Substrate and positional specificities of human and mouse lecithin-cholesterol acyltransferases. Studies with wild type recombinant and chimeric enzymes expressed in vitro

Reverse cholesterol transport in type 2 diabetes mellitus

High-density lipoprotein (HDL) plays an important protective role against atherosclerosis, and the anti-atherogenic properties of HDL include the promotion of cellular cholesterol efflux and reverse cholesterol transport (RCT), as well as antioxidant, anti-inflammatory and anticoagulant effects. RCT is a complex pathway, which transports cholesterol from peripheral cells and tissues to the liver for its metabolism and biliary excretion. The major steps in the RCT pathway include the efflux of free cholesterol mediated by cholesterol transporters from cells to the main extracellular acceptor HDL, the conversion of free cholesterol to cholesteryl esters and the subsequent removal of cholesteryl ester in HDL by the liver. The efficiency of RCT is influenced by the mobilization of cellular lipids for efflux and the intravascular remodelling and kinetics of HDL metabolism. Despite the increased cardiovascular risk in people with type 2 diabetes, current knowledge on RCT in diabetes is limited. In this article, abnormalities in RCT in type 2 diabetes mellitus and therapeutic strategies targeting HDL and RCT will be reviewed.

Amino acids 149 and 294 of human lecithin:cholesterol acyltransferase affect fatty acyl specificity

We identified two regions of human LCAT (hLCAT) that when mutated separately to the corresponding rat sequence (E149A and Y292H/W294F) and transiently expressed in COS-1 cells increased phospholipase A2 (PLA2) activity by 5.5- and 2.8-fold, respectively, and increased cholesteryl ester (CE) formation by 2.9- and 1.4-fold, respectively, relative to hLCAT using substrate particles containing 1-16:0,2-20:4-sn-glycero-3-phosphocholine (PAPC). In contrast, both activities with 1-16:0,2-18:1-sn-glycero-3-phosphocholine (POPC) substrate were similar among the three LCAT proteins. The triple mutant (E149A/Y292H/W294F) had increased PLA2 activity with PAPC similar to that observed with the E149A mutation alone; however, unlike E149A, the triple mutant demonstrated a 50% decrease in activity with POPC for both PLA2 activity and CE formation, suggesting an interaction between the two regions of LCAT. Additional mutagenesis studies demonstrated that W294F, but not Y292H, increased PLA2 activity by 3-fold with PAPC without affecting activity with POPC. The E149A/W294F double mutation mimicked the LCAT activity phenotype of the triple mutant (more activity with PAPC, less with POPC). In conclusion, separate mutation of two amino acids in hLCAT to the corresponding rat sequence increases activity with PAPC, whereas the combined mutations increase PAPC and decrease POPC activity, suggesting that these amino acids participate in the LCAT PC binding site and affect fatty acyl specificity.

Evidence for altered positional specificity of LCAT in vivo: studies with docosahexaenoic acid feeding in humans

The percentage of saturated cholesteryl esters (CEs) synthesized by human LCAT is several times higher than expected from the sn-2 acyl composition of plasma phosphatidylcholine (PC), whereas the synthesis of 20:4 CE and 22:6 CE is much lower than expected. To explain these discrepancies, we proposed that LCAT transfers some saturated fatty acids from the sn-1 position of PC species that contain 20:4 or 22:6 at sn-2. The present studies provide in vivo evidence for this hypothesis. We determined the composition and synthesis of CE species in plasma of volunteers before and after a 6 week dietary supplementation with docosahexaenoic acid (22:6; DHA). In addition to an increase in the DHA content of all plasma lipids, there was a significant (+12%; P <0.005) increase of 16:0 CE, although there was no increase in 16:0 at sn-2 of PC. The increase of DHA in CE was much lower than its increase at sn-2 of PC. Ex vivo synthesis of CE species in plasma showed a significant (+24%; P <0.005) increase in the synthesis of 16:0 CE after DHA supplementation, which correlated positively with the increase of 22:6, but not of 16:0, at sn-2 of PC. These results show that the positional specificity of human LCAT is altered when the concentration of 16:0-22:6 PC is increased by DHA supplementation.

Regulation of reverse cholesterol transport and clinical implications

Plasma levels of high-density lipoprotein (HDL) cholesterol and its major protein, apolipoprotein A-I, are inversely correlated with the incidence of atherosclerotic cardiovascular disease. Low HDL cholesterol and apolipoprotein A-I levels often are found in association with other cardiovascular risk factors, including the metabolic syndrome, insulin resistance, and type 2 diabetes mellitus. However, overexpression of apolipoprotein A-I in animals has been shown to reduce progression and even induce regression of atherosclerosis, indicating that apolipoprotein A-I is directly protective against atherosclerosis. A major mechanism by which apolipoprotein A-I inhibits atherosclerosis may be by promoting cholesterol efflux from macrophages and returning it to the liver for excretion, a process termed reverse cholesterol transport. This article focuses on new developments in the regulation of reverse cholesterol transport and the clinical implications of those developments.

Lipases and HDL metabolism

Plasma levels of high-density lipoprotein (HDL) cholesterol are strongly inversely associated with atherosclerotic cardiovascular disease, and overexpression of HDL proteins, such as apolipoprotein A-I in animals, reduces progression and even induces regression of atherosclerosis. Therefore, HDL metabolism is recognized as a potential target for therapeutic intervention of atherosclerotic vascular diseases. The antiatherogenic properties of HDL include promotion of cellular cholesterol efflux and reverse cholesterol transport, as well as antioxidant, anti-inflammatory and anticoagulant properties. The molecular regulation of HDL metabolism is not fully understood, but it is influenced by several extracellular lipases. Here, we focus on new developments and insights into the role of secreted lipases on HDL metabolism and their relationship to atherosclerosis.

Mutagenesis of highly conserved histidines in lecithincholesterol acyltransferase: identification of an essential histidine (His 377)

Lecithin-cholesterol acyltransferase (LCAT) is responsible for the formation of cholesterol esters in plasma and is implicated in the removal of excess cholesterol from peripheral tissues. It is generally accepted that the catalytic mechanism of LCAT is similar to that of serine proteases and lipases involving a Ser, a His, and an acidic amino acid residue. Ser181 in LCAT has been previously identified as a catalytic residue; however, the active site His and acidic residue have not yet been identified. In this study we have used a variety of approaches to identify the putative active site histidine. Alignments of LCAT sequences across various species indicate that the four histidines at positions 180, 263, 368, and 377 are conserved and could be involved in catalysis. Based on the observation that the members of the triad preserve the same orientation in the primary sequence of a large number of lipases, we eliminated His180 as a potential candidate. Mutational analysis along with functional assays show that, in contrast to the replacement of His263 and His368, the replacement of the His at position 377 with Gly, Ala, or Ser obliterates LCAT activity with interfacial and water-soluble substrates, thus indicating a role of His377 in catalysis.

Cloning and in vitro expression of rat lecithin:cholesterol acyltransferase

Rat lecithin:cholesterol acyltransferase (LCAT) cDNA was obtained by reverse transcriptase/polymerase chain reaction amplification of rat liver total RNA. A consensus sequence was derived from four independent clones from two strains of rats. In vitro expression of rat LCAT cDNA in COS cells resulted in secreted enzyme protein with the same fatty acyl specificity for phospholipase A2 activity and cholesterol esterification as rat plasma LCAT, but different from that of recombinant or human plasma LCAT.

Identification of a domain of lecithin-cholesterol acyltransferase that is involved in interfacial recognition

Lecithin-cholesterol acyltransferase (LCAT) is an interfacial enzyme that acts on lipid substrates on the surface of high density lipoproteins (HDL). Based on observations with other interfacial lipases, we propose that LCAT contains a surface region of 25 amino acids linked by a disulfide bond (C50-C74) that is involved in the binding of LCAT to lipoproteins. Using LCAT cDNA, we have deleted most of this region (delta 53-71) and expressed the mutant enzyme (LCAT delta 53-71) in COS-1 cells. The deletion mutant is expressed and secreted at levels similar to wildtype LCAT, suggesting that the deleted region is located on the surface of the enzyme and is not required for folding. The enzymatic activity of the mutant was tested using two interfacial substrates, reconstituted HDL (rHDL) and low density lipoprotein (LDL), as well as a water soluble substrate, p-nitrophenyl butyrate (PNPB). There was no reaction with rHDL and LDL, but 30% of the activity with PNPB was retained. This suggests that the deleted region plays a role in interfacial binding, while the active site core is not disrupted. We thus conclude that this region (C50-C74) forms part of the interfacial binding domain of LCAT.

Analysis of human lecithin-cholesterol acyltransferase activity by carboxyl-terminal truncation

Lecithin-cholesterol acyltransferase (LCAT) is a key enzyme in reverse cholesterol transport and catalyzes the esterification of cholesterol in human plasma. Human LCAT is a glycosylated protein, containing 416 amino acids and a proline-rich region at the C-terminus. To address the function of the C-terminal region of LCAT as well as that of the proline-rich region, we constructed and expressed LCAT mutants with C-terminal truncations at different positions. The expression of wild-type LCAT in COS-1 cells resulted in an enzymatically active protein that was secreted by the cells. The mutants lacking the proline-rich region at the C-terminus were expressed and secreted at levels comparable to those of wild-type (approximately 50% of wild-type concentrations in cell media). The proline-deletion mutants were similar to wild-type LCAT in terms of phospholipase or transferase activities with various interfacial substrates, including reconstituted HDL, proteoliposomes, LDL, and micelles of platelet activating factor. Thus, the binding of LCAT to the diverse interfaces is not affected by the removal of its C-terminal region. Also, the activation by apolipoproteins and access of water-insoluble substrates to the active site are not significantly affected by the deletion of the proline-rich region. However, deletions of the proline-rich region, including the five amino acids nearest to the C-terminus, resulted in approximately an 8-fold increase in the specific activity of LCAT towards the water-soluble substrate, p-nitrophenylbutyrate. This suggests that the C-terminal proline-rich region may interfere with the access of this water-soluble substrate to the active site of LCAT, and may form part of a protective covering of the active site of LCAT while in solution. Further deletions at the C-terminus, beyond the proline-rich region, impaired the secretion of the enzyme, implying that this region may play a critical role in either the secretion or folding of LCAT in COS-1 cells.

Amino acid residue 149 of lecithin:cholesterol acyltransferase determines phospholipase A2 and transacylase fatty acyl specificity

Human LCAT prefers phosphatidylcholine (PC) with sn-1-palmitoyl-2-oleoyl PC (POPC) as substrate for cholesteryl ester synthesis, whereas rat LCAT (which is 92% similar in amino acid sequence) prefers sn-1-palmitoyl-2-arachidonoyl PC (PAPC). Six recombinant human LCAT cDNA clones were constructed with unique clusters of rat sequence substitutions in the human background spanning the region encoding amino acids 121-296. Media from transfected COS cells expressing each of the constructs were assayed for LCAT cholesterol esterification (CE) or phospholipase A2 (PLA2) activity using substrate particles containing POPC or PAPC. The PAPC/POPC CE activity ratio of the cluster 1 construct (amino acids 149-158) was 1.3, resembling rat LCAT, whereas cluster 2-5 clones produced CE activity ratios <0.3, unchanged from human LCAT. The cluster 6 clone (Y292H/W294F) had an intermediate ratio (0.6). Similar results were observed for LCAT PLA2 activity. In additional studies, position 149 of human LCAT was changed to the rat sequence (hE149A) and compared to a triple mutation containing the remainder of the cluster 1 changes (G151R/E154D/R158Q). CE and PLA2 activity ratio for the hE149A construct was >1.7, similar to rat LCAT, whereas the triple mutation construct retained a ratio similar to human LCAT (<0.6). Thus, a single amino acid substitution (E149A) was sufficient to alter the fatty acyl specificity of human LCAT to that of rat LCAT, with an increase in activity toward PAPC. This is the first example of a point mutation in an enzyme with PLA2 activity that results in an increase in activity toward arachidonic acid.