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 gamma 2 subunit of laminin-332. We developed cell culture models to investigate gamma 2 dysregulation. Normal human keratinocytes displayed density-dependent repression of gamma 2, whereas premalignant keratinocytes and SCC cells overexpressed gamma 2 and secreted laminin assembly intermediates. Neoplastic cells had hyperactive EGFR/MAPK(ERK) signaling coordinate with overexpressed gamma 2, and EGFR and MEK inhibitors normalized gamma 2 expression. Keratinocytes engineered to express HPV16 E6 or activated mutant HRAS, cRAF1, or MEK1 lost density repression of gamma 2 and shared with neoplastic cells signaling abnormalities downstream of ERK, including increased phosphorylation of S6 and eIF4 translation factors. Notably, qPCR results revealed that gamma 2 overexpression was not accompanied by increased gamma 2 mRNA levels, consistent with ERIC-dependent, eIF4B-mediated translation initiation of the stem-looped, 5'-untranslated region of gamma 2 mRNA in neoplastic cells. Inhibitors of MEK, but not of TORC1/2, blocked S6 and eIF4B phosphorylation and gamma 2 overexpression. Immunostaining of oral dysplasias identified gamma 2 overexpression occurring within fields of basal cells that had elevated p-S6 levels. These results reveal a causal relationship between ERIC-dependent translation factor activation and laminin gamma 2 dysregulation and identify new markers of preinvasive neoplastic change during progression to SCC. (Am J Pathol 2012, 180:2462-2478 http://dx.doi.org/10.1016/j.ajpath.2012.02.028)