探討酮體誘導米色脂肪細胞重塑的複雜性及維生素D訊號扮演的潛在角色

Abstract

根據世界衛生組織統計，自2022年起全球約有10億人口受肥胖困擾，而肥胖相關的代謝性疾病已成為重要公共健康問題。肥胖是由於脂肪細胞的過度積聚所致，其中白色脂肪細胞在特定刺激下能褐化為具有較高代謝率的米色脂肪細胞（beige adipocyte），進而改善肥胖及代謝疾病。哺乳期是米色脂肪細胞生成關鍵時期，先前實驗室研究發現在哺乳期給予小鼠酮體前驅物1,3-butanediol（1,3BD）誘導增強哺乳期生酮作用，能促進小鼠成年後米色脂肪細胞生成。然而，增強哺乳期生酮作用是否影響脂肪前驅細胞之組成而導致此現象尚不清楚。本研究第一部分使用單細胞RNA定序（single cell RNA sequencing）分析小鼠iWAT stroma vascular fraction cells（SVFs）之組成。透過R語言分析，我們定義iWAT SVFs共由八種細胞群構成，其中主要四群為相似於幹細胞的adipocyte stromal cells (ASC) 0、前脂肪細胞ASC1及具增生能力的前脂肪細胞ASC2a和ASC2b。我們發現哺乳期增強生酮作用使ASC1細胞群比例增加、ASC0細胞群比例減少，存在ASC0細胞群轉變為ASC1細胞群的可能性，且ASC1細胞群高度表現米色脂肪前驅細胞標記Cd81。因此，ASC1細胞群比例增加可能是哺乳期增強生酮作用促進米色脂肪細胞生成的重要因素之一。此外，轉錄因子vitamin D receptor（VDR）也在ASC1細胞群高度表現，然而活化維生素D訊號途徑是否促進米色脂肪細胞生成仍不清楚。故本研究第二部分擬探討維生素D及活化VDR訊號途徑於米色脂肪生成扮演的角色，同時也想探討維生素D與酮體是否協同促進米色脂肪細胞生成。我們由in vitro模式發現維生素D（calcitriol）在分化期間促進3T3-L1米色脂肪細胞之產熱基因Ucp1表現。且哺乳期長期補充維生素D3（cholecalciferol）能顯著促進小鼠成年後受冷誘導的米色脂肪細胞生成，哺乳期短時間補充維生素D3亦趨勢性促進一般仔鼠（Ch）或肥胖子代（Ob）於P9時的米色脂肪細胞生成。然而在目前實驗設計下未觀察到維生素D與酮體協同促進的效果。綜合以上，本研究發現哺乳期增強生酮作用改變ASCs之組成，進而促進米色脂肪細胞生成，且其中維生素D及VDR訊號途徑可能扮演重要的角色。

Obesity is characterized by the expansion of white adipose tissue with excess accumulation of lipids in white adipocytes. White adipocytes can transition into beige adipocytes under specific stimuli, and beige adipocytes exhibit a higher metabolic rate, offering potential benefits in reducing body weight and improving metabolic health. The preweaning period is crucial for beige adipogenesis. Our previous research demonstrated that administering 1,3-butanediol (1,3BD), a ketone precursor, to preweaning mice enhances beige adipogenesis. However, the impact of 1,3BD on the composition of adipocyte precursor cells remains unclear. In this study, we employed single-cell RNA sequencing to analyze stromal vascular fraction cells (SVFs) isolated from the inguinal white adipose tissue (iWAT) of mice. Using R studio analysis, we identified eight distinct cell clusters based on known cell markers, with four major clusters including stem cell-like adipocyte stromal cells (ASC) 0, preadipocytes ASC1, and proliferative preadipocytes ASC2a and ASC2b. We found that postnatal ketogenesis increases the proportion of ASC1 while decreasing ASC0, suggesting a possible recruitment of ASC1 from ASC0. In addition, the beige progenitor cell marker Cd81 was highly expressed in ASC1, indicating the important role of ASC1 in beige adipogenesis during ketogenesis. Furthermore, the transcription factor vitamin D receptor (VDR) was also highly expressed in ASC1, suggesting a possible involvement of the VDR signaling pathway in beige adipogenesis. Next, we explored the role of vitamin D in beige adipogenesis and determined whether vitamin D and 1,3BD synergistically improve beige adipogenesis. We demonstrated that calcitriol promotes Ucp1 expression in the 3T3-L1 cell line during induction of adipocyte differentiation. Our in vivo experiments further revealed that long-term postnatal vitamin D3 (cholecalciferol) supplementation enhances beige adipogenesis in adult mice. Short-term postnatal vitamin D3 supplementation tends to enhance beige adipogenesis in offspring from both normal diet-fed (Ln) and diet-induced obesity (Ob) dams. However, no significant synergistic effect was observed when both vitamin D and 1,3BD were co-administered at the tested doses. In summary, enhanced ketogenesis during lactation alters the composition of ASCs, promoting beige adipogenesis. Moreover, vitamin D and VDR signaling pathway may play critical roles in regulating this process, offering insights into potential therapeutic strategies for obesity and metabolic disorders.