Intercepting IRE1 kinase-FMRP signaling prevents atherosclerosis progression

Fragile X Mental Retardation protein (FMRP), widely known for its role in hereditary intellectual disability, is an RNA‐binding protein (RBP) that controls translation of select mRNAs. We discovered that endoplasmic reticulum (ER) stress induces phosphorylation of FMRP on a site that is known to enhance translation inhibition of FMRP‐bound mRNAs. We show ER stress‐induced activation of Inositol requiring enzyme‐1 (IRE1), an ER‐resident stress‐sensing kinase/endoribonuclease, leads to FMRP phosphorylation and to suppression of macrophage cholesterol efflux and apoptotic cell clearance (efferocytosis). Conversely, FMRP deficiency and pharmacological inhibition of IRE1 kinase activity enhances cholesterol efflux and efferocytosis, reducing atherosclerosis in mice. Our results provide mechanistic insights into how ER stress‐induced IRE1 kinase activity contributes to macrophage cholesterol homeostasis and suggests IRE1 inhibition as a promising new way to counteract atherosclerosis.

Abstract 202: Zfp36l1 is a Post transcriptional Regulator of Hepatic Bile Acid and Lipid Metabolism

Bile acids are detergents and important signaling molecules that activate the nuclear receptor FXR to control key metabolic processes, including feedback mechanisms to maintain bile acid homeostasis. FXR is the central rheostat of bile acid metabolism, and activation of FXR decreases the mRNA levels of bile acid synthetic genes, including Cyp 7 a 1 , the gene encoding the rate-limiting enzyme of bile acid synthesis. We show that Cyp 7 a 1 mRNA levels were rapidly reduced after pharmacologic FXR activation in wild-type, but not Fxr – / – or liver-specific Fxr knockout mice ( Fxr L - KO ). The rapid decrease in Cyp 7 a 1 mRNA suggested a previously unidentified post-transcriptional mechanism. To identify the mechanism, we used synthetic and endogenous FXR agonists and found the RNA binding protein ZFP36L1 as a novel FXR target gene. ZFP36L1 mRNA and protein levels were increased as early as 30 minutes after FXR activation. ZFP36L1 is a bona - fide RNA binding protein that promotes degradation of mRNA targets by binding to AU-rich elements (AREs) in the 3’ UTR. We generated in vivo and in vitro gain-of-function models and we used reporter assays to show that ZFP36L1 targets the Cyp7a1 UTR . In mice, hepatic overexpression of ZFP36L1 decreased Cyp 7 a 1 mRNA and protein and decreased bile acid levels. To complement our gain-of-function studies, we generated liver-specific Zfp 36 l 1 knockout mice ( Zfp 36 l 1 L - KO ) and we show that loss of Zfp 36 l 1 resulted in elevated Cyp 7 a 1 mRNA and protein, and increased bile acid levels. Given that bile acids are important metabolites that control lipid absorption and signaling, we investigated whether loss of hepatic Zfp 36 l 1 resulted in more broad metabolic dysfunction. Western diet fed Zfp 36 l 1 L - KO mice had reduced body weight gain, specifically in adipose tissue depots compared to littermate Zfp 36 l 1 flox-flox mice. The differences in adiposity and steatosis were attributed to reduced lipid absorption, as Zfp 36 l 1 L - KO mice have increased fecal caloric content and reduced triglyceride absorption as determined by an intragastric fat tolerance test. The decreased lipid absorption is consistent with an altered bile acid metabolism. Thus, we have identified a novel pathway that controls Cyp 7 a 1 and bile acid metabolism but may also have wider implications in diseases such as obesity and hepatosteatosis.

Abstract 402: Post-transcriptional Regulation of Bile Acid Metabolism by the RNA binding protein ZFP36L1

Bile acids are detergents and important signaling molecules that activate the nuclear receptor FXR to control key metabolic processes, including feedback mechanisms to maintain bile acid homeostasis. Activation of FXR decreases the mRNA levels of several bile acid synthetic genes, including the rate-limiting enzyme Cyp7a1 . Here we show that Cyp7a1 mRNA levels are very rapidly reduced following FXR activation, indicative of a post-transcriptional mechanism. We identify the RNA binding protein Zfp36l1 as an FXR target gene and show that hepatic overexpression of ZFP36L1 in mice decreases Cyp7a1 mRNA levels. In contrast, Zfp36l1 L -KO mice have increased levels of Cyp7a1 mRNA and biliary bile acids as well as reduced plasma cholesterol levels. Zfp36l1 L -KO mice fed a Western diet have reduced diet-induced obesity and steatosis, likely due to impaired lipid absorption, consistent with increased Cyp7a1 levels. Thus, the ZFP36L1-dependent regulation of bile acid metabolism is an important metabolic contributor of dyslipidemia, obesity and hepatosteatosis.

Abstract 634: Silencing miR144 Enhances Regression and Attenutates Progression of Atherosclerosis

Cholesterol imbalance plays a central role in multiple diseases, particularly cardiovascular disease, the leading cause of death in the United States. Lipid-loaded macrophage foam cells are a critical determining factor in the pathogenesis of atherosclerosis. MicroRNAs are small non-coding RNA molecules that regulate both cholesterol and lipoprotein metabolism, and may significantly contribute to disease progression. We previously identified miR-144 as a novel regulator of lipid metabolism that is also regulated by the nuclear receptor farnesoid X receptor (FXR). We identified the cholesterol transporter ABCA1 as a miR-144 target gene, and showed that increased hepatic expression of miR-144 decreased ABCA1 protein and plasma cholesterol levels. Consequently, we hypothesized that silencing miR-144 may have therapeutic potential because it may increase the levels of atheroprotective HDL particles and enhance reverse cholesterol transport (RCT) due to increased ABCA1 levels in macrophages. Here we present en face analysis of atherosclerosis showing that silencing miR-144 both enhanced regression and attenuated progression of atherosclerosis. In addition, we demonstrate that silencing miR-144 enhances RCT, the process by which macrophage cholesterol is returned to the liver for excretion. Together, these data suggest that this pathway may represent a bona-fide strategy to treat atherosclerosis.

Abstract 328: ABCG1 Has a Critical Role in Pulmonary Type 2 Cells and Surfactant Lipid Metabolism

Pulmonary alveolar proteinosis (PAP) is a rare disease caused by the accumulation of surfactant proteins and lipids in the pulmonary alveoli resulting in respiratory distress. For almost all reported cases the underlying cause is unknown. Synthesis and secretion of surfactant in the lung is restricted to epithelial type 2 pneumocytes (T2 cells). However, subsequent clearance of surfactant from the hypophase is dependent upon both T2 cells and macrophages. Both T2 cells and macrophages express high levels of the sterol-transporter ATP Binding Cassette transporter G1 (ABCG1). Further, Abcg1-/- mice exhibit a severe pulmonary lipidosis, characterized by elevated levels of multiple lipid species and the presence of lipid-loaded macrophages and T2 cells containing abnormal lamellar bodies. Recent advances have implicated a role for specific granulocyte-macrophage colony stimulating factor (GM-CSF) autoantibodies and the innate immune system in the pathogenesis of PAP. We have recently demonstrated that the lungs of Abcg1-/- mice accumulate specific oxidized phospholipids and that this is associated with a local increase in innate immune natural antibody-secreting B-1 B cells and both local and circulating natural antibodies. Here we show that the lungs of mice containing ABCG1-deficient T2 (T2KO) cells and wild-type macrophages contain elevated levels of both phospholipids and esterified cholesterol. In addition, the T2KO cells in these mice accumulated lamellae bodies, unesterified cholesterol and exhibited abnormal gene expression. Collectively, our results suggest that ABCG1 is critical for normal surfactant metabolism and that the pulmonary expansion of B-1 B cells and the subsequent increase in natural antibodies in Abcg1-/- mice may provide a novel model in which to study the role of the innate immune system in PAP.

Abstract 76: MicroRNA-144 Regulates Hepatic ABCA1 and Plasma HDL Following Activation of the Nuclear Receptor FXR

Rationale

The bile acid receptor Farnesoid-X-Receptor (FXR) regulates many aspects of lipid metabolism by various complex and not fully understood molecular mechanisms. We set out to investigate the molecular mechanisms for FXR-dependent regulation of lipid and lipoprotein metabolism.

Objective

To identify FXR-regulated microRNAs that were subsequently involved in regulating lipid metabolism.

Methods and Results

ATP binding cassette transporter A1 (ABCA1) is a major determinant of plasma High Density Lipoprotein (HDL)-cholesterol levels. Here we show that activation of the nuclear receptor FXR in vivo increases hepatic levels of miR-144, which in turn lower hepatic ABCA1 and plasma HDL levels. We identified two complementary sequences to miR-144 in the 3’ untranslated region (UTR) of ABCA1 mRNA that are necessary for miR-144-dependent regulation. Overexpression of miR-144 in vitro decreased both cellular ABCA1 protein and cholesterol efflux to lipid-poor apolipoprotein A-I (ApoA-I) protein, whilst overexpression in vivo reduced hepatic ABCA1 protein and plasma HDL- cholesterol. Conversely, silencing miR-144 in mice increased hepatic ABCA1 protein and HDL- cholesterol. In addition, we utilized tissue-specific FXR deficient mice to show that induction of miR-144 and FXR-dependent hypolipidemia requires hepatic, but not intestinal FXR. Finally, we identified functional FXR response elements (FXREs) upstream of the miR-144 locus, consistent with direct FXR regulation.

Conclusion

In conclusion, we have identified a pathway involving FXR, miR-144 and ABCA1 that together regulate plasma HDL cholesterol. This pathway may be therapeutically targeted in the future in order to increase HDL levels.

Transcriptional regulation of human SREBP-1c (sterol-regulatory-element-binding protein-1c): a key regulator of lipogenesis

Sterol-regulatory-element-binding protein 1c (SREBP-1c) is one member of the family of transcription factors that stimulate sterol and fatty-acid biosynthesis in animal cells. Human SREBP-1c, mapped to chromosome 17p11.2, is expressed in liver, intestine, skeletal muscle and adipocytes. A section of genomic sequence from a chromosome 17 library, thought to contain the SREBP-1c promoter, was cloned. Putative transcription-factor-binding sites and a potential transcriptional start site were identified using the Genomatix Suite of sequence analysis tools (MatInspector®). Sequence analysis showed the human promoter to be 42% identical with the previously published mouse sequence. Two novel transcription-factor-binding sites were identified: those for PDX-1 (pancreatic–duodenal homoeobox-1) and HNF-4 (hepatic nuclear factor-4). Co-transfection experiments with overexpression plasmids for PDX-1 and HNF-4 suggested that both factors stimulate SREBP-1c gene expression, although further work is required to ascertain their mechanisms of action.