FIBROBLAST GROWTH FACTOR (FGF) -1 AND FGF-2 GENE REGULATION IN RAT VASCULAR SMOOTH MUSCLE CELLS

Abstract

Vascular smooth muscle cells (SMC) normally exist in a quiescent, nonproliferative state; however, SMC proliferation occurs during the pathogenesis of several cardiovascular diseases. Therefore, understanding SMC growth control is an important step in understanding cardiovascular disease pathology. Two of the most potent SMC mitogens are the structurally-related polypeptides fibroblast growth factor (FGF)-1 and FGF-2. Both growth factors have been shown to induce SMC proliferation when delivered into the vessel wall. Additionally, FGF-1 and FGF-2 promote endothelial cell (EC) growth in vitro and induce angiogenesis (new blood vessel formation) in vivo. Thus, these proteins may play a role in both the development and neovascularization of atherosclerotic plaques. The precise control of FGF gene expression in the vessel wall may be an important mechanism regulating vascular cell growth. Therefore, the purpose of this project was to determine whether two vascular-cell derived factors, angiotensin II (Ang II) and heparin-binding epidermal-like growth factor (HB-EGF) could induce FGF-1 or FGF-2 gene expression in rat SMC cultured in vitro. Ang II is an eight amino acid vasoactive peptide derived via proteolytic processing of the precursor protein angiotensinogen. Ang II has been demonstrated to promote SMC proliferation when added to cells in culture as well as when administered to the de-endothelialized rat carotid artery. The polypeptide growth factor HB-EGF is a secreted, heparin-binding member of the EGF family. It is also a SMC mitogen and is expressed in human atherosclerotic plaques as well as in rat arteries following balloon catheter injury. It was found that both Ang II and HB-EGF could induce rat aortic SMC proliferation as determined by cell growth assays. Northern blot analysis revealed that both Ang II and HB-EGF could induce the expression of FGF-2 mRNA, but not FGF-1 mRNA. Induction of FGF-2 mRNA levels was time- and dose-dependent and could be blocked by inhibitors of de novo RNA or protein synthesis. Western blot analysis indicated that FGF-2 protein levels, like FGF-2 mRNA levels, were increased by Ang II and HB-EGF treatment. Additional experiments indicated that Ang II-induced FGF-2 gene expression occurred via AT₁ receptor binding and required tyrosine kinase activity. HBEGF-induced FGF-2 gene expression was dependent on heparan sulfate proteoglycan (HSPG) binding and was inhibited by the anti-inflammatory glucocorticoid dexamethasone. In summary, both Ang II and HB-EGF can induce FGF-2 gene expression in cultured rat aortic SMC. It is possible that a similar response may occur in the vessel wall. If this is indeed the case, high extracellular concentrations of Ang II and/or HB-EGF could result in elevated levels of FGF-2 protein. This could stimulate SMC proliferation and thus play a role in the development of cardiovascular disease.