MicroRNAs (miRNAs) are single-stranded, small non-coding RNA molecules regulating gene expression by hybridization to targeted transcripts. They play key roles on biological processes such as cell differentiation, development, infection, immune response, inflammation and tumorigenesis. Therefore, the role of miRNAs has been extensively studied in the regulation of cellular processes, including proliferation and differentiation, apoptosis, cancer and viral infections. Several studies have reported that the functions of miRNAs are associated with the regulation of inflammatory responses [
1‐
3]. Recent studies have demonstrated the differences in miRNA expression between inflamed and healthy tissues [
4‐
6]. We have previously demonstrated that the three most overexpressed miRNAs in inflamed gingiva from Japanese chronic periodontitis patients were miR-150, miR-223 and miR-200b, and inflammatory cytokines induced miR-150 and miR-223 expressions in human gingival fibroblasts (HGF) [
6,
7]. MiR-223 increased the inflammation response via inhibitor of nuclear factor kappa-B (IκB) kinase α (IKKα) and mitogen-activated protein kinase phosphatase-5 (MKP-5) [
7], whereas miR-200b suppressed tumor necrosis factor-α (TNF-α) induced interleukin-8 (IL-8) secretion and tight junction disruption of intestinal epithelial cells [
8]. miR-200a/200b inhibited high-glucose induced endothelial inflammation by regulating
O-GlicNAc transferase-mediated protein O-GlcNAcylation [
9]. miR-200b/c attenuated lipopolysaccharide-induced early pulmonary fibrosis by targeting Zinc-finger E-box-binding homeobox 1/2 (ZEB1/2) via p38 mitogen-activated protein kinase (MAPK) and transforming growth factor-β (TGF-β)/smad3 signaling pathways [
10].
Periodontal disease is a chronic inflammatory disease caused by periodontopathic bacteria, such as
Porphyromonas gingivalis and viruses, which lead to inflammation, destruction of periodontium and tooth loss [
11‐
14]. The response to periodontal pathogens is determined by both innate and adaptive immune responses. The innate immune response plays a critical role in defense against putative periodontal pathogens and virulence factors [
14]. Cytokines are central regulators of the immune response which are produced by various cell types including epithelial cells, fibroblasts, dendritic cells, macrophages and T-helper cells in response to microbes [
15]. Inflammatory cytokines are regulated by activation of nuclear factor-kappa-light-chain-enhancer of activated B cells (NF-κB), interferon regulatory factor (IRF) family of transcription factors and MAPKs [
16,
17]. NF-κB consists of five family members, including RelA (p65), RelB, c-Rel, NFκB1 (p105/p50) and NFκB2 (p100/p52) that play critical roles in inflammation, immunity, differentiation, cell proliferation and apoptosis. NF-κB exists as a heterodimer consisting of p65 and p50 subunits which are associated with IκB in the cytoplasm as an inactive form. IκB is phosphorylated by IKK and NF-κB is activated by the phosphorylated IκB and induces transcription of a variety of target genes, including L-1β and IL-6 [
18‐
20]. The IKK complex is composed of three subunits each encoded by a separate gene, such as IKKα [also known as conserved helix-loop-helix ubiquitous kinase (CHUK)], IKKβ [also known as inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase beta (IKBKB)] and IKKγ [also known as NF-kappa-B essential modulator (NEMO) or inhibitor of nuclear factor kappa-B kinase subunit gamma (IKBKG)] [
21]. IKKβ is critical for the production of inflammatory cytokines and associated with the activation of cancers, including breast cancer, pancreatic cancer, melanoma and acute myeloid leukemia [
22]. MiR-200b is identified as regulator of epithelial-mesenchymal transition (EMT), which is a process that accelerates tissue remodeling and gain a motile phenotype. The induction of EMT is regulated by several transcription factors, including ZEB1 and ZEB2, which negatively control E-cadherin and activate EMT [
23‐
27]. To elucidate the mechanism how miR-200b regulates the expression of inflammatory cytokines, we transfected miR-200b expression plasmid or miR-200b inhibitor in human gingival fibroblasts (HGF) and examined the signaling pathways and the expressions of inflammatory cytokines.