Induction of TSP1 gene expression by heat shock is mediated via an increase in mRNA stability

Invoking the power of thrombospondins: regulation of thrombospondins expression

Increasing evidence suggests critical functions of thrombospondins (TSPs) in a variety of physiological and pathological processes. With the growing understanding of the importance of these matricellular proteins, the need to understand the mechanisms of regulation of their expression and potential approaches to modulate their levels is also increasing. The regulation of TSP expression is multi-leveled, cell- and tissue-specific, and very precise. However, the knowledge of mechanisms modulating the levels of TSPs is fragmented and incomplete. This review discusses the known mechanisms of regulation of TSP levels and the gaps in our knowledge that prevent us from developing strategies to modulate the expression of these physiologically important proteins.

The novel tumor suppressor NOL7 post-transcriptionally regulates thrombospondin-1 expression

Thrombospondin-1 (TSP-1) is an endogenous inhibitor of angiogenesis whose expression suppresses tumor growth in vivo. Like many angiogenesis-related genes, TSP-1 expression is tightly controlled by various mechanisms, but there is little data regarding the contribution of post-transcriptional processing to this regulation. NOL7 is a novel tumor suppressor that induces an antiangiogenic phenotype and suppresses tumor growth, in part through upregulation of TSP-1. Here we demonstrate that NOL7 is an mRNA-binding protein that must localize to the nucleoplasm to exert its antiangiogenic and tumor suppressive effects. There, it associates with the RNA-processing machinery and specifically interacts with TSP-1 mRNA through its 3'UTR. Reintroduction of NOL7 into SiHa cells increases luciferase expression through interaction with the TSP-1 3'UTR at both the mRNA and protein levels. NOL7 also increases endogenous TSP-1 mRNA half-life. Further, NOL7 post-transcriptional stabilization is observed in a subset of angiogenesis-related mRNAs, suggesting that the stabilization of TSP-1 may be part of a larger novel mechanism. These data demonstrate that NOL7 significantly alters TSP-1 expression and may be a master regulator that coordinates the post-transcriptional expression of key signaling factors critical for the regulation of the angiogenic phenotype.

Heat treatment of retinal pigment epithelium induces production of elastic lamina components and antiangiogenic activity

Age-related macular degeneration (AMD) is the leading cause of blindness in the Western world. In advanced AMD, new vessels from choriocapillaris (CC) invade through the Bruch's membrane (BrM) into the retina, forming choroidal neovascularization (CNV). BrM, an elastic lamina that is located between the retinal pigment epithelium (RPE) and CC, is thought to act as a physical and functional barrier against CNV. The BrM of patients with early AMD are characterized by decreased levels of antiangiogenic factors, including endostatin, thrombospondin-1 (TSP-1), and pigment epithelium-derived factor (PEDF), as well as by degeneration of the elastic layer. Motivated by a previous report that heat increases elastin expression in human skin, we examined the effect of heat on human ARPE-19 cell production of BrM components. Heat treatment stimulated the production of BrM components, including TSP-1, PEDF, and tropoelastin in vitro and increased the antiangiogenic activity of RPE measured in a mouse corneal pocket assay. The effect of heat on experimental CNV was investigated by pretreating the retina with heat via infrared diode laser prior to the induction of CNV. Heat treatment blocked the development of experimental CNV in vivo. These findings suggest that heat treatment may restore BrM integrity and barrier function against new vessel growth.

Regulation of thrombospondin-1 expression through AU-rich elements in the 3'UTR of the mRNA

Thrombospondin-1 (TSP-1) is a matricellular protein that participates in numerous normal and pathological tissue processes and is rapidly modulated by different stimuli. The presence of 8 highly-conserved AU rich elements (AREs) within the 3'-untranslated region (3'UTR) of the TSP-1 mRNA suggests that post-transcriptional regulation is likely to represent one mechanism by which TSP-1 gene expression is regulated. We investigated the roles of these AREs, and proteins which bind to them, in the control of TSP-1 mRNA stability. The endogenous TSP-1 mRNA half-life is approximately 2.0 hours in HEK293 cells. Luciferase reporter mRNAs containing the TSP-1 3'UTR show a similar rate of decay, suggesting that the 3'UTR influences the decay rate. Site-directed mutagenesis of individual and adjacent AREs prolonged reporter mRNA halflife to between 2.2 and 4.4 hours. Mutation of all AREs increased mRNA half life to 8.8 hours, suggesting that all AREs have some effect, but that specific AREs may have key roles in stability regulation. A labeled RNA oligonucleotide derived from the most influential ARE was utilized to purify TSP-1 ARE-binding proteins. The AU-binding protein AUF1 was shown to associate with this motif. These studies reveal that AREs in the 3'UTR control TSP-1 mRNA stability and that the RNA binding protein AUF1 participates in this control. These studies suggest that ARE-dependent control of TSP-1 mRNA stability may represent an important component in the control of TSP-1 gene expression.

CCAAT box is required for the induction of human thrombospondin-1 gene by trichostatin A

Histone deacetylase (HDAC) inhibitors have been reported to inhibit angiogenesis as well as tumor growth. Thrombospondin-1 (TSP1) has been recognized as a potent inhibitor of angiogenesis. Such an action of TSP1 may account for the effect of HDAC inhibitors. In the present study, we investigated the molecular mechanism by which trichostatin A, a HDAC inhibitor, induces the expression of TSP1 gene. Trichostatin A increased both mRNA and protein levels of TSP1 in HeLa cells. Promoter and actinomycin D chase assays showed that trichostatin A-induced TSP1 expression was regulated at the transcriptional level without changing mRNA stability. CCAAT box on the TSP1 promoter was found to primarily mediate the trichostatin A response by deletion and mutation analyses of the TSP1 promoter. Electrophoretic mobility shift assay indicated that CCAAT-binding factor (CBF) was specifically bound to the CCAAT box of TSP1 promoter. Moreover, chromatin immunoprecipitation assay showed that trichostatin A increased the binding of acetylated form of histone H3 to the CCAAT box region of TSP1 promoter. Taken together, these results strongly suggest that trichostatin A activates the transcription of TSP1 gene through the binding of transcription factor CBF to CCAAT box and the enhanced histone acetylation. Thus, the present study provides the clue that the inhibition of angiogenesis by trichostatin A is accomplished through the upregulation of TSP1, the anti-angiogenic factor.

Anti-angiogenic effect of 5-Fluorouracil-based drugs against human colon cancer xenografts

In addition to the direct cytotoxic effects of chemotherapy agents on tumor cells, the anti-angiogenic activities attained by these agents by targeting proliferating endothelial cells in tumor blood vessels has attracted much research interest. In this study, we examined the antitumor activity of 5-Fluorouracil (5-FU)-based drugs (S-1 [1M tegafur, 0.4M 5-chloro-2,4-dihydroxypyridine and 1M potassium oxonate] and capecitabine) on human colorectal cancer xenografts and evaluated their anti-angiogenic effects. Both drugs showed significant antitumor activities against COL-1 xenografts at a sub-maximum tolerated dose (sub-MTD), which was lower than the maximum tolerated dose (MTD). At the sub-MTD, a significant reduction in the microvessel number and the enhancement of tumor-associated microvessel endothelial cell apoptosis was seen in xenografts treated with S-1. In addition, we found that thrombospondin-1 (TSP-1) expression, known to be a mediator of the anti-angiogenic effects of metronomic chemotherapy, was significantly up-regulated in xenograft tumor tissues and plasma in animals treated with S-1 at a sub-MTD. Capecitabine also showed a trend toward the induction of TSP-1. These results suggest that 5-FU-based drugs inhibit tumor progression through different modes of action, including cytotoxic activity derived from 5-FU and the inhibition of angiogenesis through the induction of TSP-1.

Differential regulation of thrombospondin-1 expression and antiangiogenesis of ECV304 cells by trichostatin A and helixor A

Trichostatin A and helixor A increased thrombospondin-1 expression by ECV304 cells at both mRNA and protein levels by transcriptional activation through the enhancement of tsp-1 promoter activity. The induction of thrombospondin-1 by these agents potently reduced ECV 304 cell migration and capillary-like tube formation on Matrigel; these findings were confirmed by the neutralization of thrombospondin-1 using a specific antibody. In the presence of exogenous vascular endothelial growth factor, however, these agents had a different effect on the vascular endothelial growth factor-induced tube formation; trichostatin A remarkably inhibited tube formation regardless of the presence of exogenous vascular endothelial growth factor, whereas helixor A reduced it to 70-80% of the control level. Interestingly, when the helixor A-generated conditioned media were concentrated three-fold and the endogenous vascular endothelial growth factor was removed, tube formation was remarkably inhibited compared with the effect of three-fold concentrated conditioned media that had endogenous vascular endothelial growth factor. Additionally, in media with endogenous vascular endothelial growth factor that were concentrated five-fold, tube formation was markedly blocked regardless of the presence of exogenous or endogenous vascular endothelial growth factor. Thus, our results indicate that trichostatin A-induced or helixor A-induced antiangiogenesis is mediated by both agents; increased, absolute and relative levels of thrombospondin-1 to the vascular endothelial growth factor level are critical in angiogenesis.

Relationship between mRNA stability and length: an old question with a new twist

The half-life of individual mRNA plays a central role in controlling the level of gene expression. However, the determinants of mRNA stability have not yet been well defined. Most previous studies suggest that mRNA length does not affect its stability. Here, we show significant negative correlations between mRNA length and stability in human and Escherichia coli, but not in Saccharomyces cerevisiae or Bacillus subtilis. This finding suggests the possibility that endonucleolytic attacks by RNA endonuclease and/or mechanical damage may strongly influence mRNA stability in both prokaryotes and eukaryotes.

Relationship between mRNA stability and intron presence

Introns were found to enhance almost every steps of gene expression except increasing mRNA stability. By analyzing the genome-wide data of mRNA stability published by someone previously, we found that human intron-containing genes have more stable mRNAs than intronless genes, and the Arabidopsis thaliana genes with the most unstable mRNAs have fewer introns than other genes in the genome. After controlling for mRNA length, we found mRNA stability is still positively correlated with intron number in human intron-containing genes. But in yeast Saccharomyces cerevisiae, two different datasets on mRNA half-life gave conflicting results. The components of messenger ribonucleoprotein particles recruited during intron splicing may be retained in cytoplasmic mRNPs and act as signals of mRNA stability or simply insulators to avoid mRNA degradation.

Bile acid-mediated thrombospondin-1 induction in hepatocytes leads to transforming growth factor-beta-dependent hepatic stellate cell activation

In cholestasis, bile acids induce hepatocyte apoptosis, while activation of hepatic stellate cells (HSCs) results in fibrosis. Since transforming growth factor-beta (TGF-beta) is a critical mediator in this process, we hypothesized that bile acids may participate in TGF-beta-mediated HSC activation in cholestasis. Bile acid treatment increased TGF-beta transcription in hepatocytes, while the total TGF-beta concentration in culture media rapidly decreased following bile acid treatment. Bile acid treatment promptly induced thrombospondin-1 expression in hepatocytes, which is a potent activator of latent TGF-beta, whereas this induction was not observed in bile acid-treated HSCs. HSCs co-cultured with hepatocytes showed a significantly higher level of Smad2 phosphorylation and collagen alpha1 synthesis following bile acid treatment than cells cultured without hepatocytes. Moreover, this enhanced collagen synthesis was significantly inhibited in the presence of TGF-beta receptor inhibitor. These observations imply that bile acids induce thrombospondin-1 expression in hepatocytes, which activates latent TGF-beta leading to HSC activation.