Defects of basement membrane and hemidesmosome structure correlate with malignant phenotype and stromal interactions in HaCaT-Ras xenografts

Benign and malignant HaCaT-ras clones, derived from immortalized HaCaT cell s were grown as nude mouse surface transplants rendering a human tumor prog ression model. Searching for malignancy-related alterations, the deposition , localization and mRNA of basement membrane and hemidesmosome components w ere analysed by immunofluorescence, in situ hybridization and electron micr oscopy. Initially, at 1 week epithelia of benign and malignant cells reveal ed a similarly low polarity and an enlarged 'activated basal' compartment, reflected by partial dislocation and extended pericellular staining of the hemidesmosome constituent integrin alpha 6 beta 4 seen by immunofluorescenc e. Whereas benign grafts eventually normalized, closely resembling grafts o f HaCaT cells, malignant growth was correlated with a persisting epithelial activation state and continuing higher expression of alpha 6 (by immunoflo urescence and in situ hybridization). The basement membrane components bull ous pemphigoid antigen 1, laminin-5 and collagen TV exhibited a largely lin ear distribution at 1 week. However, in the malignant cell transplants init ially minor basement membrane discontinuities became more severe at around 2 weeks, associated with close stromal cell contacts, angiogenesis and inva sion. Most striking were basement membrane alterations seen by electron mic roscopy. At 1 week stretches of basement membrane had developed in malignan t transplants, though to a much lesser extent than in benign specimens. Wit h invasion these basement membrane structures mostly disappeared despite pe rsistent although variable immunofluorescence, suggesting high turnover wit hout ultrastructural assembly. The hemidesmosome structures were defective throughout, completely lacking anchoring plaques with keratin filaments, wh ereas they were still associated with basement membrane deposits. Thus, mal ignant HaCaT-ras transplants, while initially resembling regenerating wound s, revealed an increasing loss of tissue polarity and basement membrane str uctures, which seemed to be accelerated upon stromal cell contacts.