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

Abstract

Purpose A potent immunomodulatory role of Vitamin D in both innate and adaptive immunity has recently been appreciated. In allergic asthma, activation of NF-κB induces transcription of various cytokines and chemokines involved in allergic airway inflammation. The nuclear import of activated NF-κB p50/RelA subunit is dependent on importin α3 (KPNA4) and importin α4 (KPNA3). In this study, we examined the role of importin α3 in immunomodulatory effect of calcitriol in human bronchial smooth muscle cells (HBSMCs). Methods Cultured HBSMCs were stimulated with calcitriol in the presence and absence of cytokines, TNF-α, IL-1β, and IL-10. The mRNA transcripts of importin α3 and α4 were analyzed using qPCR while protein expression of importin α3, α4 and nuclear RelA was analyzed by immunoblotting. Results Calcitriol significantly decreased mRNA and protein expression of importin α3 as well as nuclear protein expression of NF-κB 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-α and IL-1β to upregulate mRNA and protein expression of importin α3. IL-10 significantly decreased the TNF-α induced expression of importin α3 and this effect was further potentiated by calcitriol. Conclusions These data suggest that under inflammatory conditions, calcitriol decreases the expression of importin α3 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.

Impact of Vitamin D Supplementation on Inflammatory Markers in African Americans: Results of a Four-Arm, Randomized, Placebo-Controlled Trial

“…In human and in vitro studies, vitamin D exerts a diverse array of immunomodulatory effects by interacting with vitamin D system in a complex pathway that includes precursors, active metabolites, enzymes and receptors. (27, 28)…”

Clinical view on the importance of dendritic cells in asthma

“…Vitamin D has emerged as a potent immunomodulator in recent years [144]. It modulates the immune cells through its active form calcitriol, also known as 1,25-dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ].…”

“…The inactive form of vitamin D, 25(OH)D 3 , is used as an indicator of circulating vitamin D level in an individual. However, effectiveness depends on the potential of a cell to contain the metabolizing enzyme, 1α-hydroxylase (CYP27B1), to convert inactive 25(OH)D 3 into active 1,25(OH) 2 D 3 [144,145]. Upregulation of CYP27B1 has been seen in DCs upon maturation with LPS or with T cell in an immune synapse [146].…”

“…Upregulation of CYP27B1 has been seen in DCs upon maturation with LPS or with T cell in an immune synapse [146]. Involvement of vitamin D binding protein (carrier protein), vitamin D receptor (VDR), and CYP24A1 (catabolizing enzyme) are also important in maintaining the effects of vitamin D in a particular cell [144,147]. Recently, it was demonstrated that through vitamin D binding protein, level of free 25(OH)D 3 available to DCs governs the balance between allergic and tolerogenic response to allergens [146].…”

“…In addition, it also decreases the levels of TNFα and IL12 [145,148]. This effect may be correlated with reduced nuclear transport of activated RelA, an NF-κB subunit through importin-α3 (nuclear transporter) due to a decreased expression of importin α3 under low levels of vitamin D [144]. …”