Stress Signals, Mediated by Membranous Glucocorticoid Receptor, Activate PLC/PKC/GSK-3β/β-catenin Pathway to Inhibit Wound Closure

Abstract

Glucocorticoids (GCs), key mediators of stress signals, are also potent wound healing inhibitors. To understand how stress signals inhibit wound healing, we investigated the role of membranous glucocorticoid receptor (mbGR) by using cell-impermeable BSA-conjugated dexamethasone. We found that mbGR inhibits keratinocyte migration and wound closure by activating a Wnt-like phospholipase (PLC)/ protein kinase C (PKC) signaling cascade. Rapid activation of mbGR/PLC/PKC further leads to activation of known biomarkers of nonhealing found in patients, β-catenin and c-myc. Conversely, a selective inhibitor of PKC, calphostin C, blocks mbGR/PKC pathway, and rescues GC-mediated inhibition of keratinocyte migration in vitro and accelerates wound epithelialization of human wounds ex vivo. This novel signaling mechanism may have a major impact on understanding how stress response via GC signaling regulates homeostasis and its role in development and treatments of skin diseases, including wound healing. To test tissue specificity of this nongenomic signaling mechanism, we tested retinal and bronchial human epithelial cells and fibroblasts. We found that mbGR/PLC/PKC signaling cascade exists in all cell types tested, suggesting a more general role. The discovery of this nongenomic signaling pathway, in which glucocorticoids activate Wnt pathway via mbGR, provides new insights into how stress-mediated signals may activate growth signals in various epithelial and mesenchymal tissues.

Figure 1.. GCs promote nuclearization of β-catenin through PKC.

Primary human keratinocytes control (a) or stimulated with either 30 mM LiCl (b), 1 μM Dex (c) or 100 nM Dex-BSA (f) for 24 hours in presence of 1 μM Ru486 (d and g) or 0.2 μM CC (e and h) are shown. Presence of phospho-β-catenin (Y142) was visualized by immunofluorescence (green; left panels). Nuclei were visualized by staining with PI (red; middle panels) and relative nuclearization was assessed by merging the two images using ImageJ (with co-localization appearing yellow; right panels). CC, calphostin C; Dex, dexamethasone; Dex-BSA, BSA-conjugated dexamethasone; GC, glucocorticoid; LiCl, lithium chloride; PI, propidium iodide; PKC, protein kinase C. Scale bar = 50 μm.

Figure 2.. mbGR stimulation activates PLC/PKC signaling cascade and induces phosphorylation and subsequent inactivation of GSK-3β resulting in c-myc induction.

Cells were stimulated with vehicle (DMSO), 1 μM Dex, or 100 nM Dex-BSA (DB) for 30 minutes upon treatment with GR antagonist Ru486 (1 μM), PLCγ inhibitor, genistein (50 μM) or PKC inhibitors CC (200 nM) or Go6976 (4μM). Phosphorylation of GR (a), PLCγ (b), PKC (c), and GSK-3β (d) were assessed by western blot. (e) Cells were stimulated with vehicle (DMSO) or 100 nM Dex-BSA for 4 hours upon treatment with 200 nM CC, and induction of c-myc was assessed by western blotting. All quantifications were performed using ImageJ with error bars corresponding to standard deviation from n = 3. CC, calphostin C; Dex, dexamethasone; Dex-BSA, BSA-conjugated dexamethasone (DB); GR, glucocorticoid receptor; GSK-3β, glycogen synthase kinase 3 beta; mbGR, membranous glucocorticoid receptor; PKC, protein kinase C; PLC, phospholipase C. *P ≤ 0.05 (Student t test).

Figure 3.. mbGR mediated activation of PLCγ/PKC/GSK-3β is present not only in cells of epithelial origin (eye and lung), but also in cells of mesenchymal origin (foot fibroblasts).

Presence of mbGR mediated activation of PLCy/PKC/GSK-3B signaling cascade was assessed in (a) D407-human retinal epithelial cells and (b) primary human bronchial epithelial cells, as well as in cells of nonepithelial origin like (c) primary human foot fibroblasts. Cells were stimulated with vehicle (DMSO) or 100 nM Dex-BSA (DB) for 30 minutes, and phosphorylation of PLCγ, GR, PKC pan, and GSK-3β were assessed by western blot, with Arpc2 serving as loading control. All quantifications were performed using ImageJ with error bars corresponding to standard deviation from n = 3. Dex-BSA, BSA-conjugated dexamethasone (DB); GR, glucocorticoid receptor; GSK-3β, glycogen synthase kinase 3 beta; mbGR, membranous glucocorticoid receptor; PKC, protein kinase C; PLC, phospholipase C. *P ≤ 0.05 (Student t test).

Figure 4.. Selective activation of the mbGR results in inhibition of keratinocyte migration.

Primary human keratinocytes were pretreated with 4 μg/ml mitomycin-C and stimulated with vehicle (DMSO), 25 ng/ml EGF (positive control), 1 μM Dex or 100 nM Dex-BSA (DB). Cells were wounded by a scratch and their migration was assessed at the time of the scratch (0 h) every 2 hours for 48 hours. Representative images at 0, 24, and 48 hours after the initial scratch were used to quantify migration by using Cell Migration Analysis software module (Essen Bioscience) comparing relative wound density, with light gray corresponding to initial wound scratch and dark gray corresponding to repopulation of the wound over time. Error bars correspond to standard deviation from n = 16. *P ≤ 0.05 (Student t test). Dex, dexamethasone; Dex-BSA, BSA-conjugated dexamethasone (DB); mbGR, membranous glucocorticoid receptor.

Figure 5.. Selective targeting of mbGR by Dex-BSA impedes epithelialization and can be ameliorated by inhibiting PKC in an ex vivo model of wound closure.

Normal human skin was wounded using a 3-mm biopsy punch and maintained at the air-liquid interface in presence or absence of 1 μM Dex, 100 nM Dex-BSA (DB), or PKC inhibitor CC (0.2 μM). Wound healing was assessed at day 4 after wounding, a time when exponential epithelialization occurs. Wound closure was quantified by histology using ImageJ software. Gross photos show visual signs of closure and correspond to the histology assessments with black arrows pointing to epithelial tongue location at day 4. Error bars correspond to standard deviation from n = 4. *P ≤ 0.05 (Student t test). Scale bar 1 = mm. Dex, dexamethasone; Dex-BSA, BSA-conjugated dexamethasone (DB); mbGR, membranous glucocorticoid receptor; PKC, protein kinase C.

Figure 6.. Proposed mechanism by which GC-mediated activation of mbGR contributes to inhibition of keratinocyte migration and wound healing.

A diagram summarizing findings from this study is shown. Upon binding to the membranous fraction of GR, GCs induce a phosphorylation and activation of GR, followed by activation PLCγ (via PTK), PKC, and subsequent phospho-inactivation of GSK-3β. This in turn liberates β-catenin from the inactivation complex thereby allowing it to translocate into the nucleus and induce c-myc expression, thus inducing cell proliferation while delaying keratinocyte migration and subsequent epithelialization. GC, glucocorticoid; GR, glucocorticoid receptor; GSK-3β, glycogen synthase kinase 3 beta; mbGR, membranous glucocorticoid receptor; P, phosphorylation; PKC, protein kinase C; PLC, phospholipase C; PTK, protein tyrosine kinase; U, ubiquitination.