Abstract 983: Fatty Acid Binding Protein 4 is a Target Gene of Notch Signaling in Endothelial Cells of Capillaries in Heart and Adipose Tissue
Although Notch signaling, a pathway for cell fate determination, is essential for normal vascular development and maintenance, little is known about the molecular mechanism by which Notch regulates their multiple aspects of vascular function. In search for downstream target genes of Notch in EC, we performed microarray analysis by using human umbilical vein EC (HUVEC) infected with adenovirus expressing Notch intracellular domain (Ad-NICD), an activated form of Notch. Several molecules were remarkably upregulated by NICD. One of the most intriguing molecules is fatty acid binding protein 4 (FABP4), also known as aP2, which is predominantly expressed in adipocytes and macrophages and often used as a terminal differentiation marker for adipocytes. Subsequent Northern blot analysis confirmed that FABP4 was almost absent in untreated HUVEC while it was dramatically induced in HUVEC infected with Ad-NICD. Co-culture of bovine aortic EC with cells expressing Notch ligands revealed that Dll4, but not another ligand Jagged1, induced expression of FABP4, which was blocked by a Notch signal inhibitor, DAPT. To study tissue distribution, we generated antibody against FABP4, which did not cross-react with other members such as FABP3 or FABP5. Of special interest, systemic immunohistochemistry of adult mice revealed that FABP4 was exclusively expressed in EC of cardiac capillaries and venules, but not any arteries in heart. Consistent with this, Notch1 and Dll4 were strongly expressed in EC of cardiac capillaries as well. Closer examination further found that FABP4 as well as Notch1/Dll4 were markedly expressed in EC of capillaries in adipose tissue. The same protein expression of FABP4, Notch1 and Dll4 was also observed in human heart and pericardial adipose tissue. Moreover, we observed that FABP4 was colocalized with cytoskeletal F-actin in HUVEC infected with Ad-NICD, suggesting that FABP4 functions as intracellular carrier protein of fatty acids in collaboration with F-actin. Given that fatty acids are central source of energy metabolism in heart and adipose tissue, FABP4 induced by Notch signaling may play a crucial role in active transport of fatty acids through endothelial layer to supply adequate fatty acids to those fatty acid-dependent organs.