Hydrolysis of fluorescent pyrene-acyl esters by human pancreatic carboxylic ester hydrolase and bile salt-stimulated lipase

Bile salt-dependent lipase promotes the barrier integrity of Caco-2 cells by activating Wnt/β-catenin signaling via LRP6 receptor

Bile salt-dependent lipase (BSDL) within intestinal lumen can be endocytosed by enterocytes and support the intestinal barrier function. However, the epithelial-supporting effect of this protein has not been verified in a human cell line and neither the direct signaling pathway nor the function of endocytosis in this process has been clearly identified. We sought to investigate the signaling pathway and the membrane receptor through which BSDL might exert these effects using intestinal epithelial cells. Caco-2 cells were treated with recombinant BSDL, and the barrier function, cell proliferation, and activation of the Wnt signaling pathway were assessed. The effect of Wnt signaling activation induced by BSDL and BSDL endocytosis was investigated in LRP6-silenced and non-silenced cells. Moreover, caveolae- and clathrin-dependent endocytosis inhibitors were also applied respectively to analyze their effects on Wnt signaling activation induced by BSDL. BSDL treatment increased the barrier function but not proliferation of Caco-2 cells. It also induced β-catenin nuclear translocation and activated Wnt target gene transcription. Moreover, in the Wnt pathway, BSDL increased the levels of non-phosphorylated-β-catenin (Ser33/37/Thr41) and phosphorylated-β-catenin (Ser552). Notably, the silencing of LRP6 expression impaired BSDL endocytosis and decreased BSDL-induced β-catenin nuclear translocation. The inhibition of BSDL endocytosis induced by caveolae-mediated endocytosis inhibitor was stronger than that by clathrin-mediated endocytosis inhibitor, and the Wnt signaling activation associated with its endocytosis was also most likely caveolae-dependent. Our findings suggested that LRP6, a canonical Wnt pathway co-receptor, can mediate BSDL endocytosis and then activate Wnt signaling in Caco-2 cells.

Human bile salt-dependent lipase efficiency on medium-chain acyl-containing substrates: control by sodium taurocholate

Bile salt-dependent lipase was purified to homogeneity from lyophilized human milk and used to screen the influence of the acyl chain length (2-16 carbon atoms) on the kinetic constants k(cat) and K(m) of the hydrolysis of para-nitrophenyl (pnp) ester substrates in the presence or absence of sodium taurocholate (NaTC: 0.02-20 mM). The highest k(cat) value (∼3,500 s(-1)) was obtained with pnpC(8) as substrate, whereas the lowest K(m) (<10 µM) was that recorded with pnpC(10). In the absence of NaTC, the maximal catalytic efficiency (k(cat)/K(m)) was obtained with pnpC(8), while in the presence of NaTC k(cat)/K(m) was maximal with pnpC(8), pnpC(10) or pnpC(12). The bile salt activated the enzyme in two successive saturation phases occurring at a micromolar and a millimolar concentration range, respectively. The present data emphasize the suitability of this enzyme for the hydrolysis of medium-chain acyl-containing substrates and throw additional light on how BSDL is activated by NaTC.

Adenovirus-mediated human pancreatic lipase gene transfer to rat bile: gene therapy of fat malabsorption

This study explored the potential of using the gene therapy approach, based on adenovirus-mediated expression of pancreatic lipase in the hepatobiliary tract, to increase lipid digestion in the intestinal lumen and promote lipid absorption through the gastrointestinal tract. Recombinant adenovirus containing the human pancreatic lipase cDNA (AdPL) was shown to transduce and mediate pancreatic lipase biosynthesis in rat IEC-6 epithelial cells in vitro. Retrograde infusion of recombinant adenovirus (3 x 10(8) plaque-forming units) containing the bacterial LacZ gene (AdLacZ) into the bile duct of rats resulted in positive X-gal reaction products in the periportal liver cells 7 days after AdLacZ infusion. A high level of human pancreatic lipase was detected in bile after retrograde bile duct infusion of rats with AdPL but not in the bile of animals infused with AdLacZ. Triglyceride hydrolytic activity in the bile of AdPL-infused rats was equivalent to that present in pancreatic juice. In contrast, serum obtained from these animals did not contain any detectable pancreatic lipase activity. These results suggest that ectopic expression of pancreatic enzymes in the hepatobiliary tract may be an alternative therapeutic strategy for treating fat malabsorption due to pancreatic insufficiency.

Pancreatic bile salt-dependent lipase activity in serum of normolipidemic patients

Bile salt-dependent lipase (BSDL, E.C. 3.1.1.-) is a digestive enzyme secreted by the pancreatic acinar cell. Once in the duodenum, the enzyme, upon activation by primary bile salts, hydrolyzes dietary lipid esters such as cholesteryl esters and lipid-soluble vitamin esters. This enzyme is partially transferred from the duodenum or pancreas to the circulation where it has been postulated to exert a systemic action on atheroma-generating oxidized-low density lipoprotein (LDL). In the present study, sera from 40 healthy normolipidemic volunteers were used to investigate the possible linkage between circulating BSDL, lipids, and lipoproteins. We showed, firstly, that pancreatic-like BSDL activity can be detected in these serums. Secondly, BSDL activity increased significantly with the level of LDL-cholesterol and was also positively linked to the serum concentration of Apo B100 and Apo A-I. Thirdly, we also established that BSDL was associated with LDL, in part by a specific interaction with Apo B100, while no interaction was found with Apo A-I. No linkage with other recorded parameters (triglycerides, phospholipids, and high density lipoprotein-cholesterol) was detected. Because an increase in LDL-cholesterol represents an important risk factor for atheroma, the concomitant increase in BSDL, which can metabolize atherogenic LDL, suggests for the first time that this circulating enzyme may exert a positive effect against atherosclerosis.

Enantiomeric perylene-glycerolipids as fluorogenic substrates for a dual wavelength assay of lipase activity and stereoselectivity

A new type of fluorogenic alkyldiacyl glycerols was synthesized and used as fluorogenic substrates for the analysis of lipase activities and stereoselectivities. These compounds contain perylene as a fluorophore and the trinitrophenylamino (TNP) residue as a quencher. Both substituents are covalently bound to the omega-ends of the sn-2 and sn-1 (3) acyl chains, respectively. Upon glycerolipid hydrolysis, the residues are separated from each other thus allowing determination of lipase activity by the continuous increase in fluorescence intensity which is caused by dequenching. Using enantiomeric pairs of these compounds, we were able to analyze lipase stereoselectivity depending on the reaction medium. Mixtures of enantiomeric fluorogenic alkyldiacyl glycerols, selectively labelled with pyrene or perylene as fluorophores, can be used for a dual-wavelength "stereoassay" of lipases. Since absorption and emission maxima of both labels are clearly separated, hydrolysis of the respective enantiomeric substrates can be determined simultaneously, and the difference in the rates of hydrolysis can be taken as a parameter for the stereopreference of a lipase. Hydrolysis rates measured with perylene-substituted lipids are generally lower than those obtained with the pyrene analogs. Thus, with a mixture of perylene and pyrene-substituted lipids, we observe a higher apparent stereoselectivity of lipases since we measure a combination of stereo- and substrate selectivity. In the presence of albumin, all microbial lipases tested so far exhibit stereopreference for the sn-1 glycerol position. In our assay, the apparent stereoselectivities are highest if in the presence of albumin, the sn-1 position carries pyrene and the sn-3 position is substituted with perylene. The lipase stereoselectivity assay described here requires the simultaneous measurement of the fluorescence intensities at two different wavelengths in a single cuvette and can thus be carried out using existing and cheap instrumentation that was developed for the fluorimetric analysis of Ca+2 concentrations.

Use of pyrenemethyl laurate for fluorescence-based determination of lipase activity in intact living lymphoblastoid cells and for the diagnosis of acid lipase deficiency

Pyrenemethyl laurate (PMLes), a fluorogenic substrate for determining in vitro lipase activity [Nègre, Salvayre, Dagan and Gatt (1989) Biochim. Biophys. Acta 1006, 84-88], has been administered to cultured lymphoblastoid cells from normal subjects and from a patient affected with Wolman disease, which is characterized by a deficiency of lysosomal acid lipase. The intracellular degradation of PMLes was dependent on the mode of administration of the substrate into the cells, and occurred by two separate pathways involving lysosomal and extra-lysosomal hydrolases. PMLes incorporated into LDL was taken up by normal lymphoblastoid cells through the apolipoprotein-B/E-receptor-mediated pathway and degraded in the lysosomal compartment, as suggested by the degradation block in Wolman cells. In contrast, when PMLes dissolved in 2% dimethyl sulphoxide was added directly to the culture medium, its hydrolysis was similar in lymphoblastoid cells from controls and from patients affected with Wolman disease, neutral lipid storage disease or familial hypercholesterolaemia. This suggested that the administered PMLes was degraded by a non-lysosomal enzyme which is not deficient in Wolman cells. This enzyme also differs from the neutral lipase system which is deficient in lymphoblastoid cells from patients with neutral lipid storage disease. When pyrenemethanol was administered directly to the cell culture, it was only poorly acylated and was rapidly released into the culture medium. These results and the fluorescence properties of PMLes ('monomeric' emission in a hydrophobic environment and 'excimeric' emission in a hydrophilic environment) and pyrenemethanol ('monomeric' emission in a hydrophilic environment) allowed us to design a 'direct reading' procedure by monitoring (without any lipid extraction) the fluorescence of intact living cells and that of the culture medium during pulse-chase experiments. This method allowed the direct evaluation of the time course of in situ degradation of PMLes. In pulse-chase experiments with LDL-PMLes, the fluorescence of normal cells decreased relatively rapidly with time whereas the fluorescence of the culture medium increased concomitantly. With Wolman cells, the cellular fluorescence decreased only very slightly, whereas that of the culture medium remained at the basal level; this demonstrates the catabolic block in intact living cells from patients with Wolman disease. In vitro degradation of PMLes indicated the existence of two PMLes-degrading enzymes in lymphoblastoid cell homogenates: one is the acid lipase which is involved in PMLes degradation in the lysosomal compartment (and is deficient in Wolman cells), while the second is a cytoplasmic enzyme (not deficient in Wolman cells).