8 research outputs found
Truncation of Pik3r1 causes severe insulin resistance uncoupled from obesity and dyslipidaemia by increased energy expenditure.
OBJECTIVE: Insulin signalling via phosphoinositide 3-kinase (PI3K) requires PIK3R1-encoded regulatory subunits. C-terminal PIK3R1 mutations cause SHORT syndrome, as well as lipodystrophy and insulin resistance (IR), surprisingly without fatty liver or metabolic dyslipidaemia. We sought to investigate this discordance. METHODS: The human pathogenic Pik3r1 Y657∗ mutation was knocked into mice by homologous recombination. Growth, body composition, bioenergetic and metabolic profiles were investigated on chow and high-fat diet (HFD). We examined adipose and liver histology, and assessed liver responses to fasting and refeeding transcriptomically. RESULTS: Like humans with SHORT syndrome, Pik3r1WT/Y657∗ mice were small with severe IR, and adipose expansion on HFD was markedly reduced. Also as in humans, plasma lipid concentrations were low, and insulin-stimulated hepatic lipogenesis was not increased despite hyperinsulinemia. At odds with lipodystrophy, however, no adipocyte hypertrophy nor adipose inflammation was found. Liver lipogenic gene expression was not significantly altered, and unbiased transcriptomics showed only minor changes, including evidence of reduced endoplasmic reticulum stress in the fed state and diminished Rictor-dependent transcription on fasting. Increased energy expenditure, which was not explained by hyperglycaemia nor intestinal malabsorption, provided an alternative explanation for the uncoupling of IR from dyslipidaemia. CONCLUSIONS: Pik3r1 dysfunction in mice phenocopies the IR and reduced adiposity without lipotoxicity of human SHORT syndrome. Decreased adiposity may not reflect bona fide lipodystrophy, but rather, increased energy expenditure, and we suggest that further study of brown adipose tissue in both humans and mice is warranted
Insulin resistance uncoupled from dyslipidemia due to C-terminal PIK3R1 mutations.
Obesity-related insulin resistance is associated with fatty liver, dyslipidemia, and low plasma adiponectin. Insulin resistance due to insulin receptor (INSR) dysfunction is associated with none of these, but when due to dysfunction of the downstream kinase AKT2 phenocopies obesity-related insulin resistance. We report 5 patients with SHORT syndrome and C-terminal mutations in PIK3R1, encoding the p85α/p55α/p50α subunits of PI3K, which act between INSR and AKT in insulin signaling. Four of 5 patients had extreme insulin resistance without dyslipidemia or hepatic steatosis. In 3 of these 4, plasma adiponectin was preserved, as in insulin receptor dysfunction. The fourth patient and her healthy mother had low plasma adiponectin associated with a potentially novel mutation, p.Asp231Ala, in adiponectin itself. Cells studied from one patient with the p.Tyr657X PIK3R1 mutation expressed abundant truncated PIK3R1 products and showed severely reduced insulin-stimulated association of mutant but not WT p85α with IRS1, but normal downstream signaling. In 3T3-L1 preadipocytes, mutant p85α overexpression attenuated insulin-induced AKT phosphorylation and adipocyte differentiation. Thus, PIK3R1 C-terminal mutations impair insulin signaling only in some cellular contexts and produce a subphenotype of insulin resistance resembling INSR dysfunction but unlike AKT2 dysfunction, implicating PI3K in the pathogenesis of key components of the metabolic syndrome.IHD was supported by the Raymond and Beverly Sackler Foundation via the University of Cambridge MB/PhD programme; RKS, IB, DBS, and SO were supported by the Wellcome Trust (grants WT098498, WT098051, WT107064, and WT095515, respectively and Strategic Award 100574/Z/12/Z), the MRC Metabolic Diseases Unit (MRC_MC_UU_12012/5), and the United Kingdom National Institute for Health Research (NIHR) Cambridge Biomedical Research Centre. The work was also supported by the Innovative Medicines Initiative Joint Undertaking under European Medical Information Framework (EMIF) grant agreement number 115372. UK10K was funded by the Wellcome Trust under award WT091310.This is the final version of the article. It first appeared from the American Society for Clinical Investigation via https://doi.org/10.1172/jci.insight.8876
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Mechanistic Investigation of Genotype-Phenotype Correlations in PIK3R1-Related Diseases
The PIK3R1 gene encodes three proteins - p85, p50 and p55 - that are regulatory subunits of Class IA phosphoinositide 3-kinases (PI3Ks). These regulatory subunits heterodimerise with one of three catalytic subunit isoforms, namely p110, p110, or p110. Class IA PI3Ks are critical enzymes involved in fundamental metabolic and mitogenic signalling pathways. This thesis describes the delineation of biochemical and molecular mechanisms whereby PIK3R1 mutations cause diverse disease phenotypes observed in SHORT syndrome (defined by Short stature, Hyperextensibility, Ocular depression, Rieger anomaly and Teething delay), the primary immunodeficiency Activated PI3K- Syndrome 2 (APDS2), and cancer.
Initial studies of purified wildtype or mutant PI3K complexes, utilising a modified PI3K fluorescence polarisation lipid kinase assay, established that SHORT syndrome-associated p85 mutations impaired phosphotyrosine peptide-stimulated PI3K activity when heterodimerised with either of the Class IA catalytic subunit isoforms. Two cancer-associated mutations assessed using the same assay demonstrated differential effects on PI3K function, causing either basal activation or impaired phosphotyrosine peptide-stimulated PI3K activity.
To examine the effect of SHORT syndrome-associated p85 mutations in insulin-responsive cell types, 3T3-L1 preadipocyte models with conditional overexpression of p85 Y657X or p85 R649W were generated. Doxycycline-induced overexpression of mutant p85 attenuated insulin-stimulated Akt phosphorylation due to reduced insulin-stimulated association of p85/p110 heterodimers with either IRS1 or IRS2. This in turn resulted in impaired downstream signalling as indicated by low adipogenic efficiency. Cells and tissues isolated from Pik3r1 knock-in mice also demonstrated decreased insulin-stimulated Akt phosphorylation. Observations from a system with endogenous expression of mutant p85 Y657X supported the results obtained in the 3T3-L1 p85 overexpression models.
The final part of this thesis focussed on a PIK3R1 exon skipping mutant (p85 Ex11) that confers PI3K activation in lymphocytes and causes APDS2. APDS2 patients have an immune-restricted phenotype, even though the mutation occurs within the ubiquitously expressed PIK3R1. To investigate this phenomenon, the doxycycline-inducible system was used to model overexpression of p85 Ex11, as well as an activating p110 H1047R mutation associated with cancer, in 3T3-L1 preadipocytes. Surprisingly, given that APDS2 is not normally associated with metabolic or growth problems, high overexpression of p85 Ex11 severely attenuated insulin-stimulated Akt phosphorylation and adipocyte differentiation. There was also reduced insulin-stimulated recruitment of p110 to either IRS1 or IRS2, and impaired heterodimerisation of p85 Ex11 with p110.
Collectively, the data presented in this thesis contributes to the developing knowledge of PIK3R1-related diseases. In particular, these studies provided novel insights into the biochemical and molecular mechanisms of SHORT syndrome-associated p85 mutations. Additionally, these data delivered further understanding of potential mechanisms underlying the immune-specific phenotype of APDS2 caused by PIK3R1 mutations.Funded by the Wellcome Trus
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Truncation of Pik3r1 causes severe insulin resistance uncoupled from obesity and dyslipidaemia by increased energy expenditure.
OBJECTIVE: Insulin signalling via phosphoinositide 3-kinase (PI3K) requires PIK3R1-encoded regulatory subunits. C-terminal PIK3R1 mutations cause SHORT syndrome, as well as lipodystrophy and insulin resistance (IR), surprisingly without fatty liver or metabolic dyslipidaemia. We sought to investigate this discordance. METHODS: The human pathogenic Pik3r1 Y657∗ mutation was knocked into mice by homologous recombination. Growth, body composition, bioenergetic and metabolic profiles were investigated on chow and high-fat diet (HFD). We examined adipose and liver histology, and assessed liver responses to fasting and refeeding transcriptomically. RESULTS: Like humans with SHORT syndrome, Pik3r1WT/Y657∗ mice were small with severe IR, and adipose expansion on HFD was markedly reduced. Also as in humans, plasma lipid concentrations were low, and insulin-stimulated hepatic lipogenesis was not increased despite hyperinsulinemia. At odds with lipodystrophy, however, no adipocyte hypertrophy nor adipose inflammation was found. Liver lipogenic gene expression was not significantly altered, and unbiased transcriptomics showed only minor changes, including evidence of reduced endoplasmic reticulum stress in the fed state and diminished Rictor-dependent transcription on fasting. Increased energy expenditure, which was not explained by hyperglycaemia nor intestinal malabsorption, provided an alternative explanation for the uncoupling of IR from dyslipidaemia. CONCLUSIONS: Pik3r1 dysfunction in mice phenocopies the IR and reduced adiposity without lipotoxicity of human SHORT syndrome. Decreased adiposity may not reflect bona fide lipodystrophy, but rather, increased energy expenditure, and we suggest that further study of brown adipose tissue in both humans and mice is warranted