1 Introduction

Cancer poses a serious threat to human health, having an increasing incidence and leading to mortality worldwide. Accompanied by the global aging population, cancer-related deaths would be further exacerbated according to the global statistics of the World Health Organization [1]. Cancer treatment methods mainly include surgical resection, radiation therapy, conservative drug treatment, and immunotherapy. However, treatment methods are limited because of drug resistance. In recent years, gene therapy combined with nanotechnology has been expected to overcome this shortcoming.

Small RNAs, including microRNAs (miRNAs) and small interfering RNA (siRNAs), function in posttranscriptional regulation of gene expression and powerfully regulate cell growth, development, differentiation, and apoptosis [2]. Accumulating evidence supports that small RNAs can act as attractive tools or targets for cancer therapy [3].

Unlike traditional delivery technology, nanoparticles (NPs), including organic NPs/inorganic and lipid/polymeric NPs, can specifically deliver small RNAs to cancer cells with unique physical properties [4]. Nucleases are rich in the blood and cells, and therapeutic RNAs usually require a suitable NP to protect them before they reach specific sites in the body. For example, organic NPs/inorganic NPs are used to deliver small RNAs for cancer therapy. Metal core NPs can act as delivery vectors to direct RNAs [5], while miR-21-3p-loaded gold NPs increase the roles of anti-PD-1 antibody and inhibit tumor progression by the alteration of the weight and the volume of tumor [6]. Wang et al. [7] developed multifunctional cancer-penetrating mesoporous silica NPs to deliver siRNA and miRNA, and showed that these NPs with therapeutic RNAs displayed significant cancer cell killing activity. The lipid/polymeric NPs have also been developed to deliver small RNAs for cancer therapy [8]. Lipid NPs can effectively deliver anti-KRAS siRNA to cancer cells and achieve a significant reduction in cancer growth [9]. On the basis of the roles of small RNA and NPs, we reviewed the NPs as carriers transporting small RNAs, and the application of NPs delivering small RNAs in cancer-targeted therapy (Fig. 1).

Fig. 1
figure 1

The application of NPs carrying miRNA or siRNA for therapy. NPs, including polyethylene glycol (PEG), hyaluronic acid (HA), chitosan (CS), and Gold nanorods, etc., act as carriers to deliver small RNAs for cancer therapy by targeting the oncogenes

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2 Small RNAs and targets

The common small RNAs, according to their origin, are mainly classified into two classes: miRNAs and siRNAs.

miRNAs, which are non-coding RNA molecules with 19–25 nucleotides, form from the newborn primary pri-miRNA. Then, pre-miRNA is generated and transported from export protein 5 (Xpo5) to cytoplasm, and finally processed into miRNA to form miRNA-induced silencing complex miRISC, which can degrade targeted mRNA or prevent translation, through perfect or imperfect complementation (Table 1, Fig. 2A) [10, 11]. With their high stability and utility, cell-free miRNA may be a viable diagnostic and prognostic biomarker for a wide range of illnesses, including malignancies of various origins [12,13,14,15].

Table 1 The mechanisms of action of miRNA
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Fig. 2
figure 2

The mechanism of miRNA and siRNA. A Pre-miRNA is generated from pri-miRNA by Dosha enzyme in the nucleus. Pre-miRNA is then transported from Xpo5 to cytoplasm, and cut into small RNA by Dicer and processed into miRNA by Argonaute (AGO), which finally form miRISC to combine with 3ʹ-untranslate region of mRNA to degrade mRNA or prevent translation. B siRNA is formed by the action of Dicer enzyme. The RISC complex is subsequently directed to the target mRNA by the guide strand, which then cleaves mRNA

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siRNA are double-stranded RNAs with a length of about 21–23 nucleotides. They are associated with gene silencing and involved in the regulation of gene expression with specificity. siRNA is formed by the action of the Dicer enzyme and enters RISC. Then, siRNA is subsequently directed to the target mRNA by the guide strand through complementary sequences (Fig. 2B) [16, 17]. As post-transcriptional level regulating factors, siRNA can specifically bind and cleave targeted mRNA, achieving basal interference with gene expression and the purpose of inhibiting protein synthesis.

Although both miRNA and siRNA can achieve gene disruption, they have considerable differences. The specificity of miRNA is not very rigorous, and it can work on one or several target genes. In general, the quantity of specific miRNA in cancer cells and normal tissues is different. Highly stable, detectable miRNAs in body fluids, tissues, and blood are promising biomarkers for cancer diagnosis or therapy [18]. siRNA has very strong gene specificity, and one siRNA can act on only one target gene. Both miRNA and siRNA can be used as cancer-targeted therapeutic inhibitors to reduce the expression of cancer-promoting genes as a method to suppress cancer cells.

3 Nanomaterials and their applications in cancer

3.1 Nanomaterials and gene therapy

The diameter of natural or synthetic NPs is about 1 to 100 nm [19]. Nanomaterials mainly include three components: core layer, intermediate middle layer, and surface layer. The core layer is the internal and main part of nanomaterials. The middle layer is chemically different from the core layer. The surface layer is the outermost and functional layer of nanomaterial, which interacts and functionalizes with other macromolecules [20, 21]. According to the composition of nanomaterials, they are divided into different types, such as metal NPs, polymer NPs, iron oxide NPs, and liposomes [22]. Nanomaterials have unique physical and chemical properties, such as small size effect, surface and interface effect, and strong adsorption effect, which are useful for gene therapy.

Gene therapy refers to the process of transferring defined genetic materials, including small RNAs, into specific target cells to modify the expression of individual genes or repair the abnormal genes to cure diseases. NPs have the ability to encapsulate, couple, and capture various hydrophobic and hydrophilic compounds and biomolecules, such as peptides, DNA and small RNAs. These properties ensure that small RNAs can be applied widely in gene therapy.

3.2 Applications of nanomaterials

Nanomaterials have many applications in medical treatment. They can be applied for drug delivery, resulting in an enhanced penetration and retention effect, and accumulation at the cancer sites [23]. The approach is mainly to use NP-sized drugs or nanocarriers for drug delivery, such as nano-graphene oxide, which has a remarkably large specific surface area with appropriate application of near-infrared radiation, and can be used not only for drug delivery but also for photothermal therapy [24]. Appropriate adjuvants or carriers can also be added to the vaccine configuration to enhance the immunogenicity and prolong the duration of the action of the vaccine. Graphene oxide has been reported to act as a transport carrier for vaccine antigens and has the potential to activate the immune system. The antigen-loaded graphene oxide is not only effectively internalized by dendritic cells but also promotes cross presentation of CD8+T cells [25], which greatly improves the vaccine effect.

Nanomaterials also have been applied in pulmonary medicine. The development of unique nanopackaging therapies in the form of nebulization allows the drug to go directly into the lungs. Compared with traditional oral drugs, nebulized drugs avoid absorption through the intestine, which enhances drug delivery [26]. Aerosolization drug delivery is not only adapted to traditional lung diseases, such as cancer and pulmonary fibrosis, but also can act on new emerging diseases, such as novel coronaviruses. In addition, it can be used for screening and rapid diagnosis of lung diseases [27]. For example, the diagnostic screening Corona Virus Disease 2019 nano biosensor is a graphene-based nano sheet coated with a specific antibody against the SARS-CoV-2 spike protein [28].

For treating breast and liver cancer, a thin membrane hydration technique has been developed, in which niosomes are wrapped with TPGS (7 and 2a-charged D-α-tocopherol polyethylene glycol 1000 succinate) to constitute a niosomal system. This approach improves the solubility, bioavailability, and biocompatibility of the drug compared with the uncoated compounds [29].

A major reason for the poor prognosis of cancer is multidrug resistance (MDR), where cancer cells develop resistance to drugs, overexpress drug-resistant genes to resist drug-induced cell death, and excrete drugs out of the cell, leading to increasing drug doses during treatment. In addition, the excess drugs may exacerbate adverse effects [30]. But, drug- and MDR-related small RNAs can be selectively silenced drug-resistant genes through NPs simultaneously.

Collectively, nanomaterials are currently promising and versatile options in the field of medical research. NP-sized drugs such as those mentioned above, nanodelivery platforms for drugs and even small molecule nucleic acids can be used for therapeutic applications (Table 2), as well as diagnostic screening to aid in imaging.

Table 2 Applications of nanocarriers with small RNAs in diseases
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4 Application of nano-carriers carrying small RNA in cancer targeted therapy

Small RNAs can be used to develop drugs for cancer treatment. Despite the potential of small RNAs to treat a wide range of cancers, some practical limitations remain. The stability of small RNAs is relative. The self-small RNAs in the body are relatively stable, but the synthetic small RNAs introduced into human plasma are easily degraded by the existence of RNA enzymes [45, 46]. These RNAs are negatively charged and cannot easily pass through the cell membrane into the cytoplasm, that is, the uptake by cancer cells is relatively low [47, 48]. Therefore, the problem of designing and developing vectors that can carry and effectively deliver miRNA or siRNA needs to be solved. To date, NP carriers have made great progress in carrying miRNA or siRNA, and have been applied for cancer targeted therapy.

4.1 Lung cancer

Lung cancer is the most prevalent cancer type and has the greatest fatality rate [49]. Mounting evidence supports that small RNAs play important roles in lung cancer risk and prognosis.

Chitosan (CS) has different degrees of deacetylation and molecular weight. The deacetylated amine groups throughout the CS chain will be protonated and accessible to interact with the negatively charged siRNA [50]. CS is a recognized good siRNA vector with the characteristics of non-toxicity, low immunogenicity, good biocompatibility, and low cost [51,52,53]. B-cell lymphoma/leukemia 2 (Bcl2), an important proto-oncogene [54], regulates apoptosis in many tumorigenic processes. Zhang et al. [55] synthesized cyanine 3 (Cy3)-labeled siRNA, and demonstrated the effective preparation of HA-modified CS NPs, which had minimal cytotoxicity and high siRNA encapsulation efficiency (sCS NPs-HA). The sCS NPs-HA successfully decreased the expression of BCL2 by siRNA in lung cancer cells. Moreover, in vivo results demonstrated that sCS NPs-HA was accumulated at the location of the cancer, which prevented cancer development. This finding suggests that HA-modified CS NPs might be a viable gene delivery carrier for cancer-specific gene therapy.

The silencing of the crucial mitotic checkpoint gene Mad2 leads to the termination of mitosis and large-scale cell death. Nascimento et al. [53] developed epidermal growth factor receptor-targeted CS NPs with siRNA specific to Mad2 target and silenced Mad2 expression, which induced apoptotic cell death and constructed a potent delivery system for lung cancer therapy. Nascimento et al. [56] then evaluated a CS delivery system with siRNA as a single treatment or combined with cisplatin in cisplatin-sensitive and resistant lung cancer models. The combination therapy was noticeably more effective in cisplatin-resistant cancer. Results indicated that the use of the CS delivery system with siRNA in conjunction with chemotherapeutic drugs is an effective and stable method for treating drug-resistant malignancies.

In addition to CS, other metal NPs such as gold can be implemented and used for drug monitoring. With their great X-ray attenuation and nontoxicity, gold NPs are good X-ray contrast agents that enable a more precise observation of the regulated target drug release. Yang et al. [57] designed a brand-new siRNA delivery method utilizing a gold nanocage (Au-siRNA-PAA-AS1411) that exerted high anticancer activities on lung cancer.

MiR-320a-3p, as a suppressive gene in many cancers, is expressed at low levels in lung cancer [58]. The cationic polymer polyethyleneimine (PEI) has the lysosome escape effect [59]. Lysosomes contain many hydrolytic enzymes that will degrade proteins, nucleic acids, or lipids. If lysosomes ingest the transported genes by NPs, then the genes will be damaged, making it unable to performing biological functions. PEI exerts the function of lysosome escape by its proton sponge effect. Indeed, the large number of amino groups in PEI functions as a proton sponge effect, which may facilitate the escape of PEI-nucleic acid complexes from lysosomes, and lead to the release of nucleic acids in the cytoplasm. RGD is a short peptide that contains arginine-glycine-aspartic acid [60]. It has a cancer-targeting effect and improves the cancer sensitivity to radiotherapy. It also has a recognition site for interacting integrins and ligands, which can be specifically expressed on new blood vessels or cancer cells. Gold nanorods have the common characteristics of low biological toxicity and high biocompatibility, as well as an excellent photothermal effect [61]. Therefore, our team developed a new type of gold nanorod modified with PEI, miR-320a, and RGD peptide [62], and the novel miRNA NP delivery system combines an RGD peptide for cancer targeting, PEI cationic polymer for lysosome escape, and gold nanorods for photothermal treatment. This miRNA NP delivery system can regulate a suite of cancer-related genes by miR-320a-3p directly targeting and suppressing Sp1 expression, which inhibits the proliferation of lung cancer cells but enhances the apoptosis of lung cancer. Moreover, this NP delivery system can improve the sensitivity to radiotherapy.

For the application of PEI, Wen et al. [63] synthesized a novel cancer-microenvironment-sensitive delivery polymer by conjugating hyaluronic acid (HA) to PEI, with a matrix metalloproteinase-2 (MMP-2)-sensitive peptide acting as the linker (HA-P-PEI) to deliver PD-L1–siRNA. PD-L1 is involved in intracellular anti-apoptotic signaling and affects the proliferation, apoptosis, and migration of cancer cells [64]. HA-P-PEI/siRNA NPs can significantly reduce the expression level of PD-L1 and penetrate deeper into cancer tissues, showing lower cytotoxicity and potential therapeutic characteristics.

4.2 Glioblastoma

Glioblastoma (GBM) is a type of high-grade glioma tumor that develops from brain glial cells. Forty percent of brain malignant tumors are GBMs, which pose a major threat to people's health. Primary brain tumors have increased in prevalence worldwide over the past 30 years, with a five-year survival rate of only 5% [65]

The treatment method for GBM is limited because of the biological blood–brain barrier. A new approach therefore needs to be explored in which drugs do not need to travel through the blood to enter the brain. The nasal–brain drug transport pathway can bypass the blood–brain barrier and enter the brain directly through the nasal cavity [66]. Several miRNAs that affect the p53 network, such as miR-21 and miR-10b, are dysregulated in GBM, thus promoting gliomagenesis. MiR-21 is overexpressed in most GBMs and controls key target genes involved in the apoptotic process [67, 68]. MiR-100 could induce apoptosis and decrease cell proliferation in tumor cells [69, 70]. Based on the role of these miRNAs, Sukumar et al. [71] designed multifunctional gold-iron oxide NPs (polyGIONs) encapsulating miR-100/antimiR-21 as a drug targeted carrier. The intranasal administration of polyGIONs with miR-100/antimiR-21 would significantly increase the survival of mice with GBM cell-derived orthotopic xenografts.

Costa et al. [72] designed stable nucleic acid lipid particles (GBM-targeted SNALPs), a new type of lipid-based vector, through coupling of chlorotoxin (CTX). CTX, a scorpion-derived peptide, was shown to enhance tumor cell uptake and increase gene silencing efficacy in GBM cells. The results demonstrated that SNALPs can successfully deliver anti-miRNA oligonucleotides to brain tumors, effectively reduce miR-21 levels in tumors and significantly increase the expression of RhoB, a direct target of miR-21. The SNALP with an anti-miR-21 oligonucleotide enhanced anti-tumor activity, decreased tumor cell proliferation and tumor size, and increased apoptosis activation. The results indicate its potential as a new and promising treatment for GBM.

Intra-tumoral heterogeneity (ITH) exists in GBM at the cellular level, and remains one of the most significant impediments toward development of therapeutics [73]. Khan et al. [74] used a bioinformatics approach to find miRNA that acts on signaling networks in GBM. They efficiently delivered miR-34a in vivo to orthotopic tumors by intravenous delivery of miRNAs packaged in bacterially-derived nanocells, targeting EGFR with a bispecific single-chain variable fragment antibody. The results demonstrated a significant increase in survival in mice treated in combination with temozolomide (TMZ). MiR-34a can inhibit the proliferation of a wide spectrum of GBM cells, and nano-cell-mediated delivery of miR-34a sensitizes to TMZ in GBM cell lines and primary patient-derived cultures by suppressing the levels of EGFR-related factors. Therefore, miR-34a is a powerful miRNA by targeting EGFR for the targeted treatment of GBM in combination with TMZ and can potentially counteract therapeutic resistance resulting from ITH in GBM.

4.3 Breast cancer

Breast cancer is one of the most common cancers and the leading cause of death in post-menopausal women worldwide [75]. Surgical treatment includes mastectomy or breast-conserving surgery, followed by radiotherapy. CS can also be used for gene therapy of breast cancer. To overcome the disadvantages of naked RNA, Sun et al. [52] designed polyethylene glycol (PEG) modified CS to enhance the solubility and stability of the vector and improve the transfection efficiency of siRNA. PEG is a hydrophilic polymer that creates a stable spatial effect to shield the non-specific interactions between proteins and cytoplasm [76]. Survivin plays an important role in suppressing apoptosis, and the inhibition of its expression can lead to tumor sensitization to chemical agents. Forkhead box M1 (FOXM1) can directly regulate the transcriptional levels of survivin. Therefore, the suppression of FOXM1-Survivin axis may lead to glioma cells highly susceptible to anticancer drug. It is found that PEG-CS/siRNA NPs were effectively absorbed by breast cancer cells, and significantly reduced cell proliferation of breast cancer cells by silencing the survivin gene [77]. Despite having high serum stability, PEGylated polymers have a comparatively low gene loading and gene transfection efficiency [78, 79]. Liu et al. [80] created dendrimer/siRNA nanoassemblies modified with PEG, which exhibited excellent serum resistance and gene silencing efficacy, leading to excellent anticancer effects on breast cancer in vivo.

4.4 Ovarian cancer

Ovarian cancer is one of the leading causes of cancer-related deaths among women, and it has five-year survival rates below 45% [81]. Most of the time ovarian cancer is diagnosed as advanced, and treatment options are often limited.

As for the gene therapy of ovarian cancer, some studies have applied the small RNA-targeted method mentioned above in the treatment of cancer. Cancer cells are always more or less resistant to therapeutic drugs, thus reducing the efficacy of the drug. Focal adhesion kinase (FAK) plays a crucial role in tumor invasion, migration, and chemoresistance [82]. Therefore, Byeon et al. [82] applied hyaluronic acid-labeled poly (d,l-lactide-co-glycolide) NPs (HA-PLGA-NP) to design a selective delivery strategy for paclitaxel (PTX) and siRNA specific to FAK against chemoresistant ovarian cancer. The results demonstrated that HA-PLGA-NP could simultaneously deliver PTX and siRNA to the cancer sites to overcome drug resistance and enhance the effective concentration of the drug in ovarian cancer cells. Inhibiting cancer angiogenesis is an effective cancer therapy, but this method has drawbacks and obvious side effects [83]. PLXDC1 is overexpressed in cancer endothelial cells, which can contribute to cancer angiogenesis, metastasis, migration, and invasion [84], and HA-CH-NPs/siRNA (specific to PLXDC1) is developed as a potential targeted vector system for anti-angiogenesis therapy of cancer. They demonstrated that this system is an extremely specialized siRNA delivery technology that can inhibit epithelial ovarian cancer growth by inhibiting cancer angiogenesis.

4.5 Pancreatic cancer

Pancreatic cancer (PC), characterized by late detection and frequent drug resistance, is one of the world's most lethal cancers with poor 5-year survival rates. The poor prognosis of PC often leads to the unsuccessful treatment options and tumor metastasis [85].

The targeted modification of NPs as carriers with small RNAs have been applied into the treatment of PC. A poly(lactic-co-glycolic acid;PLGA)-based nanoparticle with siRNA specific to PD-L1 (siPD-L1@PLGA) is designed for PC. The siPD-L1@PLGA can effectively suppress the expression of PD-L1 in PC, and inhibit cancer cell proliferation with increased IFN-gamma positive CD8 T cells [86]. RAS, as an oncogene, is mutated in approximately 90% of PC. The lipid NPs were designed to deliver siRNA specific to K-RAS knockdown and significantly reduced the PC cell growth [9]. Activation of scramblases plays a crucial role in tumor immunosuppression. A nanocarrier, for co-delivery of Xkr8-siRNA to suppress scramblase levels, can lead to significant inhibition of tumor growth in PC models by regulating antitumour immune [87].

5 Future prospect

Though the application of small RNAs based on NPs in cancer therapy has become increasingly popular, the NP-associated toxicity, relatively low biocompatibility, and off-targeted activities limit their utilization for cancer treatment. The adverse interactions between biological organism and NPs may also cause life-threatening toxicity [88]. Delivery of not only siRNA but other nucleic acid therapeutics, such as miRNA and pDNA, requires a long period of development. Moreover, although the combination of NPs-based carriers with the therapeutic nucleic acids (such as small RNAs) for oncogenes is a promising approach in anti-cancer strategies, the transfection efficiency of NPs-based carriers remains insufficient.

The future development trend and applications may base on the subsequent discovery of cancer-related genes and mechanisms of cancerigenesis to develop therapeutic regimens, improve the transfection efficiency or search for NP carriers. For precision targeted therapy, NPs can be designed based on the biomarkers of tumor to improve the targeting therapy. Nanomedicine can also be integrated with artificial intelligence (AI). The AI algorithms can be used to process the data from different patients, and improve the design of NPs to compensate for individual differences between patients and increase the accuracy of diagnosis or treatment. We believe that, eventually, small RNA-targeted therapy will not only exist in the laboratory but also be applied clinically.