Exosomes from MCF-7, a noninvasive human breast cancer cell line

Exosomes, tiny extracellular vesicles (EVs) with a size of between 30 and 150 nm, are gaining a lot of attention in the drug delivery space because of their properties. Exosomes are natural nanoparticles released by almost every type of cell, including cancer cells, and they function as a messenger between cells. These vesicles carry proteins, fats, RNA and other biologically active molecules from cell to cell, with the effects of affecting everything from immune systems to the growth of cancers and spreading them. The fact that they carry biomolecules far distances makes exosomes one of the best candidates for drug delivery – an exciting novel route to targeted therapy. Using the in-built cargo-carrying ability of MCF-7 cell exosomes, scientists are finding ways to inject drug agents more quickly and with greater targeting precision – a strategy that might revolutionise cancer therapy. MCF-7 cells are estrogen receptor-positive (ER+) breast cancer cells, and a common in vitro model for breast cancer biology and therapeutics. they're stem cells, originally from a human breast cancer, that have been widely employed for drug screening, chemotherapy efficacy testing, and discovering how breast cancer advances at the molecular level. Exosomes released by MCF-7 cells therefore provide an exotic, biologically valid vehicle for exosome-based drug delivery. MCF-7 cell-derived exosomes contain the proteins, lipids and nucleic acids specific to the donor cancer cells, and they can be used to communicate with these tumour cells (making them a prime candidate for targeted drug delivery). It is because these exosomes are a particular kind of entity, composed of surface markers and proteins that bind to tumour receptors, that drugs can be delivered specifically to breast cancer cells with fewer side effects than traditional chemotherapy. It is this targeted capability of the cellular system combined with the capacity to load exosomes with a host of drugs, including chemotherapy drugs, siRNAs and mRNA, that allows cancer therapies to be highly potent and personalized. Other than the ability to target, there are several other characteristics of MCF-7 cell exosomes that render them very promising for delivering drugs. Biocompatibility is probably the single greatest advantage of exosomes as drug carriers. Because exosomes are a cell-based organic compound, their lipid bilayer is intrinsically incompatibility with the human immune system, and they don't face immune rejection or toxicity. Furthermore, exosomes were not immunogenic in contrast to synthetic nanoparticles and therefore preferable for administration throughout the body. Small enough that they could pass through biological barriers (the blood-brain barrier, for example, or the endothelial lining of blood vessels), carrying medicines to the harder-to-reach places in the body. With breast cancer, for instance, exosomes' capacity to overcome the barrier of biological incompatibility, to evade immune detection and target only cancer cells is a big benefit over traditional chemotherapeutics, which aren't really able to distinguish between tumour and healthy tissue. Moreover, MCF-7 cell exosomes appear to have properties intrinsic to their uptake by the cells. That's partly because there are some surface proteins such as tetraspanins, such as CD63, CD9 and CD81, that recognise and fuse exosomes with cells. These membrane proteins, natural on MCF-7 cells' membranes, linger on the exosome membrane when extracted, and can be deployed to attack cancer receptors on the surface of other tumor cells. This allows exosomes to enter cancer cells more quickly, and to deliver their therapeutic cargo into the cytoplasm where it can take its biological charge. What's more, scientists can tweak or engineer the exosomal surface to make it even more specific. We could attach ligands or antibodies to the exosomal membrane to boost exosome recognition of specific receptors overexpressed on the surface of breast cancer cells, making it more specific and effective to deliver drugs. This is the other defining attribute of MCF-7 cell exosomes – they can be loaded with a wide variety of therapeutic payloads. The types of biomolecules that exosomes can contain range from small molecules drugs to nucleic acids (mRNA, siRNA, microRNA) and even proteins or peptides. This flexibility also permits combination therapies. Exosomes, for instance, can be designed with both chemotherapeutic and gene therapy, so that cancer cells can be inhibited by one while oncogenes or drug-resistant genes can be silenced. This combination treatment may be especially useful in dealing with the issue of drug resistance that plague the treatment of breast cancer.

Figure 1. Exosomes from MCF-7/TAM cells can be transferred into the parental MCF-7 cells.( Liu J, et al.; 2021)

How MCF-7 cell exosomes deliver drugs involves multiple components: exosome biogenesis, cargo loading, targeting and re-injection by the endogenous cells. Exosomes are formed by inward budding of endosomal membranes, called early endosomes, which develop into multivesicular bodies (MVBs) with intraluminal vesicles (ILVs). Such MVBs then cross with the plasma membrane, and the exosomes break free and enter the extracellular space. This is what happens to the contents of these exosomes, which are packaged neatly in the vesicles, proteins, lipids and RNA molecules incorporated at random. The cargo that arrives in the exosomes carries the same biomolecules as the parent MCF-7 cell so the exosomes are carrying biomolecules tailored to target breast cancer cells. After they leave MCF-7 cells, the exosomes float around in the blood and can attach to other cells, including cancer cells. This is done by receptor-ligand interactions – exosome surface proteins, like integrins and tetraspanins, attach to receptors on the surface of the cell that it is binding to. The exosomal membrane fused to the target cell membrane delivers the contents of the exosome into the target cell. That's incredibly efficient because exosomes are made for cell-cell intercommunication and cargo transfer. Once inside the target cell, the therapeutic payload (either drug, RNA or protein) can be released into the cytoplasm where it can perform its function, whether it's gene silencing, protein inhibition or modulation of cellular signalling. This direct delivery to the cytoplasm is one of the key advantages of exosomes as delivery mediums, since exosomes do not have any of the disadvantages of other types of delivery media such as endosomal escape or immune surveillance. One of the advantages of deploying MCF-7-derived exosomes to deliver drugs is that they bypass many of the fundamental limitations of current breast cancer therapy. The most pressing issue in the treatment of breast cancer is heterogeneity, including the differential pharmacokinetics among cancer subtypes. Often, the breast cancers become resistant to chemotherapy and over time are less effective at receiving other therapies. MCF-7 exosomes can be programmed to host multiple types of therapeutic cargo, though – such as drugs that inhibit resistant cancer cells or RNA-based therapies that overcome resistance to drugs. Second, by binding exosomes to receptors on cancer cells, the treatment can then be delivered at higher doses to the tumor with minimum systemic toxicity and side effects. Such targeted delivery is especially relevant in the treatment of breast cancer where keeping the tissue healthy and reducing the number of damaged normal cells is vital to the patient's quality of life.

There are several hurdles to overcome before we have exosome-based therapies in the clinic. The challenge is how to isolated and purify exosomes in a reliable and scaleable way. Exosome isolation from cell culture supernatants or other biological solutions takes time and costs money, and existing techniques don't often produce purified or well-developed exosomes of enough quality to be useful in the clinic. The scientists are tinkering with methods of isolation, including size-exclusion chromatography, ultracentrifugation and filtration, to increase exosome yield and purity. Also, exosome modification, either for targeted delivery or for loading with therapeutic agents, is a concern for stability and biological activity. There is another problem, namely immunogenicity — the surface proteins of exosomes can be immune stimuli if they are not designed correctly. Exosomes come naturally from cells and are generally biocompatible, but when foreign proteins or lipids were placed on the exosome surface, they might trigger an immune response and so cannot serve any therapeutic purpose. This can be remedied by modifying exosomes with biocompatible or "stealth" technologies (including polyethylene glycol (PEG)) that minimize immune detection and accelerate the duration of circulation). Future prospects are enticing for exosomes from MCF-7 cells to be used as drug carriers for better treatment of breast cancer. The more we learn about the biology of exosomes and their engagement with cancer cells, the more exosome engineering and enhancement options will become available for more targeted therapies. The use of exosome-derived drug delivery systems might lead to not just improved therapeutic outcomes in breast cancer treatment, but also to new delivery methods for RNA-based therapies, gene editing and immunotherapies, which promise even more powerful, selective and minimally invasive cancer therapies in the future.

Alternate Names:
MCF-7-Derived Exosomes
MCF-7 Exosome Particles
MCF-7 Cell-Derived Exosomes
MCF-7 Exosome Vesicles
MCF-7 Tumor-Derived Exosomes
Exosomes Isolated from MCF-7 Cells
MCF-7 Exosome Nanovesicles
MCF-7-Derived Extracellular Vesicles
Exosomes from MCF-7 Breast Cancer Cells
MCF-7-Derived Microvesicles

References:
1. Liu J, et al.; Exosomes from tamoxifen-resistant breast cancer cells transmit drug resistance partly by delivering miR-9-5p. Cancer Cell Int. 2021, 21(1):55.

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