Hepatic deletion of p110α and p85α results in insulin resistance despite sustained IRS1-associated phosphatidylinositol kinase activity.

Aditi Chaudhari,Yvonne Wettergren,Victoria Rotter Sopasakis,Katarina Ejeskär,C Ronald Kahn

doi:10.12688/f1000research.12418.1

Abstract

Class IA phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) is an integral mediator of insulin signaling. The p110 catalytic and p85 regulatory subunits of PI3K are the products of separate genes, and while they come together to make the active heterodimer, they have opposing roles in insulin signaling and action. Deletion of hepatic p110α results in an impaired insulin signal and severe insulin resistance, whereas deletion of hepatic p85α results in improved insulin sensitivity due to sustained levels of phosphatidylinositol (3,4,5)-trisphosphate. Here, we created mice with combined hepatic deletion of p110α and p85α(L-DKO) to study the impact on insulin signaling and whole body glucose homeostasis. Six-week old male flox control and L-DKO mice were studied over a period of 18 weeks, during which weight and glucose levels were monitored, and glucose tolerance tests, insulin tolerance test and pyruvate tolerance test were performed. Fasting insulin, insulin signaling mediators, PI3K activity and insulin receptor substrate (IRS)1-associated phosphatidylinositol kinase activity were examined at 10 weeks. Liver, muscle and white adipose tissue weight was recorded at 10 weeks and 25 weeks. The L-DKO mice showed a blunted insulin signal downstream of PI3K, developed markedly impaired glucose tolerance, hyperinsulinemia and had decreased liver and adipose tissue weights. Surprisingly, however, these mice displayed normal hepatic glucose production, normal insulin tolerance, and intact IRS1-associated phosphatidylinositol kinase activity without compensatory upregulated signaling of other classes of PI3K. The data demonstrate an unexpectedly overall mild metabolic phenotype of the L-DKO mice, suggesting that lipid kinases other than PI3Ks might partially compensate for the loss of p110α/p85α by signaling through other nodes than Akt/Protein Kinase B.

Highlights

Class IA phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) is a central mediator of a number of membrane receptor signaling pathways, including the insulin signaling pathway[1,2]
Deletion of Pik3ca and Pik3r1 in the liver resulted in markedly reduced gene and protein expression of p110α and p85α (Figures 1A, 1C, 1E), as well as impaired activation of the downstream targets Akt/Protein Kinase B (PKB), with decreased phosphorylation of serine 473 and threonine 308, and p70S6 kinase (Figure 1F). p110β gene expression was not affected by the deletion of p110α and p85α (Figure 1B). p85β gene expression was slightly, but significantly, decreased in the liver double knockout (L-DKO) livers (Figure 1D)
In the floxed control mice, there was an increase in the amount of p110α associated with IRS1 in response to insulin compared to basal conditions, whereas no p110α was associated with IRS1 in the L-DKO mice (Figure 1G)

Summary

Introduction

Class IA phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) is a central mediator of a number of membrane receptor signaling pathways, including the insulin signaling pathway[1,2]. The regulatory subunit, the most common of which is p85α, binds to phosphorylated tyrosine residues in tyrosine kinases and their substrate proteins via its SH2 domain, leading to activation of PI3K activity Both the regulatory and catalytic subunits exist as several different isoforms. Results: The L-DKO mice showed a blunted insulin signal downstream of PI3K, developed markedly impaired glucose tolerance, hyperinsulinemia and had decreased liver and adipose tissue weights. These mice displayed normal hepatic glucose production, normal insulin tolerance, and intact IRS1-associated phosphatidylinositol kinase activity without compensatory upregulated signaling of other classes of PI3K.

Methods

Results

Conclusion