TIAR and TIA-1 mRNA binding proteins co-aggregate under conditions of rapid oxygen decline and extreme hypoxia, suppress HIF-1alpha pathway and inhibit proliferation and angiogenesis
T-cell intracellular antigen (TIA)-1 and TIA-1 related protein (TIAR) are mRNA-binding proteins that aggregate within stress granules under specific stress conditions. In this study, we analyzed TIAR/TIA-1 aggregation under different hypoxic conditions, and studied the effects on hypoxia-inducible factor (HIF)-1alpha, as well as on proliferation and angiogenesis. TIAR/TIA-1 formed stress granules under acute and pronounced hypoxic conditions in A549 adenocarcinoma cells. In parallel, HIF-1alpha expression was blocked in cells displaying TIAR/TIA-1 stress granules. Silencing of TIAR and TIA-1 caused upregulation of HIF-1alpha expression, as demonstrated by western blot, immunocytochemistry, and HIF-1alpha-dependent reporter gene expression. TIAR/TIA-1 are suggested to mediate their effects by interaction with certain mRNA regions. Thus, a critical region of the 3 end of the untranslated HIF-1alpha cDNA with possible adenosine-uridine rich (AUR) elements was coupled to a reporter gene plasmid. Indeed, this coupling caused downregulation of the reporter gene expression, supporting the relevance of this mRNA region for the effects of TIAR/TIA-1 exerted on HIF-1alpha. Furthermore, employing this reporter construct inhibition of TIAR by siRNA attenuated the inhibitory cis-effect of this AUR sequence. Additionally, immunohistochemical analysis of A549 cell tumor xenografts revealed a nearly complementary expression of HIF-1alpha and TIAR. This in vivo observation may reflect the inhibitory regulation of HIF-1alpha by TIAR as revealed in the cell culture studies. In sum, rapid and severe hypoxia caused TIAR/TIA-1-dependent formation of stress granules, specifically suppressing HIF-1alpha expression. Additionally TIAR/TIA-1 was demonstrated to exhibit inhibitory effects on proliferation and angiogenesis.
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