Abstract. MicroRNA (miRNA)-126 (miR-126) was reported to be downregulated and to act as a tumor suppressor in cancers of the lung, cervix, bladder and prostate. However, the functions of miR-126 in gastric cancer appear to be diverse and are largely unknown. MiR-126 was reported to act as a tumor suppressor by targeting the Crk gene, or as an oncogene by targeting the SOX2 gene in gastric cancer. We identified that the expression of miR-126 was decreased in gastric cancer cell lines and tissues. PLK2, a tumor suppressor gene, was directly regulated by miR-126 in SGC-7901 cells. Overexpression of miR-126 not only suppressed the growth and clone formation of SGC-7901 cells, but also induced apoptosis in vitro, whereas inhibition of miR-126 slightly promoted SGC-7901 cell proliferation. The cell cycle was not affected by miR-126. Moreover, miR-126 suppressed tumor growth in vivo in a xenograft model. PLK2, PI3KR2and Crk were regulated by miR-126 in SGC-7901 cells. We infer that the functions of miR-126 in gastric cancer depend on synergistic targeting balance between oncogenes and anti-oncogenes. Our study indicates that miR-126 is a tumor suppressor, which in the future may become a therapeutic target for gastric cancer.
Methyl-CpG-binding protein 2 (MeCP2) facilitates the carcinogenesis and progression of several types of cancer. However, its role in breast cancer and the relevant molecular mechanism remain largely unclear. In this study, analysis of the Cancer Genome Atlas (TCGA) data that MeCP2 expression was significantly upregulated in breast cancer tissues, and high MeCP2 expression was correlated with poor overall survival. Knockdown of MeCP2 inhibited breast cancer cell proliferation and G1–S cell cycle transition and migration as well as induced cell apoptosis in vitro. Moreover, MeCP2 knockdown suppressed cancer cell growth in vivo. Investigation of the molecular mechanism showed that MeCP2 repressed RPL11 and RPL5 transcription by binding to their promoter regions. TCGA data revealed significantly lower RPL11 and RPL5 expression in breast cancer tissues; additionally, overexpression of RPL11/RPL5 significantly suppressed breast cancer cell proliferation and G1–S cell cycle transition and induced apoptosis in vitro. Furthermore, RPL11 and RPL5 suppressed ubiquitination-mediated P53 degradation through direct binding to MDM2. This study demonstrates that MeCP2 promotes breast cancer cell proliferation and inhibits apoptosis through suppressing RPL11 and RPL5 transcription by binding to their promoter regions.
Abstract. Polo-like kinase 2 (PLK2) is a serine/threonine protein kinase, which has vital roles during mitosis and the centrosome cycle. In acute myeloblastic leukemia and hepatocarcinogenesis, PLK2 acts as a tumor suppressor; however, the function of PLK2 in gastric cancer remains to be elucidated. In the present study, PLK2 was overexpressed in gastric cancer tissues and three types of gastric cancer cells, SGC-7901, MKN-45 and BGC-823. Transfection of SGC-7901 gastric cancer cells with small interfering (si)RNA against PLK2 exerted no effect on the ratio of cells at different stages of the cell cycle compared with that of the untransfected and control siRNA-transfected cells. In addition, silencing of PLK2 significantly enhanced the growth of SGC-7901 cells through inhibiting apoptosis. Furthermore, apoptosis-associated genes Bax and caspase 3 were found to be downregulated at the protein level. In conclusion, these results suggested that PLK2 may act as a tumor suppressor in gastric cancer, therefore indicating its therapeutic potential. IntroductionGastric cancer, also known as stomach cancer, has a high incidence rate in and is one of the most prominent causes of cancer-associated mortality in Asia (1). In addition, gastric cancer is the fourth most prevalent type of cancer worldwide and the second most frequent cause of cancer-associated mortality worldwide (2,3). The prognosis of gastric cancer patients is poor and it remains challenging to cure. Targeted gene therapy is a novel therapeutic approach for gastric cancer, for which the identification of tumor suppressors associated with this malignancy is essential.Polo-like kinase 2 (PLK2) is a member of the serine/threonine protein kinase family, which includes five members: PLK1, PLK2 (also termed SNK), PLK3 (also termed Fnk or Prk), PLK4 (also termed Sak) and PLK5 (4-6). These kinases have important roles during mitosis and the centrosome cycle (7). PLK2 is located in the centrosome and was found to be involved in embryonic development and cell cycle progression at the G 1 /S transition, as well as in skeletal development. Cell cycle analysis of cultured PLK2 -/-embryonic fibroblasts indicated that these cells proliferated more slowly than cells expressing PLK2 and exhibited delayed entry into S phase from G 1 (8). In central neurons, PLK2 was reported to be overexpressed in response to synaptic stimulation (9). Decreased expression of PLK2 was observed in B-cell malignancies; in addition, apoptosis was found to be induced in Burkitt's lymphoma cells through the ectopic expression of PLK2 (10). This therefore indicated that PLK2 may act as a tumor suppressor gene in hematologic malignancies (10,11). Pellegrino et al (12) showed that PLK2 was a tumor suppressor in hepatocarcinogenesis. In addition, Kothari et al (13) reported that PLK2 is an outlier kinase, which was highly expressed in Silencing of polo-like kinase 2 increases cell proliferation and decreases apoptosis in SGC-7901 gastric cancer cells
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