Calcitriol Decreases Expression of Importin α3 and Attenuates RelA Translocation in Human Bronchial Smooth Muscle Cells

A potent immunomodulatory role of Vitamin D in both innate and adaptive immunity has recently been appreciated. In allergic asthma, activation of NF-(DB)-B-0 induces transcription of various cytokines and chemokines involved in allergic airway inflammation. The nuclear import of activated NF-(DB)-B-0 p50/RelA subunit is dependent on importin alpha 3 (KPNA4) and importin alpha 4 (KPNA3). In this study, we examined the role of importin alpha 3 in immunomodulatory effect of calcitriol in human bronchial smooth muscle cells (HBSMCs). Cultured HBSMCs were stimulated with calcitriol in the presence and absence of cytokines, TNF-alpha, IL-1 beta, and IL-10. The mRNA transcripts of importin alpha 3 and alpha 4 were analyzed using qPCR while protein expression of importin alpha 3, alpha 4 and nuclear RelA was analyzed by immunoblotting. Calcitriol significantly decreased mRNA and protein expression of importin alpha 3 as well as nuclear protein expression of NF-(DB)-B-0 p65 (RelA). The decreased activation of RelA by calcitriol was confirmed by decreased release of RelA-inducible molecules, including IL-5, IL-6 and IL-8, by HBSMCs upon calcitriol treatment. Calcitriol attenuated the effect of TNF-alpha and IL-1 beta to upregulate mRNA and protein expression of importin alpha 3. IL-10 significantly decreased the TNF-alpha induced expression of importin alpha 3 and this effect was further potentiated by calcitriol. These data suggest that under inflammatory conditions, calcitriol decreases the expression of importin alpha 3 resulting in decreased nuclear import of activated RelA. This could be a novel mechanism by which calcitriol could exert its immunomodulatory effects to reduce allergic airway inflammation and thus may alleviate the symptoms in allergic asthma.