Bai Jin, Jiang Xue, He Mulan, Chan Ben C B, Wong Anderson O L
School of Biological Sciences, The University of Hong Kong, Hong Kong, China.
Front Endocrinol (Lausanne). 2019 Sep 3;10:605. doi: 10.3389/fendo.2019.00605. eCollection 2019.
Glucagon, a key hormone for glucose homeostasis, can exert functional crosstalk with somatotropic axis via modification of IGF-I expression. However, its effect on IGF-I regulation is highly variable in different studies and the mechanisms involved are largely unknown. Using grass carp as a model, the signal transduction and transcriptional mechanisms for IGF-I regulation by glucagon were examined in Cyprinid species. As a first step, the carp HNF1α, a liver-enriched transcription factor, was cloned and confirmed to be a single-copy gene expressed in the liver. In grass carp hepatocytes, glucagon treatment could elevate IGF-I, HNF1α, and CREB mRNA levels, induce CREB phosphorylation, and up-regulate HNF1α and CREB protein expression. The effects on IGF-I, HNF1α, and CREB gene expression were mediated by cAMP/PKA and PLC/IP/PKC pathways with differential coupling with the MAPK and PI3K/Akt cascades. During the process, protein:protein interaction between HNF1α and CREB and recruitment of RNA Pol-II to IGF-I promoter also occurred with a rise in IGF-I primary transcript level. In parallel study to examine grass carp IGF-I promoter activity expressed in αT3 cells, similar pathways for post-receptor signaling were also confirmed in glucagon-induced IGF-I promoter activation and the trans-activating effect by glucagon was mediated by the binding sites for HNF1α and CREB located in the proximal region of IGF-I promoter. Our findings, as a whole, shed light on a previously undescribed mechanism for glucagon-induced IGF-I gene expression by increasing HNF1α and CREB production via functional crosstalk of post-receptor signaling. Probably, by protein:protein interaction between the two transcription factors and subsequent transactivation via their respective cis-acting elements in the IGF-I promoter, IGF-I gene transcription can be initiated by glucagon at the hepatic level.
胰高血糖素是葡萄糖稳态的关键激素，可通过调节胰岛素样生长因子-I（IGF-I）的表达与生长激素轴发生功能性串扰。然而，其对IGF-I调节的作用在不同研究中差异很大，且涉及的机制大多未知。以草鱼为模型，研究了鲤科鱼类中胰高血糖素调节IGF-I的信号转导和转录机制。第一步，克隆了鲤鱼肝富集转录因子肝细胞核因子1α（HNF1α），并证实其为在肝脏中表达的单拷贝基因。在草鱼肝细胞中，胰高血糖素处理可提高IGF-I、HNF1α和环磷腺苷反应元件结合蛋白（CREB）的mRNA水平，诱导CREB磷酸化，并上调HNF1α和CREB蛋白表达。对IGF-I、HNF1α和CREB基因表达的影响由环磷酸腺苷/蛋白激酶A（cAMP/PKA）和磷脂酶C/肌醇三磷酸/蛋白激酶C（PLC/IP/PKC）途径介导，与丝裂原活化蛋白激酶（MAPK）和磷脂酰肌醇-3-激酶/蛋白激酶B（PI3K/Akt）级联存在差异偶联。在此过程中，HNF1α与CREB之间发生蛋白质-蛋白质相互作用，RNA聚合酶II（RNA Pol-II）募集到IGF-I启动子，同时IGF-I初级转录本水平升高。在平行研究中检测草鱼IGF-I启动子在αT3细胞中的活性，在胰高血糖素诱导的IGF-I启动子激活中也证实了类似的受体后信号传导途径，胰高血糖素的反式激活作用由位于IGF-I启动子近端区域的HNF1α和CREB结合位点介导。总体而言，我们的研究结果揭示了一种此前未描述的机制，即胰高血糖素通过受体后信号的功能性串扰增加HNF1α和CREB的产生，从而诱导IGF-I基因表达。可能是通过这两种转录因子之间的蛋白质-蛋白质相互作用，以及随后通过它们在IGF-I启动子中各自的顺式作用元件进行反式激活，胰高血糖素可在肝脏水平启动IGF-I基因转录。
