Novel roles of NM23 proteins in skin homeostasis, repair and disease.

Keratinocyte growth factor (KGF) is an important regulator of epidermal homeostasis and repair. Therefore, the identification of KGF target genes in keratinocytes should contribute to our understanding of the molecular mechanisms underlying these processes. In a search for KGF-regulated genes, we identified the gene encoding the nucleoside diphosphate kinase NM23-H1. Apart from a housekeeping function, NM23 proteins are involved in the regulation of many cellular processes as well as in tumor metastasis, but their functions in epidermal homeostasis and repair are largely unknown. Here, we show a high expression of NM23-H1 and NM23-H2 in the KGF-responsive keratinocytes of the hyperproliferative epidermis of mouse skin wounds and of patients suffering from the skin disease psoriasis. To determine if this overexpression is functionally important, we generated HaCaT keratinocyte cell lines overexpressing NM23-H1 and/or -H2. Whereas the enhanced levels of NM23 did not affect cell proliferation in monoculture, NM23-H2 and double transfectants but not NM23-H1 transfectants formed a strongly hyperthickened epithelium in three-dimensional organotypic cultures. The abnormal epithelial morphology resulted from enhanced proliferation, reduced apoptosis and alterations in the differentiation pattern. These findings suggest that epidermal homeostasis depends on a tight regulation of the levels of NM23 isoforms.