Jaulang Hwang

Characterization of the B Cell Epitopes Associated with a Truncated Form of Pseudomonas Exotoxin (PE38) Used to Make Immunotoxins for the Treatment of Cancer Patients

Recombinant immunotoxins composed of an Ab Fv fragment joined to a truncated portion of Pseudomonas exotoxin A (termed PE38) have been evaluated in clinical trials for the treatment of various human cancers. Immunotoxin therapy is very effective in hairy cell leukemia and also has activity in other hemological malignancies; however, a neutralizing Ab response to PE38 in patients with solid tumors prevents repeated treatments to maximize the benefit. In this study, we analyze the murine Ab response as a model to study the B cell epitopes associated with PE38. Sixty distinct mAbs to PE38 were characterized. Mutual competitive binding of the mAbs indicated the presence of 7 major epitope groups and 13 subgroups. The competition pattern indicated that the epitopes are discrete and could not be reproduced using a computer simulation program that created epitopes out of random surface residues on PE38. Using sera from immunotoxin-treated patients, the formation of human Abs to each of the topographical epitopes was demonstrated. One epitope subgroup, E1a, was identified as the principal neutralizing epitope. The location of each epitope on PE38 was determined by preparing 41 mutants of PE38 in which bulky surface residues were mutated to either alanine or glycine. All 7 major epitope groups and 9 of 13 epitope subgroups were identified by 14 different mutants and these retained high cytotoxic activity. Our results indicate that a relatively small number of discrete immunogenic sites are associated with PE38, most of which can be eliminated by point mutations.

The topoisomerase IIβ circular clamp arrests transcription and signals a 26S proteasome pathway

It has been proposed that the topoisomerase II (TOP2)β–DNA covalent complex arrests transcription and triggers 26S proteasome-mediated degradation of TOP2β. It is unclear whether the initial trigger for proteasomal degradation is due to DNA damage or transcriptional arrest. In the current study we show that the TOP2 catalytic inhibitor 4,4-(2,3-butanediyl)-bis(2,6-piperazinedione) (ICRF-193), which traps TOP2 into a circular clamp rather than the TOP2–DNA covalent complex, can also arrest transcription. Arrest of transcription, which is TOP2β-dependent, is accompanied by proteasomal degradation of TOP2β. Different from TOP2 poisons and other DNA-damaging agents, ICRF-193 did not induce proteasomal degradation of the large subunit of RNA polymerase II. These results suggest that proteasomal degradation of TOP2β induced by the TOP2–DNA covalent complex or the TOP2 circular clamp is due to transcriptional arrest but not DNA damage. By contrast, degradation of the large subunit of RNA polymerase II is due to a DNA-damage signal.

p53 Gene Status Modulates the Chemosensitivity of Non-Small Cell Lung Cancer Cells

This study examined the effects of p53 gene status on DNA damage-induced cell death and chemosensitivity to various chemotherapeutic agents in non-small cell lung cancer (NSCLC) cells. A mutant p53 gene was introduced into cells carrying the wild-type p53 gene and also vice versa to introduce the wild-type p53 gene into cells carrying the mutant p53 gene. Chemosensitivity and DNA damage-induced apoptosis in these cells were then examined. This study included five cell lines, NCI-H1437, NCI-H727, NCI-H441 and NCI-H1299 which carry a mutant p53 gene and NCI-H460 which carries a wild-type p53 gene. Mutant p53-carrying cells were transfected with the wild-type p53 gene, while mutant p53 genes were introduced into NCI-H460 cells. These p53 genes were individually mutated at amino acid residues 143, 175, 248 and 273. The representative cell line NCI-H1437 cells transfected with wild-type p53 gene (H1437/wtp53) showed a dramatic increase in susceptibility to three anticancer agents (7-fold to cisplatin, 21-fold to etoposide, and 20-fold to camptothecin) compared to untransfected or neotransfected H1437 cells. An increase in chemosensitivity was also observed in wild-type p53 transfectants of H727, H441, H1299 cells. The results of chemosensitivity were consistent with the observations on apoptotic cell death. H1437/wtp53 cells, but not H1437 parental cells, exhibited a characteristic feature of apoptotic cell death that generated oligonucleosomal-sized DNA fragments. In contrast, loss of chemosensitivity and lack of p53-mediated DNA degradation in response to anticancer agents were observed in H460 cells transfected with mutant p53. These observations suggest that the increase in chemosensitivity was attributable to wild-type p53 mediation of the process of apoptosis. In addition, our results also suggest that p53 gene status modulates the extent of chemosensitivity and the induction of apoptosis by different anticancer agents in NSCLC cells.

Assembly of a polymeric chain of SUMO1 on human topoisomerase I in vitro

Human (h) DNA topoisomerase I has been identified as a major SUMO1 target in camptothecin-treated cells. In response to TOP1-mediated DNA damage induced by camptothecin, multiple SUMO1 molecules are conjugated to the N-terminal domain of a single TOP1 molecule. To investigate the molecular mechanism of SUMO1 conjugation to TOP1, an in vitro system using purified SAE1/2, Ubc9, SUMO1, and TOP1 peptides was developed. Consistent with results from in vivo studies, multiple SUMO1 molecules were found to be conjugated to the N-terminal domain of a single TOP1 molecule. Systematic analysis has identified a single major SUMO1 conjugation site located between amino acid residues 110 and 125 that contains a single lysine residue at 117 (Lys-117). Using a short peptide spanning this region, we showed that a poly-SUMO1 chain was assembled in this peptide at Lys-117. Interestingly, a Ubc9-poly-SUMO1 intermediate had accumulated to a high level when the sumoylation assay was performed in the absence of hTOP1 substrate, suggesting a possibility that the poly-SUMO1 chain is formed on Ubc9 first and then transferred en bloc onto hTOP1. This is the first definitive demonstration of the assembly of a poly-SUMO1 chain on protein substrate. These results offer new insight into hTOP1 polysumoylation in response to TOP1-mediated DNA damage and may have general implications in protein polysumoylation.

Phosphorylation of Ubc9 by Cdk1 Enhances SUMOylation Activity

Increasing evidence has pointed to an important role of SUMOylation in cell cycle regulation, especially for M phase. In the current studies, we have obtained evidence through in vitro studies that the master M phase regulator CDK1/cyclin B kinase phosphorylates the SUMOylation machinery component Ubc9, leading to its enhanced SUMOylation activity. First, we show that CDK1/cyclin B, but not many other cell cycle kinases such as CDK2/cyclin E, ERK1, ERK2, PKA and JNK2/SAPK1, specifically enhances SUMOylation activity. Second, CDK1/cyclin B phosphorylates the SUMOylation machinery component Ubc9, but not SAE1/SAE2 or SUMO1. Third, CDK1/cyclin B-phosphorylated Ubc9 exhibits increased SUMOylation activity and elevated accumulation of the Ubc9-SUMO1 thioester conjugate. Fourth, CDK1/cyclin B enhances SUMOylation activity through phosphorylation of Ubc9 at serine 71. These studies demonstrate for the first time that the cell cycle-specific kinase CDK1/cyclin B phosphorylates a SUMOylation machinery component to increase its overall SUMOylation activity, suggesting that SUMOylation is part of the cell cycle program orchestrated by CDK1 through Ubc9.

Recombinant protein composed of Pseudomonas exotoxin A, outer membrane proteins I and F as vaccine against P. aeruginosa infection

Abstract We have constructed a chimeric protein composed of the receptor binding and membrane translocation domains of Pseudomonas exotoxin A (PE) with the outer membrane proteins I and F, together designated as PEIF. The potential of PEIF as a vaccine against Pseudomonas infection was evaluated in BALB/c mice and New Zealand white rabbits. We examined titers of anti-PE and anti-OprF antibodies, and the ability both to neutralize PE cytotoxicity and to increase opsonophagocytic uptake of Pseudomonas aeruginosa strain PAO1, serogroups 2 and 6. The results showed that PEIF can induce antibodies not only to neutralize the PE cytotoxicity but also to promote the uptake of various strains of P. aeruginosa by murine peritoneal macrophages. In a burned mouse model, PEIF afforded significant protection against infection by the homologous P. aeruginosa strain PAO1, heterologous serogroup 2, and the PE hyperproducing strain PA103. These observations thus indicate that PEIF may be used as a novel vaccine against P. aeruginosa infection.

UBC9 autosumoylation negatively regulates sumoylation of septins in Saccharomyces Cerevisiae

Sumoylation regulates a wide range of cellular processes. However, little is known about the regulation of the SUMO machinery. In this study, we demonstrate that two lysine residues (Lys-153 and Lys-157) in the C-terminal region of the yeast E2-conjugating enzyme Ubc9 are the major and minor autosumoylation sites, respectively. Surprisingly, mutation of Lys-157 (ubc9K157R) significantly stimulates the level of Ubc9 autosumoylation at Lys-153. The functional role of Ubc9 autosumoylation is exemplified in our findings that cell cycle-dependent sumoylation of cytoskeletal septin proteins is inversely correlated with the Ubc9 autosumoylation level and that mutation of the Ubc9 autosumoylation sites results in aberrant cell morphology. Our study elucidates a regulatory mechanism that utilizes automodification of the E2 enzyme of the sumoylation machinery to control substrate sumoylation.

Development of DNA delivery system using Pseudomonas exotoxin A and a DNA binding region of human DNA topoisomerase I.

Abstract Gene therapy is defined as the delivery of a functional gene for expression in somatic tissues with the intent to cure a disease. Thus, highly efficient gene transfer is essential for gene therapy. Receptor-mediated gene delivery can offer high efficiency in gene transfer, but several technical difficulties need to be solved. In this study, we first examined the DNA binding regions of the human DNA topoisomerase I (Topo I), using agarose gel mobility shift assay, in order to identify sites of noncovalent binding of human DNA Topo I to plasmid DNA. We identified four DNA binding regions in human DNA Topo I. They resided in aa 51–200, 271–375, 422–596, and 651–696 of the human DNA Topo I. We then used one of the four regions as a DNA binding protein fragment in the construction of a DNA delivery vehicle. Based on the known functional property of each Pseudomonas exotoxin A (PE) domain and human DNA Topo I, we fused the receptor binding and membrane translocation domains of PE with a highly positively charged DNA binding region of the N-terminal 198 amino acid residues of human DNA Topo I. The resulting recombinant protein was examined for DNA binding in vitro and transfer efficiency in cultured cells. The results show that this DNA delivery protein is a general DNA delivery vehicle without DNA sequence, topology, and cell-type specificity. The DNA delivery protein could be used to target genes of interest into cells for genetic and biochemical studies. Therefore, this technique can potentially be applied to cancer gene therapy.

A nontoxic Pseudomonas exotoxin A induces active immunity and passive protective antibody against Pseudomonas exotoxin A intoxication

Pseudomonas exotoxin A (PE) is one of the most potent cytotoxic agents produced by Pseudomonas aeruginosa. In this study, we examined the possibility of using PE with a deletion of 38 carboxyl-terminal amino acid residues, designated PE(Delta576-613), for active immunization against PE-mediated disease. We first examined the toxic effects of PE and PE(Delta576-613) on 5- and 9-week-old ICR mice. The results show that the subcutaneous administration of PE(Delta576-613) at a dose of 250 microg was still nontoxic to 5- and 9-week-old ICR mice, while native PE was lethal at a dose of 0.5 and 1 microg, respectively. PE(Delta576-613) was then used to immunize ICR mice. The minimum dose of PE(Delta576-613) that could effectively induce anti-PE antibodies in 5- and 9-week-old ICR mice was found to be 250 ng. However, immunization with 250 ng PE(Delta576-613) failed to protect the immunized mice from a lethal dose of PE. The effective immunization dose of PE(Delta576-613) that could protect mice against a 2 microg PE challenge was found to be 15 microg. In addition, sera obtained from PE(Delta576-613)-immunized ICR mice were able to neutralize PE intoxication and effectively protect mice from PE. Thus, PE(Delta576-613) may be used as an alternative route to new PE vaccine development.

Immobilizing topoisomerase I on a surface plasmon resonance biosensor chip to screen for inhibitors

BackgroundThe topoisomerase I (TopI) reaction intermediate consists of an enzyme covalently linked to a nicked DNA molecule, known as a TopI-DNA complex, that can be trapped by inhibitors and results in failure of re-ligation. Attempts at new derivative designs for TopI inhibition are enthusiastically being pursued, and TopI inhibitors were developed for a variety of applications. Surface plasmon resonance (SPR) was recently used in TopI-inhibition studies. However, most such immobilized small molecules or short-sequence nucleotides are used as ligands onto sensor chips, and TopI was used as the analyte that flowed through the sensor chip.MethodsWe established a sensor chip on which the TopI protein is immobilized to evaluate TopI inhibition by SPR. Camptothecin (CPT) targeting the DNA-TopI complex was used as a representative inhibitor to validate this label-free method.ResultsPurified recombinant human TopI was covalently coupled to the sensor chip for the SPR assay. The binding of anti-human (h)TopI antibodies and plasmid pUC19, respectively, to the immobilized hTopI was observed with dose-dependent increases in resonance units (RU) suggesting that the immobilized hTopI retains its DNA-binding activity. Neither CPT nor evodiamine alone in the analyte flowing through the sensor chip showed a significant increase in RU. The combination of pUC19 and TopI inhibitors as the analyte flowing through the sensor chip caused increases in RU. This confirms its reliability for binding kinetic studies of DNA-TopI binders for interaction and for primary screening of TopI inhibitors.ConclusionsTopI immobilized on the chip retained its bioactivities of DNA binding and catalysis of intermediates of the DNA-TopI complex. This provides DNA-TopI binders for interaction and primary screening with a label-free method. In addition, this biochip can also ensure the reliability of binding kinetic studies of TopI.