Abstract 408: Notch Signaling Induces Expression of Fatty Acid Binding Protein 4 via Induction of PPAR-γ2 in Capillary Endothelial Cells in Heart
Notch signaling, an evolutionarily conserved pathway for cell fate determination, is essential for normal vascular development and maintenance as well as other multiple aspects of vascular function. In order to find a new function of Notch signaling in vascular system, we performed microarray analysis by using human endothelial cells (EC) infected with adenovirus expressing Notch intracellular domain (NICD), an activated form of Notch. We found an intriguing candidate molecule, fatty acid binding protein 4 (FABP4), also known as aP2, which is considered to be predominantly expressed in adipocytes and macrophages. Of special interest, subsequent immunohistochemistry of adult mice with our newly generated FABP4-specific antibody revealed that FABP4 was exclusively expressed in EC of cardiac capillaries, but not any arteries in heart. Consistent with this, a Notch receptor Notch1 and a Notch ligand Dll4 were strongly expressed in EC of cardiac capillaries as well. We next studied molecular mechanism of FABP4 induction by Notch stimulation. NICD overexpression induced FABP4 expression in human umbilical vein EC and human cardiac microvessel EC (HCMEC) whereas it did not do so in vascular smooth muscle cells or even 3T3L1 cells, which are prone to differentiate into adipocytes, suggesting EC-specific induction. We found that the induction of FABP4 was not direct effect by Notch stimulation. Rather, it is mediated by induction of peroxisome proliferator activated receptor (PPAR) γ2, an isoform known to play a major role in adipocyte differentiation. NICD overexpression induced PPAR-γ2 as well as FABP4 in HCMEC and additional PPAR-γ ligand, troglitazone, strongly enhanced induction of FABP4. Luciferase reporter gene analysis showed that PPAR-γ2 promoter containing a putative RBP-Jk binding element (or Notch responsive element) was transactivated by NICD. Importantly, our immunohistochemistry revealed that PPAR-γ antigen was also detected in capillary EC of heart. Given that long chain fatty acids are central source of energy metabolism in heart, FABP4 induced by Notch-PPAR pathway in capillary EC may play a crucial role in fatty acid transport system through endothelial layer to supply adequate fatty acids to the fatty acid-burning organ.