br Materials and Methods br

Materials and Methods
Results
Discussion Psoriasis is a chronic systemic inflammatory and an autoimmune skin disease. It is histologically characterized by KC hyperproliferation, inflammation, and increased angiogenesis (Bachelez, 2005; Nestle et al., 2009). Although considerable progress in understanding the complex genetic, environmental, and immunological basis of psoriasis has been achieved, psoriasis pathogenesis remains not fully elucidated. Many biomarker candidates of psoriasis have been found, but none of them has so far met the sensitivity, specificity, and accuracy criteria that would allow their translation into clinical use (Villanova et al., 2013). Especially, limited information is available on psoriasis histopathogenesis related with chronic erythematous plaques, which is considered to be caused by KC hyperproliferation and abnormal angiogenesis. Although over-expression of vascular permeability factor/vascular endothelial growth factor and its receptors is found in psoriasis tissue, the factor responsible for contorted blood vessels in psoriatic lesions remains unknown (Detmar et al., 1994). PK2 has been recently proposed to participate in numerous important physiological processes including inflammation, neurogenesis, tissue development, angiogenesis, and even nociception (Monnier and Samson, 2010; Negri et al., 2004; Shojaei et al., 2008). It exerts the actions by binding to two G protein-coupled receptors, PKR1 and PKR2 (Cheng et al., 2012; Lattanzi et al., 2001; Negri et al., 2004). PKR1 is exclusively responsible for mediating the modulation of macrophage cytokine production. PK2 is mainly expressed in propyl but can also be found in skin, bone marrow, lymphoid organs, granulocytes, dendritic cells, macrophages, etc. (Martucci et al., 2006; Negri et al., 2007). PKRs are distributed in brain and the peripheral organs, including the spleen, and leukocytes, and they are found in most murine immune cell types (Dorsch et al., 2005; Franchi et al., 2008; Martucci et al., 2006). PK2 and its receptors are less expressed in skin but in this study, abnormally high PK2 level was found in human psoriatic skin and blood as well as mouse psoriatic skin and blood, but no significant change in other autoimmune diseases, suggesting that PK2 is a psoriasis-specific factor. PK2 is a multifunctional cytokine possessing abilities to induce epidermal, vascular and inflammatory change (Martucci et al., 2006; Monnier and Samson, 2010) as found in natural psoriasis. Why and how are PK2 overexpressed in psoriasis? One of the best-characterized psoriasis initiation mechanisms is the antimicrobial peptide LL-37-DNA/RNA complex resulted from infection or physical trauma (Di Meglio et al., 2014), which induces inflammation, but how to initiate and develop psoriasis remains unknown. As illustrated in Fig. 1e, bacterial products including LPS and DNA promoted the production of PK2 besides inflammatory factors, suggesting that infection is a primary inducer of PK2 and inflammation. Our results suggested that PK2 aggravates psoriasis by promoting KC and FB proliferation and differentiation, inflammation, and angiogenesis. Besides the well-known angiogenesis function of PK2, how did PK2 play the roles in psoriasis? PK2 significantly induced production of IL-1 in KC and macrophage. It is well known that TNF-a, IL-23, IL-17 and some other cytokines play important roles in psoriasis. These cytokines were up-regulated by PK2 in vivo but not obvious in KC or macrophage in our study. IL-1 plays multiple roles in the initiation of an inflammatory response to injury. In psoriasis, dysregulation of IL-1α and IL-1β plays significant roles in orchestrating the immune response (Cooper et al., 1990; Johnston et al., 2011; Perera et al., 2012). IL-1 is a direct chemoattractant to lymphocytes and exerts a further paracrine effect (Chung et al., 2009; Nourshargh et al., 1995). This may be why TNF-a, IL-23, IL-17 and some other cytokines were up-regulated by PK2 in vivo but not in vitro. IL-1 is the most likely initiator of the KC activation cycle (Perera et al., 2012). IL-1 is the inducer of nearly all other proinflammatory cytokines and is able to activate secondary cytokines in the skin (Schroder, 1995). The secreted IL-1 induced by PK2, which acts on adjacent fibroblast (FB), promoted the secretion of mitogenic and proinflammatory factors and consequently exerted proliferative and differentiation effects on KC and FB. Why was the co-culture of KC and FB required for the proliferation effect of PK2 as the result showed? The predominant IL-1 receptor in KC is IL-1R2, which is incapable of participating in IL-1 signaling, while that in FB is IL-1R1, which mediates all IL-1 biological response (Chong et al., 2009). Therefore, IL-1 produced by KC and macrophage likely acted in a paracrine fashion on adjacent FB, triggering the release of other cytokines to regulate proliferation of both KC and FB (Chong et al., 2009; Kovacs et al., 2005). This possibility was supported by observations that many mitogenic and proinflammatory factors including KGF, GM-CSF, IL-6, -8, and -10 were induced by PK2 when KC was co-cultured with FB but not when KC or FB was cultured alone.