Differential contribution of M(r) 120 kDa rasGTPase-activating protein and neurofibromatosis type 1 gene product during the transition from growth phase to arrested state in human fibroblasts accompanied by a unique rasGTPase-activating activity

The Nf1 tumor suppressor regulates mouse skin wound healing, fibroblast proliferation, and collagen deposited by fibroblasts

Neurofibromatosis type 1 patients develop peripheral nerve tumors (neurofibromas) composed mainly of Schwann cells and fibroblasts, in an abundant collagen matrix produced by fibroblasts. Trauma has been proposed to trigger neurofibroma formation. To test if loss of the neurofibromatosis type 1 gene (Nf1) compromises fibroblast function in vivo following trauma, skin wounding was performed in Nf1 knockout mice. The pattern and amount of collagen-rich granulation bed tissue, manufactured by fibroblasts, was grossly abnormal in 60% of Nf1+/- wounds. Nf1 mutant fibroblasts showed cell autonomous abnormalities in collagen deposition in vitro that were not mimicked by Ras activation in fibroblasts, even though some Nf1 effects are mediated through Ras. Nf1+/- skin wound fibroblasts also proliferated past the normal wound maturation phase; this in vivo effect was potentiated by muscle injury. In vitro, Nf1+/- fibroblasts showed higher proliferation in 10% serum than Nf1+/+ fibroblasts. Macrophage-conditioned media or epidermal growth factor potentiated Nf1+/- fibroblast proliferation in vitro, demonstrating abnormal response of mutant fibroblasts to wound cytokines. Thus Nf1 is a key regulator of fibroblast responses to injury, and Nf1 mutation in mouse fibroblasts causes abnormalities characteristic of human neurofibromas.

Partial nuclear localization of a bovine phosphoprotein, BCNT, that includes a region derived from a LINE repetitive sequence in Ruminantia

BCNT, named after Bucentaur, is a protein that contains a 324-amino-acid region derived from part of a long interspersed DNA sequence element (LINE) in Ruminantia. However, the unique portion is completely missing in human and mouse BCNTs. Since no significant information on their function has been obtained by homology search, we at first examined cellular localization and biochemical characteristics of bovine BCNT to get a hint on its function. Subcellular fractionation and immunohistochemical analyses using a normal bovine epithelial cell line and bovine brain revealed that a significant amount of bovine BCNT is localized in the nuclei, while the major portion is present in the cytosol. Furthermore, it was shown that bovine BCNT is a phosphoprotein and that both bovine and human BCNTs are phosphorylated by casein kinase II in vitro. These results show that BCNTs consist of a unique family, probably a substrate of casein kinase II, which may contribute further to the understanding of gene evolution.

Upregulation of tumor suppressor protein neurofibromin in normal human wound healing and in vitro evidence for platelet derived growth factor (PDGF) and transforming growth factor-beta1 (TGF-beta1) elicited increase in neurofibromin mRNA steady-state levels in dermal fibroblasts

We first studied expression of neurofibromin by immunohistochemistry in scars obtained from operations involving areas of healing wounds. The results demonstrated increased immunoreactivity for neurofibromin in the fibroblastic cell population of the lesions when compared with fibroblasts of apparently healthy perilesional skin, or those of intact control skin. Furthermore, dermal fibroblasts of 19 and 34 wk-old fetuses displayed a clearly detectable immunosignal for neurofibromin. In vitro studies were designed to investigate the potential effects of selected growth factors--known to be operative in wound healing--on neurofibromin mRNA steady-state levels in cultured fibroblasts. Northern transfer analyses revealed that different isoforms of platelet derived growth factor (PDGF) exerted selective effects on the neurofibromin mRNA levels: PDGF isoform AB elevated neurofibromin mRNA levels in a concentration-dependent manner when concentrations of 0.1, 1, 10, and 30 ng per ml were used. The maximal upregulatory effect of PDGF BB was reached at a concentration of 1 ng per ml. In contrast, PDGF AA did not alter the steady-state levels of neurofibromin mRNA. As estimated by RNase protection assay, transforming growth factor-beta1 (TGF-beta1) upregulated neurofibromin gene expression when concentrations of 0.5 and 5 ng per ml were used. Reverse transcription followed by polymerase chain reaction did not detect apparent alterations in the ratio of type I/type II neurofibromin isoforms in PDGF- or TGF-beta1-treated cultures. Taken together, our results suggest that expression of tumor suppressor protein neurofibromin is upregulated in response to skin injury, and that this upregulation can be mediated through PDGF and TGF-beta.

Effect of growth factor on GTPase-activating protein (Ras GAP) in Chinese hamster ovary cells

GTPase-activating proteins (GAPs) stimulate the hydrolysis of GTP bound to small G-proteins and regulate the signal transduction pathway. Changes in the expression of p21-Ras p120-GAP induced by growth factor treatment were examined in cultured Chinese hamster ovary (CHO) and human choriocarcinoma (BeWo) cells. Expression of p120-GAP and GAP activity were measured. Fetal bovine serum induced a significant increased level of GAP in CHO cells, but did not increase GAP in BeWo cells. The results suggest that growth factors affect Ras GAP expression in CHO cells, while they do not in other cells such as BeWo cells.

An Alu-linked repetitive sequence corresponding to 280 amino acids is expressed in a novel bovine protein, but not in its human homologue

A novel protein harboring a 280-amino acid region from an Alu-linked repetitive sequence (bovine Alu-like dimer-driven family) was isolated from a bovine brain S-100 fraction using monoclonal antibodies against a rat GTPase-activating protein that shares the same epitope. The protein has an apparent molecular mass of 97 kDa (p97). Western blot analysis using extracts prepared from various tissues showed p97 to be predominantly detected in brain and moderately in liver and lung. From sequence analysis of the cDNA encoding p97, it was found that the 840-base pair sequence homologous to a part of the bovine Alu-like dimer-driven family, which has never been shown to be expressed, occurs in the middle of the protein coding region. The protein also contains a pair of intramolecular repeats composed of 40 highly hydrophilic amino acids at the C terminus. Human cDNA homologous to p97 was cloned, and its nucleotide sequence demonstrates that the 840-base pair repetitive sequence and one of the intramolecular repeats are missing. We named p97 bovine BCNT after Bucentaur. These results show that bovine BCNT is a unique molecule and suggest that an analysis of the relationship between bovine bcnt and its human homologue may help further the understanding of gene organization and evolution.

Human rasGTPase-activating protein (human counterpart of GAP1m): sequence of the cDNA, primary structure of the protein, production and chromosomal localization

When cDNA encoding rat rasGTPase-activating protein (rat GAP1m) was used as a probe, two partial cDNA clones of a human counterpart of rat GAP1m were isolated from a cDNA library derived from growth-arrested normal human ectocervical epithelial cells. One clone was found to be a cDNA of premature mRNA with two introns. A complete cDNA of human GAP1m was constructed by a series of reverse transcription-polymerase chain reaction (RT-PCR) using total RNA from human epidermoid carcinoma A431 cells. Human GAP1m shows 87.7% nucleotide identity to rat GAP1m in open reading frame and encodes an 850-amino acid protein that shows 89.2% identity to rat GAP1m. A 100-kDa protein was detected in A431 cells by Western blotting with anti-rat GAP1m antibody. The human GAP1m gene was mapped to chromosome 3q24-q26.

Ras-GTP regulation is not altered in cultured melanocytes with reduced levels of neurofibromin derived from patients with neurofibromatosis 1 (NF1)

As derivatives of the neural crest, epidermal melanocytes are supposed to be clinically affected by NF1 gene defects. The NF1 gene shares sequence homology with the p120 GTPase activating protein (p120-GAP) and neurofibromin has been shown to participate in Ras-regulation. By immunoprecipitation and Western blotting, neurofibromin was found to be expressed in melanocytes from the unaffected skin and café au lait macules of NF1 patients, but the intensity of the neurofibromin band was decreased compared to control cultures. The Ras-GTP/Ras-GDP ratios of NF1 derived melanocyte cultures were comparable to those derived from healthy donors. Furthermore, the total GAP-activity of cell lysates was not altered in NF1 melanocyte cultures compared to controls. However, lysates of proliferating melanocytes, both from NF1 patients and from healthy donors, showed an about 2-fold higher GAP-activity than poorly growing cells. Neurofibromin contributed approximately one third of total GAP-activity, in both control and NF1 melanocytes, indicating that it is not the major regulator of Ras in these cells. These results suggest that the function of neurofibromin in melanocytes is not limited to regulation of Ras activity.

Phosphorylation of a proline-directed kinase motif is responsible for structural changes in myogenin

Myogenin, a member of the MyoD family which governs skeletal muscle differentiation, was identified as a pair of phosphorylated bands on SDS-PAGE during myogenesis. The slow migrating form was found to be hyperphosphorylated myogenin. In vitro phosphorylation by CDC2 kinase caused a prominent reduction in electrophoretic mobility of myogenin. Furthermore, we demonstrated that phosphorylation of the serine residue at position 43 contributes to the modification of myogenin in vivo and in vitro resulting in the reduction in electrophoretic mobility. We propose here that a CDC2-like proline-directed kinase regulates myogenin activity through its phosphorylation.