A proper amino terminus of diphtheria toxin is important for cytotoxicity

A series of deletions and substitutions were made at the 5' end of the gene fusion between the first 388 codons of diphtheria toxin (DT) and a cDNA encoding human IL2. The chimeric protein (DT388-IL2) was expressed and purified from E. coli and found to be very cytotoxic to a human T cell line, HUT 102, that expresses a large number of IL2 receptors. Deletion of the first five amino acids of DT resulted in a non-cytotoxic chimeric protein that had both ADP-ribosylation activity and IL2 receptor binding activity. Deletion of the first two amino acids of DT had little effect on cytotoxicity, while deletion of the first four amino acids or of two acidic residues at positions 3 and 4 greatly reduced cytotoxicity. Unexpectedly, a mutant containing a single leucine in place of the first two amino acids (gly, ala) was 2-3 fold more active. The amino terminus of DT may participate in the translocation of the A chain to the cytosol in a manner similar to Pseudomonas exotoxin (PE) in which a specific C-terminal sequence has been proposed to be involved in its cytotoxicity.     

Agents which reverse multidrug-resistance are inhibitors of [3H]vinblastine transport by isolated vesicles.

Resistance of human cancer cells to multiple cytotoxic hydrophobic agents (multidrug resistance) is due to overexpression of the MDR1 gene whose product is the ATP-dependent multidrug transporter, P-glycoprotein. We have previously reported that plasma membrane vesicles partially purified from multidrug-resistant human KB carcinoma cells, but not from drug-sensitive cells, accumulated [3H]vinblastine in an ATP-dependent manner (Horio, M., Gottesman, M.M. and Pastan, I. (1988) Proc. Natl. Acad. Sci. USA 85, 3580-3584). Certain calcium-channel blockers, quinidine, and phenothiazines are able to overcome multidrug resistance in cultured cells. In this work, the effect of these reversing agents on ATP-dependent vinblastine (VBL) transport by vesicles from drug-resistant KB cells has been characterized. Azidopine was the most potent inhibitor of ATP-dependent VBL uptake tested (ID50: concentration of inhibitor such that the transport of vinblastine is inhibited by 50%, less than 1 microM). Verapamil, quinidine, and the tiapamil analogue RO-11-2933 were potent but less effective inhibitors (ID50 less than 5 microM). Diltiazem, nifedipine and trifluoperazine were even less effective. These agents had no effect on Na(+)-dependent and Na(+)-independent L-leucine uptake by the vesicles, indicating that the inhibition of ATP dependent VBL transport by these agents is not a non-specific effect, as might result from leaks in the vesicle membrane. Verapamil, quinidine, azidopine and trifluoperazine increased the apparent Km value of vinblastine transport, suggesting that these agents may be competitive inhibitors of vinblastine transport.     

Substitution of foreign protein sequences into a chimeric toxin composed of transforming growth factor alpha and Pseudomonas exotoxin.

TGF alpha-PE40 is a chimeric toxin made by replacing domain Ia of Pseudomonas exotoxin (PE) with transforming growth factor alpha (TGF alpha). We have now replaced a portion of domain Ib of PE with different polypeptides or an extra domain III of PE in transforming growth factor alpha-PE40 and maintained cell killing. Thus, TGF alpha-PE40 can be used to transport foreign protein sequences into the cytosol of cells.     

Single-chain immunotoxins directed at the human transferrin receptor containing Pseudomonas exotoxin A or diphtheria toxin: anti-TFR(Fv)-PE40 and DT388-anti-TFR(Fv).

Two single-chain immunotoxins directed at the human transferrin receptor have been constructed by using polymerase chain reaction-based methods. Anti-TFR(Fv)-PE40 is encoded by a gene fusion between the DNA sequence encoding the antigen-binding portion (Fv) of a monoclonal antibody directed at the human transferrin receptor and that encoding a 40,000-molecular-weight fragment of Pseudomonas exotoxin (PE40). The other fusion protein, DT388-anti-TFR(Fv), is encoded by a gene fusion between the DNA encoding a truncated form of diphtheria toxin and that encoding the antigen-binding portion of antibody to human transferrin receptor. These gene fusions were expressed in Escherichia coli, and fusion proteins were purified by conventional chromatography techniques to near homogeneity. In anti-TFR(Fv)-PE40, the antigen-binding portion is placed at the amino terminus of the toxin, while in DT388-anti-TFR(Fv), it is at the carboxyl end of the toxin. Both these single-chain immunotoxins kill cells bearing the human transferrin receptors. However, anti-TFR(Fv)-PE40 was usually more active than DT388-anti-TFR(Fv), and in some cases it was several-hundred-fold more active. Anti-TFR(Fv)-PE40 was also more active on cell lines than a conjugate made by chemically coupling the native antibody to PE40, and in some cases it was more than 100-fold more active.     

Alanine scanning mutagenesis identifies surface amino acids on domain II of Pseudomonas exotoxin required for cytotoxicity, proper folding, and secretion into periplasm.

Pseudomonas exotoxin A (PE) is a single polypeptide chain that contains 613 amino acids and is arranged into three major structural domains. Domain Ia is responsible for cell recognition, domain II for translocation of PE across the membrane, and domain III for ADP-ribosylation of elongation factor 2. Recombinant PE can be produced in Escherichia coli and is efficiently secreted into the periplasm when an OmpA signal sequence is present. To investigate the role of the amino acids located on the surface of domain II in the action of the toxin against mammalian cells, we substituted alanine for each of the 27 surface amino acids present in domain II. Surprisingly, all 27 mutant proteins had some alteration in cytotoxicity when tested on human A431 or MCF7 cells or mouse L929 cells. Native PE has a compact structure and therefore is relatively protease resistant and very little ADP-ribosylation activity is detected in the absence of the denaturing agents like urea and dithiothreitol. Several of the mutations resulted in altered protease sensitivity of the toxin. Seven of the mutant molecules exhibited ADP-ribosylation activity without urea and dithiothreitol, indicating they are partially unfolded. Out of these seven mutants, six had increased cytotoxic activity on at least one of the target cell lines and the other retained its native cytotoxic potency.     

Mik-beta 1(Fv)-PE40, a recombinant immunotoxin cytotoxic toward cells bearing the beta-chain of the IL-2 receptor.

Mik-beta 1 is a mAb that binds to the beta subunit of the IL-2R. We have constructed a recombinant single chain immunotoxin Mik-beta 1(Fv)-PE40 by genetically fusing the H and L V domains of Mik-beta 1 to each other via a peptide linker, and then to PE40, a derivative of Pseudomonas exotoxin. Mik-beta 1(Fv)-PE40 was selectively cytotoxic for cells expressing high levels of IL-2R beta (p75) subunit. Mik-beta 1(Fv)-PE40 was cytotoxic to the NK cell line YT-S, which expresses p75 but not p55 subunits, with an IC50 of 6 ng/ml. The ATL line HUT-102 was less sensitive, with an IC50 of 200 ng/ml. However, the IC50 could be lowered to 11 ng/ml when Mik-beta 1(Fv)-PE40 was allowed to bind to HUT-102 cells at 4 degrees C for 4 h before overnight incubation at 37 degrees C. An excess of Mik-beta 1 but not of anti-Tac, the anti-p55 mAb, prevented the cytotoxicity of Mik-beta 1(Fv)-PE40. We constructed a more active version of Mik-beta 1(Fv)-PE40, designated Mik-beta 1(Fv)-PE40KDEL, by converting the carboxyl-terminus of the toxin from -REDLK to -KDEL. Mik-beta 1(Fv)-PE40KDEL showed an IC50 of 2 ng/ml toward YT-S cells and 35 ng/ml toward HUT-102 cells. Binding studies using radioiodinated Mik-beta 1 showed that Mik-beta 1(Fv)-PE40 bound to the p75 receptor subunit with 11% of the affinity of the native Mik-beta 1 antibody. Mik-beta 1(Fv)-PE40 may be a useful reagent to study cells that express IL-2R, and it deserves further study as a possible treatment for cancers in which the malignant cells express high numbers of p75 subunit.     

A method for increasing the yield of properly folded recombinant fusion proteins: single-chain immunotoxins from renaturation of bacterial inclusion bodies.

Many proteins produced in Escherichia coli accumulate in inclusion bodies. We have systematically evaluated the parameters that affect the refolding and renaturation of enzymatically active molecules from bacterial inclusion bodies containing a recombinant single-chain immunotoxin, B3(Fv)-PE38KDEL. This recombinant molecule is composed of the variable domains of monoclonal antibody B3 (B3(Fv)) fused to a truncated mutant form of Pseudomonas exotoxin A (PE38KDEL). This immunotoxin kills carcinoma cells in vitro, causes tumor regression in animal tumor models, and is being developed as an anti-cancer therapeutic agent (Brinkmann et al., 1991, Proc. Natl. Acad. Sci. USA 88, 8616-8620). Like many other recombinant proteins, B3(Fv)-PE38KDEL is produced in E. coli in inclusion bodies and must be denatured and refolded to become active. This requires correct folding, formation of native disulfide bonds, and the association of different domains. All these steps are strongly dependent on the renaturation conditions used. Optimum conditions of refolding were obtained by the addition of reduced and oxidized thiol reagents to promote disulfide bond formation and the addition of a labilizing agent such as L-arginine. Furthermore, the necessity to reactivate proteins at low protein concentrations due to its tendency to aggregate at high concentrations was overcome by a step-by-step addition of denatured and reduced protein into the refolding solution. This approach should be useful for the production of active forms of other recombinant proteins.     

Adaptations to training at the individual anaerobic threshold

The individual anaerobic threshold (Th(an)) is the highest metabolic rate at which blood lactate concentrations can be maintained at a steady-state during prolonged exercise. The purpose of this study was to test the hypothesis that training at the Th(an) would cause a greater change in indicators of training adaptation than would training "around" the Th(an). Three groups of subjects were evaluated before, and again after 4 and 8 weeks of training: a control group, a group which trained continuously for 30 min at the Th(an) intensity (SS), and a group (NSS) which divided the 30 min of training into 7.5-min blocks at intensities which alternated between being below the Th(an) [Th(an) -30% of the difference between Th(an) and maximal oxygen consumption (VO2max)] and above the Th(an) (Th(an) +30% of the difference between Th(an) and VO2max). The VO2max increased significantly from 4.06 to 4.27 l.min-1 in SS and from 3.89 to 4.06 l.min-1 in NSS. The power output (W) at Th(an) increased from 70.5 to 79.8% VO2max in SS and from 71.1 to 80.7% VO2max in NSS. The magnitude of change in VO2max, W at Th(an), % VO2max at Th(an) and in exercise time to exhaustion at the pretraining Th(an) was similar in both trained groups. Vastus lateralis citrate synthase and 3-hydroxyacyl-CoA-dehydrogenase activities increased to the same extent in both trained groups. While all of these training-induced adaptations were statistically significant (P < 0.05), there were no significant changes in any of these variables for the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nucleotide sequence and analysis of the mgl operon of Escherichia coli K12

Summary The nucleotide sequence of the Escherichia coli K12 -methylgalactoside transport operon, mgl, was determined. Primer extension analysis indicated that the synthesis of mRNA initiates at guanine residue 145 of the determined sequence. The operon contains three open reading frames (ORF). The operator proximal ORF, mglB, encodes the galactose binding protein, a periplasmic protein of 332 amino acids including the 23 residue amino-terminal signal peptide. Following a 62 nucleotide spacer, the second ORF, mglA, is capable of encoding a protein of 506 amino acids. The amino-terminal and carboxyl-terminal halves of this protein are homologous to each other and each half contains a putative nucleotide binding site. The third ORF, mglC, is capable of encoding a hydrophobic protein of 336 amino acids which is thought to generate the transmembrane pore. The overall organization of the mglBAC operon and its potential to encode three proteins is similar to that of the ara FGH high affinity transport operon, located approximately 1 min away on the E. coli K12 chromosome.     

Interdomain hydrolysis of a truncated Pseudomonas exotoxin by the human immunodeficiency virus-1 protease

The specificity of HIV-1 (human immunodeficiency virus-1) protease has been evaluated relative to its ability to cleave the three-domain Pseudomonas exotoxin (PE66) and related proteins in which the first domain has been deleted or replaced by a segment of CD4. Native PE66 is not hydrolyzed by the HIV-1 protease. However, removal of its first domain produces a molecule which is an excellent substrate for the enzyme. The major site of cleavage in this truncated exotoxin, called LysPE40, occurs in a segment that connects its two major domains, the translocation domain (II), and the ADP-ribosyltransferase (III). This interdomain region contains the sequence ...Asn-Tyr-Pro-Thr... which is similar to that surrounding the scissile Tyr-Pro bond in the gag precursor polyprotein, a natural substrate of the HIV-1 protease. Nevertheless, it is not this sequence that is recognized and cleaved by the enzyme, but one 6 residues away, ...Ala-Leu-Leu-Glu... in which the Leu-Leu peptide bond is hydrolyzed. A second, slower cleavage takes place at the Leu-Ala bond 3 residues in from the NH2 terminus of LysPE40. When domain I of PE66 is replaced by a segment comprising the first two domains of CD4, the resulting chimeric protein is hydrolyzed at the same Leu-Leu bond by HIV-1 protease. Enzyme activities toward synthetic peptides modeled after the sequences defined above in LysPE40 are in complete accord, relative to specificity, kinetics, and pH optimum, with results obtained in the hydrolysis of the parent protein. These findings demonstrate that ideas concerning the specificity of the HIV-1 protease that are based solely upon its processing of natural viral polyproteins can be expanded by evaluation of other multidomain proteins as substrates. Moreover, it would appear that it is not a particular conformation, but sequence and accessibility that play the dominant role in defining sites in a protein substrate that are susceptible to hydrolysis by the enzyme.