**3.5 Fibroblasts**

*Extracellular Vesicles and Their Importance in Human Health*

Macropinocytosis was blocked with an inhibitor of Na<sup>+</sup>

environment.

multiple epithelial cell lines.

are anchored into lipid rafts.

to control exosome internalization. Some studies described previously have shown that fusion of exosomes to the cell membrane, without endocytosis, can influence intracellular signaling [63], but these are the first to show how intracellular signaling specifically impacts the endocytosis mechanism of exosomes. These results illustrate the complexity of exosome-cell interactions and where additional research is needed. The interdependence of exosome-cell interactions and intracellular signaling are unexplored areas with vast therapeutic potential and are necessary to better understand how extracellular vesicles influence their

Other characteristics are influential in directing endocytosis in epithelial cells including vesicle size, lipid profile, and protein profile (**Figure 4A**). In epithelial cells, particle size dictates entry mechanism with macropinocytosis as one of the pathways operative at a size range that corresponds with exosomes [88]. This pattern is supported by multiple studies where exosome internalization was decreased when key aspects of macropinocytosis were targeted.

affects Rac1 activation and actin reorganization) in human cerebral microvascular endothelial cells (hCMEC/D3) [89] and HeLa cells, as well as with an inhibitor of phosphoinositide 3-kinase (PI3K) (influences membrane ruffling and macropinosome formation) [19, 90] with concomitant decreases in exosome internalization. Assessing the same pathway but from an activating instead of inhibiting direction, exosome internalization was stimulated by activation of epidermal growth factor receptor (which activates Rac family members) in HeLa cells [19]. Membrane extensions, or filopodia, that facilitate the formation of the macropinosome and are regulated by Rac1 activation have also been shown to influence exosome internalization in hepatocyte (Huh7) and kidney (Hek293) cells [91], furthering the support that exosomes utilize macropinocytosis in

The lipid profile of the exosomes and membrane integrity of the cell are also important contributors to vesicle uptake in several different types of epithelial and endothelial cells. While macrophages readily recognize external-facing PS, these cells can also utilize exosomal PS in the process of internalization, as was shown when pre-incubating exosomes with Annexin V inhibited uptake by HeLa cells (cervical cancer epithelial cells), A375 and A431 cells (squamous skin cancer cells) [92] and in human umbilical vein endothelial cells (HUVEC) [93]. Disruption of cellular lipid raft integrity through cholesterol depletion or sequestration reduced exosome uptake in U87 human glioblastoma epithelial cells [43], hCME/D3 human cerebral microvascular cells [89], HeLa cells [43, 90], HUVECs [43, 46], and A375 cells [46]. Lipid rafts play a key role in many of the functions of epithelial cells, including the protein binding interactions between cell and extracellular environment. Also, some of the most central components to epithelial cell function are proteins that interact closely with the environment such as integrins and adhesion molecules, and

Protein interactions are essential to epithelial and endothelial function and are closely tied to several of the most common endocytosis pathways used by these cells. Clathrin-dependent endocytosis has been shown in gastric [94], nasopharyngeal [95], breast [96], ovarian cancer epithelial cells [97] and HUVECs [98]. Caveolin-dependence was seen in breast [96] and nasopharyngeal cancer [95], however, caveolin-1 showed negative regulation in glioblastoma cell lines [43] (**Figure 4B**). General receptor-mediated uptake has been shown with several proteins including heparan sulfate peptidoglycan (HSPG) in glioblastoma cells and HUVECs [99, 100] and in the transitional epithelial cells of the bladder [101];

/H<sup>+</sup>

exchange (which

**54**

The extracellular matrix (ECM) and stroma are important contributors to cellular homeostasis and function. This is particularly evident in tumors when evaluating the role of the tumor microenvironment (TME) on the survival and progression of the tumor cells. Fibroblasts are the major component of this extracellular environment. In normal physiology, they promote stromal stability, while in cancer, they contribute to altered ECM, increased angiogenesis, and metastasis [108]. These cells are in a pivotal position to interact with circulating exosomes and their internalization can have a compounding effect on the surrounding environment. Fibroblasts have been shown to participate primarily in clathrin-mediated endocytosis [109, 110] and occasionally receptor-mediated endocytosis [111]. Interestingly, RME [48, 106] and macropinocytosis [91] are the mechanisms by which fibroblasts have been shown to internalize exosomes (**Figure 5**). Tetraspanins are important proteins in fibroblast function and migration [112]. This protein family is well represented on the exosomal surface and is key to the uptake in many different cell types [48]. Additionally, evidence shows that the smaller the size of the vesicle, the more likely the fibroblast is to use receptors to internalize particles [111]. These three qualities lend support to the evidence of RME as a key pathway for fibroblasts to endocytose exosomes.

#### **Figure 5.**

*Cell-specific internalization of exosomes: fibroblasts. Fibroblasts take up exosomes with tetraspanins and utilize multiple endocytic pathways.*
