Transrepression function of the glucocorticoid receptor regulates eyelid development and keratinocyte proliferation but is not sufficient to prevent skin chronic inflammation

Proteomic Analysis of Brain Regions Reveals Brain Regional Differences and the Involvement of Multiple Keratins in Chronic Alcohol Neurotoxicity

AIMS

Alcohol abuse has attracted public attention and chronic alcohol exposure can result in irreversible structural changes in the brain. The molecular mechanisms underlying alcohol neurotoxicity are complex, mandating comprehensive mining of spatial protein expression profile.

METHODS

In this study, mice models of chronic alcohol intoxication were established after 95% alcohol vapor administration for 30 consecutive days. On Day 30, striatum (the dorsal and ventral striatum) and hippocampus, the two major brain regions responsible for learning and memorizing while being sensitive to alcohol toxicity, were collected. After that, isobaric tags for relative and absolute quantitation -based quantitative proteomic analysis were carried out for further exploration of the novel mechanisms underlying alcohol neurotoxicity.

RESULTS

Proteomic results showed that in the striatum, 29 proteins were significantly up-regulated and 17 proteins were significantly down-regulated. In the hippocampus, 72 proteins were significantly up-regulated, while 2 proteins were significantly down-regulated. Analysis of the overlay proteins revealed that a total of 102 proteins were consistently altered (P < 0.05) in both hippocampus and striatum regions, including multiple keratins such as Krt6a, Krt17 and Krt5. Ingenuity pathway analysis revealed that previously reported diseases/biofunctions such as dermatological diseases and developmental disorders were enriched in those proteins. Interestingly, the glucocorticoid receptor (GR) signaling was among the top enriched pathways in both brain regions, while multiple keratins from the GR signaling such as Krt1 and Krt17 exhibited significantly opposite expression patterns in the two brain nuclei. Moreover, there are several other involved pathways significantly differed between the hippocampus and striatum.

CONCLUSIONS

Our data revealed brain regional differences upon alcohol consumption and indicated the critical involvement of keratins from GR signaling in alcohol neurotoxicity. The differences in proteomic results between the striatum and hippocampus suggested a necessity of taking into consideration brain regional differences and intertwined signaling pathways rather than merely focusing on single nuclei or molecule during the study of drug-induced neurotoxicity in the future.

Roles of the Glucocorticoid and Mineralocorticoid Receptors in Skin Pathophysiology

The nuclear hormone receptor (NR) superfamily comprises approximately 50 evolutionarily conserved proteins that play major roles in gene regulation by prototypically acting as ligand-dependent transcription factors. Besides their central role in physiology, NRs have been largely used as therapeutic drug targets in many chronic inflammatory conditions and derivatives of their specific ligands, alone or in combination, are frequently prescribed for the treatment of skin diseases. In particular, glucocorticoids (GCs) are the most commonly used compounds for treating prevalent skin diseases such as psoriasis due to their anti-proliferative and anti-inflammatory actions. However, and despite their therapeutic efficacy, the long-term use of GCs is limited because of the cutaneous adverse effects including atrophy, delayed wound healing, and increased susceptibility to stress and infections. The GC receptor (GR/NR3C1) and the mineralocorticoid receptor (MR/NR3C2) are members of the NR subclass NR3C that are highly related, both structurally and functionally. While the GR is ubiquitously expressed and is almost exclusively activated by GCs; an MR has a more restricted tissue expression pattern and can bind GCs and the mineralocorticoid aldosterone with similar high affinity. As these receptors share 95% identity in their DNA binding domains; both can recognize the same hormone response elements; theoretically resulting in transcriptional regulation of the same target genes. However, a major mechanism for specific activation of GRs and/or MRs is at the pre-receptor level by modulating the local availability of active GCs. Furthermore, the selective interactions of each receptor with spatio-temporally regulated transcription factors and co-regulators are crucial for the final transcriptional outcome. While there are abundant genome wide studies identifying GR transcriptional targets in a variety of tissue and cell types; including keratinocytes; the data for MR is more limited thus far. Our group and others have studied the role of GRs and MRs in skin development and disease by generating and characterizing mouse and cellular models with gain- and loss-of-function for each receptor. Both NRs are required for skin barrier competence during mouse development and also play a role in adult skin homeostasis. Moreover, the combined loss of epidermal GRs and MRs caused a more severe skin phenotype relative to single knock-outs (KOs) in developing skin and in acute inflammation and psoriasis, indicating that these corticosteroid receptors play cooperative roles. Understanding GR- and MR-mediated signaling in skin should contribute to deciphering their tissue-specific relative roles and ultimately help to improve GC-based therapies.

Glucocorticoid receptor enhances involucrin expression of keratinocyte in a ligand-independent manner

In this study, we investigated the role of glucocorticoid receptor (GR) in epidermal keratinocytes. In adult normal human skin, GR was highly expressed in the upper layers of the epidermis. Consistent with normal skin, GR expression was increased after calcium treatment of HaCaT keratinocytes cultured in vitro, suggesting that GR is involved in keratinocyte differentiation process. Overexpression of GR using an adenovirus showed that expression of involucrin, an early differentiation marker of keratinocytes, was markedly increased due to GR overexpression. However, treatment with dexamethasone, a GR agonist, did not increase involucrin expression. Overexpression of GR led to phosphorylation of c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinases (ERK) in the absence of glucocorticoid, suggesting that the GR effect on involucrin expression is related to activation of intracellular signaling cascades. This idea was supported by the fact that GR-mediated involucrin induction was abolished after treatment with JNK and ERK inhibitors. In addition, GR mutants lacking the ligand-binding domain increased involucrin expression concomitantly with increase of ERK phosphorylation. Together, these results suggest that GR modulates involucrin expression of keratinocytes by regulating the intracellular signaling network in a ligand-independent manner.

Cortisol promotes differentiation of epidermal ionocytes through Foxi3 transcription factors in zebrafish (Danio rerio)

Glucocorticoid regulates epidermal cell proliferation, and is used to treat certain skin disorders. Cortisol, a glucocorticoid, is also linked to skin development in teleost fish. Cortisol increases the number of epithelial ionocytes during environmental acclimation in euryhaline fishes, but it is unclear whether this is due to increased differentiation or proliferation. To investigate, we treated zebrafish embryos with exogenous cortisol (20mg/L). The densities of the ionocytes Na(+)-K(+)-ATPase rich cells (NaRCs) and H(+)-ATPase rich cells (HRCs) were significantly increased by cortisol, and this was accompanied by an increase in the respective marker genes. Expression of the glucocorticoid receptor (GR) gene was decreased. Cortisol treatment also increased ionocytes in cultured adult zebrafish gills, and up-regulated expression of genes encoding forkhead box I3 (foxi3a and foxi3b) transcription factors, which regulate ionocyte progenitor development. GR expression was up-regulated by cortisol in vitro; as such, the observed decrease in vivo reflects a regulatory systemic-negative feedback. Notably, in situ hybridization revealed that foxi3a/b mRNA expression was increased by cortisol at 24-48h post-fertilization. Cortisol also decreased keratinocytes, but did not affect epidermal stem cells or mucus cells. We conclude that foxi3a/b transactivation by cortisol-GR favors differentiation of ionocyte progenitors, thereby facilitating proliferation of mature ionocytes.

Keratinocyte-targeted overexpression of the glucocorticoid receptor delays cutaneous wound healing

Delayed wound healing is one of the most common secondary adverse effects associated to the therapeutic use of glucocorticoid (GC) analogs, which act through the ligand-dependent transcription factor GC-receptor (GR). GR function is exerted through DNA-binding-dependent and –independent mechanisms, classically referred to as transactivation (TA) and transrepression (TR). Currently both TA and TR are thought to contribute to the therapeutical effects mediated by GR; however their relative contribution to unwanted side effects such as delayed wound healing is unknown. We evaluated skin wound healing in transgenic mice with keratinocyte-restricted expression of either wild type GR or a mutant GR that is TA-defective but efficient in TR (K5-GR and K5-GR-TR mice, respectively). Our data show that at days (d) 4 and 8 following wounding, healing in K5-GR mice was delayed relative to WT, with reduced recruitment of granulocytes and macrophages and diminished TNF-α and IL-1β expression. TGF-β1 and Kgf expression was repressed in K5-GR skin whereas TGF-β3 was up-regulated. The re-epithelialization rate was reduced in K5-GR relative to WT, as was formation of granulation tissue. In contrast, K5-GR-TR mice showed delays in healing at d4 but re-established the skin breach at d8 concomitant with decreased repression of pro-inflammatory cytokines and growth factors relative to K5-GR mice. Keratinocytes from both transgenic mice closed in vitro wounds slower relative to WT, consistent with the in vivo defects in cell migration. Overall, the delay in the early stages of wound healing in both transgenic models is similar to that elicited by systemic treatment with dexamethasone. Wound responses in the transgenic keratinocytes correlated with reduced ERK activity both in vivo and in vitro. We conclude that the TR function of GR is sufficient for negatively regulating early stages of wound closure, while TA by GR is required for delaying later stages of healing.

Glucocorticoid receptor mutants: man-made tools for functional research

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that can bind to glucocorticoids (GCs). Upon ligand binding, GR sheds its cytoplasmic chaperoning complex and translocates to the nucleus, where it can act as a ligand-dependent transcription factor, transactivating or transrepressing specific gene promoters. Often, GR interacts with specific cofactors to implement a variety of gene promoter effects. GR activity and function is further modulated by post-translational modifications. To assess the diverse aspects of GR mechanisms of activation and gene regulation, researchers continue to use a range of artificial GR mutants. In this review we analyze the characteristics of GR mutants with the aim of assisting the design and interpretation of GR mutant-based experiments.

Glucocorticoid receptors, epidermal homeostasis and hair follicle differentiation

Glucocorticoids (GCs) exert their biological and therapeutical actions through the GC receptor (GR), a ligand-dependent transcription factor. Synthetic GC derivatives are widely prescribed for treating numerous cutaneous inflammatory and immune diseases due to their great efficacy. However, chronic treatment with GCs produces adverse side-effects including skin atrophy, delayed wound healing, and in certain cases, GC resistance. The mechanisms underlying the therapeutic actions of the GR in skin have been extensively studied; in contrast, the role of GR as a modulator of epidermal development and homeostasis has received less attention. The ubiquitous functional inactivation of GR results in defective epidermal formation although the underlying mechanisms have not been fully characterized. The use of transcriptomic approaches both in vitro and in vivo allowed the identification of genes that are regulated by GR in developing and adult skin. A main goal to understand the role of GR in skin biology is to identify primary transcriptional targets as well as the signaling pathways mediating GR action. Furthermore, it will be important to decipher the contribution of GR in the different cellular compartments of the skin, including keratinocytes of the interfollicular epidermis and hair follicles, and their respective stem cell progenitors. Additionally, recent findings indicating that the skin acts as a true peripheral endocrine organ implies greater complexity than originally thought. The local production of GCs and other steroid hormones should be considered as a modulator of skin function under homeostatic and diseased conditions. Finally, studying GR function in skin should take into account that the mineralocorticoid receptor may also mediate GC actions and/or regulate transcription either by itself or in combination with GR. Addressing these issues should help to elucidate the mechanisms by which Gr contributes to establishment of a competent epidermal barrier and may also have implications in the context of dermatological treatments based on GC-analogs.

Glucocorticoid receptor regulates overlapping and differential gene subsets in developing and adult skin

We have previously shown that the glucocorticoid receptor (GR) is required for skin homeostasis and epidermal barrier competence. To understand the transcriptional program by which GR regulates skin development, we performed a microarray analysis using the skin of GR(-/-) and GR(+/+) mice of embryonic d 18.5 and identified 442 differentially expressed genes. Functional clustering demonstrated overrepresentation of genes involved in ectoderm/epidermis development. We found strong repression of genes encoding proteins associated with the later stages of epidermal differentiation, such as several small proline-rich proteins (Sprrs) and corneodesmosin (Cdsn). This, together with the up-regulation of genes induced earlier during epidermal development, including the epithelial-specific gene transcripts E74-like factor 5 (Elf5) and keratin 77 (Krt77), fits with the phenotype of defective epidermal differentiation observed in the GR(-/-) mice. We also found down-regulation of the antimicrobial peptide defensin β 1 (Defb1) and FK506-binding protein 51 (Fkbp51). Skin developmental expression profiling of these genes and studies in cultured keratinocytes from GR(-/-) and wild type embryos demonstrated that gene regulation occurred in a cell-autonomous manner. To investigate the consequences of GR loss in adult epidermis, we generated mice with inducible inactivation of GR restricted to keratinocytes (K14-cre-ER(T2)//GR(loxP/loxP) mice). K14-cre-ER(T2)//GR(loxP/loxP) mice featured thickened skin with increased keratinocyte proliferation and impaired differentiation. Whereas Krt77 and Elf5 expression remained unaffected by loss of GR in adult epidermis, Fkbp51, Sprr2d, and Defb1 were strongly repressed. Importantly, we have identified both Fkbp51 and Defb1 as direct transcriptional targets of GR, and we have shown that GR-mediated regulation of these genes occurs in both developing and adult epidermis. We conclude that both overlapping and differential GR targets are regulated in developing vs. adult skin.

Minireview: latest perspectives on antiinflammatory actions of glucocorticoids

Taking into consideration that glucocorticoid (GC) hormones have been used clinically for over half a century and that more than 20 yr have passed since the cloning of the GC receptor (GR), it is hard to imagine that novel aspects in the molecular mechanism by which GCs mediate their antiinflammatory actions are still being unveiled today. Partly, this is because almost on a daily basis, novel insights arise from parallel fields, e.g. nuclear receptor cofactor and chromatin regulation and their concomitant impact on gene transcription events, eventually leading to a revisitation or refinement of old hypotheses. On the other hand, it does remain striking and puzzling why GCs use different mechanisms in so many different cell types and on many different target genes to elicit an antiinflammatory effect. Meanwhile, the obvious question for the clinic remains: is the separation of GR functionalities through differential ligand design the strategy of choice to avoid most GC-mediated side effects? This minireview aims to highlight some of the latest findings on aspects of the antiinflammatory working mechanisms of GCs.