豬脂締素及其受體表現調控機制與功能之探討

Abstract

脂締素（adiponectin）為一種脂肪組織所大量表現的脂肪細胞激素，adiponectin藉由膜上的受體來調節細胞對脂質與葡萄糖的代謝，在脂締素受體剔除小鼠中，指出脂締素受體一（AdipoR1）與受體二（AdipoR2），為媒介脂締素功能的重要受體。因此本論文著重在選殖豬的脂締素、AdipoR1與AdipoR2的cDNA全長。AdipoR1與AdipoR2廣泛表現於豬隻各組織中，我們發現八小時的禁食可增加AdipoR2 mRNA在皮下脂肪組織的表現，所以認為AdipoR2可能受禁食影響參與豬隻脂肪酸的氧化。由於八小時的禁食無法體現豬隻真實飢餓的生理與代謝，我們重複豬隻飢餓試驗並延長時間為二十四小時，我們發現AdipoR1與AdipoR2 mRNA皆顯著增加於肝臟與皮下脂肪組織中，長期飢餓亦會增加脂締素mRNA於臟器間脂肪組織的表現，顯示脂肪細胞結合素可能藉由增加其受體的活性，來增加身體對能量的代謝與利用。由於生理的飢餓參與了眾多內泌素與營養分的調控，這些因子可能參與對豬脂肪細胞結合素及其受體的調控，因此我們利用豬脂肪前身細胞體外培養與分化系統，發現AdipoR2 mRNA表現受胰島素抑制，而治療第二型糖尿病藥物（rosiglitazone）可增加AdipoR2 mRNA的表現，經由PI3K抑制物的添加，得知胰島素與rosiglitazone對AdipoR2 mRNA表現的調控，乃是經由PI3K訊息傳導路徑所達成，因此認為脂締素可能藉由AdipoR1維持基礎能量的代謝，而AdipoR2則扮演受內泌素與營養分調控的角色。 為更瞭解AdipoR1與AdipoR2調控與功能上的差異，我們亦產製全身型表現豬隻脂締素受體轉基因小鼠，經高飼糧油脂與蔗糖所誘導的肥胖小鼠模式中，我們發現AdipoR1轉基因小鼠體重較輕，而AdipoR2轉基因小鼠體重較胖，符合其他脂脂締素受體基因剔除小鼠的觀察外，AdipoR1轉基因小鼠亦可預防由飼糧所引起的肥胖、脂肪肝以及胰島素阻抗，並增加對葡萄糖的耐受性， 而AdipoR2轉基因小鼠則具有較弱的效果。此外，AdipoR1與AdipoR2轉基因小鼠同時減少血漿脂締素與脂肪組織中mRNA的表現量，顯示脂締素與其受體間存在負回饋調控機制。另外，AdipoR2轉基因小鼠降低與脂締素同屬C1q家族的脂肪細胞激素（C1qtnf9）的mRNA表現量，由於C1qtnf9可與脂締素形成異型多體，並活化脂締素下游AMPK訊息傳導路徑，我們推測對低分子量脂締素有較高親和力的AdipoR1，可能較不受此異型多體變化量的影響，使得AdipoR1轉基因小鼠具有較佳預防肥胖的能力。我們亦發現AdipoR1轉基因小鼠於脂肪組織中的Pck1 mRNA表現量，並不受由高飼糧油脂與蔗糖所影響，由於脂肪組織中的Pck1扮演甘油新生合成的重要角色，因此AdipoR1轉基因小鼠亦可能藉由影響體內甘油酯與甘油間的流動，來達成預防肥胖的作用。簡言之，我們清楚地定義AdipoR1在預防飼糧所引起代謝症候群的功能，這些發現將有助於開發對代謝症候群與肥胖治療的新策略。

Adiponectin is an adipocyte-derived hormone that plays important roles in regulating glucose and lipid metabolism. Adiponectin receptor 1 and 2 (AdipoR1 & AdipoR2) are the major physiological receptors to mediate the actions of adiponectin in vivo. We successfully cloned the porcine adiponectin and its receptors. The amino acid sequences predicted for the full-length cDNA of porcine adiponectin, AdipoR1, and AdipoR2 were similar to those of the human and mouse, suggesting similar functions of these genes in pigs. In young growing pigs, adiponectin mRNA was up-regulated by fasting in visceral, but not s.c., adipose tissues, whereas AdipoR1 and AdipoR2 mRNA were increased in s.c., but not visceral, adipose tissues. We also found that insulin inhibited the expression of AdipoR2 through the phosphatidylinositol 3-kinase pathway, but the AdipoR1 was not regulated by insulin. Therefore, we speculate that AdipoR1 mediates the function of adiponectin to maintain energy metabolism and AdipoR2 responses to hormonal and nutritional regulation. However, the physiological differences of these two receptors in diet-induced metabolic syndromes are still unclear. Thus, we generated porcine AdipoR1 and R2 transgenic mice and fed the mice with long-term high fat and high sucrose diet to induce obesity. We found that AdipoR1 prevented mice from diet-induced obesity, fatty liver and insulin resistance whereas AdipoR2 had minor effects. Both AdipoR1 and R2 mice fed with high fat and high sucrose diet showed negative feedback regulation of adiponectin. In addition, we found that the multimeric partner of adiponectin (C1qtnf9) in the adipose tissue was decreased when mice fed with the high fat and high sucrose diet. AdipoR2 but not AdipoR1 mice tended to downregulate the expression of C1qtnf9 in the adipose tissue. In contrast, AdipoR1 but not AdipoR2 transgenic mice have stronger effects on preventing diet-induced insulin resistance. Only AdipoR1 transgene increases the expression of Pck1, a glyceroneogenesis enzyme in the adipose tissue. AdipoR1 transgenic mice may increase Pck1 to promote glyceroneogenesis in the adipose tissue to reduce glucose intolerance. Such effect may activate the flux of glyceride-glycerol function to increase glucose tolerance in the adipose tissue. We clearly defined the function of AdipoR1 on preventing diet-induced metabolic syndromes and such findings could lead to development of new therapeutic strategy for the metabolic syndrome and obesity.