MicroRNA-18a Enhances the Interleukin-6-mediated Production of the Acute-phase Proteins Fibrinogen and Haptoglobin in Human Hepatocytes

Abstract

The acute-phase response is an inflammatory process triggered mainly by the cytokine IL-6. Signaling of IL-6 is transduced by activation of STAT3 (signal transducer and activator of transcription 3), which rapidly induces the production of acute-phase proteins such as haptoglobin and fibrinogen. Another target of the IL-6/STAT3 signal transduction pathway is the microRNA cluster miR-17/92. Here, we investigated the interplay of miR-17/92 and STAT3 signaling and its impact on the acute-phase response in primary human hepatocytes and hepatoma (HepG2) cells. Employing a reporter gene system consisting of STAT3-sensitive promoter sequences, we show that the miR-17/92 cluster member miR-18a enhanced the transcriptional activity of STAT3. IL-6 stimulation experiments in miR-18a-overexpressing hepatocytes and HepG2 cells revealed an augmented acute-phase response indicated by increased expression and secretion of haptoglobin and fibrinogen. This effect was due, at least in part, to repression of PIAS3 (protein inhibitor of activated STAT, 3), a repressor of STAT3 activity, which we identified as a novel direct target of miR-18a. Finally, we demonstrate that the expression of miR-17/92 in primary hepatocytes and HepG2 cells is modulated by IL-6. Our data reveal, for the first time, a microRNA-mediated positive feedback loop of IL-6 signal transduction leading to an enhanced acute-phase response in human hepatocytes.Severe inflammation is observed in several conditions, including overwhelming infection, systemic inflammatory response syndrome, sepsis, and septic shock, and results in the activation of a signaling cascade that is commonly referred to as the acute-phase response. Acute-phase proteins are involved in a great variety of physiological and biochemical processes, including growth inhibition of microbes (e.g. complement factors), blood coagulation (e.g. fibrinogen), and binding of proteins (e.g. haptoglobin-mediated binding of hemoglobin) (1).During inflammation, the production of plasma proteins by hepatocytes is altered either by increasing the levels of plasma proteins (positive acute-phase reaction) or by decreasing their levels (negative acute-phase reaction).IL-6 is one of the most important cytokines that orchestrates the hepatic production of acute-phase proteins (2). For signal transduction, IL-6 binds to a membrane-associated receptor complex, which subsequently leads to the phosphorylation of Tyr-705 of STAT3 (signal transducer and activator of transcription 3) (3). Phosphorylated STAT3 is actively transported to the nucleus, where it activates the transcription of IL-6 target genes such as the acute-phase genes haptoglobin (Hp) 2 (4) and fibrinogen ␥-chain (FGG) (5). Several negative feedback loops antagonize the activation of STAT3 to avoid persistent downstream signaling and subsequent dysregulation in the production of acute-phase proteins. A well characterized feedback pathway includes the blocking of the phosphorylation of STAT proteins by the action of the cytokine-inducible family of SOCS (sup...

Interleukin-6: Biology, signaling and strategies of blockade

“…IL-6 induces miR-18a expression. This micro RNA suppresses PIAS3 protein expression, consequently leading to enhanced STAT3-dependent acute-phase gene expression [143]. Surprisingly, this study also indicates enhanced STAT3 phosphorylation and STAT3 protein expression due to miR-18a expression, which is in clear contrast to the initial idea of PIAS3-mediated inhibition of STAT3 by Shuai and coworkers [139].…”

Interleukin-6 influences stress-signalling by reducing the expression of the mTOR-inhibitor REDD1 in a STAT3-dependent manner

“…To test whether miRNA-dependent regulation of gene expression is involved in IL-6-dependent repression of REDD1-mRNA we depleted Dicer to impair miRNA processing by an siRNA approach. To examine whether knock-down of Dicer reduces the processing of miRNAs we analysed exemplarily the expression of miR-18a_2 which is constitutively expressed in liver cells [26]. Suppl.…”

Hepatic Premalignant Alterations Triggered by Human Nephrotoxin Aristolochic Acid I in Canines

“…In the immune system, enhanced regulation of NF-kB signaling is crucial for maintaining the normal function of immune cells and avoidance of tumorigenesis (28,29). Although the importance of miRNAs in the signal transduction of IL6 and NF-kB has been highlighted in human HCC (19,28,30) and in human acute-phase response of hepatocytes (31), to date, whether IL6R/NF-kB contributes to miRNA dysregulation, thereby causing premalignance in liver by short-term AAI exposure has not yet been illustrated.…”