Overexpression of Akt1 Enhances Adipogenesis and Leads to Lipoma Formation in Zebrafish

BackgroundObesity is a complex, multifactorial disorder influenced by the interaction of genetic, epigenetic, and environmental factors. Obesity increases the risk of contracting many chronic diseases or metabolic syndrome. Researchers have established several mammalian models of obesity to study its underlying mechanism. However, a lower vertebrate model for conveniently performing drug screening against obesity remains elusive. The specific aim of this study was to create a zebrafish obesity model by over expressing the insulin signaling hub of the Akt1 gene. Methodology/Principal FindingsSkin oncogenic transformation screening shows that a stable zebrafish transgenic of Tg(krt4Hsa.myrAkt1)cy18 displays severely obese phenotypes at the adult stage. In Tg(krt4:Hsa.myrAkt1)cy18, the expression of exogenous human constitutively active Akt1 (myrAkt1) can activate endogenous downstream targets of mTOR, GSK-3α/β, and 70S6K. During the embryonic to larval transitory phase, the specific over expression of myrAkt1 in skin can promote hypertrophic and hyperplastic growth. From 21 hour post-fertilization (hpf) onwards, myrAkt1 transgene was ectopically expressed in several mesenchymal derived tissues. This may be the result of the integration position effect. Tg(krt4:Hsa.myrAkt1)cy18 caused a rapid increase of body weight, hyperplastic growth of adipocytes, abnormal accumulation of fat tissues, and blood glucose intolerance at the adult stage. Real-time RT-PCR analysis showed the majority of key genes on regulating adipogenesis, adipocytokine, and inflammation are highly upregulated in Tg(krt4:Hsa.myrAkt1)cy18. In contrast, the myogenesis- and skeletogenesis-related gene transcripts are significantly downregulated in Tg(krt4:Hsa.myrAkt1)cy18, suggesting that excess adipocyte differentiation occurs at the expense of other mesenchymal derived tissues. Conclusion/SignificanceCollectively, the findings of this study provide direct evidence that Akt1 signaling plays an important role in balancing normal levels of fat tissue in vivo. The obese zebrafish examined in this study could be a new powerful model to screen novel drugs for the treatment of human obesity.背景：肥胖是一种复杂的多因素失调症，受遗传、表观遗传与环境因素的相互作用影响。肥胖会提升罹患多种慢性疾病或代谢综合征的风险。研究者已建立多种肥胖哺乳动物模型，用以探究其潜在发病机制，但目前仍缺乏可便捷用于抗肥胖药物筛选的低等脊椎动物模型。本研究的核心目标是通过过表达胰岛素信号转导关键枢纽基因Akt1，构建斑马鱼肥胖模型。 方法与主要结果：皮肤致癌转化筛选实验表明，稳定转基因品系Tg(krt4:Hsa.myrAkt1)cy18的成体斑马鱼呈现严重肥胖表型。在该转基因品系中，外源性人组成型激活型Akt1（myrAkt1）的表达可激活mTOR、GSK-3α/β及70S6K的内源性下游靶标。在胚胎向幼体的过渡阶段，皮肤中特异性过表达myrAkt1可促进细胞肥大与细胞增生。自受精后21小时（hpf）起，myrAkt1转基因在多种间充质来源组织中出现异位表达，这一现象可能由整合位点效应导致。成体Tg(krt4:Hsa.myrAkt1)cy18斑马鱼出现体重快速增加、脂肪细胞增生、脂肪组织异常蓄积及血糖耐量异常。实时逆转录PCR（real-time RT-PCR）分析显示，该转基因品系中多数调控脂肪生成、脂肪细胞因子及炎症反应的关键基因均显著上调。与之相反，成肌与成骨相关的基因转录本在Tg(krt4:Hsa.myrAkt1)cy18中显著下调，这表明过量的脂肪细胞分化以其他间充质来源组织的发育为代价。 结论与意义：综上，本研究结果直接证明Akt1信号通路在维持体内脂肪组织的正常水平中发挥关键作用。本研究构建的肥胖斑马鱼模型可作为新型高效模型，用于筛选治疗人类肥胖的新型候选药物。