Analysis of serum miRNAs expression profiling between malaria patients and healthy controls revealed that 64 of the 341 detected miRNAs were differentially expressed. Among them, expression of the miRNA, hsa-miR-106b-5p, was significantly higher in malaria patients than in the healthy controls (the expression level is 14.871 times that of the control group), which had the smallest P value and adjusted. P.value, and largest logFC value. Thus, we confirmed that the downregulation of miR-106b is positively correlated with a decrease in the immune protection of malaria vaccines. It has been reported that increasing the expression of miR-106b in vitro can enhance the expression of transmembrane activator and CAML interactor (TACI) on the surface of memory B cells (MBCs) []. This indicated that hsa-miR-106b-5p is closely related to malaria, therefore we focused on its analysis.
We used bioinformatic analysis such as target gene prediction, GO and KEGG analyses to understand the biological functions of hsa-miR-106b-5p and its association with malaria comprehensively and systematically. First, in order to make up for the inevitable false positives and false negatives of bioinformatic predictions, four authoritative miRNA databases, TargetScan, DIANA-microT, miRDB, and miRTarbase, were used to predict the target genes of hsa-miR-106b-5p. The intersection of these results was analyzed using Venn, and a more reliable target gene set, containing 78 overlapping predicted target genes was obtained (Fig. 1). Subsequently, Metascape was used to perform GO and KEGG analyses for overlapping target genes.
GO results revealed that the target genes were significantly enriched in the related functions of small GTPases (Additional file 2). Small GTPases are key participants in a series of pathophysiological processes, most of which are associated with the regulation of protein secretion, endocytosis and vesicular transport [, ]. Previous studies have shown that the dysfunction and negative regulation of certain small GTPases, such as members of the Ras and Arf subfamilies, are related to the promotion and progression of tumors [, ], and recent studies have confirmed that GO:0007264 ~ small GTPase mediated signal transduction is related to the occurrence of breast cancer []. Studies have shown that miR-106b-5p is the most regulated miRNA in cancer tissues. It is involved in the emergence and development of pancreatic cancer, lung cancer, and breast cancer among others [,,,], which not only verifies the accuracy of our approaches and directions, but also suggests a link between malaria and tumors.
In 2011, antitumor effects were observed in a Lewis lung cancer model mouse after infection with Plasmodium yoelii. The proliferation of tumor cells was significantly inhibited and the survival of tumor-bearing mice was remarkably prolonged []. An epidemiological survey also showed a significantly negative correlation between the incidence of malaria and tumor mortality []. In general, there are three reasons: (1) Periodic high fevers bouts occur in the host in the acute phase of malaria infection, which can kill certain non-heat-resistant cancer cells. Hyperthermia has been clinically used for the treatment of some cancers [, ]. (2) Plasmodium infection can stimulate the innate immunity of host, activating and enhancing antitumor immunity. It mainly occurs through activation of NK and DC cells to promote the production of a large number of cytokines such as IFN-γ and TNF-α, which antagonize the tumor immunosuppressive microenvironment, leading to an antitumor effect. In fact, malaria infection can enhance the host immune response and it has been used as an adjuvant to treat cancer []. GO analysis showed that GO:0002376 ~ immune system process (P < 0.01), GO:0048518 ~ positive regulation of biological process (P < 0.01) and GO:0008283 ~ cell proliferation (P < 0.01) are all related to immune system. (3) Excluding immune responses, malaria infection also acts on VEGFR2 through miRNAs in exosomes and lncRNA F63 in tumor tissues, thereby inhibiting tumor angiogenesis. Angiogenesis is the formation of new blood vessels from pre-existing blood vessels. It plays an important role in tumor development, and it is responsible for the transportation of nutrients needed for tumor growth. Cutting off the nutrient supply from tumor blood vessels will cause the cancer cells to "starved to death" [25, ]. This is strongly associated with GO:0048514 ~ blood vessel morphogenesis (P < 0.01), suggesting that malaria infection inhibits tumor progression. Besides, GO:0051781 ~ positive regulation of cell division and GO:003052 ~ intracellular receptor signaling pathway are also important for further investigation.
KEGG analysis results revealed that the overlapping target gene set was enriched in hsa04144: Endocytosis (P < 0.01), hsa01521: EGFR tyrosine kinase inhibitor resistance (P < 0.01), hsa05212: Pancreatic cancer (P < 0.01) and other pathways (Table 3). The most significant enrichment was observed in endocytosis, which plays an essential role in malaria infection. For Plasmodium, the importance of endocytosis is self-evident []. In the pre-erythrocytic stage of malaria, endocytosis mediates the uptake and degradation of up to 80% of host cell cytoplasm which is mainly composed of hemoglobin by the malaria parasite. However, the molecular mechanism of this process remains unclear []. In general, endocytosis requires core transport factors, including Rab-GTPase [, ], which is similar to the results of GO analysis, and thus, it further provides evidence for the link between hsa-miR-106b-5p and malaria. Interestingly, a recent study found that mutations of Kelch13 (K13) protein in Plasmodium are resistant to artemisinin (ART) and its derivatives due to a reduction of hemoglobin endocytosis []. Artemisinin is a frontline antimalarial drug. However, drug resistance has gradually emerged in Cambodia and the Greater Mekong Subregion due to long-term use and abuse. The main clinical manifestation of artemisinin resistance is that the clearance of malaria parasite is delayed or reduced after treatment with ART [, ]. The mutated K13 gene in Plasmodium is related to a delayed parasite clearance after treatment with Artemisinin-based combination therapy (ACT), as recommended by WHO []. Molecular characteristics of the highly conserved K13 C-terminal region in P.falciparum, kelch13 propeller region (Pfk-13), are closely linked to the Plasmodium phenotype from Cambodia having a delayed ART clearance []. Moreover, K13 is related to hemoglobin internalization and catabolism. Thus, reducing K13 impairs hemoglobin catabolism and reduces artemisinin activation []. Studies on compartment proteins of K13 and their functions revealed the entire pathway of ART resistance, that is, K13 and its compartment proteins play important role in the endocytosis of hemoglobin uptake by malaria parasite. Hemoglobin digestion products are necessary for the activation of ARTs. Therefore, an inactivation of all kinds of K13 proteins will reduce hemoglobin uptake by malaria parasites, thereby reducing the activation and concentration of ART decreasing the susceptibility of malaria parasites to ART, and eventually resulting in the resistance of malaria parasite to ART. Moreover, the main manifestation observed in wild-type anti-ART P.falciparum explains that changing the stability of K13 protein can affect endocytosis and lead to resistance to ARTs from both negative and positive sides. An opportunistic inactivation of K13 gene will cause the malaria parasite to remain in the ring stages and no longer grow and divide. However, the inactivation of K13 only affects the ring stage and does not work for trophozoite stages. This may be the reason for the frequent mutations found in drug-resistant Plasmodium isolates due to K13 gene, out of all proteins that affect endocytosis [36, , ]. Since the main reason for the emergence of ART resistance among Plasmodium is the reduction of endocytosis and the predicted target genes of hsa-miR-106b-5p are significantly concentrated in the signaling pathway of endocytosis, hsa-miR-106b-5p is the key miRNA in malaria. Is it possible to enhance endocytosis by targeting hsa-miR-106b-5p and improve ART resistance?
Finally, we constructed a PPI network using STRING, containing 39 predicted target genes of hsa-miR-106b-5p. Additionally, we identified five hub genes, VEGFA, STAT3, RACGAP1, OCRL, and RBBP7, which may play vital roles in the regulation of biological effects in hsa-miR-106b-5p (Fig. 5). The most critical hub gene was VEGFA, a key factor for vascular endothelial activation, which induces the survival, migration, and proliferation of endothelial cells during angiogenesis, and promotes vascular penetration and endothelial activation during inflammation []. It has been proposed as a biomarker for malaria severity, and it is expressed in the plasma and brain tissues of malaria patients [, ]. Endothelial activation is an early feature of malaria. It is probably beneficial for the isolation of infected RBCs and it also leads to thrombocytopenia. The number of platelets was negatively correlated with parasitemia and malaria severity. These results reveal a correlation between hsa-miR-106b-5p and malaria. Furthermore, the expression of VEGFA increased in many cancers. When its production is inhibited, tumor angiogenesis reduces, thus, decreasing the source of nutrition for cancer cells and finally achieving an anti-tumor effect. Monoclonal antibodies against VEGFA have been used to treat cancer. However, they cause some side effects and redundancy of other antigenic factors []. Therefore, using miRNA to target genes is highly specific and safe. An equally important gene, STAT3, regulates cell growth and apoptosis, whereas RACGAP1, a GTPase-activating protein belonging to the Rho GTPase family, is highly expressed in malignant tumor cells. These hub genes are of great significance in malaria and tumors.
In brief, the GO results of our bioinformatic analysis suggest that hsa-miR-106b-5p may mediate malaria to inhibit tumor progression by controlling vascular morphogenesis and other complex functions. The KEGG results indicate that hsa-miR-106b-5p can improve ART resistance and affect the growth and division in Plasmodium through endocytosis. Likewise, the PPI of predicted target genes revealed the relationship between VEGFA and malaria severity, and the link between malaria parasite and tumor growth and metastasis.
However, our study had some limitations. First, the number of volunteers was small, and it may not be representative due to large individual differences in humans. However, the study detected 341 miRNAs, obtained serum miRNA expression profiling of malaria patients, and identified certain differentially expressed miRNAs. To reduce errors, we will expand the sample size in future experiments. Second, disease factors of volunteers, which may affect the results were not considered. Confounding factors can be increased in information statistics. Third, false positives or false negatives inevitably exist in bioinformatics analysis approaches. Therefore, a further experimental verification for the predicted target genes and their biological functions is needed. The dual-luciferase reporter assay can be used to identify binding sites of miRNA target genes and choose miRNA mimics or inhibitors for in vitro induction and in vivo experiments for functional verification.
In summary, this study analyzed the changes in serum miRNA expression profiling of malaria patients using bioinformatic analysis, identified differentially expressed miRNAs, and conducted a comprehensive and systematic analysis of hsa-miR-106b-5p, which shows the maximum statistical difference and has attracted tremendous research interest. Additionally, we constructed a PPI network for the predicted target genes of hsa-miR-106b-5p. The results demonstrate that it plays a role in inhibiting tumor progression and the emergence of ART resistance in Plasmodium, which may be achieved by regulating vascular morphogenesis, endocytosis, and VEGFA. The study findings will provide a systematic understanding of the biological processes associated with hsa-miR-106b-5p, and they will provide data support and theoretical basis for further studies to verify the link between malaria and tumors, design malaria vaccines, and regulate miRNA functions to achieve disease treatment.