Cell density and culture factors regulate keratinocyte commitment to differentiation and expression of suprabasal K1/K10 keratins

Irreversible growth arrest and commitment to differentiation are among the earliest events in the program of cellular terminal differentiation. The transition from highly proliferative human keratinocytes in subconfluent culture to stationary cells in confluent culture was studied in a serum-free culture system to identify conditions that regulate the initiation of terminal differentiation in keratinocytes. We observed that culture confluence strongly induced commitment to terminal differentiation, as demonstrated by a dramatic loss of keratinocyte clonogenicity. Commitment was accompanied by the rapid induction of early differentiation markers, represented by expression of suprabasal keratin 1 (K1) and 10 (K10) genes. Induction of differentiation was independent of low (0.1 mM) or high (1.5 mM) calcium concentration in the medium. Epidermal growth factor suppressed expression of K1 and K10 mRNA in cultures induced to differentiate. Suspension of keratinocytes in methylcellulose medium failed to induce in subconfluent cultures, or enhance in confluent cultures, the expression of K1 and K10 genes. Subconfluent cells cultured in medium containing high calcium and no exogenous growth factor induced expression of K1 and K10 transcripts, but commitment and loss of proliferative potential were not observed. Confluent cell density primarily controlled keratinocyte commitment to terminal differentiation and differentiated keratin gene expression. However, suprabasal K1 and K10 gene expression also was regulated by medium calcium and exogenous growth-factor concentrations in subconfluent cultures that promoted cell-cell association. Epidermal growth factor inhibited the expression of suprabasal keratins but not the commitment to terminal differentiation mediated by cell confluence. Control of keratinocyte commitment and expression of selected differentiation genes are mediated by cell confluence and, at subconfluence, by specific culture factors.