PPAR? is a ligand activated transcription factor best known for its role in regulating insulin sensitivity and adipose tissue development and expansion. There is now convincing evidence to support an important beneficial role for endothelial PPAR? in the regulation of endothelial function. However, little is known about the transcriptional targets for PPAR?, how PPAR? selectively activates some targets over others, and their mechanisms of action in the endothelium. Selective expression of a dominant negative mutant of PPAR? in the endothelium (E-V290M) has no effect on endothelial function in aorta, carotid or basilar arteries in young transgenic mice at baseline, but induces severe dysfunction if the mice are challenged by a high fat diet, Ang-II, or are old. The cerebral circulation is particularly sensitive to oxidative stress in response to the interference with PPAR? signaling. Conversely, increased expression of wildtype PPAR? specifically in the endothelium (E- PPAR?-WT) reduced vasoconstriction to Ang-II. These data strongly suggest that PPAR? plays a protective role in the endothelium under stressed conditions, and the dysfunction resulting from PPAR? impairment is caused by oxidative stress. We identified retinol-binding protein 7 (RBP7) as a PPAR? target gene in the endothelium. RBP7 is an intracellular retinol binding protein belonging to the family of intracellular lipid and fatty acid-binding proteins. Expression of RBP7 is endothelial cell-specific. RBP7-deficient mice exhibit the same high fat diet-induced and Ang-II-induced phenotype as E-V290M mice which selectively express a dominant negative mutant of PPAR? in the endothelium. This led us to consider the innovative concept that the beneficial effects of PPAR? in the endothelium may be mediated by RBP7. Intracellular lipid and fatty acid binding proteins have been reported to bind ligands which activate other ligand activated transcription factors. Thus conceptually, we hypothesize that PPAR? and RBP7 may form a transcriptional regulatory loop (or hub) in endothelial cells, which is required to support an anti oxidant state and when impaired induces a pro-oxidant state. We will examine this concept in two Specific Aims.
Specific Aim 1 will test the hypotheses that a) the beneficial effects of wildtype PPAR? over-expression in the endothelium require RBP7, b) that the effects of RBP7 are mediated by its retinol-binding and lipid transport activity, and c) RBP7 is required for the PPAR?- mediated anti-oxidant response, including the expression and function of endothelial adiponectin.
Specific Aim 2 will evaluate the molecular mechanisms by which RBP7 regulates PPAR? transcriptional activity by testing the hypothesis that RBP7 with its intrinsic retinol binding and nuclear translocation activity is required for transcriptional activity of PPAR?.
PAR? is an important transcription factor which as anti-oxidant and anti-inflammatory effects in the endothelium and provides protection against risk factors such as a high fat diet. Little is known about the transcriptional targets for PPAR?, how PPAR? selectively activates some targets over others, and their mechanisms of action in the endothelium. We identified retinol-binding protein 7 (RBP7) as a PPAR? target gene in the endothelium. We hypothesize that PPAR? and RBP7 form a transcriptional regulatory loop in endothelial cells, which is required to support an anti-oxidant state and when impaired induces a pro-oxidant state leading to endothelial dysfunction.
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