ABSTRACT
OBJECTIVE: Bone morphogenetic protein (BMP)-9, a member of the transforming growth factor-ß family of cytokines, is constitutively produced in the liver. Systemic levels act on many organs and tissues including bone and endothelium, but little is known about its hepatic functions in health and disease. DESIGN: Levels of BMP-9 and its receptors were analysed in primary liver cells. Direct effects of BMP-9 on hepatic stellate cells (HSCs) and hepatocytes were studied in vitro, and the role of BMP-9 was examined in acute and chronic liver injury models in mice. RESULTS: Quiescent and activated HSCs were identified as major BMP-9 producing liver cell type. BMP-9 stimulation of cultured hepatocytes inhibited proliferation, epithelial to mesenchymal transition and preserved expression of important metabolic enzymes such as cytochrome P450. Acute liver injury caused by partial hepatectomy or single injections of carbon tetrachloride (CCl4) or lipopolysaccharide (LPS) into mice resulted in transient downregulation of hepatic BMP-9 mRNA expression. Correspondingly, LPS stimulation led to downregulation of BMP-9 expression in cultured HSCs. Application of BMP-9 after partial hepatectomy significantly enhanced liver damage and disturbed the proliferative response. Chronic liver damage in BMP-9-deficient mice or in mice adenovirally overexpressing the selective BMP-9 antagonist activin-like kinase 1-Fc resulted in reduced deposition of collagen and subsequent fibrosis. CONCLUSIONS: Constitutive expression of low levels of BMP-9 stabilises hepatocyte function in the healthy liver. Upon HSC activation, endogenous BMP-9 levels increase in vitro and in vivo and high levels of BMP-9 cause enhanced damage upon acute or chronic injury.
Subject(s)
Acute Lung Injury/physiopathology , Growth Differentiation Factor 2/metabolism , Growth Differentiation Factor 2/pharmacology , Hepatic Stellate Cells/metabolism , Hepatocytes/physiology , Liver Cirrhosis/metabolism , Liver Regeneration/drug effects , Acute Lung Injury/genetics , Animals , Cell Proliferation/drug effects , Cells, Cultured , Disease Models, Animal , Down-Regulation/drug effects , Epithelial-Mesenchymal Transition/drug effects , Growth Differentiation Factor 2/antagonists & inhibitors , Growth Differentiation Factor 2/genetics , Hepatectomy , Hepatocytes/drug effects , Hepatocytes/enzymology , Lipopolysaccharides/pharmacology , Liver Cirrhosis/genetics , Male , Mice , Mice, Inbred C57BL , Mice, KnockoutABSTRACT
The hepatic hormone hepcidin is the master regulator of systemic iron homeostasis. Its expression level is adjusted to alterations in iron levels, inflammatory cues, and iron requirements for erythropoiesis. Bone morphogenetic protein 6 (BMP6) contributes to the iron-dependent control of hepcidin. In addition, TGF-ß1 may stimulate hepcidin mRNA expression in murine hepatocytes and human leukocytes. However, receptors and downstream signaling proteins involved in TGF-ß1-induced hepcidin expression are still unclear. Here we show that TGF-ß1 treatment of mouse and human hepatocytes, as well as ectopic expression of TGF-ß1 in mice, increases hepcidin mRNA levels. The hepcidin response to TGF-ß1 depends on functional TGF-ß1 type I receptor (ALK5) and TGF-ß1 type II receptor (TßRII) and is mediated by a noncanonical mechanism that involves Smad1/5/8 phosphorylation. Interestingly, increasing availability of canonical Smad2/3 decreases TGF-ß1-induced hepcidin regulation, whereas the BMP6-hepcidin signal was enhanced, indicating a signaling component stoichiometry-dependent cross-talk between the two pathways. Although ALK2/3-dependent hepcidin activation by BMP6 can be modulated by each of the three hemochromatosis-associated proteins: HJV (hemojuvelin), HFE (hemochromatosis protein), and TfR2 (transferrin receptor 2), these proteins do not control the ALK5-mediated hepcidin response to TGF-ß1. TGF-ß1 mRNA levels are increased in mouse models of iron overload, indicating that TGF-ß1 may contribute to hepcidin synthesis under these conditions. In conclusion, these data demonstrate that a complex regulatory network involving TGF-ß1 and BMP6 may control the sensing of systemic and/or hepatic iron levels.
