Cells of the human keratinocyte line HaCaT were shifted to a mesenchymal/myogenic phenotype (DTHMZ cells) by MyoD1 transfection, 5-aza-2' deoxycytidine treatment, and selection for reduced adhesion on plastic. Since this correlated with loss of stratification (inability to form a multilayered tissue), we determined the status of cell-cell and cell-matrix adhesion molecules involved in epidermal morphogenesis. Expression of desmosomal proteins (plakoglobin, desmoglein, desmoplakin) and uvomorulin was no longer detectable at the mRNA and protein level in the DTHMZ cells while both HaCaT cells and malignant variants (transfected with c-Ha-ras oncogene) expressed uvomorulin in vitro and in transplants in vivo, the latter even in invasively growing tumor nodules. Furthermore, HaCaT cells stained positive for the integrin subunits beta 1, alpha 2, alpha 3, and alpha 5, typical for cultured keratinocytes. In contrast, the putative fibronectin receptor alpha 5 beta 1, common also in fibroblasts, was the only integrin showing strong staining in DTHMZ cells. The integrin subunits alpha v and a6, clearly expressed at the mRNA level, weakly stained HaCaT cultures and led to a dotlike fluorescence in DTHMZ cells, possibly representing focal adhesion plaques. The respective integrin status correlated well with the growth behavior on different matrices. While HaCaT cells readily attached and proliferated on collagen (type I), fibronectin-coated, and laminin-coated collagen gels, DTHMZ cells formed monolayers only on fibronectin-coated collagen. This was, however, not sufficient to allow stratification in vivo. Altogether, the status of adhesion molecules in DTHMZ cells more likely reflects that seen in mesenchymal cells as compared to the pattern of keratinocytes displayed by HaCaT cells. Moreover, since the DTHMZ cells were clearly HaCaT descendants, the results support our hypothesis of a "trans-differentiation" process from an epidermal (HaCaT) to a mesenchymal/myogenic phenotype (DTHMZ).

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