Regulation and Role of GLI1 in Cutaneous Squamous Cell Carcinoma Pathogenesis.

Joanna Pyczek,Anja Uhmann,Steffen Emmert,Hans-Ulrich Schildhaus,Maria Kuzyakova,Frauke Nitzki,Natalia Khizanishvili,Sebastian Zabel,Julia Bauer,Michael P Schön,Heidi Hahn,Petra Boukamp

doi:10.3389/fgene.2019.01185

Abstract

Cutaneous squamous cell carcinoma (cSCC) is the second most common skin tumor in humans. Although current therapies are sufficient to clear the tumor in many cases, the overall risk of cSCC metastasis is still 5%. Alternative treatment options could help to overcome this situation. Here we focused on the role of the Hedgehog (HH) signaling pathway and its interplay with epidermal growth factor receptor (EGFR) signaling in cSCC. The analyses revealed that, despite lack of Sonic HH (SHH) expression, a subset of human cSCC can express GLI1, a marker for active HH signaling, within distinct tumor areas. In contrast, all tumors strongly express EGFR and the hair follicle stem cell marker SOX9 at the highly proliferative tumor-stroma interface, whereas central tumor regions with a more differentiated stratum spinosum cell type lack both EGFR and SOX9 expression. In vitro experiments indicate that activation of EGFR signaling in the human cSCC cell lines SCL-1, MET-1, and MET-4 leads to GLI1 inhibition via the MEK/ERK axis without affecting cellular proliferation. Of note, EGFR activation also inhibits cellular migration of SCL-1 and MET-4 cells. Because proliferation and migration of the cells is also not altered by a GLI1 knockdown, GLI1 is apparently not involved in processes of aggressiveness in established cSCC tumors. In contrast, our data rather suggest a negative correlation between Gli1 expression level and cSCC formation because skin of Ptch +/- mice with slightly elevated Gli1 expression levels is significantly less susceptible to chemically-induced cSCC formation compared to murine wildtype skin. Although not yet formally validated, these data open the possibility that GLI1 (and thus HH signaling) may antagonize cSCC initiation and is not involved in cSCC aggressiveness, at least in a subset of cSCC.

Highlights

Cutaneous squamous cell carcinoma is the second most common human skin tumor after basal cell carcinoma (BCC) with increasing incidence
Incubation of SCL-1 with HhA resulted in a paradoxical increase in GLI1 expression, whereas this drug inhibited GLI1 transcription in MET-1 and MET-4 cells (Figure 2C, middle panel). These results show that i) canonical HH signaling in Cutaneous squamous cell carcinoma (cSCC) cell lines is neither inducible with Sonic HH (SHH) or SAG nor inhibitable with cyclopamine or vismodegib, at least under our experimental conditions and ii) that the SMO inhibitor HhA can paradoxically induce the expression of GLI1, which has been seen in other tumor cell lines (Ridzewski et al, 2015)
On the other hand numerous clinical data demonstrate that inhibition of canonical HH signaling i.e. BCC treatment with vismodegib can result in cSCC development (e.g. Mohan et al, 2016)

Summary

Introduction

Cutaneous squamous cell carcinoma (cSCC) is the second most common human skin tumor after basal cell carcinoma (BCC) with increasing incidence. CSCC are characterized by malignant proliferation of epidermal keratinocytes. They are heterogeneous invasive tumors that show proliferation at the tumor-stroma interface and usually have an inner differentiating cell mass resembling the stratum spinosum. CSCC express SOX9, which induces proliferation of keratinocytes (Shi et al, 2013), deregulates hair follicle stem cell maintenance and suppresses epidermal differentiation (Kadaja et al, 2014). CSCC show phosphorylation of the EGFR-downstream signaling targets ERK (Rittie et al, 2007; Zhang et al, 2007; Sonavane et al, 2012), AKT (Rittie et al, 2007; Barrette et al, 2014), and S6 (Khandelwal et al, 2016). The EGFRdirected monoclonal antibody cetuximab is currently applied in clinical trials (Dereure et al, 2016; Wollina et al, 2018)

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