NF1 Regulates a Ras-Dependent Vascular Smooth Muscle Proliferative Injury Response

Abstract

Background-Neurofibromatosis type I (NF1) is a common autosomal dominant disorder with a broad array of clinical manifestations, including benign and malignant tumors, osseous dysplasias, and characteristic cutaneous findings. In addition, NF1 patients have an increased incidence of cardiovascular diseases, including obstructive vascular disorders. In animal models, endothelial expression of the disease gene, NF1, is critical for normal heart development. However, the pathogeneses of the more common vascular disorders are not well characterized. Methods and Results-To examine the role of NF1 in vascular smooth muscle, we generated mice with homozygous loss of the murine homolog Nf1 in smooth muscle (Nf1smKO). These mice develop and breed normally. However, in response to vascular injury, they display a marked intimal hyperproliferation and abnormal activation of mitogenactivated protein kinase, a downstream effector of Ras. Vascular smooth muscle cells cultured from these mice also display enhanced proliferation and mitogen-activated protein kinase activity. Smooth muscle expression of the NF1 Ras-regulatory domain (GTPase activating protein-related domain) rescues intimal hyperplasia in Nf1smKO mice and normalizes vascular smooth muscle cell Ras effector activity and proliferation in vitro, similar to blockade of downstream effectors of Ras. Conclusions-In this in vivo model of NF1 obstructive vascular disease, we have shown that Nf1 regulation of Ras plays a critical role in vascular smooth muscle proliferation after injury. These results suggest opportunities for targeted therapeutics in the prevention and treatment of NF1-related vascular disease and in the treatment of neointimal proliferation in other settings.

Giant Extracranial Aneurysms of Both Internal Carotid Arteries with Aberrant Jugular Veins in a Patient with Neurofibromatosis Type 1

“…Neurofibromin normally acts as a tumor-suppressor product through downregulation of reticular activating system (RAS)-induced signaling. 11,12 In normal vascular tissue, neurofibromin is strongly expressed by both endothelial and smooth muscle cells. 6,11,12 Mice engineered to have increased RAS signaling through a different mutation than the one underlying NF-1 develop neointimal hyperplasia and luminal occlusion similar to what is seen in patients with NF-1 who have cerebrovascular complications.…”

“…11,12 In normal vascular tissue, neurofibromin is strongly expressed by both endothelial and smooth muscle cells. 6,11,12 Mice engineered to have increased RAS signaling through a different mutation than the one underlying NF-1 develop neointimal hyperplasia and luminal occlusion similar to what is seen in patients with NF-1 who have cerebrovascular complications. 11 Work with mouse vascular smooth muscle cells and with cells obtained from patients with NF-1 shows that loss of neurofibromin results in increased proliferation and migration of vascular smooth muscle cells.…”

“…11 Work with mice engineered to have homozygous loss of the homolog NF-1 in smooth muscle cells has produced several findings with significant implications for patients with NF-1. 12 Under normal conditions, these mice develop and live comparably to their genetically healthy peers. However, they show a markedly pathologic response to vascular injury.…”

The ecology of brain tumors: lessons learned from neurofibromatosis-1

“…Proliferation or survival is mediated through Ras/MAPK signaling in Nf1-deficient or heterozygous mast cells (Khalaf et al, 2007;McDaniel et al, 2008) and vascular smooth muscles Xu et al, 2007). Multiple pathways contribute to increased osteoclast activity and gain of function (Yang et al, 2006; Yan et al, 2008;Li et al, 2009) as well as abnormal myeloid cell survival and proliferation (Bollag et al, 1996;Donovan et al, 2002).…”

Ras-Directed N-Glycoproteins are Novel Early Biomarkers for Tumorigenesis and Malignant Transformation, and Therapeutic Targets of Neurofibromatosis Type I

“…The Nf1 gene product, neurofibromin, functions as a GTPase activation protein (GAP), a negative regulator of the cellular Ras kinase [18]. It has been reported that the levels of activated Ras-GTP due to the loss of neurofibromin in NF1 plexiform neurofibromas and neurogenic sarcomas were approximately 5 and 15 times higher respectively, than the levels present in non-NF1 schwannomas, supporting the hypothesis that an aberrant Ras signaling pathway is the initial event in the tumorigenesis of NF1 [19,20]. However, the molecular mechanisms of the mutational inactivation of Nf1 resulting in hyperactive Ras that leads to alterations in uncontrolled growth and dedifferentiation of Schwann cells have not been elucidated in neurofibromas and MPNSTs.…”