Formaldehyde, a common indoor air pollutant, exacerbates asthma and synergizes with allergen to induce airway hyperresponsiveness (AHR) in animal models. The mechanisms mediating formaldehyde-induced AHR remain poorly understood. We posit that formaldehyde modulates agonist-induced contractile response of human airway smooth muscle (HASM) cells to elicit AHR. HASM cells were exposed to formaldehyde or vehicle and agonist-induced intracellular Ca²⁺ ([Ca²⁺]_i) and myosin light-chain phosphatase (MYPT1) phosphorylation were determined. Air-liquid interface-differentiated human bronchial epithelial (HBE) cells were exposed to formaldehyde or vehicle and cocultured with HASM cells. Agonist-induced [Ca²⁺]_i and MYPT1 phosphorylation were determined in the cocultured HASM cells. Precision-cut human lung slices were exposed to PBS or varying concentrations of formaldehyde, and then carbachol-induced airway narrowing was determined 24 hours after exposure. HASM cells were transfected with nontargeting or nuclear factor erythroid-derived 2, like 2 (Nrf-2)-targeting small interfering RNA and exposed to formaldehyde or vehicle, followed by determination of antioxidant response (quinone oxido-reductase 1 and thioredoxin 1) and basal and agonist-induced MYPT1 phosphorylation. Formaldehyde enhanced the basal Rho-kinase activity and MYPT1 phosphorylation with little effect on agonist-induced [Ca²⁺]_i in HASM cells. Formaldehyde induced Nrf-2-dependent antioxidant response in HASM cells, although the MYPT1 phosphorylation was independent of Nrf-2 induction. Although HBE cells exposed to formaldehyde had little effect on agonist-induced [Ca²⁺]_i or MYPT1 phosphorylation in cocultured HASM cells, formaldehyde enhanced carbachol-induced airway responsiveness in precision-cut human lung slices. In conclusion, formaldehyde induces phosphorylation of the regulatory subunit of MYPT1, independent of formaldehyde-induced Nrf-2 activation in HASM cells. The findings suggest that the Rho kinase-dependent Ca²⁺ sensitization pathway plays a role in formaldehyde-induced AHR.

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