It has become increasingly clear that microRNAs (miRNAs) play important roles in tumorigenesis and metastasis. Recently, miR-203 was reported as a suppressor microRNA often silenced in different malignancies including hepatocellular carcinoma, prostate cancer, oral cancer, and hematopoietic malignancy, but little is known about its potential role in breast carcinogenesis. In this study, we found that in breast cancer, miR-203 was upregulated in primary tumors and some nonmetastatic cell lines but was significantly downregulated in metastatic cell lines including BT549, Hs578T, and MDA-MB-231, as measured by regular and real-time PCR. Downregulation of miR-203 in metastatic breast cancer cells appeared to be caused by hypermethylation of its promoter. Functionally, ectopic expression of miR-203 in BT549 and MDA-MB-231 breast cancer cell lines caused cell cycle arrest and apoptosis and inhibited cell invasion and migration in vitro. Bioinformatic analysis predicted the snail homolog 2 (SNAI2 or SLUG), a transcription factor that promotes cell invasion and tumor metastasis, as a target of miR-203, and the prediction was validated by expression analysis and luciferase reporter assay of the 3′ untranslated region of SNAI2 that contains the miR-203 target sequences. These results suggest that in malignant breast cancer cells, miR-203 is epigenetically silenced, and the silencing promotes tumor cell growth and invasion at least in part by upregulating the SNAI2 transcription factor.

Myeloid-derived suppressor cells (MDSCs) were one of the major components of the immune suppressive network. STAT3 has an important role in regulating the suppressive potential of MDSCs. In this study, we found that the expression of STAT3 could be modulated by both miR-17-5p and miR-20a. The transfection of miR-17-5p or miR-20a remarkably reduces the expression of reactive oxygen species and the production of H₂O₂, which are regulated by STAT3. MDSCs transfected with miR-17-5p or miR-20a are less able to suppress Ag-specific CD4 and CD8 T cells. Importantly, both miR-17-5p and miR-20a alleviate the suppressive function of MDSCs in vivo. The expression of miR-17-5p and miR-20a in tumor-associated MDSCs was found to be lower than in Gr1⁺CD11b⁺ cells isolated from the spleens of disease-free mice. Tumor-associated factor downregulates the expression of both miR-17-5p and miR-20a. The modulation of miR-17-5p and miR-20a expression may be important for the process by which patients with a tumor can overcome the immune tolerance mediated by MDSCs. Our results suggest that miR-17-5p and miR-20a could potentially be used for immunotherapy against diseases, especially cancer, by blocking STAT3 expression.

Gut mucosal layers are crucial in maintaining the gut barrier function. Gut microbiota regulate homeostasis of gut mucosal layer via gut immune cells such as RORγt (+) IL-22(+) ILC3 cells, which can influence the proliferation of mucosal cells and the production of mucin. However, it is unclear how gut microbiota execute this regulation. Here we show that lactobacilli promote gut mucosal formation by producing L-Ornithine from arginine. L-Ornithine increases the level of aryl hydrocarbon receptor ligand L-kynurenine produced from tryptophan metabolism in gut epithelial cells, which in turn increases RORγt (+)IL-22(+) ILC3 cells. Human REG3A transgenic mice show an increased proportion of L-Ornithine producing lactobacilli in the gut contents, suggesting that gut epithelial REG3A favors the expansion of L-Ornithine producing lactobacilli. Our study implicates the importance of a crosstalk between arginine metabolism in Lactobacilli and tryptophan metabolism in gut epithelial cells in maintaining gut barrier.

KLF5 is an essential basic transcriptional factor that regulates a number of physiopathological processes. In this study, we tested whether and how KLF5 modulates the epithelial-mesenchymal transition (EMT). Using transforming growth factor β (TGF-β)- and epidermal growth factor (EGF)-treated epithelial cells as an established model of EMT, we found that KLF5 was downregulated during EMT and that knockdown of KLF5 induced EMT even in the absence of TGF-β and EGF treatment, as indicated by phenotypic and molecular EMT properties. Array-based screening suggested and biochemical analyses confirmed that the microRNA 200 (miR-200) microRNAs, a group of well-established EMT repressors, were transcriptionally activated by KLF5 via its direct binding to the GC boxes in miR-200 gene promoters. Functionally, overexpression of miR-200 prevented the EMT induced by KLF5 knockdown or by TGF-β and EGF treatment, and ectopic expression of KLF5 attenuated TGF-β- and EGF-induced EMT by rescuing the expression of miR-200. In mouse prostates, knockout of Klf5 downregulated the miR-200 family and induced molecular changes indicative of EMT. These findings indicate that KLF5 maintains epithelial characteristics and prevents EMT by transcriptionally activating the miR-200 family in epithelial cells.

Tumor‐associated factors are related to increased accumulation of CD11b⁺Gr1⁺myeloid‐derived suppressor cells (MDSCs). However, the exact mechanism of how genetic factors control the expansion of MDSCs in tumor‐bearing hosts remains elusive. Herein, we found that tumor‐associated MDSCs and their subsets, mononuclear MDSCs and polymorphonuclear MDSCs, have decreased expression of miR‐223 when compared to CD11b⁺Gr1⁺ cells from the spleen of disease‐free mice. With the differentiation of CD11b⁺Gr1⁺MDSCs from bone marrow cells (BMCs) upon exposure to tumor‐associated factors, the expression of both pri‐miR‐223 and mature miR‐223 was downregulated, indicating that the expression of miR‐223 could be regulated by tumor‐associated factors. Interestingly, miR‐223 remarkably inhibits differentiation of BMCs into CD11b⁺Gr1⁺MDSCs in the presence of tumor‐associated factors by targeting myocyte enhancer factor 2C (MEF2C). Using reconstituted s.c. tumor models, miR‐223 also suppresses accumulation of CD11b⁺Gr1⁺MDSCs, whereas its targeting molecule MEF2C increases the number of MDSCs. Tumor growth is slower in mice infused by miR223‐engineered BMCs than in mice infused with control transfected BMCs. As miR‐223 and its target molecule MEF2C are highly conserved between mice and humans, the modulation of miR‐223 in tumor‐induced CD11b⁺Gr1⁺MDSCs may exert an important role in controlling the increased accumulation of CD11b⁺Gr1⁺MDSCs in patients with tumor.

Accumulating evidence has indicated that microRNAs (miRNAs) play an important role in the occurrence and progression of ovarian cancer (OC). However, the function of miRNAs implicated in OC remains unclear. This study investigated the potential role of miR‐211 in OC. Gene Expression Omnibus database analysis indicated that miR‐211 expression was significantly down‐regulated in OC tissues compared with normal specimens. In addition, miR‐211 overexpression apparently inhibited proliferation, migration, xenograft growth, and induced apoptosis in HEY‐T30 and SKOV3 cells. Moreover, PHF19, a component of the polycomb group of proteins, was found to be a direct target of miR‐211 based on the luciferase reporter assay and Western blot analysis. Consistently, survival analysis indicated that high PHF19 expression was associated with shorter survival time in patients with OC. Importantly, silence of PHF19 reduced proliferation, induced cell cycle arrest, promoted apoptosis, suppressed migration, and inhibited xenograft growth in SKOV3 cells. Restoration of PHF19 expression markedly reversed the inhibitory effect of miR‐211 on OC. Moreover, our results indicate that the long noncoding RNA MALAT1 could sponge miR‐211 as a competing endogenous RNA and potentially up‐regulate PHF19 expression, thus facilitating the OC progression. These findings suggest that the MALAT1/miR‐211/PHF19 axis may act as a key mediator in OC and provide new insight into the prevention of this disease.—Tao, F., Tian, X., Ruan, S., Shen, M., Zhang, Z. miR‐211 sponges lncRNA MALAT1 to suppress tumor growth and progression through inhibiting PHF19 in ovarian carci noma. 32, 6330–6343 (2018). www.fasebj.org