Overexpression of YAP1 induces immortalization of normal human keratinocytes by blocking clonal evolution.

YAP1 is a transcriptional co-activator able to bind several transcription factors. YAP1 was termed a candidate oncogene after it was shown to be in human chromosome 11q22 amplicon; besides the genomic amplification, several experiments indicated that it has oncogenic function. However, YAP1 was also reported to be a tumor suppressor as its gene locus is deleted in some breast cancers. To clarify the role of this protein in the physiology of rapidly renewal cells, we investigated YAP1 in human keratinocytes. Here, we show that YAP1 overexpression in primary human keratinocytes blocks clonal evolution and induces cell immortalization, but not malignant transformation. YAP1 overexpression led to an increase in cell proliferation, colony forming efficiency and holoclone percentage. Cells escaped from senescence, immortalized but still remained unable to grow in soft agar or express mesenchymal markers, suggesting that YAP1 overexpression is not sufficient to promote a complete epithelial-mesenchymal transition and tumorigenic transformation. Protein analysis showed an increase in epithelial proliferation markers and a decrease in epithelial differentiation markers. The expression of LEKTI, a late differentiation marker, dramatically dropped to undetectable levels. Taken together, these data suggest that YAP1-overexpressing keratinocytes are maintained in the proliferative compartment.

Telomerase activity is sufficient to bypass replicative senescence in human limbal and conjunctival but not corneal keratinocytes.

The human ocular surface is covered by the conjunctival, corneal and limbal stratified epithelia. While conjunctival stem cells are distributed in bulbar and forniceal conjunctiva, corneal stem cells are segregated in the basal layer of the limbus, which is the transitional zone between the cornea and the bulbar conjunctiva. Keratinocyte stem and transient amplifying (TA) cells when isolated in culture give rise to holoclones and paraclones, respectively. Keratinocyte replicative senescence ensues when all holoclones have generated paraclones which express high levels of p16(INK4a). In the present study, we show that enforced telomerase activity induces the bypass of replicative senescence in limbal and conjunctival keratinocytes, without the inactivation of the p16(INK4a)/Rb pathway or the abrogation of p53 expression. hTERT-transduced limbal and conjunctival keratinocytes are capable to respond to both growth inhibitory and differentiation stimuli, since they undergo growth arrest in response to phorbol esters, and activate p53 upon DNA damage. Following a sustained PKC stimulation, occasional clones of p16(INK4a)-negative cells emerge and resume ability to proliferate. Telomerase activity, however, is unable to induce the bypass of senescence in corneal TA keratinocytes cultured under the same conditions. These data support the notion that telomere-dependent replicative senescence is a general property of all human somatic cells, including keratinocytes, and suggest that telomerase activity is sufficient to extend the lifespan only of keratinocytes endowed with high proliferative potentials (which include stem cells), but not of TA keratinocytes.

Erbium:YAG laser and cultured epidermis in the surgical therapy of stable vitiligo.

OBJECTIVE: To induce complete and reproducible repigmentation of large "stable" vitiligo lesions by means of autologous cultured epidermal grafts using a rapid, simple, and minimally invasive surgical procedure. DESIGN: Achromic epidermis was removed by means of appropriately settled erbium:YAG laser, and autologous epidermal grafts were applied onto the recipient bed. Melanocyte content was evaluated by dopa reaction. The percentage of repigmentation was calculated using a semiautomatic image analysis system. SETTING: A biosafety level 3-type cell culture facility, a surgical ambulatory department, and a dermatological department in a hospital. PATIENTS: Twenty-one patients with different types of vitiligo were admitted to the study and treated with autologous cultured epidermal grafts. Inclusion criteria were failure of at least 2 standard medical approaches; no therapy for at least 12 months; no progression of old lesions or appearance of new lesions; no Koebner phenomenon within the past 18 months; and no autoimmune disorders. RESULTS: The average percentage of repigmentation in 21 patients was 75.9% (1759.7 cm2 repigmented/2315.8 cm2 transplanted). Three patients showed a reactivation of their vitiligo and did not show repigmentation. The remaining 18 patients, with 43 distinct lesions, showed an average percentage of repigmentation of 90% (1759.7 cm2 repigmented/1953.4 cm2 transplanted). CONCLUSIONS: Under appropriate conditions, cultured epidermal grafts induce complete repigmentation of stable vitiligo lesions. Erbium:YAG laser surgery can supply a fast and precise tool for disepithelialization, hence allowing treatment of large vitiligo lesions during a single surgical operation.