Psoriasis is a chronic skin inflammatory disease mediated by the dysregualted immune and wound healing responses, however, the specific pathogenesis is not completely understood [16]. Growing evidences confirms the central role of IL-17A in the pathogenesis of Ps [2, 16]. IL-17A influence the keratinocytes, fibroblasts, endothelial cells, and immune cells as downstream targets in the Ps. Recent studies have identified Plet1 as a novel target gene of IL-17A [10], however the expression and the role of Plet1 in the pathogenesis of Ps is not yet studied.
GlcNAc exhibit multiple therapeutic benefits including its anti-inflammatory, wound healing and skin care effects by promoting the proliferation of keratinocytes and fibroblasts and, increases the production of hyaluronic acid in the skin [17]. In addition, GlcNAc also found to have a inhibiting role on the IL-17 cytokines (IL-17A and IL-17F). However, the mechanisms involved in the beneficial therapeutic effects and possible repurposing of GlcNAc for treatment of Ps are yet to be explored. Therefore, in this study we aimed to investigate the role of Plet1 expression in the pathogenesis of Ps and the effects of therapeutic intervention by GlcNAc on the expression of Plet1 in IMQ induced mice model of Ps.
The established IMQ induced Ps model is used in this study to evaluate the effect of GlcNAc on the Ps severity is assesed by PSAI erythma, thickness and scarring. As expected, group of mice which received IMQ application (Ps) on back skin of mice gradually induced Ps-like skin inflammation while the control groups (control) with base cream retained the normal skin throughout the experimental period (D0- D8) (Fig. 1). The mice in the treatment group (Ps + GlcNAc) significantly reduced the PSAI score gradually and the skin thickness on D8. In addition, the H&E skin sections from the Ps group of mice showed reduced inflammation (Fig. 1). These results suggest an anticipated anti-inflammatory effects of GlcNAc in IMQ induced mice model of Ps.
During the pathogenesis of Ps, the initial chemokine and pro-inflammatory cytokine including IL-17A triggers infiltration of immune cells like neutrophils, macrophages and T lymphocytes [16, 18]. Additional IL-17A secreted by these recruited neutrophils, excerbate the response and leads to the recruitment of more neutrophils. These infiltrated cells further contributes to the worseing of the disease condition. While Th17 cells were previously thought to be the primary source of IL-17A, however, a new findings suggests that mast cells and neutrophils are the predominant cell type containing and releasing IL-17A in psoriatic skin [19]. Therefore, in next step, we have evaluated the frequencies of infiltrated neutrophils in the skin tissues of all groups (control, Ps and Ps + GlcNAc). The flow cytometry based immune phentyping suggested an increased percentages of neutrophils (CD45+CD11b+GR-1hi) in the mice of Ps group (Fig. 2). However, the treatment with GlcNAc significantly reduced the percentages of infiltrated neutrophils (Fig. 2). Studies have highlighted the key role of mononucelar phogocytes incuding macrophages and CD11c+ DCs [20, 21] in pathogenesis of Ps. The myeloid cells provide essential triggers that supports the survival, differentiation, and activation of Th17 cells, which later secretes IL-17A cytokines. Consistent with these findings, our data demonstrated an increase in the myeloid CD11c + DCs inflammatory mononuclear phagocytes (CD11b+CD11chi) in Ps group, while GlcNAc treatment significantly reduced the frequency of these cell populations (Fig. 2). However, in the treated mice skin CD11bhiCD11c+ populations were observed to be reversed to the baseline as in the control group (Fig. 2). Together, these data suggests an indirect effect of GlcNAc in interfering with the infiltration of pro-inflammatory cells including neutrophils and CD11c+ DCs into the Psoriatic skin tissues. Furthermore, IL-17A also upregulates pro-inflammatory effects on macrophages and dendritic cells [22]. Several studies demonstrated the impaired function of regulatory T-cells (Tregs) mediated by IL-17A in Ps condition. This suppressive effect of Tregs induce an imbalance between conventional CD4 + T cells (Tcons) and Tregs [23–25]. Considering these conditions,, we therefore evaluated the ratio of Tcons: Tregs in the mice skin from all the experimental groups. Our data showed an increased frequencies of Tregs (CD4+FOXP3+) when treated with GlcNAc and reversed the Tcons (CD4+FOXP3neg): Treg ratio compared to the Ps group (Fig. 2). This data providce evidence that GlcNAc can positively regulate the Tcon:Treg ratio and may support in attenuating Ps.
IL-17A can act on keratinocytes directly and results in an increased proliferation of keratinocytes and release of various pro-inflammatory cytokines associated with Ps such as IL-6, IL-17A and TNF-α [26]. In addition to the Ps associated cytokines, keratinocyte hyperplasia in Ps may be explained in part by overproduction of GFs which stimulate epidermal proliferation and altere metabolism of GF receptors in affected skin [27]. Several studies have observed an increased expressions of GFs including VEGF, PDGF and TGF-β in psoriatic skin [8, 9, 28]. Interestingly, in additon to these reported roles of GFs in psoriatic lesions, recent studies have found that progranulin (PGRN) expression in human psoriatic lesions and serum. Therefore we studied the mRNA expression of these GFs, along with the psoriastic-cytokines. Our data demonstrated an increase in expression of all the proinflammatory cytokines including IL-17 and GFs like granulin, VEGF, PDGF and TGF-β in Ps skin, while GlcNAc treatment significantly reduced their expressions (Fig. 3). This data supports the existing evidence on the role of GlcNAc in inhibiting pro-inflammatory cytokines like IL-17, IL-6 and TNF-α [15]. In addition, the role of GlcNAc in reducing the expression of GFs in IMQ induced Ps mice model, propose GlcNAc as a promising therapeutic agent in treating Psoriatic leasions.
MAPK is one of the IL-17A dependent signaling pathways mediated kinases that are activated during the pathogenesis of Ps [29, 30], which eventually triggers the expression of GFs. In our study, we have obseved the significant downregulation of GFs by GlcNAc treatment, therefore, we speculated that GlcNAc could have a role in inhibition of MAPK. Our data showed an increased expression of MAPK in mice skin tissues of Ps group and a significantly reduced the MAPK expression in the GlcNAc treated group.
A recent study demonstrated that IL-17A induced expression of Plet1 and proposed Plet1 as a target gene of IL-17A [10]. In addition, it is well established that IL-17A plays a key role in the pathogenesis of Ps. However, underlying mechanisms of IL-17A induced Plet1 expression in the pathogenesis of Ps is not yet reported. Therefore, we next evaluated the expression of Plet1 in the skin tissues of all the groups. The mRNA and protein expression of Plet1 showed an increased expression in Ps skin compared to the skin from the control group. Strickingly, GlcNAc treatment significantly reduced the expression of Plet1 in the treated group (Fig. 4). This data highlights for the first time the expression of Plet1 in Ps and the role of GlcNAc in downregualting the expression of Plet1. Despite, the exact mechanism of Plet1 inhibition by GlcNAc is not yet understood, we could speculate from the existing data, that the downregulation of Plet1 expression may be mediated by modulating IL-17-MAPK axis by GlcNAc (Fig. 6). In consistant with this proposed mechanism of GlcNAc inhibition of Plet1 in Ps, inhibition of an upstream modulator of MAPK showed a significant reduction in Plet1 expression [10, 31]. In concurrence with a potential profibrotic role, IL-17 was shown to enhance fibroblast proliferation in humans [32]. Therefore, we have validated the effect of GlcNAc on the apoptosis and proliferation of IL-17A stimulated HDFs. In additon, the proinflammatory cytokines including IL-17A have been observed to be significantly reduced upon GlcNAc treatment and Plet1 is significantly reduced in the IL-17A stimulated HDFs (Fig. 5). These results demonstrated the efficiency of GlcNAc as a promising agent in treating the Ps disease conditions.
In conclusion, Ps is an inflammatory skin disease that is associated with multiple comorbidities and substantially diminishes patients' quality of life. Topical therapies remain the cornerstone for treating mild Ps. Recent evidences suggust that targeted therapies could be a turnaround in the management of Ps. TNF-α blocking agents have initially paved the way, followed by monoclonal antibodies directed against IL-12/23, IL-17, and IL-23. This highlights the crucial role of the IL-17/23 cytokine pathway in Ps pathogenesis. Therapeutic advancements for moderate to severe plaque Ps include biologics that inhibit TNF-α, p40IL-12/23, IL-17, and p19IL-23, as well as an oral phosphodiesterase 4 inhibitor [3]. Constant efforts are required to decipher the molecular mechanisms behind this disease, since new treatments are still needed for refractory and severe cases. Therefore, GlcNAc in addition to its anti-inflammatory effects must be further investigated for the development of an effective and durable therapeutic alternative for treating Ps. In addition, antibodies agaist Plet1 could be an additonal promising treatment strategy for the traetment of Ps. Future studies are required to investigate such a possibility to decipher the detailed mechanism.