miRNAs are a class of small non-coding RNAs, typically 19–25 nucleotides long, which mediate post-transcriptional gene silencing by binding to the 3′-UTR region or to the Open Reading Frame of the target mRNA ^[1][8]. Therefore, miRNAs have not only the remarkable ability to regulate different biological processes—such as proliferation, angiogenesis, and metastasis—by altering gene expression, but also to remodel the TME in order to signal to the immune system ^[2][167]. It is also known that changes in the expression of miRNAs, during chemotherapy treatments, have been proposed as a tool to monitor the efficacy of the therapy and the risk of relapse ^[3][168]. At the same time, the TME modulates the expression of miRNAs by releasing SF (cytokines and growth factors) by both the tumor and non-tumor cells, or by the presence of mechanisms such as hypoxia, starvation, and pH alterations, as described above. A sequence of two 4-nucleotide sequences, recognized by the RNA binding protein Heterogeneous Nuclear Ribonucleoproteins A2/B1 (hnRNPA2B1), is used for the sorting of miRNAs in exosomes, as indicated by data in the literature. A study by Wang et al. has shown that, in hypoxic conditions of the TME, exosomes relate to transformation of the tumor phenotype by the transfer of miR-193-3p, miR-210-3p, and miR-510-5p ^[4][19] and are capable of activating the transition between epithelium and mesenchyme—one of the mechanisms used for DR ^[5][125]. In Table 1 ,we thsummarize miRNAs and the pathway that will be discussed in the following paragraph.

^][193]. miRNA expression has been widely associated with different diseases, including several cancers such as hematological malignancies, through the targeting of mRNAs coding for proteins influencing drug-resistance (such as drug efflux pumps, increased resistance to apoptotic stimuli, and modulation of the tumor microenvironment) ^[36][194]. Table 2 summarizes the principal action type of exosomes and related miRNA, with their specific mechanisms.

Exosomes released by drug resistant cancer cells typically have a different cargo than exosomes released by sensitive cancer cells, hereby mediating the horizontal transfer of DR traits from drug-resistant cancer cells to drug-sensitive ones. The different exosomal miRNAs exert their role in DR through the following mechanisms: I) by alteration of the drug efflux pump, II) increased resistance to apoptotic stimuli, and III) modulation of the tumor microenvironment ^[17][18][5,50].

Many studies suggested that exosome secretion acts as a signal to change the sensitivity of tumors to chemotherapy drugs and tumor progression. Meanwhile, exosomes can expel a variety of compound drugs out of tumor cells, reduce tumor cells’ DR, and thus lead to the failure of chemotherapy ^[19][178]. Levchenko and colleagues demonstrated that the expression of exosomal protein P-gp is regulated by miR-451 and miR-27a. They can maintain a prolonged expression of the P-gp proteins, with consequent upregulation, which contributes to the activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB). These cells are known to be involved in DR mechanisms ^[4][19], through the alteration of drug efflux pumps ^[20][179]. In fact, P-gp, the most important drug transporter, can induce an acquired resistance phenotype associated with miRNAs, through blocking apoptosis by increasing the expression of the MDR1 gene, which encodes P-gp ^[21][180].

Overall, miR-21 is a key regulator of the oncogenic processes that promote cell survival, and the formation and activation of CAFs ^[1][8] by regulating TGFβ1 signaling ^[22][181]. Isolated exosomes derived from hepatocellular carcinoma (HCC) patients can convert normal hepatic stellate cells to CAFs via miR-21, which downregulates the phosphatase and tensin homolog (PTEN) tumor suppressor gene, and consequently upregulates the PI3K/Akt signaling pathway ^[23][182]. The Yan-Ran group has instead shown that miR-21, released from gastric tumor cells, promotes resistance to apoptosis through the PTEN/PI3K/Akt pathway; it is known for its role in the regulation of various biological processes such as transcription, protein synthesis, metabolism, autophagy, proliferation, apoptosis, angiogenesis, and migration ^[24][183]. The regulation of miRNAs involves a loss of function of PTEN, which acts as a gatekeeper of the pathway, with a consequent increase in the invasion capacity and aggressiveness of the tumor. On the other hand, a gain of function of PI3K and Akt promotes survival and anti-apoptotic mechanisms ^[25][184]. miR-21, released from exosomes derived from cells resistant to cisplatin, induces resistance to sensitive cells by acting on the pathway of phosphatases and the homolog of tensin (PTEN) to regulate the mechanism of programmed cell death ^[26][117]. Fu et al. demonstrated that MDR liver cells transfer miR-32-5p to sensitive cells by exosomes, which in turn activates the PI3K pathway through the Akt pathway, capable of inducing resistance ^[27][185].

This bidirectional communication between the tumor cells and the fibroblasts reprogrammed in CAFs, through the exosomes, allows for the creation of an optimal microenvironment for tumor growth, giving a more aggressive phenotype to tumor cells; thus, allowing them to adapt to the “new” microenvironment ^[28][186]. A further study showed that miR-222, released from exosomes in lung cancer cells, is correlated with a high tumor aggressiveness, as these miRNAs act on the downregulation of the target gene PDLIM2, which in turn activates the nuclear factor NF-kB ^[29][187]. Further studies have been carried out on miR-1246, which is highly present in solid tumors, and is found in the microenvironment and released by exosomes. The overexpression of Caveolin 1 (Cav1), the direct target of miR-1246, sensitizes cells to paclitaxel, by reducing the levels of the p-gp protein (MDR1) ^[30][188]. Furthermore, a study using exosomal human melanoma miRNAs showed that miR-155 and miR-210 induce metabolic changes in adult human fibroblasts to increase aerobic glycolysis, promoting a pre-metastatic microenvironment ^[31][189]. In particular, miR-155 and miR-210 are referred to as oncogenic miRNAs that drive resistance to therapy in different types of cancer ^[32][33][34][190,191,192]. While cancer-derived exosomal miRNAs can promote CAFs differentiation, CAFs-released exosomal miRNAs in the TME play an important role in therapy resistance. Next-generation sequencing technology demonstrated that exosomal transfer of miR-21, from CAFs to ovarian cancer cells, inhibited apoptosis and promoted resistance to treatment with different types of drugs. ^[1][8]

miRNAs can be transferred from tumor cells to adjacent cells in their cell-free circulating exosome ^[35

In CML, the majority of patients can be treated with TKIs, such as IM, but some patients will develop DR. Studies have identified exosomes as promoters of DR in cancer ^[37][195]. Jaiswal et al. demonstrated that several miRNAs (miR-1228*, miR-1246, miR-1308, miR-149*, miR-455-3p) were more abundant in the exosomes of the ALL cell line CCRF-CEM (designated CEM for simplicity), compared to healthy donor cells. The same miRNAs were found to be significantly increased in drug-sensitive cells upon co-culturing with drug-resistant exosomes ^[6][169], confirming the role of exosomes in tumor progression. Min et al. demonstrated that IM-sensitive CML cells exhibited DR after having been incubated with exosomes derived from IM-resistant CML cells. They identified a differential miRNA expression profile in exosomes, deriving from either IM-resistant or IM-sensitive CML cells. Moreover, they observed the highest miR-365 expression level in exosomes of IM-sensitive CML cells, and identified that exosomes mediated the transfer of miR-365 between IM resistant and sensitive CML cells, and that miR-365 induced DR by inhibiting expression of pro-apoptosis proteins BAX and Cleaved Caspase-3 in IM-sensitive CML cells ^[7][170]. Additionally, another study confirmed that the transfer between resistant and sensitive CML cells of the drug resistant phenotype is mediated through exosomes ^[8][171]. Exosomes deriving from resistant cells, containing miR-27a, miR-451, and miR-21, have been shown to be related to P-gp expression and can also contribute to chemotherapy resistance in tumor cells. Another study suggests that exosomes released by IM-resistant CML cells (K562/G01) were internalized by sensitive CML cells (K562), thus horizontally transferring miR-365 that confers DR to the recipient cells ^[7][170].

The upregulation of miR-7977 in AML exosomes was found to have a role in the disruption of normal hematopoiesis, and in the aberrant production of hematopoietic growth factors ^[9][172]. P-gp can be expressed on cancer cells of AML patients and contributes to resistance to anthracyclines in vitro. The study of Bouvy demonstrated that exosomes from the drug-resistant HL-60 cell line (HL-60/AR) can interact with their sensitive (HL-60) counterpart cells and enable resistant cells to transfer their drug-resistance phenotype to sensitive cells ^[10][173]. In particular, two miRNAs, miR-19b and miR-20a, were found to be more expressed (> 4-fold change) in exosomes of sensitive cells as compared to resistant cells. These increased P-gp expression, targeted PTEN, and decreased the activation of the PI3K/Akt signaling pathway; this signaling pathway is often constitutively activated in AML ^[10][173].

CLL-derived exosomes have been demonstrated to deliver miRNAs to stromal cells that acquired features of cancer-associated fibroblasts, promoting CLL progression. In particular, miR-146a was specifically enriched in CLL exosomes as compared to a healthy subject’s cells (ratio 15:1) and was able to induce activation of the Akt pathway and NF-kB ^[10][173].

Follicular dendritic cells (FDCs) are stromal cells that support lymphoma progression, and their adhesion to NHL cells is associated with DR ^[11][174]. Lwin et al. showed that NHL cell adhesion to FDC induced the upregulation of miR-181a, through B-cell lymphoma-2 (BCL-2)–interacting mediator of cell death (BIM) downregulation and, consequently, protected lymphoma cells from drug-induced apoptosis ^[11][174]. The same group discovered that miR-548m downregulation, and subsequent histone deacetylase 6 (HDAC6) overexpression, enhances the adhesion of NHL to lymphoma stroma cells, promoting sustained c-Myc activation. Interestingly, c-Myc is a target of miR-548m, forming an amplification loop in these cancers. Both mechanisms, regulated by the tumor microenvironment, suggest that alterations in specific miRNA levels lead to DR and lymphoma survival ^[12][175]. Additionally, in NHL and diffuse large B-cell lymphoma (DLBCL), miR-99a-5p and miR-125b-5p were found significantly upregulated in exosomes from resistant cells, when compared to sensitive parental cells’ exosomes ^[13][176]. The expression levels of miR-99a-5p and miR-125b-5p were higher in exosomes from the serum of chemo-resistant DLBCL patients, as compared to the sensitive ones, thus being associated with a worse prognosis ^[13][176]. Additionally, it has been suggested that exosomal miRNAs could be used as predictive biomarkers of chemotherapeutic efficacy in DLBCL patients ^[13][176].

Thanks to the novel therapy for MM patients using Btz, panobinostat, melphalan, and lenalidomide, the survival rate of these patients has significantly improved in the past years. The interaction between myeloma cells and the BMME plays a pivotal role in MM initiation, progression, and DR. BMME-derived exosomes antagonize Btz-induced apoptosis and decrease the viability of MM cells ^[14][40]. Tang and co. employed KEGG analysis on Btz-resistant MM cells and found four enriched terms associated with DR, including the mechanistic target of rapamycin (mTOR), and the cAMP and PI3K–Akt signaling pathways. Moreover, bone marrow stromal cell derived exosomes (BMSC) influence multiple signal transduction pathways, such as JNK and p38 MAP kinases (MAPK), p53, and Akt, that affect the survival of MM cells, thus stimulating MM-cell growth and inducing Btz resistance ^[14][40]. Their results also indicate that miR-627-3p and miR-642a-5p target the transcriptional factor SP9, and that these miRNAs may bind differentially expressed exosomal lncRNA involved in PI3K-Akt signaling pathways ^[14][40]. Zhang et al. analyzed the cargo of exosomes from the plasma of MM patients treated with Btz and identified several miRNAs ^[15][43]. Specifically, they found exosomes with significantly increased internal RNAs and a downregulation of miR16-5p, miR-15a-5p, miR-20a-5p, and miR-17-5 in exosomes from patients resistant to Btz, as compared to patients sensitive to Btz. This suggests the use of exosomal miRNAs as DR biomarkers for MM ^[15][43]. Moreover, Zhao et al., knowing of the correlation between miR-30 and B-cell lymphoma 9 (BCL9), observed another correlation between MM cell lines and cells from MM patients. In fact, H929 cells (MM cell lines) co-cultured with BMSCs showed miR-30 downregulation, associated with an enhanced expression of BCL9, which is a transcriptional coactivator of the Wnt signaling pathway, known to promote multiple myeloma cell proliferation and DR ^[16][177].