellular and physiological effects, however, Gcs are clinically used for the treatment of inflammatory conditions due to the potent anti-inflammatory actions of GR. The anti-inflammatory effects come with serious side effects e.g. metabolic disease. I examine the role of lipid rafts in modulating the anti-inflammatory actions of Gcs, and the role of circadian rhythms in the control of Gc side effects.I tested the role of caveolin-1 (Cav1), a constituent of membrane lipid rafts, and its role in Gc suppression of inflammation. Gene expression analysis of mouse lung tissue showed that genetic depletion of Cav1 (CAV1KO) results in increased transactivation of Gc target genes. The increased Gc action, however, does not result in an increased effect on suppression of inflammation in a model of innate immunity: aerosolised lipopolysaccharide (LPS) induced lung inflammation or in a model of adaptive immunity: Ovalbumin. CAV1KO mice were protected from LPS induced inflammation, despite increased cytokine production. This suggests a differential response to LPS in lung parenchyma and alveolar macrophages dependent on Cav1. CAV1KO results in a pro-inflammatory phenotype in macrophages, and the opposite in parenchymal tissue. These data suggest that while Cav1 is an upstream regulator of Gc response, it does not have a strong enough effect to alter the ability of GR to repress inflammation in vivo.Gc treatment results in a strong metabolic phenotype, with aberrant energy metabolism, insulin resistance and hepatosteaotosis, I investigated how this side effect interacts with circadian rhythms, another key determinant of energy metabolism. Using transcriptomics of whole lung and liver taken during the day or the night, I demonstrate that the metabolic actions of Gc in the liver can be temporally separated, whilst maintaining consistent anti-inflammatory actions in both liver and lung. This temporal gene regulation by Gc is controlled by REV-ERB, a rhythmically expressed, orphan nuclear receptor, part of the core clock machinery, via a direct interaction with GR at key regulatory DNA loci. Genetic deletion of REV-ERB protects mice from the hepatosteotosis associated with Gc treatment. Taken together, these data suggest that Gcs are regulated upstream of the receptor by the core consitutent of membrane lipid rafts; Cav1, which modulates the Gc response in vivo. Also, that the GR action can be controlled by dosing at different times of day, separating the detrimental metabolic effects of Gcs from the beneficial anti-inflammatory effects. This is enabled through a direct interaction between GR and REV-ERB at key gene regulatory sites.

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UCSC genome browser tracks of RNA-seq dataExcel files of RNA-seq data and analysis of ChIP-seq dataPrimary comparisons of ChIP-seq data from publicly available datasets

Keyword(s)

Caveolin; Glucocorticoid; Lipid raft; NR1D1; NR3C1; REV-ERB; circadian

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