*3.2. Angiogenesis*

Angiogenesis has a crucial role in tissue regeneration after damage. In this process, endothelial cells invade injured tissues to form buds and ultimately establish a capillary network. It is well accepted that MSCs exert therapeutic effects on ischemic diseases by directly producing or stimulating endogenous factors, such as vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF) and stromal-derived factor-1 (SDF-1) [62,63]. These factors facilitate angiogenesis in damaged tissues [64]. Recent evidence has revealed that MSC-exosomes also have pro-angiogenic properties [60,65] (Table 2).

*Cells* **2021**, *10*, 42


**Table 2.** MSC Exosomes Facilitate Angiogenesis in Various Disorders.

*Cells* **2021**, *10*, 42


Based on proteomic analysis, MSC-exosomes contain various factors that are involved in angiogenesis, such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF) and proteins associated with NF-κB activation [61]. PDGF, FGF and EGF function as common factors in mediating angiogenesis [77], and the role of proteins associated with NF-κB activation in mediating angiogenesis should be addressed. To our knowledge, NF-κB activation is conventionally associated with inducing pro-inflammatory responses. Nonetheless, intriguingly, blocking NF-κB activation abrogated tube formation by endothelial cells in vitro [61]. Consistent with this finding, exosomes from bone marrow MSCs were found to activate STAT3 signaling cascades in target cells, thus upregulating expression of genes encoding HGF, insulin-like growth factor-1 (IGF1), nerve growth factor (NGF) and SDF-1 [63]. Similarly, several other studies have revealed the mechanisms by which MSC-exosomes induce angiogenesis. For example, exosomes from umbilical cord MSCs reportedly activate Wnt/β-catenin to increase angiogenesis [75], but those from bone marrow MSCs promote angiogenesis by activating the HIF-1α/VEGF axis in target cells [73]. Furthermore, bone marrow MSCs increase the survival of pulmonary endothelial cells via exosomal miRNA-21-5p, which targets the antioncogenes *PDCD4* and *PTEN* in a mouse model of ischemia/reperfusion [78]. In addition, placental MSC-exosomes are capable of upregulating expression of genes encoding Ang2 and Tie2 by endothelial cells [60]. The details of the angiogenic features of MSC-exosomes are provided in Table 2 and Figure 2.

Similar to their immunomodulatory features, the pro-angiogenic potency of MSCexosomes can be impacted by foreign stimuli [55,69]. For example, it was found that MSCs conditioned with PDGF showed increased production of exosomes containing angiogenic molecules, such as c-kit and stem cell factor [55]. However, preconditioning MSCs with pro-inflammatory cytokines, such as TNF-α and IL-6, increased the exosomal cargo content of miRNA-196a-5p and miRNA-17-5p, inactivating the PI3K-AKT, MAPK and VEGF-related pathways and impairing angiogenesis [79]. In addition to such bioactive substances, environmental factors impact the proangiogenic properties of MSC-exosomes. In fact, exposure of MSCs to blue light resulted in the increased content of miRNA-135b-5p and miRNA-499a-3p as exosomal cargo, which promoted angiogenesis in vitro by repressing myocyte enhancer factor 2C (MEF2C) [66]. Consistent with this finding, hypoxia enhanced the cargo content of miRNA-126 in MSC-exosomes. In this context, miRNA-126 was able to stimulate SPRED1/Ras/Erk/HIF-1α, thus increasing angiogenesis in injured tissues [38]. More intriguingly, HIF-1α was found to upregulate expression of the gene encoding RAB22A, which participates in vesicle formation in cells [80]. This event partially illustrates why HIF-1α-overexpressing MSCs can increase their production of exosomes [38]. Functionally, exosomes from cells such MSCs promote angiogenesis by activating the Jagged-1/Notch signaling pathway [55]. As the pro-angiogenic potency of MSC-exosomes can be improved by using the above methods, we can purposely generate them and utilize them to treat ischemic diseases (Table 2).

**Figure 2.** MSC-exosomes are capable of protecting against radiation-induced damage to hematopoietic and nonhematopoietic systems. In hematopoietic reconstruction post irradiation, MSC-exosomes enhance hematopoietic cell survival and proliferation by carrying functional molecules, such as the pro-regeneration miRNAs miRNA221, miRNA451 and miRNA654, the anti-apoptosis-related miRNAs miRNA210, miRNA106b and miRNA155, the hematopoiesis-related cytokines G-CSF and IL-8, and the hematopoiesis-related growth factor VEGF. In addition, MSC-exosomes can protect irradiated bone marrow MSCs from radiation-induced DNA and oxidative stress damage by activating the Wnt/β-catenin signaling pathway. With regard to the non-hematopoietic system, MSC-exosomes reduce apoptosis of skin epidermal, lung alveolar epithelium and intestinal epithelium cells, as MSC-exosomal miRNAs likely mediate repair of DNA double-strand breaks in damaged cells. Oxidative stress reaction and DNA damage are the major processes in radiation damage. MSC-exosomes can overcome these crucial events effectively, and have potential to suppress the development of acute and chronic radiation damage from several aspects. MSC-exosomes also facilitate vascular endothelium proliferation owing to their bioactive cargo molecules, such as PDGF, FGF and EGF.
