Cell Adhesion, Migration and Cancer

Résumé

The skin epidermis is an excellent example for exploring homeostasis and injury repair in a stratified epithelium. The epidermis maintains a single inner (basal) layer of proliferative cells that adhere to an underlying basement membrane rich in extracellular matrix (ECM) and growth factors (Fuchs 2007). Basal cells express a number of characteristic markers including keratins and transcription factors. Periodically, these cells withdraw from the cell cycle, commit to differentiate terminally, move outward and are eventually shed from the skin surface. This architecture allows the epidermis to generate a self-perpetuating barrier that keeps harmful microbes out and essential body fluids in.

Upon commitment to terminally differentiate, an epidermal keratinocyte progresses through three distinct differentiation stages: spinous, granular and stratum corneum. Major changes in transcription, morphology and function occur at the basal/spinous layer transition and again at the granular/stratum corneum transition, such that differentiated cells reaching the skin surface are enucleated, cellular skeletons that are packed with cables of keratin filaments encased by an indestructible envelope of proteins. An additional final step in the differentiation process is the extrusion of a lipid bilayer that seals and protects the body surface from dehydration and harmful microbes. The process is in a continual homeostasis, so that surface cells are continually sloughed and replaced by inner cells differentiating and moving outward. In human epidermis, the self-renewing capacity of epidermal stem cells is enormous, and within 4 weeks, a basal cell has terminally differentiated and exited at the skin surface. In mice, the epidermis becomes thinner and proliferation slows substantially as the hair coat develops.

To coordinate epidermal homeostasis and wound-repair and to maintain a single layer of dividing cells and multiple layers of differentiating cells, the epidermis displays an elaborate cytoskeletal architecture. Ten nanometer wide intermediate filaments composed of keratin proteins are the major cytoskeletal component of the epidermis and its appendages. Dividing cells express keratins 5 and 14, while differentiating epidermal cells express keratins 1 and 10 (Fuchs and Green 1980). The basic subunit structure of the keratin filament is an obligatory heterodimer of type I and type II keratins (Fuchs et al. 1981; Hanukoglu and Fuchs 1982; Hanukoglu and Fuchs 1983; Coulombe and Fuchs 1990)). The function of these keratins is to impart mechanical integrity to the epidermis, without which the cells become fragile and prone to rupturing upon physical stress (Albers and Fuchs 1987; Albers and Fuchs 1989; Coulombe et al. 1990; Vassar et al. 1991; Letai et al. 1992)). First discovered in mice and then in humans, the blistering skin disease epidermolysis bullosa simplex (EBS) is a genetic disorder of keratins 14 and 5 (Bonifas et al. 1991; Coulombe et al. 1991; Lane et al. 1992; Fuchs and Weber 1994), and the blistering disorder epidermolytic hyperkeratosis is a genetic disorder of keratins 1 and 10 (Cheng et al. 1992; Chipev et al. 1992; Rothnagel et al. 1992). There are now more than 20 different IF disorders in humans, and many of these set the paradigm first discovered for EBS (Fuchs and Cleveland 1998; Omary et al. 2004).

To form a cytoskeleton, keratin filaments associate with α6β4 integrin-rich hemidesmosomes at the base of the basal epidermal cell, and desmosomal-cadherin-rich desmosomes to make cell-cell junctions. When these IF-adhesive connections are defective, mechanical fragility and degenerative disorders also occur. By contrast, the actin cytoskeleton associates with α3β1 integrin-rich focal adhesions and to E-cadherin rich adherens junctions (Perez-Moreno et al. 2006). We’ve used gene targeting to conditionally mutate the genes encoding E-cadherin, α-catenin and p120-catenin from the skin epidermis (Vasioukhin et al. 2000; Vasioukhin et al. 2001; Tinkle et al. 2004; Kobielak and Fuchs 2006; Perez-Moreno et al. 2006). Intriguingly, mutations in all of these genes render the skin epithelium prone to squamous cell carcinomas and/or proinflammatory responses. We’ve shown that α-catenin is particularly important not only for coordinating adhesion-actin dynamics but also proliferation, invasion, and inflammation (Vaezi et al. 2002; Jamora et al. 2003; Kobielak and Fuchs 2006).

Interestingly, it is also required for proper spindle orientation in the epidermis, a process which requires actin-microtubule polarization. During embryonic development as the epidermis transits from a single layer to a stratified layered epithelium, spindle orientation changes to become asymmetric relative to the basement membrane. Mechanistically, the process appears to involve many of the proteins used by fly neuroblasts in as ymmetric divisions that generate neurons (Lechler and Fuchs 2005).

In addition to requiring α-catenin, the process of asymmetric divisions in the epidermis also relies upon β1 integrin (Lechler and Fuchs 2005). We’ve targeted β1 and its downstream tyrosine kinase effector, focal adhesion kinase (FAK) for ablation in the skin (Raghavan et al. 2000; Raghavan et al. 2003; Schober et al. 2007). Without β1 integrin, basement membrane assembly is defective and hair follicles cannot invaginate. Epidermal cells also fail to activate FAK and focal adhesion turnover, necessary for efficient cell migration, is impaired (Schober et al. 2007). Without FAK, the skin epidermis becomes more resistant to tumorigenesis (McLean et al. 2004; Schober et al. 2007). Interestingly, TGFβ signaling may also be linked to focal adhesions, as without TGFβ signaling, the skin is more susceptible to tumorigenesis and FAK is upregulated (Guasch et al. 2007).

In summary, in the nearly three decades of skin biology research conducted by my laboratory, an understanding is beginning to emerge of how multipotent stem cells receive external signals to change their programs of transcription and gene expression, remodel their cytoskeletal-adhesive contacts and generate tissues. In the case of skin, one of the remarkable features of these multipotent stem cells is their ability to generate the epidermis, hair follicle and sebaceous gland, three fascinating and strikingly distinct tissue structures. With the future promise of the skin as a possible source for generating embryonic stem cells, the skin may well not only prove to be our largest organ and our largest immune system of the body, but also our most important source of material for the future of regenerative medicine.