過度表現脂締素受體一型抑制小鼠棕色與米色脂肪之活性

Abstract

成人與小鼠具顯而易見之棕色脂肪，且經由冷刺激下，某些白色脂肪部位可發現具類似棕色脂肪之細胞，此過程稱為白色脂肪棕色化 (white fat browning) 或該棕色化之白色脂肪細胞被稱作米色脂肪細胞 (beige adipocyte) 。活化棕色脂肪或增加白色脂肪棕色化/米色脂肪可增加產熱以促進能量消耗，具抵抗肥胖或治療糖尿病之潛能。脂締素 (adiponectin) 為脂肪組織分泌之細胞激素，可調控全身代謝。於高脂飼糧下，脂締素可藉由增加白色脂肪堆積以減少游離脂肪酸於其他器官之傷害與堆積，進而改善肥胖造成之胰島素阻抗。然而，脂締素與其受體 (adiponectin receptors) 對冷刺激下棕色脂肪之活性與白色脂肪棕色化之功能仍未全然了解。首先，本實驗先改良脂肪幹細胞分離之流程，增加產量與效率，並且建立條件可以分化出白色與米色脂肪細胞以進行後續實驗。另外，此研究並建立FVB小鼠品系下，冷刺激對該FVB品系小鼠，低溫處理之強度及時間長度。接著利用此條件探討野生型 (wild-type) 小鼠與基因轉殖小鼠大量表現脂締素受體一型 (adiponectin receptor 1; AdipoR1) ，於冷刺激下棕色脂肪與白色脂肪棕色化之活性與功能。本研究利用3D正子掃描 (positron emission tomography/computed tomography, PET/CT scanning) 發現，AdipoR1小鼠於冷刺激下葡萄糖之攝入較WT小鼠少，棕色脂肪與米色脂肪之脂肪細胞大小較大，並有小鼠全身體表溫度或棕色脂肪部位表面溫度較低之情形，此現象皆顯示AdipoR1小鼠產熱功能可能較低。另外，棕色或米色脂肪組織中生熱蛋白基因 (uncoupling protein 1, Ucp1) 與棕色脂肪標記 (brown adipocyte marker) 基因、棕色脂肪細胞粒線體基因標記、脂肪分解基因、脂肪氧化基因於棕色或米色脂肪組織中也較WT小鼠低。此外，AdipoR1小鼠所分離出白色脂肪幹細胞分化之米色脂肪細胞，其產熱基因Ucp1與棕色脂肪標記基因、棕色脂肪細胞粒線體基因標記基因，皆較WT小鼠之脂肪幹細胞分化之米色脂肪低。因此，本研究發現AdipoR1減少冷刺激誘導下棕色脂肪與米色脂肪之生熱功能 (cold-induced thermogenesis) ，調節脂締素或其受體功能或調控其下游途徑可能為日後肥胖與相關疾病治療之契機。

Adult humans and mice possess significant classical brown adipose tissues (BAT) and, upon cold-induction, acquire brown-like adipocytes in certain depots of white adipose tissues (WAT), known as beige adipose tissues or WAT browning/beiging. Activation of thermogenic classical BAT or WAT beiging to generate heat limits diet-induced obesity or type-2 diabetes in mice. Adiponectin is a beneficial adipokine resisting diabetes, and causing “healthy obese” by increasing WAT expansion to limit lipotoxicity in other metabolic tissues during high-fat feeding. However, the role of its receptors and signaling, especially adiponectin receptor 1 (AdipoR1), on cold-induced thermogenesis in vivo in BAT and in WAT beiging is still elusive. First, we streamlined the isolation process for purifying high quality adipose stem cells that are able to differentiate into white or beige adipocytes from different species such as human, pig or mice. Next, we established a cold-induction procedure in transgenic mice over-expressing AdipoR1 and applied a live 3-D [18F] fluorodeoxyglucose-PET/CT (18F-FDG PET/CT) scanning to measure BAT activity by determining glucose uptake in cold-acclimated transgenic mice. Results showed that cold-acclimated mice over-expressing AdipoR1 had diminished cold-induced glucose uptake, enlarged adipocyte size in BAT and in browned WAT, and reduced surface BAT/body temperature in vivo. Furthermore, decreased gene expression related to thermogenic Ucp1, BAT-specific markers, BAT-enriched mitochondrial markers, lipolysis and fatty acid oxidation and increased expression of whitening genes in BAT or in browned subcutaneous inguinal WAT of AdipoR1 mice are congruent with results of PET/CT scanning and surface body temperature in vivo. Moreover, differentiated brown-like beige adipocytes isolated from pre-adipocytes in subcutaneous WAT of transgenic AdipoR1 mice also had similar effects of lowered expression of thermogenic Ucp1, BAT selective markers, and BAT mitochondrial markers. Therefore, this study combines in vitro and in vivo results with live 3-D scanning and reveals one of the many facets of the adiponectin receptors in regulating energy homeostasis, especially in the involvement of cold-induced thermogenesis.