Cellular uptake properties of a 2'-amino/2'-O-methyl-modified chimeric hammerhead ribozyme targeted to the epidermal growth factor receptor mRNA

Evaluation of generation 2 and 3 poly(propylenimine) dendrimers for the potential cellular delivery of antisense oligonucleotides targeting the epidermal growth factor receptor

PURPOSE

To evaluate low generation, G2 and G3, poly(propylenimine) dendrimers for the potential cellular delivery of antisense oligonucleotides (ODNs) targeting the epidermal growth factor receptor (EGFR) in A431 epidermoid carcinoma cells.

METHODS

Cell cytotoxicity of the dendrimers was evaluated using trypan blue exclusion assays. Cellular uptake studies of fluorescently labeled ODNs were performed using fluorescence-activated cell sorting analysis. Intracellular fate of dendrimer-delivered ODNs was assessed in both fixed and live cells using fluorescent microscopy. Antisense ODN activity was assessed in terms of cancer cell growth, inhibition of target EGFR protein, and reduction in mRNA levels.

RESULTS

G2 dendrimer (DAB-8) was less toxic than G3 (DAB-16) dendrimer in A431 cells, with IC50 of >175 and approximately 30 microg/ml, respectively. Uptake of fluorescently labeled ODN:dendrimer complexes was increased by up to 100-fold compared to a marker of fluid-phase endocytosis and up to 9-fold over free ODN at the optimal dendrimer:ODN (w/w) ratio of 5:1. Uptake of dendrimer:ODN complexes was significantly reduced at 4 degrees C (p < 0.05). Live cell fluorescent microscopy resulted in an intracellular distribution of dendrimer:ODN complexes that was suggestive of endocytic uptake; in contrast, cell fixation resulted in an artefactual nuclear localization. Treatment of A431 cells with anti-EGFR antisense ODN:dendrimer complexes inhibited cell growth, protein, and mRNA expression to levels comparable to Oligofectamine-mediated delivery.

CONCLUSIONS

G2 and G3 poly(propylenimine) dendrimers markedly improved the delivery and activity of ODNs and thus may represent general reagents for the delivery of ODNs to cells in culture.

Modular nanotransporters: a versatile approach for enhancing nuclear delivery and cytotoxicity of Auger electron-emitting 125I

Background

This study evaluates the potential utility of a modular nanotransporter (MNT) for enhancing the nuclear delivery and cytotoxicity of the Auger electron emitter ¹²⁵I in cancer cells that overexpress the epidermal growth factor receptor (EGFR).

Methods

MNTs are recombinant multifunctional polypeptides that we have developed for achieving selective delivery of short-range therapeutics into cancer cells. MNTs contain functional modules for receptor binding, internalization, endosomal escape and nuclear translocation, thereby facilitating the transport of drugs from the cell surface to the nucleus. The MNT described herein utilized EGF as the targeting ligand and was labeled with ¹²⁵I using N-succinimidyl-4-guanidinomethyl-3-[¹²⁵I]iodobenzoate (SGMIB). Membrane binding, intracellular and nuclear accumulation kinetics, and clonogenic survival assays were performed using the EGFR-expressing A431 epidermoid carcinoma and D247 MG glioma cell lines.

Results

[¹²⁵I]SGMIB-MNT bound to A431 and D247 MG cells with an affinity comparable to that of native EGF. More than 60% of internalized [¹²⁵I]SGMIB-MNT radioactivity accumulated in the cell nuclei after a 1-h incubation. The cytotoxic effectiveness of [¹²⁵I]SGMIB-MNT compared with ¹²⁵I-labeled bovine serum albumin control was enhanced by a factor of 60 for D247 MG cells and more than 1,000-fold for A431 cells, which express higher levels of EGFR.

Conclusions

MNT can be utilized to deliver ¹²⁵I into the nuclei of cancer cells overexpressing EGFR, significantly enhancing cytotoxicity. Further evaluation of [¹²⁵I]SGMIB-MNT as a targeted radiotherapeutic for EGFR-expressing cancer cells appears warranted.

Complexity in interpretation of embryonic epithelial-mesenchymal transition in response to transforming growth factor-beta signaling

Epithelial-mesenchymal transition (EMT) is a highly conserved and fundamental process that governs morphogenesis in development and may also contribute to cancer metastasis. Transforming growth factor (TGF-beta) is a potent inducer of EMT in various developmental and tumor systems. The analysis of TGF-beta signal transduction pathways is now considered a critically important area of biology, since many defects occur in these pathways in embryonic development. The complexity of TGF-beta signal transduction networks is overwhelming due to the large numbers of interacting constituents, complicated feedforward, feedback and crosstalk circuitry mechanisms that they involve in addition to the cellular kinetics and enzymatics that contribute to cell signaling. As a result of this complexity, apparently simple but highly important questions remain unanswered, that is, how do epithelial cells respond to such TGF-beta signals? System biology and cellular kinetics play a crucial role in cellular function; omissions of such a critical contributor may lead to inaccurate understanding of embryonic EMT. In this review, we identify and explain why certain conditions need to be considered for a true representation of TGF-beta signaling in vivo to better understand the controlled, yet delicate mechanism of embryonic EMT.

Toxicogenomics of non-viral drug delivery systems for RNAi: potential impact on siRNA-mediated gene silencing activity and specificity

RNA interference (RNAi) is an evolutionary conserved cellular process for the regulation of gene expression. In mammalian cells, RNAi is induced via short (21-23 nt) duplexes of RNA, termed small interfering RNA (siRNA), that can elicit highly sequence-specific gene silencing. However, synthetic siRNA duplexes are polyanionic macromolecules that do not readily enter cells and typically require the use of a delivery vector for effective gene silencing in vitro and in vivo. Choice of delivery system is usually made on its ability to enhance cellular uptake of siRNA. However, recent gene expression profiling (toxicogenomics) studies have shown that separate from their effects on cellular uptake, delivery systems can also elicit wide ranging gene changes in target cells that may impact on the 'off-target' effects of siRNA. Furthermore, if delivery systems also alter the expression of genes targeted for silencing, then siRNA activity may be compromised or enhanced depending on whether the target gene is up-regulated or down-regulated respectively. Citing recent examples from the literature, this article therefore reviews the toxicogenomics of non-viral delivery systems and highlights the importance of understanding the genomic signature of siRNA delivery reagents in terms of their impact on gene silencing activity and specificity. Such information will be essential in the selection of optimally acting siRNA-delivery system combinations for the many applications of RNA interference.

Angiogenesis inhibitors in lung cancer: a promise fulfilled

Sustained angiogenesis is one of the hallmarks of carcinogenesis. Vascular endothelial growth factor (VEGF) is a crucial molecule mediating proangiogenic signals against which a number of therapeutic approaches have been designed, such as monoclonal antibodies, small-molecule receptor kinase inhibitors, and nucleic acid inhibitors. The VEGF signaling pathway as a target in lung cancer therapy was validated by a randomized phase III study of platinum agent-based combination chemotherapy with or without bevacizumab, a recombinant humanized monoclonal antibody against VEGF-A, in first-line, nonsquamous, metastatic non-small-cell lung cancer. This trial demonstrated an improvement in overall survival among patients who received bevacizumab in comparison with those who received carboplatin and paclitaxel alone. In this review, we will discuss various aspects of this pivotal trial and highlight issues relevant to angiogenesis inhibition in the treatment of non-small-cell lung cancer.

The design and exogenous delivery of siRNA for post-transcriptional gene silencing

RNA interference (RNAi) is a natural cellular process that effects post-transcriptional gene silencing in eukaryotic systems. Small interfering RNA (siRNA) molecules are the key intermediaries in this process which when exogenously administered can inhibit or "silence" the expression of any given target gene. Thus, siRNA molecules hold great promise as biological tools and as potential therapeutic agents for targeted inhibition of disease-causing genes. However, key challenges to the effective and widespread use of these polyanionic, macromolecular duplexes of RNA are their appropriate design and efficient delivery to cells in vitro and in vivo. This review highlights the current strategies used in the design of effective siRNA molecules and also summarises the main strategies being considered for the exogenous delivery of siRNA for both in vitro and in vivo applications.

A novel anionic dendrimer for improved cellular delivery of antisense oligonucleotides

The optimal design of hybridisation-competent antisense oligonucleotides (ODNs) coupled with an efficient delivery system appear to be important prerequisites for the successful use of antisense reagents for gene silencing. We selected an antisense ODN complementary to an accessible region of the epidermal growth factor receptor (EGFR) mRNA with the aid of an antisense oligonucleotide scanning array. The scanning array comprised 2684 antisense ODN sequences targeting the first 120 nts in the coding region of EGFR mRNA. The array-designed antisense ODN was covalently conjugated to a novel anionic dendrimer using a pentaerythritol-based phosphoroamidite synthon via automated DNA synthesis and the ability of this conjugate to effectively deliver and down-regulate EGFR expression in cancer cells was evaluated. Each dendrimeric structure had nine ODN molecules covalently linked to a common centre at their 3' termini. This dendrimer conjugate was markedly more stable to serum nucleases compared to the free ODNs and the cellular uptake of ODN-dendrimer conjugates was up to 100-fold greater as compared to mannitol, a marker for fluid phase endocytosis, and up to 4-fold greater than naked ODN in cancer cells. ODN-dendrimer uptake was energy-dependent and mediated, at least in part, via binding to cell surface proteins; a process that was inhibited by self-competition and by competition with free ODN, salmon sperm DNA, heparin and dextran sulphate. Fluorescent microscopy studies showed a combination of punctate and more diffuse cytosolic distribution pattern for fluorescently labelled ODN-dendrimer conjugate in A431 cells implying internalization by endocytosis followed by release and sequestration of the conjugate into the cytosol. Little or no conjugate appeared to be present in the nuclei of A431 cells. In vitro RNase H-mediated cleavage assays confirmed that covalently conjugated antisense ODNs in the dendrimer conjugate were able to hybridize and cleave the array-defined hybridisation target site within the EGFR mRNA without the need for ODN dissociation from the conjugate. In cell culture, ODN-dendrimer conjugates were effective in inhibiting cancer cell growth that correlated with a marked knockdown in EGFR protein expression. These data highlight a novel anionic dendrimer delivery system for gene silencing oligonucleotides that improved their biological stability, cellular delivery and antisense activity in cultured cancer cells.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Sequence and chemistry requirements for a novel aptameric oligonucleotide inhibitor of EGF receptor tyrosine kinase activity

We have previously identified a phosphorothioate oligonucleotide (PS-ODN) that inhibited epidermal growth factor receptor tyrosine kinase (TK) activity both in cell fractions and in intact A431 cells. Since ODN-based TK inhibitors may have anti-cancer applications and may also help understand the non-antisense mediated effects of PS-ODNs, we have further studied the sequence and chemistry requirements of the parent PS-ODN (sequence: 5'-GGA GGG TCG CAT CGC-3') as a sequence-dependent TK inhibitor. Sequence deletion and substitution studies revealed that the 5'-terminal GGA GGG hexamer sequence in the parent compound was essential for anti-TK activity in A431 cells. Site-specific substitution of any G with a T in this 5'-terminal motif within the parent compound caused a significant loss in anti-TK activity. The fully PS-modified hexameric motif alone exhibited equipotent activity as the parent 15-mer whereas phosphodiester (PO) or 2'-O-methyl-modified versions of this motif had significantly reduced anti-TK activity. Further, T substitutions within the two 5'-terminal G residues of the hexameric PS-ODN to produce a sequence, TTA GGG, representing the telomeric repeats in human chromosomes, also did not exhibit a significant anti-TK activity. Multiple repeats of the active hexameric motif in PS-ODNs resulted in more potent inhibitors of TK activity than the parent ODN. These results suggested that PS-ODNs, but not PO or 2'-O-methyl modified ODNs, containing the GGA GGG motif can exert potent anti-TK activity which may be desirable in some anti-tumor applications. Additionally, the presence of this previously unidentified motif in antisense PS-ODN constructs may contribute to their biological effects in vitro and in vivo and should be accounted for in the design of the PS-modified antisense ODNs.

Receptor-mediated endocytosis of phosphodiester oligonucleotides in the HepG2 cell line: evidence for non-conventional intracellular trafficking

Having identified an oligonucleotide (ON) receptor in the HepG2 cell line, we have re-examined here the kinetics of ON uptake, subcellular distribution and intracellular localisation in these cells, at concentrations relevant for the study of a receptor-dependent process. Kinetic parameters of ON endocytosis were comparable with those of the receptor-mediated endocytosis tracer, transferrin (uptake equilibrium, saturation with concentration, specific competition and rapid efflux) and were clearly distinct from those of fluid-phase endocytosis. By analytical subcellular fractionation, particulate ON showed a bimodal distribution after 2 h of uptake, with a low-density peak superimposed on the distribution of endosomes, and a high-density peak overlapping lysosomes. After an overnight chase, only the high-density peak remained, but it could be dissociated from lysosomes, based on its refractoriness to displacement upon chloroquine-induced swelling. After 2 h of uptake at 300 nM ON-Alexa, a punctate pattern was resolved, by confocal microscopy, from those of transferrin, of a fluid-phase tracer, and of vital staining of lysosomes by LysoTracker. At 3 microM ON-Alexa, its pattern largely overlapped with the fluid-phase tracer and LysoTracker. Taken together, these data suggest that ON may be internalised at low concentrations by receptor-mediated endocytosis into unique endosomes, then to dense structures that are distinct from lysosomes. The nature of these two compartments and their significance for ON effect deserve further investigation.

Angiozyme: a novel angiogenesis inhibitor

Several inhibitors of angiogenesis are being developed for the treatment of cancer. One dominant strategy involves disruption of the vascular endothelial growth factor (VEGF) pathway by inhibition of the receptors for VEGF. Inhibition of the VEGF receptor activity can be accomplished using catalytic RNA molecules known as ribozymes, which downregulate VEGF receptor function by specifically cleaving the mRNAs for the primary VEGF receptors, Flt-1 and KDR. Significant inhibition of angiogenesis using ribozymes against both receptors has been demonstrated. In animal tumor models, antitumor effects are most pronounced with the anti-Flt-1 ribozyme known as Angiozyme (Ribozyme Pharmaceuticals, Boulder, CO). Extensive preclinical studies have demonstrated no significant toxicities. Clinical trials of Angiozyme are currently in progress for patients with advanced malignancy. Preliminary results demonstrate Angiozyme to be well tolerated, without significant side effects. Several phase II trials are underway for patients with advanced malignancy to test therapeutic efficacy.

The cellular delivery of antisense oligonucleotides and ribozymes

The design and development of antisense oligonucleotides and ribozymes for the treatment of diseases arising from genetic abnormalities has become a real possibility over the past few years. Improvements in oligonucleotide chemistry have led to the synthesis of nucleic acids that are relatively stable in the biological milieu. However, advances in cellular targeting and intracellular delivery will probably lead to more widespread clinical applications. This review looks at recent advances in the in vitro and in vivo delivery of antisense oligodeoxynucleotides and ribozymes.

Alternative interpretations of the oligonucleotide transport literature: insights from nature

Elucidation of the mechanism of oligonucleotide (ON) cellular internalization has met an impasse at the lipid penetration stage. ON internalization is commonly regarded to involve endocytosis, yet the method by which the ON penetrates the endosome membrane remains a mystery despite more than 10 years of research by multiple laboratories. In addition, the literature regarding this topic is fraught with discrepancies and inconsistencies. Therefore, the goal of this review is to propose and illustrate the feasibility of the notion that the literature discrepancies are perhaps an indication of a complex transport mechanism involving more than one uptake pathway. Accordingly, ON- and cell-differences in uptake may be attributed to differences in the relative importance of these pathways for different cell types and ONs. An example of one such pathway is reviewed and critiqued in this communication with respect to its hypothetical role in ON uptake. Other innovative mechanisms should similarly be considered to stimulate new ideas, discussion and research in this unique and interesting field.

The delivery of antisense therapeutics

Antisense oligonucleotides, ribozymes and DNAzymes have emerged as novel, highly selective inhibitors or modulators of gene expression. Indeed, their use in the treatment of diseases arising from genetic abnormalities has become a real possibility over the past few years. The first antisense drug molecule is now available for clinical use in Europe and USA. However, their successful application in the clinic will require improvements in cellular targeting and intracellular delivery. This review aims to look at recent advances in the in vitro and in vivo delivery of antisense oligodeoxynucleotides and ribozymes.

Studies on uptake, sub-cellular trafficking and efflux of antisense oligodeoxynucleotides in glioma cells using self-assembling cationic lipoplexes as delivery systems

The cellular uptake of antisense oligodeoxynucleotides (ODNs) may be enhanced by the use of carriers such as cationic liposomes or lipoplexes, but little is known about the intracellular fate and subcellular trafficking of these systems in target cells. In this study, we report on the cellular uptake and biodistribution of ODNs in the presence and absence of optimised self-assembled cationic lipoplexes using the C6 glioma cell line as an in vitro model. Biotin or radiolabelled 15-mer phosphorothioate (PS) ODNs were synthesised and their cellular uptake and subcellular biodistribution characterised in the presence and absence of an optimised cationic lipoplex delivery system using studies ranging from cellular association, cellular efflux and transmission electron microscopy (TEM). Ultrastructural studies clearly showed PS ODNs in the absence of liposomal delivery to be sequestered within endosomal and lysosomal vesicular bodies indicative of endocytic uptake. ODNs were also visible, to a lesser extent, in the nucleus and cytoplasm. By employing DOSPA (2'-(1",2"-dioleoyloxypropyldimethyl-ammonium bromide)-N-ethyl-6-amidospermine tetra trifluoroacetic acid) and DOPE (dioleoylphosphatidylethanolamine) complex in a 3 : 1 ratio, as a delivery system for ODNs at a optimal lipid/DNA charge ratio of 1 : 1, the level of ODN cellular association was significantly increased by approximately 10-12 fold with a concomitant change in subcellular distribution of PS ODN. TEM studies indicated enhanced penetration of ODN within the cytosol and the cell nucleus with reduced presence in vesicular compartments. Efflux studies confirmed that cationic lipoplexes promoted entry of ODNs into 'deeper' cellular compartments, consistent with endosomal release. Optimised cationic lipoplexes improved cellular delivery of ODNs by enhancing cell association, uptake and by favourably modulating the intracellular trafficking and distribution of ODNs into non-vesicular compartments including the cytosol and nucleus.

Inhibition of luciferase expression by synthetic hammerhead ribozymes and their cellular uptake

Two synthetic hammerhead ribozymes, one unmodified and the other with 2"-modifications and four phosphorothioate groups, targeting a single GUA site in the luciferase mRNA, were compared for their inhibition of gene expression in cell cultureand their cellular uptake was also analysed. A HeLa X1/5 cell line stably expressing luciferase, under an inducible promoter, was treated with these ribozymes by liposome-mediated transfection to determine their activity. Luciferase expression in cells was inhibited to approximately 50% with little difference between the unmodified and the 2"-modified ribozyme. A similar degree of inhibition was observed with two catalytically inactive ribozymes, indicating that inhibition was mainly due to an antisense effect. A ribozyme carrying a cholesterol moiety, applied to the cells without carrier, showed no inhibition. Northern blotting indicated a similar amount of cellular uptake of all ribozymes. The unmodified ribozyme was essentially evenly distributed between cytoplasm and nucleus, whereas a higher proportion of the phosphorothioate-containing ribozyme was observed in the nucleus. Fluorescence microscopy, including confocal microscopy using 5"-fluorescein-labelled ribozymes, showed that the unmodified and 2"-modified ribozymes were present in the cytoplasm and in the nucleus to a similar extent, whereas the fluorescence of the phosphorothioate-containing ribozyme was much stronger in the nucleus. Both ribozymes inhibited luciferase expression to a comparable degree, suggesting that the ribozyme in the nucleus did not contribute significantly to the inhibition. Ribozymes with a cholesterol moiety were predominantly trapped in the cell membrane, explaining their inability to interfere with gene expression.

Enzymatic and antisense effects of a specific anti-Ki-ras ribozyme in vitro and in cell culture

Due to their mode of action, ribozymes show antisense effects in addition to their specific cleavage activity. In the present study we investigated whether a hammerhead ribozyme is capable of cleaving mutated Ki-ras mRNA in a pancreatic carcinoma cell line and whether antisense effects contribute to the activity of the ribozyme. A 2[prime]-O-allyl modified hammerhead ribozyme was designed to cleave specifically the mutated form of the Ki- ras mRNA (GUU motif in codon 12). The activity was monitored by RT-PCR on Ki- ras RNA expression by determination of the relative amount of wild type to mutant Ki-ras mRNA, by 5-bromo-2[prime]-deoxy-uridine incorporation on cell proliferation and by colony formation in soft agar on malignancy in the human pancreatic adenocarcinoma cell line CFPAC-1, which is heterozygous for the Ki-ras mutation. A catalytically inactive ribozyme was used as control to differentiate between antisense and cleavage activity and a ribozyme with random guide sequences as negative control. The catalytically active anti-Ki-ras ribozyme was at least 2-fold more potent in decreasing cellular Ki-ras mRNA levels, inhibiting cell proliferation and colony formation in soft agar than the catalytically inactive ribozyme. The catalytically active anti-Ki-ras ribozyme, but not the catalytically inactive or random ribozyme, increased the ratio of wild type to mutated Ki-ras mRNA in CFPAC-1 cells. In conclusion, both cleavage activity and antisense effects contribute to the activity of the catalytically active anti-Ki-ras hammerhead ribozyme. Specific ribozymes might be useful in the treatment of pancreatic carcinomas containing an oncogenic GTT mutation in codon 12 of the Ki-ras gene.

Designing ribozymes for the inhibition of gene expression

Ribozymes are being increasingly used for the sequence-specific inhibition of gene expression by the cleavage of mRNAs encoding proteins of interest. However, particular attention must be paid to the following points: the identification of regions on the mRNA accessible to the ribozyme; the delivery of ribozymes to cells by either exogenous or endogenous delivery; colocalization of the ribozyme with the target RNA in the cell; and differentiation between closely related sequences. This field is advancing rapidly, and results obtained with transgenic animals demonstrate the power of this strategy for the inhibition of gene expression.

Synthetic 2'-O-methyl-modified hammerhead ribozymes targeted to the RNA component of telomerase as sequence-specific inhibitors of telomerase activity

Telomerase is a ribonucleoprotein that synthesizes tandem arrays of the hexameric DNA sequence TTAGGG at chromosome termini using its RNA component as a template. As most normal cells lack telomerase activity, a progressive shortening of chromosome length occurs with each cell division because of incomplete DNA replication. Cell senescence ensues when a critical telomere length is reached, but importantly, senescence bypass and life span extension occur in normal cells transfected with functional telomerase activity. Almost 90% of all tumors express telomerase activity, implying that telomerase is an important determinant in tumor progression and cell immortalization. However, the exact role and regulation of the individual components of the telomerase complex are not fully understood and would benefit from the availability of specific inhibitors. In this study, we investigated the potential use of chemically stabilized, catalytic RNA molecules (hammerhead ribozymes) to inhibit telomerase activity by cleaving the RNA component in a sequence-specific manner. Catalytically competent (active) hammerhead ribozymes containing 2'-O-methyl ribonucleotides for enhanced biologic stability and designed to be complementary to the RNA component of human telomerase exhibited dose-dependent inhibition of telomerase activity in human glioma U87-MG cell lysates with an IC50 of around 0.4 microM. Catalytically incompetent (inactive) ribozymes or mismatched ribozymes with reduced hybridization capability to telomerase RNA did not inhibit telomerase activity, as detected by a PCR-based telomeric repeat amplification protocol (TRAP) assay. In vitro cleavage reactions using short substrates and RT-PCR analyses of the full-length RNA substrate in U87-MG cell lysates confirmed a sequence-specific catalytic cleavage of the targeted RNA component of telomerase. Exogenously administrable, synthetic ribozymes may have important uses in further understanding the role and regulation of this ribonucleoprotein in normal and diseased tissues as well as in the potential therapy of telomerase-positive tumors.