Retinoic acid inhibits downregulation of DeltaNp63alpha expression during terminal differentiation of human primary keratinocytes.

Recently, the p53 homolog p63 has been implicated in sustaining the epidermal stem cell population. The p63 gene encodes six major products with transactivating or dominant-negative properties. The expression pattern of these isoforms in keratinocytes was investigated here. Northern blot, ribonuclease protection assay, reverse transcription-polymerase chain reaction, and western blot techniques sensitive for all six p63 isotypes verified the predominant expression of the truncated and potentially dominant-negative isotype DeltaNp63alpha in human keratinocytes. The expression of this isoform is downregulated when proliferating human primary keratinocytes begin to differentiate after growth factor withdrawal. The onset of differentiation does not change the ratio of two other weakly expressed isotypes DeltaNp63gamma and TAp63alpha relative to DeltaNp63alpha. Treatment of primary human keratinocytes with all-trans retinoic acid does not alter the expression pattern of p63 isotypes but prevents its downregulation as observed in control cell cultures. These data suggest that p63 expression in human keratinocytes is affected by all-trans retinoic acid and this influence might contribute to the fine tuned keratinocyte proliferation and differentiation equilibrium in the mammalian epidermis.