肥胖為現代常見之慢性疾病、且與代謝異常密切相關，其特徵為脂肪組織過度擴張，包含脂肪細胞體積增大（hypertrophy）與數量增加（hyperplasia），並常伴隨胰島素阻抗與代謝異常。胰島素（insulin）為調控脂肪細胞生長之重要賀爾蒙之一，然而其相關分子機制仍有待進一步釐清。 MicroRNA（miRNA）為一類小型非編碼 RNA，可於轉錄後層級調控基因表現。其中 miR-let-7a 已被證實具有抑制細胞增生之功能，其主要標的基因之一為 HMGA2，與細胞生長與發育密切相關。過去本實驗室研究已證實，綠茶主要活性成分Epigallocatechin gallate（EGCG）可透過調控 miR-let-7a/HMGA2 訊息傳遞路徑抑制白色前脂肪細胞生長。然而，在胰島素刺激條件下，EGCG 是否仍可透過此訊息路徑發揮調控作用，尚未被釐清。 因此，本研究以 3T3-L1 白色前脂肪細胞為模型，探討 EGCG 對胰島素刺激細胞 生長之影響及其分子機制。研究結果顯示，胰島素可顯著促進細胞增生，並抑制 miRlet-7a 表現，同時提升 HMGA2 表現，且具時間與劑量依賴性。EGCG 預處理可有效逆轉上述現象，恢復 miR-let-7a 表現並降低 HMGA2 表現，進而抑制細胞生長。此外，透過 miR-let-7a mimic 與 inhibitor 之功能性分析進一步證實，miR-let-7a/HMGA2 訊息傳遞路徑為 EGCG 抑制胰島素誘導細胞生長之關鍵機制。 綜上所述，本研究延伸既有研究成果，證實 EGCG 即使在胰島素刺激條件下，仍 可透過調控 miR-let-7a/HMGA2 訊息傳遞路徑抑制 3T3-L1 白色前脂肪細胞生長，提供肥胖與代謝疾病調控之重要分子基礎。;Obesity is a prevalent metabolic disorder characterized by excessive expansion of adipose tissue， involving both adipocyte hypertrophy and hyperplasia. Development of obesity and fat cell functions are often associated with insulin resistance and metabolic dysfunction， and can be regulated by insulin， microRNAs (miRNAs)， and green tea epigallocatechin gallate (EGCG). Previous studies from our laboratory have shown that EGCG suppressed insulin stimulation of white preadipocyte growth and that EGCG inhibited white preadipocyte growth through the pathways of miR-let-7a/high mobility group AT-hook 2 (HMGA2). Thus， the hypothesis arose in the thesis that EGCG may inhibit insulin stimulation of white preadipocyte growth through modulations of the miR-let-7a/HMGA2 signaling pathways. To test the hypothesis， we utilized 3T3-L1 white preadipocytes as a model to investigate the effects of EGCG on insulin-induced cell growth and its underlying molecular mechanisms involved in the miR-let-7a/HMGA2 signaling cascades. We found that insulin significantly promoted cell proliferation， suppressed miR-let-7a expression， and increased HMGA2 expression in a time- and dose-dependent manner. In the absence of EGCG， overexpression of let-7a by transfection of let-7a mimic could antagonize these effects of insulin by restoring miR-let-7a expression and reducing HMGA2 levels， thereby inhibiting cell growth， as evidenced by changes in both cell number and cell viability. In addition， pretreatment with EGCG reversed these effects of insulin on let-7a and HMGA2 gene expression and preadipocyte growth. However， pretreatment with the let-7a inhibitor could block the antagonistic effects of EGCG on insulin-induced alterations in the number of preadipocytes and levels of let-7a and HMGA2 gene expressions. In conclusions， the results from this study demonstrates that EGCG suppresses insulin-stimulated growth of 3T3-L1 white preadipocytes through modulations of the miR-let-7a/HMGA2 signaling pathways， as well as may provide important molecular insights into the interactive relationships among insulin， green tea EGCG， and miR molecules to regulate fat cell-associated obesity and metabolic disorders.
