HuR post-transcriptionally regulates TNF-α-induced IL-6 expression in human pulmonary microvascular endothelial cells mainly via tristetraprolin

HuR and tristetraprolin (TTP) are both RNA-binding proteins, which are characterized as binding to the AU-rich elements (AREs) in the 3′-untranslated regions (3′-UTRs) of target mRNAs. Studies have shown that some ARE-containing mRNAs are stabilized by HuR, whereas are destabilized by TTP. Our previous study showed that HuR upregulated tumor necrosis factor-α (TNF-α)-induced interleukin-6 (IL-6) expression by stabilizing its mRNA in human pulmonary microvascular endothelial cells (HPMECs). Considering IL-6 mRNA has AREs, we decided to examine whether TTP was also involved in the regulation of TNF-α-induced IL-6 expression in HPMECs and whether HuR and TTP influenced each other at protein and mRNA level. Here, we report that TTP silencing increased IL-6 levels. HuR silencing increased TTP expression. TTP had no effect on HuR expression and subcellular localization. Compared to TTP silencing alone, double knockdown of HuR and TTP did not significantly reduce IL-6 release. The RNA-binding protein immunoprecipitation (RIP) results further showed that TTP but not HuR bound to intracellular IL-6 mRNA in HPMECs. We demonstrate for the first time that HuR post-transcriptionally regulates IL-6 expression mainly via TTP.

Graphical abstractDownload full-size imageHighlights► Tristetraprolin (TTP) negatively regulates TNF-α-induced IL-6 expression. ► HuR knockdown increases TTP levels, whereas TTP has no effect on HuR expression and subcellular localization. ► TTP silencing abrogates the regulation of IL-6 expression by HuR, which implies that HuR regulates IL-6 expression mainly via TTP. ► TTP but not HuR binds to intracellular IL-6 mRNA in HPMECs.