Skin immunosenescence: decreased receptor for activated C kinase-1 expression correlates with defective tumour necrosis factor-α production in epidermal cells

Immunosenescence and Skin: A State of Art of Its Etiopathogenetic Role and Crucial Watershed for Systemic Implications

Immunosenescence is a complex multifactorial phenomenon consisting of wide-ranging remodeling of the immune system during the life span, resulting in an age-related qualitative-quantitative decline of immune cells and cytokines. A growing body of evidence in the international literature is highlighting the etiopathogenetic role of skin immunosenescence in the onset of various dermatologic conditions. Skin immunosenescence also serves as an interesting watershed for the onset of system-wide conditions in the context of allergic inflammation. Moreover, in recent years, an increasingly emerging and fascinating etiopathogenetic parallelism has been observed between some mechanisms of immunosenescence, both at cutaneous and systemic sites. This would help to explain the occurrence of apparently unconnected comorbidities. Throughout our review, we aim to shed light on emerging immunosenescent mechanisms shared between dermatologic disorders and other organ-specific diseases in the context of a more extensive discussion on the etiopathogenetic role of skin immunosenescence. A promising future perspective would be to focus on better understanding the mutual influence between skin and host immunity, as well as the influence of high inter-individual variability on immunosenescence/inflammaging. This can lead to a more comprehensive "immunobiographic" definition of each individual.

Skin Immunosenescence and Type 2 Inflammation: A Mini-Review With an Inflammaging Perspective

Skin-resident stromal cells, including keratinocytes, fibroblasts, adipocytes, and immune cells including Langerhans cells, dendritic cells, T cells, and innate lymphoid cells, and their functional products work in concert to ensure the realization of skin barrier immunity. However, aging-induced immunosenescence predisposes the elderly to pruritic dermatoses, including type 2 inflammation-mediated. Inflammaging, characterized by chronic low level of pro-inflammatory cytokines released from senescent cells with the senescence-associated secretory phenotype (SASP), may drive immunosenescence and tangle with type 2 inflammatory dermatoses. The present mini-review summarizes current evidence on immunosenescence and type 2 inflammation in the skin and further focuses on future needs from an inflammaging perspective to clarify their complexity.

RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics

Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs. This article has an associated First Person interview with the first author of the paper.

Immunostimulatory effects of RACK1 pseudosubstrate in human leukocytes obtained from young and old donors

Aims of this study were to investigate the ability of RACK1 pseudosubstrate alone or in combination with classical immune stimuli to activate human leukocytes, and to restore age-associated immune defects.A total of 25 donors (17 old donors, 77-79 yrs; 8 young donors, 25-34 yrs) were enrolled. To evaluate the effect of RACK1 pseudosubstrate on cytokine production and CD86 expression the whole blood assay was used. Cultures were treated with RACK1 pseudosubstrate in the presence or absence of lipopolysaccharide (LPS) or phytohaemagglutinin (PHA) and incubated for 24 h or 48 h for LPS-induced CD86 expression, TNF-α, IL-6, IL-8, IL-10 production, and PHA-induced IL-4, IL-10, IFN-γ, respectively. RACK1 pseudosubstrate alone induced IL-6, IL-8, and CD86 expression in both young and old donors, and IFN-γ in old donors. In combination with LPS an increase in IL-8, IL-10 and TNF-α was observed, also resulting in restoration of age-associated defective production, while no changes in the other parameters investigated were found.Even if based on a small sample size, these results suggest the possibility to by-pass some of age-associated immune alterations, which may be beneficial in situations were natural immune stimulation is required, and highlight a different role of PKCβ in immune cells activation.

Corticosteroids modulate the expression of the PKC-anchoring protein RACK-1 and cytokine release in THP-1 cells

We demonstrated that cortisol reduces the expression of RACK-1 (Receptor for Activated C Kinase-1), a protein required for immune cell activation. The aim of this study was to evaluate whether and to what extent other clinically relevant corticosteroids may modulate RACK-1 expression. We used the human promyelocytic cell line THP-1 to investigate the effects of cortisol, prednisone, prednisolone, budesonide, betamethasone and methylprednisolone on RACK-1 expression and cytokine production. As anticipated, all corticosteroids inhibited at non-cytotoxic concentrations in a dose and time related manner LPS-induced TNF-α and IL-8 release, with budesonide, betamethasone and methylprednisolone being the most active followed by prednisolone, cortisol and prednisone. To a similar extent, all corticosteroids also reduced RACK-1 mRNA expression and RACK-1 protein levels as assessed by Real Time PCR and Western blot, respectively. Prednisone was the least potent compound while betamethasone and methylprednisolone where the most active. A good correlation was observed between RACK-1 mRNA or protein levels and cytokine release (Pearson r=0.7376, p=0.0471 for RACK-1 mRNA and TNF-α release, and Pearson r=0.8108, p=0.0252 for RACK-1 protein and IL-8 release). Mifepristone, a potent glucocorticoid receptor (GR) antagonist, completely prevented the effect of cortisol, demonstrating that RACK-1 downregulation is via GR. Furthermore, to by-pass the defective PKC activation due to the decrease in RACK-1, we used a RACK-1 pseudosubstrate, that directly activates PKC-beta. RACK-1 pseudosubstrate was able to restore LPS-induced cytokine production affected by cortisol, supporting the role of RACK-1 in the anti-inflammatory effect of corticosteroids. These results confirm the involvement of RACK-1 in immune cell activation and identify this protein as a novel transcriptional target of corticosteroid-induced anti-inflammatory effects.

Role of PKC-β in chemical allergen-induced CD86 expression and IL-8 release in THP-1 cells

We previously demonstrated an age-related decrease in receptor for activated C-kinase (RACK-1) expression and functional deficit in Langerhans cells' responsiveness. This defect specifically involves the translocation of protein kinase C (PKC)-β. The purpose of this study was to investigate the role of RACK-1 and PKC-β in chemical allergen-induced CD86 expression and IL-8 release in the human promyelocytic cell line THP-1 and primary human dendritic cells (DC). Dinitrochlorobenzene, p-phenylenediamine and diethyl maleate were used as contact allergens. The selective cell-permeable inhibitor of PKC-β and the broad PKC inhibitor GF109203X completely prevented chemical allergen- or lipopolysaccharide (LPS)-induced CD86 expression and significantly modulated IL-8 release (50 % reduction). The selective cell-permeable inhibitor of PKC-ε (also known to bind to RACK-1) failed to modulate allergen- or LPS-induced CD86 expression or allergen-induced IL-8 release, while modulating LPS-induced IL-8 release. The use of a RACK-1 pseudosubstrate, which directly activates PKC-β, resulted in dose-related increase in CD86 expression and IL-8 release. Similar results were obtained with human DC, confirming the relevance of results obtained in THP-1 cells. Overall, our findings demonstrate the role of PKC-β and RACK-1 in allergen-induced CD86 expression and IL-8 production, supporting a central role of PKC-β in the initiation of chemical allergen-induced DC activation.

Expression of epidermal CAMP changes in parallel with permeability barrier status

Two critical defensive functions of the outer epidermis, the permeability barrier and antimicrobial defense, share certain structural and biochemical features. Moreover, 3antimicrobial peptides (AMP); i.e., mouse beta-defensin 3 (mBD3), mouse cathelicidin protein (mCAMP), and the neuroendocrine peptide, catestatin, all localize to the outer epidermis, and both mBD3 and mCAMP are secreted from epidermal lamellar bodies with other organelle contents that subserve the permeability barrier. These 3 AMP are up-regulated in response to acute permeability barrier disruption, while conversely, mCAMP−/− mice (unable to combatgram-positive pathogens) also display abnormal barrier homeostasis. To determine further whether these two functions are co-regulated, we investigated changes in immunostaining for these 3 AMP in skin samples in which permeability barrier function in mice had been either compromised or enhanced. Compromised or enhanced barrier function correlated with reduced or enhanced immunohistochemical expression of mCAMP, respectively, but conversely with Cst expression likely due to the role of this AMP as an endogenous inhibitor of cathelicidin expression. mBD3 expression correlated with experimental barrier perturbations, but poorly with developmental changes in barrier function. These studies show that changes in cathelicidin and Cst expression parallel changes in permeability barrier status, with a less clear relationship with mBD3 expression.

Opposing effects of cortisol and dehydroepiandrosterone on the expression of the receptor for Activated C Kinase 1: implications in immunosenescence

Aging is associated to a decline in immune functions that are in part related to a defective protein kinase C dependent signal transduction machinery. RACK-1 (Receptor for Activated C Kinase 1) is a scaffold protein for different kinases and membrane receptors. We have previously demonstrated, in the elderly, a defective PCKβII (Protein Kinase C βII) translocation related to a decrease in RACK-1 protein expression, which is correlated to the age-associated decline in DHEA (dehydroepiandrosterone) levels. As a consequence of this signal transduction impairment, a significant decrease in immune cells functionality was observed. Furthermore, we could demonstrate that in vivo and in vitro DHEA administration restored RACK-1 level and immune functions, indicating that this hormone behaved as a positive RACK-1 regulator. We have most recently characterized the human GNB2L1 promoter region, coding for RACK-1 protein. Although no direct DHEA responsive elements were found, a glucocorticoid responsive element (GRE) was identified. The purpose of this work was to investigate, in the human pro-myelocytic cell line THP-1, whether physiological cortisol concentrations were able to modulate GNB2L1 promoter activity, RACK-1 transcription as well as cytokine production. As DHEA is endowed of anti-glucocorticoid properties in several cellular systems, and as cortisol:DHEA ratio imbalance is relevant in aging, we also investigated their possible interaction at the RACK-1 expression level. We could demonstrate that cortisol acted in a dose-related manner as a GNB2L1 promoter repressor, reducing RACK-1 mRNA expression and protein level. Probably by interfering with glucocorticoid receptor binding to GRE sequence, prolonged DHEA exposure counteracted cortisol effects, restoring RACK-1 levels and cytokine production, as assessed by LPS-induced TNF-α release.