Abstract
BackgroundThe epithelial isoform of the fibroblast growth factor receptor 2 (FGFR2b) controls the entire program of keratinocyte differentiation via the sequential involvement of protein kinase C (PKC) δ and PKCα. In contrast, the FGFR2 isoform switch and the aberrant expression of the mesenchymal FGFR2c isoform leads to impairment of differentiation, epithelial-mesenchymal transition (EMT) and tumorigenic features. Aim of our present study was to contribute in clarifying the complex network of signaling pathways involved in the FGFR2c-mediated oncogenic outcomes focusing on PKCε, which appears to be involved in the induction of EMT and tumorigenesis in several epithelial contexts.MethodsBiochemical and molecular analysis, as well as in vitro invasion assays, combined with the use of specific small interfering RNA (siRNA), were performed in human keratinocytes stably expressing FGFR2c or FGFR2b isoforms.ResultsOur results showed that aberrant expression and signaling of FGFR2c, but not those of FGFR2b, in human keratinocytes induced a strong phosphorylation/activation of PKCε. The use of siRNA approach showed that PKCε is the hub signaling downstream FGFR2c responsible for the modulation of EMT markers and for the induction of the EMT-related transcription factors STAT3, Snail1 and FRA1, as well as for the acquisition of the invasive behavior. Moreover, experiments of depletion of ESRP1, responsible for FGFR2 splicing in epithelial cells, indicated that the activation of PKCε is the key molecular event triggered by FGFR2 isoform switch and underlying EMT induction.ConclusionsOverall, our results point to the identification of the downstream PKC isoform responsible for the FGFR signaling deregulation occurring in epithelial tissues from the physiological oncosoppressive to the pathological oncogenic profile.DoUzGs4sbo_jjVe6wgq-stVideo Graphical abstract
Highlights
The epithelial isoform of the fibroblast growth factor receptor 2 (FGFR2b) controls the entire program of keratinocyte differentiation via the sequential involvement of protein kinase C (PKC) δ and PKCα
PKCε signaling is responsible for FGFR2c-mediated modulation of epithelial-mesenchymal transition (EMT)-related markers In order to verify whether PKCε could be responsible for the multiple oncogenic outcomes of aberrant FGFR2c expression, we first assayed the ability of this receptor to
HaCaT pBp and HaCaT pBp-FGFR2c clones were transfected with PKCε small interfering RNA or with an unrelated siRNA (Cx siRNA) as control, and left untreated or stimulated with FGFR2 ligands as above. a Western blot shows that in untreated cells PKCε siRNA transfection induces an efficient depletion of PKCε
Summary
The epithelial isoform of the fibroblast growth factor receptor 2 (FGFR2b) controls the entire program of keratinocyte differentiation via the sequential involvement of protein kinase C (PKC) δ and PKCα. It is known that most of the human genes undergo alternative splicing and many studies have suggested that the isoform switch represent a crucial event in cancer [1] In this regard, several studies have demonstrated that the Ranieri et al Cell Communication and Signaling (2020) 18:76 changes in FGFR ligand specificity, leading to impairment of differentiation [8], EMT and early tumorigenic features [9, 10]. PKCε shows the greatest oncogenic potential among PKC family members [13] and it has been proposed to play a relevant role in EMT induction [14, 15]: PKCε overexpression alone is sufficient to dramatically increase growth rate and motility in human keratinocytes (HKs) [16], as well as to induce EMT-related phenotype in non-tumorigenic mammary epithelial cells [14, 15], strongly encouraging us to go deeper inside on its possible function as key molecular player in the context of aberrant FGFR2c expression and signaling
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