Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Abstract

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Figure 1

Inflammation at a molecular level: a simplified scheme. TNFR activation by TNF, IL1RI by IL1β, TLR3 by double-stranded RNA (dsRNA), TLR4 by bacterial LPS, and activation of other TLRs can signal via specific intermediary factors such as TRADD, TRAF2, RIP1, MEKK3, TAK1, TAB2/3, and NIK (for the TNFR), MyD88, IRAKs, TRAF6 and TAK1 (for IL1RI), Trif, RIP1, TRAF6 and TAK1 (for TLR3) and MyD88, Mal, Trif, Tram, RIP1, IRAKs, TRAF6 and TAK1 (for TLR4) to the MAPK pathway and to the activation and regulation of NF-κB and AP-1. Additionally, triggering TLR3 or TLR4 signaling cascades can instigate IKKε and TBK1 activation and subsequent IRF3-regulated gene transcription (10,11,12,29). TAB, TAK1-binding protein.

Figure 2

Structure and domain functions of the GR. NTD, N-Terminal domain; HR, hinge region; TF, transcription factor.

Figure 3

Activation and nuclear actions of the GR. The unactivated, cytoplasmic GR is complexed with chaperone proteins. Binding of GCs to the GR instigates the nuclear translocation of GR. The binding of dimeric, activated GR onto GREs, DNA binding of GR in a concerted manner with another transcription factor (TF), or binding of GR onto a TF via a tethering mechanism can all result in GC-directed promoter activation. This transactivation results in the expression of metabolic gene products, associated with the occurrence of side effects, and to the expression of a number of antiinflammatory proteins. The antiinflammatory effects of GCs are predominantly mediated via interference of monomeric GR with the transactivation capacity of TFs, such as NF-κB, via a tethering mechanism. Otherwise, GC-activated GR can also negatively regulate gene transcription via competition for an overlapping binding site (competitive GRE) or via DNA-binding crosstalk with another TF (composite GRE), or else via the sequestration of a DNA-bound TF. Although the nature of these sequences is not well-defined, gene repression via direct binding of GR onto a so-called nGRE has also been reported.

Figure 4

Structure and posttranslational modifications of the GR. A, The reported phosphomodulated sites for the hGR, mGR, and rGR are depicted in relation to the known functional domains of this receptor. B, The reported posttranslational modifications for the hGR, exempt from the phosphorylated sites, are depicted in relation to the known functional domains of this receptor. C, The reported phosphomodulated sites for human NF-κB-p65 are depicted in relation to the known functional domains of this transcription factor. NTD, N-Terminal domain; HR, hinge region; P, phosphorylation site; h, human; m, murine; r, rat; aa, amino acids; SUMO, SUMOylation site; Ub, ubiquitination site; Ac, acetylation site; Rel-HD, Rel-homology domain; TA, transactivation.

Figure 5

The layered MAPK signaling cascade. A schematic representation of the signaling cascades initiated by mitogens and stressors that lead to the activation of the MAP2Ks/MKKs, MAPKs, and ultimately the various MKs.