Transcellular processing of disulfide- and thioether-linked peroxidase--polylysine conjugates in cultured MDCK epithelial cells

Preparation of catalytically active, covalent α-polylysine-enzyme conjugates via UV/vis-quantifiable bis-aryl hydrazone bond formation

Covalent UV/vis-quantifiable bis-aryl hydrazone bond formation was investigated for the preparation of conjugates between α-poly-d-lysine (PDL) and either α-chymotrypsin (α-CT) or horseradish peroxidase (HRP). PDL and the enzymes were first modified via free amino groups with the linking reagents succinimidyl 6-hydrazinonicotinate acetone hydrazone (S-HyNic, at pH 7.6) and succinimidyl 4-formylbenzoate (S-4FB, at pH 7.2), respectively. The modified PDL and enzymes were then conjugated at pH 4.7, whereby polymer chains carrying several enzymes were obtained. Kinetics of the bis-aryl hydrazone bond formation was investigated spectrophotometrically at 354 nm. Retention of the enzymatic activity after conjugate formation was confirmed by using the substrates N-succinimidyl-l-Ala-l-Ala-l-Pro-l-Phe-p-nitroanilide (for α-CT) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS, for HRP). Thus, not only a mild and efficient preparation and convenient quantification of a conjugate between the polycationic α-polylysine and enzymes could be shown, but also the complete preservation of the enzymatic activity.

Comparison of pharmacokinetic parameters of a polypeptide, the Bowman-Birk protease inhibitor (BBI), and its palmitic acid conjugate

PURPOSE

The alteration of the pharmacokinetic parameters of the polypeptide BBI through conjugation with palmitic acid was examined.

METHODS

125I-BBI or 125I-Pal-BBI was administered iv to 6 week old CF-1 mice at a dose of 3 mg/kg. The mice were sacrificed at 5, 10, 20, 60, 120, 240, 360, and 480 min and the total radioactivity was determined for blood and each organ. The blood was analyzed on a Sephadex G-50 size-exclusion column to determine the amount of intact polypeptide present in the blood. From the amount of intact polypeptide at each time point, the pharmacokinetic parameters were determined.

RESULTS

By conjugating three palmitic acids to each BBI molecule, the area under the curve (AUC) and mean residence time (MRT) increase by a factor of 10.8 and 2.8, respectively. There was also a difference in the organ distribution between the two treatments; while 125I-BBI was rapidly cleared from the kidneys, 125I-Pal-BBI was predominantly to the liver. Subsequent studies suggested that the binding of the conjugate to non-albumin serum proteins was most likely the cause of the altered pharmacokinetics.

CONCLUSIONS

The residence time in the blood and the lipophilicity of BBI were increased upon conjugation with palmitic acid through a reversible disulfide linkage. Pharmacokinetic studies showed an increase in the AUC and a decrease in kidney clearance in palmitic acid conjugates, indicating a potential increase of the therapeutic efficacy of the polypeptide drug.

Water-soluble fatty acid derivatives as acylating agents for reversible lipidization of polypeptides

A novel method allowing the conjugation of a fatty acid to a peptide or protein in aqueous buffer is described in this paper. L-Cysteinyl 2-pyridyl disulfide (CPD) (III), which was obtained by reacting L-cysteine (I) with 2,2-dithiopyridine (II), was reacted with the N-hydroxysuccinimide ester of palmitic acid (IV) to yield a water-soluble derivative of palmitic acid, termed Pal-CPD (V). Pal-CPD (V) could be reacted with a sulfhydryl-containing peptide or protein in aqueous buffer to yield the palmitic acid-derivatized conjugate (VI). The palmitic acid-derivatized Bowman-Birk protease inhibitor (BBI), synthesized using this conjugation method, was demonstrated to have 140-fold higher uptake into Caco-2 cell monolayers compared to native-BBI. The biological activity of the conjugate, as assessed using an in vitro transformation assay, was retained.

Disposition of positively charged Bowman-Birk protease inhibitor conjugates in mice: influence of protein conjugate charge density and size on lung targeting

The influence of conjugate charge density and size on the targeting of cationic Bowman-Birk inhibitor (BBI) conjugates to the lungs was studied in mice. The biodistribution of BBI, either as the native protein or in the conjugated form (conjugated to a dicationic, tetracationic, or polycationic carrier), indicated that by increasing the charge density of BBI conjugates, the lung accumulation of the conjugates administered intravenously (i.v.) can be increased. The order of lung accumulation in these studies was as follows: polycationic- > tetracationic- > dicationic-conjugated BBI > BBI. The influence of conjugate size on lung accumulation was studied in three experiments. First, the biodistribution of poly(D-lysine) carriers of equal charge density but different molecular weight demonstrated that lung accumulation of polycationic carriers increases with an increase in carrier size. Second, the biodistributions of BBI, tyramine-derivatized poly(D-lysine)3 kDa, and poly(D-lysine)3 kDa conjugated to BBI indicated that an increase in conjugate size alone is not sufficient to promote the lung accumulation of cationic BBI conjugates. Finally, the biodistribution poly(D-lysine) complexed with heparin showed that targeting of a conjugate to the lungs can be abolished by neutralizing the charge on the carrier. Collectively, data in this paper demonstrate that the carrier-mediated targeting of BBI to the lungs is dependent on (a) cationization of BBI, (b) the conjugate positive charge density, and (c) the size of the cationic conjugate if the charge density is maintained. Also, the data show the size of the conjugate alone does not make a significant impact on lung accumulation.

Carbamylation decreases the cytotoxicity but not the drug-carrier properties of polylysines

The charge density of Poly(D-lysine) was reduced by the carbamylation of the lysyl residues with potassium cyanate. A decrease in the charge density of poly(D-lysine) by 25% and 50% reduced the cytotoxicity of the ligand to cultured L929 cells by a 5-, and a 20 to 25-fold level, respectively, as estimated by using either the viability or the protein assay. The uptake of cyanate-modified poly(D-lysine) ligands in cultured L929 cells was not reduced, while the uptake of poly(D-lysine)/Heparin complex was reduced by 80%, as compared to that of unmodified poly(D-lysine). The in vivo biodistribution of cyanate-modified poly(D-lysine) ligands in the lungs and the liver of mice was not altered in comparison to that of unmodified poly(D-lysine), whereas the poly(D-lysine)/Heparin complex was only accumulated in the liver but not in the lungs. The data in this paper indicate that a 50% decrease in the positive charge density of poly(D-lysine) reduces the toxicity, but not the carrier potential of this polycationic ligand.

Transepithelial transport of tyramine across filter-grown MDCK cells via a poly(D-lysine) carrier

In order to investigate the advantage of using membrane-adsorptive carriers to mediate drug transport across epithelial tissue, we have prepared disulfide- and thioether-linked conjugates of tyramine (tyn) as a model drug to a cationic, nondegradable carrier, poly(D-lysine) (PDL). The transport properties were evaluated using microporous filter-grown Madin-Darby canine kidney (MDCK, strain I) epithelial cells, and we have determined that: (a) the [125I]tyn-SS-PDL conjugate predominantly transported [125I]tyn in the apical-to-basal direction (20-fold greater transport vs. basal-to-apical); (b) [125I]tyn-SS-PDL elicits a 10-fold greater degree of [125I]tyn transport than [125I]tyn-S-PDL, thus demonstrating the importance of the reducible disulfide linkage for [125I]tyn transport to occur; (c) 125I-radioactivity recovered in the basal media was determined to be 95% [125I]tyn-containing small molecules, thus demonstrating the release of [125I]tyn from its PDL carrier; (d) the apical addition of an anionic species, heparin, completely blocks apical-to-basal transport of [125I]tyn, indicating the importance of PDL-mediated binding to the apical membrane for transport to occur; (e) apical-to-basal transport proceeds via non-lysosomal pathways, as lysosomal inactivation by NH4Cl exposure does not inhibit transport, and (f) the addition of a membrane-impermeable inhibitor of disulfide reduction, bisdithionitrobenzoic acid (DTNB), to the apical media inhibits transport by approximately 70%, indicating the importance of apically-localized disulfide reducing reactions for transport of [125I]tyn. Pulse-chase studies indicate that there is no paracellular leakage of conjugate, and the ratio of recycled:membrane-associated:transported [125I]tyn fragment following chase is 4:2:1.

Receptor-mediated peptide delivery in pulmonary epithelial monolayers

The present study investigated the feasibility of utilizing receptor-mediated endocytosis as a means to enhance peptide delivery to the pulmonary epithelium. The strategy employs a molecular conjugate consisting of a cognate moiety, transferrin (TF), covalently-linked to a model polypeptide, horseradish peroxidase (HRP), via a reversible disulfide linkage. A cultured alveolar epithelial monolayer system was used to simulate the conditions of the pulmonary epithelium and to allow accurate quantitation of intra- and transcellular peroxidase transport. The alveolar cells were isolated from rat lungs by enzymatic digestion and grown on microporous tissue culture-treated polycarbonate filters. A significant increase in the uptake of HRP by the cell monolayer was observed upon its conjugation with TF. The effect was found to be concentration-dependent, being more pronounced at low concentrations, i.e., 3.9- and 1.2-fold increase over unconjugated HRP controls at the concentration levels of 0.05 and 1.50 U/ml respectively. Effective peroxidase uptake was shown to require the TF cognate moiety for the cell surface receptor. Specific internalization of the conjugate by the TF endocytic pathway was verified by competition for the TF receptor. Conjugate internalization was not followed by a proportional increase in transcytosis, i.e., at 0.05 U/ml conjugate level, a 1.7-fold increase in transcytosis was observed as compared to 3.9-fold for endocytosis. Effective enhancement of transcytosis was achieved by treating the monolayers with brefeldin A (BFA), a compound known to affect intracellular transport of TF receptor complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

The establishment of polarity and enhanced transcytosis of transferrin receptors in enterocyte-like Caco-2 cells

The effect of enterocyte-like differentiation on the transferrin receptor (TfR) polarity in filter-grown Caco-2 cells was studied. The ratio of apical to basolateral TfRs which was found to be approximately 1:1 on the first day after the cells had reached confluence, changed to 1:40 eight days after reaching confluence. The transepithelial electrical resistance (TEER), transport of horseradish peroxidase (HRP) across the monolayer, and total cellular TfR number remained constant over this period. However, the activity of brush border membrane-associated alkaline phosphatase, an established marker for enterocyte differentiation, increased over this 8-day period concurrent with a decrease in apical TfR number. These results suggest that enterocyte-like differentiation rather than tight junction formation is most likely responsible for the polarized distribution of TfRs in Caco-2 cells. The effects of the fungal metabolite brefeldin A (BFA) on TfR distribution and TfR-mediated transcytosis in Caco-2 cells were also studied. BFA caused a marked decrease in the number of basolateral TfRs along with a slight increase in the number of apical TfR. BFA enhanced the TfR-mediated transcytosis of both 125I-Tf and the horseradish peroxidase-Tf conjugate across Caco-2 cells in both apical-to-basolateral and basolateral-to-apical directions. These findings imply a potential application of BFA as an enhancer for TfR-mediated delivery of protein drugs across the intestinal epithelium.

Regulation of pathways within cultured epithelial cells for the transcytosis of a basal membrane-bound peroxidase-polylysine conjugate

A conjugate of horseradish peroxidase (HRP) to poly(L-lysine) (PLL) was used as a non-specific adsorptive probe to study transcytosis in MDCK strain I and Caco-2 epithelial cells. As we have shown previously, HRP-PLL transcytosis proceeds via an intracellular, non-lysosomal proteolytic compartment in MDCK cells; yet, this compartment is utilized for transcytosis only in the basal-to-apical direction (Taub, M. E. and Shen, W.-C. J. Cell. Physiol., 150, 283–290, 1992). Using size exclusion chromatography, we demonstrate that the PLL moiety of the conjugate is effectively cleaved during transcytosis, thus releasing free HRP from the apical surface of the cells. Pulse-chase studies indicate that approximately 6% of basolateral surface-associated HRP-PLL conjugate in Transwell-grown cell monolayers enters the basal-to-apical transcytotic pathway. Brief (1 hour) treatment with 160 nM phorbol ester (PMA), a protein kinase C stimulator, elicits a 2-fold increase in the rate and amount of HRP-PLL transcytosis following a 1 hour lag time. Treatment with 1.6 micrograms/ml brefeldin A (BFA) inhibits HRP-PLL transcytosis by approximately 30%; additionally, BFA is able to abolish completely the PMA stimulatory effect. Removal of BFA causes a re-establishment of the normal rate of transcytosis within 2 hours, demonstrating the reversibility of BFA inhibition with respect to HRP-PLL transcytosis. Thus, PMA most likely elicits an increase in the amount of basally internalized conjugate delivered to BFA-sensitive transcytotic compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarity in the transcytotic processing of apical and basal membrane-bound peroxidase-polylysine conjugates in MDCK cells

A conjugate of horseradish peroxidase (HRP) to poly(L-lysine) (PLL) was used to characterize a non-lysosomal proteolytic compartment in the MDCK Strain I epithelial cell line. This compartment is expressed in a polar fashion, and is capable of degradation of the PLL moiety in the conjugate followed by release of HRP via a basal-to-apical, but not apical-to-basal, transcytotic pathway. This uptake, cleavage, and transport process appears to require approximately 2 hr, as there is a 2 hr lag-time between conjugate administration to the basal surface and HRP release to the apical medium. Monensin (10 microM) failed to inhibit this process, indicating that participation of the trans-Golgi network (TGN) in the trafficking of internalized conjugate is not the rate-determining step. Inhibition of HRP transport was found to be elicited by 50 micrograms/ml leupeptin, but only when applied to the basal surface. Brief trypsinization of either the basal or apical surfaces of cells preloaded with HRP conjugate showed no appreciable inhibitory effect on the apical release of HRP, indicating that an intracellular compartment rather than surface-bound enzymes is responsible for the degradation of the PLL moiety in the conjugate. Our results demonstrate the presence of an intracellular proteolytic compartment which is accessible in the basal-to-apical, but not apical-to-basal, transport pathway; and this compartment can be exploited for the transcytosis of membrane-bound molecules.