UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II. Scuola di Dottorato in Medicina Molecolare - PDF

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UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II Scuola di Dottorato in Medicina Molecolare Dottorato di Ricerca in Genetica e Medicina Molecolare Analysis of the p63 function in cell proliferation and differentiation

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UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II Scuola di Dottorato in Medicina Molecolare Dottorato di Ricerca in Genetica e Medicina Molecolare Analysis of the p63 function in cell proliferation and differentiation through the study of the mechanisms regulating p63 protein stability and transcriptional activity. Coordinatore: Prof. Carmelo Bruno Bruni Candidato: Dott. Antonella Di Costanzo Anno UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II Dip. Biologia Strutturale e Funzionale, Università degli Studi di Napoli Federico II Complesso Universitario Monte S. Angelo Dottorato di Ricerca in Genetica e Medicina Molecolare Coordinatore Prof. Carmelo Bruno Bruni Sede amministrativa: Dipartimento di Biologia e Patologia Cellulare e Molecolare Luigi Califano 2 UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II Dip. Biologia Strutturale e Funzionale, Università degli Studi di Napoli Federico II Complesso Universitario Monte S. Angelo Tesi di Dottorato di Ricerca in Genetica e Medicina Molecolare XX ciclo Analysis of the p63 function in cell proliferation and differentiation through the study of the mechanisms regulating p63 protein stability and transcriptional activity. Candidata: Antonella Di Costanzo Docente guida: Girolama La Mantia 3 Index Introduction p53 family members pag.6 p63 knock-out and transgenic mouse models: toward a comprehension of p63 physiological function. pag.11 P63 human associated disorders P63 protein regulation pag.22 pag.28 Preliminary data and aim of the thesis Results pag.30 pag.35 1) DIFFERENTIAL GENE EXPRESSION ANALYSIS TO IDENTIFY P63 SPECIFIC TRANSCRIPTIONAL TARGETS 1.1) Production and characterization of TAp63α, TAp63αQ540L and Np63α stable cell lines. pag ) Subcellular localization of mutant TAp63αQ540L pag ) Functional analysis of stable clones expressing TAp63α, Np63α and TAp63αQ540L pag ) Microarray analysis pag ) TAp63αQ540L binds to p21 promoter sequences in vitro and interacts with Sp1 in vivo. pag ) TAp63αQ540L does not bind to p21 promoter sequences in vivo pag.53 4 2) INVESTIGATION ON P63 PROTEIN DEGRADATION Dlx3-MEDIATED. 2.1) Dlx3 downregulates p63 protein level. pag ) Dlx3-induced Np63 degradation requires specific Serine and Threonine residues located in the p63 α and β carboxyterminal tails. pag ) Dlx3-mediated p63 degradation is impaired by inhibiting Raf signaling pathway. pag.66 3) Investigation on mechanisms throught which p14arf regulates p63 transcriptional activity. pag.73 Discussion pag.78 Matherials and Methods pag.92 References pag.102 5 Introduction 6 p53 family membersat the end of the last century, the discovery of p53 protein homologues named p63 and p73, engendered the concept of a new family of p53-like transcription factors. The research focus expanded from a single protein named the guardian of the genome to a family of transcription factors that likely have distinct roles through the diverse collection of genes they regulate. p53 was discovered 25 years ago as a protein interacting with the oncogenic T antigen from SV40 virus. P53 transcription factor is the product of a pivotal tumor-suppressor gene, whose inactivaction is the most frequent single gene event in human cancer, and germline mutation in human p53 gene are cause of enhanced risk of developing cancer ( Li- Fraumeni syndrome). The p53 gene encodes a protein with a central DNA binding domain, flanked by an N-terminal transactivation domain, and a C-terminal tetramerization domain ( Levine et al;1997). The active form of p53 is a tetramer and, consistent with its tetrameric state, p53 binds DNA sites that contain four repeats of the pentamer sequence motif 5 -Pu-Pu-Pu-C-A/T-3. Until now, the p53 gene structure was considerate much simpler, with only one promoter and transcribing three mrna splicing variants encoding, respectively, full-length p53, p53i9 ( Flaman et al;1996), and 40p53 ( Courtois et al.2002; Ghosh et al.2004). Recent studies have proposed for the p53 gene a structure more complex than has been previously thought. The p53 gene, as the other two family members p63 and p73, contains two promoters and can generate six different mrnas, that encode at least six p53 isoforms ( Bourdon et al. 2005). P53 isoforms are expressed in several normal human tissue. The functions of p53 are primarily the regulation of cell cycle 7 checkpoints, apoptosis, and genome stability. A substantial number of genes, involved in cell cycle arrest or in induction of apoptosis, are activated by p53. These include MDM2, p21waf, GADD45, bax and IGF-BP3. p53 has also been reported to negatively regulate the transcription of a number of genes such as presenilin 1, topoisomerase IIα, bcl2 and hsp70. The transcriptional activity and stability of p53 are highly regulated by posttranslational mechanisms involving protein-protein interaction, phosphorylation, acetylation, ubiquitination, and sumoylation. MDM2 is an ubiquitin ligase and a p53 transcriptional target; it binds to the p53 transactivation domain, and inhibits its transcriptional activity. MDM2 shuttles p53 out of the nucleus targeting the protein for ubiquitin-mediated proteolysis ( Vogelstein et al. 2000). MDM2, thus, is assumed to be the principale regulatorof p53 protein levels. The p14arf tumor suppressor protein, one of the alternative products of the INK4A locus, antagonizes MDM2 activity leading to p53 stabilization. Several mechanisms have been postulated to inactivate p53 such as amplification of MDM2 gene, deletion of ARF gene, expression of some viral oncogenes that stimulates p53 degradation or missense mutation in DNA binding domain that disrupt the DNA binding capability of p53. P63 and p73 are genes structurally related to p53. In fact, also p63 and p73 proteins contain an N-terminal transactivation domain, which shares 25% homology with N- terminal part of p53, the DNA binding domain, which shares 65% of homology with the corresponding p53 domain, and the tetramerization domain, which shares 35% of homology with the oligomerization domain of p53. The p63 and p73 genes are 8 transcribed from two distinct promoters, giving rise to proteins that either contain ( TA isoforms) or lack ( N isoforms) the N-terminal transactivating domain. In addition, both p63 and p73 genes, are subject to alternative splicing event that generate three ( α, β, γ ) and seven ( α, β, γ, δ, ε, ζ,η ) different C-termini respectively for p63 and p73 encoded proteins ( Yang and McKeon; 2002). The α isoforms contain a sterile α motif ( SAM ) and a transactivation inhibitory domain ( TID ). The SAM domain, which is absent in p53, is a protein-protein interaction domain also found in other developmentally important protein, such as several Eph receptor tyrosine kinase ( Schultz et al; 1997 ). TA-p63 N-p63 γ i α β TAp63α TA DBD OD SA TI TAp63β TA DBD OD TAp63γ TA DBD OD Np63α DBD OD SA TI Np63β DBD OD Np63γ DBD OD p53 TA DBD 25% 65% OD 35% Structure of both TP63 and major protein isotypes. TP63 uses several transcription initiation sites and extensive alternative splicing, to generate different mrna. Several protein domains can be distinguished; of these, the TA domains, the DBD, and the OD domain are highly homologous to the corresponding domain in p53. The SAM domain and the TID are not contained in the p53 protein. 9 The Transactivation inhibitory domain of p63 binds to the N-terminal TA domain masking residues that are important for transactivation ( Serber et al; 2002 ). In fact, p63 isoforms that contain the γ and β C-termini are associated with higher transactivation competency that ones with α terminus ( Shimada et al; 1999 ). The lack of TA domain in Np63 isoforms suggest that they are transcriptionally competent. Since Np63 isoforms retain the oligomerization and DNA binding domains, it is plausible that they act as dominant negative inhibitorsof p53 and TAcontaining p53 family members ( Yang et al;1998. Westfall et al; 2003). Indeed, numerous studies show that co-expression of Np63 with either TAp63, TAp73, or p53 has inhibitory effect on TAp63-mediated transcription. A plausible mechanism is the formation of transcriptionally inactive N-TA heterotypic or homotypic tetramers (composed of either all-ta or all- N monomers) that compete for the same DNA binding sites. Despite the well-documented role of Np63 as a dominant negative transcriptional repressor, several studies have shown that Np63 isoforms directly transactivated a set of genes including Hsp70 and p57kip ( Beretta et al; 2005; Wu et al; 2005; King et al; 2003 ). This is possible thanks to existence of two cryptic transactivation domains in Np63 isoforms: a region encompassing the first 26 N-terminal amino acids named TA2 domain and a prolin rich sequence corresponding to exon 11/12 present in p63 β and α isoforms ( Ghioni et al; 2002 ). Surveillance of cellular integrity might be achieved throught a network of these p53- like tumor suppressors. This speculation was, further, fueled by the observation that the p73 gene is localized to chromosome 1p36.3, a region that is frequently lost in 10 neuroblastomas and in other types of cancers ( Kaghad et al; 1997; Takahashi et al; 1998 ), while the p63 chromosomal location, 3q27-29, is deleted in some bladder cancers and amplified in some cervical, ovarian, lung, and squamous cell carcinoma ( Yang et al; 1998 ) where Np63 was the predominant isoform expressed at protein level ( Cui R et al; 2005). Moreover, both p73 and p63 can bind to p53 DNA-binding sites and activate transcription of genes that mediates cell cycle arrest or apoptosis in vivo. Despite all this circumstantial evidence, however, only a very few mutation in p63 and in p73 have been found in human tumors, and a direct link to carcinogenesis similar to that for p53 has so far not established ( Ichimiya et al; 2001; Nomoto et al; 1998 ). Moreover the analysis of p63 and p73 deficient mice and studies on human tumors have often led to conflicting results with regards to its role in tumorigenesis ( Westfall et al; 2004; Flores et al; 2005; Keyes et al; 2006). 11 p63 knock-out and transgenic mouse models: toward a comprehension of p63 physiological function. Target gene disruption studies in mice have established an important role for p63 in development and differentiation. Mice lacking p63 are born alive but have striking developmental defects. Their limbs are absent or truncated, defects that are caused by a failure of the apical ectodermal ridge ( AER ) to differentiate. p63-/- mice on postnatal day have hypoplastic upper and lower jaws, and have no eyelids, whisker pads, skin and related appendages, including vibrissae, pelage follicles and hair shaft. Homozygous mutants lack distal components of the forelimb, including the radius, carpals and digits, as well as all components of the hindlimb., The lack of a proper AER in p63 -/- limb buds results from a failure of the ectoderm to undergo growth and differentiation that give rise to this stratified epithelium ( Mills et al; 1999 ). The skin of the p63 deficient mice does not progress past an early developmental stage: it lacks stratification and does not express differentiation markers. 12 a b a)defects in stratified epithelial differentiation in p63-deficient mice. p63-/- mice lacking squamous stratification in the epidermis (top) and tongue epithelium (bottom).b) Immunohistochemical staining with the 4A4 anti-p63 antibody, showing p63 protein expression in the apical ectodermal ridge. The AER is absent in the p63-/-. At birth, p63-deficient mice have striking and visible skin defects, in fact, they die within a day of birth from dehydration. Structures dependent upon epidermalmesenchymal interactions during embryonic development, such as hair follicles, teeth and mammary glands, are also absent in p63 deficient mice. The surface of the skin is covered by a single layer of flattened cells, without the spinosum, granulosum and stratum corneum. Two contrasting models have been advanced to explain the absence of stratified epithelia ( McKeon et al; 2004). One model posits that p63 is required for simple epithelial cells to commit to a stratified epithelial lineage during development ( Mills et al; 1999). The second model argues that the primary defect resides not in the acquisition of stratified epithelial cell fate but rather in an inability of epidermal stem cells to sustain epidermal self-renewal ( Yang et al; 1999). 13 Model for p63 in maintaining the proliferative capacity of epithelial progenitor cells. Stem cells in the basal layer of stratified squamous epithelia express high levels of p63 and undergo asymmetric division to enable both selfrenewal and progression to transient amplifying cells (TACs). TACs, which may express less p63, are also capable of limited proliferation and self-renewal, but are ultimately destined for terminal differentiation. The absence of p63 results in the failure to maintain a basal cell population, suggesting a requirement for p63 in the regenerative aspect of stem cell division. The finding that p63 is specifically expressed in epithelial cells that have adopted an epidermal fate suggested that p63 is involved in development of the embryonic basal layer, the first layer of embryonic epidermis. The epidermis is an example of stratified epithelium. It functions as a barrier protecting the organism from dehydration, mechanical trauma, and microbial insults. This barrier function is established during embryogenesis through a complex and tightly controlled stratification program. The epidermis, the outermost component of the skin, is the primary barrier that protects the body from dehydration, mechanical trauma, and microbial insults. The epidermis is separated from the underlying dermis by the basement membrane, which consists of proteins secreted by epidermal keratinocytes and by dermal fibroblasts (McMillan et al. 2003). The two compartments of the skin, the dermis and the epidermis, function cooperatively and 14 together are responsible for the development of epidermal appendages, including hair follicles and mammary glands (Chuong et al; 1998). Therefore, a failure to properly develop either the dermis or the epidermis may result in defects in appendage development. This is, for example, illustrated by ectodermal dysplasias, in which primary defects in epidermal development are the cause of subsequent defects in epidermal appendages (Koster and Roop; 2004; Priolo et al; 2000). The barrier function of the epidermis is established during embryogenesis and is the result of a complex and precisely coordinated stratification program. In mice, the execution of this program occurs in a period of approximately 10 days, between E8.5 and E18.5, and initiates when cells of the surface ectoderm commit to an epidermal fate. (a) Schematic illustrating epidermal morphogenesis. During epidermal morphogenesis, the single-layered surface ectoderm that initially covers the developing embryo initiates a stratification program culminating in the formation of the epidermal barrier. (b) In wild-type mice, epidermal stratification and barrier formation are completed by birth. The surface ectoderm of mice lacking the transcription factor p63 fails to adopt an epidermal fate, and therefore stratification and barrier formation do not occur. As a consequence, mice lacking p63 are born with a single layer of ectodermal cells covering their bodies and die shortly after birth due to dehydration. (c) Images of p63 / and wild-type mice. 15 After this initial commitment step, keratinocytes in the newly established embryonic basal layer give rise to a second layer of cells, the periderm (M Boneko and Merker; 1988). The periderm is shed before birth in conjunction with the acquisition of epidermal barrier function (Hardman et al. 1998). The next layer of the epidermis to form is the intermediate cell layer, which develops between the basal layer and the periderm (Smart et al; 1970). Development of this layer is associated with asymmetric cell division of embryonic basal keratinocytes (Lechler and Fuchs; 2005, Smart et al; 1970). Like basal keratinocytes, intermediate cells undergo proliferation, and the loss of this proliferative capacity is associated with the maturation of intermediate cells into spinous cells (Koster et al. 2007, Smart 1970). Spinous cells subsequently undergo further maturation into granular and cornified cells. The morphological changes that are a hallmark of epidermal stratification are associated with changes in the expression of keratin differentiation markers (Koster and Roop; 2004). Primary p63 / surface epithelial cells are blocked in their commitment to a stratified epithelial lineage. Differentiation markers K5 and K14, which are expressed in epithelia that have committed to a stratification program, are not expressed in primary p63 / cells. These cells do, however, express K18, a marker for singlelayered epithelia 16 For example, whereas the uncommitted surface ectoderm expresses keratins K8 and K18 (Moll et al. 1982), K5 and K14 are induced as these cells commit to an epidermal fate (Byrne et al. 1994). Subsequently, the initiation of terminal differentiation results in the induction of K1 and K10 expression in the newly formed suprabasal keratinocytes (Bickenbach et al. 1995, Fuchs and Green; 1980). The final step in epidermal stratification involves the formation of the epidermal barrier. During normal development, barrier acquisition is patterned and initiates at the dorsal surface, spreading laterally to the ventral surface in approximately one day (Hardman et al; 1998). The process of barrier formation is characterized by the formation of cornified cell envelopes, composed of proteins crosslinked into a rigid scaffold and of lipids covalently attached to the exterior surface (Rice and Green; 1977, Steven and Steinert; 1994). Although established in utero, the barrier function of the epidermis is maintained during postnatal life owing to the continuous selfrenewal of the epidermis, a process mediated by epidermal stem cells (Blanpain and Fuchs; 2006). Under homeostatic conditions, the epidermal stem cells that are located in the interfollicular epidermis are responsible for the maintenance of this structure (Ito et al. 2005). These stem cells represent a small proportion of basal keratinocytes and, through asymmetric cell division, give rise to a daughter stem cell and a transit-amplifying cell, which ultimately undergoes terminal differentiation (Dunnwald et al. 2003, Potten andmorris; 1988, Schneider et al. 2003). 17 Attempts to establish unequivocally the role of p63 in epithelial development are complicated by the fact that this protein exists in multiple isoforms with different, often contradictory, biological activities. Koster and Roop reported that TAp63 was expressed earlier than Np63 ( Koster et al; 2004 ) during epidermal development. Thus, TAp63, but not Np63α, was proposed to be required for the initiation of epidermal stratification. TAp63 was reputed to be the molecular switch responsible for epithelial stratification while Np63 was believed to counteract the TAp63 isoform allowing keratinocytes terminal differentiation( Koster et al; 2006 ). Accordingly, studies on transgenic mice, demonstrate that upregulated TAp63α expression resulted in skin hyperplasia and a failure of keratinocytes to properly differentiate (Candi et al; 2006 ). These data are in contrast with other works where Np63 was reported to be the only isoform expressed in epidermal development, until E13 embryonal stage ( Mikkola et al; Laurikkala et al; 2006 ), and that Np63 is expressed in the basal undifferentiated layer of the skin, in particular in the stem cell compartment, and was rapidly degraded when keratinocytes are induced to differentiate ( Yang et al; Pellegrini et al; Rossi et al; 2006 ). 18 The down-regulation of Np63 in suprabasal keratinocytes was recently shown to be mediated, at
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