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Carriers of the hydroxysteroid 17-β dehydrogenase 13 (HSD17B13) gene variant (rs72613567:TA) have a reduced risk of NASH and cirrhosis but not steatosis. We determined its effect on liver histology, lipidome, and transcriptome using ultra performance liquid chromatography-mass spectrometry and RNA-seq. In carriers and noncarriers of the gene variant, we also measured pathways of hepatic fatty acids (de novo lipogenesis [DNL] and adipose tissue lipolysis [ATL] using 2H2O and 2H-glycerol) and insulin sensitivity using 3H-glucose and euglycemic-hyperinsulinemic clamp) and plasma cytokines. Carriers and noncarriers had similar age, sex and BMI. Fibrosis was significantly less frequent while phospholipids, but not other lipids, were enriched in the liver in carriers compared with noncarriers. Expression of 274 genes was altered in carriers compared with noncarriers, consisting predominantly of downregulated inflammation-related gene sets. Plasma IL-6 concentrations were lower, but DNL, ATL and hepatic insulin sensitivity were similar between the groups. In conclusion, carriers of the HSD17B13 variant have decreased fibrosis and expression of inflammation-related genes but increased phospholipids in the liver. These changes are not secondary to steatosis, DNL, ATL, or hepatic insulin sensitivity. The increase in phospholipids and decrease in fibrosis are opposite to features of choline-deficient models of liver disease and suggest HSD17B13 as an attractive therapeutic target.

IgE has a key role in the pathogenesis of allergic responses through its ability to activate mast cells via the receptor FcεR1. In addition to mast cells, many cell types implicated in atherogenesis express FcεR1, but whether IgE has a role in this disease has not been determined. Here, we demonstrate that serum IgE levels are elevated in patients with myocardial infarction or unstable angina pectoris. We found that IgE and the FcεR1 subunit FcεR1α were present in human atherosclerotic lesions and that they localized particularly to macrophage-rich areas. In mice, absence of FcεR1α reduced inflammation and apoptosis in atherosclerotic plaques and reduced the burden of disease. In cultured macrophages, the presence of TLR4 was required for FcεR1 activity. IgE stimulated the interaction between FcεR1 and TLR4, thereby inducing macrophage signal transduction, inflammatory molecule expression, and apoptosis. These IgE activities were reduced in the absence of FcεR1 or TLR4. Furthermore, IgE activated macrophages by enhancing Na+/H+ exchanger 1 (NHE1) activity. Inactivation of NHE1 blocked IgE-induced macrophage production of inflammatory molecules and apoptosis. Cultured human aortic SMCs (HuSMCs) and ECs also exhibited IgE-induced signal transduction, cytokine expression, and apoptosis. In human atherosclerotic lesions, SMCs and ECs colocalized with IgE and TUNEL staining. This study reveals what we believe to be several previously unrecognized IgE activities that affect arterial cell biology and likely other IgE-associated pathologies in human diseases.

Degranulated mast cells are present in human fatty streaks. Chymase in granules released from degranulated rat serosal mast cells, i.e., in granule remnants, proteolyzes human high density lipoprotein3 (HDL3), and so reduces its ability to induce cholesterol efflux from macrophage foam cells in vitro. In this study we found that remnant chymase, by proteolyzing human serum and human aortic intimal fluid, prevents these two physiologic fluids from effectively inducing cholesterol efflux from cultured macrophage foam cells. Inhibition was strongest when remnants were added to apolipoprotein AI (apoAI)-containing lipoproteins; the remnants had no effect on the weaker efflux produced by apoAI-deficient serum. Western blot analysis showed that granule remnants degrade apoAI in serum and in internal fluid. When released from remnants, chymase lost its ability to proteolyze HDL3 in the presence of serum. Thus, remnant chymase (but not isolated chymase) was able to resist the natural protease inhibitors present in serum and in intimal fluid. The results imply participation of exocytosed mast cell granules in foam cell formation in atherogenesis.

A mutation of the LDL receptor gene very common among Finnish patients with heterozygous familial hypercholesterolemia (FH) was identified. This mutation, designated as FH-North Karelia, deletes seven nucleotides from exon 6 of the LDL receptor gene, causes a translational frameshift, and is predicted to result in a truncated receptor protein. Only minute quantities of mRNA corresponding to the deleted gene were detected. Functional studies using cultured fibroblasts from the patients revealed that the FH-North Karelia gene is associated with a receptor-negative (or binding-defective) phenotype of FH. Carriers of the FH-North Karelia gene showed a typical xanthomatous form of FH, with mean serum total and LDL cholesterol levels of 12 and 10 mmol/liter, respectively. This mutation was found in 69 (34%) out of 201 nonrelated Finnish FH patients and was especially abundant (prevalence 79%) in patients from the eastern Finland. These results, combined with our earlier data on another LDL receptor gene deletion (FH-Helsinki), demonstrate that two "Finnish-type" mutant LDL receptor genes make up about two thirds of FH mutations in this country, reflecting a founder gene effect. This background provides good possibilities to examine whether genetic heterogeneity affects the clinical presentation or responsiveness to therapeutic interventions in FH.

Patients with familial dysbetalipoproteinemia (F. Dys.), also called familial type 3 hyperlipoproteinemia, are homozygous for a mutant allele, Ed, that specifies an abnormal form of apoprotein (apo) E, a prominent constituent of remnant lipoproteins derived from very low density lipoproteins (VLDL) and chylomicrons. Apo E is thought to mediate the removal of remnant lipoproteins from the plasma by virtue of its ability to bind to hepatic lipoprotein receptors. In F. Dys. patients, remnant-like lipoproteins accumulate, apparently because of delayed clearance by the liver. In the current studies, we show that the abnormal protein specified by the Ed allele (apo E-D) from some, but not all, patients with F. Dys. has a markedly deficient ability to bind to low density lipoprotein (LDL) receptors. Apo E was isolated from eight control subjects and nine patients with F. Dys. and incorporated into phospholipid complexes. The complexes were tested for their ability to compete with human 125I-LDL or rabbit 125I-beta-VLDL fo binding to LDL receptors in four assay systems: cultured human fibroblasts, solubilized receptors from bovine adrenal cortex, liver membranes from rats treated with 17 alpha-ethinyl estradiol, and liver membranes from normal rabbits. The apo E-D from six of the nine patients with F. Dys. showed binding affinities for LDL receptors that were reduced by greater than 98% in all receptor assays (group 1 patients). All of these group 1 patients were unequivocally of phenotype apo E-D/D by the criterion of isoelectric focussing. The apo E from the three other F. Dys. patients showed a near normal binding ability in all four of the receptor assays (group 2 patients). One of these group 2 patients appeared to have the apo E-D/D phenotype by isoelectric focussing. In the other two patients in group 2, apo E-D was the predominant protein (phenotype, apo E-D/D), but traces of protein in the region corresponding to normal apo E (apo E-N) were also present. The difference between group 1 and group 2 patients was also apparent when the apo E was iodinated and tested directly for binding to liver membranes from rats treated with 17 alpha-ethinyl estradiol. The 125I-labeled apo E from a group 2 patient, but not a group 1 patient, showed enhanced uptake when perfused through the liver of an estradiol-treated rate, indicating that the receptor binding ability of apo E correlated with uptake in the intact liver. The current studies allow the subdivision of patients with F. Dys. into two groups. In group 1, the elevated plasma level of remnants appears to be due to a diminished receptor binding activity of the abnormal protein specified by the Ed allele; in group 2 patients, the cause of the elevated plasma level of remnants remains to be explained.

Salla disease is a lysosomal storage disorder characterized by mental retardation and disturbed sialic acid metabolism. To study endogenous synthesis and breakdown of sialic acid, fibroblasts were incubated for 5 d in the presence and then in the absence of N-[3H]acetylmannosamine. Labeling of free sialic acid was 5-10 times higher in mutant than in normal cells. Radioactivity decreased in 4 d by 75% in normal but only by 30% in mutant fibroblasts. The labeling pattern was not normalized upon coculture of mutant and normal cells. To study the metabolism of extracellular sialic acid, low-density lipoprotein (LDL) was labeled in the sialic acid moiety (periodate-NaB3H4) or in the protein moiety (125I). Binding, internalization, lysosomal degradation, and exit of products of protein catabolism were similar in normal and mutant fibroblasts. Upon incubation with LDL labeled in the sialic acid moiety, mutant cells accumulated 2-3 times more free sialic acid radioactivity than normal fibroblasts, mostly in the lysosomal fraction. After a 24-h chase incubation, radioactivity in free sialic acid decreased by 70-80% in normal but only by 10-30% in mutant cells. In mutant fibroblasts, 40% of the radioactivity remained in lysosomes, whereas no labeled free sialic acid was detected in lysosomes from normal fibroblasts. We conclude that in Salla disease, fibroblast endogenous synthesis of sialic acid and lysosomal cleavage of exogenous glycoconjugates is normal, but free sialic acid cannot leave the lysosome. These findings suggest that the basic defect in Salla disease is deficient transport of free sialic acid through the lysosomal membrane.

A specific type of gene mutation affecting the LDL receptor has been found in many Finnish patients with familial hypercholesterolemia (FH). The mutant allele is characterized by a 9.5-kb deletion extending from intron 15 to exon 18. Molecular cloning and sequencing of a cDNA segment corresponding to the deleted allele indicated that the mutant receptor differs radically from the normal one because of loss of the domains encoded by exons 16, 17, and 18. The carboxy-terminal portion of the normal receptor, comprising the amino acids 750-839, has been replaced by an unrelated stretch of 55 amino acids. The mutant allele was found to occur in 23 (50%) of 46 unrelated FH patients with an established functional defect in the LDL receptor. In cultured fibroblasts from the FH patients with the 9.5-kb deletion, both receptor-mediated binding and internalization of 125I-LDL were lower than normal, the former, on average, by 25%, and the latter, on average, by 50%. This combined functional defect probably results from both impaired attachment and impaired internalization of the mutated receptor. It remains to be investigated whether this Finnish type of LDL receptor gene mutation, here designated FH-Helsinki, occurs in other ethnic groups.