Dual use of combustible and electronic cigarettes progressively disrupts tight junctions and elevates oxidative stress, leading to compromised blood–brain barrier integrity

Despite the adverse effects of combustible (C-cig) and electronic cigarettes (E-cig) being well studied, their combined impact as dual product use on the blood-brain barrier (BBB) remains underexplored. This study uses both in vitro and in vivo models to examine the effect of dual use of C-cig and E-cig products on BBB integrity. For short (24 h) and prolonged (5 d) duration, brain endothelial cell (bEnd.3) and primary astrocytes were exposed to C-cig, E-cig, and three dual combinations. Assessments included cell viability, sodium fluorescein (NaF) permeability assay across monolayer, astrocyte-conditioned media, and co-culture models, western-blot analysis of tight junction (TJ) proteins (zonula occludens-1 (ZO-1), occludin, and claudin-5), and antioxidative markers (NAD(P)H quinone dehydrogenase 1 (NQO1), heme oxygenase-1 (HO-1), and superoxide dismutase 2 (SOD2)). In vivo, male mice (C57BL/6) were acutely exposed (7 d), and outcomes included changes in body weight, plasma nicotine concentration using LCMS/MS, western-blot analysis of TJ proteins, and cytokine profiles. A significant increase in the NaF permeability was observed with Dual 1 exposure (1:1 C-cig:E-cig ratio), with significant downregulation of ZO-1 after short and claudin-5 expression after prolonged exposure duration. Dual exposure groups also elevated NQO1 and HO-1 levels, indicating a shift in oxidative stress, while SOD2 levels remained unchanged. In vivo, dual use resulted in weight loss, reduced ZO-1 expression, elevated plasma nicotine concentration, and an increase in proinflammatory cytokines (IL-13, KC). Dual use of C-cig and E-cig is often misinterpreted as a safer alternative due to perceived reduction in C-cig use. Our data indicate that this might not be the case, as dual use, particularly with a 1:1 ratio, significantly alters BBB integrity.