NF1 tumor suppressor in skin : expression in response to tissue trauma and in cellular differentiation

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Type 1 neurofibromatosis (NF1) syndrome is caused by a mutation of the NF1 gene. NF1 protein (neurofibromin) contains a domain which is related to the GTPase activating protein (GAP) and accelerates the switch of active Ras-GTP to inactive Ras-GDP. The clinical symptoms of NF1 patients include e.g. the formation of benign neurofibroma tumors and hyperpigmented lesions of the skin. The NF1 protein has been referred to as a tumor suppressor since cells of malignant schwannomas of NF1 patients may display loss of heterozygosity of the NF1 gene. </p><p>In the present study, the expression of the NF1 gene was investigated during tissue repair in human skin. Elevated NF1 protein levels were seen in a fibroblastic cell population of healing wounds. <em>In vitro</em> studies were designed to investigate <em>NF1</em> expression in dermal fibroblasts under the influence of growth factors that are operative during wound healing. Platelet-derived growth factor (PDGF) isoforms AB and BB and transforming growth factor β1 (TGFβ1) elevated NF1 mRNA levels in cultured dermal fibroblasts. In further studies, histological examination on apparently healthy skin of NF1 patients revealed frequently small masses of neurofibromatous tissue at the vicinity of hair follicles. Thus, action of the NF1 gene appears to be an integral part of normal tissue repair. Enhanced NF1 tumor suppressor expression may serve to limit excessive fibrosis in wound healing.</p><p>As Ras proteins play a role in the regulation of cell differentiation and formation of cell junctions, the functional expression of NF1 protein was elucidated using differentiating keratinocytes as an <em>in vitro</em> model system. The results demonstrate that an intense NF1 tumor suppressor signal on intermediate filaments was temporally limited to the period in which the formation of desmosomes takes place. In analogy to NF1 protein, a rapid elevation of NF1 mRNA level was detected following initiation of differentiation. Interestingly, NF1 mRNA hybridization signal polarized towards cell-cell contact zones. This finding recognizes a potential way for post-transcriptional modification of <em>NF1</em> expression and targeting of translation to subplasmalemmal location. The results demonstrate that the function of NF1 protein is associated with the formation of cell junctions, and thus to cellular communication.</p></div>