Abstract
Mtss1 is located within chromosomal region 8q23-24, which is one of the three most commonly amplified regions in head and neck squamous cell carcinoma (HNSCC). Mtss1 is lost in metastatic cells, but confusingly is commonly overexpressed in primary tumors. Here we address possible reasons why Mtss1 is positively selected for in primary tumors. We find that Mtss1 enhances the localization of the epidermal growth factor (EGF) receptor to the plasma membrane, prolonging EGF signaling and resulting in enhanced proliferation in HNSCC. Depletion of Mtss1 results in decreased EGF receptor levels and decreased phosphorylation of Erk1/2 and Akt. However, when cells are at high density and adherent to each other, analogous to conditions in a solid tumor, Mtss1 does not confer any growth advantage, either in basal conditions or following EGF stimulation. This could indicate why Mtss1 might be lost in metastases, but preserved in early primary tumors. This is supported by an organotypic assay showing that Mtss1-expressing cells display a less proliferative more epithelial-like morphology on top of a collagen matrix. Furthermore, xenograft tumors expressing Mtss1 initially grow more rapidly, but later show less proliferation and more differentiation. Mtss1 positively modulates EGF signaling at low cell densities to promote proliferation and, therefore, may be beneficial for the early stages of primary HNSCC tumor growth. However, at high cell densities, Mtss1 impacts negatively on EGF signaling and this suggests why it inhibits metastasis.