Multivesicular endosomes (MVEs) in cells can merge with the lysosome, sending their contents to be degraded, or they can fuse with the plasma membrane to send their contents out into the extracellular environment as exosomes (van Niel et al., 2011; Kowal et al., 2014). Secretion of exosomes is thought to be controlled by certain Rab GTPases (Stenmark, 2009). For instance, Rab27a, Rab27b (Ostrowski et al., 2010), and Rab35 (Yang et al., 2019a) are involved in MVE docking at the plasma membrane. If Rab27a or/and Rab27b silenced, exosome release is suppressed without noticeable changes to the exosomes’ protein composition or morphology. Additionally, two of Rab27’s effectors, Slp4 and Slac2b, are responsible for a decrease in exosome secretion upon their silencing (Ostrowski et al., 2010). Likewise, by preventing Rab27a from being degraded by the proteasome, KIBRA regulates exosome secretion (Song et al., 2019). Furthermore, vesicle-membrane SNAREs (v-SNAREs) and target-membrane SNAREs (t-SNAREs) worked together to control MVE fusion with the plasma membrane (Jahn and Scheller, 2006).

Exosomes are made by almost every cell (Zhao et al., 2021). They are detected to be circulating in fluids and secretions of the body, like urine, blood, bile, tears, semen, saliva, cerebrospinal and amniotic fluids (Geng et al., 2020). Moreover, exosomes play crucial regulatory roles in intercellular communications, locally and systematically (Wang et al., 2019a) by acting as signalling vesicles in autocrine, endocrine, juxtracrine, and paracrine (EL Andaloussi et al., 2013). Moreover, they can transport cargo from cell to cell, causing phenotypic alterations in recipient cells (Mathieu et al., 2019).

Exosomes have been implicated in many biological processes and show an essential role in pathological and physiological situations, including cancer, circulatory and metabolic abnormalities, immune responses, and inflammatory illnesses (Wang et al., 2019a; Kalluri and LeBleu, 2020). For example, through releasing cytokines and other bioactive molecules, exosomes create an appropriate microenvironment for tumor growth and regulate cell reproduction, metastasis, and drug resistance (Ye et al., 2022). Exosomes also contribute to early tumorigenesis in BC, can carry signalling molecules to cancer cells inside the TME, assist cancer cells in evading an immune response, promote angiogenesis, and alter the tumor microenvironment among other functions (Jia et al., 2017).

Expression and regulation of circRNAs take place specifically by cells and tissues at different developmental stages (Conn et al., 2015). The majority primarily exists in the cytoplasm. Scientists have studied their function as post-transcriptional regulators (Han et al., 2018). Although there is no comprehensive description of circRNAs properties and biological implications (Tran et al., 2020), thus far, it has been shown to act like miRNA sponges, interact with proteins, regulate the splicing or transcription of genes, translate proteins, and exert epigenetic regulations (Han et al., 2018). Having diverse functions in the body, circRNAs involvement in various body diseases, such as cardiovascular diseases (Altesha et al., 2019), renal diseases (Jin et al., 2020a), chronic liver diseases (Zeng et al., 2021), skin diseases (Wu et al., 2020) and cancers (Ghafouri-Fard et al., 2022a; Ghafouri-Fard et al., 2022b).

Studies continuously demonstrate that abnormally expressed circRNAs are associated with several features of cancer (Li J. et al., 2020), in particular carcinogenesis, programmed cell death, proliferation, invasiveness, metastatic abilities, resistance to radiotherapy and chemotherapy, and cancer prognosis (Geng et al., 2020).

Dysregulation in circRNA gene expression is considered one of the important factors causing the onset and progression of gynecological cancers (Tran et al., 2020). In addition, numerous studies over the last years have shown circRNAs to be tightly linked to BC, and these studies can be categorized into two major groups: those that investigate the regulatory effect in BC development and those focused on detecting different patterns of expression to elucidate possible biomarkers for diagnosis of BC or molecular subtypes (Ghafouri-Fard et al., 2021b).

Exo-circRNAs’ contribution to tumor regulation, proliferation, and angiogenesis has been shown in many studies. When Li et al. studied tumor and paracancerous sample tissues of BC patients, they detected significant overexpression of circ_002178 in cancer tissue compared to surrounding cells. They also found that overexpression of circ_002178 in vitro caused a considerable increase in the rate of survival and the number of SUM149PT cell clones, which facilitates tumorigenesis through targeting miR-1258 and modulating the expression of KDM7A (Li W. et al., 2020). Further, Liu et al. used the gain and loss of function approach to study the functional role of hsa_circRNA_002178 in BC angiogenesis. They revealed that overexpression of has_circRNA_002178 was associated with poor prognosis in their microarray-based gene expression analysis. Later they proved that hsa_circRNA_002178 knockdown inhibited angiogenesis by decreasing cell survival, energy usage, and the ability to form tubes (Liu et al., 2020a). Likewise, Cai et al. used the same gain and loss of function to study hsa_circ_0000515. First, they detected overexpressed hsa_circ_0000515 in BC tissues. Afterward, they silenced hsa_circ_0000515 in the MCF-7 cell line. Finally, they found that MCF-7 cells were neither able to progress through their cell cycle nor able to proliferate or invade, and their angiogenetic potential was diminished through sponging miR-296-5p and modulating CXCL10 expression (Cai et al., 2021).

Furthermore, Liang and his team performed a circRNA microarray to screen for dysregulated circRNA in BC tissues. Among 2,587 abnormally expressed circRNA, there was significant overexpression of circ-ABCB10 with five to ten times more in BC cells than normal ones. Then, their bioinformatics study found that miR-1271 could represent a target for circ-ABCB10. They also found that knocking down circ-ABCB10 led to cell cycle arrest in the G0/G1 phase, suppression of colony formation, and inhibition of cell proliferation in vitro (Liang et al., 2017). Circ-Dnmt1 (hsa_circRNA_102,439) was overly expressed in BC cells, which caused inducing of proliferation and survival of cells. It also led to stimulation of cellular autophagy, inhibition of cellular biological aging, and increment of tumor growth (Du et al., 2018). These outcomes resulted from circ-Dnmt1 binding to p53 and Auf1, which triggered nuclear translocation of these tumor suppressor proteins and lowered p53 transcription (Tesfaye et al., 2021). Table 1 shows the expression levels of several different carcinogenic exo-circRNAs in BC along with their targets and hallmarks.

Cancer cells utilize a significant amount of their energy using anaerobic glycolysis (Warburg effect), a phenomenon of changing glucose to lactate for energy generation, succeeded by lactate fermentation in the absence of oxygen (Fadaka et al., 2017). In BC cells, circRNF20 promoted anaerobic glycolysis by increasing glucose uptake, lactate production, and ATP levels, in addition to reducing apoptosis and increasing proliferation (Cao et al., 2020).

Despite this, several circRNAs are extensively downregulated in vitro and in vivo studies of BC; these circRNAs inhibit tumor growth by sponging oncogenic miRNA or targeting other oncogenic products (Table 2). For example, Zhang and Ling Mao found circ_0000442 to be downregulated in cell lines and patient specimens. Using a variety of methods, they were able to identify the tumor suppressor circ_0000442 by its ability to sponge miR-148b-3p and decrease its proliferative effect. Furthermore, the G1 arrest effect was enhanced by circ_0000442 overexpression in cell lines, and colony formation was inhibited (Zhang and Mao, 2021). In the same way, Peng and Wen found that circDDX17 expression was low and there was a direct interaction between circDDX17 and miR-605, which controlled the expression of cell cycle genes (increased p21 expression and suppressed CDK1 expression) (Peng and Wen, 2020).

Furthermore, Li et al. compared the expression of circ-VRK1 in 350 BC tumor tissues and 163 adjacent tissues, and the results showed lower circ-VRK1 expression in tumor tissues. They also found that patients with a low level of circVRK1 expression had unfavorable clinicopathological features (Li and Li, 2020). Likewise, Liu and his colleagues used plasmid transfection to overexpress circRNA_103809 in cell lines, results showed that overexpression of circRNA_103809 could significantly suppress the growth of these cell lines. The team also studied proteins that are involved in the cell cycle (CyB1 and Cyd1) and found that overexpression of circRNA_103809 could stop the cell cycle at the G2/M phase (Liu et al., 2020b). Additionally, Xiao et al. showed a low level of circAHNAK1 in TNBC patients and cell lines. Then, they overexpressed circAHNAK1 in BC cells and found that it inhibited cancer proliferation and suppressed colony formation in two different cell lines by affecting miR-421 and RASA1 (Xiao et al., 2019).

These findings shed light on previously unknown functions of circRNAs in the BC tumor microenvironment, particularly in tumor growth regulation.

Breast cancer is a systemic disease that commonly leads to metastatic spread (Hüsemann et al., 2008). Metastasis is a complex and multi-step process (Lambert et al., 2017) that is responsible for the majority of cancer patients’ deaths (Wittekind and Neid, 2005). Therefore, knowing the mechanisms behind metastasis can be crucial to improve alternative therapies and creating new approaches for better managing metastatic diseases (Parker et al., 2022).

Tumor exo-circRNAs are more highly expressed in BC (De Palma et al., 2022). Their significance in cancer metastasis and EMT was demonstrated through sponging miRNAs and modifying tumor suppressor genes or specific signalling pathways (Figure 3). For instance, Zeng and his colleagues showed that patients with overexpressed circANKS1B had a higher risk of metastasis to lymph nodes and higher clinical stage, as in vitro and in vivo research outcomes demonstrate that circANKS1B enhanced metastasis and invasion by inducing epithelial-to-mesenchymal transition (EMT) through TGF-β1 signalling pathway (Zeng et al., 2018). Meanwhile, Chen et al. detected overexpression of FECR1 circRNA in BC cell lines and tissue samples. Circular RNA FECR1 made tumors more aggressive and spread to other parts of the body by controlling how genes methylated and demethylated their DNA. It could do this by interacting with the FLI1 promoter in cis to start TET1 demethylase and by turning down DNMT1 in trans (Chen N. et al., 2018). Zhou et al. selected overexpressed hsa_circ_0011946 among 152 differentially expressed circRNA in BC. Based on miR26a/b sponging, they hypothesized that hsa_circ_0011946 targeted replication factor RFC3. When hsa_circ_0011946 was knocked down, RFC3 mRNA and protein expression were silenced. Thus, migration and invasion abilities were inhibited (Zhou et al., 2018). Moreover, it was discovered by Song et al. that hsa_circ_0007534, when elevated, promoted BC cell proliferation and invasion via MUC19 to modify miR-593 production, whereas hsa_circ_0007534, when downregulated, lost its oncogene properties and increased apoptosis (Song and Xiao, 2018).

Fu and his colleagues showed that circBCBM1 encourages cancer proliferation and migration. In vivo studies with mice showed that circBCBM1 also helps growth of cells and spread to the brain. Their finding suggested that circBCBM1 sponges miR-125a and modulates BRD4 regulation, leading to changes in the MMP9 expression via the Sonic hedgehog (SHH) signalling pathway (Fu et al., 2021). Also, Zhou et al. looked at BC and lung tissues that had spread to other parts of the body, and they found that metastatic lung samples had the most circFBXL5 upregulation. They showed that the knockdown of circFBXL5 in mouse models diminished the growth and lung metastasis of tumors. Furthermore, they revealed that circFBXL5 modulated the expression of SRSF6 via sponging miR-660 (Zhou et al., 2020a). Similarly, Song et al. detected circHMCU upregulation in BC cell lines, which was linked to the fast proliferative process and metastasize to the lungs. Furthermore, they found that circHMCU could impact the G1 phase cell cycle checkpoint and epithelial-mesenchymal transition (EMT) pathway of BC cells, enhancing proliferation, migration, and invasion (Song et al., 2020).

On the other hand, the expression levels of certain exo-circRNAs are reduced or inhibited in BC (Figure 4). For example, Xu and his team performed high-throughput circular RNA microarray assays of primary BC tissues and lung metastatic tissues samples and found circNFIC to be most downregulated in metastatic lung tissues among 20 dysregulated circRNAs. Then, they overexpressed circNFIC in mouse models and detected suppression of tumor growth and lung metastasis (Xu et al., 2020a). Hou and his team detected the most downregulation of circASS1 in 1,137 dysregulated circRNAs in BC cells. Overexpression of circASS1 was discovered to have anti-invasion and anti-migration effects. The group hypothesized that circASS1 promotes expression of its parent gene ASS1 via sponging miR-443 (Hou et al., 2019). Furthermore, significant downregulation of circNR3C2 in TNBC was discovered by Fan et al., and this was inversely linked with distant metastasis and tumor invasiveness. Overexpression of circNR3C2 inhibits tumor development and metastasis by degrading vimentin, as shown by gain-of-function studies. It also promotes expression of the tumor suppressor gene HRD1 (Fan et al., 2021). Moa et al. determined a low level of circRNA_000554 and ZFP36 while a high level of miR-182 in BC tissues. In addition, an immunofluorescence assay was performed to evaluate expressions of EMT markers Vimentin, N-cad, and E-cad in MCF-7 cells and the results showed reduced expression of Vimentin and N-cad, and increased expression of E-cad (Mao et al., 2020).

Additionally, circular mitochondrial RNAs (circ-mtRNAs) have been found to regulate mitochondrial function and cellular metabolism in BC. For example, a specific circ-mtRNA, called circRNA_103809, was significantly down-regulated in BC tissues compared to adjacent normal tissues (Liu et al., 2020b). In breast cancer cells, the overexpression of circRNA_103809 could interfere with the EMT signalling pathway, causing miR-532-3p to function improperly, resulting in G2/M phase arrest and a reduction of cell proliferation and metastasis (Liu et al., 2020b). However, further studies are required to better understand circ-mtRNAs’ functions and their potential as diagnostic or therapeutic targets in breast cancer.

These studies show that circRNAs can influence the spread of tumor cells to distant parts of the body through different mechanisms and pathways by suppressing or promoting invasion and metastasis.

Apoptosis is programmed cell death that normally occurs through the developmental and aging stages of cells as a natural body mechanism to control the population of cells in tissues (Kaczanowski, 2016). Apoptosis occurs just in a single cell and does not trigger an immune response. The relative abundance of pro-apoptotic and anti-apoptotic signals within and around a cell determines whether the cell will survive or die by apoptosis (Henne et al., 2013).

Through interactions with downstream signalling pathways, circRNA can control the apoptotic process and contribute to BC pathogenesis (Figure 5). For example, Jiang and Cheng detected overexpression of circABCC4 in BC tissues, and they found circABCC4 to be involved in cell viability, migration and apoptosis via sponging miR-154-5p. Then, with the downregulation of circABCC4, they discovered enhancement of apoptosis, as well as significant downregulation of Bc1-2 and upregulation of Cleaved/Caspase-3 (Jiang and Cheng, 2020). Zheng et al. investigated the role of circSEPT9, and they found upregulation of circSEPT9 was associated with advanced clinical stage and bad prognosis in TNBC. They also revealed circSEPT9’s role in tumor growth, migration, invasion, and apoptosis via miR-637/LIF axis (Zheng et al., 2020). Likewise, Xie et al. detected high expression of hsa_circ_0004771 and ZEB2 in BC tumor tissues, with a reduction in the expression of miR-653. Differential knockdown of circ_0004771 and ZEB2, as well as overexpression of miR-653 in BC cells, decreased cell growth and triggered apoptosis (Xie et al., 2019).

PI3K/AKT signalling pathway is one of the main signal transition pathways inside cells, and they are remarkably important in apoptosis and cell survival regulation (Meng et al., 2017). According to recent evidence, the expression of cancer-related genes modulated by the circRNA/PI3K/AKT axis and suppressed apoptosis. For instance, Xu et al. detected significant upregulation of hsa_circ_001569 both in cell lines and BC tissues. When they glanced at the effects of silencing has_circ_001569 in cell lines, they found that has_circ_001569 knockdown not only stopped migration and invasion but also changed EMT markers and caused apoptosis by preventing the activation of the PI3K/AKT signalling pathway (Xu et al., 2019a).

On the other hand, the upregulation of circRNAs may induce the apoptosis process through sponging miRNAs. For example, Hu et al. found that the upregulation of circRNA_0001283 significantly inhibited cell growth and invasion while promoting apoptosis by sponging miR-187 and decreasing its expression (Hu Y. et al., 2020). Additionally, Yuan et al. detected downregulated hsa_circ_0068033 in BC tissues, which was significantly associated with the TNM stage and tumor size of patients. Furthermore, Yuan and his colleagues performed in vitro experiments and also found overexpressed hsa_circ_0068033 to sponge miR-659 and inhibit tumor growth in addition to inducing apoptosis through activation of intrinsic apoptotic pathways (Yuan et al., 2020). Furthermore, Wang et al. detected circRNA_000911 to be downregulated in BC cells. When they overexpressed circRNA_000911 in cell lines, they observed suppression of proliferation, migration, and invasion, with increased apoptotic abilities of the cells through sponging miR-449a, thus enhancing Notch1 and NF-κB signalling pathway (Wang et al., 2018a). Focusing on mechanisms in which circRNAs affect apoptosis is essential in BC since studies show that treating BC through promoting apoptosis is a promising approach.

Therapy for breast cancer consists of multifaceted strategies, including surgery, radiotherapy, neoadjuvant and adjuvant therapy. It is crucial that the chosen therapy has an optimal therapeutic effect for the proper treatment of BC (Fisusi and Akala, 2019).

Tamoxifen is known as an endocrine therapy that is most frequently used for hormone receptor-positive (HR positive) patients suffering from BC. Sang and his team used RNA-seq to detect the downregulation of circRNA_0025202 in tamoxifen-resistant cells, they performed further analysis and found that circRNA_0025202 acted as a tumor suppressor. Then they treated BC cells with circRNA_0025202 alongside tamoxifen, which led to tumor growth suppression and enhanced tamoxifen sensitivity in vivo (Sang et al., 2019). Similarly, Liang et al. detected downregulation of circBMPR2 in BC tissues. They knocked down circBMPR2 in tamoxifen-treated cell lines, and assay results showed enhanced tamoxifen resistance by suppressing tamoxifen influenced apoptosis (Liang et al., 2019a).

On the other hand, Hu et al. found that circ_UBE2D2 was upregulated in BC tamoxifen-resistant cells and promoted resistance through combination with miR-200a-3p to modulate viability of cells, metastasis, and Erα level in vivo and in vitro studies (Hu K. et al., 2020).

Monastrol is a chemotherapeutic agent that suppresses tumors by hindering mitotic kinesin Eg5 needed for forming bipolar spindle. Liu et al. performed a human circRNA microarray on cancer cells resistant to monastrol and found circRNA-MTO1 to be downregulated. The team then upgraded circRNA-MTO1 expression in BC cells resistant to monastrol and showed that circRNA-MTO1 not only reversed resistance but also had a synergetic effect when used with monastrol through binding to TRAF4 and inhibiting Eg5 protein (Liu et al., 2018). Even though BC cells showed over-expression of circFBXL5, even more upregulation was detected in antitumor agent 5-fluorouracil (5-FU) resistant BC cells. According to study results performed by Zhu and his colleagues, circFBXL5 appeared to regulate cell migration, invasion, and apoptosis by modulating the miR-216b/HMGA2 axis (Zhu et al., 2021).

In contrast, Liang et al. detected circKDM4C downregulation in metastasized tissues of BC, which was involved in both BC advancement and chemotherapy resistance. Resistant cells to doxorubicin showed a low level of circKDM4C, while transfection of MDA-MB-231/DOX cells with circKDM4C expression vector led to reduced cell proliferation and lowered resistance (Liang et al., 2019b). Likewise, Zhang et al. found downregulation of circKDM4C in BC tissues and intended overexpression of circKDM4C increased sensitivity of doxorubicin-resistant cells (Zhang et al., 2020). Another circular RNA that impacted therapy with doxorubicin was circUBE2D2, but negatively, as it induced doxorubicin resistance. When Dou and his colleagues knocked down circUBE2D2 in TNBC cells, it reversed the chemoresistance via downregulation of miR-512-3p or upregulation of CDCA3 (Dou et al., 2020). Similarly, overexpression of hsa_circ_0092276 was detected in BC doxorubicin-resistant cells by Wang et al., and this overexpression contributed to the resistance through altering autophagy-related gene 7 (ATG7) via sponging miR-384 (Wang et al., 2021).

Resistance to paclitaxel is one of the concerns in TNBC patients undergoing chemotherapy. Ma and his colleagues overexpressed circAMOTL1 in BC cells through plasmid construct, which improved cell viability, reduced apoptosis, and promoted invasion ability in MDA-MB-231 cells treated with paclitaxel. CircAMOTL contributed to resistance via AKT pathway regulation, promotion of anti-apoptotic protein, and suppression of pro-apoptotic protein (Ma et al., 2019). Meanwhile, Zang et al. observed circ-RNF111 upregulation in cells resistant to paclitaxel compared to cells responding to paclitaxel. When the team knocked down Circ-RNF111, the resistance of BC cells to paclitaxel was suppressed both in vivo and in vitro by upregulating E2F3 via sponging miR-140-5p (Zang et al., 2020). Li et al. detected 3.34 times increase of hsa_circ_0000199 in TNBC tissues compared to non-TNBC tissues. Breast cancer cells with silenced hsa_circ_0000199 became more sensitive to paclitaxel, cisplatin, gemcitabine, and adriamycin therapies in TNBC. Improved chemotherapeutic outcomes were achieved through enhanced expression of miR-206/miR-613 and inactivated PI3K/Akt/mTOR signalling (Li et al., 2021). In addition, Goa and his colleagues spotted a higher level of circ_0006528 in BC cells resistant to adriamycin compared to sensitive cells, and even though the mechanism was not known, there was a noticeable increase in sensitivity to adriamycin with downregulation of circ0006528 (Gao et al., 2017) (Table 3).

It can be observed through these studies that circRNAs have essential roles in the efficacy of a wide variety of treatment options for BC through their contribution to resistance or sensitivity of therapies.