Neurofibromin: expression by normal human keratinocytes in vivo and in vitro and in epidermal malignancies

Neurofibromin is the product of the NF1 gene, the mutations of which have been linked with type 1 neurofibromatosis. The expression of neurofibromin in human skin has not been analyzed in detail.

Polyclonal Ab were raised against synthetic peptides corresponding to three different sites of neurofibromin. One of the Ab selectively recognized type II neurofibromin. The localization of neurofibromin was first studied in normal human skin. Further studies concentrated on neurofibromin expression in basal cell and squamous cell carcinomas. Reverse transcription-PCR (RT-PCR) and molecular hybridizations and immunocytochemistry were used to characterize the expression of neurofibromin in cultured keratinocytes.

All neurofibromin-specific Ab immunolabeled the epidermis. The basal keratinocytes displayed the most prominent immunosignal for type II neurofibromin. RT-PCR demonstrated the presence of both type I and II neurofibromin mRNA transcripts in cultured keratinocytes. Keratinocytes induced to differentiate and to arrest division by a high (1.4 mM) Ca2+ concentration of the culture medium displayed a down-regulation of neurofibromin expression at the mRNA and protein levels. This was most strikingly demonstrated by a reduction of immunoreactivity for type II neurofibromin. Basal cell carcinomas displayed a weak immunosignal for type II neurofibromin. In contrast, particularly the central areas of squamous cell carcinoma, islands were intensely immunolabeled.

The results suggest that neurofibromin acts as a regulator of the basal keratinocytes in normal skin and that cultured keratinocytes offer a human model for studies aimed to elucidate the regulation of neurofibromin gene expression. Furthermore, aberrations in neurofibromin expression may play a role in the pathogenesis of epidermal cancers.