Our study examined the expression of
AP-1 family members in keratinocytes derived 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, more aggressive tumour phenotype compared with the well-differentiated, less aggressive keratinocytes, whereas the expression of other
AP-1 family members (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
protein as the well-differentiated cells. Both untransfected and transfected cell lines were
keratin negative and
vimentin positive. Increased expression of JunB in the transfected cells resulted in up-regulation of c-Jun and Fra1 and an enhanced
AP-1 activity as demonstrated by transcriptional activation of the prototypic
AP-1 dependent promoter, MMP-1. JunB transfected cells grew more quickly than vector-only controls and were refractory to the growth inhibitory effects of
TGF-beta1. Over-expression of JunB resulted in the elevated expression of the
AP-1 dependent
proteinase, MMP-9, whereas the expression of the
AP-1 independent
enzyme, MMP-2, was unaffected. JunB transfected 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 keratinocytes does not alter epithelial differentiation but enhances the malignant phenotype in vitro, possibly by altering the dynamics of the
AP-1 complex.