Unterschiedliche Sensitivität von Kopf-Halskarzinom Zelllinien auf den EMT Masterregulator TGF-β1

Head and neck squamous cell carcinomas (HNSCC) are the sixth most common tumors world wide. Despite improved treatments the 5-year survival rate has remained merely at 50%. Crucial for prognosis of HNSCC patients is the level of metastatic spread that has often already occurred at the time of diagnosis. A central mechanism underlying the invasive potential of HNSCC allows to transform tumor cells from a stationary, epithelial-like into a motile form thereby aquiring mesenchymal cell properties. This so-called epithelial mesenchymal transition (EMT) is characterized by the reduced expression of typical epithelial marker proteins such as E-cadherin and keratins as well as an increase in typical mesenchymal factors such as N-cadherin and vimentin. Transforming growth factor (TGF)-β1 was identified as a central regulator of EMT. Head and neck tumor cells, under the influence of TGF-β1, develop a mesenchymal phenotype associated with increased migration and clinically poor prognosis. TGF-β1 may act as a tumor suppressor as well as a tumor promoter, depending on the cellular context. TGF-β1, via its receptor TGF-β RII, activates -besides the Smad-dependent signaling pathway- other Smad-independent signaling pathways, such as Erk/MEK, PI3K/MAPK and Ras. This results in the modulation of E-cadherin expression, formation of actin stress fibers via modulation of the cytoskeleton, and increased tumor cell migration and invasion. Numerous studies have found evidence of a relevant role of EMT in HNSCC, being involved in the observed different morphological HNSCC phenotypes. This raises the question of whether the observed varying histopathological pattern of differentiation or invasiveness can be attributed to a difference in the activation status of EMT inducing factors. HNSCC cell lines with differences in their epithelial/mesenchymal phenotypes were used to investigate how they differ in their sensitivity to EMT-inducing cytokines in vitro. The morphological phenotype without and with the exogenous supply of EMT inducing factors, the activation (phosphorylation) of the TGF-β receptor type II as well as changes in the migration behavior, E-cadherin status and cytoskeleton architecture were assessed. Upon addition of TGF-β1, the rather epithelial cell lines adapted a more mesenchymal phenotype, whereas in cells that already carried mesenchymal features, this effect was less pronounced. Transformation of the cellular phenotype by TGF-β1 was documented by microscopic imaging, alteration of in vitro proliferation, activation of the TGF-β RII, and subcellular redistribution of typical EMT proteins as well as in changes of the cytoskeleton architecture. In particular, a modulation of the actin cytoskeleton, as well as the E-cadherin relocalization and E-cadherin degradation appeared to be involved in the morphological change. Since TGF-β1 and other cytokines are partially secreted by cells of the stroma (e.g. immune cells), the tumor microenvironment may be involved in the variable cellular phenotypes observed in HNSCC tumors, which correlate with the ability of the tumor cells to invade and metastasize. In view of the present investigations, the question arises to which extent an autocrine secretion of TGF-β1 (or other EMT cytokines) in HNSCC tumor cells contributed to the observed variable sensitivity of the cells to exogenous TGF-β1. For further investigations, it seems meaningful to, after evaluation of the cell-line-specific TGF-β1 expression (including other EMT cytokines) or other relevant components of the EMT, to specifically knockdown these components as this may potentially lead to new therapeutic targets that can be used to inhibit invasiveness and metastasis of HNSCC tumors.