The mixed lineage kinase leucine-zipper protein kinase exhibits a differentiation-associated localization in normal human skin and induces keratinocyte differentiation upon overexpression

Leucine-zipper protein kinase/dual leucine zipper bearing kinase/mitogen-ac tivated protein kinase-upstream kinase is a recently described protein seri ne/threonine kinase which belongs to the mixed lineage kinase family. The o verall pattern of expression of the leucine-zipper protein kinase/dual leuc ine zipper bearing kinase/mitogen-activated protein kinase-upstream kinase gene in embryonic and adult mouse tissues suggested that this kinase could be involved in the regulation of epithelial cell proliferation and differen tiation. In order to get more insights into the potential role of leucine-z ipper protein kinase in these cellular processes, we characterized its expr ession in normal human skin, both at the mRNA and protein levels. In situ h ybridization, western blotting, and indirect immunofluorescence studies wer e conducted to localize leucine-zipper protein kinase on various human skin tissues. This is one of the first reports that leucine-zipper protein kina se has a very precise pattern of expression in human skin epithelia, as bot h mRNA and protein are restricted to the granular layer of the epidermis an d inner root sheath of hair follicles. Detection of leucine-zipper protein kinase protein on skin from various body sites, donors of different ages as well as on reconstructed human skin always reveals that leucine-zipper pro tein kinase is present only in the very differentiated keratinocytes of epi dermis and hair follicles. To determine directly whether leucine-zipper pro tein kinase exhibits any effect on cell growth and differentiation, keratin ocytes were transfected with an expression vector harboring the leucine-zip per protein kinase cDNA. The presence of this construct in keratinocytes re sults in growth arrest together with a concomitant increase in filaggrin ex pression. Collectively, our results indicate that leucine-zipper protein ki nase plays an active part in cellular processes related to terminal differe ntiation of epidermal keratinocytes.