Isolation and characterization of the gene for mouse hormone-sensitive lipase

Hormone-sensitive lipase (HSL) is the rate-limiting enzyme in hydrolysis of triglycerides in adipose tissue and of cholesteryl esters in steroidogenic tissues and macrophages. The gene encoding mouse HSL has been isolated and characterized from two overlapping lambda clones. The gene spans approximately 10.4 kb and comprises 9 exons interrupted by 8 introns. The deduced amino acid sequence specifies a protein of 759 amino acids with a Mr of 83,297 in the absence of posttranslational modifications. The known functional domains of the HSL protein are encoded by discrete exons, with the putative catalytic site (Ser423) encoded by exon 6, and the basal and regulatory phosphorylation sites (Ser557 and Ser559) encoded by exon 8. In addition, a putative lipid binding domain occurs in exon 9. The mouse protein shows 94% identity with the previously determined rat sequence and 85% identity with the recently determined human sequence. Interestingly, despite the high degree of similarity, the three species diverge significantly for a stretch of 16 amino acid residues upstream of the phosphorylation sites. In addition, an error was discovered in the carboxyl-terminal portion of the previously reported rat sequence, which produced a frame shift and premature termination of the coding sequence. The corrected rat sequence alters the identity of 12 amino acid residues and extends the protein an additional 11 residues. We have also examined the mouse HSL gene and 5' flanking region for nucleotide sequences that may modulate HSL gene transcription. Using primer extension, we identified a major transcription initiation site 593 nucleotides upstream of the protein coding sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the apolipoprotein AI and CIII genes in the domestic pig

The apolipoproteins (apo) AI and CIII are important constituents of triglyceride-rich lipoproteins and high-density lipoproteins. In humans, apo AI is believed to play an important protective role in the pathogenesis of arteriosclerosis, whereas apo CIII might be involved in the development of hypertriglyceridemia. Both human genes are located within a gene cluster on chromosome 11. Although the domestic pig has been widely used as an animal model in arteriosclerosis and lipid research, the porcine apolipoproteins genes are poorly characterized. In this report, the complete nucleotide sequences of the porcine apo AI and CIII genes are presented and we demonstrate, for the first time, apo CIII expression in the pig. Both genes are composed of four exons and three introns and resemble closely their human counterparts with regard to the transcriptional start sites, exon sizes, intron sizes, exon-intron borders, and the size of the intergenic region. The predicted pig apo AI is a protein of 241 amino acids, which is 2 amino acids shorter than human apo AI. The protein sequence was found to be very homologous to apo AI sequences in other mammalian species. Apo AI expression was detected on the mRNA level in porcine liver and intestine. The apo CIII gene encodes a protein with 73 amino acids, which is 6 amino acids shorter than human apo CIII. In contrast to the three isoforms of apo CIII found in humans, only one major isoform was detected in the pig. Presumably this isoform is unglycosylated. In addition to apo CIII expression in the liver and the intestine, a truncated form of apo CIII mRNA was also found in porcine kidney. Our studies demonstrate the presence of an apo CIII gene, an apo CIII mRNA, and an apo CIII protein in the pig and, therefore, exclude a hypothesized apo CIII deficiency in these animals.

Metabolism of very low density lipoproteins in the pig. An in vivo study

1. The metabolism of apolipoprotein B (apoB) was investigated in pigs injected with [125I]very low density lipoproteins (VLDL) to determine to which extent the two distinct low density lipoprotein subclasses (LDL1 and LDL2) derive from VLDL. 2. The lipoproteins were isolated by density gradient ultracentrifugation and the transfer of radioactivity from VLDL into LDL1 and LDL2 apoB was measured. 3. Only a minor portion of VLDL apoB was converted to LDL1 (7.7 +/- 3.2%) and LDL2 (3.6 +/- 1.5%), respectively. Thus, we conclude that the major portion of LDL, especially LDL2, is synthesized independently from VLDL catabolism.

Action of lecithin-cholesterol acyltransferase on low-density lipoproteins in native pig plasma

The action of lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) on the different pig lipoprotein classes was investigated with emphasis on low-density lipoproteins (LDL). It was demonstrated previously that LDL can serve as substrate for LCAT, probably because they contain sufficient amounts of apoA-I and other non-apoB proteins, known as LCAT activators. Upon a 24-h incubation of pig plasma in vitro in the presence of active LCAT, both pig LDL subclasses, LDL-1 and LDL-2, fused together, forming one fraction, as revealed by analytical ultracentrifugation. This fusion was time dependent, becoming visible after 3 h and complete after 18 h of incubation. Concomitantly, free cholesterol and phospholipids decreased and cholesteryl esters increased. When isolated LDL-1 and LDL-2 were incubated with purified pig LCAT for 24 h, LDL-1 floated toward higher densities and LDL-2 toward lower densities, although this effect was not as pronounced as in incubations of whole serum. In further experiments, pig serum was incubated for various periods of time in the presence and absence of the LCAT inhibitor sodium iodoacetate. The individual lipoproteins then were separated by density gradient ultracentrifugation or by specific immunoprecipitation and chemically analyzed. Both methods revealed that in the absence of active LCAT there was a transfer of free cholesterol from LDL to high-density lipoproteins (HDL) and a small transfer of cholesteryl esters in the opposite direction. In the presence of LCAT the loss of free cholesterol started immediately in all three lipoprotein classes, was most prominent in LDL, and was proportional to the newly synthesized cholesteryl esters incorporated in each fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the substrate specificity of human and pig lecithin: cholesterol acyltransferase: role of low-density lipoproteins

The substrate properties of low-density lipoprotein (LDL) fractions from human and pig plasma and of lipoprotein a [Lp(a)] upon incubation with either pig or human lecithin:cholesterol acyltransferase (LCAT, EC 2.3.1.43) were investigated and compared with those of pig high-density lipoproteins (HDL) or human HDL-3. The cholesterol esterification using purified native pig LDL-1, human LDL, or Lp(a) as a substrate was approximately 36-42% that of pig HDL or human HDL-3, while cholesteryl ester formation with pig LDL-2 was 41-47%. No significant difference was found in the substrate activity between pig HDL and human HDL-3, and between human LDL and Lp(a), respectively. After depletion of pig LDL-1, pig LDL-2, and human LDL from apolipoprotein A-I (apoA-I), cholesteryl ester formation decreased to about 22-28% of the value found with pig HDL. Depletion of human LDL from apolipoprotein E (apoE) did not result in significantly different esterification rates in comparison to native LDL. Total removal of non-apoB proteins from human LDL resulted in esterification rates of approximately 10-15% that of HDL. Readdition of apoA-I to all these LDL fractions produced solely in apoA-I-depleted LDL fractions an increase of cholesteryl ester formation, whereas in those LDL fractions that were additionally depleted from apoE and/or from apoC polypeptides, a further decrease in the esterification rate occurred.(ABSTRACT TRUNCATED AT 250 WORDS)