Flavored e-liquids increase cytoplasmic Ca ²⁺ levels in airway epithelia

Abstract

E-cigarettes are noncombustible, electronic nicotine-delivery devices that aerosolize an e-liquid, i.e., nicotine, in a propylene glycol-vegetable glycerin vehicle that also contains flavors. While the effects of nicotine are relatively well understood, more information regarding the potential biological effects of the other e-liquid constituents is needed. This is a serious concern, because e-liquids are available in >7,000 distinct flavors. We previously demonstrated that many e-liquids affect cell growth/viability through an unknown mechanism. Since Ca 2+ is a ubiquitous second messenger that regulates cell growth, we characterized the effects of e-liquids on cellular Ca 2+ homeostasis. To better understand the extent of this effect, we screened e-liquids for their ability to alter cytosolic Ca 2+ levels and found that 42 of 100 flavored e-liquids elicited a cellular Ca 2+ response. Banana Pudding (BP) e-liquid, a representative e-liquid from this group, caused phospholipase C activation, endoplasmic reticulum (ER) Ca 2+ release, store-operated Ca 2+ entry (SOCE), and protein kinase C (PKCα) phosphorylation. However, longer exposures to BP e-liquid depleted ER Ca 2+ stores and inhibited SOCE, suggesting that this e-liquid may alter Ca 2+ homeostasis by short- and long-term mechanisms. Since dysregulation of Ca 2+ signaling can cause chronic inflammation, ER stress, and abnormal cell growth, flavored e-cigarette products that can elicit cell Ca 2+ responses should be further screened for potential toxicity.