Haptoglobin 在動脈硬化中的功能性角色

Abstract

平滑肌Haptoglobin (Hp) 為強抗氧化及抗發炎分子。人類Hp 主要分為三種表現型: Hp 1-1、2-1及 2-2。臺灣地區Hp 表現型分佈為8.2%、43%、48.5%。Hp 主要於肝細胞合成，且為一個急性期蛋白，當遭受發炎反應時。氧化壓力與發炎反應皆參與於動脈硬化過程中。本實驗室首先發現有大量Hp及低密度脂蛋白 (LDL)堆積於人類冠狀動脈硬化血管中病變區內同一位置。探討LDL 於平滑肌細胞 (SMC)及肝細胞 (HepG2)中Hp 表現上扮演之角色。利用細胞免疫染色發現經LDL 刺激後SMC細胞將大量分泌Hp蛋白。LDL在不同劑量下有效增加SMC細胞內Hp基因表現或分泌其蛋白質濃度。具有意義地氧化低密度脂蛋白(ox-LDL)比 LDL 更有效刺激Hp mRNA表現。在HepG2 中Hp基因表現或分泌其蛋白質皆與LDL濃度具正相關。在免疫分析發現經各種脂蛋白刺激過後HepG2細胞Hp基因表現或分泌其蛋白質皆與脂蛋白濃度具正相關。HepG2之表現型為Hp 2-1會表現出α1β及α2β基因及蛋白質。 首次發現當HepG2內Hp基因表現量增高時，α1β mRNA較α2β mRNA 先行表現。於LDL 刺激後Hp 表現量隨之增高，證實Hp於動脈硬化形成過程中扮演抗動脈粥樣硬化 (anti-atherogenic)之角色。

Haptoglobin (Hp) is a potent antioxidant and an anti-inflammatory molecule being classified as three phenotypes: Hp 1-1, 2-1 and 2-2. They distribute as 8.2%, 43%, and 48.5%, respectively, among the population in Taiwan. Hp is primarily synthesized in the liver as one of the acute-phase proteins response to inflammation. Oxidative stress and inflammatory process are well-known to be involved in the formation of atherosclerosis. Initially in the present study, we found the colocalization of Hp and low density lipoprotein (LDL) in some of the areas of the coronary atherosclerotic lesions. We further investigated the possible roles of LDL in the biosynthesis of Hp using human smooth muscle cells (SMC) and HepG2. By immunostaining, we showed a drastic Hp expression in SMC upon the treating of LDL. The level of Hp mRNA was also increased by LDL with a dose-dependent manner. It is of interest that the oxidatively modified LDL could stimulate more expression of Hp mRNA than did native LDL. A similar increase was found in LDL-treated HepG2 cells. Consequently, a striking level of Hp in HepG2 was secreted and increased in the presence of LDL as determined by immunoassays. On the molecular level, there are two genes in HepG2, namely α1β and α2β that are responsible for the assembly Hp. Most interestingly, we found that rate of α1β mRNA synthesis was significantly greater than that of α2β. Taking together, the elevated expression level induced by the LDL suggests that Hp may play an antiatherogeneic role during the formation atherosclerosis.