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The integrin ligands are shown in Table 1.6. o2f>\ has been found in focal contacts when kératinocytes are grown on collagen and a3pl when cells are grown on laminin (Carter et al., 1990a, b). Guo et al. (1990) observed co-localisation of pi

integrins with vinculin in cells that had spread on collagen and Marchisio et al. (1991) observed av, but not a2, a3 or pi in focal contacts in cells cultured on plastic. aSpl

has also been demonstrated in focal adhesions (Grinnell 1990).

In vivo, a6p4 is a component of hemidesmosomes (Stepp et al., 1990). In

culture, hemidesmosomes do not form, neither is a6p4 found in focal contacts. Instead

a6p4 is organised in the ventral plasma membrane into stable anchoring contacts (SACs) (Carter et al., 1990b; Marchisio et al., 1991). It is possible that SACs are

precursors of hemidesmosomes since they contain bullous pemphigoid antigen, which is a component of hemidesmosomes (Carter et al., 1990b).

DiPersio et al. (1997) demonstrated that a3pi is required for the normal

development of epidermal basement membrane. a3 -/- mice showed neonatal blistering at the epidermal-dermal junction consistent with basement membrane rupture (rather than detachment which occurs in some blistering disorders such as epidermolysis bullosa). Kératinocytes from a3 -/- mice attached to laminin 5 through a6p4, but spread poorly, also indicating a post-attachment requirement for a3pi. Hodivala-Dilke et al. (1998) found that the absence of a3pi in these kératinocytes led to increased stress fibre formation in vitro, and allowed an increase in fibronectin and collagen type IV receptor activities. This demonstrates that a3pi can act as a trans-dominant

inhibitor of the functions of other integrins in mouse kératinocytes.

1.4.2.2 Keratinocyte cell-cell adhesion

Several studies have suggested that the pericellular distribution of a2 p i and a 3 p l in vivo^ indicate a functional role in cell-cell adhesion (Peltonen et a l, 1989; Adams and Watt, 1990,1991; Carter et al., 1990a, 1991; De Luca et al., 1990).

However, Laijava et al. (1990) found that antibodies directed against the p i subunit and aSpi only perturbed cell-cell adhesion when added to adherent monolayers in low calcium medium (when cadherins are non-functional), but did not prevent cell-cell adhesion or cell stratification when the concentration of calcium was raised to that found in standard medium. These observations were confirmed by Tenchini et al. (1993) who further suggested that the effects observed in low calcium could be an indirect consequence of perturbation of adhesion to the extracellular matrix deposited by kératinocytes. Jensen and Wheelock (1995) also suggested that the effect of anti-pi antibodies on inhibiting colony formation in subconfluent keratinocyte cultures was a consequence of inhibition of cell migration, and concluded that p i integrins do not play a significant role in intercellular adhesion.

1.4.3.3 Keratinocyte stratification

When basal kératinocytes become committed to terminal differentiation there is a marked reduction in their abihty to adhere to extracellular matrix and this ensures that the cell moves out of the basal layer (Stanley et al., 1980; Watt 1984; Adams and Watt,

1990). The loss of adhesiveness precedes loss of integrins from the cell surface (Adams and Watt, 1990), and the decreased suprabasal expression of integrins is mediated through transcriptional and post-translational mechanisms (Nicholson and Watt, 1991).

1.4.3.4 Keratinocyte commitment to terminal differentiation

Cultured kératinocytes always contain proliferating and terminally

differentiating cells making study of differentiation difficult. Integrin occupancy by extracellular matrix ligands suppresses terminal differentiation, and differentiation can be induced by depriving kératinocytes of contact with the culture substratum (Adams and Watt, 1989; Watt et al., 1993). One technique, which induces the majority of cells to undergo terminal differentiation, is suspension culture (Watt et al., 1988; Adams and Watt, 1989). In this system, induction of differentiation can be blocked by adding fibronectin, or antibodies against the p i subunit to the cell suspension (Adams and Watt, 1989; Nicholson and Watt, 1991; Watt et al., 1993). When kératinocytes are placed on an adhesive substrate that restricts spreading, terminal differentiation is stimulated (Watt et al., 1988). Symington and Carter (1995) found that blocking a 3 p i induced terminal differentiation. A cocktail of function blocking antibodies against (x2, a3 and o5 has been shown to inhibit differentiation, as has ligand binding of laminin and type IV collagen (Watt et al., 1993). Interestingly, Jones et al. (1996) demonstrated that transfection of av into an av deficient neoplastic keratinocyte cell line increased the capacity for terminal differentiation.

1.4.3.5 Keratinocyte proliferation

Keratinocyte stem cells expresses higher levels of p i integrins than transit amplifying cells and adhere to fibronectin, type IV collagen or keratinocyte extracellular proteins more rapidly (Jones and Watt, 1993).

1.4.3.6 Keratinocyte migration

Kératinocytes freshly isolated from skin are relatively immobile and their ability to migrate develops with time in culture (Guo et a l, 1990). avpS, avp6 and a 5 p l are weakly expressed or not detectable on kératinocytes in vivo but all three are strongly expressed by kératinocytes in culture where they have been shown to have a role in cell migration ( Toda et al., 1987; Adams and Watt, 1991; Kim et al., 1994; Haapasalmi et al., 1996; Huang et al., 1998). The principal ligands for these integrins are fibronectin and vitronectin, proteins which have been shown to be important for cell migration in other cell types, and which are found in pathological and physiological situations where cells are migrating (Liotta et al., 1986a; Tomasini and Mosher, 1991; Morla et al., 1994; Pujuguet et al., 1996). avpS has been shown to modulate migration of kératinocytes on vitronectin (Brown et al., 1991; Kim et al., 1994). Brown et al. (1991) observed that this increased motility was not directed, linear movement, but appeared random. Keratinocyte migration on fibronectin has been shown to be modulated through a 5 p l and avP6 (Kim et al., 1992a; Lange et al., 1995; Huang et al., 1998). Huang et al. (1998) demonstrated that avp6-mediated keratinocyte migration on fibronectin was dramatically augmented by protein kinase C activators, and also found avp6 was critical for keratinocyte migration on vitronectin.

Zhang and Kramer (1996) showed that laminin 5 deposition promoted keratinocyte motility and that this migration could be blocked with anti-a3pl antibodies. A role for aS pi in keratinocyte migration has also been suggested by

Tenchini et al. (1993) and Jensen and Wheelock (1995). Conversely, Kim et al. (1992b) demonstrated that antibodies directed against a 3 p i enhanced keratinocyte migration on fibronectin and collagen matrices. Migration on collagen matrices has been shown to be modulated through a 2 p i (Chen et al., 1993).