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Tissue Characterization after a New Disaggregation Method for Skin Micro-Grafts Generation

Several new methods have been developed in the field of biotechnology to obtain autologous cellular suspensions during surgery, in order to provide one ...

Burns Centre and Emilia Romagna Regional Skin Bank

Membrane and actin cytoskeleton dynamics during phagocytosis can be triggered and amplified by the signal transduction of receptor tyrosine kinases. The epidermal keratinocytes appear to use the phagocytic mechanism of uptake to ingest melanosomes released by the melanocytes and play a pivotal role in the transfer process. We have previously demonstrated that the keratinocyte growth factor KGF/FGF7 promotes the melanosome uptake through activation of its receptor tyrosine kinase FGFR2b/KGFR. The aim of the present study was to investigate the contribution of KGFR expression, activation, and signaling in regulating the phagocytic process and the melanosome transfer. Phagocytosis was analyzed in vitro using fluorescent latex beads on human keratinocytes induced to differentiate. Melanosome transfer was investigated in keratinocyte-melanocyte cocultures. KGFR depletion by small interfering RNA microinjection and overexpression by transfection of wild type or defective mutant KGFR were performed to demonstrate the direct effect of the receptor on phagocytosis and melanosome transfer. Colocalization of the phagocytosed beads with the internalized receptors in phagolysosomes was analyzed by optical sectioning and 3-dimensional reconstruction. KGFR ligands triggered phagocytosis and melanosome transfer in differentiated keratinocytes, and receptor kinase activity and signaling were required for these effects, suggesting that FGFR2b/KGFR expression/activity and PLCγ signaling pathway play crucial roles in phagocytosis.

Expression and signaling of the tyrosine kinase FGFR2b/KGFR regulates phagocytosis and melanosome uptake in human keratinocytes.

The FGFRs trigger divergent responses, such as proliferation and differentiation, and the cell type as well as the context-dependent signaling are crucial for the functional outcome. The FGFR2b/KGFR is expressed exclusively on epithelial cells and plays a key role in skin homeostasis. Here we analyzed in vitro the role of KGFR in the early differentiation of keratinocytes modulating its expression by KGFR cDNA transient transfection or KGFR siRNA microinjection and inducing a synchronous wave of differentiation in pre-confluent cells. Immunofluorescence, biochemical and molecular approaches demonstrated that KGFR overexpression increased the early differentiation marker keratin 1 at both transcriptional and translational levels, while receptor depletion reduced it. Ligand-dependent receptor activation and signaling were required for this differentiative effect. Overexpression of kinase negative KGFR mutant or Tyr769 KGFR signaling mutant, which is not able to recruit and activate PLC-γ, showed that the receptor kinase activity, but not its PLCγ-mediated signaling, is required for differentiation. Reduction of K1 expression, obtained by AKT inhibition, demonstrated that the PI3K/Akt signaling pathway is involved in the control of KGFR-mediated keratinocyte differentiation. This in vitro experimental model indicates that FGFR2b/KGFR expression represents a key event regulating keratinocyte early differentiation during the switch from undifferentiated to differentiating cells.

The receptor tyrosine kinase FGFR2b/KGFR controls early differentiation of human keratinocytes.

The E5 oncogenic protein of the human papillomavirus type 16 (HPV16 E5) cooperates in epithelial transformation perturbing the behaviour of differentiating suprabasal cells. Among the receptor tyrosine kinases deregulated by 16E5 expression, the key paracrine mediator of epithelial homeostasis keratinocyte growth factor receptor (KGFR/FGFR2b) is altered in its signaling and endocytic traffic in undifferentiated keratinocytes expressing 16E5 and it would represent a major target of the viral protein in differentiated cells. With the aim to specifically address the possible interplay of 16E5 with KGFR/FGFR2b in cells already committed to differentiation, we took advantage of an in vitro model for forced overexpression or depletion of KGFR in E5 expressing human keratinocytes under synchronous waves of differentiation. Quantitative RT-PCR, biochemical and immunofluorescence analysis showed that KGFR down-modulation is responsible for a E5-mediated decrease of the early differentiation marker K1 and that the receptor re-expression as well as triggering of its kinase activity and signaling are able to efficiently counteract the impairment of differentiation, providing a further demonstration of the tumor-suppressive role of KGFR in the new unexplored context of HPV16 E5-mediated carcinogenesis. In addition, KGFR induced a ligand-dependent decrease of p63 through a miR-203 independent mechanism and this effect was blocked by inhibition of the PI3K/Akt signaling, which is the main pathway involved in KGFR-dependent keratinocyte differentiation, suggesting that alterations of the KGFR/p63 crosstalk are responsible for the impairment of keratinocyte differentiation induced by 16E5 and that the opposite tumor-suppressive action of KGFR and oncogenic role of E5 might both involve p63.

HPV16 E5 and KGFR/FGFR2b interplay in differentiating epithelial cells.

Autophagy is a degradative pathway through which cells overcome stressful conditions and rapidly change their phenotype during differentiation. Despite its protective role, when exacerbated, autophagy may lead to cell death. Several growth factors involved in cell survival and in preventing differentiation are able to inhibit autophagy. Here we investigated the autophagic role of FGF7/KGF, an important player in epithelial cell protection and differentiation. Biochemical and quantitative fluorescence approaches showed that FGF7 and its signaling induce autophagy in human keratinocytes and the use of specific inhibitors indicated that this effect is independent of the PI3K-AKT-MTOR pathway. The selective block of autophagosome-to-lysosome fusion clarified that FGF7 induces autophagy stimulating autophagosome formation. However, quantitative fluorescence approaches also indicated that, upon a prolonged autophagic stimulus, FGF7 is able to accelerate autophagosome turnover. Moreover, in differentiating keratinocytes, the use of the autophagic inhibitor 3-MA as well as the depletion of BECN1 and ATG5, 2 essential regulators of the process, counteracted the FGF7-induced increase of the differentiation marker KRT1/K1, suggesting that autophagy is required for the FGF7-mediated early differentiation. These results provide the first evidence of a role of FGF7 in the regulation of sequential steps of the autophagic process and strengthen the hypothesis of a direct interplay between autophagy and differentiation. On the other hand, the ability of FGF7 to accelerate autophagosome turnover, preventing their dangerous accumulation, is consistent with the well-established protective role played by the growth factor in epithelial cells.

Valeria Purpura

Burns Centre and Emilia Romagna Regional Skin Bank

Tissue Characterization after a New Disaggregation Method for Skin Micro-Grafts Generation