Endogenous Mechanisms of PPAR Modulation

描述(申请人提供)：过氧化物酶体增殖物激活受体(PPAR)是配体激活的转录因子，调节许多关键的代谢途径。对PPAR的大部分洞察来自对合成PPAR激动剂的研究，包括临床使用的那些。PPARa被降脂的贝特类药物激活，调节脂肪酸代谢。PPARy由胰岛素敏感型噻唑烷二酮类化合物激活，控制脂肪生成和葡萄糖稳态。PPARa和PPARy的激活都可以限制动脉粥样硬化和炎症。这一组合成激动剂激活PPAR的数据确立了理解内源性PPAR激活的重要性。最近，我们报道了脂蛋白脂酶(LPL)以一种特异性和选择性的方式作用于循环中的脂蛋白来产生PPARpha配体。这一建议侧重于中心假设，即内源性PPAR拮抗的直接和间接途径在决定代谢反应中发挥重要作用。提供了三种不同的内源性途径的数据，这些途径可能对PPAR活性进行负面调节。这些机制将通过研究它们对已建立的PPAR激活的体外和体内模型的调节来研究。LPL介导的PPARa激活提示，结构不同的脂肪酸对内皮的不同影响可能是由于不同的PPARa激活，包括拮抗作用。在目标1中，将在体外和体内研究特定脂肪酸对PPAR的激活和抑制，将omega-3脂肪酸与饱和脂肪酸和反式脂肪酸进行对比。肝脏核因子4α(HNF4α)是一种知之甚少但却是关键的脂肪酸激活受体，它调节脂质代谢、血栓形成和血糖控制。通过使用我们的LPL/PPARa研究中使用的脂代谢操作，我们已经确定了HNF4α调节的新机制以及PPARpha和HNF4α对脂代谢途径的不同反应。在目标2中，将探索HNF4pha和PPARpha之间的这种差异。PPAR调节的一个新颖而有效的机制是存在一个直接的内源性PPAR拮抗剂。虽然β胡萝卜素的对称裂解产生了RXR核受体的天然配体，但我们已经确定，不对称的β胡萝卜素裂解产生一种特定的载胡萝卜素，直接拮抗PPAR的反应。在目标3中，这种直接拮抗剂将在体外和体内进行表征。总之，这些研究整合了生化、生物和体内模型，以更好地理解PPAR活性的内源性调节如何决定生物反应。

DESCRIPTION (provided by applicant): Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors regulating many key metabolic pathways. Much of the insight into PPARs derives from studies with synthetic PPAR agonists, including those in clinical use. PPARa, activated by lipid-lowering fibrates, regulates fatty acid metabolism. PPARy, activated by insulin-sensitizing thiazolidinediones, controls adipogenesis and glucose homeostasis. Both PPARa and PPARy activation may limit atherosclerosis and inflammation. This body of data for PPAR activation by synthetic agonists establishes the importance of understanding endogenous PPAR activation. Recently, we reported lipoprotein lipase (LPL) acts on circulating lipoproteins in a specific and selective manner to generate PPARalpha ligands. This proposal focuses on the central hypothesis that direct and indirect pathways for endogenous PPAR antagonism play an important part in determining metabolic responses. Data is provided for three different endogenous pathways that may negatively regulate PPAR activity. These mechanisms will be studied by examining their modulation of well established in vitro and in vivo models of PPAR activation. LPL-mediated PPARa activation suggests the discrepant endothelial effects of structurally diverse fatty acids may be due to differential PPARa activation, including antagonism. In Aim 1, PPAR activation and inhibition by specific fatty acids will be studied in vitro and in vivo, contrasting omega-3 fatty acids to saturated and trans-fatty acids. Hepatic nuclear factor 4 alpha (HNF4alpha) is a poorly understood but critical fatty acidactivated receptor that regulates lipid metabolism, thrombosis, and glucose control. By using the manipulations of lipid metabolism employed in our LPL/PPARa studies, we have identified novel mechanisms of HNF4alpha modulation and divergent responses between PPARalpha and HNF4alpha to pathways of lipid metabolism. In Aim 2, this divergence between HNF4alpha and PPARalpha will be explored. A novel but powerful mechanism for PPAR modulation would be the existence of a direct endogenous PPAR antagonist. While symmetric cleavage of beta carotene generates natural ligands for the RXR nuclear receptor, we have identified that asymmetric beta carotene cleavage produces a specific apocarotenal that directly antagonizes PPAR responses. In Aim 3, this direct antagonist will be characterized in vitro and in vivo. Together, these studies integrate biochemical, biologic, and in vivo models to better understand how endogenous modulation of PPAR activity may determine biologic responses.