Mechanisms of p42/44MAPK-induced LDL receptor expression

在所有已知的促进冠状动脉疾病的危险因素中，高血清低密度脂蛋白(LDL)水平是最重要的。肝脏低密度脂蛋白受体表达的调节是饮食和体液因素改变血浆低密度脂蛋白水平的潜在机制，上调低密度脂蛋白受体表达是目前治疗高胆固醇血症的基础。甾醇对低密度脂蛋白受体转录的负调控已被广泛研究，但其诱导和控制关键核因子(S)活性的信号级联反应的分子机制尚不清楚。最近，我们首次提供证据表明，p42/44丝裂原活化蛋白激酶(MAPK)的特异性激活不仅是必需的，而且足以充分诱导低密度脂蛋白受体的表达。我们最近的观察支持在p42/44MAPK诱导的低密度脂蛋白受体转录中需要CREB结合蛋白(CBP)，这是特异性目标1的基础，拟议的研究将把CBP乙酰转移酶活性与固醇反应元件结合蛋白(SREBPs)的修饰和/或启动子区域的染色质重塑联系起来。CBP-SREBP蛋白质-蛋白质相互作用在诱导过程中对反式激活和染色质结构的影响将通过p42/44MAPK反应的哺乳动物双杂交系统和体内足迹技术来检测。具体目标2将研究白细胞介素1β和肝细胞生长因子诱导的低密度脂蛋白受体的表达，以检测p42/44MAPK如何参与通过类固醇敏感和非类固醇非依赖性机制诱导。具体目标3将建立应激激活的p38MAPKα亚型与低密度脂蛋白受体表达之间的负相关关系，然后通过抑制低密度脂蛋白受体的表达来研究p38MAPK激活在应激诱导的高胆固醇血症中的作用。这一目的是基于我们观察到的特异性抑制p38MAPKα亚型通过抑制p42/44MAPK诱导低密度脂蛋白受体的表达。最后，根据MAPKs在肝细胞中的关键作用，具体目标4将研究p42/44MAPK和p38MAPK级联信号通路在调节低密度脂蛋白和清道夫受体表达方面的作用，而低密度脂蛋白和清道夫受体是巨噬细胞中脂质积累和转化为泡沫细胞的关键决定因素。明确调控诱导过程的分子机制和信号通路将有助于理解受体通路被扰乱导致高胆固醇血症的病理状态。这一知识可以被用来开发改进的高胆固醇血症疗法，并减少泡沫细胞的形成。

Of all known risk factors promoting coronary artery disease, a high serum low density lipoprotein (LDL) level is the most important. The regulation of hepatic LDL receptor expression is a potential mechanism by which dietary and humoral factors alter plasma LDL levels, and upregulation of LDL receptor expression is the basis for current treatment of hypercholesterolemia. The negative regulation of LDL receptor transcription by sterols has been extensively delineated; however, the molecular mechanisms of induction and the signaling cascades controlling activity of critical nuclear factor(s) are not known. Recently, we provided the first evidence that specific activation of the p42/44mitogen-activated protein kinase (MAPK) is not only required but is sufficient to fully induce LDL receptor expression. Our recent observations supporting the requirement of CREB-binding protein (CBP) in p42/44MAPK-induced LDL receptor transcription are the basis of Specific Aim 1, and the studies proposed will link CBP acetyltransferase activity with modification of sterol responsive element binding proteins (SREBPs) and/or chromatin remodeling in the promoter region. Effects of CBP-SREBP protein-protein interactions on transactivation and on chromatin structures during the induction process will be examined by using a p42/44MAPK-responsive mammalian two-hybrid system and in vivo footprinting techniques. The Specific Aim 2 will study interleukin-1beta- and hepatocyte growth factor-induced LDL receptor expression to examine how p42/44MAPK participates during induction by sterol-sensitive and sterol-independent mechanisms. The Specific Aim 3 will establish a negative relationship between stress-activated p38MAPK alpha- isoform and LDL receptor expression and then examine the role of p38MAPK activation in stress-induced hypercholesterolemia through suppression of LDL receptor expression. This aim is based on our observation that specific inhibition of p38MAPK alpha-isoform induces LDL receptor expression via suppression of p42/44MAPK. Finally, in light of the crucial roles of MAPKs in hepatic cells, the Specific Aim 4 will examine the roles of p42/44MAPK and p38MAPK cascades in regulating expression of LDL and scavenger receptors that are a critical determinant of lipid accumulation in the macrophages and their conversion to foam cells. Defining the molecular mechanisms and the signaling pathways regulating the induction process will help in understanding the pathologic states under which the receptor pathways are perturbed, resulting in hypercholesterolemia. This knowledge could be exploited to develop improved hypercholesterolemia therapies and to reduce foam cell formation.