Overexpression of JunB in undifferentiated malignant rat oral keratinocytes enhances the malignant phenotype in vitro without altering cellular differentiation

Our study examined the expression of AP-I family members in keratinocytes d erived from the rat-4NQO model of oral carcinogenesis in which extremes of epithelial differentiation and tumour cell aggressiveness are evident. The constitutive expression of JunB was diminished in the undifferentiated, mor e aggressive tumour phenotype compared with the well-differentiated, less a ggressive keratinocytes, whereas the expression of other AP-I family member s (c-jun, junD, c-fos, fra1 fra2 and fosB) was either very weak or variable , After transfection of the undifferentiated keratinocytes with junB cDNA, clonal populations were isolated that expressed similar levels of JunB prot ein as the well-differentiated cells, Both untransfected and transfected ce ll lines were keratin negative and vimentin positive. Increased expression of JunB in the transfected cells resulted in up-regulation of c-Jun and Fra 1 and an enhanced AP-I activity as demonstrated by transcriptional activati on of the prototypic AP-I dependent promoter, MMP-I. JunB transfected cells grew more quickly than vector-only controls and were refractory to the gro wth inhibitory effects of TGF-PI, Overexpression of JunB resulted in the el evated expression of the AP-I dependent proteinase, MMP-9, whereas the expr ession of the AP-I independent enzyme, MMP-2, was unaffected, JunB transfec ted keratinocytes were highly invasive in an in vitro assay of tumour cell invasion compared with vector controls. The results indicate that increased expression of JunB above baseline levels in undifferentiated rat keratinoc ytes does not alter epithelial differentiation but enhances the malignant p henotype in vitro, possibly by altering the dynamics of the AP-I complex, ( C) 2001 Wiley-Liss, Inc.