Post-transcriptional gene regulation by HuR promotes a more tumorigenic phenotype

Abstract

In a breast tumor xenograft model, the MCT-1 oncogene increases the in vivo tumorgenicity of MCF7 cells by promoting angiogenesis and inhibiting apoptosis. Increases in the tumor microvascular density are accompanied by a strong reduction in the levels of the angiogenesis inhibitor thrombospondin-1 (TSP1), but the mechanisms underlying this process are unknown. We show that TSP1 expression is controlled, at least in part, by post-transcriptional events. Using RNA interference to knock down the expression of the RNA-binding protein HuR in MCF7 cells as well as HuR overexpression, we demonstrate that HuR plays an important role in translation of the TSP1 mRNA. Furthermore, employing the RIP-Chip assay yielded 595 transcripts with significantly altered binding to HuR in the more tumorigenic breast cancer clones compared with the weakly tumorigenic clones. These mRNAs clustered in several pathways implicated in the transformed phenotype, such as the RAS pathway (involved in mitogenesis), the PI3K pathway (evasion of apoptosis) and pathways mediating angiogenesis and the cellular response to hypoxia. These findings demonstrate for the first time that global changes in HuR-bound mRNAs are implicated in the evolution to a more tumorigenic phenotype in an in vivo tumor model and underscore the role of global mRNA-protein interactions toward tumor progression.

Overexpression of the RNA binding protein HuR impairs tumor growth in triple negative breast cancer associated with deficient angiogenesis

“…33,34 As previously mentioned HuR has been described to stabilize TSP1 and VEGF mRNA resulting in greater levels and increased protein expression. 21,27 In contradistinction to published reports of HuR stabilization of VEGF mRNA, our findings indicate that under conditions of HuR overexpression, there are decreases in both VEGF mRNA and protein levels. The reasons for this discrepancy are presently unknown, though the results may be dependent upon levels of HuR overexpression and hypoxia.…”

“…TSP1 is a well known anti-angiogenetic factor and has been described to be regulated by HuR. 27 Enumeration of angiogenesis by CD34 staining confirmed that HuR overexpression resulted in significant decreases in new blood vessel formation. This decrease in neovascularization may, in part, explain why tumors formed by HA-HuR overexpression were much smaller than EV controls.…”

“…We conducted experiments which targeted well known HuR target genes involved in angiogenesis which had previously been described in the literature: thrombospondin 1 (TSP1), VEGF and HIF1α. 21,22,27 We therefore performed real-time PCR to measure mRNA levels of TSP1, VEGF and HIF1α. As seen in Figure 3A, HuR overexpression caused an increase in TSP1 mRNA and protein (Fig.…”

Rapid Insulin-like Growth Factor-1-induced Changes in Granulosa Cell Thrombospondin-1 Expression In Vitro

“…Other AUBPs such as tristetraprolin (TTP) and additional members of the TTP family have been shown to have similar effects, while still other AUBPs such as HuR appear to promote enhanced transcript stability (reviewed in [14]). Very recently, HuR has been shown to bind to the TSP-1 mRNA and increase its stability, although the site of this interaction was not characterized [17]. The conditions under which AUBPs such as AUF1 and HuR bind to TSP-1 mRNA in granulosa cells, and whether such interactions are regulated by IGFs or other factors relevant to the growth and differentiation of granulosa or other follicular cells, remains a subject of active investigation.…”

“…The presence of these elements mediates a strong destabilizing effect on TSP-1 mRNA transcripts, resulting in a rapid reduction of TSP-1 mRNA. Recently, HuR, an AUBP known to stabilize target mRNAs and enhance their translation, has been shown to associate with the TSP-1 mRNA in breast cancer cells, although the HuR binding site on the mRNA was not identified in that study [17]. The stability of the TSP-1 mRNA is also influenced by cellular expression of the oncogene c-myc, although this has not been shown to involve ARE-medicated decay [18].…”