ELABORACION DE TARJETA ELECTRÓNICA EN LA

1.4.1 Keratinocyte - derived cytokines: IL-1 as a primary cytokine

Epidermal cytokines vary in their abilities to trigger cutaneous inflammation, as illustrated by overexpression of individual cytokines from epiderm al - specific promoters in transgenic mice (see Table 1.2). For example, overexpression of MCP-1 or lL-6 does not induce either a spontaneous inflammatory infiltrate or significant epidermal alterations. However, overexpression of either IL -la or T N F a is sufficient to induce both a cutaneous inflammatory infiltrate and epidermal abnormalities. I L - l a and T N F a are thus referred to

as primary cytokines whilst MCP-1 and IL-6 should be referred to as secondary cytokines (Kupper and Groves, 1995). It is also of interest that overexpression of the adhesion molecule ICAM-1 in the epidermis of transgenic mice is insufficient to induce cutaneous inflammation, given its roles in T cell recruitment and its upregulation in inflammatory skin diseases such as psoriasis (Griffiths et al., 1989; Singer et al., 1990; W illiams and Kupper, 1994). Primary cytokines can mediate their pleiotropic effects on diverse cell types either directly or indirectly via induction of a range of secondary cytokines (see below).

IL-1 is a critical primary cytokine produced in abundance by kératinocytes. Mouse kératinocytes primarily produce the a form and human kératinocytes are able to produce a and P forms, encoded by different genes (Ansel et al., 1988). Both a and p are produced as 31-34 kDa precursors which can be cleaved to mature 17 kDa forms. I L - la is active in both precursor and mature form whereas IL -ip requires cleavage by caspase-1 for activity (Thompson, 1998). It is thought that human kératinocytes normally lack caspase-1 activity and are therefore unable to produce active, m ature I L - ip (M izutani et al., 1991). Synthesised IL-1 remains in the cytoplasm of kératinocytes as it has no secretory signal peptide; however, it can be released upon disruption of the epidermis (Lee et al., 1997; Wood et al., 1996). Released IL-1 triggers the recruitment of CLA-i- T lymphocytes to sites of cutaneous injury by inducing E-selectin expression in underlying endothelial cells and then acting as a chemoattractant for lymphocytes tethered to these activated endothelial cells (Freedberg et al., 2001; Kupper and Groves, 1995; Murphy et al., 2000).

Chapter 1. Introduction

W hilst IL-1 by itself can initiate inflammation, many of its actions in this process are via the autocrine and paracrine induction of secondary cytokines and growth factors including GM-CSF, IL-6, IL-8, T N F a and ligands for the epidermal growth factor receptor (TG Fa and amphiregulin) (Boxman et al., 1996; Chen et al., 1995; Chung et al., 1996; Freedberg et al., 2001; Kupper et al., 1988; Larsen et al., 1989). Besides its effect on initiating cutaneous inflammation, IL-1 is also able to activate kératinocytes in an autocrine fashion to initiate a “wound-healing” type response from the cells: IL-1 causes kératinocytes to become more migratory and express keratins associated with a hyperproliferative state (e.g. keratin 6) (Chen et al., 1995; Komine et al., 2001). A model has been proposed whereby release of IL-1 in response to cutaneous injury triggers a “keratinocyte activation cycle” of induction of multiple cytokines and keratins associated with various stages of wound healing (Freedberg et al., 2001).

Recent work has uncovered a vital role for IL-1 in epidermal homeostasis. Via a so-called “double-paracrine” mechanism, keratinocyte-derived IL-1 stimulates dermal fibroblasts to produce both keratinocyte growth factor (KGF) and granulocyte-m acrophage colony- stimulating factor (GM-CSF) which act in concert on the kératinocytes themselves to stimulate appropriate proliferation and terminal differentiation (Maas-Szabowski et al., 1999; Maas-Szabowski et al., 2000; Szabowski et al., 2000; W erner and Smola, 2001). Fibroblasts are also stimulated to produce IL-6 in response to keratinocyte IL-1 (Boxman et al., 1996), which given the phenotype of keratin 14 / IL-6 transgenic mice (see Table 1.2) may also play a role in regulating keratinocyte differentiation. Therefore, keratinocyte - derived IL-1 not only potently induces inflammation in response to cutaneous injury but is involved in the regulation of epidermal proliferation and differentiation. IL-1 is thus a

central regulator of skin integrity and suggests that the altered expression of IL-1 found in psoriatic lesions may have a pathogenic role (see Table 1.1).

The mechanism by which binding of IL-1 to its cell surface receptor transduces signals into the cell has received much attention of late and is summarised in Fig. 1.3 (for reviews see Auron, 1998; Janssens and Beyaert, 2002). In particular, a central role has been described for the IL -I receptor-associated kinase (IRAK) family members of which four have been characterised in humans and mice (IR A K I, 2, M and 4) (Gao et al., 1996a; Croston et al., 1995; Kobayashi et al., 2002; Li et al., 2002; Muzio et al., 1997; Rosati and M artin, 2002; Suzuki et al., 2002; W esche et al., 1999). IL-1 is known to efficiently activate the NFkB pathway and also the stress-activated M APKs, p38 and JNK (Auron, 1998; Janssens and Beyaert, 2002) (see also e.g. Holtmann et al., 2001; Ninomiya-Tsuji et al., 1999) similar to the proinflammatory cytokine T N F a (Baud and Karin, 2001).

M ost likely due to the potent pro-inflammatory effects of IL-1, there are multiple negative regulators of IL-1 dependent stimulation. These include a non-transducing decoy receptor for IL-1, the type 2 IL-1 receptor (IL-IR) (Mantovani et al., 2001) and the IL-1 receptor antagonist (IL -lra), which binds to the transducing type 1 IL -IR but does not trigger receptor activation (Arend, 1993; Arend et al., 1998) (see Fig. 1.3). Kératinocytes express an intracellular variant of the IL -lra (icIL-lra) (Corradi et al., 1995; Haskill et al., 1991) which is released in response to cutaneous injury alongside the pre-formed pools of IL-1 (Lee et al., 1997) and is also induced in response to IL-1 stimulation (La and Fischer, 2001; La et al., 1999). Furthermore, kératinocytes can be induced to express the type 2 IL- IR (Groves et al., 1995a). In psoriatic lesions there are reports of an upregulation of both

IL -la /p