Co-regulation of p16INK4a and migratory genes in culture conditions that lead to premature senescence in human keratinocytes

Cellular stasis, also known as telomere-independent senescence, prevents many epithelial cells from becoming immortalized by telomerase alone. As human keratinocytes age in culture, protein levels of the tumor suppressor p16(INK4a) continue to increase, resulting in growth arrest independent of telomere length. Differences in culture conditions have been shown to modulate both p16(INK4a) expression and replicative capacity of human keratinocytes; however, the mechanism of p16(INK4a) induction under these conditions is unknown. Using multiple primary keratinocyte cell strains, we verified a delay in p16(INK4a) induction and an extended lifespan of human keratinocytes when grown in co-culture with post-mitotic fibroblast feeder cells as compared with keratinocytes grown on tissue culture plastic alone. Evaluation of gene expression levels in the two culture conditions by microarray analysis, and subsequent validation, demonstrated that keratinocytes cultured on plastic alone had significantly increased expression of many genes involved in keratinocyte migration and reduced expression levels of genes involved in keratinocyte differentiation. Higher levels of p16(INK4a) expression were present in cells that also displayed increased amounts of autophosphorylated focal adhesion kinase and urokinase plaminogen activator receptor (uPAR), both markers of keratinocyte migration. Furthermore, when tyrosine phosphorylation or urokinase-type plasminogen activator (uPA)/uPAR function was inhibited, both keratinocyte migration and p16(INK4a) expression were reduced. Our results indicate that keratinocytes cultured in the absence of feeder cells exhibit a migratory phenotype and suggest that p16(INK4a) is selectively induced under these conditions by a mechanism involving tyrosine kinase activity and the urokinase plasminogen activation system.