Table 2.
Major Findings and Toxicities in the 53 Included Studies
| Upper Airway Site | Study | Major Findings | Toxicity |
|---|---|---|---|
| Oral Cavity | Ji EH, et al12 | Dose-dependent decrease in intracellular GSH, decreased cell viability, and increased gene expression of HO-1 in oral keratinocytes exposed to flavored e-cigarette aerosols (+ nicotine) vs air controls. | Cytotoxicity |
| Franco T, et al13 | Decreased micronuclei in oral epithelial cells of e-cigarette users compared to cigarette smokers; no difference between non-smoking controls and e-cigarette users. | None | |
| Ganapathy V et al14 | Dose-dependent increase in DNA damage, increased intracellular ROS, decreased intracellular TAC, and decreased gene expression of the DNA repair proteins ERCC1 and OGG1 in oral keratinocytes exposed to flavored e-cigarette aerosol condensate (+/− nicotine) vs vehicle controls. Cigarette smoke exerted increased DNA damage but similar intracellular oxidative stress and protein expression changes compared to e-cigarettes | Genotoxicity, Cytotoxicity | |
| Tommasi S, et al15 | Significantly increased numbers of differentially expressed genes in oral epithelial cells of cigarette smokers and e-cigarette users (1726 genes, 1152 genes; respectively) vs non-smokers. IPA showed that “Cancer” was the top disease pathway associated with differentially expressed genes in both e-cigarette users and cigarette smokers. The most enriched canonical pathway was “Wnt/Ca+” in e-cigarette users, and “integrin signaling” in cigarette smokers. Oral epithelial cells from e-cigarette users and cigarette smokers exhibited downregulation of tumor suppressors NOTCH1 and HERC2, and upregulation of BCL-2 and its binding heat-shock protein HSPA1B compared to non-smokers. | Pro-carcinogenic | |
| Iskander AR, et al16 | No changes in histopathology in buccal epithelial cells exposed to e-cigarette aerosols (+/− nicotine) compared to air controls. Increased secretion of IL-1α, decreased secretion of IL-13, and increased expression of genes associated with “inflammatory process” network family in buccal epithelial cells exposed to e-cigarette aerosols (+/− nicotine) vs air controls. Cigarette smoke exposure resulted in buccal epithelial cell morphological changes, increased secretion of IL-1β, and increased number of differentially expressed genes compared to e-cigarettes. | Pro-inflammatory | |
| Ureña JF, et al17 | Increased intracellular ROS and decreased cell viability in SCC-25 cells that were exposed to Lava Flow flavored e-cigarette aerosols (+/− nicotine) vs air controls. No cytotoxicity seen following exposure to other flavored e-cigarette aerosols. | Cytotoxicity | |
| Tsai KYF, et al18 | Increased Ca9-22 invasion and decreased Cal-27 invasion following exposure to Red Hot flavored e-liquid (+/− nicotine) vs media-only treated controls. Decreased Ca9-22 invasion following exposure to Green Apple flavored e-liquid (+/− nicotine) vs controls. Increased RAGE expression and differential expression of secreted cytokines IL-1α, IL-8, MMP-13 in both cell lines following exposure to Red Hot or Green Apple. Nicotine addition to both e-liquid flavors enhanced RAGE expression, however reduced secretion of IL-1α, IL-8. | Pro-carcinogenic, Pro-inflammatory | |
| Pop AM, et al19 | Increased micronuclei in oral epithelial cells of e-cigarette users and cigarette smokers vs non-smoking controls; no difference between e-cigarette users and cigarette smokers. | Genotoxicity | |
| Guo J, et al20 | Significantly lower apurinic/apyramidinic sites in e-cigarette users vs nonsmokers and cigarette smokers. | None | |
| Muqawwi et al21 | Decreased number of micronucleated cells and micronucleous in oral epithelial cells of e-cigarette users and non-smoking controls vs cigarette smokers (no difference between e-cigarette users and non-smoking controls). | None | |
| Tellez CS, et al22 | Significant heterogeneity across different flavors; however, oral epithelial cells exposed to flavored e-cigarette aerosols (+/− nicotine) generally displayed increased intracellular oxidative stress, lipid peroxidation, cellular toxicity, and micronuclei formation compared to cells exposed to unflavored e-cigarette aerosols. Cigarette smoke exposure induced dose-dependent cytotoxicity, intracellular oxidative stress, and DNA damage, which exceeded any flavored or unflavored e-cigarette aerosol. | Cytotoxicity, Genotoxicity | |
| Schwarzmeier LAT, et. al23 | Oral epithelial cells from e-cigarette users displayed increased metanuclear abnormities as evidenced by: increased karolysis (vs never-smokers and former cigarette smokers), increased binucleation (vs never-smokers and former smokers), increased “broken egg” (vs cigarette smokers, never-smokers, and former cigarette smokers), and increased “nuclear bud” (vs never-smokers and former smokers). | Genotoxicity | |
| Hamad S, et. al24 | Increased expression of genes related to DNA damage (TP53, FEN1, TREX1, XRCC2, AIFM1) in oral epithelial cells following scripted vaping session. Specifically, TP53 expression was puff volume and flow rate dependent. IPA showed that “cancer”, “cell cycle”, and “DNA repair” pathways were the most enriched pathways following e-cigarette exposure. | Genotoxicity, Pro-carcinogenic | |
| Cátala-Valentín AR, et al25 | Oral epithelial cells exposed to e-cigarette condensate (+/− nicotine) showed a dose-dependent decrease in cell viability, increased COX2 expression, increased pro-inflammatory pERK1/2 and NF-kB nuclear translocation and signaling, increased immunofluorescence of DNA damage marker pH2AX, and reduced secreted cytokines (IL-8, IL-6, IL-1β) vs controls who were treated with media exposed to air. | Cytotoxicity, Genotoxicity, Pro-inflammatory | |
| Marinucci L et al26 | Oral epithelial cells exposed to e-cigarette aerosol condensate (+nicotine) displayed no changes in cell viability, morphology, apoptosis rate, or gene expression compared to media-treated controls. Oral epithelial cells exposed to cigarette smoke condensate showed significant alterations in cellular morphology, increased apoptosis rate and cellular migration vs media-treated controls. | None | |
| Cheng G et al27 | Oral epithelial cells from e-cigarette users displayed higher levels of acrolein-DNA adducts compared to non-smokers, but less than cigarette smokers. | Genotoxicity | |
| Robin H et al28 | Human oral squamous cell carcinoma cells exposed to Green Apple flavored e-cigarette aerosol condensate (+ nicotine) displayed increased protein expression of NF-kB, TNF-α, ERK, JNK, MMP-13 and increased cell invasion vs media-exposed controls. Human tongue squamous cell carcinoma cells exposed to Green Apple or Red Hot flavored-cigarette aerosol condensate (+ nicotine) showed increased protein expression of TNF-α and JNK vs media-exposed controls. | Pro-inflammatory, Pro-carcinogenic | |
| Reeve G et al29 | There were no statistically significant histological or transcriptomic changes in the oral epithelium of e-cigarettes users vs non-smokers. | None | |
| Mandour D et al30 | Albino rats exposed to nicotine-containing e-cigarette aerosols demonstrated widening of submandibular acini and ducts along with epithelial degeneration, cytoplasm vacuolization, connective tissue septa thickening, and increased TNF-α immunostaining compared to submandibular glands from air controls. | Histological, Pro-inflammatory | |
| Elmahdi F et al31 | E-cigarette users displayed higher rates of oral epithelial cytological atypia (4.8 vs 0.4%) and HPV infection rates (3.2 vs 0.8%) vs non-smokers. Among e-cigarette users, higher atypia rates and HPV infection rates were seen in individuals with a longer duration of e-cigarette use (> 7 years). | Histological, Immunosupression or Infectivity | |
| Tommasi S et al32 | Oral epithelial cells from e-cigarette users displayed a dose-dependent increase in DNA damage vs non-smoking controls. There was no difference in DNA damage levels between e-cigarette users and smokers. Nicotine content was not associated with increased DNA damage. Among e-cigarette flavors, fruit, sweet, and mint/menthol showed the highest amount of DNA damage. | Genotoxicity | |
| Muniz de Lima J, et al33 | Increased cytotoxicity in normal oral epithelial cells treated with e-liquid (+ nicotine) vs untreated control. Human and murine oral SCCa cells treated with e-liquid (+ nicotine) showed increased cell proliferation, cell migration and invasion, as well as gene expression changes consistent with an epithelial to mesenchymal transition when compared to untreated controls. | Cytotoxicity, Pro-carcinogenic | |
| Oropharynx | Welz C, et al34 | Increased cytotoxicity and DNA fragmentation in oropharyngeal epithelial cells exposed to e-cigarette condensates (fruit flavors > tobacco) vs controls. | Cytotoxicity, Genotoxicity |
| Nasal Cavity/ Nasopharynx | Martin EM, et al35 | Decreased expression of immune-related genes (ZBTB16, PIGR, PTGS2, FKBP5) and transcription factors (NFKB1, ETS1, NOTCH1, BCL3, XBP1, BCL6, EGR1) in nasal epithelial cells from current e-cigarette users and cigarette smokers compared to non-smokers (> suppression in e-cigarette users vs. cigarette smokers). Top IPA canonical pathways in both e-cigarette users and cigarette smokers included: “cytokine-cytokine receptor interaction,” “apoptosis,” “Toll-like receptor signaling pathway,” and “NOD-like receptor signaling pathway.” | Immunosuppression or Infectivity, Pro-inflammatory |
| Kumral TL, et al36 | After 3 months of cigarette smoking cessation, both e-cigarette users and non-smokers displayed lower SNOT-22 scores vs baseline (SNOT-22 of non-smokers < e-cigarette users), whereas after 3 month of smoking cessation, only non-smokers had significantly lower mucociliary clearance times. | Ciliary dysfunction | |
| Carson JL, et al37 | Human nasal epithelial cells exposed to cigarette smoke or e-cigarette aerosols displayed similar declines in ciliary beat frequency with gradual return of function within 1 hour. Nitric oxide production was far greater in nasal epithelial cells exposed to cigarette smoke vs e-cigarette aerosol or unexposed controls. Following exposure to e-cigarette aerosols, nasal epithelial cells displayed secretory material overlying ciliary beds indicating mucous hypersecretion; however, this finding was much more prominent in nasal epithelial cells exposed to cigarette smoke. | Histological, Ciliary dysfunction | |
| Miyashita L et al38 | Nasal epithelial PAFR expression increased following vaping session in e-cigarette users. Pneumococcal adhesion increased in human nasal epithelial cells following exposure to e-cigarette aerosol condensate (+/− nicotine) compared to vehicle controls. In CD1 mice, e-cigarette aerosol condensate (+ nicotine) exposure increased nasal PAFR expression and nasopharyngeal pneumococcal colonization vs vehicle controls. | Immunosupression or infectivity | |
| Jabba S et al39 | Human nasal epithelial cells exposed to the benzaldehyde-propylene glycol adduct (berry/fruit e-cigarette flavoring) showed increased cell death vs benzaldehyde exposure alone. | Cytotoxicity | |
| Rouabhia M, et al40 | Nasal epithelial cells exposed to e-cigarette aerosol (+/− nicotine) or cigarette smoke showed increased LDH levels, fewer Ki67+ cells, reduced cellular proliferation rate, and increased secretion of IL-6, IL-8, TNF-α, and MCP-1 compared to controls (Magnitude of effects: cigarette smoke > nicotine-rich e-cigarette aerosols > nicotine-free e-cigarette aerosols). | Pro-inflammatory, Cytotoxicity | |
| Rebuli ME, et al41 | Compared to non-smokers, e-cigarette users and cigarette smokers showed suppression of live attenuated influenza vaccine (LAIV)-induced nasal immune response as evidenced by decreased gene expression, decreased cytokine/chemokine secretion, and decreased LAIV-specific IgA levels (Magnitude of effects: e-cigarette users > cigarette smokers > non-smokers) | Immunosuppression or infectivity | |
| Escobar YN et al42 | Human nasal epithelial cells from non-smokers exposed to e-cigarette aerosols (+/− nicotine) demonstrated increased protein expression of MUC5AC vs air controls. Human nasal epithelial cells from non-smokers exposed to e-cigarette aerosol (+ nicotine) showed elevated MUC5AC and MUC5B protein levels vs air controls. Human nasal epithelial cells from smokers exposed to e-cigarette aerosols (+/− nicotine) showed elevated pro-inflammatory cytokine secretion. | Pro-inflammatory | |
| Kwak S et al43 | Human nasal epithelial cells exposed to the e-cigarette compounds (glyoxal or methylglyoxal) demonstrated increased secretion of IL-1β, IL-6, MUC5AC, MUC5B, and increased activation of nuclear factors (ERK1/2, p38, and NF-κB) compared to culture-exposed controls. | Pro-inflammatory | |
| Hinds D et al44 | Golden Syrian mice exposed to 2 days of e-cigarette aerosols displayed nicotine-dependent increases in nasal epithelial gene expression of CCL-5, CXCL-10, TNF-α, IL-1β, TGF-β, and nicotine-independent increases in gene expression of SOD-2 and decreases in gene expression of VEGF vs air controls. There were no changes in the histology of the nasal epithelium of e-cigarette aerosol exposed mice compared to air controls. | Pro-inflammatory | |
| Karey E et al45 | Pro-inflammatory cytokine levels (IL-2, IL-4, IL-8, IL-10, IL-12p70, IL-13, TNF-α, IL-1β, IFN-γ) were significantly increased in the nasal fluid of e-cigarette users compared to cigarette users and non-smoking controls. | Pro-inflammatory | |
| Pozuelos et al46 | Significantly increased number of differentially expressed genes in primary human nasal epithelial cells of cigarette smokers and e-cigarette users (407 genes, 1817 genes; respectively) vs non-smokers. Enriched gene ontology terms for the downregulated genes between e-cigarette users and non-smokers included “cilium assembly and function” and for the upregulated genes included “immune response”, “neutrophil activation”, “leukocyte degranulation”, and “granulocyte activation”. | Pro-inflammatory, Ciliary dysfunction, Immunosupression or Infectivity | |
| Nicholas BD, et al47 | BALB/cJ mice exposed to 8 weeks of e-cigarette aerosols displayed decreased goblet cells in Eustachian tube mucosa vs controls (no significant differences in cilia, mucin, and squamous metaplasia). In BALB/cJ mice exposed to e-cigarette aerosol, trans-tympanic application of anti-IL-13 or AG1478 led to increased goblet cells in Eustachian tube mucosa. | Histological | |
| Larynx | Salturk Z, et al48 | Wistar albino rats exposed to 4 weeks of e-cigarette aerosols (+ nicotine) displayed increased vocal fold hyperplasia and metaplasia, however results were not statistically significant. | None |
| Ha TN, et al49 | C57BL/6 mice exposed to 16 weeks of e-cigarette aerosols (+ nicotine) or cigarette smoke displayed significantly elevated levels of IL-4 in larynx homogenates compared to control mice exposed to air or e-cigarette aerosols without nicotine. | Pro-inflammatory | |
| Hassan NH, et al50 | Wistar albino rats exposed to 4 weeks of e-cigarette aerosols (+ nicotine) displayed increased MDA levels and decreased TAC levels in laryngeal tissue homogenates vs control rats exposed to air. Histologically in the larynx, e-cigarette-exposed rats showed increased cilia loss, increased epithelial hyperplasia, increased vascular dilation of the lamina propria, increased p53 immunostaining, and decreased Ki67 immunostaining compared to control rats exposed to air. | Cytotoxicity, Histological, Ciliary dysfunction | |
| Martinez JD, et al51 | Human vocal fold fibroblasts (hVFFs) exposed to e-cigarette condensate (+/− nicotine) showed decreased cell viability vs untreated controls. Compared to e-cigarette condensate, hVFFs exposed to cigarette smoke condensate showed a greater reduction in cell viability and increased DNA damage. Gene expression was not significantly altered in hVFFs following either e-cigarette or cigarette smoke condensate exposure. | Cytotoxicity, Genotoxicity | |
| Easwaran M et al52 | There were no differences in vocal fold epithelial thickness, cellular proliferation, glandular surface area, surface topography between C57BL6/J rats exposed to flavored e-cigarette aerosol (+ nicotine) or air controls. There was a slight increase in acidic mucus content in the subglottis and increase in macrophage and CD3+ T cell infiltration in the vocal cords of e-cigarette exposed mice vs air controls. | Histological | |
| Trachea | Wu Q et al53 | Tracheobronchial epithelial cells exposed to tobacco flavored e-liquid (+/− nicotine) showed increased IL-6 secretion, enhanced human rhinovirus infection, and decreased SPLUNC1 expression. There was no change in cytotoxicity of tracheobronchial epithelial cells exposed to flavored e-liquid (+/− nicotine) at 24 or 48 hours post exposure. | Pro-inflammatory, Immunosupression or Infectivity |
| El-Merhie et al54 | Maternal Drosophilia Melanogaster flies exposed to nicotine-containing e-cigarette aerosols prior to mating produced offspring with abnormalities in tracheal length compared to sham controls. | Other | |
| Multisite | Manyanga J, et al55 | E-cigarette (+/− nicotine) or cigarette smoke condensate exposure in various oral cavity and oropharyngeal cancer cell lines reduced cisplatin-induced cytotoxicity, reduced expression of DNA repair genes MMS19 and ERCC1, decreased expression of drug influx protein CTR1, and increased expression of drug efflux proteins ABCG2 and ATP7A compared to cells exposed to saline treated controls. | Pro-carcinogenic, Genotoxicity |
| Werley MS, et al56 | Sprague-Dawley rats chronically exposed to 13 weeks of e-cigarette aerosols (+/− nicotine or flavor) displayed increased nasal secretions, nasal mucous cell hyperplasia and epithelial vacuolization, and intraluminal mucin exudate in the larynx; histological changes did not resolve following 42-day recovery period. | Histological | |
| Phillips B, et al57 | Sprague-Dawley rats chronically exposed to 13 weeks of e-cigarette aerosols (+ nicotine) displayed mild adaptive squamous cell metaplasia and basal cell hyperplasia in the larynx vs air controls. No statistically significant histological changes in nasal epithelium. There were no changes in nasal epithelial gene or protein expression between rats exposed to e-cigarettes or filtered air. | Histological | |
| Lee KM, et al58 | C57BL/6 exposed to 13 days of e-cigarette aerosols had a minimal increase in microscopic squamous metaplasia of the nasal turbinates and epiglottis compared to sham controls. All histological findings were significantly more severe in cigarette smoke-exposed group. | Histological | |
| Ho J, et al59 | Sprague-Dawley rats exposed to 13 weeks of e-cigarette aerosols (+ nicotine) displayed mild adaptive squamous cell metaplasia and lamina propria infiltration in the larynx. No statistically significant histological changes in nasal epithelium. No biologically significant gene expression differences in nasal epithelium of rats exposed to e-cigarettes aerosols vs PBS vehicle controls. | Histological | |
| Ni F, et al60 | C57BL/6 mice exposed to flavored e-cigarette aerosols (+/− nicotine) displayed increased numbers of stimulated nociceptive neurons (nasal cavity > trachea) vs air controls. | Other | |
| Wong ET et al61 | ApoE−/− mice exposed to 6 months of e-cigarette aerosols (+/− nicotine) displayed no differences in nasal or tracheal histology and only minimal squamous metaplasia at the epiglottis compare to the sham control group. All histological findings were significantly more severe in cigarette smoke-exposed group. E-cigarette exposed mice showed no significant changes in gene expression in the nasal epithelium or trachea following exposure. | Histological | |
| Kim M, et al62 | Reduced nasal CFTR function was seen in human subjects after 7 days of vaping vegetable glycerol (VG)-containing e-cigarettes vs baseline. Primary human nasal epithelial cells exposed to VG-containing e-liquid displayed increased secretions of pro-inflammatory mediators (IL-6, IL-8, MMP-2, MMP-9, TGF-β) vs baseline. Tracheal secretions from sheep exposed to 5 days of VG-containing e-cigarette aerosols showed increased mucus concentrations and MMP-9 activity vs baseline | Pro-inflammatory, Other | |
| Wong ET, et al63 | A/J mice exposed to 5 weeks of flavored e-cigarette aerosols (+ nicotine) displayed a mild adaptive increase in nasal and laryngeal epithelial hyperplasia and laryngeal squamous metaplasia. No significant histological changes in trachea. All histological findings were significantly more severe in cigarette smoke-exposed group. | Histological | |
| Desai R, et al64 | Sprague-Dawley rats exposed to 13 weeks of e-cigarette aerosols (+ nicotine) displayed mild squamous cell metaplasia in the larynx vs air controls. No statistically significant histological changes were seen in the nasal epithelium. All histological findings were significantly more severe in cigarette smoke-exposed group | Histological |
Abbreviations: GSH, glutathione; ROS, reactive oxidative species, TAC, total antioxidant capacity; IPA, Ingenuity pathway analysis; RAGE, receptor for advanced glycation end-products; HPV, human papilloma virus; SNOT-22, Sino-nasal Outcome Test; MDA, malondialdehyde