MicroRNAs and p63 in epithelial stemness

Cell Death and Differentiation (2015) 22, 12–21 OPEN & 2015 Macmillan Publishers Limited All rights reserved 1350-9047/15 www.nature.com/cdd Review MicroRNAs and p63 in epithelial stemness E Candi1,2, I Amelio3, M Agostini3 and G Melino*,1,2,3 MicroRNAs (miRs) are a class of small noncoding RNAs that suppress the expression of protein-coding genes by repressing protein translation. Although the roles that miRs and the miR processing machinery have in regulating epithelial stem cell biology are not fully understood, their fundamental contributions to these processes have been demonstrated over the last few years. The p53-family member p63 is an essential transcription factor for epidermal morphogenesis and homeostasis. p63 functions as a determinant for keratinocyte cell fate and helps to regulate the balance between stemness, differentiation and senescence. An important factor that regulates p63 function is the reciprocal interaction between p63 and miRs. Some miRs control p63 expression, and p63 regulates the miR expression profile in the epidermis. p63 controls miR expression at different levels. It directly regulates the transcription of several miRs and indirectly regulates their processing by regulating the expression of the miR processing components Dicer and DGCR8. In this review, we will discuss the recent findings on the miR–p63 interaction in epidermal biology, particularly focusing on the DNp63-dependent regulation of DGCR8 recently described in the DNp63  /  mouse. We provide a unified view of the current knowledge and discuss the apparent discrepancies and perspective therapeutic opportunities. Cell Death and Differentiation (2015) 22, 12–21; doi:10.1038/cdd.2014.113; published online 29 August 2014 Facts  Epidermal morphogenesis is governed by sets of microRNAs(miRs) with different expression patterns.  Epidermal-conditional deletion of miR processing machinery components, such as DICER, Drosha and DGCR8, has underscored the importance of miRs in epidermal morphogenesis and homeostasis.  p63 is a master regulator of epidermal homeostasis; p63  /  and DNp63  /  mice fail to develop a functional epidermis.  p63 directly, via transcriptional regulation, and indirectly, by promoting the transcription of Dicer and DGCR8, controls the epidermal miR expression profile.  The complex network between miRs and p63 regulates the fate of epidermal cells. Open Questions  Where and when are epidermal/epithelial miRs precisely expressed during epidermal/epithelial morphogenesis, homeostasis and in different diseases?  How do the isoforms DNp63 and TAp63 contribute to epidermal homeostasis? And to what extent do miRs contribute to the functioning of the p63 isoforms?  Will the miR expression profiles provide innovative biomarker tools in epidermal cancers?  Will drugs targeting miR expression provide alternative therapeutic approaches in epidermal oncology? MicroRNAs (miRs) are a class of small noncoding RNA (single-stranded RNAs consisting of 19–22 nucleotides) that suppress the expression of protein-coding genes by repressing protein translation by interacting with the 30 UTR region of messenger RNAs.1 The epidermal-conditional deletion of components of the enzymatic complexes responsible for miR maturation, such as DICER, Drosha and DGCR8, has underscored the importance of miRs in the morphogenesis and adult homeostasis of the epidermis.2–5 The epidermis is the first barrier of an animal that protects against water loss and the biological and physical stress caused by the external environment. The epidermis consists of a keratinised stratified epithelium (interfollicular epidermis, IFE) that can also include appendages such as hair follicles (HFs) and sebaceous glands (SG). Together, these structures help to protect and thermoregulate the organism. The epidermis originates from a single layer of ectodermal cells during embryonic development. This single layer of ectodermal precursor cells undergoes a stratification process that forms the different layers of the differentiated cells (basal, spinous, granular and cornified layers), which all together function as a barrier. The basal layer consists of highly proliferative cells that express high levels of basal integrins, keratin 5 (K5) and keratin 14 (K14). The mature epidermis 1 Biochemistry Laboratory, IDI-IRCCS, Rome 00133, Italy; 2Department of Experimental Medicine and Surgery, University of Rome ‘Tor Vergata’, Rome 00133, Italy and Medical Research Council, Toxicology Unit, Hodgkin Building, Leicester University, Lancaster Road, P.O. Box 138, Leicester LE1 9HN, UK *Corresponding author: G Melino, Department of Experimental Medicine, University of Rome ‘Tor Vergata’, via Montpellier 1, Rome 00133, Italy. Tel: +39 6 204 27 299; Fax: +39 06 204 27 290; E-mail: Abbreviations: miR, microRNA; IFE, interfollicular epidermis; HFs, hair follicles; SG, sebaceous glands; SC, stem cell; PAK, p21-activated kinases; IPSC, induced pluripotent stem cell; MEF, mouse embryonic fibroblast; ECM, extracellular matrix; K, keratin 3 Received 24.3.14; revised 07.7.14; accepted 08.7.14; Edited by RA Knight; published online 29.8.14 MicroRNAs and p63 E Candi et al 13 Figure 1 (a) Schematic diagram of the mouse skin epithelium. Basal layer keratinocytes (violet) adhere to the underlying basement membrane (basal lamina, grey). Basal keratinocytes lose their proliferative ability when they detach from the basement membrane and migrate towards the epidermal surface (spinous layer, orange; granular layer, green; stratum corneum, dark green). An epidermal interfollicular proliferative unit (basal layer) is composed of stem cells and transiently amplifying cells, which divide a few times and then leave the basal layer to execute their differentiation programme. K5, K14, TG2 and BPAG1 are specific markers for the basal layer. Different intermediate differentiation steps occur in the spinous and granular layers. Markers for the spinous layer include K10, K1, TG1 and TG5. Markers for the granular layer – Loricrin, Filaggrin and Involucrin. The cornified layer represents the final phase of terminal differentiation. miR-205 is exclusively and specifically expressed in the basal layer. miR-24, miR-203, miR-574, miR-720 and mir-34a are pro-differentiation miRs expressed in the epidermal suprabasal compartment. (b) Schematic diagram illustrating the following different epidermal SC populations found in the skin epidermis: the IFE SCs (shown in violet), the bulge SCs (Bu in orange), the isthmus SCs (Is in green). IFE SCs specifically express K5 and K14; bulge SCs specifically express K15, K19 and Lgr5; and isthmus SCs specifically express MTS24 and Kgr6. miR-205 is expressed in IFE and bulge SCs, and miR-125b is only expressed in bulge SC. Panel b is a modified version of a previously published illustration.69 K5, cytokeratin-5; K14, cytokeratin-14; K10, TG2, transglutaminase-2; cytokeratin-10; K1, cytokeratin-1; TG1, transgl (...truncated)