Lipid-based carriers of microRNAs and intercellular communication

Purpose: Extracellular miRNAs are unusually stable in plasma and their circulating levels change with disease, making them promising biomarkers. They are associated with lipid-based carriers—exosomes, microvesicles, apoptotic bodies, and lipoproteins—as well as lipid-free proteins. Whether these carriers mediate functional miRNA transfer between cells and act as a form of intercellular communication was the central gap addressed.

Hypothesis: If miRNAs are packaged into lipid-based carriers, then they can be protected from plasma RNases, transported through the circulation, delivered to recipient cells by endocytosis, membrane fusion, or scavenger receptors, and alter gene expression in those cells as a form of intercellular communication.

Aims: Summarize recent evidence that miRNAs are exported from cells in lipid-based carriers. - Describe the diversity of lipid-based miRNA carriers: exosomes, microvesicles, apoptotic bodies, and lipoproteins. - Review mechanisms of cellular miRNA export and uptake. - Highlight examples of functional miRNA transfer between cells and the potential relevance to biomarkers and therapeutics.

Delivery system: Carrier types: exosomes (~40–100 nm), microvesicles/microparticles (~50–4000 nm), apoptotic bodies, lipoproteins (HDL and LDL), and large microparticles. - Payload: microRNAs (miRNAs), small noncoding RNAs; also viral miRNAs and plant miRNAs in some studies. - Export pathway: regulated in part by the ceramide pathway and neutral sphingomyelinase 2 (nSMase2). - Uptake routes: endocytosis, macropinocytosis, phagocytosis, membrane fusion, and scavenger receptor SR-BI for HDL-associated miRNAs. - Non-lipid carriers also noted: Argonaute 2 (AGO2) and other protein complexes; a significant fraction of circulating miRNAs may be non-vesicle and protein-bound.

Approach: Review and synthesis of in vitro and in vivo literature. Cell models include dendritic cells, T cells, antigen-presenting cells, hepatocellular carcinoma cells (Hep3B, HepG2, Huh7), adipocytes, macrophages (THP-1), breast cancer cells, endothelial cells (HMEC-1, HUVEC), epithelial cells (Caco-2), renal cancer stem cells, and embryonic stem cells. Animal models include mice and SCID mice. As a review, it reports no primary experimental groups, n values, doses, or controls.

Key methods: No primary methods. The review discusses data generated by cited studies using: - miRNA profiling and sequencing. - Luciferase reporter assays for miRNA function. - RNase protection assays. - Ultracentrifugation, size-exclusion chromatography, and mass spectrometry for carrier isolation/characterization. - Matrigel angiogenesis and cell migration/invasion assays. - Gene expression and protein target validation (e.g., c-Myb, TAK1).

Key results: Over 1,500 human miRNAs have been curated; each cell type typically contains ~150–300 miRNAs. - HDL was shown to contain miRNAs and deliver them to Huh7 hepatocellular carcinoma cells and SR-BI–overexpressing BHK cells. - THP-1 macrophage-derived microvesicles delivered miR-150 to HMEC-1 endothelial cells, causing a 12-fold increase in cellular miR-150 and reduction of c-Myb, with increased endothelial migration. - Plant MIR168a from Caco-2 microvesicles was transferred to HepG2 cells, producing a 100-fold increase in MIR168a and loss of its target protein. - Hep3B hepatocellular carcinoma exosomes contained >100 unique miRNAs and reduced TAK1 protein expression after 72 h. - Tumor-associated macrophage exosomes containing miR-223 increased breast cancer cell invasion. - nSMase2 inhibition with GW4869 decreased exosomal miR-16/146a export but increased cellular miRNA export to HDL. - A major fraction of circulating miRNAs may be non-vesicle and associated with protein complexes such as AGO2.

Interpretation: The authors conclude that cellular miRNAs are exported in and to lipid-based carriers and can be transferred to recipient cells with gene-expression changes, supporting a role in intercellular communication. Circulating miRNAs remain promising disease biomarkers, and if in vitro findings are confirmed in vivo, they may offer new therapeutic intervention points. However, the physiological relevance to health and disease remains to be definitively established.

Limitations: Review article; no primary data, effect sizes, n values, doses, or controls. - Many findings are from in vitro systems; in vivo physiological relevance is not fully established. - Isolation methods often cannot definitively distinguish exosomes from microvesicles. - The majority of circulating miRNAs may be protein-bound rather than lipid-carrier-associated. - Cell-surface receptors and ligands mediating carrier recognition are largely unknown. - Delivery routes and whether transferred miRNAs are functional in vivo require further study. - Potential therapeutic and diagnostic applications remain speculative at this stage.