Background

Extracellular vesicles (EVs) are nanoscale cellular secretions that act as key mediators in many pathological/physiological processes [145]. Another method that uses electricity and acoustic forces to manipulate biological particles and submicron particles for deterministic sorting has been applied to the purification of exosomes. The purity of the exosomes purified by this method is more than 95% and the recovery rate is 81% [146].

Cargos in exosomes

Exosome cargos are the core components that confer biological effects on exosomes. Nearly 100,000 proteins and over 1,000 lipids have been reported to be associated with exosomes [147]. Nucleic acids mainly mRNAs and ncRNAs were enriched in exosomes, including more than 27,000 mRNAs and more than 10,000 ncRNAs were identified in sEV [147]. Genomic DNA (gDNA) and mitochondria DNA are present in exosomes in the form of s single-stranded or double-stranded [148, 149]. Together, these biologically active substances make up the exosomal cargos. (Fig. 2).

Fig. 2
figure 2

Biomarkers of exosomes and main components of exosomal cargos. Several exosome surface proteins are considered to be biomarkers of exosomes, including transmembrane 4 superfamily (CD9, CD63, CD81), lipid raft protein (flotillin-1), and Ceramide. Exosome content proteins HSP70, TSG101 and ALIX are also biomarkers of exosomes. Exosomes carry a variety of cargos, such as nucleic acids, proteins, enzymes and metabolites (mainly lipids)

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Nucleic acids

The role of exosomal RNAs in tumors has been widely reported, mainly including mRNAs and ncRNAs. Regarding functional exosomal mRNAs, an early study reported that exosomal mRNAs can complete translation in receptor cells [150]. Recently, exosomal mRNAs CUL9, KMT2D, PBRM1, PREX2 and SETD2 were found to be possible novel potential biomarkers for clear cell renal cell carcinoma (ccRCC) [151]. Studies on exosomal ncRNAs have focused on miRNAs, lncRNAs & circRNAs. Usually exosomal ncRNAs are transported to the recipient cells as molecular sponges.

Exosomal DNA mainly includes gDNA and mtDNA [149]. The TME of "hibernating" cancer cells secretes EVs containing mtDNA, leading to endocrine therapy resistance in breast cancer cells [152]. T cells secrete EVs containing gDNA and mtDNA, which activate the cGAS/STING signaling pathway and induce antiviral responses in DCs [153].

Lipids

The lipid cargos of exosomes mainly include sphingolipids, cholesterol, phosphatidylserine, saturated fatty acids, and ceramides, which are mainly associated with exosome biogenesis [154]. The bilayer lipid membrane structure of exosomes determines the enrichment of membrane lipid components (phosphoglycerolipids, sphingolipids, and sterols) [147]. The fact that neutral sphingomyelinase inhibitors can reduce exosome secretion further illustrates the importance of membrane lipids for exosomes [155]. Cholesterol enrichment in exosomes is associated with MVB. Different subcellular organelles have different cholesterol concentrations. Oxysterol-binding protein-related proteins (ORPs) are involved in cholesterol transport and are able to maintain the proper cholesterol concentration required for MVB biogenesis [156]. In addition, when low-density lipoprotein-cholesterol is low in endosomes, endoplasmic reticulum stress-derived cholesterol can be transferred to MVB [157].

Some biologically active lipids are important cargos for exosomes to perform their biological functions. LTB4 is packaged and released in exosomes [158]. This LTB-enriched exosome biogenesis originates from the nuclear envelope of centrocytes and is an unconventional pathway of exosome production [159]. Granulocyte myeloid-derived suppressor cells can secrete exosomal PGE2 to ameliorate collagen-induced arthritis [160]. Ubiquitination of 15-LO2 in hypoxia promotes 15-LO2 sorting to exosomes, which are involved in the regulation of pulmonary vascular homeostasis [176]. Recent studies have shown that two RNA binding proteins, Alyref and Fus, mediate miRNA sorting into sEVs, enriching the understanding of exosomal miRNA biogenesis [99, 177]. In addition, activation of the NLRP3 inflammasome and cleavage of RILP increase exosome production, and the cleaved form of RILP interacts with FMR1 to regulate exosomal miR-155 content [178].

Some lncRNAs are also found in exosomes, probably by forming lncRNA–RBP complexes [179]. Although the specific mechanism of lncRNA sorting to exosomes is still unclear. Interestingly, lncRNA-encoded microproteins were identified in glioma-derived exosomes, indicating the biological functional diversity of exosomal lncRNAs [188].

CircRNA is a component of exosomal ncRNAs, and different circRNAs were found in exosomes originating from different cells, indicating that the sorting process of exosomal circRNAs is selective [189]. SNF8, a key component of the ESCRT-II complex, sorts circRHOBTB3 into exosomes by binding to specific sequences (141-240nt) on circRHOBTB3 [180]. After ectopic expression of miR-7 in HEK293T and MCF-7 cells, the level of circRNA CDR1as was significantly downregulated in exosomes but slightly increased in cells. This result partially suggests that sorting of circRNAs to exosomes was regulated by changes of associated miRNA levels [181].

The sorting mechanism of exosomal DNA is still unclear. Certain physiological pathways may be involved in this sorting process. MtDNA sorting to exosomes may be related to the endosomal pathway [153], and gDNA sorting to exosomes may involve micronuclei (MN) [182].

Protein sorting to exosomes

Typically, proteins can enter cells together with cell surface proteins by endocytosis and invagination of the plasma membrane [12]. The protein sorting process can be done in organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus [190]. During the budding stage of exosome biogenesis, early-sorting endosome (ESE) may fuse and communicate with mitochondria, endoplasmic reticulum and trans-Golgi network, indicating the reason why the protein can be detected in exosomes of host cells [12]. In addition, some regulatory proteins in the formation, transport and secretion of exosomes may be involved in the sorting process of exosomal proteins, such as Rab GTPase, ESCRT proteins, tetraspanins and SNARE protein complexes [172, 191,192,193].

Some non-canonical mechanisms are also involved in the regulation of exosomal protein sorting. A recent study shows that proteins containing the KFERQ pentapeptide sequence can be sorted to exosomes by a process dependent on the membrane protein LAMP2A, a novel mechanism independent of ESCRT [183, 184]. Epigenetic modifications can be involved in the sorting of exosomal proteins. Cav1 can be sorted to MVB and ILV in a phosphorylation and ubiquitination-dependent manner, regulates exosome biogenesis by regulating MVB cholesterol contents, and delivers specific ECM-associated proteins (Tenascin-C, fibronectin, nidogen, elilin, EDIL3 and heparan sulfate proteoglycans) to exosomes [185]. Some proteins have UBL domains (ubiquitin-like sequences), among which UBL3/MUB proteins, as one of the conserved UBLs, can act as post-translational modification (PTM) factors to regulate the process of protein sorting to sEVs [186]. In addition, endosomal microautophagy is also involved in the sorting of exosomal proteins, and the chaperone HSC70-mediated proteins are sorted into exosomes under the electrostatic interaction between the cationic domain of HSC70 and the MVB membrane [187, 194].

Metabolites sorting to exosomes

Exosomes contain intact metabolites (mainly lipid metabolites) with characteristic of the host cells [195]. For example, exosomes secreted by granulocyte-myeloid derived suppressor cells (G-MDSCs) are enriched in PGE2, exosomes secreted by neutrophils are enriched in LTB4 [158, 160]. Triglycerides (TG) and sphingomyelin were found in T cell-derived exosomes isolated from human plasma [196]. Metabolomics of cancer stem cell-derived exosomes from melanoma revealed that multiple lipid metabolites, such as glycerophosphoglycerol (PG), glycerophosphatidylserine (PS), TG, and glycerophosphorylcholine (PC) in exosomes [197]. Currently, the mechanism by which these metabolites are sorted to exosomes remains unclear, but may be related to the interaction of lyn and fotillin-1 through the lipid domains of exosomal lipid membranes [159].

Biology of tumor metabolic reprogramming

Metabolic abnormalities are one of the hallmarkers of tumor cells, which are metabolically reprogrammed to meet their rapid proliferation requirements [198]. Given the specific physicochemical characteristics of high pressure, high pH and hypoxia within the TME, as well as the heterogeneity of the tumor vasculature, local tumor cells often have limited metabolic resources, which accelerates the digestion of nutrients and the accumulation of metabolites. In this particular environment, tumor cells adjust their metabolism in order to maintain their growth, which not only allows them to re-meet their energy supply needs, but also regulates their gene expression and protein modifications, facilitating the spread of tumor cells [199].

Glucose metabolism

Glucose is the main source of energy for cellular metabolism and biosynthesis. Glucose metabolism includes the glycolytic pathway, the pentose phosphate pathway (PPP), the serine synthesis pathway (SSP) and the tricarboxylic acid (TCA) cycle pathway [200]. Tumor cells are able to re-edit these pathways to obtain ATP with various biological macromolecules. In contrast to normal cells, tumor cells produce large amounts of lactate via the aerobic glycolytic pathway, resulting in an acidic TME that contributes to the proliferation [201]. In addition, increased uptake of glucose leads to the accumulation of intermediate metabolites in the glycolytic pathway, activating the cellular PPP while inhibiting the intracellular TCA[297]. During the release of exosomes, phosphorylated PKM2 acts as a protein kinase to promote the formation of SNARE complexes by enhancing the phosphorylation of SNAP23 [298]. During the release of exosomes, phosphorylated PKM2 acts as a protein kinase to promote the formation of SNARE complexes by enhancing the phosphorylation of SNAP23. PKM2 combines metabolic regulation with non-metabolic regulation of exosome secretion, is an ideal target for exosome and tumor metabolic therapy. Shikonin is the active ingredient of Comfrey, a naphthoquinone compound [290]. Shikonin is a specific PKM2 inhibitor that not only inhibits glucose uptake and lactate production in tumor cells, but also inhibits glycolysis by reducing extracellular secretion of exosomal PKM2 and enhances cisplatin sensitivity in NSCLC cells [290]. A study on bladder cancer found that highly-expression of PKM2 was associated with cisplatin resistance. Shikonin can promote cisplatin sensitivity of bladder cancer cells by reducing the release of exosomes, but the specific mechanism remains to be explored [291].

Exosomal ncRNAs, as common cargos in TME, play an important role in the metabolic reprogramming of TME. It was found that oxaliplatin-resistant CRC cells-derived exosomal circRNA ciRS-122 was delivered to sensitive cells, which enhanced glycolysis and chemoresistance in sensitive cells via miR-122/PKM2 signaling axis [292]. Development of exosome-transported si-ciRS-122 can reverse the ciRS-122/miR-122/PKM2 signaling axis to inhibit glycolysis and enhance chemosensitivity in CRC cells. In addition to tumor cells, targeting exosomal circRNAs derived from CAFs in TME has antitumor effects. It was found that exosomal circCCT3 secreted by CAFs could enhance glucose metabolism by regulating the expression of HK2. It was found that exosomal circCCT3 secreted by CAFs could enhance glucose metabolism by regulating the expression of HK2. Treatment of CAFs with coptisine inhibited the secretion of exosomal circCCT3 and suppressed HCC cell proliferation and invasion [293]. In addition, docosahexaenoic acid (DHA) as an omega 3 free fatty acid has been reported to exert anti-angiogenesis effects. DHA can alter the expression of angiogenesis-related exosomal miRNAs in breast cancer cells, inhibits angiogenesis by up-regulating exosomal miR-101, miR-199, and miR-342, and down-regulating exosomal mir-382 and miR-21 to exert anti-tumor effects [294, 299].

Benefiting from the targeting and biocompatibility of exosomes, exploitation of exosomes as carriers for drug delivery targeting tumor metabolism has a bright future. Although there are currently no engineered exosomes to directly target various metabolic pathways in the TME, exosomes can be combined with classical drugs or modalities as an adjuvant therapy to improve the efficacy of anti-tumor therapy. It has been shown that combination of exosome-mediated cPLA2 siRNA and metformin reduces the growth of glioblastoma xenografts by impairing the energy metabolism of mitochondria [295]. Photodynamic therapy (PDT) is a novel method of treating tumors with photosensitizing drugs and laser activation [300]. Aggregation-induced emission luminogens (AIEgens) are photosensitizers for PDT whose efficacy is limited by GSH. A recent work developed TEX for the co-delivery of AIEgens and proton pump inhibitor (PPI) for tumor combination therapy. TEX can specifically deliver AIEgens and PPI to tumor sites, and PPI inhibits GSH and ATP produced by glutamine metabolism in tumor cells, which contribute to the efficacy of AIEgens [296]. The combination of exosomes, glutamine metabolism and PDT may be a new option for future tumor treatment, but treatments that inhibit glutamine metabolism still need to be approached with caution. Glutamine depletion may stimulate release from Rab11a compartments of exosomes with pro-tumorigenic functions [301]. Therefore, exosome-based tumor metabolic therapy still needs further refinement to find the balance between pro-tumorigenesis and anti-tumorigenesis.

Conclusion

This review highlights the multiple roles and molecular mechanisms of exosome-mediated metabolic reprogramming in TME reprogramming. The field of exosomes (or EVs) has made great progress in recent years benefiting from technological breakthroughs in isolation, purification, in vivo tracking and content analysis [2]. This has led to the identification of other types of EVs besides exosomes and their functions becoming a novel hotspot in the field of EVs. In the future, the understanding of exosomes will be enriched by how to precisely distinguish exosomes from other EVs subtypes and exclude contaminants to further obtain high purity exosomes. This will also help to improve the targeting and biosafety of antitumor therapies developed with exosomes as vectors.

We describe the cell–cell communication mediated by exosomal cargos in TME and how these cargos are sorted to exosomes. Along with technological advances, the way of sorting various types of cargos into exosomes (or specific subtypes of EVs) is the bottleneck for further development in the field of exosomes. The bioactive cargos are the key to the function of exosomes. In addition to the cell-derived exosomal cargos in human TME, milk exosomes have a bright future as an oral drug delivery system, due to the biocompatibility of milk exosomes with exogenous cargos [302].

The heterogeneity of TME promotes tumor proliferation, metastasis, stemness and drug resistance. We summarized the main components and characteristics of TME, and highlighted the role and mechanism of exosomal cargos-mediated metabolic reprogramming in the heterogeneity of TME. Improving TME becomes an emerging strategy for anti-tumor treatment. The plasticity of tumor metabolism is both promising and challenging. Given the complex composition of TME, targeting one component for metabolic remodeling is difficult, and we need to consider more whether altered metabolism has the same therapeutic effects on multiple components of TME. Application of tumor organoid platforms to exosomes may be used to simulate the effect of exosomes on TME.

Exosomal cargos-mediated abnormalities metabolism in TME remains to be extensively studied. Considering the widespread of exosomal cargos, exploring the molecular mechanisms of exosomal cargos-induced metabolic reprogramming is beneficial for tumor precision treatment. As more and more biologic companies are entering the exosome field, the development of exosome-based drug delivery modalities to reshape metabolism in TME is promising. Combining chemotherapy, radiotherapy or targeted therapy with novel metabolic therapies may be the future trend in tumor treatment.