Control of cytokine mRNA expression by RNA-binding proteins and microRNAs

Abstract

Cytokines are critical mediators of inflammation and host defenses. Regulation of cytokines can occur at various stages of gene expression, including transcription, mRNA export, and post- transcriptional and translational levels. Among these modes of regulation, post-transcriptional regulation has been shown to play a vital role in controlling the expression of cytokines by modulating mRNA stability. The stability of cytokine mRNAs, including TNFα, IL-6, and IL-8, has been reported to be altered by the presence of AU-rich elements (AREs) located in the 3'-untranslated regions (3'UTRs) of the mRNAs. Numerous RNA-binding proteins and microRNAs bind to these 3'UTRs to regulate the stability and/or translation of the mRNAs. Thus, this paper describes the cooperative function between RNA-binding proteins and miRNAs and how they regulate AU-rich elements containing cytokine mRNA stability/degradation and translation. These mRNA control mechanisms can potentially influence inflammation as it relates to oral biology, including periodontal diseases and oral pharyngeal cancer progression.

Figure 1.

Diagram of cytokine mRNA and sites of post-transcriptional regulation. Inflammatory cytokines are regulated post-transcriptionally through both the 5′- and 3′-untranslated regions (UTRs). The 5′UTR dictates mRNA translation initiation, whereas the 3′UTR dictates mRNA turnover. Cytokines are highly regulated via their AU-rich elements (AREs). TNF-α, IL-6, IL-8, and COX-2 AREs are described in the diagram.

Figure 2.

Signaling pathways that regulate cytokine AREs post-transcriptionally. The p38 pathway is the best-described signaling pathway that regulates AU-rich element (ARE)-mediated cytokine degradation. Activation of IL-1β receptors and the Toll-like receptor (TLR) family stimulates p38 MAPK signaling via signaling intermediates, including MKK3/6. Active p38 dictates mRNA stability through activation of MK2, which subsequently phosphorylates and inactivates TTP. Once inactivated, phosphorylated TTP dissociates from the ARE region of cytokine transcripts to enhance mRNA stability and translation, thereby causing an increase in cytokine secretion that can promote chronic inflammation and inflammatory disease progression if not properly regulated. In addition, the phosphatidylinositol-3 kinase (PI3K) pathway can also post-transcriptionally regulate mRNAs through AKT phosphorylation of RBPs. This phosphorylation alleviates the mRNAs of RBPs that normally destabilize the mRNA and results in increased mRNA stability and thus increased cytokine protein production and secretion, which, if not properly controlled, can induce chronic inflammation.

Figure 3.

Model of the cooperative function between RNA-binding proteins (RBPs) and miRNAs in the regulation of cytokine mRNAs. Deregulation of miRNAs or RBPs in cancer can be due to the altered expression, localization, activity, or stability of these regulators. The mRNA is presented linearly for simplicity. CR represents a coding region. Under normal conditions, both the AU-rich element-binding protein TTP and miRNAs are involved in the destabilization of cytokine mRNAs, and the non-phosphorylated form of HuR disassociates from mRNAs. However, in response to inflammation and cancer progression, HuR becomes phosphorylated and translocates to the cytoplasm, where it stabilizes mRNAs along with miRNAs that are known to repress translation. This action releases the mRNAs from TTP- and miRNA-mediated destability and repression, respectively. Moreover, inflammation induces TTP phosphorylation and disassociates it from mRNAs along with destabilizing miRNAs.