Área de Endocrinología Molecular y Celular, Instituto de Investigación Sanitaria de Santiago, Complejo Hospitalario Universitario de Santiago, Servicio Gallego de Salud, Santiago de Compostela, Spain.
Int J Biochem Cell Biol. 2013 Jul;45(7):1281-92. doi: 10.1016/j.biocel.2013.03.014. Epub 2013 Apr 1.
β-Arrestins were identified as scaffold-proteins that have the capacity to desensitize G protein-coupled receptors. However, it has been found that β-arrestins activate signaling pathways independent of G protein activation. The diversity of these signaling pathways has also been recognized for receptor tyrosine kinase. The aim of the present study was to validate the β-arrestin-dependent signaling mechanism(s) responsible for regulation of adipogenesis. Two signal models were selected, ghrelin and insulin, based on its β-arrestin-associated Akt activity. Herein, we found that β-arrestin 1 and 2 were essential molecules for adipocyte differentiation. More specifically, the role of these scaffolding proteins was demonstrated by depletion of β-arrestin 1 and 2 during ghrelin-induced adipogenesis in 3T3-L1 cells, which decreased the adipocyte differentiation and the expression levels of master regulators of early, the CCAAT/enhancer-binding protein β (C/EBPβ) and the CCAAT/enhancer-binding protein δ (C/EBPδ), and terminal, the peroxisome proliferator-activated receptor (PPARγ) and the CCAAT/enhancer-binding protein α (C/EBPα), adipogenesis. Accordingly ghrelin-induced Akt activity and its downstream targets, the mammalian target of rapamycin complex 1 (mTORC1) and the ribosomal protein S6 kinase beta-1 (S6K1), were inhibited by β-arrestin 1 and 2 siRNAs. By contrast, assays performed during insulin-activated adipogenesis showed an intensifying effect on the adipocyte differentiation as well as on the expression of C/EBPβ, C/EBPδ, PPARγ and C/EBPα. The increase in insulin-induced adipogenesis by β-arrestin knock-down was concomitant to a decrease in the insulin receptor susbtrate-1 (IRS-1) serine phosphorylation, proving the loss of the negative feedback loop on IRS-1/phosphoinositide 3-kinase (PI3K)/Akt. Therefore, β-arrestins control the extent and intensity of the lipogenic and adipogenic factors associated to Akt signaling, although the mechanistic and functional principles that underlie the connection between signaling and β-arrestins are specifically associated to each receptor type.
β-arrestins 被鉴定为支架蛋白，具有使 G 蛋白偶联受体脱敏的能力。然而，已经发现β-arrestins 可以独立于 G 蛋白激活激活信号通路。受体酪氨酸激酶的这些信号通路的多样性也得到了认可。本研究的目的是验证负责调节脂肪生成的β-arrestin 依赖性信号机制。基于其β-arrestin 相关 Akt 活性，选择了两种信号模型，即 ghrelin 和胰岛素。在此，我们发现β-arrestin 1 和 2 是脂肪细胞分化所必需的分子。更具体地说，通过在 3T3-L1 细胞的 ghrelin 诱导脂肪生成过程中耗尽β-arrestin 1 和 2，证明了这些支架蛋白的作用，这降低了脂肪细胞分化和早期主调控因子的表达水平，CCAAT/增强子结合蛋白β (C/EBPβ) 和 CCAAT/增强子结合蛋白 δ (C/EBPδ)，以及终末，过氧化物酶体增殖物激活受体 (PPARγ) 和 CCAAT/增强子结合蛋白 α (C/EBPα)，脂肪生成。相应地，ghrelin 诱导的 Akt 活性及其下游靶标，哺乳动物雷帕霉素靶蛋白复合物 1 (mTORC1) 和核糖体蛋白 S6 激酶β-1 (S6K1)，被β-arrestin 1 和 2 siRNA 抑制。相比之下，在胰岛素激活脂肪生成过程中进行的测定显示，对脂肪细胞分化以及 C/EBPβ、C/EBPδ、PPARγ 和 C/EBPα 的表达具有增强作用。β-arrestin 敲低对胰岛素诱导脂肪生成的增加伴随着胰岛素受体底物-1 (IRS-1) 丝氨酸磷酸化的减少，证明了 IRS-1/磷酸肌醇 3-激酶 (PI3K)/Akt 负反馈环的丧失。因此，β-arrestins 控制与 Akt 信号相关的脂肪生成和脂肪生成因子的程度和强度，尽管信号和β-arrestins 之间的连接所依据的机制和功能原则具体与每种受体类型相关。
