Association of bile-salt-dependent lipase with membranes of human pancreatic microsomes is under the control of ATP and phosphorylation

Pancreatic adenocarcinoma, chronic pancreatitis, and MODY-8 diabetes: is bile salt-dependent lipase (or carboxyl ester lipase) at the crossroads of pancreatic pathologies?

Pancreatic adenocarcinomas and diabetes mellitus are responsible for the deaths of around two million people each year worldwide. Patients with chronic pancreatitis do not die directly of this disease, except where the pathology is hereditary. Much current literature supports the involvement of bile salt-dependent lipase (BSDL), also known as carboxyl ester lipase (CEL), in the pathophysiology of these pancreatic diseases. The purpose of this review is to shed light on connections between chronic pancreatitis, diabetes, and pancreatic adenocarcinomas by gaining an insight into BSDL and its variants. This enzyme is normally secreted by the exocrine pancreas, and is diverted within the intestinal lumen to participate in the hydrolysis of dietary lipids. However, BSDL is also expressed by other cells and tissues, where it participates in lipid homeostasis. Variants of BSDL resulting from germline and/or somatic mutations (nucleotide insertion/deletion or nonallelic homologous recombination) are expressed in the pancreas of patients with pancreatic pathologies such as chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We discuss the possible link between the expression of BSDL variants and these dramatic pancreatic pathologies, putting forward the suggestion that BSDL and its variants are implicated in the cell lipid metabolism/reprogramming that leads to the dyslipidemia observed in chronic pancreatitis, MODY-8, and pancreatic adenocarcinomas. We also propose potential strategies for translation to therapeutic applications.

Macrophage cholesteryl ester mobilization and atherosclerosis

Accumulation of cholesteryl esters (CE) stored as cytoplasmic lipid droplets is the main characteristic of macrophage foam cells that are central to the development of atherosclerotic plaques. Since only unesterified or free cholesterol (FC) can be effluxed from the cells to extracellular cholesterol acceptors, hydrolysis of CE is the obligatory first step in CE mobilization from macrophages. This reaction, catalyzed by neutral cholesteryl ester hydrolase (CEH), is increasingly being recognized as the rate-limiting step in FC efflux. CEH, therefore, regulates the process of reverse cholesterol transport and ultimate elimination of cholesterol from the body. In this review, we summarize the earlier controversies surrounding the identity of CEH in macrophages, discuss the characteristics of the various candidates recognized to date and examine their role in mobilizing cellular CE and thus regulating atherogenesis. In addition, physiological requirements to hydrolyze lipid droplet-associated substrate and complexities of interfacial catalysis are also discussed to emphasize the importance of evaluating the biochemical characteristics of candidate enzymes that may be targeted in the future to attenuate atherosclerosis.

Binding of the viral immunogenic octapeptide VSV8 to native glucose-regulated protein Grp94 (gp96) and its inhibition by the physiological ligands ATP and Ca2+

The molecular chaperone Grp94 (gp96) of the endoplasmic reticulum (ER) lumen plays an essential role in the structural maturation and/or secretion of proteins destined for transport to the cell surface. Its proposed role in binding and transferring peptides for immune recognition is, however, controversial. Using SPR spectroscopy, we studied the interaction of native glycosylated Grp94 at neutral pH and 25 and 37 degrees C with the viral immunogenic octapeptide RGYVYQGL (VSV8), derived from vesicular stomatitis virus nucleoprotein (52-59). The peptide binds reversibly with low affinity ([A]0.5 approximately 640 microM) and a hyperbolic binding isotherm, and the binding is partially inhibited by ATP and Ca2+ at concentrations that are present in the ER lumen, and the effects are explained by conformational changes in the native chaperone induced by these ligands. Our data present experimental support for the recent proposal that, under native conditions, VSV8 binds to Grp94 by an adsorptive, rather than a bioselective, mechanism, and thus further challenge the proposed in vivo peptide acceptor-donor function of the chaperone in the context of antigen-presenting cell activation.

Monoclonal antibody 16D10 to the C-terminal domain of the feto-acinar pancreatic protein binds to membrane of human pancreatic tumoral SOJ-6 cells and inhibits the growth of tumor xenografts

Feto-acinar pancreatic protein (FAPP) characterized by mAbJ28 reactivity is a specific component associated with ontogenesis and behaves as an oncodevelopment-associated antigen. We attempted to determine whether pancreatic tumoral SOJ-6 cells are expressed at their surface FAPP antigens and to examine if specific antibodies directed against these FAPP epitopes could decrease the growth of pancreatic tumors in a mice model. For this purpose, we used specific antibodies against either the whole FAPP, the O-glycosylated C-terminal domain, or the N-terminal domain of the protein. Our results indicate that SOJ-6 cells expressed at their surface a 32-kDa peptide corresponding to the C-terminal domain of the FAPP. Furthermore, we show, by using endoproteinase Lys-C or geldanamycin, a drug able to impair the FAPP secretion, that this 32-kDa peptide expressed on the SOJ-6 cell surface comes from the degradation of the FAPP. Finally, an in vivo prospective study using a preventative tumor model in nude mice indicates that targeting this peptide by the use of mAb16D10 inhibits the growth of SOJ-6 xenografts. The specificity of mAb16D10 for pancreatic tumors and the possibility to obtain recombinant structures of mucin-like peptides recognized by mAb16D10 and mAbJ28 are promising tools in immunologic approaches to cure pancreatic cancers.

The Combinatorial Extension Method Reveals a Sphingolipid Binding Domain on Pancreatic Bile Salt-Dependent Lipase

Structure similarity searches using a combinatorial extension approach revealed that a protein fold structurally related to the sphingolipid binding domain (SBD) of HIV-1 gp120 (V3 loop) is present on pancreatic bile salt-dependent lipase (BSDL). A synthetic peptide derived from the predicted V3-like domain of BSDL interacted with reconstituted monolayers of sphingolipids such as GalCer and GlcCer. Using Chinese hamster ovary cells stably transfected with the cDNA encoding the rat BSDL (CHO-3B clone) or pancreatic SOJ-6 cells expressing the human BSDL as models, we showed that the enzyme cofractionates with caveolin-1. The secretion of BSDL by CHO-3B cells was inhibited by permeable drugs affecting rafts structure (D609, PDMP, and filipin). Data suggest that the functional interaction between the BSDL SBD and lipid rafts is physiologically relevant and could be essential for sensing the BSDL folding prior to secretion. A tentative model accounting for the phosphorylation-induced dissociation of BSDL from rafts is presented.

Protein kinase activity is central to rat germ cell apoptosis induced by methoxyacetic acid

Methoxyacetic acid (MAA) is a major metabolite of ethylene glycol monomethyl ether (EGME). Previous investigations of the testicular lesion induced by EGME have found that dividing meiotic cells are the most sensitive, although several stages of spermatocytes are also vulnerable. Preliminary data from this lab suggested the involvement of protein kinase activity in the development of this lesion, a hypothesis explored in the present studies. We used cultured seminiferous tubules (STs) from juvenile rats (25-day-old), exposed in vitro to MAA and several inhibitors of protein kinases. Nineteen h following a 5-h exposure to 5 mM MAA (the plasma level in vivo after a toxic dose of EGME), apoptotic spermatocytes were seen in early- and late-stage STs. Cell death was prevented by cotreatment with broad-spectrum inhibitors of protein kinases such as H-7, H-8, K-252a, W-7, and genistein. In corroboration, immunocytochemistry with antibodies to various kinases (PKCmu, zeta, and gamma, AKAP220, CaMKII, MLCK, and Src) showed increased staining around dying spermatocytes following EGME treatment in vivo. 2D-PAGE, autoradiography, and nanospray mass spectrometry was used to separate and identify proteins whose phosphorylation status was most greatly changed following exposure to MAA. One protein was identified by sequence analysis as being glucose-regulated protein 94 (grp94). Westem blotting and immunocytochemistry confirmed this finding. The data we present implicate kinase activities in the pathogenesis of this lesion and suggest the involvement of Sertoli cells.

Impairment of bile salt-dependent lipase secretion in AR4-2J rat pancreatic cells induces its degradation by the proteasome

Bile salt-dependent lipase (BSDL, EC 3.1.1.13) is a lipolytic enzyme normally secreted by the pancreatic acinar cell. Co- and post-translational modifications, such as N- and O-linked glycosylation, regulate the secretion of this enzyme; therefore it was of first importance to determine the behaviour of BSDL under conditions that impaired its secretion. Using AR4-2J pancreatic cells as model, we showed, particularly when BSDL secretion is impaired, that proteasome inhibitors increased the amount of intracellular BSDL, suggesting that the proteasome is involved in the degradation of this protein. This was strengthened by the detection of ubiquitinated BSDL and of degradation product. Our results suggested that both ubiquitination and degradation of the enzyme occurred at the level of the cytosolic side of microsome membranes. ATP hydrolysis appears essential in ubiquitinated BSDL association with membranes and degradation. Furthermore, under normal secretory conditions, we have shown that a fraction of ubiquitinated BSDL is neither O-glycosylated nor N-glycosylated, suggesting that the N-glycosylation-deficient proteasome substrate does not reach the Golgi and could be degraded by the ER-associated degradation machinery. However, another fraction of ubiquitinated BSDL that is deficient in O-glycosylation, carries out endoglycosidase H-insensitive N-linked glycans, meaning that a second system, that detects abnormal BSDL molecules, could also operate at the level of the Golgi compartment. Consequently, it appears that impairment of BSDL secretion consecutive to secretion inhibition or to a deficient glycosylation leads to the proteasome-ubiquitin-dependent degradation of the protein. Therefore, this pathway is part of the quality control involved in BSDL secretion.

Control of pancreatic bile-salt-dependent-lipase secretion by the glucose-regulated protein of 94 kDa (Grp94)

Bile-salt-dependent lipase (BSDL; EC 3.1.1.13) is an enzyme expressed by the pancreatic acinar cell and secreted as a component of the pancreatic juice. During its route towards secretion, BSDL is associated with intracellular membranes by means of a multiprotein folding complex, which includes the glucose-regulated protein of 94 kDa (Grp94). We have postulated that the association of BSDL with membranes is required for the complete O-glycosylation of the protein, which diverts BSDL from a degradation route and consequently allows its secretion. To further characterize the role of Grp94 in BSDL secretion, we have studied the effect of a ribozyme specifically targeted to Grp94 mRNA. This ribozyme has been transfected into AR4-2J cells, and we have shown that a decrease in Grp94 expression leads to a concomitant decrease in BSDL secretion and expression. Geldanamycin (GA), which alters Grp94 functions, also affects the release of BSDL into the culture medium of AR4-2J cells. BSDL expressed in GA-treated AR4-2J cells is unstable. Furthermore, under conditions that decrease the level of BSDL secretion, no intracellular accumulation of the enzyme was observed, suggesting that BSDL that cannot associate with (or be structured by) Grp94 could be rapidly degraded. We have further shown that this degradation probably occurs via the ubiquitin-dependent pathway. Altogether, these results indicate that Grp94 has a pivotal role in BSDL folding and in the sorting of this pancreatic enzyme.

Impairment of bile salt-dependent lipase secretion in human pancreatic tumoral SOJ-6 cells

Bile salt-dependent lipase (BSDL) was detected in human SOJ-6 and rat AR4-2J pancreatic cells. Whereas AR4-2J cells actively secreted the enzyme, BSDL was retained within the Golgi compartment of SOJ-6 cells. Because Rab6 is involved in vesicle transport in the Golgi apparatus and the trans-Golgi network, we confirmed the presence of Rab6 in these cells. In rat AR4-2J cells, Rab6 as well as Rab1A/B and Rab2, partitioned between the cytosol and microsomes. In SOJ-6 cells Rab1A/B and Rab2 also partitioned between the cytosol and microsomes, but Rab6 was strictly associated with microsome membranes, suggesting a specific defect of Rab6 cycling in human SOJ-6 cells. The apparent defect of cycling in these cells is not due to the expression of a defective Rab6 since its correct sequence was confirmed. We further demonstrated that AR4-2J and SOJ-6 cells express the Rab-GDIbeta and Rab-GDIalpha isoforms, respectively. However, the sequence of Rab-GDIbeta, which may be the main form expressed by SOJ-6 cells, identified a few substitutions located in regions that are essential for Rab-GDI function. We conclude that the deficient secretion of BSDL by SOJ-6 cells could be due to the expression of defective Rab-GDIbeta. In spite of the alterations in Rab-GDIbeta, membrane proteins such as CD71 and NHE3 were correctly localized to the cell plasma membrane of SOJ-6 cells, suggesting that two functional distinct secretory pathway coexist in pancreatic cells.

Phosphorylation of the rat pancreatic bile-salt-dependent lipase by casein kinase II is essential for secretion

Bile-salt-dependent lipase (BSDL, EC 3.1.1.-) is an enzyme expressed by the pancreatic acinar cells and secreted as a component of the pancreatic juice of all examined species. During its secretion route BSDL is associated with intracellular membranes. This association allows the complete glycosylation of the enzyme or participates in the inhibition of the enzyme activity, which can deleterious for the acinar pancreatic cell. Thereafter, the human BSDL is phosphorylated by a serine/threonine protein kinase and released from intracellular membranes. In the present study, we show that the rat pancreatic BSDL, expressed by AR4-2J cells used as a model, is phosphorylated by a protein kinase that is insensitive to inhibitors of protein kinases A, C or G and that the phosphorylation process is favoured by okadaic acid (an inhibitor of protein phosphatases 1 and 2A). However, 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB), which is a specific inhibitor of casein kinase II, abolishes the phosphorylation in vitro of BSDL within micro- somes of AR4-2J pancreatic cells. We showed further that the alpha-subunit of casein kinase II co-locates with BSDL within the lumenal compartment of the Golgi. Genistein, which perturbs the trans-Golgi network, also inhibits the phosphorylation of BSDL, suggesting that this post-translational modification of BSDL probably occurred within this cell compartment. The inhibition of the phosphorylation of BSDL by DRB also decreases the rate at which the enzyme is secreted. Under the same conditions, the rate of alpha-amylase secretion was not modified. These data strongly suggest that phosphorylation is a post-translational event, which appears to be essential for the secretion of BSDL.

Molecular cloning of the oncofetal isoform of the human pancreatic bile salt-dependent lipase

Specific transcripts for bile salt-dependent lipase (BSDL), a 100-kDa glycoprotein secreted by the human pancreas, were immunodetected in BxPC-3 and SOJ-6 pancreatic tumoral cell lines. Sequencing of fragments, obtained by mRNA reverse transcription and amplification, confirmed the presence of BSDL transcripts in these cancer cells. The protein was detected in lysates of pancreatic tumoral cells, where it was mainly associated with membranes. Only a minute amount of the enzyme was detected in the culture media. Immunofluorescence studies demonstrated that in SOJ-6 cells, BSDL colocates with the p58 Golgi protein and suggested that the protein may be sequestrated within the Golgi compartment. These results demonstrated that BSDL is expressed in human pancreatic tumoral cells and cannot be secreted (or for the least very poorly). Subsequently, a cDNA covering the entire sequence of BSDL was obtained by reverse transcription-polymerase chain reaction. The sequence of this cDNA indicated that the N-terminal domain encoded by exons 1-10 was identical to that of BSDL expressed by the human normal pancreas. However, the sequence corresponding to exon 11, which should code for the 16 tandem-repeated identical mucin-like sequences of BSDL, was deleted by 330 base pairs (bp) and encoded only 6 of these repeated sequences. We conclude that this truncated variant of BSDL would be its oncofetal form, referred to as feto-acinar pancreatic protein. We then investigated whether the deletion of 330 bp affected the secretion of the protein. For this purpose, the cDNA corresponding to the mature form of the BSDL variant expressed in SOJ-6 cells was cloned into an expression/secretion vector and transfected into CHO-K1 cells. Results indicated that the variant of BSDL isolated from SOJ-6 cells was expressed and secreted by transfected cells. However, the level of BSDL secreted by these transfected CHO-K1 cells was significantly higher than that observed for SOJ-6 cells. Consequently, the retention of the oncofetal variant of BSDL observed in human pancreatic tumoral cells might not result from inherent properties of the protein.

Molecular cloning of the bile salt-dependent lipase of ferret lactating mammary gland: an overview of functional residues

Ferret lactating mammary gland bile salt-dependent lipase (BSDL, EC 3.1.1.-) has been cloned by RT-PCR. The open reading frame consists of 1869 nucleotides which encode 623 amino acids of the functional enzyme. When compared to other species, the greatest homology is observed between residues 1 and 484, with little or no homology at the C-terminal end where seven repeated segments of similar sequence are located. Ferret mammary gland BSDL retains residues involved in the active site and the tentative heparin binding site at similar positions in comparison to other milk or pancreatic BSDL. Other important items, such as binding peptide to chaperone molecular, phosphorylation site(s) or bile salt binding sites, were also tentatively located in well conserved regions of seven available BSDL sequences.