Growth factor receptor binding protein 14 inhibition triggers insulin-induced mouse hepatocyte proliferation and is associated with hepatocellular carcinoma

Metabolic diseases such as obesity and type 2 diabetes are recognized as independent risk factors for hepatocellular carcinoma (HCC). Hyperinsulinemia, a hallmark of these pathologies, is suspected to be involved in HCC development. The molecular adapter growth factor receptor binding protein 14 (Grb14) is an inhibitor of insulin receptor catalytic activity, highly expressed in the liver. To study its involvement in hepatocyte proliferation, we specifically inhibited its liver expression using a short hairpin RNA strategy in mice. Enhanced insulin signaling upon Grb14 inhibition was accompanied by a transient induction of S-phase entrance by quiescent hepatocytes, indicating that Grb14 is a potent repressor of cell division. The proliferation of Grb14-deficient hepatocytes was cell-autonomous as it was also observed in primary cell cultures. Combined Grb14 down-regulation and insulin signaling blockade using pharmacological approaches as well as genetic mouse models demonstrated that Grb14 inhibition-mediated hepatocyte division involved insulin receptor activation and was mediated by the mechanistic target of rapamycin complex 1-S6K pathway and the transcription factor E2F1. In order to determine a potential dysregulation in GRB14 gene expression in human pathophysiology, a collection of 85 human HCCs was investigated. This revealed a highly significant and frequent decrease in GRB14 expression in hepatic tumors when compared to adjacent nontumoral parenchyma, with 60% of the tumors exhibiting a reduced Grb14 mRNA level.

CONCLUSION

Our study establishes Grb14 as a physiological repressor of insulin mitogenic action in the liver and further supports that dysregulation of insulin signaling is associated with HCC. (Hepatology 2017;65:1352-1368).

Class III PI3K regulates organismal glucose homeostasis by providing negative feedback on hepatic insulin signalling

Defective hepatic insulin receptor (IR) signalling is a pathogenic manifestation of metabolic disorders including obesity and diabetes. The endo/lysosomal trafficking system may coordinate insulin action and nutrient homeostasis by endocytosis of IR and the autophagic control of intracellular nutrient levels. Here we show that class III PI3K--a master regulator of endocytosis, endosomal sorting and autophagy--provides negative feedback on hepatic insulin signalling. The ultraviolet radiation resistance-associated gene protein (UVRAG)-associated class III PI3K complex interacts with IR and is stimulated by insulin treatment. Acute and chronic depletion of hepatic Vps15, the regulatory subunit of class III PI3K, increases insulin sensitivity and Akt signalling, an effect that requires functional IR. This is reflected by FoxO1-dependent transcriptional defects and blunted gluconeogenesis in Vps15 mutant cells. On depletion of Vps15, the metabolic syndrome in genetic and diet-induced models of insulin resistance and diabetes is alleviated. Thus, feedback regulation of IR trafficking and function by class III PI3K may be a therapeutic target in metabolic conditions of insulin resistance.

[Cross-talk between insulin signaling and cell proliferation pathways]

Epidemiological studies provide evidence for a close relationship between diabetes and cancer. Insulin is in fact a growth factor, and its binding to its membrane receptor activates intracellular signaling pathways involved in the regulation of both metabolism and cell proliferation. The balance between mitogenic and metabolic actions of insulin can be modulated by various mechanisms, including the way the ligand binds to its receptor or to the closely related insulin-like growth factor-1 (IGF-1) receptor. Cross-talks with other signaling pathways implicated in cell proliferation have also been described, like the Wnt/β catenin pathway, and involve the activation of common downstream effectors such as insulin receptor substrate-1 (IRS-1). Finally, the identification of new proteins activated by insulin and involved in intracellular signaling would allow a better understanding of the complex connections linking metabolic and proliferative regulatory pathways. As an example, the molecular adaptor Grb14, which is a specific inhibitor of insulin receptor catalytic activity, also controls insulin-induced metabolic and mitogenic signaling pathways through post-receptor mechanisms that remain to be fully elucidated.