Paradoxical regulation of IGF2 in promoting lipid metabolism in adipose tissues

Background and aims: Metabolic diseases, including diabetes, obesity, and dyslipidemia, are significant public health concerns worldwide. The insulin-like growth factor 2 (IGF2) gene have been implicated in various physiological processes, but its specific role in lipid metabolism remains unclear. We aim to elucidate the role of IGF2 in regulating lipid metabolism in adipose tissues and its association with metabolic syndrome (MetS). Methods: The research employed a multidisciplinary approach to investigate the role of IGF2 in lipid metabolism. We investigated the correlation between genetic variations within the IGF2 gene and metabolic parameters. We conduct a cross-sectional human study to evaluate relationships between varying IGF2 serum concentrations and the incidence of MetS. Additionally, manipulation of IGF2 expression levels in mouse and cell models via overexpression and knockdown to assess impacts on lipid metabolism in adipose tissue, specifically lipid accumulation, insulin resistance, and the balance between lipogenesis and lipolysis. Furthermore, the study employs metabolomics techniques to scrutinize the broader metabolic profiles in adipose tissues in response to IGF2 modulation. Results: Multiple SNP loci in the IGF2 gene were significantly associated with BMI, HbA1c, and diabetes. Insufficient or excessive expression of IGF2 was identified as a risk factor for hyperlipidemia, low HDL-c, and central obesity in MetS. We observed that IGF2 was mainly concentrated in adipose tissues and adipocytes. Enhanced IGF2 expression stimulated adipogenesis and lipid accumulation, whereas IGF2 knockdown hindered lipolysis, exacerbating ectopic lipid accumulation and insulin resistance. There is a substantial enlargement of pancreatic tissue and heightened insulin generation in mice deficient in IGF2. Activation of the PI3K/Akt pathway through IGF1R in IGF2 excess or INSR in conditions of IGF2 scarcity, along with inhibition of AMPK, implies a common downstream process that favors lipid accumulation and metabolic reprogramming in adipocytes. Conclusions: Our study demonstrated that upregulation of IGF2 enhanced adipogenesis and lipogenesis, while knockdown of IGF2 inhibited lipolysis, which resulting in accelerating lipid accumulation through PI3K/Akt-AMPK pathway. Global transcriptome profiling was performed by RNA-Seq using epididymal white adipose tissue of Ad-IGF2 (n=2, 2 replicate sequencing) or Ad-GFP mice (n=3, 2 replicate sequencing) and IGF2-RNAi (n=2, 2 replicate sequencing) or NC-RNAi mice (n=3, 2 replicate sequencing). RNA was extracted, sequenced, and analyzed by a custom service provided by Novogene using an Illumina NovaSeq 6000. The bioinformatics analysis was conducted utilizing the OmicStudio tools available at the following website: https://www.omicstudio.cn/tool. The differential expression cut-off threshold was determined relied on |log2fold change (log2FC) | > 1 and p-value < 0.05.

研究背景与目的：代谢性疾病包括糖尿病、肥胖症及血脂异常，是全球范围内亟待应对的重大公共卫生问题。胰岛素样生长因子2（insulin-like growth factor 2, IGF2）基因已被证实参与多种生理过程，但其在脂质代谢中的具体作用仍未明确。本研究旨在阐明IGF2在脂肪组织脂质代谢调控中的作用，以及其与代谢综合征（metabolic syndrome, MetS）的关联。

研究方法：本研究采用多学科策略探究IGF2在脂质代谢中的功能。首先分析IGF2基因内遗传变异与代谢参数的相关性；开展横断面人体研究，评估不同IGF2血清浓度与代谢综合征发病风险的关联；此外，通过过表达与敲低技术在小鼠及细胞模型中调控IGF2的表达水平，以考察其对脂肪组织脂质代谢的影响，具体包括脂质蓄积、胰岛素抵抗以及脂肪生成与脂解的动态平衡；本研究还利用代谢组学技术，系统分析脂肪组织在IGF2调控下的整体代谢谱变化。

研究结果：IGF2基因上的多个单核苷酸多态性（single nucleotide polymorphism, SNP）位点与体重指数（body mass index, BMI）、糖化血红蛋白A1c（glycated hemoglobin A1c, HbA1c）及糖尿病显著相关。IGF2表达不足或过量均被确定为代谢综合征患者伴发高脂血症、高密度脂蛋白胆固醇（high-density lipoprotein cholesterol, HDL-c）降低及中心性肥胖的危险因素。研究发现IGF2主要富集于脂肪组织及脂肪细胞中：上调IGF2表达可促进脂肪生成与脂质蓄积，而敲低IGF2则会抑制脂解，加重异位脂质蓄积与胰岛素抵抗。在IGF2缺陷小鼠中，可见胰腺组织显著增大且胰岛素生成水平升高。在IGF2过量时，可通过胰岛素样生长因子1受体（insulin-like growth factor 1 receptor, IGF1R）激活PI3K/Akt通路；而在IGF2缺乏时，则通过胰岛素受体（insulin receptor, INSR）激活该通路，同时伴随AMPK通路的抑制，这提示存在一条共同的下游通路，可促进脂肪细胞的脂质蓄积与代谢重编程。

研究结论：本研究证实，上调IGF2可增强脂肪生成与脂质合成，而敲低IGF2则会抑制脂解，最终通过PI3K/Akt-AMPK通路加速脂质蓄积。本研究对Ad-IGF2组（n=2，每组进行2次重复测序）、Ad-GFP组（n=3，每组进行2次重复测序）、IGF2-RNAi组（n=2，每组进行2次重复测序）及NC-RNAi组（n=3，每组进行2次重复测序）小鼠的附睾白色脂肪组织开展RNA测序（RNA-Seq），并进行全转录组分析。总RNA提取、测序及数据分析由诺禾致源（Novogene）提供的定制化服务完成，测序平台采用Illumina NovaSeq 6000。生物信息学分析使用OmicStudio工具完成，工具网址为https://www.omicstudio.cn/tool。差异表达基因的筛选阈值设定为|log2倍变化（log2FC）|>1且p值<0.05。