APOBEC3 regulates keratinocyte differentiation and expression of Notch3

Apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family consists of deaminases. Some isozymes of APOBEC3 are induced upon human papillomavirus infection or development of psoriasis skin lesions. However, the involvement of APOBEC3 in keratinocyte differentiation has not been addressed. We herein sought to evaluate the roles of APOBECs in mouse primary keratinocyte differentiation. We found that expression levels of APOBEC1 and APOBEC3 were increased during calcium-induced keratinocyte differentiation. Unexpectedly, however, the expression levels of keratinocyte differentiation markers keratin 1/10, involucrin, loricrin and filaggrin were higher in keratinocytes treated with APOBEC3 siRNAs than in those treated with control RNAs. In addition, the treatment of keratinocytes with APOBEC3 siRNAs increased the gene expression levels of Notch3, a master regulator of keratinocyte differentiation. Moreover, calcium-induced increase in Notch3 expression and keratinocyte differentiation were impaired by transfection with an APOBEC3 expression plasmid. Furthermore, co-treatment with Notch3 siRNAs reduced the APOBEC3 siRNA-mediated upregulation of Notch3 expression and in part attenuated the increased expression levels of keratinocyte differentiation markers. These results suggest that APOBEC3 is induced upon keratinocyte differentiation and negatively regulates the keratinocyte differentiation in part by its inhibitory role for Notch3 expression.