Primary hypocholesterolemia is associated with an increased risk of hepatic complications in the general population.

BACKGROUND & AIMS: Beyond cardiovascular disease protection, the health consequences of very low concentrations of low-density lipoprotein-cholesterol (LDL-C) remain a matter of debate. In primary hypobetalipoproteinemia (HBL), liver steatosis and cirrhosis have occasionally been reported. Here, we aimed to investigate the association between HBL and the risk of hepatic complications (cirrhosis complications and/or primary liver cancer) in the general population. METHODS: A cohort study was conducted in the French population-based cohort CONSTANCES. Participants with primary HBL (LDL-C <5th percentile for age and sex, [HBL]) were compared with those with normal LDL-C concentrations (40th-60th percentile, [Control]). Participants on lipid-lowering therapies were excluded. For hepatic complications, follow-up events were compared by calculating the incidence density ratio (IDR). The same analyses were replicated in the UK Biobank (UKBB) cohort. RESULTS: In the CONSTANCES and UKBB cohorts, 34,653 and 94,666 patients were analyzed, with median ages of 45 and 56 years, mean LDL-C concentrations (HBL vs. control) of 71 vs. 128 mg/dl and 86 vs. 142 mg/dl, and mean follow-up durations of 5.0 and 11.5 years, respectively. The HBL group presented a higher incidence of hepatic complications than the control group: 0.32/ vs. 0.07/1,000 person-years (IDR = 4.50, 95% CI 1.91-10.6) in CONSTANCES, and 0.69/ vs. 0.21/1,000 person-years (IDR = 3.27, 95% CI 2.63-4.06) in the UKBB. This risk proved to be independent of classic risk factors for liver disease (obesity, alcohol consumption, diabetes, viral hepatitis), including in a 5-year landmark analysis excluding early events. Sensitivity analyses based on apoliprotein-B levels (instead of LDL-C levels) or genetically defined HBL showed similar results. CONCLUSIONS: HBL is associated with a markedly increased risk of hepatic complications. HBL must be considered as a substantial independent risk factor for liver diseases which justifies specific prevention and screening. IMPACT AND IMPLICATIONS: Hypobetalipoproteinemia (HBL) is a lipid disorder characterized by permanent, inherited low levels (below the 5th percentile) of low-density lipoprotein-cholesterol. While HBL is associated with a lower risk of cardiovascular events, some studies suggest that it may be associated with a potential risk of hepatic steatosis and hepatic complications. Here, we studied the association between HBL and hepatic complications (defined as cirrhosis complications and/or primary liver cancer) in two populations of several hundred thousand people, both in France (CONSTANCES cohort) and the United Kingdom (UKBB). The results show that HBL is associated with a significant and independent excess risk of hepatic complications, including primary liver cancer. Thus, in people with HBL, the value of regular liver monitoring must be studied.

Identification of a Gain-of-Function LIPC Variant as a Novel Cause of Familial Combined Hypocholesterolemia.

BACKGROUND: Atherosclerotic cardiovascular disease is the main cause of mortality worldwide and is strongly influenced by circulating low-density lipoprotein (LDL) cholesterol levels. Only a few genes causally related to plasma LDL cholesterol levels have been identified so far, and only 1 gene, ANGPTL3, has been causally related to combined hypocholesterolemia. Here, our aim was to elucidate the genetic origin of an unexplained combined hypocholesterolemia inherited in 4 generations of a French family. METHODS: Using next-generation sequencing, we identified a novel dominant rare variant in the LIPC gene, encoding for hepatic lipase, which cosegregates with the phenotype. We characterized the impact of this LIPC-E97G variant on circulating lipid and lipoprotein levels in family members using nuclear magnetic resonance-based lipoprotein profiling and lipidomics. To uncover the mechanisms underlying the combined hypocholesterolemia, we used protein homology modeling, measured triglyceride lipase and phospholipase activities in cell culture, and studied the phenotype of APOE*3.Leiden.CETP mice after LIPC-E97G overexpression. RESULTS: Family members carrying the LIPC-E97G variant had very low circulating levels of LDL cholesterol and high-density lipoprotein cholesterol, LDL particle numbers, and phospholipids. The lysophospholipids/phospholipids ratio was increased in plasma of LIPC-E97G carriers, suggestive of an increased lipolytic activity on phospholipids. In vitro and in vivo studies confirmed that the LIPC-E97G variant specifically increases the phospholipase activity of hepatic lipase through modification of an evolutionarily conserved motif that determines substrate access to the hepatic lipase catalytic site. Mice overexpressing human LIPC-E97G recapitulated the combined hypocholesterolemic phenotype of the family and demonstrated that the increased phospholipase activity promotes catabolism of triglyceride-rich lipoproteins by different extrahepatic tissues but not the liver. CONCLUSIONS: We identified and characterized a novel rare variant in the LIPC gene in a family who presents with dominant familial combined hypocholesterolemia. This gain-of-function variant makes LIPC the second identified gene, after ANGPTL3, causally involved in familial combined hypocholesterolemia. Our mechanistic data highlight the critical role of hepatic lipase phospholipase activity in LDL cholesterol homeostasis and suggest a new LDL clearance mechanism.

Plasma apolipoprotein concentrations and incident diabetes in subjects with prediabetes.

BACKGROUND: The identification of circulating biomarkers associated with the risk of type 2 diabetes (T2D) is useful for improving the current prevention strategies in the most at-risk patients. Here, we aimed to investigate the association of plasma apolipoprotein concentrations in prediabetes subjects with the incidence of new-onset T2D during follow-up. METHODS: In the IT-DIAB prospective study, 307 participants with impaired fasting glucose levels (fasting plasma glucose [FPG]: 110-125 mg/dL) were followed yearly for 5 years. The onset of T2D was defined as a first FPG value ≥ 126 mg/dL during follow-up. Apolipoprotein (apo)A-I, A-II, A-IV, B100, C-I, C-II, C-III, C-IV, D, E, F, H, J, L1, M, and (a) plasma concentrations were determined by mass spectrometry. Correlations between apolipoproteins and metabolic parameters at baseline were assessed by Spearman's coefficients. Kaplan-Meier curves were drawn using a ternary approach based on terciles and incident T2D. The association between plasma apolipoproteins concentrations and the incidence of T2D was determined using Cox proportional-hazards models. RESULTS: During a median follow-up of 5-year, 115 participants (37.5%) developed T2D. After adjustment for age, sex, body mass index, FPG, HbA1c, and statin use, the plasma levels of apoC-I, apoC-II, apoC-III, apoE, apoF, apoH, apoJ, and apoL1 were positively associated with a high risk for T2D. After further adjustment for plasma triglycerides, only apoE (1 SD natural-log-transformed hazard ratio: 1.28 [95% confidence interval: 1.06; 1.54]; p = 0.010), apoF (1.22 [1.01; 1.48]; p = 0.037), apoJ (1.24 [1.03; 1.49]; p = 0.024), and apoL1 (1.26 [1.05; 1.52]; p = 0.014) remained significantly associated with the onset of T2D. Kaplan-Meier survival curves also showed that the lower third of plasma apoE levels (< 5.97 mg/dL) was significantly associated with a lower risk of conversion to T2D (log-rank test, p = 0.002) compared to the middle and upper thirds. CONCLUSIONS: The plasma apoE levels are positively associated with the risk of T2D in prediabetes subjects, independently of traditional risk factors. The possible associations of apoF, apoJ, and apoL1 with T2D risk also pave the way for further investigations. Trial registration This trial was registered at clinicaltrials.gov as NCT01218061 and NCT01432509.

Nutritional biomarkers and heart failure requiring hospitalization in patients with type 2 diabetes: the SURDIAGENE cohort.

BACKGROUND: Heart failure (HF) is a growing complication and one of the leading causes of mortality in people living with type 2 diabetes (T2D). Among the possible causes, the excess of red meat and the insufficiency of vegetables consumption are suspected. Such an alimentation is associated with nutritional biomarkers, including trimethylamine N-oxide (TMAO) and its precursors. Here, we aimed to study these biomarkers as potential prognostic factors for HF in patients with T2D. METHODS: We used the SURDIAGENE (SURvival DIAbetes and GENEtics) study, a large, prospective, monocentric cohort study including 1468 patients with T2D between 2001 and 2012. TMAO and its precursors (trimethylamine [TMA], betaine, choline, and carnitine) as well as thio-amino-acids (cysteine, homocysteine and methionine) were measured by liquid chromatography-tandem mass spectrometry. The main outcome was HF requiring Hospitalization (HFrH) defined as the first occurrence of acute HF leading to hospitalization and/or death, established by an adjudication committee, based on hospital records until 31st December 2015. The secondary outcomes were the composite event HFrH and/or cardiovascular death and all-cause death. The association between the biomarkers and the outcomes was studied using cause-specific hazard-models, adjusted for age, sex, history of coronary artery disease, NT-proBNP, CKD-EPI-derived eGFR and the urine albumin/creatinine ratio. Hazard-ratios (HR) are expressed for one standard deviation. RESULTS: The data of interest were available for 1349/1468 of SURDIAGENE participants (91.9%), including 569 (42.2%) women, with a mean age of 64.3 ± 10.7 years and a median follow-up of 7.3 years [25th-75th percentile, 4.7-10.8]. HFrH was reported in 209 patients (15.5%), HFrH and/or cardiovascular death in 341 (25.3%) and all-cause death in 447 (33.1%). In unadjusted hazard-models, carnitine (HR = 1.20, 95% CI [1.05; 1.37]), betaine (HR = 1.34, [1.20; 1.50]), choline (HR = 1.35, [1.20; 1.52]), TMAO (HR = 1.32, [1.16; 1.50]), cysteine (HR = 1.38, [1.21; 1.58]) and homocysteine (HR = 1.28, [1.17; 1.39]) were associated with HFrH, but not TMA and methionine. In the fully adjusted models, none of these associations was significant, neither for HFrH nor for HFrH and/or CV death, when homocysteine only was positively associated with all-cause death (HR = 1.16, [1.06; 1.27]). CONCLUSIONS: TMAO and its precursors do not appear to be substantial prognosis factors for HFrH, beyond usual cardiac- and kidney-related risk factors, whereas homocysteine is an independent risk factor for all-cause death in patients with T2D.

Transintestinal cholesterol excretion in health and disease.

PURPOSE OF REVIEW: The transintestinal cholesterol efflux (TICE) pathway is the second described route for plasma cholesterol fecal elimination. This article summarizes recent TICE research progresses, involving TICE inducers, molecular determinants of this pathway, and its role in lipoprotein metabolism. RECENT FINDINGS: TICE is an active pathway in mice, rats, and humans. Kinetic measurements showed that under basal conditions, the relative contribution of TICE in fecal elimination of plasma cholesterol is quantitatively less important than the hepatobiliary pathway. However, the amplitude of TICE can be induced by numerous nutritional factors and pharmacological drugs. More importantly, by contrast with the stimulation of biliary cholesterol excretion that is associated with an increased risk of gallstone formation, TICE appears as a safer therapeutical target. Finally, several independent studies have demonstrated that TICE is actively contributing to the anti-atherogenic reverse cholesterol pathway reinforcing the interest to better understand its mode of action. The discovery of TICE and the understanding of its mode of action open new therapeutical perspectives for patients at high risk of cardiovascular diseases.

PCSK9 is not secreted from mature differentiated intestinal cells.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes lysosomal degradation of the LDL receptor and is a key regulator of cholesterol metabolism. After the liver, the small intestine is the second organ that highly expresses PCSK9. However, the small intestine's ability to secrete PCSK9 remains a matter of debate. While liver-specific PCSK9-deficient mice present no PCSK9 in systemic blood, human intestinal Caco-2 cells can actively secrete PCSK9. This raises the possibility for active intestinal secretion via the portal blood. Here, we aimed to determine whether enterocytes can secrete PCSK9 using in vitro, ex vivo, and in vivo approaches. We first observed that PCSK9 secretion from Caco-2 cells was biphasic and dependent on Caco-2 maturation status. Transcriptional analysis suggested that this transient reduction in PCSK9 secretion might be due to loss of SREBP2-mediated transcription of PCSK9. Consistently, PCSK9 secretion was not detected ex vivo in human or mouse intestinal biopsies mounted in Ussing chambers. Finally, direct comparison of systemic versus portal blood PCSK9 concentrations in WT or liver-specific PCSK9-deficient mice confirmed the inability of the small intestine to secrete PCSK9 into the portal compartment. Altogether, our data demonstrate that mature enterocytes do not secrete PCSK9 and reinforce the central role of the liver in the regulation of the concentration of circulating PCSK9 and consequently of cellular LDL receptors.

Circulating Rather Than Intestinal PCSK9 (Proprotein Convertase Subtilisin Kexin Type 9) Regulates Postprandial Lipemia in Mice.

OBJECTIVE: Increased postprandial lipemia (PPL) is an independent risk factor for atherosclerotic cardiovascular diseases. PCSK9 (Proprotein convertase subtilisin kexin type 9) is an endogenous inhibitor of the LDLR (low-density lipoprotein receptor) pathway. We previously showed that PCSK9 inhibition in mice reduces PPL. However, the relative contribution of intracellular intestinal PCSK9 or liver-derived circulating PCSK9 to this effect is still unclear. Approach and Results: To address this issue, we generated the first intestine-specific Pcsk9-deficient (i-Pcsk9-/-) mouse model. PPL was measured in i-Pcsk9-/- as well as in wild-type and streptozotocin-induced diabetic mice following treatment with a PCSK9 monoclonal antibody (alirocumab). Blocking the circulating form of PCSK9 with alirocumab significantly reduced PPL, while overexpressing human PCSK9 in the liver of full Pcsk9-/- mice had the opposite effect. Alirocumab regulated PPL in a LDLR-dependent manner as this effect was abolished in Ldlr-/- mice. In contrast, i-Pcsk9-/- mice did not exhibit alterations in plasma lipid parameters nor in PPL. Finally, PPL was highly exacerbated by streptozotocin-induced diabetes mellitus in Pcsk9+/+ but not in Pcsk9-/- mice, an effect that was mimicked by the use of alirocumab in streptozotocin-treated Pcsk9+/+ mice. CONCLUSIONS: Taken together, our data demonstrate that PPL is significantly altered by full but not intestinal PCSK9 deficiency. Treatment with a PCSK9 monoclonal antibody mimics the effect of PCSK9 deficiency on PPL suggesting that circulating PCSK9 rather than intestinal PCSK9 is a critical regulator of PPL. These data validate the clinical relevance of PCSK9 inhibitors to reduce PPL, especially in patients with type 2 diabetes mellitus.

VLDL (Very-Low-Density Lipoprotein)-Apo E (Apolipoprotein E) May Influence Lp(a) (Lipoprotein [a]) Synthesis or Assembly.

OBJECTIVE: To clarify the association between PCSK9 (proprotein convertase subtilisin/kexin type 9) and Lp(a) (lipoprotein [a]), we studied Lp(a) kinetics in patients with loss-of-function and gain-of-function PCSK9 mutations and in patients in whom extended-release niacin reduced Lp(a) and PCSK9 concentrations. Approach and Results: Six healthy controls, 9 heterozygous patients with familial hypercholesterolemia (5 with low-density lipoprotein receptor [LDLR] mutations and 4 with PCSK9 gain-of-function mutations) and 3 patients with heterozygous dominant-negative PCSK9 loss-of-function mutations were included in the preliminary study. Eight patients were enrolled in a second study assessing the effects of 2 g/day extended-release niacin. Apolipoprotein kinetics in VLDL (very-low-density lipoprotein), LDL (low-density lipoprotein), and Lp(a) were studied using stable isotope techniques. Plasma Lp(a) concentrations were increased in PCSK9-gain-of-function and familial hypercholesterolemia-LDLR groups compared with controls and PCSK9-loss-of-function groups (14±12 versus 5±4 mg/dL; P=0.04), but no change was observed in Lp(a) fractional catabolic rate. Subjects with PCSK9-loss-of-function mutations displayed reduced apoE (apolipoprotein E) concentrations associated with a VLDL-apoE absolute production rate reduction. Lp(a) and VLDL-apoE absolute production rates were correlated (r=0.50; P<0.05). ApoE-to-apolipoprotein (a) molar ratios in Lp(a) increased with plasma Lp(a) (r=0.96; P<0.001) but not with PCSK9 levels. Extended-release niacin-induced reductions in Lp(a) and VLDL-apoE absolute production rate were correlated (r=0.83; P=0.015). In contrast, PCSK9 reduction (-35%; P=0.008) was only correlated with that of VLDL-apoE absolute production rate (r=0.79; P=0.028). CONCLUSIONS: VLDL-apoE production could determine Lp(a) production and/or assembly. As PCSK9 inhibitors reduce plasma apoE and Lp(a) concentrations, apoE could be the link between PCSK9 and Lp(a).

Unexpected metabolic disorders induced by endocrine disruptors in Xenopus tropicalis provide new lead for understanding amphibian decline.

Despite numerous studies suggesting that amphibians are highly sensitive to endocrine disruptors (EDs), both their role in the decline of populations and the underlying mechanisms remain unclear. This study showed that frogs exposed throughout their life cycle to ED concentrations low enough to be considered safe for drinking water, developed a prediabetes phenotype and, more commonly, a metabolic syndrome. Female Xenopus tropicalis exposed from tadpole stage to benzo(a)pyrene or triclosan at concentrations of 50 ng⋅L-1 displayed glucose intolerance syndrome, liver steatosis, liver mitochondrial dysfunction, liver transcriptomic signature, and pancreatic insulin hypersecretion, all typical of a prediabetes state. This metabolic syndrome led to progeny whose metamorphosis was delayed and occurred while the individuals were both smaller and lighter, all factors that have been linked to reduced adult recruitment and likelihood of reproduction. We found that F1 animals did indeed have reduced reproductive success, demonstrating a lower fitness in ED-exposed Xenopus Moreover, after 1 year of depuration, Xenopus that had been exposed to benzo(a)pyrene still displayed hepatic disorders and a marked insulin secretory defect resulting in glucose intolerance. Our results demonstrate that amphibians are highly sensitive to EDs at concentrations well below the thresholds reported to induce stress in other vertebrates. This study introduces EDs as a possible key contributing factor to amphibian population decline through metabolism disruption. Overall, our results show that EDs cause metabolic disorders, which is in agreement with epidemiological studies suggesting that environmental EDs might be one of the principal causes of metabolic disease in humans.

A high-throughput mass spectrometry-based assay for large-scale profiling of circulating human apolipoproteins.

Apolipoproteins govern lipoprotein metabolism and are promising biomarkers of metabolic and cardiovascular diseases. Unlike immunoassays, MS enables the quantification and phenotyping of multiple apolipoproteins. Hence, here, we aimed to develop a LC-MS/MS assay that can simultaneously quantitate 18 human apolipoproteins [A-I, A-II, A-IV, A-V, B48, B100, C-I, C-II, C-III, C-IV, D, E, F, H, J, L1, M, and (a)] and determined apoE, apoL1, and apo(a) phenotypes in human plasma and serum samples. The plasma and serum apolipoproteins were trypsin digested through an optimized procedure and peptides were extracted and analyzed by LC-MS/MS. The method was validated according to standard guidelines in samples spiked with known peptide amounts. The LC-MS/MS results were compared with those obtained with other techniques, and reproducibility, dilution effects, and stabilities were also assessed. Peptide markers were successfully selected for targeted apolipoprotein quantification and phenotyping. After optimization, the assay was validated for linearity, lower limits of quantification, accuracy (biases: -14.8% to 12.1%), intra-assay variability [coefficients of variation (CVs): 1.5-14.2%], and inter-assay repeatability (CVs: 4.1-14.3%). Bland-Altman plots indicated no major statistically significant differences between LC-MS/MS and other techniques. The LC-MS/MS results were reproducible over five repeated experiments (CVs: 1.8-13.7%), and we identified marked differences among the plasma and serum samples. The LC-MS/MS assay developed here is rapid, requires only small sampling volumes, and incurs reasonable costs, thus making it amenable for a wide range of studies of apolipoprotein metabolism. We also highlight how this assay can be implemented in laboratories.

ABCB1 (P-glycoprotein) regulates vitamin D absorption and contributes to its transintestinal efflux.

Efficient intestinal absorption of dietary vitamin D is required in most people to ensure an adequate status. Thus, we investigated the involvement of ATP binding cassette subfamily B member 1 (ABCB1) in vitamin D intestinal efflux. Both cholecalciferol (D3) and 25-hydroxycholecalciferol [25(OH)D3] apical effluxes were decreased by chemical inhibition of ABCB1 in Caco-2 cells and increased by ABCB1 overexpression in Griptites or Madin-Darby canine kidney type II cells. Mice deficient for the 2 murine ABCB1s encoded by Abcb1a and Abcb1b genes ( Abcb1-/-) displayed an accumulation of 25(OH)D3 in plasma, intestine, brain, liver, and kidneys, together with an increased D3 postprandial response after gavage compared with controls. 25(OH)D3 efflux through Abcb1-/- intestinal explants was markedly decreased compared with controls. This reduction of 25(OH)D3 transfer from plasma to lumen was further confirmed in vivo in intestine-perfused mice. Docking experiments established that both D3 and 25(OH)D3 could bind with high affinity to Caenorhabditis elegans P-glycoprotein, used as an ABCB1 model. Finally, in a group of 39 healthy male adults, a single-nucleotide polymorphism (SNP) in ABCB1 (rs17064) was significantly associated with the fasting plasma 25(OH)D3 concentration. Thus, we showed here for the first time that ABCB1 is involved in neo-absorbed vitamin D efflux by the enterocytes and that it also contributes to vitamin D transintestinal excretion and likely impacts vitamin D status.-Margier, M., Collet, X., le May, C., Desmarchelier, C., André, F., Lebrun, C., Defoort, C., Bluteau, A., Borel, P., Lespine, A., Reboul, E. ABCB1 (P-glycoprotein) regulates vitamin D absorption and contributes to its transintestinal efflux.

Glycaemic control influences the relationship between plasma PCSK9 and LDL cholesterol in type 1 diabetes.

Pro-protein convertase subtilisin/kexin type 9 (PCSK9) is a critical regulator of LDL cholesterol metabolism. Little is known, however, about the regulation of PCSK9 in patients with type 1 diabetes (T1D). In the present study, we aimed to determine the relationship between circulating PCSK9 and metabolic variables in T1D. Plasma PCSK9 levels were measured in 195 people with T1D (mean age 38.8 years, mean diabetes duration 17.2 years, mean glycated haemoglobin [HbA1c] 8.3%), who were free of any lipid-lowering agent. Plasma PCSK9 was positively correlated with LDL cholesterol (P = .0007), triglycerides (P = .004), apolipoprotein B (P = .005), HbA1c (P = .003), systolic (P = .003) and diastolic (P = .001) blood pressure and body mass index (0.02). In multivariate analysis, PCSK9 concentration was independently associated with HbA1c (P = .02) and LDL cholesterol (P = .03). After classifying patients according to HbA1c tertile, the correlation between PCSK9 and LDL cholesterol was only observed in the highest tertile (P = .0006; Rho = 0.43), whereas no correlation was found in the lowest and intermediate tertiles. This study suggests that good glycaemic control abolishes the positive relationship between PCSK9 and LDL cholesterol in patients with T1D; however, the underlying molecular mechanisms remain to be established.

The Role of Estrogens in Pancreatic Islet Physiopathology.

In rodent models of insulin-deficient diabetes, 17ß-estradiol (E2) protects pancreatic insulin-producing ß-cells against oxidative stress, amyloid polypeptide toxicity, gluco-lipotoxicity, and apoptosis. Three estrogen receptors (ERs)-ERα, ERß, and the G protein-coupled ER (GPER)-have been identified in rodent and human ß-cells. This chapter describes recent advances in our understanding of the role of ERs in islet ß-cell function, nutrient homeostasis, survival from pro-apoptotic stimuli, and proliferation. We discuss why and how ERs represent potential therapeutic targets for the maintenance of functional ß-cell mass.

PPARγ Modulates Long Chain Fatty Acid Processing in the Intestinal Epithelium.

Nuclear receptor PPARγ affects lipid metabolism in several tissues, but its role in intestinal lipid metabolism has not been explored. As alterations have been observed in the plasma lipid profile of ad libitum fed intestinal epithelium-specific PPARγ knockout mice (iePPARγKO), we submitted these mice to lipid gavage challenges. Within hours after gavage with long chain unsaturated fatty acid (FA)-rich canola oil, the iePPARγKO mice had higher plasma free FA levels and lower gastric inhibitory polypeptide levels than their wild-type (WT) littermates, and altered expression of incretin genes and lipid metabolism-associated genes in the intestinal epithelium. Gavage with the medium chain saturated FA-rich coconut oil did not result in differences between the two genotypes. Furthermore, the iePPARγKO mice did not exhibit defective lipid uptake and stomach emptying; however, their intestinal transit was more rapid than in WT mice. When fed a canola oil-rich diet for 4.5 months, iePPARγKO mice had higher body lean mass than the WT mice. We conclude that intestinal epithelium PPARγ is activated preferentially by long chain unsaturated FAs compared to medium chain saturated FAs. Furthermore, we hypothesize that the iePPARγKO phenotype originates from altered lipid metabolism and release in epithelial cells, as well as changes in intestinal motility.

Role of PCSK9 beyond liver involvement.

PURPOSE OF REVIEW: Proprotein convertase subtilisin kexin type 9 (PCSK9) acts as an endogenous natural inhibitor of the LDL receptor pathway, by targeting the receptor to lysosomes for degradation. Beside the liver, PCSK9 is also expressed at significant levels in other tissues, where its function remains unclear. The current review focuses on the extrahepatic actions of PCSK9. RECENT FINDINGS: The generation of liver-specific PCSK9 knockout mice has clearly indicated that PCSK9 affects cholesterol homeostasis via its action on extrahepatic organs. PCSK9 is highly expressed in the intestine, where it controls the production of triglyceride-rich lipoproteins and the transintestinal cholesterol excretion. The role of PCSK9 in the endocrine pancreas and glucose homeostasis remains unclear because conflicting data exist concerning the metabolic phenotype of PCSK9-deficient mice. Sparse data suggest that PCSK9 might also play a role in kidneys, vascular smooth muscle cells, and neurons. SUMMARY: Based on the virtuous combination of genetic and pharmacological approaches, the major function of PCSK9 as a key regulator of hepatic LDL receptor metabolism had quickly emerged. Accumulating evidence indicates that intestinal PCSK9 is also involved in the modulation of lipid homeostasis. Additional studies are warranted to decipher the physiological function of PCSK9 in other extrahepatic tissues and thus to better assess the safety of PCSK9 inhibitors.

The loss-of-function PCSK9 p.R46L genetic variant does not alter glucose homeostasis.

AIMS/HYPOTHESIS: Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a critical regulator of cholesterol homeostasis. PCSK9 inhibitors are being actively developed to lower LDL-cholesterol levels. However, there are conflicting data regarding the consequences of Pcsk9 deficiency on glucose homeostasis in mouse models. Here, we analysed in humans the association between the PCSK9 p.R46L loss-of-function (LOF) variant and (1) glucose homeostasis variables; (2) type 2 diabetes status; and (3) the risk of 9 year incident type 2 diabetes in a prospective study. METHODS: PCSK9 p.R46L was genotyped in 4630 French participants from the Data from an Epidemiological Study on the Insulin Resistance Syndrome (DESIR) prospective study and in 1342 French participants with type 2 diabetes. The association between p.R46L and metabolic traits or type 2 diabetes risk was assessed through linear or logistic regression models adjusted for age, sex and BMI. The association between p.R46L and incident type 2 diabetes was assessed using a Cox regression model adjusted for sex, age and BMI at baseline. RESULTS: Significant associations (p < 10(-6)) were found between p.R46L and lower total cholesterol (-0.394 mmol/l), LDL-cholesterol (-0.393 mmol/l) and apolipoprotein B concentrations (-0.099 g/l). However, no significant association was observed between p.R46L and markers of glucose homeostasis (including fasting glucose, fasting insulin, HbA1c, HOMA-B, HOMA-IR) or type 2 diabetes risk. Furthermore, no significant association between p.R46L variant and risk of incident type 2 diabetes was observed in DESIR. CONCLUSIONS/INTERPRETATION: The PCSK9 p.R46L LOF variant was not associated with impaired glucose homeostasis in humans. These data are reassuring regarding the safety of PCSK9 inhibitors.

Congenital lipodystrophies and dyslipidemias.

Lipodystrophies are rare acquired and genetic disorders characterized by the selective loss of adipose tissue. One key metabolic feature of patients with congenital inherited lipodystrophy is hypertriglyceridemia. The precise mechanisms by which the lack of adipose tissue causes dyslipidemia remain largely unknown. In recent years, new insights have arisen from data obtained in vitro in adipocytes, yeast, drosophila, and very recently in several genetically modified mouse models of generalized lipodystrophy. A common metabolic pathway involving accelerated lipolysis and defective energy storage seems to contribute to the dyslipidemia associated with congenital generalized lipodystrophy syndromes, although the pathophysiological changes may vary with the nature of the mutation involved. Therapeutic management of dyslipidemia in patients with lipodystrophy is primarily based on specific approaches using recombinant leptin therapy. Preclinical studies suggest a potential efficacy of thiazolidinediones that remains to be assessed in dedicated clinical trials.

Transintestinal cholesterol excretion is an active metabolic process modulated by PCSK9 and statin involving ABCB1.

OBJECTIVE: Transintestinal cholesterol excretion (TICE) is an alternate pathway to hepatobiliary secretion. Our study aimed at identifying molecular mechanisms of TICE. APPROACH AND RESULTS: We studied TICE ex vivo in mouse and human intestinal explants, and in vivo after bile diversion and intestinal cannulation in mice. We provide the first evidence that both low-density lipoprotein (LDL) and high-density lipoprotein deliver cholesterol for TICE in human and mouse jejunal explants at the basolateral side. Proprotein convertase subtilisin kexin type 9 (PCSK9)(-/-) mice and intestinal explants show increased LDL-TICE, and acute injection of PCSK9 decreases TICE in vivo, suggesting that PCSK9 is a repressor of TICE. The acute repression was dependent on the LDL receptor (LDLR). Further, TICE was increased when mice were treated with lovastatin. These data point to an important role for LDLR in TICE. However, LDLR(-/-) mice showed increased intestinal LDL uptake, contrary to what is observed in the liver, and tended to have higher TICE. We interpret these data to suggest that there might be at least 2 mechanisms contributing to TICE; 1 involving LDL receptors and other unidentified mechanisms. Acute modulation of LDLR affects TICE, but chronic deficiency is compensated for most likely by the upregulation of the unknown mechanisms. Using mice deficient for apical multidrug active transporter ATP-binding cassette transporter B1 a and b, and its inhibitor, we show that these apical transporters contribute significantly to TICE. CONCLUSIONS: TICE is operative in human jejunal explants. It is a metabolically active process that can be acutely regulated, inversely related to cholesterolemia, and pharmacologically activated by statins.