Regulation of mucin gene expression by CREB via a nonclassical retinoic acid signaling pathway

Vitamin A and its metabolite retinoic acid (RA) are essential elements for normal lung development and the differentiation of lung epithelial cells. We previously showed that RA rapidly activated cyclic AMP response element-binding protein (CREB) in a nonclassical manner in normal human tracheobronchial epithelial (NHTBE) cells. In the present study, we further demonstrated that this nonclassical signaling of RA on the activation of CREB plays a critical role in regulating the expression of airway epithelial cell differentiation markers, the MUC2, MUC5AC, and MUC5B genes. We found that RA rapidly activates the protein kinase Calpha isozyme and transmits the activation signal to CREB via the Raf/MEK/extracellular signal-regulated kinase/p90 ribosomal S6 kinase (RSK) pathway. Activated RSK translocated from the cytoplasm to the nucleus, where it phosphorylates CREB. Activated CREB then binds to a cis-acting replication element motif on the promoter (at nucleotides [nt] -878 to -871) of the MUC5AC gene. The depletion of CREB using small interfering RNA abolished not only the RA-induced MUC5AC but also RA-induced MUC2 and MUC5B. Taken together, our findings demonstrate that CREB activation via this nonclassical RA signaling pathway may play an important role in regulating the expression of mucin genes and mediating the early biological effects of RA during normal mucous differentiation in NHTBE cells.     

Cyclophilin A is required for retinoic acid-induced neuronal differentiation in p19 cells

Stable transfectants with expression of small interfering RNA for targeting cyclophilin A (CypA) in p19 cells lose their potential for retinoic acid (RA)-induced neuronal differentiation but not Me(2)SO-induced mesodermal differentiation. This difference suggests that CypA is specifically required for the RA-induced neuronal pathway. In addition to the loss of RA-induced RA receptor beta expression and retinoic acid response element (RARE)-binding activity, a dramatic reduction in RA-induced RARE-mediated luciferase activity in the CypA knockdown cell line suggests that CypA affects RARE-mediated regulation of gene expression. Silent mutation of target sequences confirms the specificity of RNA interference in p19 embryonal carcinoma cells. Collectively, our data reveal that a novel function of CypA is required in the processing of RA-induced neuronal differentiation in p19 embryonal carcinoma cells.     

An intronic Ikaros-binding element mediates retinoic acid suppression of the kappa opioid receptor gene, accompanied by histone deacetylation on the promoters

The mouse kappa opioid receptor (KOR) gene is constitutively expressed in mouse embryonal carcinoma P19 stem cells and suppressed by retinoic acid (RA) in cells undergoing neuronal differentiation. A negative regulatory element is located within intron 1 of the KOR gene, which contains an Ikaros (Ik)-binding site (GGGAAgGGGAT). This sequence is an Ik-1 respondive, functionally negative element as demonstrated in the context of both natural KOR and heterologous promoters. The two underlined G residues of the second half-site are critical for Ik-1 binding and Ik-mediated repression of the KOR gene. RA induces Ik-1 expression within 1 day of treatment and suppresses KOR expression between 2 and 3 days. Overexpression of Ik-1 in P19 suppresses endogenous KOR gene expression, accompanied by increased binding of Ik-1 to the Ik-binding site and chromatin histone deacetylation on KOR promoters. It is proposed that in an RA-induced P19 differentiation model, RA elevates Ik-1 expression, which recruits histone deacetylase to intron 1 of the KOR gene and silences KOR gene promoters.