MAPK/ERK-dependent translation factor hyperactivation and dysregulated laminin γ2 expression in oral dysplasia and squamous cell carcinoma.

Lesions displaying a variety of dysplastic changes precede invasive oral and epidermal squamous cell carcinoma (SCC); however, there are no histopathological criteria for either confirming or staging premalignancy. SCCs and dysplasias frequently contain cells that abnormally express the γ2 subunit of laminin-332. We developed cell culture models to investigate γ2 dysregulation. Normal human keratinocytes displayed density-dependent repression of γ2, whereas premalignant keratinocytes and SCC cells overexpressed γ2 and secreted laminin assembly intermediates. Neoplastic cells had hyperactive EGFR/MAPK(ERK) signaling coordinate with overexpressed γ2, and EGFR and MEK inhibitors normalized γ2 expression. Keratinocytes engineered to express HPV16 E6 or activated mutant HRAS, cRAF1, or MEK1 lost density repression of γ2 and shared with neoplastic cells signaling abnormalities downstream of ERK, including increased phosphorylation of S6 and eIF4 translation factors. Notably, qPCR results revealed that γ2 overexpression was not accompanied by increased γ2 mRNA levels, consistent with ERK-dependent, eIF4B-mediated translation initiation of the stem-looped, 5'-untranslated region of γ2 mRNA in neoplastic cells. Inhibitors of MEK, but not of TORC1/2, blocked S6 and eIF4B phosphorylation and γ2 overexpression. Immunostaining of oral dysplasias identified γ2 overexpression occurring within fields of basal cells that had elevated p-S6 levels. These results reveal a causal relationship between ERK-dependent translation factor activation and laminin γ2 dysregulation and identify new markers of preinvasive neoplastic change during progression to SCC.