Novel Mechanisms for IGF-I Regulation by Glucagon in Carp Hepatocytes: Up-Regulation of HNF1α and CREB Expression via Signaling Crosstalk for IGF-I Gene Transcription.

Jin Bai,Xue Jiang,Ben C B Chan,Anderson O L Wong,Mulan He

doi:10.3389/fendo.2019.00605

Abstract

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/IP3/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.

Highlights

Glucagon released from the pancreas by functional interaction with insulin is known to play a key role in glucose homeostasis [1], mainly through regulation of gluconeogenesis and glycogenolysis in the liver [2]
As a first step to study the functional role of Hepatic nuclear factor 1α (HNF1α) in Insulin-like growth factor I (IGF-I) gene expression in grass carp, the structural identity of grass carp HNF1α was established by 3′/5′ RACE using total RNA from the liver as the template (Supplemental Figure 1)
Among the transcription factors involved in IGF-I gene expression, the role of HNF1α appears to be well-conserved and its binding site(s), namely HNF1α binding element (HBE), can be identified within the IGF-I promoter from fish to mammals [22]

Summary

Introduction

Glucagon released from the pancreas by functional interaction with insulin is known to play a key role in glucose homeostasis [1], mainly through regulation of gluconeogenesis and glycogenolysis in the liver [2]. There is only a single report in salmon hepatocytes showing that glucagon did not alter basal but reduced GH-induced IGF-I mRNA expression at the hepatic level [15]. These findings suggest that, similar to mammals, glucagon may play a role in IGF-I regulation in fish models

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Conclusion