Cell Type-specific Post-transcriptional Regulation of Production of the Potent Antiangiogenic and Proatherogenic Protein Thrombospondin-1 by High Glucose

Abstract

Hyperglycemia is an independent risk factor for development of vascular diabetic complications. Vascular dysfunction in diabetics manifests in a tissue-specific manner; macrovasculature is affected by atherosclerotic lesions, and microvascular complications are described as "aberrant angiogenesis": in the same patient angiogenesis is increased in some tissues (e.g. retinal neovascularization) and decreased in others (e.g. in skin). Molecular cell-and tissue-specific mechanisms regulating the response of vasculature to hyperglycemia remain unclear. Thrombospondin-1 (TSP-1), a potent antiangiogenic and proatherogenic protein, has been implicated in the development of several vascular diabetic complications (atherosclerosis, nephropathy, and cardiomyopathy). This study examines cell type-specific regulation of production of thrombospondin-1 by high glucose. We previously reported the increased expression of TSP-1 in the large arteries of diabetic animals. mRNA and protein levels were up-regulated in response to high glucose. Unlike in macrovascular cells, TSP-1 protein levels are dramatically decreased in response to high glucose in microvascular endothelial cells and retinal pigment epithelial cells (RPE). This downregulation is post-transcriptional; mRNA levels are increased. In situ mRNA hybridization and immunohistochemistry revealed that the level of mRNA is up-regulated in RPE of diabetic rats, whereas the protein level is decreased. This cell type-specific posttranscriptional suppression of TSP-1 production in response to high glucose in microvascular endothelial cells and RPE is controlled by untranslated regions of TSP-1 mRNA that regulate coupling of TSP-1 mRNA to polysomes and its translation. The cellspecific regulation of TSP-1 suggests a potential mechanism for the aberrant angiogenesis in diabetics and TSP-1 involvement in development of various vascular diabetic complications.Despite the significant advances in the therapeutic methods to control blood glucose and insulin levels in diabetic patients, the precise regulation of these levels remains a problem. Vascular diabetic complications remain most prevalent and dangerous and account for the greatest numbers of deaths and hospitalizations in diabetic patients. The molecular basis for the vascular complications of diabetes is not well understood. Recent reports indicate that both microvascular and macrovascular complications of diabetes correlate directly with glucose levels in both patients and animal models (1-5). Some of these reports revealed the pathogenic role of impaired glucose tolerance and post-prandial hyperglycemia even in the absence of diabetes, e.g. (6). In vascular cells, glucose regulates expression of many genes that have been linked to the development of atherosclerosis or abnormal angiogenesis (reviewed in Ref. 7). One of them is thrombospondin-1 (TSP-1), 3 a cell matrix protein implicated in both atherogenesis (8 -12) and angiogenesis (13)(14)(15)(16)(17)(18)(19). Several lines of evidence indicate that TSP-1 may represent a lin...

Glucose and insulin modify thrombospondin 1 expression and secretion in primary adipocytes from diet-induced obese rats

“…This stimulation seems to be mediated by glycosylation of specific nuclear proteins that participate in the hexosamine pathway of glucose catabolism [34]. However, in another report from the same investigation group, a cell-type specific modulation by HG treatment was observed [2]. In such work, the authors described that, unlike in macrovascular cells, TSP-1 levels dropped as a result of the HG treatment in microvascular endothelial cells and retinal pigment epithelial cells.…”

“…Also, gene expression pattern differed drastically from the protein secretion levels under HG conditions, being inhibited and induced by this treatment, respectively. These phenomena could be attributed to post-translational mechanisms [2].…”

“…More recently, this protein has been studied as a potential link between vasculature mechanisms and hyperglycemia [2,39]. An increase in TSP-1 production has been related with atherogenesis and restenosis in diabetes, since this increase could be a response of vascular cells to higher levels of glucose [39].…”

Embracing the complexity of matricellular proteins: the functional and clinical significance of splice variation

“…In support of the importance of UTR regions in matricellular protein expression, TSP-1 in particular has a long 3′ UTR that has been analysed in detail, and is known to contain AU-rich elements that regulate mRNA stability (17). Indeed, the regulation of TSP-1 expression by glucose occurs at the level of translation and is mediated by miR-467 binding to the 3′-UTR (57,58). SPARC expression is also regulated by non-coding RNA, and in particular is regulated by miR029a/b in nasopharangeal cancer (59).…”

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

“…The best-characterized regulatory mechanisms for these changes in expression involve inducible changes in TSP-1 transcription [6]. However, changes in mRNA stability have recently been suggested to play a role in the modulation of TSP-1 gene expression [12][13][14][15]. mRNAs that are post-transcriptionally regulated often encode proteins with potent biological activities that peak shortly after an extracellular stimulus and then undergo rapid decay [16][17][18][19].…”