Applications of Cell-penetrating Peptides

Cell-penetrating peptides (CPPs) are short peptides that facilitate cellular intake and uptake of molecules ranging from nano-size particles to small chemical compounds to large fragments of DNA. As a versatile peptide, CPP has been widely used for diagnostic and therapeutic applications.

1. 1. Oligo and siRNA Transport

As an important means for targeted therapy of specific genes, oligonucleotides and siRNA can regulate protein activation and post-transcriptional gene expression. Besides, they’re also broadly utilized in the research of signal transmission systems, upstream and downstream molecular interaction, and so forth. However, the clinical development of siRNA has been hindered by the lack of an effective delivery system. Therefore, researchers are constantly trying new ways to change its ability to enter cells.

Yuhuan Li and his colleagues constructed a novel siRNA delivery system, sTOLP, which was based on cell-penetrating peptide oleoyl-octaarginine (OA-R8) modified multifunctional lipid nano-particles. Experiments revealed that the siRNA encapsulated in the sTOLP delivery system showed strong tumor inhibition (61.7%) and can be preferentially absorbed by hepatocytes and tumor cells in HepG2-containing mice without inducing immunogenicity or hepatic or renal toxicity, which brings promise to tumor-specific therapies.

1. Treatment of Tumors

CPPs can use their membrane-penetrating effects to carry chemotherapeutic drugs or biologic agents into the body and release the carrier in the cytoplasm or target action sites to maximize the effectiveness of the treatment of the resulting tumor cells.

Through a biolysable disulfide ligand, the cell-penetrating peptide R8 can be linked to paclitaxel (PTX), a chemotherapeutic drug. After entering the cell, the CPP-paclitaxel compound can be cleaved when it encountered the chemical bond of the coupling with a high concentration of glutathione.

Studies have also shown that CPP-paclitaxel compounds are more effective than paclitaxel alone in both in vitro cell cultures and animal models. In addition, drug activity was also observed in paclitaxel-resistant cells.

1. Targeted Treatment of Inflammation

The nuclear transcription factor kappaB (NF-κB) is a vital transcriptional regulator in cells. It usually binds to its inhibitory protein in a non-activated state in the form of a P50-P65 dimer.

It’s reported that persistent activation of NF-κB can lead to chronic inflammation, such as rheumatoid arthritis, Parkinson's disease, and inflammatory bowel disease. However, NF-κB activation requires interactions with NF-κB essential modulators and kinase complexes.

A test was carried out by combining CPP (8K, Octylysine) with NF-κB essential regulator binding domain (NBD), and then treating mice with inflammatory bowel disease by intraperitoneal injection. The results showed that the intestinal inflammation was improved in the treated mice since the activation of NF-κB was inhibited and the inflammatory cytokines TNF-α, IL-6, and IL-1 were reduced.

1. Bactericidal Effect

Broad-spectrum antibiotics are difficult to inhibit or kill intracellular bacteria, which cause great harm to animal and human health, especially zoonotic infectious diseases. Due to the selective permeability of the cell membrane, drugs cannot penetrate the barrier which weakens their effectiveness.

In this case, some researchers attempted to connect CPP to peptide nucleic acids (PNAs), which could effectively kill intracellular listeria and C-elegans (Milletti,2012), providing insights into the design of intracellular bactericidal drugs introduced by CPP.

It was found that the (KFF)3K cell-penetrating peptide had a specific inhibitory effect on Amyloid mildew after it was linked to 10 nucleotide oligomers of PNA in the initiation codon region of the targeted gene ACPP. More interestingly, the specific bactericidal active substances binding with CPPs can play a selective therapeutic role in cells after entering into cells through direct transport or endocytosis, which provides a more feasible solution for the treatment of intracellular bacteria.

Perspectives of CPPs

Despite the role of CPPs in transporting exogenous substances into cell membranes is well understood, the knowledge about its cellular penetration mechanisms is limited. Several common transmembrane processes have been found in previous studies, but the specific transmembrane processes are not completely clear and what mechanisms are involved remain to be explored.

Furthermore, when CPPs are linked to exogenous substances, the latter’s influence on the transmembrane mechanism, entry site, and targeting as well as the metabolism and degradation of lysosomes need to be further studied and improved.

CPPs are believed to matter a lot in the treatment of animal and human diseases as more interdisciplinary research goes deeper and its potential effectiveness and specificity continue to be explored.