Vaping and tumor metastasis: current insights and progress

Abstract

Tumor metastasis is the primary cause of cancer-related mortality and remains a major hurdle in cancer treatment. Traditional cigarette smoking has been extensively studied for its role in promoting metastasis. However, the impact of e-cigarette (e-cig) on cancer metastasis is not well understood despite their increasing popularity as a supposedly safer alternative. This mini review synthesizes current literature on the effects of e-cig on cancer metastasis, focusing on the processes of dissemination, dormancy, and colonization. It also incorporates recent findings from our laboratory regarding the role of e-cig in tumor progression. E-cig exposure enhances metastatic potential through various mechanisms: it induces epithelial-mesenchymal transition (EMT), increasing cell migratory and invasive capabilities; promotes lymphangiogenesis, aiding tumor cell spread; and alters the pre-metastatic niche to support dormant tumor cells, enhancing their reactivation and colonization. Furthermore, e-cig induce significant epigenetic changes, such as DNA methylation and histone modifications, which regulate genes involved in metastasis. Our data suggest that e-cig upregulate histone demethylases like KDM6B in macrophages, impacting the TME and promoting metastasis. These findings underscore the need for further research to understand the long-term health implications of e-cig use and inform public health policies to reduce e-cig use.

Graphical Abstract

1. Introduction

Tumor metastasis, the spread of neoplastic cells to distant locations from the primary tumor site, is the leading cause of cancer-related deaths and poses significant challenges for cancer treatment [1]. Conceptually, metastasis can be divided into three temporally overlapping stages: dissemination, dormancy, and colonization [2]. During this three-step metastatic cascade, metastatic tumor cells undergo a series of complex genetic, epigenetic, and phenotypic changes while continuously interacting with immune cells and extracellular matrix components [3]. The role of cigarette smoking in promoting tumor metastasis is well-documented [4]. E-cigarette (e-cig) were introduced as a putatively healthier alternative to traditional smoking and have gained widespread popularity [5]. However, due to the high levels of toxins in e-cig aerosols, increasing concerns are being raised about their purported health benefits, including the claims of lesser carcinogenicity [6, 7]. Research on the impact of e-cig on tumor metastasis is limited, and no large-scale epidemiological results directly link e-cig usage to cancer. However, one recent case report suggests a link between extensive vaping and oral squamous cell carcinoma [8]. Current literature suggests that e-cig may play an important role on enhancing stemness, inducing EMT, promoting lymphangiogenesis, and shaping the TME, all contributing to metastasis. This mini review aims to summarize the current literature on this topic, supplemented by observations from our laboratory.

2. Current status of E-cigarette use

2.1. Trends in E-cigarette usage

Since their introduction to the US market in 2007, e-cig have rapidly gained acceptance, particularly among young individuals who have never used traditional tobacco products [5]. The prevalence of vaping is increasing, suggesting that e-cig may soon become the primary nicotine product in the USA. From 1997 to 2020, the percentage of traditional tobacco users aged 18–24 years decreased from 29.1 to 5.4% [9]. Meanwhile, current e-cig use among young adults exhibited a significant quadratic trend over 2014–2018 (5.1%, 5.2%, 4.7%, 5.2%, 7.6%), with approximately 13 million American adults currently using e-cig [10–13]. Among adults aged 18–24, 14.5% use e-cig compared to only 6.1% who smoke traditional cigarettes [10]. E-cig usage among adults aged 18–29 has increased from 8.8% in 2019 to 10.2% in 2021, with the increase being greatest among those who have never smoked (p = 0.01) [14]. Additionally, e-cig are often used as tools for smoking reduction or cessation [15–17]. In 2021, former smokers made up the largest group of e-cig users among adults over 30, accounting for about 50% of this demographic [15, 18]. Overall, as more adults in the USA start using e-cig, it is crucial to pay closer attention to the potential harmful effects of e-cig.

2.2. Composition of E-cigarettes and aerosols

E-liquids, the fluid that is vaporized in e-cig, generally contain solvents such as vegetable glycerin (VG) and propylene glycol (PG), along with nicotine or nicotine salts and various flavorings. As a result, the aerosol produced by e-cig contain numerous harmful substances, including carbonyls, volatile organic compounds, trace heavy metal elements, polycyclic aromatic hydrocarbons, and tobacco-specific nitrosamines, all classified by the FDA as harmful or potentially harmful. Many of these toxic compounds are generated through chemical reactions that occur during the high-temperature vaporization process, which can reach approximately 250 °C [19]. Compared with traditional tobacco, the effects of these components on health less understood and warrant further study (Fig. 1, Composition of E-Cigarettes and Aerosols).

Fig. 1.

Composition of E-cigarettes and aerosols

2.3. Health concerns and toxicants

Despite being marketed as a safer alternative to smoking, e-cig use exposes individuals to significant levels of toxicants, including heavy metals that increase cancer risk. Some research also indicates that e-cig use is associated with respiratory injury, altered breath-control [20], increased arterial stiffness, and an altered response to hypoxia [21], potentially leading to peripheral tissue hypoxia, immune evasion and reactivation of disseminated tumor cells. E-cig use can also lead to various oral health issues, including mouth and throat irritation, periodontal disease, dental damage, and changes in the oral microbiome. Some components of e-cig vapor have cytotoxic, genotoxic, and carcinogenic properties [22, 23]. Most importantly, e-cig use has been associated with acute lung toxicity, with over 2800 cases reported in the USA, some of which resulted in death [24]. E-cigarette use is associated with an increased risk of lung cancer (LCa) and bladder cancer (BCa), although the risk is lower compared to traditional smoking. E-cigarette users are diagnosed with BCa at a younger age compared to non-users [25]. Emerging evidence in animal models suggests that e-cig inhalation enhances the development and progression of carcinoma and/or premalignant lesions in the lung and bladder, probably through increased DNA damage [26]. E-cigarette users have a lower overall prevalence of cancer compared to traditional smokers. However, specific cancers such as cervical cancer, leukemia, non-melanoma skin cancer, other skin cancers, and thyroid cancer show a higher prevalence among e-cigarette users compared to traditional smokers [27]. Our group is the first to assess the effects of e-cig on tumor metastasis; our results from animal models point to a metastasis-promoting effect of e-cig on breast carcinoma [28], the recent study also found that exposure to the main components of electronic cigarettes, PG/VG and nicotine, significantly promotes tumor invasion, metastasis, and immunosuppression, while also increasing immune checkpoint markers like CTLA4 and PD-1, although these effects could be mitigated through immune checkpoint blockade therapy. This suggests that, while not direct carcinogens, these components of e-cigarettes pose significant risks by accelerating tumor progression and impairing antitumor immunity [29]. While other groups found that compared with tobacco-Cig condensate exerts, e-Cig condensate has less cytotoxic effects on 16HBE cells and less influence on exosomal protein expression, suggesting a lower potential for tumor development and metastasis. Although e-cig are often marketed as safer alternatives to traditional smoking, they pose several health risks, particularly to cardiovascular, respiratory, and oral health. Further research is needed to fully understand the long-term health implications of e-cig use, especially for tumor initiation and metastasis [30].

3. Mechanisms and impact of E-cigarette on metastasis

3.1. Dissemination

Dissemination is the initial stage of metastasis, where tumor cells detach from the primary tumor, invade surrounding tissues, and enter the bloodstream or lymphatic system to travel to distant sites [31, 32]. E-cigarettes have been shown to impact various cellular processes and signaling pathways that may facilitate this stage, including epithelial-mesenchymal transition, lymphangiogenesis, and stemness, as well as cancer cell plasticity.

3.1.1. Epithelial-mesenchymal transition (EMT)

Epithelial-Mesenchymal Transition (EMT) is a well-studied process in cancer development characterized by dynamic changes in cellular organization. It involves the loss of cell polarity and downregulation of epithelial cell adhesion molecules, leading to enhanced migratory and invasive capabilities that facilitate metastasis in vivo [33, 34]. In vitro studies have demonstrated that e-cig exposure can induce EMT in various cancer cell types. For example, long-term exposure of A549 lung adenocarcinoma cells to e-cig aerosols resulted in a fibroblast-like morphology, loss of cell–cell junctions, and upregulation of EMT markers, accompanied by the nuclear translocation of active β-catenin [35]. Additionally, nicotine, a key component of e-cig, has been shown to promote EMT in pancreatic and non-small cell lung cancer cells via miRNA-mediated pathways [36, 37]. In an orthotopic mouse tongue cancer model, in vivo nicotine treatment promoted cervical lymph node metastasis of implanted oral squamous cell carcinoma (SCC) cells. This treatment downregulated E-cadherin expression and upregulated EMT markers vimentin and Snail [38]. An in vitro study also showed that e-cigarette liquid (e-liquid) induces gene expression changes indicative of EMT, evidenced by decreased levels of epithelial markers like E-cadherin and increased levels of mesenchymal proteins such as vimentin and β-catenin in both OSCC cell lines and normal oral epithelium cells [39].

3.1.2. Lymphangiogenesis

Accumulating evidence indicates that cancer cells can stimulate the development of lymphatic vessels within and near tumors in a process known as tumor-induced lymphangiogenesis, thereby facilitating the dissemination of tumor cells to lymph nodes (LN) [40]. Retrospective studies have shown that a history of smoking is associated with LN metastasis status at diagnosis and post-therapy in patients with esophageal, oral, lung, and cutaneous cancers [41–44]. In a recent study of esophageal cancer, it was demonstrated that nicotine exposure decreases the expression of OUT domain-containing protein 3 (OTUD3). OTUD3 directly interacts with ZFP36 ring finger protein (ZFP36) and stabilizes it by inhibiting FBXW7-mediated K48-linked polyubiquitination. ZFP36 binds to the VEGF-C 3′ untranslated region (UTR) and recruits the RNA degrading complex, leading to rapid mRNA decay. The downregulation of OTUD3 and ZFP36 is crucial for nicotine-induced VEGF-C expression, lymphangiogenesis, and subsequent lymphatic metastasis of esophageal cancer [45]. The available research indicates that e-cigarette flavors, particularly menthol, tobacco, and cinnamaldehyde, can induce inflammatory responses, cytotoxicity, and endothelial dysfunction. These effects are likely to impair lymphangiogenesis by disrupting the function of lymphatic endothelial cells and altering the immune environment [46–49]. Further studies are needed to fully understand the specific mechanisms by which e-cigarette flavors, or other specific e-cigarette components influence lymphangiogenesis.

3.1.3. Stemness and cancer cell plasticity

The current understanding of tissue stem cells extends beyond cancer cells to encompass a state that many cells can enter and exit through phenotypic plasticity during tissue regeneration [50, 51]. Stemness differentiation of cancer cells is a prime example of cancer cell plasticity, playing a crucial role in all stages of metastasis: dissemination, dormancy, and colonization. In mouse models of lung adenocarcinoma, a high-plasticity cell state with multilineage differentiation potential has been identified as a precursor to metastasis [52]. Additionally, the reacquisition of more developmentally primitive transcriptional gene programs has been observed in metastasis of both mouse and human lung adenocarcinoma [53].

Recently, Schaal et al. demonstrated that both e-cig vapor and nicotine can enhance the self-renewal capabilities of a subset of lung adenocarcinoma cells enriched in stem-like cell populations. This enhancement is accompanied by upregulation of classic embryonic stem cell transcription factors such as Sox2, Oct4, and Nanog, likely mediated through the induction of c-Kit ligand/Stem Cell factor [54].

In summary, compelling experimental evidence indicates e-cig as an important factor to promote metastasis by inducing epithelial-mesenchymal transition (EMT), enhancing cancer cell stemness and plasticity, and increasing lymphangiogenesis, all of which facilitate tumor cell dissemination to distant sites.

3.2. Dormancy and colonization

Dormancy refers to a period when disseminated tumor cells (DTCs) remain in a quiescent state, evading immune surveillance and resisting therapeutic interventions. Reactivation of dormant cells can lead to metastatic outgrowth and disease relapse. Cigarette exposure may influence the tumor microenvironment (TME) to support dormancy and subsequent reactivation. Colonization is the final stage of metastasis, where DTCs exit dormancy, proliferate, and establish secondary tumors in distant organs. Cigarette exposure has been linked to changes in the TME that facilitate this process.

3.2.1. Tumor microenvironment (TME)

The TME is composed of extracellular matrix (ECM), stromal cells, and immune cells, including tumor-associated macrophages (TAMs). TAMs play a crucial role in tumor progression, metastasis, and immunosuppression. In human breast cancer (BC), TAM infiltration is associated with poor prognosis and metastatic disease. Single-cell RNA sequencing of 165,753 cells from 22 treatment-naive lung adenocarcinoma (LUAD) patients revealed key differences in the TME between never-smokers and smokers. In smokers, the dysfunction of alveolar cells significantly contributes to the aggressiveness of LUAD, while in never-smokers, an immunosuppressive microenvironment plays a more prominent role. The study also identified SPP1hi pro-macrophages as an independent source of monocyte-derived macrophages. Additionally, higher expression of immune checkpoint CD47 and lower expression of MHC-I in cancer cells of never-smoker LUADs suggest that CD47 may be a more effective immunotherapy target for lung cancer in never-smokers (LCINS). This study highlights the distinct tumorigenesis mechanisms between never-smoker and smoker LUADs and proposes a potential immunotherapy strategy for LCINS [55].

Some research indicates that e-cigarette use is associated with increased arterial stiffness and an altered response to hypoxia, potentially leading to adverse cardiovascular outcomes [21]. Additionally, chronic intermittent hypoxia can induce serotonin-dependent changes in breathing control [56], which may have implications for e-cigarette users experiencing intermittent hypoxic conditions. It is also a common feature of the TME in solid tumors and has been implicated in promoting tumor progression, metastasis and resistance to therapy. In the TME, hypoxia leads to decreased proliferation and cytotoxicity of immune cells, such as CD8 T cells and Natural Killer (NK) cells, contributing to an immunosuppressive environment [57–59]. At the same time, hypoxia can promote immune escape by creating a physical barrier and altering the TME to inhibit immune cell infiltration and function [58, 60]. It also contributes to resistance against immune checkpoint inhibitors by fostering an immune suppressive TME [61]. As the outcomes of the e-cig exposure, hypoxia is closely associated with an immunosuppressive TME, contributing to immune cell dysfunction, immune escape, and resistance to immunotherapies. Targeting hypoxia within the TME holds promise for enhancing the efficacy of cancer immunotherapies and improving patient outcomes.

3.2.2. Immune evasion of dormant tumor cells

PD-L1, mainly expressed on tumor cells and TME TAMs, myeloid-derived suppressor cells and DCs, is a ligand for the inhibitory immune checkpoint receptor PD-1. The PD-L1-PD-1 interaction inhibits T cell activity [62]. Some investigators have found that smoking is associated with elevated PD-L1 expression in tumors, showing a dose-dependent relationship [63, 64]. It is hypothesized that tobacco carcinogens, such as benzo[a]pyrene (BAP), may play a role in upregulating PD-L1 expression [63]. With respect to the impact of e-cig exposure on BAP, one study found that e-cig aerosol condensate (EAC) can increase the metabolism of BAP to genotoxic products. EAC can activate the aryl hydrocarbon receptor (AhR), inducing CYP1A1 and CYP1B1 mRNA and protein, and increasing the rate of conversion of PAHs to genotoxic products [65]. Remarkably, e-cig emissions reduce BAP by 99.7% compared to cigarette smoke [66]. Therefore, further research is needed to investigate the relationship between e-cig effects on PD-L1 function and expression.

3.2.3. Reactivation of dormant tumor cells

In murine metastasis models, cancer cells have been shown to induce neutrophils to produce neutrophil extracellular traps (NETs), which support metastasis. These NETs stimulate breast cancer cell invasion, migration, and lung metastasis [67]. Factors such as tobacco smoke have been shown to trigger reactivation of dormant DTCs [68–70]. E-cig exposure can alter the TME, making it more conducive to tumor cell survival and reactivation. Nicotine exposure skews macrophages and microglia towards a pro-tumor M2 phenotype, enhancing the secretion of factors like IGF-1 and CCL20, which promote tumor cell survival and reactivation [71]. Chronic nicotine exposure promotes the formation of pre-metastatic niches by recruiting pro-tumor neutrophils, facilitating the survival and colonization of disseminated cancer cells [72]. Additionally, e-cig exposure also promotes the pro-Inflammatory changes in TME by activating tumor cells. For instance, treatment of oral SCC with e-cig vapor extracts resulted in increased cell invasion and upregulation of inflammatory markers like NF-kB, TNF-α, and MMP-13 [73]. These changes in the TME can promote the establishment and growth of metastatic tumors.

The key role of TAM or its relatives in e-cig-promoted tumor metastasis has been suggested by a series of recent studies including results from our own lab. Nicotine has been shown to enhance brain metastasis by promoting microglial activation and reducing phagocytic ability, aiding tumor cell colonization [71]. In an earlier publication, we showed that e-cigs promote human breast cancer cell mobility in vitro and lung colonization in vivo [74]. One of our other publications also indicated that e-cig exposure significantly promoted lung metastasis of xenograft breast carcinoma in all animals (6/6) compared to the control cohort (2/6). During metastasis, e-cig exposure aids the survival and colonization of disseminated breast cancer cells through interactions with infiltrated macrophages, regulated by VCAM-1 and integrin α4β1 [28]. Our latest data also showed that subcutaneous oral SCC (MOC-2) injection mouse models exposed to e-cig exhibit increased cancer cell migration to lung tissues (6/16) compared to air exposure (1/16). In this study, we generate an immunocompetent mouse model of HNSCC in which the mouse oral squamous cell carcinoma (OSCC) cell line, Moc-2, has been orthotopically transplanted subcutaneously into C57BL/6 wild-type mice. 7 days after the Moc-2 injection, the mice were exposed to the e-cig by using the InExpose rodent exposure system (2 h/day, 5 days/week, 50% PG and 50% VG, 36 mg/ml nicotine salt) for 20 days, the filtered air exposure as the control group. e-cig significantly aggravate tumor growth, necrosis and skin ulcer formation in exposed mice induced by Moc-2 tumor cell injection. e-cig exposed subcutaneous Moc-2 tumors and their associated lung metastasis colonies contain more TAM compared to the air control (Fig. 2). These result parallel findings from our published study where breast cancer mouse models treated with e-cig also displayed cancer cell migration to lung [28].

Fig. 2.

The impact of E-cig exposure on lung metastasis of Moc-2 mouse oral squamous cell carcinoma cancer cell. E-cig exposure promote metastatic colonies in lung parenchyma, as well as F4/80 positive macrophage number in tumor microenvironment (Unpublished data). N = 16 for each group, two-tailed T test, * < 0.05

4. Epigenetic modifications of E-cigarette exposure

4.1. Epigenetic mechanisms in metastasis

Epigenetic modifications, such as DNA methylation, histone alterations, and ncRNA interactions, play a central role in cancer metastasis. These changes facilitate key processes such as EMT, invasion, and colonization of distant tissues [75]. The advent of CRISPR-based epigenomic editing tools has provided new opportunities to dissect and manipulate these alterations, offering insights into their roles and potential as therapeutic targets [76]. The DNA methylome of primary breast cancers contributes to transcriptomic heterogeneity and different metastatic behaviors [77]. Epigenetic and post-translational modifications, including E-cadherin repression via promoter hypermethylation and histone modifications, are important in regulating the epithelial-mesenchymal transition (EMT) [78]. Post-translational modifications of histones, such as methylation and acetylation, alter chromatin structure and gene expression. For example, EZH2-mediated H3K27 trimethylation suppresses metastasis-suppressing genes [79–81]. LncRNAs and microRNAs are also involved in post-transcriptional regulation of gene expression. LncRNAs can regulate STAT3 signaling, while miRNAs modulate STAT3 expression, which are important in affecting cancer cell proliferation and metastasis [82].

4.2. Impact of E-cig on epigenetic modifications

Although, there is no direct evidence to show that e-cig influence the metastatic phenotype via epigenetic modifications, both e-cig and traditional tobacco exposure have been shown to induce significant epigenetic changes [83]. For instance, IGFBP-3, a tumor suppressor gene that is often suppressed in lung cancer, was downregulated by NNK (a tobacco-specific carcinogen, which also found in e-cig) treatment via DNA methylation. This suppression of IGFBP-3 in BEAS-2B normal lung epithelial cells finally led to increased cell proliferation [84]. Cigarette smoke affects EZH2 expression and reduces DAB2IP via H3K27me3 in COPD patients, promoting the progression of airway inflammation toward lung cancer [85]. Changes of cigarette smoked induced DNA methylation are most extensively documented for genes including AHRR, F2RL3, DAPK, and p16. Similarly, miRNAs like miR16, miR21, miR146, and miR222 were identified to be differentially expressed in smokers and exhibit potential as biomarkers for determining susceptibility to COPD. In our previous study, long term exposure to e-cig aerosols in mouse also led to global WBCs DNA methylation changes, which may affect MAPK signaling pathway [86]. Furthermore, our latest data also show that e-cig exposure up-regulates a series of histone demethylases including KDM6B in macrophages (Fig. 3), which may influence the TME and reactivation of dormant tumor cells at distant metastatic niche.

Fig. 3.

E-cigarette exposure can influence KDM6B mRNA expression in macrophages. We treated mouse macrophage cells (Raw 264.7) with e-cigarette condensate medium, with and without the KDM6B inhibitor GSK-J4. After 48 h, cells were collected, and RNA was isolated for RT-qPCR analysis of target gene expression. A E-cigarette exposure upregulates histone demethylase KDM6B in Raw 264.7 cells. B The KDM6B inhibitor GSK-J4 reverses the pro-inflammatory cytokine IL6 upregulation induced by e-cigarette exposure (unpublished data). N = 4, two-tailed T test

In summary, e-cig generate cellular stress with the potential to induce epigenetic changes; when these changes affect the EZH2-mediated metastasis-suppressing genes [85], or foster EMT [78], the risk of metastasis may rise(Fig. 4).

Fig. 4.

Epigenetic modifications of E-cigarettes exposure and related cancer metastasis

5. Public health implications

5.1. Impact of E-cigarette on cancer metastasis

The evidence presented in this mini-review suggests that e-cig use can promote various mechanisms that facilitate tumor metastasis, including enhancing stemness, inducing EMT, promoting lymphangiogenesis, and altering the TME. These findings underscore the potential public health risks associated with e-cig use, particularly for individuals at risk of or currently battling cancer. The widespread use of e-cig, especially among young adults and teenagers, raises significant public health concerns. The potential for e-cig to exacerbate cancer metastasis adds to the growing list of health risks associated with vaping. Public health initiatives should focus on educating the public about these risks and implementing policies to reduce e-cig use.

5.2. Recommendations for research and policy

Given the growing prevalence of e-cig use and the emerging evidence of its impact on cancer metastasis, further research is urgently needed to fully understand the long-term effects of e-cig exposure on cancer progression. Studies should focus on elucidating the molecular mechanisms underlying these effects and exploring potential preventative/therapeutic targets to mitigate the risks associated with e-cig use. Additionally, regulatory policies should be developed to address the potential health risks of e-cig, including stricter regulations on e-cig marketing, especially towards youth, and increased public awareness of the potential dangers of vaping. Research should also investigate the effects of different e-cig components and flavors on cancer metastasis. Understanding the specific toxicants and mechanisms involved can inform the development of safer alternatives and targeted interventions. Collaboration between researchers, healthcare providers, and policymakers is essential to address the public health challenges posed by e-cig.

6. Conclusion

This mini-review has summarized the current literature on the impact of e-cig exposure on tumor metastasis, highlighting the various mechanisms through which e-cig can promote tumor spread and growth. E-cig exposure has been shown to enhance stemness, induce EMT, promote lymphangiogenesis, and modify the TME, all of which contribute to metastasis. Our preliminary data suggest that targeting epigenetic modifications, such as inhibiting KDM6B, could be a potential therapeutic strategy to counteract the pro-metastatic effects of e-cig exposure in TAMs (Fig. 3). As e-cig use continues to rise, understanding its effects on cancer progression is crucial for developing effective public health strategies and regulatory policies. Future research should focus on elucidating the molecular mechanisms underlying these effects and exploring potential preventative/therapeutic targets to mitigate the risks associated with e-cig use.

Data availability

No datasets were generated or analysed during the current study.