A non-covalent peptide-based strategy for protein and peptide nucleic acid transduction

A new fluorescence/PET probe for targeting intracellular human telomerase reverse transcriptase (hTERT) using Tat peptide-conjugated IgM

Despite an increasing need for methods to visualize intracellular proteins in vivo, the majority of antibody-based imaging methods available can only detect membrane proteins. The human telomerase reverse transcriptase (hTERT) is an intracellular target of great interest because of its high expression in several types of cancer. In this study, we developed a new probe for hTERT using the Tat peptide. An hTERT antibody (IgG or IgM) was conjugated with the Tat peptide, a fluorescence dye and (64)Cu. HT29 (hTERT+) and U2OS (hTERT-) were used to visualize the intracellular hTERT. The hTERT was detected by RT-PCR and western blot. Fluorescence signals for hTERT were obtained by confocal microscopy, live cell imaging, and analyzed by Tissue-FAXS. In nude mice, tumors were visualized using the fluorescence imaging devices Maestro™ and PETBOX. In RT-PCR and western blot, the expression of hTERT was detected in HT29 cells, but not in U2OS cells. Fluorescence signals were clearly observed in HT29 cells and in U2OS cells after 1 h of treatment, but signals were only detected in HT29 cells after 24 h. Confocal microscopy showed that 9.65% of U2OS and 78.54% of HT29 cells had positive hTERT signals. 3D animation images showed that the probe could target intranuclear hTERT in the nucleus. In mice models, fluorescence and PET imaging showed that hTERT in HT29 tumors could be efficiently visualized. In summary, we developed a new method to visualize intracellular and intranuclear proteins both in vitro and in vivo.

PEP-1-FK506BP12 inhibits matrix metalloproteinase expression in human articular chondrocytes and in a mouse carrageenan-induced arthritis model

The 12 kDa FK506-binding protein (FK506BP12), an immunosuppressor, modulates T cell activation via calcineurin inhibition. In this study, we investigated the ability of PEP-1-FK506BP12, consisting of FK506BP12 fused to the protein transduction domain PEP-1 peptide, to suppress catabolic responses in primary human chondrocytes and in a mouse carrageenan-induced paw arthritis model. Western blotting and immunofluorescence analysis showed that PEP-1-FK506BP12 efficiently penetrated chondrocytes and cartilage explants. In interleukin-1β (IL-1β)-treated chondrocytes, PEP-1-FK506BP12 significantly suppressed the expression of catabolic enzymes, including matrix metalloproteinases (MMPs)-1, -3, and -13 in addition to cyclooxygenase-2, at both the mRNA and protein levels, whereas FK506BP12 alone did not. In addition, PEP-1-FK506BP12 decreased IL-1β-induced phosphorylation of the mitogen-activated protein kinase (MAPK) complex (p38, JNK, and ERK) and the inhibitor kappa B alpha. In the mouse model of carrageenan-induced paw arthritis, PEP-1-FK506BP12 suppressed both carrageenan-induced MMP-13 production and paw inflammation. PEP-1-FK506BP12 may have therapeutic potential in the alleviation of OA progression. [BMB Reports 2015; 48(7): 407-412]

Fusion with pep-1, a cell-penetrating peptide, enhances the transmembrane ability of human epidermal growth factor

Administration of macromolecule compositions in medicine and cosmetics always exhibited low bioavailability due to the limitation of transmembrane transport. Here, human epidermal growth factor (hEGF) was fused with glutathione S-transferase (GST) and Pep-1, the first commercial cell-penetrating peptide, in Escherichia coli. The fusion protein was firstly purified with the affinity chromatography, and then the GST tag was released by TEV protease. Final purification was achieved by the ion exchange chromatography. The biological activities and the transmembrane ability of the obtained products were determined using scratch wound-healing assay, MTT analysis, and immunofluorescence assay. The results showed that both rhEGF and Pep-1-fused hEGF were soluble expressed in E. coli. The fusion of Pep-1 could markedly increase the transmembrane ability of EGF, whereas it did not interfere with the growth-stimulating and migration-promoting functions of hEGF on fibroblasts. This research provided a novel strategy for the transmembrane transport of protein-derived cosmetics or drugs.

Synthetic SiRNA Delivery: Progress and Prospects

Small interfering RNA (siRNA) is a powerful tool for modulating gene expression by RNA interference (RNAi). Duplex RNA oligonucleotides induce cleavage of homologous target transcripts, thereby enabling posttranscriptional silencing of potentially any gene. As such, siRNAs may have utility as novel pharmaceuticals for a wide range of diseases. However, a lack of "drug-likeness," physiological barriers, and potential toxicities have meant that systemic delivery of SiRNAs in vivo remains a major challenge. Here we discuss various strategies that have been employed to solve the problem of SiRNA delivery. These include chemical modification of the SiRNA, direct conjugation to bioactive moieties, and nanoparticle formulations.

GFP-complementation assay to detect functional CPP and protein delivery into living cells

Efficient cargo uptake is essential for cell-penetrating peptide (CPP) therapeutics, which deliver widely diverse cargoes by exploiting natural cell processes to penetrate the cell’s membranes. Yet most current CPP activity assays are hampered by limitations in assessing uptake, including confounding effects of conjugated fluorophores or ligands, indirect read-outs requiring secondary processing, and difficulty in discriminating internalization from endosomally trapped cargo. Split-complementation Endosomal Escape (SEE) provides the first direct assay visualizing true cytoplasmic-delivery of proteins at biologically relevant concentrations. The SEE assay has minimal background, is amenable to high-throughput processes, and adaptable to different transient and stable cell lines. This split-GFP-based platform can be useful to study transduction mechanisms, cellular imaging, and characterizing novel CPPs as pharmaceutical delivery agents in the treatment of disease.

Cell Penetrating Peptide Conjugated Chitosan for Enhanced Delivery of Nucleic Acid

Gene therapy is an emerging therapeutic strategy for the cure or treatment of a spectrum of genetic disorders. Nevertheless, advances in gene therapy are immensely reliant upon design of an efficient gene carrier that can deliver genetic cargoes into the desired cell populations. Among various nonviral gene delivery systems, chitosan-based carriers have gained increasing attention because of their high cationic charge density, excellent biocompatibility, nearly nonexistent cytotoxicity, negligible immune response, and ideal ability to undergo chemical conjugation. However, a major shortcoming of chitosan-based carriers is their poor cellular uptake, leading to inadequate transfection efficiency. The intrinsic feature of cell penetrating peptides (CPPs) for transporting diverse cargoes into multiple cell and tissue types in a safe manner suggests that they can be conjugated to chitosan for improving its transfection efficiency. In this review, we briefly discuss CPPs and their classification, and also the major mechanisms contributing to the cellular uptake of CPPs and cargo conjugates. We also discuss immense improvements for the delivery of nucleic acids using CPP-conjugated chitosan-based carriers with special emphasis on plasmid DNA and small interfering RNA.

Development of protein mimics for intracellular delivery

Designing delivery agents for therapeutics is an ongoing challenge. As treatments and desired cargoes become more complex, the need for improved delivery vehicles becomes critical. Excellent delivery vehicles must ensure the stability of the cargo, maintain the cargo's solubility, and promote efficient delivery and release. In order to address these issues, many research groups have looked to nature for design inspiration. Proteins, such as HIV-1 trans-activator of transcription (TAT) and Antennapedia homeodomain protein, are capable of crossing cellular membranes. However, due to the complexities of their structures, they are synthetically challenging to reproduce in the laboratory setting. Being able to incorporate the key features of these proteins that enable cell entry into simpler scaffolds opens up a wide range of opportunities for the development of new delivery reagents with improved performance. This review charts the development of protein mimics based on cell-penetrating peptides (CPPs) and how structure-activity relationships (SARs) with these molecules and their protein counterparts ultimately led to the use of polymeric scaffolds. These scaffolds deviate from the normal peptide backbone, allowing for simpler, synthetic procedures to make carriers and tune chemical compositions for application specific needs. Successful design of polymeric protein mimics would allow researchers to further understand the key features in proteins and peptides necessary for efficient delivery and to design the next generation of more efficient delivery reagents.

Cationic Cell-Penetrating Peptides Are Potent Furin Inhibitors

Cationic cell-penetrating peptides have been widely used to enhance the intracellular delivery of various types of cargoes, such as drugs and proteins. These reagents are chemically similar to the multi-basic peptides that are known to be potent proprotein convertase inhibitors. Here, we report that both HIV-1 TAT_47-57 peptide and the Chariot reagent are micromolar inhibitors of furin activity in vitro. In agreement, HIV-1 TAT_47-57 reduced HT1080 cell migration, thought to be mediated by proprotein convertases, by 25%. In addition, cyclic polyarginine peptides containing hydrophobic moieties which have been previously used as transfection reagents also exhibited potent furin inhibition in vitro and also inhibited intracellular convertases. Our finding that cationic cell-penetrating peptides exert potent effects on cellular convertase activity should be taken into account when biological effects are assessed.

Peptide-mediated delivery: an overview of pathways for efficient internalization

Poor cellular delivery and low bioavailability of novel potent therapeutic molecules continue to remain the bottleneck of modern cancer and gene therapy. Cell-penetrating peptides have provided immense opportunities for the intracellular delivery of bioactive cargos and have led to the first exciting successes in experimental therapy of muscular dystrophies. This review focuses on the mechanisms by which cell-penetrating peptides gain access to the cell interior and deliver cargos. Recent advances in augmenting delivery efficacy and facilitation of endosomal escape of cargo are presented, and the cell-penetrating peptide-mediated delivery of two of the most popular classes of cargo molecules, oligonucleotides and proteins, is analyzed. The arsenal of tools for oligonucleotide delivery has dramatically expanded in the last decade enabling harnessing of cell-surface receptors for targeted delivery.

Efficient Cargo Delivery into Adult Brain Tissue Using Short Cell-Penetrating Peptides

Zebrafish brains can regenerate lost neurons upon neurogenic activity of the radial glial progenitor cells (RGCs) that reside at the ventricular region. Understanding the molecular events underlying this ability is of great interest for translational studies of regenerative medicine. Therefore, functional analyses of gene function in RGCs and neurons are essential. Using cerebroventricular microinjection (CVMI), RGCs can be targeted efficiently but the penetration capacity of the injected molecules reduces dramatically in deeper parts of the brain tissue, such as the parenchymal regions that contain the neurons. In this report, we tested the penetration efficiency of five known cell-penetrating peptides (CPPs) and identified two- polyR and Trans - that efficiently penetrate the brain tissue without overt toxicity in a dose-dependent manner as determined by TUNEL staining and L-Plastin immunohistochemistry. We also found that polyR peptide can help carry plasmid DNA several cell diameters into the brain tissue after a series of coupling reactions using DBCO-PEG4-maleimide-based Michael's addition and azide-mediated copper-free click reaction. Combined with the advantages of CVMI, such as rapidness, reproducibility, and ability to be used in adult animals, CPPs improve the applicability of the CVMI technique to deeper parts of the central nervous system tissues.

Modifications of natural peptides for nanoparticle and drug design

Natural products serve as an important source of novel compounds for drug development. Recently, peptides have emerged as a new class of therapeutic agents due to their versatility and specificity for biological targets. Yet, their effective application often requires use of a nanoparticle delivery system. In this chapter, we review the role of natural peptides in the design and creation of nanomedicines, with a particular focus on cell-penetrating peptides, antimicrobial peptides, and peptide toxins. The use of natural peptides in conjunction with nanoparticle delivery systems holds great promise for the development of new therapeutic formulations as well as novel platforms for the delivery of various cargoes.

Effects of cell-penetrating peptides on transduction efficiency of PEGylated adenovirus

BACKGROUND

Adenovirus (Ad) is one of the viral vectors most widely used for gene delivery. The virus, however, has serious shortcomings such as immunogenicity, promiscuous tropism, and the inability to efficiently infect certain types of cells. The goal of this study was to improve the ability of an Ad-based vector to efficiently transform cells that lack the native coxsackie-adenovirus receptor (CAR(-)) by modifying the virus with CPP-PEG conjugates.

METHODS

The vector was produced by PEGylating Ad, which packages a lacZ reporter gene, and then conjugating CPPs to form CPP-PEG-Ad particles. The study compared the effectiveness of four different CPPs: Pen, Tat, Pep1, and pArg. The effects of CPP amount per virus, degree of PEGylation, and PEG molecular weight on transduction efficiency were studied on CAR(-) NIH/3T3 cells.

RESULTS

CPP-PEG-Ad particles transduced CAR(-) cells significantly better than unmodified Ad. Pen, the most effective CPP, produced an 80-fold improvement in transduction compared to the unmodified virus. The Pen peptide utilized a combination of electrostatic and hydrophobic interactions with the cell membrane to maximize cellular association while the other CPPs used only electrostatic or hydrophobic interactions but not both. Lastly, higher degrees of PEGylation, which prompted PEG to adopt a "brush" conformation, resulted in more efficient CPP-PEG-Ad particles because of both better conjugation of CPPs to the PEGylated virus and better exposure of the conjugated CPPs on the surface of the particle.

CONCLUSIONS

CPP-PEG-Ad particles efficiently deliver genes to cells that Ad alone would not efficiently infect, thereby extending potential gene therapy treatments to a much broader range of cell types and diseases.

Enhanced cellular uptake of engineered spider silk particles

Drug delivery systems allow tissue/cell specific targeting of drugs in order to reduce total drug amounts administered to an organism and potential side effects upon systemic drug delivery.

Design and evaluation of endosomolytic biocompatible peptides as carriers for siRNA delivery

Gene therapy using RNA interference (RNAi) technology has been explored to treat cancers, by regulating the expression of oncogene. However, even though small interfering RNA (siRNA), which triggers RNAi, may have great therapeutic potential, efforts at using them in vivo have been hampered by the difficulty of effective and safe delivery into cells of interest. In this study, to develop a safe and efficient carrier for in vitro and in vivo siRNA delivery, we designed a peptide library. These peptides are improved variants of a known peptide based siRNA carrier C6. All the modifications improved the transfection efficiency of C6 to some degree. After completing prescreening for activity, several promising candidates were used for further evaluation. Selected peptides C6M3 and C6M6 could form stable complexes with siRNA. These complexes could be greatly uptaken by cells and showed a punctate perinuclear distribution. Moreover, peptide/siRNA complexes achieved high transfection efficiency in vitro without inducing substantial cytotoxicity. We have validated the therapeutic potential of this strategy for cancer treatment by targeting Bcl-2 gene in mouse tumor models, and demonstrated that tumor growth was inhibited. In order to address possible immune side effects of these peptide carriers, biocompatibility study in terms of complement activation and cytokine activation assay were carried out, whereas none of the peptides induced such effects. In conclusion, these results support the potential of these peptides as therapeutic siRNA carrier.

Physicochemical and biological characterization of pep-1/elastin complexes

Transdermal drug delivery of proteins is challenging because the skin acts as a natural and protective barrier. Several techniques including using the cell-penetrating peptides have been studied to increase the penetration of therapeutic proteins into and through the skin. Cell-penetrating peptides facilitate and improve the transduction of large and hydrophilic cargo molecules through plasma membrane. We have recently reported an efficient skin delivery of elastin protein in complex with a cell-penetrating peptide called Pep-1. As the biophysical characteristics of cell-penetrating peptide/protein complexes have been linked with their biological responses, in this study, we investigated biophysical properties of Pep-1/elastin complexes (ratio 10:1) stored in three temperatures (-20 °C, 4 °C and 25 °C) by photon correlation spectroscopy, circular dichroism and isothermal denaturation. We also evaluated the ability of transduction of this complex into cells and skin tissue using both fluorescence microscopy and Kodak In-Vivo FX Pro Imaging System.

Controlling the actuation of therapeutic nanomaterials: enabling nanoparticle-mediated drug delivery

The implementation of biofunctionalized nanoparticles (NPs) as potential therapeutic materials has seen exponential growth in recent years due to their unique ability to overcome the constraints of current medicine. This has been largely driven by significant advances on a number of basic research fronts including high-quality NP synthesis, bioconjugation, cellular delivery and the controlled release or ‘actuation’ of NP-associated cargos. Cumulatively, these are the key enabling tools for the full realization of NP-mediated drug delivery. In this review, the authors’ focus is on recent developments in methodologies for the controlled actuation of therapeutic NPs. The authors discuss the critical requirements for their integration into biological systems and highlight examples from the recent literature where controlled NP actuation has been successfully demonstrated. The current state of therapeutic NPs in the clinical setting is summarized and the article concludes with a brief perspective of how we can expect to see this emerging field develop in the coming years.

Cell-penetrating peptides: achievements and challenges in application for cancer treatment

One of the major hurdles to cure cancer lies in the low potency of currently available drugs, which could eventually be solved by using more potent therapeutic macromolecules, such as proteins or genes. However, although these macromolecules possess greater potency inside the cancer cells, the barely permeable cell membrane remains a formidable barrier to exert their efficacy. A widely used strategy is to use cell penetrating peptides (CPPs) to improve their intracellular uptake. Since the discovery of the first CPP, numerous CPPs have been derived from natural or synthesized products. Both in vitro and in vivo studies have demonstrated that those CPPs are highly efficient in transducing cargoes into almost all cell types. Therefore, to date, CPPs have been widely used for intracellular delivery of various cargoes, including peptides, proteins, genes, and even nanoparticles. In addition, recently, based on the successes of CPPs in cellular studies, their applications in vivo have been actively pursued. This review will focus on the advanced applications of CPP-based in vivo delivery of therapeutics (e.g., small molecule drugs, proteins, and genes). In addition, we will highlight certain updated applications of CPPs for intracellular delivery of nanoparticulate drug carriers, as well as several "smart" strategies for tumor targeted delivery of CPP-cargoes.

The enhanced membrane interaction and perturbation of a cell penetrating peptide in the presence of anionic lipids: toward an understanding of its selectivity for cancer cells

Cell penetrating peptides (CPPs) are usually short, highly cationic peptides that are capable of crossing the cell membrane and transport cargos of varied size and nature in cells by energy- and receptor-independent mechanisms. An additional potential is the newly discovered anti-tumor activity of certain CPPs, including RW16 (RRWRRWWRRWWRRWRR) which is derived from penetratin and is investigated here. The use of CPPs in therapeutics, diagnosis and potential application as anti-tumor agents increases the necessity of understanding their mode of action, a subject yet not totally understood. With this in mind, the membrane interaction and perturbation mechanisms of RW16 with both zwitterionic and anionic lipid model systems (used as representative models of healthy vs tumor cells) were investigated using a large panoply of biophysical techniques. It was shown that RW16 autoassociates and that its oligomerization state highly influences its membrane interaction. Overall a stronger association and perturbation of anionic membranes was observed, especially in the presence of oligomeric peptide, when compared to zwitterionic ones. This might explain, at least in part, the anti-tumor activity and so the selective interaction with cancer cells whose membranes have been shown to be especially anionic. Hydrophobic contacts between the peptide and lipids were also shown to play an important role in the interaction. That probably results from the tryptophan insertion into the fatty acid lipid area following a peptide flip after the first electrostatic recognition. A model is presented that reflects the ensemble of results.

Relationships between Cargo, Cell Penetrating Peptides and Cell Type for Uptake of Non-Covalent Complexes into Live Cells

Modulating signaling pathways for research and therapy requires either suppression or expression of selected genes or internalization of proteins such as enzymes, antibodies, nucleotide binding proteins or substrates including nucleoside phosphates and enzyme inhibitors. Peptides, proteins and nucleotides are transported by fusing or conjugating them to cell penetrating peptides or by formation of non-covalent complexes. The latter is often preferred because of easy handling, uptake efficiency and auto-release of cargo into the live cell. In our studies complexes are formed with labeled or readily detectable cargoes for qualitative and quantitative estimation of their internalization. Properties and behavior of adhesion and suspension vertebrate cells as well as the protozoa Leishmania tarentolae are investigated with respect to proteolytic activity, uptake efficiency, intracellular localization and cytotoxicity. Our results show that peptide stability to membrane-bound, secreted or intracellular proteases varies between different CPPs and that the suitability of individual CPPs for a particular cargo in complex formation by non-covalent interactions requires detailed studies. Cells vary in their sensitivity to increasing concentrations of CPPs. Thus, most cells can be efficiently transduced with peptides, proteins and nucleotides with intracellular concentrations in the low micromole range. For each cargo, cell type and CPP the optimal conditions must be determined separately.

The potential role of cell penetrating peptides in the intracellular delivery of proteins for therapy of erythroid related disorders

The erythroid related disorders (ERDs) represent a large group of hematological diseases, which in most cases are attributed either to the deficiency or malfunction of biosynthetic enzymes or oxygen transport proteins. Current treatments for these disorders include histo-compatible erythrocyte transfusions or allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy delivered via suitable viral vectors or genetically modified HSCs have been under way. Protein Transduction Domain (PTD) technology has allowed the production and intracellular delivery of recombinant therapeutic proteins, bearing Cell Penetrating Peptides (CPPs), into a variety of mammalian cells. Remarkable progress in the field of protein transduction leads to the development of novel protein therapeutics (CPP-mediated PTs) for the treatment of monogenetic and/or metabolic disorders. The “concept” developed in this paper is the intracellular protein delivery made possible via the PTD technology as a novel therapeutic intervention for treatment of ERDs. This can be achieved via four stages including: (i) the production of genetically engineered human CPP-mediated PT of interest, since the corresponding native protein either is missing or is mutated in the erythroid progenitor cell (ErPCs) or mature erythrocytes of patients; (ii) isolation of target cells from the peripheral blood of the selected patients; (iii) ex vivo transduction of cells with the CPP-mediated PT of interest; and (iv) re-administration of the successfully transduced cells back into the same patients.

Therapeutic delivery using cell-penetrating peptides

Intracellular delivery of promising therapeutic agents as well as nanocarriers presents a unique challenge. However, with the discovery of the cell-penetrating peptides (CPPs), overcoming this obstacle seems more plausible. In many cases, CPPs conjugated with therapeutic agent or therapeutic agent loaded-nanoparticles have shown promising results via increased cellular uptake. In this review, the current status of CPPs for the intracellular delivery of not just potential therapeutic small molecules but also large molecules like peptides, nucleic acids and nanocarriers is discussed. In addition, the design of ‘smart stimuli-sensitive nanocarrier’ to overcome the non-target-specificity of CPPs is also described.

A peptide carrier for the delivery of elastin into fibroblast cells

BACKGROUND

Transmembrane delivery of active peptides and proteins, including skin delivery of cosmeceutical proteins such as collagen and elastin, has been a challenging issue. Amphipathic cell-penetrating peptides (CPPs) have been proposed as carrier peptides to mediate cellular uptake of proteins without covalent binding.

MATERIALS AND METHODS

In this study, we have used a short peptide, Pep-1, as our CPP to transport elastin into fibroblast cells. Different ratios of Pep-1/elastin complexes were produced by using a fixed amount of elastin and different molar ratio of Pep-1. The ability of transduction into cells was determined by fluorescence microscopy. The characteristics of Pep-1/elastin complexes were monitored using scanning electron microscopy and photon correlation spectroscopy.

RESULTS

No cellular toxicity was observed in cells treated with Pep-1/elastin complex. Finally, we determined a Pep-1 : elastin ratio of 10 : 1 as the most effective ratio in cellular delivery of elastin.

CONCLUSION

Pep-1 mediated fast and effective delivery of elastin as a cosmetic protein into fibroblast cells in the treatment of skin-aging symptoms.

Solid formulation of cell-penetrating peptide nanocomplexes with siRNA and their stability in simulated gastric conditions

Cell-penetrating peptides (CPPs) are short cationic peptides that have been extensively studied as drug delivery vehicles for proteins, nucleic acids and nanoparticles. However, the formulation of CPP-based therapeutics into different pharmaceutical formulations and their stability in relevant biological environments have not been given the same attention. Here, we show that a newly developed CPP, PepFect 14 (PF14), forms non-covalent nanocomplexes with short interfering RNA (siRNA), which are able to elicit efficient RNA-interference (RNAi) response in different cell-lines. RNAi effect is obtained at low siRNA doses with a unique kinetic profile. Furthermore, the solid dispersion technique is utilized to formulate PF14/siRNA nanocomplexes into solid formulations that are as active as the freshly prepared nanocomplexes in solution. Importantly, the nanocomplexes are stable and active in mediating RNAi response after incubation with simulated gastric fluid (SGF) that is highly acidic. These results demonstrate the activity of PF14 in delivering and protecting siRNA in different pharmaceutical forms and biological environments.

Peptides in cancer nanomedicine: drug carriers, targeting ligands and protease substrates

Peptides are attracting increasing attention as therapeutic agents, as the technologies for peptide development and manufacture continue to mature. Concurrently, with booming research in nanotechnology for biomedical applications, peptides have been studied as an important class of components in nanomedicine, and they have been used either alone or in combination with nanomaterials of every reported composition. Peptides possess many advantages, such as smallness, ease of synthesis and modification, and good biocompatibility. Their functions in cancer nanomedicine, discussed in this review, include serving as drug carriers, as targeting ligands, and as protease-responsive substrates for drug delivery.

Cell penetrating peptides in the delivery of biopharmaceuticals

The cell membrane is a highly selective barrier. This limits the cellular uptake of molecules including DNA, oligonucleotides, peptides and proteins used as therapeutic agents. Different approaches have been employed to increase the membrane permeability and intracellular delivery of these therapeutic molecules. One such approach is the use of Cell Penetrating Peptides (CPPs). CPPs represent a new and innovative concept, which bypasses the problem of bioavailability of drugs. The success of CPPs lies in their ability to unlock intracellular and even intranuclear targets for the delivery of agents ranging from peptides to antibodies and drug-loaded nanoparticles. This review highlights the development of cell penetrating peptides for cell-specific delivery strategies involving biomolecules that can be triggered spatially and temporally within a cell transport pathway by change in physiological conditions. The review also discusses conjugations of therapeutic agents to CPPs for enhanced intracellular delivery and bioavailability that are at the clinical stage of development.

Molecular interactions between cell penetrating peptide Pep-1 and model cell membranes

We investigated the molecular interactions of a cell penetrating peptide (CPP) Pep-1 with model cell membranes using sum frequency generation (SFG) vibrational spectroscopy, supplemented by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Hydrogenated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG and dDPPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) were used in the experiments to represent gel-phase and fluid-phase lipid bilayers, respectively. Our SFG results indicated that Pep-1 molecules adopted a β-sheet conformation when adsorbed to the surface of gel-phase DPPG lipid bilayers. When interacting with fluid-phase POPG lipid bilayers, Pep-1 adopted a mix of α-helical and β-sheet structures over a broad range of peptide concentrations. The orientation distribution of the α-helical Pep-1 segment associated with the fluid-phase bilayers was found to depend on the peptide concentration. SFG orientation analysis showed that Pep-1 molecules adopted an orientation nearly perpendicular to the plane of the bilayer for peptide concentrations of 0.28 and 1.4 μM. When the Pep-1 concentration was increased to 7.0 μM, combined SFG and ATR-FTIR measurements showed that Pep-1 molecules were associated with the bilayer with a broad orientation distribution. Our results demonstrated that lipid bilayer phase and peptide concentration affect the conformation and orientation of Pep-1 molecules associated with model cell membranes, which is crucial to the translocation process of CPPs. A combination of SFG and ATR-FTIR studies can be used to determine the conformation and orientation of CPPs interacting with model cell membranes in situ.

Evaluation of the use of amphipathic peptide-based protein carrier for in vitro cancer research

Intracellular delivery of proteins offers an alternative to genetic modification or siRNA transfection for functional studies of proteins in live cells, especially for studies in cancer cells for therapeutics development. However, lack of efficient and biocompatible delivery system has limited the use of protein for in vitro cancer research. In this study, we design and evaluate an amphipathic peptide RV24, composing of a hydrophobic domain for protein binding, a flexible linker, and a hydrophilic domain to facilitate cell penetration. When using β-galactosidase as a cargo protein for comparison with commercially available peptide- and lipid-based carriers, RV24 peptide provides up to 5-fold increase in quantity delivered into 3 different cancer cell lines. Green fluorescent protein could also be delivered rapidly within 4h and transduced up to 83% of tested cancer cell lines. Although having a cell penetrating domain, RV24 peptide did not compromise cell viability, morphology and granularity significantly. These findings suggest the feasibility of using biocompatible amphipathic peptide to efficiently deliver protein-based molecules intracellularly for in vitro cancer research.

LAH4 enhances CD8+ T cell immunity of protein/peptide-based vaccines

It is now clear that CD8+ T cells are crucial for therapeutic immunity against chronic viral infections and/or tumors. We reason that a strategy capable of improving CD8+ T cell activation would improve the efficacy of protein-based vaccines, which predominantly generate CD4+ T cell-mediated responses. Herein, we explore the ability of a novel cell-penetrating peptide (CPP), LAH4, to facilitate intracellular delivery of protein-based vaccines adjuvanted with Toll-like receptor 9 agonist CpG oligonucleotide (CpG) to generate enhanced CD8+ T cell immune responses and antitumor effects. LAH4 was found to mediate the intracellular delivery of both protein and nucleotide cargo and facilitate protein internalization using mechanisms involving endosomal acidification and processing through the proteasome pathway, leading to enhanced cross presentation of protein antigen by dendritic cells to CD8+ T cells. LAH4 also improved the internalization of CpG, resulting in NFkB activation, thus potentiating the adjuvant effect of CpG. We found that protein-based vaccine comprised of LAH4 mixed with model antigen and CpG generated significantly improved antigen-specific CD8+ T cell immune responses and/or antitumor effects. Furthermore, we found that LAH4 was able to enhance the ability of a tyrosinase-related protein 2 (TRP-2) peptide-based vaccine to generate TRP2-specific CD8+ T cells and antitumor effects against TRP2-expressing tumors. Thus, our results suggest that CPP technology using LAH4 is able to enhance both protein-based and peptide-based vaccine potency to generate antigen-specific CD8+ T cells and antitumor effects. Our findings serve as an important foundation for future clinical applications of CPP technology to improve protein/peptide-based vaccine potency.

Novel p65 binding glucocorticoid-induced leucine zipper peptide suppresses experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a neurological disease characterized by inflammatory demyelination in the brain and spinal cord. The immune-mediated inflammation involves well orchestrated intermolecular interactions that exhibit rapid binding kinetics. The binding interfaces of transient interactions frequently include proline residues that favor an extended conformation for molecular recognition. Linear interface peptides are excellent lead inhibitors of specific protein-protein interactions because only small segments of the interface contribute to the binding. Glucocorticoid-induced leucine zipper (GILZ), a recently identified molecule exhibits potent anti-inflammatory properties. Mechanistically, a proline-rich segment in the carboxyl terminus of GILZ physically binds the p65 subunit of nuclear factor-κB and inhibits the transactivation of inflammatory cytokines. Integrating knowledge derived from the mechanism of action of GILZ with in silico structure prediction identified an immunomodulatory peptide, the GILZ-P. Treatment with GILZ-P exhibited therapeutic efficacy in experimental autoimmune encephalomyelitis, a model for human MS.

Transduction of human recombinant proteins into mitochondria as a protein therapeutic approach for mitochondrial disorders

Protein therapy is considered an alternative approach to gene therapy for treatment of genetic-metabolic disorders. Human protein therapeutics (PTs), developed via recombinant DNA technology and used for the treatment of these illnesses, act upon membrane-bound receptors to achieve their pharmacological response. On the contrary, proteins that normally act inside the cells cannot be developed as PTs in the conventional way, since they are not able to "cross" the plasma membrane. Furthermore, in mitochondrial disorders, attributed either to depleted or malfunctioned mitochondrial proteins, PTs should also have to reach the subcellular mitochondria to exert their therapeutic potential. Nowadays, there is no effective therapy for mitochondrial disorders. The development of PTs, however, via the Protein Transduction Domain (PTD) technology offered new opportunities for the deliberate delivery of human recombinant proteins inside eukaryotic subcellular organelles. To this end, mitochondrial disorders could be clinically encountered with the delivery of human mitochondrial proteins (engineered via recombinant DNA and PTD technologies) at specific intramitochondrial sites to exert their function. Overall, PTD-mediated Protein Replacement Therapy emerges as a suitable model system for the therapeutic approach for mitochondrial disorders.

Potent enhancement of GFP uptake into HT-29 cells and rat skin permeation by coincubation with tat peptide

The delivery enhancements of green fluorescent protein (GFP), a model reporter protein into/transepithelial colon adenocarcinoma (HT-29) cells and excised rat skin by coincubation, by simple mixing or as fusion protein with HIV1-trans-activating transcriptional (Tat), a cell-penetrating peptide, have been described. By simple mixing, Tat/GFP mixture could increase the cellular uptake of GFP into HT-29 cells by 4.25-fold and 1.79-fold of GFP and Tat-GFP fusion protein, respectively. The incubation time showed no effect on the cellular uptake of Tat/GFP and Tat-GFP. In transepithelial study, Tat-GFP demonstrated the highest ability to penetrate through the HT-29 cells of about 1.3-fold and 1.2-fold of GFP and Tat/GFP, respectively. Only Tat/GFP gave lower cytotoxicity than Tat or GFP alone. In transdermal delivery study, Tat/GFP showed better transdermal delivery profile with higher cumulative amount than Tat-GFP in stratum corneum (SC), viable epidermis and dermis, and the receiver compartment of the diffusion cell with the highest fluxes of 7.42 and 35.6; 8.87-fold and 5.57-fold of GFP and Tat-GFP in SC and receiver compartment, respectively. This study demonstrated an efficient enhancement of GFP uptake into cells and through excised rat skin by simple mixing with Tat peptide, which can be further applied for the development of protein drug delivery systems.

Internalization of nucleoside phosphates into live cells by complex formation with different CPPs and JBS-nucleoducin

Nucleoside phosphates can bind to many functional proteins like G-proteins or other GTP-binding proteins in signal transduction or translation processes. Till now internalization of nucleoside phosphates into live cells remains a challenge. We study the internalization of a fluorescent-labelled deoxyuridine triphosphate into HeLa cells and other adhesion and suspension cells. We use different cell-penetrating peptides and a cocktail suitable for formation of non-covalent complexes with the nucleotide. Internalization is observed by fluorescence microscopy, and the uptake efficiency is quantitatively estimated by fluorescence spectroscopy. The applied concentrations of CPPs and the cocktail were checked on cell viability (MTT test) and membrane integrity (bioluminescence test with peptidyl-luciferin), indicating that the CPPs and the complexes with the nucleotide are cytotoxic above certain concentrations. These concentrations depend on CPP and cell type and are the limiting factors for the cargo uptake.