Expression of proviral integration site for Moloney murine leukemia virus 1 (Pim-1) is post-transcriptionally regulated by tristetraprolin in cancer cells.

The proviral integration site for Moloney murine leukemia virus 1 (Pim-1) is an oncogenic serine/threonine kinase that is up-regulated in several human cancers, facilitates cell cycle progression, and suppresses apoptosis. Previously, it has been reported that the Pim-1 3'-UTR plays important roles in the regulation of Pim-1 mRNA stability. However, the mechanisms explaining how Pim-1 mRNA stability is determined by its 3'-UTR are not well known. Here, we demonstrate that tristetraprolin (TTP) plays a critical role in the regulation of Pim-1 mRNA stability. Our results show that the level of Pim-1 expression is inversely correlated with TTP expression in human cancer cells. Pim-1 mRNA contains two AU-rich elements (ARE1 and ARE2) in the 3'-UTR. TTP bound to ARE2 and enhanced the decay of Pim-1 mRNA. Overexpression of TTP decreased Pim-1 expression and p21 and p27 phosphorylation and inhibited cell growth. Overexpression of Pim-1 cDNA without the 3'-UTR attenuated the inhibitory effects of TTP on p21 phosphorylation and cell growth. In addition, inhibition of p21 by siRNA attenuated the inhibitory effect of TTP on cell growth. Our results suggest that TTP post-transcriptionally down-regulates Pim-1 expression and that the overexpression of TTP may contribute to tumor suppression in part by down-regulating Pim-1 expression.

Anticancer Activity of Self-Assembled Molecular Rectangles via Arene-Ruthenium Acceptors and a New Unsymmetrical Amide Ligand.

Two new and large molecular rectangles 4 and 5 were synthesized from two different arene-ruthenium [Ru(2)(µ-η(4)-C(2)O(4))(MeOH)(2)(η(6)-p-Pr(i)C(6)H(4)Me)(2)][O(3)SCF(3)](2) (2), and [Ru(2) (p-cymene)(2) (donq) (OH(2))(2)] [O(3)SCF(3)](2) (donq = 5,8-dioxydo-1,4-naphthaquinonato) (3) acceptors and a new unsymmetrical N-(4-(pyridin-4-ylethynyl)phenyl) isonicotinamide (1) donor ligand. X-ray crystallography of 4 confirmed a molecular rectangle. The (1)H NMR spectra of both rectangles 4 and 5 showed a mixture of two structural, head-to-tail (HTL) and head-to-head (HTH) type, isomers in a 1:1 ratio. The cytotoxicities of both rectangles have been established against Colo320 (colorectal cancer), A549 (lung cancer), MCF-7(breast cancer) and H1299 (lung cancer) human cancer cell lines. The cytotoxicity of rectangle 5 was found to be considerably stronger against all cancer cell lines than that of the reference drug cisplatin.

Ectopic over-expression of tristetraprolin in human cancer cells promotes biogenesis of let-7 by down-regulation of Lin28.

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.

Tristetraprolin controls the stability of cIAP2 mRNA through binding to the 3'UTR of cIAP2 mRNA.

cIAP2 is a key regulator of programmed cell death and the NF-κB pathway. Here, we investigated the post-transcriptional regulation of cIAP2 expression by tristetraprolin (TTP). Our results showed that overexpression of TTP reduced the stability of cIAP2 mRNA and the expression level of cIAP2. In addition, TTP destabilized a luciferase mRNA containing cIAP2 mRNA 3'UTR. cIAP2 mRNA 3'UTR contains four AU-rich elements (AREs) and the 2nd ARE was responsible for the TTP-mediated destabilization of the cIAP2 mRNA. RNA EMSA revealed that TTP directly bound to 42 nucleotides from the 3'UTR of cIAP2 mRNA containing the 2nd ARE. However, the 42 nucleotides did not promote TTP-dependent destabilization of mRNA and did not recruit the decapping enzyme Dcp2 and the 5'-3' exonuclease Xrn1. When we used a 52 nucleotide sequence containing an additional 5 nucleotides from cIAP2 mRNA 3'UTR at both ends, this long nucleotide sequences recruited Dcp2 and Xrn1 and promoted TTP-dependent destabilization of mRNA. Collectively, our results suggest that TTP can bind to the 2nd ARE of cIAP2 mRNA 3'UTR and destabilize cIAP2 mRNA by forming complexes with Dcp2 and Xrn1. However, while a short nucleotide sequence containing the 2nd ARE of cIAP2 mRNA can recruit the TTP binding, this cannot recruit Dcp2 and Xrn1 and cannot induce TTP-mediated destabilize the mRNA. Instead, additional nucleotide sequences are required to recruit Dcp2 and Xrn1 and to destabilize mRNA.

Stability of the LATS2 tumor suppressor gene is regulated by tristetraprolin.

LATS2 is a tumor suppressor gene implicated in the control of cell growth and the cell cycle. Here, we investigated the post-transcriptional 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 AU-rich 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.