Penetration of human metastatic melanoma cells through an authentic dermal-epidermal junction is associated with dissolution of native collagen types IV and VII.

The events occurring during the penetration of melanoma cells through the dermal-epidermal junction, which is the first crucial step in the process of metastasis, are poorly understood, partly because no suitable tissue models exist. In the in vitro model reported here, two melanoma clones (T1C3, which generates lung metastases in experimental animals, and IC8, which does not) derived from a single parental cell line were co-seeded with normal allogenic keratinocytes onto acellular human de-epidermized dermis with preserved intact basement membrane and cultured for up to 1 month at an air-liquid interface. Histological, immunohistochemical and ultrastructural studies showed that melanoma cells from the metastatic clone (T1C3), but not from the non-metastatic clone (IC8), penetrated the dermal-epidermal junction to invade the dermis after 3 weeks of culture. Local invasion was associated with the dissolution of the native epidermal basement membrane collagens type IV and VII. Confocal laser scanning microscopy analysis demonstrated that numerous T1C3 cells were able to colonize the interstitial dermis and to rapidly penetrate empty dermal cavities. Our model represents a significant technical advance over others currently available since: (i) the organized three-dimensional architecture of the native dermal-epidermal junction is preserved; (ii) the active invasion process coincides with the dissolution of native components of the epidermal basement membrane, i.e. collagen types IV and VII; and (iii) the ability of melanoma cells to cross the dermal-epidermal junction correlates with their metastatic potential. This model provides a valuable tool for the study of the time-course of the cellular and molecular events that occur during the earliest steps of invasion in cutaneous melanoma. It also offers new opportunities to study the possible role of the keratinocyte environment in melanoma invasion.