Biology subjects

Zvetkova, I.

Publications and source records attributed to Zvetkova, I..

5 recordsLinked to original sources

bioRxiv2024

Damaging mutations in LXRα uncouple lipogenesis from hepatotoxicity and implicate hepatic cholesterol sensing in human liver health

The nuclear receptor Liver X Receptor- (LXR) activates lipogenic gene expression in hepatocytes. Its inhibition has therefore been proposed as a strategy to treat metabolic-dysfunction-associated steatotic liver disease (MASLD). In order to understand the impact of reducing LXR activity on human health we first examined the association between the carriage of rare loss of function mutations in NR1H3 (encoding LXR) and metabolic and hepatic phenotypes. We identified 63 rare predicted damaging variants in the ligand binding domain of LXR in 454,787 participants in UK Biobank. On functional characterisation, 42 of these were found to be severely impaired. Consistent with loss of the lipogenic actions of LXR, carriers of damaging mutations in LXR had reduced serum triglycerides ({beta}=-0.13 s.d. {+/-}0.03, P=2.7x10-5, N(carriers)=971). Surprisingly, these carriers also had elevated concentrations of serum liver enzymes (e.g. ALT: {beta}=0.17s.d. {+/-}0.03, P=1.1x10-8, N(carriers)=972) with a 35% increased risk of clinically significant elevations in ALT (OR=1.32, 95%CI:1.15-1.53, P=1.2x10-4, N(carriers)=972), suggestive of hepatotoxicity. We generated a knock-in mouse carrying one of the most severely damaging mutations (Nr1h3 p.W441R) which we demonstrated to have dominant negative properties. Homozygous knock-in mice rapidly developed severe hepatitis and fibrotic liver injury following exposure to western diet despite markedly reduced steatosis, liver triglycerides and lipogenic gene expression. This phenotype was completely rescued by viral over-expression of wildtype LXR specifically in hepatocytes, indicating a cell-autonomous effect of the mutant on hepatocyte health. While homozygous LXR knockout mice showed some evidence of hepatocyte injury under similar dietary conditions, the phenotype of the LXRW441R/W441R mouse was much more severe, suggesting that dominant negative mutations that actively co-repress target genes can result in pathological impacts significantly more severe than those seen with simple absence of the receptor. In summary, our results show that loss of function mutations in LXR occur in at least 1/450 people and are associated with evidence of liver dysfunction. These findings implicate LXR in the maintenance of human liver health, identify a new murine model of rapidly progressive fibrotic liver disease and caution against LXR antagonism as a therapeutic strategy for MASLD.

genetics↗

bioRxiv2022

The imprinted Mir483 is a growth suppressor and metabolic regulator functioning through IGF1

Mir483 is a conserved and highly expressed microRNA in placental mammals, embedded within the Igf2 gene. Here, we uncover the control mechanisms and physiological functions of Mir483 in vivo, by generating constitutive loss-of-function and over-expressing mice. Mir483 expression is imprinted and dependent on the Igf2 promoters and Igf2/H19 imprinting control region. Over-expression of Mir483 causes severe mid-gestation fetal, but not placental, growth restriction, and late lethality. Fetal death is prevented by restoring Mir483 to endogenous levels using an inducible transgenic system. Continuous postnatal Mir483 over-expression induces growth stunting, elevated hepatic lipid content, increased adiposity, reduced local and systemic IGF1 levels and increased GH. The growth phenotypes are rescued by IGF1 infusion. Our findings provide evidence for a novel functional antagonism between a growth-suppressor microRNA and its growth-promoter host gene, and suggest that Mir483 evolved to limit excessive tissue growth through repression of IGF ligand signalling.

developmental biology↗

bioRxiv2022

Endogenous GDF15 and FGF21 additively alleviate hepatic steatosis and insulin resistance in obese mice.

Obesity in mice and humans is associated with elevated levels of at least two hormones responsive to cellular stress, namely GDF15 and FGF21. Over-expression of each of these is associated with weight loss and beneficial metabolic changes but where they are secreted from and what they are required for physiologically in the context of overfeeding remains unclear. Here we used tissue selective knockout mouse models to establish that, like FGF21, circulating GDF15 is primarily derived from the liver, rather than adipose tissue, muscle or macrophages in high fat fed mice. Combined whole body deletion of FGF21 and GDF15 does not result in any additional weight gain in high fat fed mice but is associated with significantly greater hepatic steatosis and insulin resistance. Collectively the data suggest that activation of the integrated stress response in hepatocytes is a major driver for GDF15 and FGF21 secretion in the context of overfeeding, and that they both act to alleviate this metabolic stress.

physiology↗

bioRxiv2019

Mesenchymal Igf2 is a major paracrine regulator of pancreatic growth and function

The genetic mechanisms that determine the size of the adult pancreas are poorly understood. Here we demonstrate that many imprinted genes are highly expressed in the pancreatic mesenchyme, and explore the role of Igf2 in-vivo. Mesenchyme-specific Igf2 deletion results in acinar and beta-cell hypoplasia, postnatal whole-body growth restriction and maternal glucose intolerance during pregnancy. Surprisingly, mesenchymal mass is unaffected, suggesting that the mesenchyme is a developmental reservoir of IGF2 used for paracrine signalling. The unique actions of mesenchymal IGF2 are demonstrated by the absence of phenotypes upon Igf2 deletion in the developing pancreatic epithelium. Furthermore, increased IGF2 activity specifically in the mesenchyme, through Igf2 loss-of-imprinting or Igf2r deletion, leads to pancreatic acinar overgrowth. Ex-vivo exposure of primary acinar cells to exogenous IGF2 increases cell proliferation and amylase production through AKT signalling. We propose that mesenchymal Igf2, and perhaps other imprinted genes, are key developmental regulators of adult pancreas size and function.

developmental biology↗

bioRxiv2017

C-terminal truncation of Pik3r1 in mice models human lipodystrophic insulin resistance uncoupled from dyslipidemia

Heterodimeric class IA phosphatidylinositol-3-kinases (PI3K) transduce signals from many receptor tyrosine kinases including the insulin receptor. PI3K recruitment to phosphotyrosines is mediated by Pik3r1 gene products including the most intensely studied PI3K regulatory subunit, p85, which also binds and regulates the PIP3 phosphatase Pten, and the lipogenic transcription factor Xbp1. Mutations in human PIK3R1 cause SHORT syndrome, featuring lipodystrophy and severe insulin resistance which, uniquely, are uncoupled from fatty liver and dyslipidemia. We describe a novel mouse model of SHORT syndrome made by knock in of the Pik3r1 Y657X mutation. Homozygous embryos die at E11.5, while heterozygous mice exhibit pre-and postnatal growth impairment with diminished placental vascularity. Adipose tissue accretion on high fat feeding was reduced, however adipocyte size was unchanged and preadipocyte differentiation ex vivo unimpaired. Despite severe insulin resistance, heterozygous mice were hypolipidemic, and plasma adiponectin, liver weight, cholesterol, glycogen and triglyceride content were unchanged. Mild downregulation of lipogenic Srebp1, Srebp2 and Chrebp transcriptional activity but no suppression of Xbp1 target genes was seen after fasting. These findings give new insights into the developmental role of Pik3r1, and establish a model of lipodystrophic insulin resistance dissociated from dyslipidemia as seen in SHORT syndrome.

biochemistry↗