Abstract 332: Retinol-binding Protein 7 (RBP7) Regulates Endothelial Function, Independent of Metabolic Responses in High-fat Diet Fed Mice

Peroxisome proliferator-activated receptor gamma (PPARG), a ligand-activated transcription factor, plays a protective role in the endothelium through mechanisms that remain unclear. RBP7 is an intracellular retinoid binding protein belonging to the family of fatty acid-binding proteins (FABP) and is a PPARG target. RBP7 expression was increased 2.6-fold by rosiglitazone (RZ), a PPARG agonist in mouse lung endothelial cells, which was blocked by GW9662, a PPARG antagonist. We hypothesized that the protective effects of PPARG on the endothelium are mediated by RBP7. Studies were performed in: 20-week HFD-fed RBP7-deficient (KO) and wild type (WT) and age-matched chow-fed KO and WT mice. HFD increased fasting glucose in both WT and KO mice compared to chow-fed groups, and caused similar hepatic steatosis. HFD-fed mice were more glucose-intolerant and insulin-resistant compared with chow-fed mice, but there were no differences comparing KO with WT mice. Weight gain in response to HFD was also similar in KO and WT mice. Thus KO and WT mice exhibited similar HFD-induced metabolic dysfunction. Endothelium-dependent relaxation to acetylcholine (Ach) was normal in chow-fed KO mice, but was impaired in carotid artery of HFD-fed KO mice (49±6% KO vs 70±4% WT, p<0.05, 30μM Ach). Endothelial-independent relaxation was similar in all groups. Superoxide production in carotid artery (DHE-fluorescence) was elevated 1.5-fold in HFD-fed KO mice compared with the other 3 groups. Normal relaxation to Ach was restored by anti-oxidant treatment, suggesting HFD causes oxidative stress in the absence of RBP7. Systolic blood pressure was similar in HFD-fed KO and WT mice (110±4 mmHg in KO vs 105±0 mmHg in WT). Because other members of the FABP family facilitate steroid hormone receptor function, we hypothesized that RBP7 may facilitate PPARG function. To initially test this, we examined the cellular localization of an RBP7-GFP fusion protein. In vehicle treated COS7 cells, RBP7 was distributed both in cytoplasm and nucleus. RZ treatment resulted in a robust shift of RBP7 into the nucleus. In conclusion, RBP7 protects the endothelium from HFD-induced oxidative and vascular dysfunction, independent of changes to systemic metabolism.