Overexpression of extracellular epimorphin leads to impaired epidermal differentiation in HaCaT keratinocytes

Epimorphin (also known as syntaxin2) is a stromal signaling factor that is temporally secreted via a non-classical route to regulate the morphogenesis of various epithelia, including skin epidermis. In this study, we show that epimorphin signaling also regulates the differentiation program in the keratinocyte. The extracellular presentation of this molecule is detectable predominantly in the dermal compartments of the skin and its secretion is increased by cell stress such as UVB irradiation, which generates an epimorphin signaling gradient in the epidermis. Artificial stimulation of functionally normal keratinocyte HaCaT cells with extracellular epimorphin triggered initiation of their differentiation program with a dramatic depression of metabolic turnover. Intriguingly, however, sustained epimorphin signaling appeared to severely attenuate the terminal cornification in the cells induced by calcium influx and anchorage-dependent anoikis. In the organotypic culture of HaCaT cells, overexpression of epimorphin impaired the successive differentiation program in the stratified epidermis-like structures; the cells underwent aberrant multicellular arrangement with a presentation of mid-differentiation markers throughout all the cell layers. These results demonstrate that inadequate epimorphin elicits an abnormal differentiation response in keratinocytes, and indicate a causal function of the epimorphin signaling gradient for the establishment of differentiated epidermal structure in the skin.