No abstract

Tristetraprolin (TTP) is a AU-rich element (ARE) binding protein and exhibits suppressive effects on cell growth through down-regulation of ARE-containing oncogenes. The let-7 microRNA has emerged as a significant factor in tumor suppression. Both TTP and let-7 are often repressed in human cancers, thereby promoting oncogenesis by derepressing their target genes. In this work, an unexpected link between TTP and let-7 has been found in human cancer cells. TTP promotes an increase in expression of mature let-7, which leads to the inhibition of let-7 target gene CDC34 expression and suppresses cell growth. This event is associated with TTP-mediated inhibition of Lin28, which has emerged as a negative modulator of let-7. Lin28 mRNA contains ARE within its 3′-UTR and TTP enhances the decay of Lin28 mRNA through binding to its 3′-UTR. This suggests that the TTP-mediated down-regulation of Lin28 plays a key role in let-7 miRNA biogenesis in cancer cells.

LATS2 is a tumor suppressor gene implicated in the control of cell growth and the cell cycle. Here, we investigated the posttranscriptional regulation of LATS2 expression by tristetraprolin (TTP). Our results show that the expression level of LATS2 is inversely correlated with TTP expression in human cancer cell lines. Overexpression of TTP reduced the expression level of LATS2. Conversely, treatment with small interfering RNA against TTP increased the expression level of LATS2 through stabilization of LATS2 mRNA and suppressed the proliferation of A549 human lung cancer cells. LATS2 mRNA contains AUrich elements (AREs) within the 3-untranslated region, and TTP destabilized a luciferase mRNA containing LATS2 ARE. In addition, RNA electrophoretic mobility shift assay revealed that TTP directly bound to the ARE of LATS2 mRNA. These results establish LATS2 mRNA as a physiological target of TTP and suggest the possibility that TTP controls cell growth through regulation of LATS2 mRNA stability.LATS2 is a new member of the LATS tumor suppressor family (1-3). The LATS2 gene is located at chromosome 13q11-12 and encodes a 1046-amino acid protein that contains a C-terminal serine/threonine kinase domain (4). According to recent publications, LATS2 appears to coordinate the cell cycle, cell proliferation, and cell death. LATS2 promotes the stabilization of p53 by inactivating MDM2 (5), plays a role in maintenance of mitotic fidelity (6), and regulates spindle organization through recruitment of ␥-tubulin to the centrosome (7). Overexpression of LATS2 results in G 2 /M arrest via inhibition of Cdc2 kinase activity (8), inhibition of G 1 /S transition via down-regulation of cyclin E/Cdk2 kinase activity (9), and induction of apoptosis via down-regulation of apoptotic inhibitors such as Bcl-2 and Bcl-x L (10). Deletion of LATS2 in flies (11, 12) and mice (13) accelerates cell proliferation and tumor development, indicating that the reduced function of the LATS2 gene leads to accelerated cell proliferation.Expression of LATS2 is down-regulated in several human cancer cells (14 -18). The mechanisms for down-regulation have been the subject of substantial interest. It has been reported that the expression of LATS2 in breast cancer (14), astrocytoma (17), and acute lymphoblastic leukemia (18, 19) is down-regulated by hypermethylation of the promoter. In addition, LATS2 mRNA is down-regulated at the post-transcriptional level by microRNA-372 and microRNA-373 in testicular germ cell tumors (20), gastric cancer cells (21), and esophageal cancer cells (22), suggesting that the expression of the LATS2 gene is controlled at the post-transcriptional level as well as the transcriptional level.Post-transcriptional regulation of gene expression is also mediated by AU-rich elements (AREs) 4 located in the 3Ј-untranslated region (3Ј-UTR) of a variety of short-lived mRNAs such as cytokines and proto-oncogenes (23). The destabilizing function of AREs is believed to be regulated by ARE-binding proteins (24). Numerous ARE-binding proteins ha...