Human Pancreatic Lipase-related Protein Research Articles

Spontaneous lid closure and substrate-induced lid opening dynamics of human pancreatic lipase-related protein 2: A computational study

Pancreatic lipases are essential dietary enzymes with a well-conserved catalytic triad covered by a lid. Unlike classical lipases, pancreatic lipase-related proteins 2 (PLRP2) are active in aqueous medium without any colipase. As their substrate accessibility is controlled by the lid in N-terminal domain, we investigated the lid dynamics of human PLRP2 (HPLRP2) in its closed and open conformational states using molecular dynamics (MD) simulation. The free energy landscape constructed from the MD trajectory of HPLRP2 in the open conformation suggests that the well-conserved W254 along with the hydrophobic region G256-V261 on the lid, which differs from the classical lipases, show larger conformational changes during the lid closure pathway of HPLRP2. Further, HPLRP2 in the closed conformation could attain the open state only after introducing a double mutation D251S/E255C and upon the addition of substrate molecules, tributyrin (Tbt). This initially disrupted the stable interactions between the lid and β6 loop, and the lid opening movement was further facilitated by the interactions of Tbt with β8 and β9 loops. Tbt also bound to C-terminal domain and significantly altered its orientation. The lid opening pathway of HPLRP2 was found to be distinct from its closing trajectory and tributyrin could access the active site even when the lid did not attain its fully-extended open conformation. The results indicate that the lid dynamics of HPLRP2 is different from the classical lipases and facilitates its function in the absence of any colipase.

The common truncation variant in pancreatic lipase related protein 2 (PNLIPRP2) is expressed poorly and does not alter risk for chronic pancreatitis.

A nonsense variant (p.W358X) of human pancreatic lipase related protein 2 (PNLIPRP2) is present in different ethnic populations with a high allele frequency. In cell culture experiments, the truncated protein mainly accumulates inside the cells and causes endoplasmic reticulum stress. Here, we tested the hypothesis that variant p.W358X might increase risk for chronic pancreatitis through acinar cell stress. We sequenced exon 11 of PNLIPRP2 in a cohort of 256 subjects with chronic pancreatitis (152 alcoholic and 104 non-alcoholic) and 200 controls of Hungarian origin. We observed no significant difference in the distribution of the truncation variant between patients and controls. We analyzed mRNA expression in human pancreatic cDNA samples and found the variant allele markedly reduced. We conclude that the p.W358X truncation variant of PNLIPRP2 is expressed poorly and has no significant effect on the risk of chronic pancreatitis.

Kinetic and structural characterization of triacylglycerol lipases possessing phospholipase A1 activity

The pancreatic lipase gene family displays various substrate selectivities for triglycerides and phospholipids. The structural basis for this difference in substrate specificity has not been definitively established. Based on a kinetic comparative study between various pancreatic lipase family members, we showed here that porcine pancreatic lipase (PPL), which was so far classified as “classical lipase”, was able to hydrolyze phosphatidylcholine (PC). Amino acid sequence alignments revealed that Val260 residue in PPL lid could be critical for the interaction with lipid substrate. Molecular dynamics was applied to investigate PC binding modes within the catalytic cavity of PPL and human pancreatic lipase (HPL), aiming to explain the difference of specificity of these enzymes towards phospholipids. Results showed that with HPL, the oxyanion hole was not able to accommodate the PC molecule, suggesting that no activity could be obtained. With PPL, the formation of a large pocket involving Val260 allowed the PC molecule to come near the catalytic residues, suggesting that it could be hydrolyzed. One more interesting finding is that human pancreatic lipase related protein 2 could hydrolyze phospholipids through its PLA1 and PLA2 activities. Overall, our study shed the light on new structural features of the phospholipase activity of pancreatic lipase family members.

Inhibition of phospholipase A1, lipase and galactolipase activities of pancreatic lipase-related protein 2 by methyl arachidonyl fluorophosphonate (MAFP)

Methyl arachidonyl fluorophosphonate (MAFP) is a known inhibitor of cytosolic phospholipase A2 and some other serine enzymes. MAFP was found here to be an irreversible inhibitor of human pancreatic lipase-related protein 2 (HPLRP2), an enzyme displaying lipase, phospholipase A1 and galactolipase activities. In the presence of MAFP, mass spectrometry analysis of HPLRP2 revealed a mass increase of 351Da, suggesting a covalent binding of MAFP to the active site serine residue. When HPLRP2 was pre-incubated with MAFP before measuring residual activity, a direct inhibition of HPLRP2 occurred, confirming that HPLRP2 has an active site freely accessible to solvent and differs from most lipases in solution. HPLRP2 activities on tributyrin (TC4), phosphatidylcholine (PC) and monogalactosyl dioctanoylglycerol (C8-MGDG) were equally inhibited under these conditions. Bile salts were not required to trigger the inhibition, but they significantly increased the rate of HPLRP2 inhibition, probably because of MAFP micellar solubilization. Since HPLRP2 is active on various substrates that self-organize differently in the presence of water, HPLRP2 inhibition by MAFP was tested in the presence of these substrates after adding MAFP in the course of the lipolysis reaction. In this case, the rates of inhibition of lipase, phospholipase A1 and galactolipase activities were not equivalent (triglycerides>PC>MGDG), suggesting different enzyme/inhibitor partitioning between the aqueous phase and lipid aggregates. The inhibition by MAFP of a well identified phospholipase A1 (HPLRP2), present in pancreatic juice and also in human monocytes, indicates that MAFP cannot be used for discriminating phospholipase A2 from A1 activities at the cellular level.

Kinetic properties of mouse pancreatic lipase-related protein-2 suggest the mouse may not model human fat digestion

Genetically engineered mice have been employed to understand the role of lipases in dietary fat digestion with the expectation that the results can be extrapolated to humans. However, little is known about the properties of mouse pancreatic triglyceride lipase (mPTL) and pancreatic lipase-related protein-2 (mPLRP2). In this study, both lipases were expressed in Pichia Pastoris GS115, purified to near homogeneity, and their properties were characterized. Mouse PTL displayed the kinetics typical of PTL from other species. Like mPTL, mPLRP2 exhibited strong activity against various triglycerides. In contrast to mPTL, mPLRP2 was not inhibited by increasing bile salt concentration. Colipase stimulated mPLRP2 activity 2- to 4-fold. Additionally, mPTL absolutely required colipase for absorption to a lipid interface, whereas mPLRP2 absorbed fully without colipase. mPLRP2 had full activity in the presence of BSA, whereas BSA completely inhibited mPTL unless colipase was present. All of these properties of mPLRP2 differ from the properties of human PLRP2 (hPLRP2). Furthermore, mPLRP2 appears capable of compensating for mPTL deficiency. These findings suggest that the molecular mechanisms of dietary fat digestion may be different in humans and mice. Thus, extrapolation of dietary fat digestion in mice to humans should be done with care.

Continuous measurement of galactolipid hydrolysis by pancreatic lipolytic enzymes using the pH-stat technique and a medium chain monogalactosyl diglyceride as substrate

Galactolipids are the main lipids from plants and galactolipases play a major role in their metabolism. These enzymes were however poorly studied so far and only few assays have been developed. A specific and continuous galactolipase assay using synthetic medium chain monogalactosyl diacylglycerol (MGDG) as substrate was developed using the pH-stat technique and recombinant human (rHPLRP2) and guinea pig (rGPLRP2) pancreatic lipase-related protein 2 as model enzymes. PLRP2s are the main enzymes involved in the digestion of galactolipids in the gastrointestinal tract. Monogalactosyl di-octanoylglycerol was mixed with bile salt solutions by sonication to form a micellar substrate before launching the assay. The nature of the bile salt and the bile salt to MGDG ratio were found to significantly affect the rate of MGDG hydrolysis by rHPLRP2 and rGPLRP2. The maximum galactolipase activity of both enzymes was recorded with sodium deoxycholate (NaDC) and at a NaDC to MGDG ratio of 1.33 and at basic pH values (8.0–9.0). The maximum rates of hydrolysis were obtained using a MGDG concentration of 10 − 2 M and calcium chloride was found to be not necessary to obtain the maximum of activity. Under these conditions, the maximum turnovers of rGPLRP2 and rHPLRP2 on mixed NaDC/MGDG micelles were found to be 8000 ± 500 and 2800 ± 60 μmol/min/mg (U/mg), respectively. These activities are in the same order of magnitude as the activities on triglycerides of lipases and they are the highest specific activities ever reported for galactolipases. For the sake of comparison, the hydrolysis of mixed bile salt/MGDG micelles was also tested using other pancreatic lipolytic enzymes and only native and recombinant human carboxyl ester hydrolase were found to display significant but lower activities (240 ± 17 and 432 ± 62 U/mg, respectively) on MGDG.

Structure of Human Pancreatic Lipase-Related Protein 2 with the Lid in an Open Conformation,

Access to the active site of pancreatic lipase (PL) is controlled by a surface loop, the lid, which normally undergoes conformational changes only upon addition of lipids or amphiphiles. Structures of PL with their lids in the open and functional conformation have required cocrystallization with amphiphiles. Here we report two crystal structures of wild-type and unglycosylated human pancreatic lipase-related protein 2 (HPLRP2) with the lid in an open conformation in the absence of amphiphiles. These structures solved independently are strikingly similar, with some residues of the lid being poorly defined in the electron-density map. The open conformation of the lid is however different from that previously observed in classical liganded PL, suggesting different kinetic properties for HPLRP2. Here we show that the HPLRP2 is directly inhibited by E600, does not present interfacial activation, and acts preferentially on substrates forming monomers or small aggregates (micelles) dispersed in solution like monoglycerides, phospholipids and galactolipids, whereas classical PL displays reverse properties and a high specificity for unsoluble substrates like triglycerides and diglycerides forming oil-in-water interfaces. These biochemical properties imply that the lid of HPLRP2 is likely to spontaneously adopt in solution the open conformation observed in the crystal structure. This open conformation generates a large cavity capable of accommodating the digalactose polar head of galactolipids, similar to that previously observed in the active site of the guinea pig PLRP2, but absent from the classical PL. Most of the structural and kinetic properties of HPLRP2 were found to be different from those of rat PLRP2, the structure of which was previously obtained with the lid in a closed conformation. Our findings illustrate the essential role of the lid in determining the substrate specificity and the mechanism of action of lipases.

Lipolysis of the semi-solid self-emulsifying excipient Gelucire ® 44/14 by digestive lipases

Gelucire ® 44/14 is a semi-solid self-emulsifying excipient used for the oral delivery of poorly water-soluble drugs. It is composed of C8-C18 acylglycerols and PEG-32 esters, all of which are potential substrates for digestive lipases. Here we studied the lipolysis of Gelucire ® 44/14 by porcine pancreatic extracts, human pancreatic juice and several purified digestive lipases. Human pancreatic lipase (HPL), the main lipase involved in the digestion of triacylglycerols, did not show any significant activity on Gelucire ® 44/14 or on either of its individual compounds, C8-C18 acylglycerols and PEG-32 esters. Other pancreatic lipases such as human pancreatic lipase-related protein 2 (HPLRP2) showed low activity on Gelucire ® 44/14 although the highest activity of HPLRP2 was that observed on the C8-C18 acylglycerol fraction, which accounts for 20% (w/w) of Gelucire ® 44/14. In addition, HPLRP2 showed low activities on the PEG-32 esters, whether these were tested individually or mixed together. Carboxyl ester hydrolase (CEH) showed high activity on Gelucire ® 44/14, and the highest activities of CEH were those recorded on the total PEG-32 ester fraction and on each individual PEG-32 ester, except for PEG-32 monostearate. The highest activity of all the enzymes tested was that of dog gastric lipase (DGL) on Gelucire ® 44/14, although DGL showed low activity on the PEG-32 ester fraction and on each individual PEG-32 ester. We compared the lipolysis of Gelucire ® 44/14 with that of Labrasol ®, another self-emulsifying excipient, which is liquid at room temperature. Human pancreatic juice showed similar rates of activity on both Gelucire ® 44/14 and Labrasol ®. This finding means that these excipients are hydrolyzed in vivo during pancreatic digestion, mainly by CEH in the case of Gelucire ® 44/14 and by both HPLRP2 and CEH in that of Labrasol ®, whereas HPL showed very low activities on each of these two excipients. This is the first time the effects of PEG and acyl chain length on the lipolytic activity of digestive lipases on PEG esters have been investigated.

Further biochemical characterization of human pancreatic lipase-related protein 2 expressed in yeast cells

Recombinant human pancreatic lipase-related protein 2 (rHPLRP2) was produced in the protease A-deficient yeast Pichia pastoris. A major protein with a molecular mass of 50 kDa was purified from the culture medium using SP-Sepharose and Mono Q chromatography. The protein was found to be highly sensitive to the proteolytic cleavage of a peptide bond in the lid domain. The proteolytic cleavage process occurring in the lid affected both the lipase and phospholipase activities of rHPLRP2. The substrate specificity of the nonproteolyzed rHPLRP2 was investigated using pH-stat and monomolecular film techniques and various substrates (glycerides, phospholipids, and galactolipids). All of the enzyme activities were maximum at alkaline pH values and decreased in the pH 5-7 range corresponding to the physiological conditions occurring in the duodenum. rHPLRP2 was found to act preferentially on substrates forming small aggregates in solution (monoglycerides, egg phosphatidylcholine, and galactolipids) rather than on emulsified substrates such as triolein and diolein. The activity of rHPLRP2 on monogalactosyldiglyceride and digalactosyldiglyceride monomolecular films was determined and compared with that of guinea pig pancreatic lipase-related protein 2, which shows a large deletion in the lid domain. The presence of a full-length lid domain in rHPLRP2 makes it possible for enzyme activity to occur at higher surface pressures. The finding that the inhibition of nonproteolyzed rHPLRP2 by tetrahydrolipstatin and diethyl-p-nitrophenyl phosphate does not involve any bile salt requirements suggests that the rHPLRP2 lid adopts an open conformation in aqueous media.

Human pancreatic lipase-related protein 2: Tissular localization along the digestive tract and quantification in pancreatic juice using a specific ELISA

Human pancreatic lipase-related protein 2 (HPLRP2) was previously found to be secreted by the exocrine pancreas. HPLRP2 shows a high level of activity on galactolipids, and might be involved in the digestion of these common vegetable lipids. Specific antibodies were raised in rabbits using a synthetic HPLRP2 peptide selected for its weak amino acid homology with the corresponding peptides of classical human pancreatic lipase (HPL) and human pancreatic lipase-related protein 1 (HPLRP1). ELISA and Western blotting data showed that these antibodies did not react with HPL or HPLRP1. Various tissues from the digestive tract were subjected to Western blotting analysis with the specific anti-peptide HPLRP2 antibody and the expression of HPLRP2 was detected in the pancreas and colon. An ELISA was developed for specifically measuring the HPLRP2 levels in pure pancreatic juice. This procedure was performed using the anti-peptide HPLRP2 antibody as the captor antibody and a biotinylated anti-HPLRP2 polyclonal antibody as the detector antibody. The lowest HPLRP2 quantification limit was found to be 50 μg/L and the reference range for the present assay was 50 μg–500 μg/L. HPL and HPLRP2 levels were measured using specific ELISAs in pancreatic juice from patients with and without pancreatic disorders. Patients with chronic calcifying pancreatitis (CCP) had significantly lower levels of both HPL and HPLRP2 than the controls subjects. The mean HPLRP2 to HPL ratio was estimated to be 28.30% (w/w) and 23.96% (w/w) in controls subjects and CCP patients, respectively, and the difference was not significant. The levels of HPL and HPLRP2 are therefore similarly reduced in both healthy patients and CCP patients.

High-level expression of nonglycosylated human pancreatic lipase-related protein 2 in Pichia pastoris

The human pancreatic lipase-related protein 2 (HPLRP2) was produced in the methylotrophic yeast Pichia pastoris. The HPLRP2 cDNA corresponding to the protein coding sequence including the native signal sequence, was cloned into the pPIC9K vector and integrated into the genome of P. pastoris. P. pastoris transformants secreting high-level rHPLRP2 were obtained and the expression level into the liquid culture medium reached about 40 mg/L after 4 days of culture. rHPLRP2 was purified by a single anion-exchange step after an overnight dialysis. N-terminal sequence analysis showed that the purified rHPLRP2 mature protein possessed a correct N-terminal amino acid sequence indicating that its signal peptide was properly processed. Mass spectrometry analysis showed that the recombinant HPLRP2 molecular weight was 52,532 Da which was 2451 Da greater than the mass calculated from the sequence of the protein (50,081 Da) and 1536 Da greater than the mass of the native human protein (50,996 Da). In vitro deglycosylation experiments by peptide: N-glycosidase F (PNGase F) indicated that rHPLRP2 secreted from P. pastoris was N-glycosylated. Specific conditions were setup in order to obtain a recombinant protein free of glycan chain. We observed that blocking glycosylation in vivo by addition of tunicamycin in the culture medium during the production resulted in a correct processing of the rHPLRP2 mature protein. The lipase activity of glycosylated or nonglycosylated rHPLRP2, which was about 800 U/mg on tributyrin, was inhibited by the presence of bile salts and not restored by adding colipase. In conclusion, the experimental procedure which we have developed will allow us to get a high-level production in P. pastoris of glycosylated and nonglycosylated rHPLRP2, suitable for subsequent biophysical and structural studies.

Constitutive expression of human pancreatic lipase-related protein 1 in Pichia pastoris

High-level constitutive expression of the human pancreatic lipase-related protein 1 (HPLRP1) was achieved using the methylotrophic yeast Pichia pastoris. The HPLRP1 cDNA, including its original leader sequence, was subcloned into the pGAPZB vector and further integrated into the genome of P. pastoris X-33 under the control of the glyceraldehyde 3-phosphate dehydrogenase (GAP) constitutive promoter. A major protein with a molecular mass of 50 kDa was found to be secreted into the culture medium and was identified using anti-HPLRP1 polyclonal antibodies as HPLRP1 recombinant protein. The level of expression reached 100–120 mg of HPLRP1 per liter of culture medium after 40 h, as attested by specific and quantitative enzyme-linked immunosorbent assay. A single cation-exchange chromatography sufficed to obtain a highly purified recombinant HPLRP1 after direct batch adsorption onto S-Sepharose of the HPLRP1 present in the culture medium, at pH 5.5. N-terminal sequencing and mass spectrometry analysis were carried out to monitor the production of the mature protein and to confirm that its signal peptide was properly processed.

Human pancreatic lipase-related protein 2 is a galactolipase.

Human pancreatic lipase-related protein 2 (HPLRP2) was found to be expressed in the pancreas, but its biochemical properties were not investigated in detail. A recombinant HPLRP2 was produced in insect cells and the yeast Pichia pastoris and purified by cation exchange chromatography. Its substrate specificity was investigated using pH-stat and monomolecular film techniques and various lipid substrates (triglycerides, diglycerides, phospholipids, and galactolipids). Lipase activity of HPLRP2 on trioctanoin was inhibited by bile salts and poorly restored by adding colipase. In vivo, HPLRP2 therefore seems unlikely to show any lipase activity on dietary fat. In human pancreatic lipase (HPL), residues R256, D257, Y267, and K268 are involved in the stabilization of the open conformation of the lid domain, which interacts with colipase. These residues are not conserved in HPLRP2. When the corresponding mutations (R256G, D257G, Y267F, and K268E) are introduced into HPL, the effects of colipase are drastically reduced in the presence of bile salts. This may explain why colipase has such weak effects on HPLRP2. HPLRP2 displayed a very low level of activity on phospholipid micelles and monomolecular films. Its activity on monogalactosyldiglyceride monomolecular film, which was much higher, was similar to the activity of guinea pig pancreatic lipase related-protein 2, which shows the highest galactolipase activity ever measured. The physiological role of HPLRP2 suggested by the present results is the digestion of galactolipids, the most abundant lipids occurring in plant cells, and therefore, in the vegetables that are part of the human diet.

Characterization of pancreatic lipase-related protein 2 isolated from human pancreatic juice

Human pancreatic lipase-related protein 2 (HPLRP2) was identified for the first time in pancreatic juice using specific anti-peptide antibodies and purified to homogeneity. Antibodies were raised in the rabbit using a synthetic peptide from the HPLRP2 protein sequence deduced from cDNA. Western blotting analysis showed that these antibodies did not react with classical human pancreatic lipase (HPL) or human pancreatic lipase-related protein 1 (HPLRP1) but cross-reacted with native rat PLRP2 (RPLRP2), as well as with recombinant rat and guinea-pig PLRP2 (GPLRP2). Immunoaffinity chromatography was performed on immobilized anti-recombinant HPLRP2 polyclonal antibodies to purify native HPLRP2 after conventional chromatographic steps including gel filtration and chromatrography on an anion-exchanger. The substrate specificity of HPLRP2 was investigated using various triglycerides, phospholipids and galactolipids as substrates. The lipase activity on triglycerides was inhibited by bile salts and weakly restored by colipase. The phospholipase activity of HPLRP2 on phospholipid micelles was very low. A significant level of galactolipase activity was measured using monogalactosyldiglyceride monomolecular films. These data suggest that the main physiological function of HPLRP2 is the hydrolysis of galactolipids, which are the main lipids present in vegetable food.

Genomic Organization of the Murine CTL Lipase Gene

Murine cytotoxic T-lymphocyte (CTL) lipase was originally identified as an IL-4-inducible gene in CD8-positive T cells. To further our understanding of both the function and the regulation of CTL lipase in T cells, we have cloned and characterized the murine gene. Two overlapping phage clones spanning 29 kb contain the entire CTL lipase gene. The exon structure is similar to those characterized for the human and canine pancreatic lipase-related protein 1 genes, with notable differences in the 5′ end. Transcripts initiate from a site that matches a consensus for an initiator sequence. Potentialcis-regulatory elements in the CTL lipase 5′ regulatory region that would confer dual tissue specificity in exocrine pancreas and cytotoxic T lymphocytes are identified. The implications of this promoter organization are discussed.

Announcements

Murine cytotoxic T-lymphocyte (CTL) lipase was originally identified as an IL-4-inducible gene in CD8-positive T cells. To further our understanding of both the function and the regulation of CTL lipase in T cells, we have cloned and characterized the murine gene. Two overlapping phage clones spanning 29 kb contain the entire CTL lipase gene. The exon structure is similar to those characterized for the human and canine pancreatic lipase-related protein 1 genes, with notable differences in the 5′ end. Transcripts initiate from a site that matches a consensus for an initiator sequence. Potentialcis-regulatory elements in the CTL lipase 5′ regulatory region that would confer dual tissue specificity in exocrine pancreas and cytotoxic T lymphocytes are identified. The implications of this promoter organization are discussed.