Exosomal miRNAs play pivotal roles in intercellular communication and various biological processes [60, 61]. Dysregulation of exosomal miRNAs could link to tumorigenesis through regulation changes in central cellular process including cell proliferation, cell survival, and apoptosis. Current research has emphasized the potential of exosomal miRNAs as prognostic and diagnostic biomarkers for diseases [62]. While short-term adverse effects of e-cigarettes on human health have been well-documented, the long-term health effects of e-cigarette use remains unknown due to the relatively short period these products have been on the market [63, 64]. Few studies have examined the impact of e-cigarette use, particularly exclusive e-cigarette use, on exosomal miRNAs.
Using plasma samples from 30 participants in the PATH Wave 1 study, we investigated differences in exosomal miRNA expression levels between exclusive e-cigarette users and non-users. We identified four significant exosomal miRNAs (hsa-miR-100-5p, hsa-miR-125a-5p, hsa-miR-125b-5p, and hsa-miR-99a-5p) that were upregulated in exclusive e-cigarette users compared to non-users. Further gene enrichment analysis of predicted target genes regulated by these four significant miRNAs revealed their involvement in various cancer pathways. Our results suggest potential risks associated with exclusive e-cigarette use.
The involvement of hsa-miR-100-5p in numerous pathological processes associated with diseases has been documented. Overexpression of hsa-miR-100-5p in human ovarian endometriotic stromal cells has been linked to the promotion of invasion, contributing to the pathogenesis of endometriosis [65]. Additionally, hsa-miR-100-5p has been identified as an independent risk factor and a prognostic signature for patients with stomach adenocarcinoma [66]. Elevated expression levels of hsa-miR-100-5p have been associated with extracapsular extension and poorer survival in patients with oral squamous cell carcinoma compared to their normal counterparts [67]. In Alzheimer’s disease, hsa-miR-100-5p is known to regulate neuron survival by targeting the Mammalian Target of Rapamycin (mTOR) pathway, a central player in regulating many fundamental cell processes and a critical factor in tumor metabolism [68–70]. The upregulation of hsa-miR-100-5p in exclusive e-cigarette users, when compared to non-users, indicates potential elevated risks of various cancers.
The miRNA has-miR-125a-5p has been recognized as a tumor suppressor in various malignancies, including those affecting the breast, ovary, lung, and central nervous system [71, 72]. It plays a role in cell proliferation through cell cycle regulation and has potential as a therapeutic target for treating squamous cell carcinoma of the head and neck [71, 72]. Prior study has indicated that has-miR-125a-5p can induce apoptosis through a p53-dependent pathway in human lung cancer cells [73]. Furthermore, its associated with the pathological stage or lymph node metastasis in non-small cell lung cancer has been reported [74].
The miRNA hsa-miR-125b-5p has been identified as a regulator of inflammatory genes, targeting MAPKs and NF-kB signaling pathways in human osteoarthritic chondrocytes [75, 76]. Additionally, miR-125b-5p inhibits the expression of TNFR2, demonstrating immunosuppressive activity to enhance the antitumor efficacy in human colon adenocarcinoma patients [77]. In liver cancer, miR-125b-5p has been shown to inhibit cell proliferation, migration, and invasion [78]. The miRNA hsa-miR-99a-5p, recognized as a tumor suppressor in tumors like bladder cancer and breast cancer, suppresses cell proliferation, migration, and invasion [79]. Interestingly, a previous study found that the hsa-miR-99a-5p expression level in breast cancer tissues were significantly lower than healthy breast tissue, while the expression level in plasma samples were significantly higher in breast cancer patients than in healthy controls [80].
Although has-miR-125a-5p, miR-125b-5p, and hsa-miR-99a-5p are known tumor suppressors, it remains uncertain whether the upregulation of these exosomal miRNAs in plasma samples indicates an elevated risk in exclusive e-cigarette users. Our experimental validation showed that overexpression of miRNA hsa-miR-125b-5p can promote DNA damage in bronchial epithelia cells. Further research is necessary to evaluate the role of exosomal miRNAs in the association between exclusive e-cigarette use and adverse health effects.
Limited research has explored the impact of e-cigarette use on exosomal miRNA profiles. A prior study in 2019 investigated plasma exosomal miRNA expression levels, revealing a different set of miRNAs significantly differentially expressed between exclusive e-cigarette users and non-smokers [47]. This variation may stem from differences in e-liquid and e-cigarette devices. Our samples were collected during the PATH Wave 1 study from 2013 to 2014, aligning with the era of the first generation of e-cigarette devices [81]. During this period, most e-cigarettes were usually disposable single units containing natural nicotine in the e-liquid. In contrast, the predominant e-cigarette devices on the market in 2019 were fourth generation devices, largely disposable with nicotine salt in the e-liquid to mitigate nicotine harshness [81]. Given the large differences in e-liquid and e-cigarette devices, it is highly plausible that distinct exosomal miRNAs are influenced by e-cigarette use.