探討西方飼糧對李宋豬代謝異常與內質網壓力之影響

Abstract

現代人對於養分攝取與能量消耗間之不平衡導致肥胖，而肥胖不只是脂質過度累積於脂肪組織，過多脂質異常堆積亦發生在非脂肪組織中。肥胖與許多代謝性疾病具有關聯性，其中包含第二型糖尿病、脂肪肝與心血管疾病等。內質網的基本功能包含將蛋白質摺疊成正確之構型、鈣離子恆定與脂質生合成。氧化壓力、缺血與鈣離子失衡皆會使得過多未摺疊的蛋白質堆積於內質網內進而形成內質網壓力 (ER stress)，而內質網壓力會誘導下游未摺疊蛋白質反應 (UPR)，用以降低未摺疊蛋白質的累積，使得內質網恢復其正常功能。然而，若是延長內質網壓力將會誘發細胞凋亡反應，進而造成細胞死亡，研究顯示細胞內過多的脂肪堆積亦會造成內質網壓力。 本試驗目的為以西方飼糧誘發李宋豬產生代謝症侯群，探討代謝症候群豬隻組織內質網壓力之變化。本試驗以9月齡李宋豬為試驗模式，逢機分為2組，分別餵飼一般飼糧 (C)或西方飼糧 (W)，為期5個月。結果顯示，W組之體重顯著高於C組 (P < 0.05)。W組之背脂厚度與全身脂肪率顯著高於C組 (P < 0.05)。血液中之血糖、三酸甘油酯、LDL、HDL與總膽固醇濃度皆以W組顯著較高 (P < 0.05)。血中的胰島素含量亦是以W組有顯著較高之數值 (P < 0.05)。W組於試驗末期已產生葡萄糖不耐之徵狀，而C組則無。不論收縮壓或是舒張壓也都是以W組有較高之表現 (P < 0.05)。蛋白質陣列分析的結果指出W組具有較高的第二型糖尿病、動脈粥狀硬化與發炎反應相關的蛋白質表現 (P < 0.05)。 為了釐清內質網壓力是否在西方飼糧所造成的代謝異常中扮演著關鍵要角，我們分析了肝臟、肌肉與心臟中，內質網壓力相關的標誌基因表現 (GRP78、XBP1、IRE1α、ATF6與CHOP)。結果顯示，不論是肝門靜脈近端或是肝門靜脈遠端，W組中之GRP78、XBP1與CHOP於皆顯著高於C組 (P < 0.05)。且於內質網壓力所調控下游代謝相關基因表現中，W組之SREBP1與FAS皆有較高之表現量 (P < 0.1)。肌肉組織中C組XBP1與IRE1α有較高之基因表現量 (P < 0.05），且脂質代謝相關基因DGAT也是以C組有較高表現量 (P < 0.1）。但W組肌肉中的CHOP表現量顯著高於C組 (P < 0.05）。在心臟的基因表現中，不論是內質網壓力相關之基因表現或是內質網壓力下游相關之代謝基因表現，W組與C組皆無差異。 綜觀上述，長期餵飼李宋豬西方飼糧，確實增加豬隻體重與體脂肪堆積，並造成高血糖、高血脂、高胰島素血症、高血壓與葡萄糖不耐之代謝症候群反應。西方飼糧之處理將造成豬隻肝臟產生內質網壓力。據此，我們推測內質網壓力可能於代謝症候群中扮演著重要的角色，然而，相關的機制仍待更多的實驗加以釐清。

Nutrient uptake and energy expenditure unbalance which causes excess lipid accumulation and abnormal energy metabolism result in the prevalence of obesity in the western world. Obesity is associated with numerous metabolic diseases, including type II diabetes (T2DM), fatty liver, and cardiovascular diseases. Endoplasmic reticulum (ER) is involved in protein folding, calcium homeostasis and lipid biosynthesis. Oxidative stress, ischemia and disturbance of calcium homeostasis that interfere ER function lead to accumulation of unfolded proteins so-called ER stress that tend to hamper ER functions. When the unfolded protein response (UPR) is accumulated, a compensatory regulation is then initiated to target the unfold proteins for degradation through ubiquitin associated mechanisms to reduce the ER stress. However, if ER stress is prolonged, it tends to trigger apoptosis signaling to lead to cell death. The objective of this study was to establish using Lee-Sung pigs as model of Western diet-induced metabolic syndrome and examine the change of ER stress during the development of the metabolic syndrome. 9-month-old Lee-Sung pigs were randomly assigned to 2 groups: control diet (C) and Western diet (W), in a 5-month experimental period. Results showed that W pigs were heavier than C pigs (P < 0.05). W group had higher back fat thickness and body fat ratio than C group (P < 0.05). Compared with C group, W group also exhibited an elevated level of plasma glucose, triglyceride, LDL, HDL, total cholesterol and insulin (P < 0.05). Furthermore, W pigs also displayed glucose intolerance at the end of experiment. Both systolic pressure and diastolic pressure were higher in W group than those in C group (P < 0.05). In obesity antibody array analysis, W pigs had higher T2DM-, atherosclerosis- and inflammation-related protein expressions. (P < 0.05). In order to elucidate whether Western diet caused the metabolic dysfunctions through ER stress-dependent mechanisms, we analyzed ER stress-related gene (GRP78, XBP1, IRE1α, ATF6 and CHOP) in the liver, muscle and heart. The transcript abundance of GRP78, XBP1 and CHOP were higher in both periportal and non-periportal zone of liver in W pigs than those in C group (P < 0.05). W group also had increased of SREBP1 and FAS expression in non-periportal zone and periportal zone (P < 0.1). In the muscle, transcripts of ER stress-related gene including XBP1 and IRE1α were lower in W group (P < 0.05). The metabolic related gene expressions downstream ER stress including DGAT (diacylgycerol acyltransferase) were lower in W group as well (P < 0.1), but CHOP (C/EBP homologous protein) was higher in W group compared to C group. In the heart, neither ER stress related genes nor downstream metabolic related genes were regulated by Western diet. In conclusion, long-term feeding of Western diet in Lee-Sung miniature pigs not only increased body weight and fat accumulation, but also induced metabolic syndromes including hyperglycemia, hyperlipidemia, hyperinsulinemia, hypertension and glucose intolerance, and moreover hepatic ER stress. The results suggest that ER stress may mediate a fundamental machinery of metabolic syndrome induced by Western diet. However, the related mechanisms need further studies.