The Pro-inflammatory Effects of Glucocorticoids in the Brain

Abstract

Glucocorticoids are a class of steroid hormones derived from cholesterol. Their actions are mediated by the glucocorticoid and mineralocorticoid receptors, members of the superfamily of nuclear receptors, which, once bound to their ligands, act as transcription factors that can directly modulate gene expression. Through protein-protein interactions with other transcription factors, they can also regulate the activity of many genes in a composite or tethering way. Rapid non-genomic signaling was also demonstrated since glucocorticoids can act through membrane receptors and activate signal transduction pathways, such as protein kinases cascades, to modulate other transcriptions factors and activate or repress various target genes. By all these different mechanisms, glucocorticoids regulate numerous important functions in a large variety of cells, not only in the peripheral organs but also in the central nervous system during development and adulthood. In general, glucocorticoids are considered anti-inflammatory and protective agents due to their ability to inhibit gene expression of pro-inflammatory mediators and other possible damaging molecules. Nonetheless, recent studies have uncovered situations in which these hormones can act as pro-inflammatory agents depending on the dose, chronicity of exposure, and the structure/organ analyzed. In this review, we will provide an overview of the conditions under which these phenomena occur, a discussion that will serve as a basis for exploring the mechanistic foundation of glucocorticoids pro-inflammatory gene regulation in the brain.

Keywords: brain; cytokines; glucocorticoids; glucocorticoids receptors; inflammation; pro-inflammatory; steroid hormones.

Figure 1

Glucocorticoid-mediated modulation of pro-inflammatory pathways. (A) LPS binds to TLR4 and activates the MAP kinase pathway, activating NFκB and increasing the gene expression of pro-inflammatory cytokines (IL-1β, TNF-α, and IL-6). Elevated and prolonged GC exposure increases and potentiates the pro-inflammatory response related to MAP kinase–NFκB pathway. (B) GR activation regulates the gene and protein expression of TLR2, which recognizes PAMPS. This event recruits intracellular proteins, ultimately leading to the downstream signaling activation of NFκB, increasing the expression of inflammatory cytokines, including IL-6, TNF-α, and IL-1β. (C) GR activation regulates the gene and protein expression of NLRP3, which recognizes DAMPS and regulates the immune system response in a mechanism similar to the proposed in (B). Schematic figures based on Ref. (39, 70).