Secretion of proalbumin by canavanine-treated Hep-G2 cells

L-Canavanine potentiates the cytotoxicity of doxorubicin and cisplatin in arginine deprived human cancer cells

The non-protein amino acid L-canavanine (L-CAV), an antimetabolite of L-arginine (L-ARG), can alter the 3D conformation of proteins when incorporated into a protein instead of L-ARG. L-CAV inhibits the proliferation of some tumour cells. The deprivation of L-ARG in the culture medium enhances the response of cells to L-CAV. This study aimed to investigate the interaction of L-CAV in combination with the chemotherapeutic drugs, doxorubicin (DOX) or cisplatin (CIS), in cancer cells, especially in the absence of L-ARG. A combination method based on the median-effect principle and mass-action law was used. The following cancer cells were employed: HeLa and Caco-2 cells, overexpressing argininosuccinate synthase (ASS), pancreatic cells (MIA PaCa-2 and BxPC-3) and hepatocellular carcinoma cells (Hep G2 and SK-HEP-1), with down-regulated ASS. When constant and non-constant ratios of L-CAV were combined with DOX and CIS, a synergistic potentiation of cytotoxicity was recorded. Cells expressing high levels of ASS were more sensitive to the treatment as compared to the cells with reduced ASS levels. Overall, this study may provide a new approach to targeting some cancer cells with L-CAV in combination with DNA-targeting drugs such as DOX and CIS, especially those cells which overexpress ASS, such as human cervical and colorectal carcinoma cells.

Molecular structures of citrus flavonoids determine their effects on lipid metabolism in HepG2 cells by primarily suppressing apoB secretion

This study investigated the underlying mechanisms of action for blood lipid lowering effects of citrus flavonoids and their methoxylated analogues (n = 19; dose range: 0-100 μM) in HepG2 cells. Cholesterol (CH) and triglyceride (TG) syntheses were assessed by measuring the incorporation of (14)C-acetate and (14)C-glycerol, respectively, whereas apoB secretion was determined by ELISA. Results show that two polymethoxylated citrus flavonoids (PMFs), tangeretin and nobiletin, potently inhibited apoB secretion (IC(50) = 13 and 29 μM, respectively) and modestly inhibited CH synthesis (IC(50) = 49 and 68 μM) and TG synthesis (IC(50) = 14 and 73 μM), without effecting LDL-receptor activity. Other PMFs (e.g., sinensetin) and non-PMFs (e.g., hesperetin and naringenin) had only weak effects on CH and TG syntheses and apoB secretion (IC(50) > 100 μM). The structure-activity analysis indicated that a fully methoxylated A-ring of the flavonoid structure was associated with a potent inhibitory activity on hepatic apoB secretion. In conclusion, this study using HepG2 cells indicates that citrus flavonoids with a fully methoxylated A-ring may lower blood CH and TG concentrations primarily by suppressing hepatic apoB secretion as a main underlying mode of action.

The Mechanism of l-Canavanine Cytotoxicity: Arginyl tRNA Synthetase as a Novel Target for Anticancer Drug Discovery

There is a clear need for agents with novel mechanisms of action to provide new therapeutic approaches for the treatment of pancreatic cancer. Owing to its structural similarity to L-arginine, L-canavanine, the beta-oxa-analog of L-arginine, is a substrate for arginyl tRNA synthetase and is incorporated into nascent proteins in place of L-arginine. Although L-arginine and L-canavanine are structurally similar, the oxyguanidino group of L-canavanine is significantly less basic than the guanidino group of L-arginine. Consequently, L-canavanyl proteins lack the capacity to form crucial ionic interactions, resulting in altered protein structure and function, which leads to cellular death. Since L-canavanine is selectively sequestered by the pancreas, it may be especially useful as an adjuvant therapy in the treatment of pancreatic cancer. This novel mechanism of cytotoxicity forms the basis for the anticancer activity of L-canavanine and thus, arginyl tRNA synthetase may represent a novel target for the development of such therapeutic agents.

Covalent binding of the flavonoid quercetin to human serum albumin

Quercetin is an abundant flavonoid in the human diet with numerous biological activities, which may contribute to the prevention of human disease but also may be potentially harmful. Quercetin is oxidized in cells to products capable of covalently binding to cellular proteins, a process that may be important for its biological activities. In the present study, using radiolabeled drug and quantifying the products after electrophoretic separation, proteins to which oxidized quercetin is binding irreversibly were identified. The binding of quercetin to human serum albumin (HSA) in human blood and the effect of stimulation of neutrophilic myeloperoxidase on this binding were also measured. The in vitro binding of quercetin to eight proteins in the presence of catalytic amounts of horseradish peroxidase and hydrogen peroxide was highly selective for HSA. For all proteins the binding was dramatically decreased by reduced L-glutathione. In the blood samples, the release of neutrophilic myeloperoxidase by phorbol ester caused a 3-fold increase in the binding of quercetin to HSA. This study shows that quercetin in the presence of peroxidase/hydrogen peroxide covalently links to proteins with a particularly high affinity for HSA and that this also may occur in vivo after exposure to quercetin. This provides further insights into the complex behavior of this major dietary flavonoid.

Expression and secretion of chicken apolipoprotein AI in transfected COS cells

A full-length chicken apolipoprotein A-I (apoAI) cDNA has been cloned into an expression vector, pRSVapoAI. This plasmid was transfected into a monkey kidney (COS-1) cell line in order to study apolipoprotein-lipid assembly. Chicken apoAI is the major apolipoprotein of chicken high-density lipoprotein (HDL), which is less complex in apolipoprotein content than the HDL of human plasma. The transient transfected COS-1 cells synthesized and secreted authentic plasma apoAI. Under serum-free medium conditions, COS cells secreted only proapoAI. A small portion (15%) of the secreted apoAI floated at a density 1.07-1.20 g/ml. Upon incubation with fetal bovine serum at 10 degrees C, a majority of the apoAI was recovered in the HDL density (1.06-1.20 g/ml) region. Secreted apoAI was labeled when transfected COS cells were incubated with [U-14C]palmitate, but the incorporation of radioactivity was not the result of fatty acid acylation through ester bond formation. These results indicate that heterologous COS-1 cells are capable of synthesizing and secreting apoAI, and that intracellular association of apoAI with lipids is not necessary for secretion.

Trapidil derivatives and low density lipoprotein metabolism by human skin fibroblasts and by human hepatoma cell line Hep G2

The effect of trapidil (RocornalR) and some of its newly developed derivatives (AR 12456, AR 12463, AR 12465, AR 12464) on the receptor-mediated low density lipoprotein (LDL) binding, uptake and degradation was studied in human skin fibroblasts (HSF) and in human hepatoma cell line Hep G2. Compound AR 12456 influenced this pathway in a selective way: it enhanced the uptake and degradation of 125I-LDL by Hep G2 cells in a dose-dependent manner, but inhibited it in HSF. Scatchard analysis of the saturable LDL binding in Hep G2 indicates that the effect of compound AR 12456 is the result of an increased number of LDL binding sites. Compound AR 12465 was less effective on LDL catabolism. Trapidil and the other derivatives were inactive under the same experimental conditions. When Ar 12456 was preincubated with Hep G2 cells and then the incubation medium was transferred to HSF, a stimulation of specific LDL pathway occurred also in this cell line. These findings suggest that a metabolite(s) of AR 12456 might be responsible for the enhanced expression of LDL receptors in cultured human cells.

Differential secretion of proteins and glycoproteins by livers of immature and adult rats. Effect of antimicrotubule drugs

This study was initiated to re-examine reported differences in the action of antimicrotubule agents on plasma protein secretion from livers of immature versus adult rats. The aim was (1) to determine the composition and to monitor the secretion of various plasma proteins and glycoproteins from liver slices labeled in vitro with specific amino acids and sugar residues, and (2) to correlate observed differences in secretion of these proteins with structural changes in the hepatocytes of the different aged animals. For the most part, slices of liver from fetal (term), neonatal (4- to 5 days old), and adult rats (70 days old) were incubated with radioactive amino acids or various tritiated sugars specific for N-linked core oligosaccharide and/or N-linked terminal oligosaccharide chains. Our findings indicate that liver slices of fetal and neonatal rats are efficient in synthesizing plasma proteins including fully glycosylated glycoproteins. The secretion of glycosylated and nonglycosylated proteins believed to be processed through Golgi complexes was inhibited to the same extent (approximately 70-80%) by antimicrotubule agents, regardless of the age of the host animal. However, other proteins and glycoproteins secreted by livers of immature rats were found to be relatively insensitive (i.e. inhibited to only 30-40%) to the action of various antimicrotubule drugs. The glycoproteins were found to lack N-linked terminal sugars (although the glycoproteins did contain N-linked core sugars), and it is likely that the drug-insensitive proteins bypassed critical glycosylating sites in the Golgi compartment prior to release. Overall, these findings support earlier data showing that antimicrotubule drugs have a special impact on Golgi-associated events in liver cells. To what extent these findings are related to the action of microtubules remains to be seen.

Proteolytic processing and compartmentalization of the primary translation products of mammalian apolipoprotein mRNAs

The steps involved in the initial assembly of apolipoproteins and lipids into supramolecular arrays (nascent lipoprotein particles) are largely unknown. Examination of the proteolytic processing and compartmentalization of the primary translation products of apolipoprotein mRNAs represents one approach to deciphering the molecular details of lipoprotein assembly. The structures of the primary translation products of seven mammalian apolipoprotein mRNAs has been determined in the past several years. The organization of apolipoprotein signal peptides is typical of eukaryotic prepeptides, although an unusual degree of sequence conservation is present among the signal segments of apo AI, AIV, and E. For those apolipoprotein sequences studied in detail, SRP-dependent cotranslational translocation and proteolytic processing appears to be highly efficient and results in sequestration of the processed protein within the lumen of the endoplasmic reticulum (ER). However the mechanism by which these lipid-binding proteins avoid arrest during their translocation through the lipid bilayer of the ER membrane remains obscure. The two principal human HDL apolipoproteins undergo novel extracellular post-translational proteolytic processing, which results in removal of nonhomologous propeptides. The proteases responsible for proapo AI and AII processing appear to be different. The processing of these proapolipoproteins provides a potential series of steps for regulating the ordered assembly of HDL constituents.

Calcium antagonists and low density lipoproteins metabolism by human fibroblasts and by human hepatoma cell line HEP G2

The effect of Ca2+ antagonists (CA) on the receptor-mediated low density lipoprotein pathway has been investigated "in vitro" in human skin fibroblasts (HSF) and in human hepatoma cell line Hep G2. The specific binding and internalization of human 125I-labeled LDL are dose-dependently increased in HSF by CA of the verapamil series (verapamil, anipamil, gallopamil, ronipamil, and diltiazem), but neither by CA of the dihydropyridine series (nifedipine, nitrendipine) nor by flunarizine. BAY K 8644, a Ca2+ agonist, elicited an opposite effect. In the presence of the tested CA, LDL degradation is either unaffected (lower concentrations) or inhibited (higher concentrations). 125I-LDL uptake is stimulated also in fibroblasts from type IIa hypercholesterolemic patients, heterozygous for defective expression of LDL receptor. The enhanced cellular uptake of 125I-LDL was prevented by cycloheximide and by alpha-amanitin. CA of the verapamil series including diltiazem retained their effect in human hepatoma cell line Hep G2, a model proposed for hepatic metabolism of LDL. Our studies show that a) CA stimulate the high affinity binding and internalization of LDL in HSF and in human hepatoma cell line Hep G2; b) this stimulation involves DNA transcription and new protein synthesis; c) this effect is specific to one subgroup of Ca2+ antagonists (the verapamil class only).

Effects of canavanine on the secretion of plasma proteins by Hep G2 cells

Many secretory proteins contain an amino-terminal propeptide extension which is removed prior to secretion. The point of cleavage is usually marked by a basic pair of amino acids containing arginine. Canavanine, an analogue of arginine, is incorporated into protein and has been shown to inhibit the proteolytic processing of several of these prosecretory proteins. The addition of 3 mM canavanine to Hep G2 cells incubated with L-[35S]methionine inhibited the secretion of 11 plasma proteins studied. Of the secretory proteins studied only albumin is thought to contain a propeptide, which is marked by a pair of arginine residues at its point of proteolytic processing. Canavanine had varying effects on the secretion of plasma proteins; ranging from a 43-53% inhibition of secretion of alpha 1 antitrypsin and alpha 1 anti-chrymotrypsin to nearly abolishing (93% inhibition) secretion of transferrin. Canavanine also caused most of the proteins studied to migrate slower on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Two of the canavanine-treated proteins (albumin and transferrin) which underwent marked changes in electrophoretic mobility were more sensitive than untreated proteins to proteolysis by Staphylococcus Aureus V8 proteinase. The slower electrophoretic migration and the greater sensitivity to proteolysis of these proteins may be attributed to marked structural changes caused by the incorporation of canavanine. This suggests that the inhibition of plasma protein secretion by canavanine is not only due to an inhibition of the processing of proteins but may be caused by structural distortions of the secretory proteins.

Biosynthesis of complement protein D by HepG2 cells: a comparison of D produced by HepG2 cells, U937 cells and blood monocytes

The biosynthesis of complement protein D of the alternative pathway by HepG2 cells, a human hepatocyte cell line, was studied and compared to the biosynthesis of D by U937 cells and blood monocytes. Increasing amounts of antigenic D were detected in HepG2 cell culture supernatants by radioimmunoassay. The kinetics of D synthesis and secretion by HepG2 cells was followed in a pulse-chase study using [35S]cysteine. As analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography, only a single D band was seen intra- and extracellularly and both forms had the same apparent molecular weight as D synthesized by U937 cells or purified from serum. Treatment of HepG2 and U937 cells with canavanine, an arginine amino acid analog, to inhibit intracellular processing resulted in slight depression of the apparent molecular weight of D synthesized by these cells. D synthesized by blood monocytes had an apparent molecular weight similar to that synthesized by HepG2 and U937 cells, suggesting that these cell lines do not synthesize and process D differently than normal monocytes. The data demonstrate that the hepatocyte is a site of D synthesis and suggest that D is not synthesized as a precursor molecule.

Regulation of low-density lipoprotein receptors in the human hepatoma cell line Hep G2

The human hepatoma cell line Hep G2 can be maintained in continuous culture and secretes numerous plasma proteins and lipoproteins into the medium. To better characterize cholesterol homeostasis in these cells we have examined the binding, internalization and degradation of [125I]LDL by cultured Hep G2 cells. Hep G2 cells express high-affinity low-density lipoprotein (LDL) receptors which facilitate the binding, internalization and degradation of [125I]LDL; these receptors can be induced by growth in LDL-depleted medium and repressed by further incubation in medium supplemented with LDL. The degradation of [125I]LDL by derepressed Hep G2 cells was inhibited by greater than 90% by monensin. Incubation of Hep G2 cells in the presence of increasing concentrations of LDL also inhibited cholesterol biosynthesis. Our results indicate that Hep G2 cells possess high affinity LDL receptors which are subject to metabolic regulation and suggest that this cell line affords a valuable model to further examine cholesterol and lipoprotein metabolism in human liver cells.

Variable denaturation of ovalbumin by incorporation of amino acid analogs

Denaturation of hen ovalbumin synthesized in a cell-free system was assayed by examining its sensitivity to trypsin. The native ovalbumin resisted digestion by trypsin, and it remained resistant to digestion when some amino acid analogs, including azetidine-2-carboxylic acid and meta-flourotyrosine, were incorporated into its peptide chain. However, when other amino acid analogs such as beta-hydroxyleucine and 4-thiaisoleucine were incorporated during protein systhesis, ovalbumin became very lablie to trypsin. These experiments demonstrate a sensitive system for detecting protein denaturation and suggest a variable effect of different amino acid analogs on the native conformation of a protein.