Epidermal hyperproliferation and abnormal differentiation

The regulation of keratinocyte proliferation and differentiation is perturbed in psoriatic lesions as mentioned above. The epidermis is massively thickened due to an increase in the number of keratinocyte layers (acanthosis) and there is an elongation o f the rete ridges (Fig. 1.2 A compared to Fig. 1.1 A). Squames of the com ified layer retain their nuclei (parakeratosis) and there is no observable granular layer (agranulosis) (Fig. 1.2 A and B). When sections of psoriatic lesions are stained for the proliferative marker Ki67, it is found that there is a substantial increase in the numbers of proliferating cells compared to normal

Features of lesional epidermis Parakeratosis Absence of granular layer Enhanced spinous layer markers Thickened epidermis (acanthosis) Hyperproliferation (expansion of transit compartment?)

K

Figure 1.2: Psoriatic epidermis.

(A) Hematoxylin and Eosin - stained section of a human psoriatic lesion. Note the thickened epidermis, nucleated squames (parakeratosis) and dermal inflammatory infiltrate.

Magnification lOx.

(B) Diagram representing the epidermis of a psoriatic lesion with the main abnormalities/ features compared to normal epidermis indicated (compare with Figure 1.1). Key to coloured cells is as in Figure 1.1. For more details see main text.

Chapter 1. Introduction

epiderm is (Gerritsen et al., 1997; Mils et al., 1994). These K i67 positive cells are predominantly localised to the basal layer of the psoriatic epidermis although some are present in the immediately suprabasal layers (Mils et al., 1994).

It has been proposed that the observed elongation of rete ridges in psoriasis is due to the increased numbers of proliferating cells in the basal layer causing this compartment to expand (lizuka et al., 1997; lizuka et al., 1999; lizuka et al., 1996). Furthermore, due to the location of transit amplifying cells in the rete ridges and stem cells at the tips of the dermal papillae (Jensen et al., 1999; Jones et al., 1995) (see preceding section), it has also been suggested that the hyperproliferation of psoriatic epidermis is due to an expansion of the transit amplifying compartment (lizuka et al., 1996) (Fig. 1.2 B). This proposal is supported by the fact that the majority of Ki67 positive kératinocytes in psoriatic lesions are present along the rete ridges and not at the tips of the dermal papillae (Asumalahti et al., 2002).

The hyperproliferative nature of the psoriatic epidermis can also be demonstrated by the expression of keratins K6, K16 and K17. The type II keratin K6 and the type I keratins K16 and K17 are constitutively expressed within the hair follicle and sporadically expressed in palmar/plantar epidermis but absent from normal interfollicular epidermis (Moll et al., 1983; Troyanovsky et al., 1989). H owever these keratins are induced in hyperproliferative epidermis such as found during wound healing and also in squamous cell carcinomas (SCCs) (Coulombe, 1997; M ansbridge and Knapp, 1987; Moll et al., 1983; Paladini et al., 1996; Stoler et al., 1988). As expected, these keratins are present in psoriatic epidermis (Leigh et al., 1995; M ils et al., 1994; Stoler et al., 1988). The

expression of K17 has even been used as a marker for evaluating the treatment of psoriasis (de Jong et al., 1991). Forced K l 6 expression has been dem onstrated to trigger a reorganisation of the keratin network within kératinocytes and its induction at wound edges may play a vital role in the re-epithelialisation process (Paladini et al., 1996). K l7 expression has been suggested to regulate keratinocyte contractility and possibly lead to the re-organisation of provisional basement membranes form ed during wound healing processes (Freedberg et al., 2001; Troyanovsky et al., 1992). It is also of interest that mutations within keratins 6, 16 and 17 have been found to cause the keratinising disorder Pachyonychia congenita (McLean et al., 1995; Smith et al., 1999; Terrinoni et al., 2001). Psoriatic lesions display an increase in the number of basal K14 positive layers and a delay in the onset of suprabasal K1 expression (Bernard et al., 1988; Mils et al., 1994) and these are also regarded as features of hyperproliferative epidermis (Haase and Hunzelmann, 2002; Wongwaisayawan et al., 1991).

Aside from alterations in the expression of keratins within psoriatic lesions, the expression of comified envelope precursors is also abnormal. In particular, the localisation of spinous and granular layer markers are different from that seen in normal epidermis (Fig. 1.2 B). It is well docum ented that whilst involucrin can be detected in the upper spinous and granular layers of normal epidermis, in psoriatic lesions it shows an extended expression and is present in most of the lower spinous cell layers (Bernard et al., 1988; Bernard et al., 1986; Bernard et al., 1985; Mils et al., 1994; W atanabe et al., 1991). Furthermore, there is an increase in involucrin mRNA in psoriatic epidermis (Takahashi et al., 1996). Through the use of dansylcadaverine to detect transglutaminase activity in sections of psoriatic and normal skin sections, it was found that the activity was present in most suprabasal layers of

Chapter 1. Introduction

the psoriatic epidermis but localised to upper spinous and granular layers only of normal epidermis (Bernard et al., 1986). This was confirmed through use of antibodies to TG-1 (Bernard et al., 1988). Comifin was also found to have an extended expression pattern in psoriatic versus normal epidermis, similar to involucrin, (Fujimoto et al., 1993; Fujimoto et al., 1997) thus confirm ing that m ultiple spinous layer m arkers are precociously expressed in psoriatic lesions. Conversely, the expression of granular layer markers is much reduced. Loricrin protein and mRNA are absent or reduced in psoriatic lesions (Hohl, 1993; Juhlin et al., 1992; Takahashi et al., 1996) and there is also much less detectable filaggrin (Bernard et al., 1988; Mils et al., 1994; W atanabe et al., 1991).

These observations establish that the differentiation program o f kératinocytes within psoriatic lesions is significantly altered. It has been suggested that due to the delayed expression of the early differentiation marker keratin K1 and the extended spinous layer marker expression that these two stages of the differentiation program are inverted in psoriatic epidermis (Bernard et al., 1988). It is of interest that this abnormal differentiation of kératinocytes in psoriasis is also seen transiently during wound healing although not in another hyperproliferative skin disease, lichen planus (Bernard et al., 1988; Démarchez et al., 1986; Mansbridge and Knapp, 1987). Psoriasis is regarded as a chronic wound-healing response of the skin, lacking resolution and return of the epiderm is to a steady state condition.

A further key feature of psoriasis is the alteration o f integrin expression w ithin the epidermis. It was found that in psoriatic epidermis expression of p i integrins extends into the suprabasal layers as compared to the predominantly basal location in normal epidermis

(Hertle et al., 1992). The a3 and a 2 subunit partners of p i are also found in the suprabasal layers. The oc6 integrin subunit is expressed suprabasally but its partner subunit p4 remains basal. It is considered possible that when a 6 is present in the suprabasal layers it may partner with the suprabasal p i integrin subunit (Hertle et al., 1992); alternatively it may reach the cell surface without a P subunit partner (Owens and W att, unpublished observations). Forced expression of the p i integrin subunit alone in the suprabasal layers of mouse epidermis from the involucrin promoter causes an upregulation of endogenous a 6 integrin in those same layers under some circumstances (Carroll et al., 1995). The de novo expression of a 5 p l integrin is also observed in psoriatic epidermis and the polarised distribution of p l integrins (predominantly to cell-cell contact sites) and a6 P 4 integrin (to basal membrane in hemidesmosomes) is lost (De Luca et al., 1994; Pellegrini et al., 1992). Furthermore, the changes in integrin distribution around the keratinocyte membrane are reported to be present in uninvolved psoriatic epidermis and have been suggested as an initial trigger of the skin lesions (Pellegrini et al., 1992). The presence of p l integrins in the suprabasal layers of psoriatic epidermis is also seen during wound healing of the epidermis (Cavani et al., 1993; Hertle et al., 1992), thus demonstrating again that psoriasis is very much like a chronic wound-healing response. Keratinocyte suprabasal p i integrin expression has been found to have a possible role in the generation of many of the features of psoriasis and this is discussed in later sections.

Chapter 1. Introduction

1.3 Pathogenesis of psoriasis