Antiproliferative effects of steric blocking phosphorodiamidate morpholino antisense agents directed against c-myc

The phosphorodiamidate Morpholino oligomers (PMO) are a new class of antisense agents that inhibit gene expression by binding to RNA and sterically blocking processing or translation. In a search for a Morpholino agent that would inhibit cell proliferation, it was found that oligomers directed against c-myc, a gene involved in control of the cell cycle, were effective. The sequence specificity and mechanism of action of one agent were determined. The 20-mer 126 lowers c-myc protein levels in treated cells and arrests cells in G0/G1 of the cell cycle. It also acts at the RNA level to inhibit normal pre-mRNA splicing and instead produces an aberrantly spliced mRNA. Irrelevant and mispair control oligomers indicated that the observed antiproliferative effect was sequence specific. This was confirmed in a reporter gene model system using a c-myc 5'-untranslated region (5'-UTR) fused to a cDNA copy of the insect luciferase gene. We conclude that 126 is acting through an antisense mechanism involving Watson-Crick hydrogen bonding to its target RNA. A specific antisense agent directed against a cell cycle-associated gene mRNA may be useful as a therapeutic in diseases characterized by excess cell proliferation, such as restenosis following balloon angioplasty or cancer.

Resistance of morpholino phosphorodiamidate oligomers to enzymatic degradation

Oligomers possessing the Morpholino phosphorodiamidate backbone were evaluated for resistance to a variety of enzymes and biologic fluids. A 25-mer was incubated with nucleases, proteases, esterases, and serum, and the reaction mixtures were directly analyzed by MALDI-TOF mass spectrometry. The 25-mer was completely resistant to 13 different hydrolases and serum and plasma. The excellent resistance of Morpholino phosphorodiamidates to enzymatic attack indicates their suitability for in vivo use.

Targeting c-erbB-2 expressing tumors using single-chain Fv monomers and dimers

Single-chain Fv proteins containing a COOH-terminal cysteine (sFv') were constructed by using an antidigoxin 26.10 sFv and an anti-c-erbB-2 741F8 sFv. The fully active sFv' proteins were prepared by expression in Escherichia coli as insoluble inclusion bodies, followed by in vitro refolding using glutathione redox buffers and purification. The COOH-terminal cysteines of the refolded sFv' proteins were protected by a blocking group presumed to be the glutathionyl peptide, which was easily and selectively removed by gentle reduction. Air oxidation of the reduced sFv' monomers resulted in the efficient formation of disulfide-linked sFv' homodimers, designated (sFv')2, which were stable under oxidizing conditions and relatively slow to be disrupted under reducing conditions. The (26-10-1 sFv')-(741F8-1 sFv') heterodimer was prepared and possessed dual-antigen specificity; the active bispecific (sFv')2 dimerized under native conditions, apparently as a manifestation of self-association by the 741F8 sFv' subunit. Biodistribution and imaging studies that were performed on mice bearing human SK-OV-3 tumor xenografts that express the c-erbB-2 as a cell surface antigen were reviewed. Radioiodinated 741F8-2 (sFv')2 homodimer localized to the tumors with high specificity, as evidenced by excellent tumor:normal tissue ratios. Sagittal section autoradiography of whole animals 24 h after administration of antibody species revealed that 741F8 (sFv')2 produced a stronger tumor image than comparable doses of the 741F8 Fab, monomeric sFv', and the 26-10 (sFv')2 control without the high nonspecific background distribution of the 741F8 IgG.

Engineering disulfide-linked single-chain Fv dimers [(sFv')2] with improved solution and targeting properties: anti-digoxin 26-10 (sFv')2 and anti-c-erbB-2 741F8 (sFv')2 made by protein folding and bonded through C-terminal cysteinyl peptides

Single-chain Fv fusions with C-terminal cysteinyl peptides (sFv') have been engineered using model sFv proteins based upon the 26-10 anti-digoxin IgG and 741F8 anti-c-erbB-2 IgG monoclonal antibodies. As part of the 741F8 sFv construction process, the PCR-amplified 741F8 VH gene was modified in an effort to correct possible primer-induced errors. Genetic replacement of the N-terminal beta-strand sequence of 741F8 VH with that from the FR1 of anti-c-erbB-2 520C9 VH resulted in a dramatic improvement of sFv folding yields. Folding in urea-glutathione redox buffers produced active sFv' with a protected C-terminal sulfhydryl, presumably as the mixed disulfide with glutathione. Disulfide-bonded (sFv')2 homodimers were made by disulfide interchange or oxidation after reductive elimination of the blocking group. Both 26-10 (sFv')2 and 741F8 (sFv')2 existed as stable dimers that were well behaved in solution, whereas 741F8 sFv and sFv' exhibited considerable self-association. The 741F8 sFv binds to the extracellular domain (ECD) of the c-erbB-2 oncogene protein, which is often overexpressed in breast cancer and other adenocarcinomas. The recombinant ECD was prepared to facilitate the analysis of 741F8 binding site properties; the cloned ECD gene, modified to encode a C-terminal Ser-Gly-His6 peptide, was transfected into Chinese hamster ovary cells using a vector that also expressed dihydrofolate reductase to facilitate methotrexate amplification. Optimized cell lines expressed ECD-His6 at high levels in a cell bioreactor; after isolation by immobilized metal affinity chromatography, final ECD yields were as high as 47 mg/l. An animal tumor model complemented physicochemical studies of 741F8 species and indicated increased tumor localization of the targeted 741F8 (sFv')2 over other monovalent 741F8 species.

Mammalian cell expression of single-chain Fv (sFv) antibody proteins and their C-terminal fusions with interleukin-2 and other effector domains

The production of several single-chain Fv (sFv) antibody proteins was examined by three modes of mammalian cell expression. Our primary model was the 741F8 anti-c-erbB-2 sFv, assembled as either the VH-VL or VL-VH, and expressed alone, with C-terminal cysteine for dimerization, or as fusion proteins with carboxyl-terminal effector domains, including interleukin-2, the B domain of staphylococcal protein A, the S-peptide of ribonuclease S, or hexa-histidine metal chelate peptide. Constructs were expressed and secreted transiently in 293 cells and stably in CHO or Sp2/0 cell lines, the latter yielding up to 10 mg per liter. Single-chain constructs of MOPC 315 myeloma and 26-10 monoclonal antibodies were also expressed, as were hybrids comprising unrelated VH and VL regions. Our results suggest that mammalian expression is a practical and valuable complement to the bacterial expression of single-chain antibodies.

Tumor targeting in a murine tumor xenograft model with the (sFv')2 divalent form of anti-c-erbB-2 single-chain Fv

This investigation has utilized novel forms of the single-chain Fv (sFv), wherein a cysteine-containing peptide has been fused to the sFv carboxyl terminus to facilitate disulfide bonding or specific cross-linking of this sFv' to make divalent (sFv')2. The 741F8 anti-c-erbB-2 monoclonal antibody was used as the basis for construction of 741F8 sFv, from which the sFv' and (sFv')2 derivatives were prepared. Recombinant c-erbB-2 extracellular domain (ECD) was prepared in CHO cells and the bivalency of 741F8 (sFv')2 demonstrated by its complex formation with ECD. The tumor binding properties of 125I-labeled anti-c-erbB-2 741F8 sFv, sFv', and (sFv)2 were compared with radiolabeled antidigoxin 26-10 sFv' and (sFv')2 controls. Following intravenous administration of radiolabeled species to severe combined immune-deficient (SCID) mice bearing SK-OV-3 tumors (which over-express c-erbB-2), blood and organ samples were obtained as a function of time over 24 h. Comparative analysis of biodistribution and tumor-to-organ ratios demonstrated the 741F8 sFv, sFv', and (sFv')2 had excellent specificity for tumors, which improved with time after injection. This contrasted with nonspecific interstitial pooling in tumors observed with the 26-10 sFv, sFv', and (sFv')2, which decreased with time after administration. Tumor localization was significantly better for disulfide or peptide crosslinked 741F8 (sFv')2 having Gly4Cys tails than for monovalent 741F8 sFv' or Fab. The superior properties of the 741F8 (sFv')2 in targeting SK-OV-3 tumors in SCID mice suggests the importance of further investigations of divalent sFv analogs for immunotargeting.

Cell transformation potential of a HER2 transmembrane domain deletion mutant retained in the endoplasmic reticulum

The HER2 (c-erbB-2) gene encodes a protein, p185HER2, which possesses all of the structural characteristics and functional properties of a growth factor receptor, although its ligand has not yet been well characterized. HER2 is the human homolog of the rat proto-oncogene neu and is closely related to the gene encoding the epidermal growth factor receptor. Amplification of this gene and overexpression have been found to be a prognostic criterion for a 30% subpopulation of human breast cancer patients. In this study, we investigated the role of the transmembrane-spanning sequence in the biosynthesis and localization of p185HER2. A truncation mutant lacking the cytoplasmic and transmembrane domains was glycosylated and efficiently secreted. However, a mutant lacking only the transmembrane-spanning sequence was incompletely glycosylated and failed to reach the cell surface. Unexpectedly, although this deletion mutant was retained in the endoplasmic reticulum membrane, it was still able to transform NIH 3T3 cells when expressed at high levels.

Characterization of murine monoclonal antibodies reactive to either the human epidermal growth factor receptor or HER2/neu gene product

High levels of expression of either the epidermal growth factor receptor or the receptor-like HER2/neu gene product p185HER2 have been observed in a variety of human malignancies. Because of the association of this high level expression with certain human tumors, we have generated a panel of monoclonal antibodies specific for either the epidermal growth factor receptor or p185HER2 to study their structure, function, and antigenic domains in the normal and neoplastic states. We used the epidermoid carcinoma line A431 to generate five monoclonal antibodies which immunoprecipitate the epidermal growth factor receptor. These monoclonal antibodies bind to the extracellular domain of the epidermal growth factor receptor and demonstrate variable effects on epidermal growth factor binding. We used a stably transfected NIH 3T3 cell line expressing the HER2/neu gene to produce and characterize 10 monoclonal antibodies which immunoprecipitate p185HER2. These monoclonal antibodies bind to the extracellular domain of p185HER2 and do not cross-react with the epidermal growth factor receptor. The characteristics and potential applications of these monoclonal antibodies will be discussed.

Selection for transformation and met protooncogene amplification in NIH 3T3 fibroblasts using tumor necrosis factor alpha

Alterations in the structure and expression of protein tyrosine kinases are associated with cellular transformation and tumorigenicity. These enzymes may contribute to tumor progression by interfering with host mechanisms of antitumor surveillance. In the present work, we show that selection of NIH 3T3 fibroblasts for resistance to the cytotoxic effects of tumor necrosis factor alpha (TNF-alpha), a major mediator of macrophage-induced tumor cell cytotoxicity, leads to enrichment in the remaining cells for those with transformed morphology and is often associated with amplified copy number and expression of the met protooncogene. We further substantiate the relationship between met protooncogene amplification and resistance to TNF-alpha by showing that spontaneous (non-TNF-alpha-selected) NIH 3T3 cell transformants which have amplified met protooncogene copy number have increased resistance to the growth-inhibitory activity of this cytokine. These results provide evidence for one mechanism by which the activated macrophage may select for tumor cells within a developing focus with properties associated with tumor progression. In addition, our ability to select cells with such properties, as demonstrated using the met protooncogene as a model system, may also provide a unique means (i.e., selection with TNF-alpha) for identifying other gene products, including other tyrosine kinases, associated with aggressive tumor growth.

p185HER2 monoclonal antibody has antiproliferative effects in vitro and sensitizes human breast tumor cells to tumor necrosis factor

The HER2/c-erbB-2 gene encodes the epidermal growth factor receptorlike human homolog of the rat neu oncogene. Amplification of this gene in primary breast carcinomas has been show to correlate with poor clinical prognosis for certain cancer patients. We show here that a monoclonal antibody directed against the extracellular domain of p185HER2 specifically inhibits the growth of breast tumor-derived cell lines overexpressing the HER2/c-erbB-2 gene product and prevents HER2/c-erbB-2-transformed NIH 3T3 cells from forming colonies in soft agar. Furthermore, resistance to the cytotoxic effect of tumor necrosis factor alpha, which has been shown to be a consequence of HER2/c-erbB-2 overexpression, is significantly reduced in the presence of this antibody.

Amplified expression of the HER2/ERBB2 oncogene induces resistance to tumor necrosis factor alpha in NIH 3T3 cells

Functional characterization of oncogene products that induce cellular transformation has progressed rapidly in recent years. However, less is known about the mechanism(s) by which the transformed cells may escape destruction by host immune defenses and form tumors. A recently described oncogene that has an important association with aggressive human breast carcinoma is "HER2," for human epidermal growth factor receptor 2. The oncogene has also been called NGL and human c-erbB-2 (ERBB2). In this paper we show that amplification of HER2 oncogene expression can induce resistance of NIH 3T3 cells to the cytotoxic effects of recombinant tumor necrosis factor alpha (rTNF-alpha) or macrophages. Resistance is accompanied by an increased dissociation constant for rTNF-alpha binding to high-affinity receptors on the HER2-transformed NIH 3T3 cells. The resistance phenotype is independent of transformation since NIH 3T3 cells transformed by the activated human homologue of the Harvey-ras oncogene (HRAS) retain high-affinity binding sites for rTNF-alpha as well as sensitivity to its cytotoxic effects. These results suggest that HER2 may potentiate tumorigenesis by inducing tumor cell resistance to host defense mechanisms.

Increased expression of the putative growth factor receptor p185HER2 causes transformation and tumorigenesis of NIH 3T3 cells

The HER2 gene encodes a cell-surface glycoprotein with extensive homology to the epidermal growth factor receptor. Recently it was found to be amplified in about 30% of primary human breast malignancies. In experiments designed to assess the role of the HER2 gene in oncogenesis, we found that overexpression of unaltered HER2 coding sequences in NIH 3T3 cells resulted in cellular transformation and tumorigenesis.

Synthesis of an ochre suppressor tRNA gene and expression in mammalian cells

We have used site-specific mutagenesis to change the anticodon of a Xenopus laevis tyrosine tRNA gene so that it would recognize ochre codons. This tRNA gene is expressed when amplified in monkey cells as part of a SV40 recombinant and efficiently suppresses termination at both the ochre codon separating the adenovirus 2 hexon gene from a 23-kd downstream gene and the ochre codon at the end of the NS1 gene of influenza virus A/Tex/1/68. Termination at an amber codon of a NS1 gene of another influenza virus strain was not suppressed by the (Su+) ochre gene suggesting that in mammalian cells amber codons are not recognized by ochre suppressor tRNAs. Finally, microinjection into mammalian cells of both (Su+) ochre tRNA genes and selectible genes containing ochre nonsense mutations gives rise to colonies under selective conditions. We conclude that it should be possible to isolate a wide assortment of mammalian cell lines with ochre suppressor activity.

Establishment of mammalian cell lines containing multiple nonsense mutations and functional suppressor tRNA genes

We describe the generation of mammalian cell lines carrying amber suppressor genes. Nonsense mutants in the herpes simplex virus thymidine kinase (HSV tk) gene, the Escherichia coli xanthine-guanine phosphoribosyl transferase (Eco-gpt) gene and the aminoglycoside 3' phosphotransferase gene of the Tn5 transposon (NPT-II) were isolated and characterized. Each gene was engineered with the appropriate control signals to allow expression in both E. coli and mammalian cells. Expression in E. coli made possible the use of well developed bacterial and phage genetic manipulations to isolate and characterize the nonsense mutants. Once characterized, the nonsense mutants were transferred into mammalian cells by microinjection and used, in turn, to select for amber suppressor genes. Xenopus laevis amber suppressor genes, prepared by site-specific mutagenesis of a normal X. laevis tRNA gene, were microinjected into the above cell lines and selected for the expression of one or more of the amber mutant gene products. The resulting cell lines, containing functional amber suppressor genes, are stable and exhibit normal growth rates.