The pulmonary epithelium separates the gaseous intraluminal space of the airways and the aqueous interstitium. This compartimentalization is required for appropriate lung function, it is established during perinatal periods and can be disturbed in lung edema. Herein we elaborated the impact of the air-liquid interface (ALI) on the function of the pulmonary epithelium. We used NCI-H441 epithelia as a well-established and characterized model of distal airway epithelia, which were cultivated either at ALI or (at submerged conditions) at liquid-liquid interface conditions (LLI). Our study revealed that paracellular permeability was increased and claudin 1 (CLDN1) expression levels were reduced under LLI conditions. This was accompanied by elevated c-FOS, c-JUN and retinoic acid receptor α (RARA) expression, as well as cellular retinoic acid (RA) content. Exposure of epithelia to RA derivatives of ALI cultivated epithelia mimicked effects of LLI. The increase in RA content was in line with the identified upregulation of retinoic acid anabolizing enzymes ALDH1A3 and DHRS3. CLDN1 promoter analysis revealed c-FOS and c-JUN as activating transcription factors, whereas activation of RARA reduced CLDN1 promoter activity. We then concluded that ALI/LLI dependent modulation of CLDN1 expression and TJ permeability is under the control of RA synthesis. Activation of RARA results in an inhibition of c-FOS/c-JUN dependent CLDN1 promoter activation and increased TJ permeability. Our results underscore RA signalling as a pivotal mechanism in adjusting TJ properties, which could play a role during birth when the lung changes from LLI to ALI conditions.
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