Research ArticleSOCS1 controls liver regeneration by regulating HGF signaling in hepatocytes
Introduction
Hepatocellular carcinoma (HCC) is an often lethal tumor with limited therapeutic options. Understanding the molecular mechanisms of hepatocarcinogenesis could lead to development of strategies to arrest, retard or even reverse the disease process in HCC patients [1]. Development of HCC follows a series of events involving inflammation, chronic liver injury, and hepatocyte proliferation [2]. The mutagenic environment created by inflammation is believed to facilitate activation of proto-oncogenes and/or inactivation of tumor suppressors, leading to deregulated hepatocyte proliferation and progression toward HCC [1], [2]. The Socs1 gene, encoding suppressor of cytokine signaling 1 (SOCS1), is frequently repressed in human HCC [3], and is suppressed by the core protein of hepatitis C virus, an etiologic agent of HCC [4]. Furthermore, Socs1+/− mice display increased HCC formation in response to the hepatocarcinogen diethylnitrosamine [5]. Despite such compelling evidence for a tumor suppressor function of SOCS1 in the liver, the underlying molecular mechanisms remain unknown.
Liver regeneration (LR) following partial hepatectomy (PH) is widely used to study regulatory mechanisms of hepatocyte proliferation that are also relevant for neoplastic growth [6], [7]. Liver regeneration involves coordinated action of distinct cytokines and growth factors, which regulate three temporal stages of hepatocyte proliferation, namely, priming, DNA synthesis, and cell division, followed by growth termination. TNFα and IL-6 are critical priming factors, which facilitate G0 to G1 transition of hepatocytes, rendering them competent to respond to growth factors. Mice lacking TNF receptor 1 show delayed liver regeneration, which could be reversed by administration of IL-6, whereas IL-6 deficiency induces severe apoptosis because IL-6-induced STAT3 activation is essential for liver regeneration [8], [9], [10]. Following priming, growth factors provide mitogenic signals that facilitate competent hepatocytes to progress through the cell cycle. Among these mitogenic factors, hepatocyte growth factor (HGF) plays an important role in hepatocyte proliferation and in the pathogenesis of HCC [1], [7]. Conditional ablation of HGF or its receptor c-Met in the adult liver impairs liver regeneration [11], [12].
In this study, we investigated whether SOCS1 controls hepatocyte proliferation by regulating cytokines and growth factors involved in hepatocyte priming and/or proliferation. To this end, we first evaluated liver regeneration in SOCS1-deficient mice. In parallel, we stimulated SOCS1-deficient and control primary hepatocytes with IL-6 or HGF and compared downstream signaling events. We also examined IL-6 and HGF signaling in murine and human hepatoma cells overexpressing SOCS1. Our findings suggest that the anti-tumor function of SOCS1 in the liver could result, at least partly, from the regulation of c-Met receptor signaling.
Section snippets
Mice and cell lines
Socs1+/−Ifng+/− mice, kindly provided by Dr. J. Ihle [13], have been backcrossed with Ifng−/− mice in C57BL/6 background (The Jackson Laboratory). All experiments were approved by the institutional Ethics Committee. Murine Hepa1-6 and human Hep3B cells were purchased from ATCC.
Reagents and antibodies
Cytokines and growth factors were from R&D Systems. LPS, bromodeoxyuridine (BrdU), collagen, and Hoechst nuclear stain were from Sigma–Aldrich. Collagenase (Blendzyme), and anti-HA, and anti-BrdU antibodies were from
SOCS1 deficiency accelerates liver regeneration
To investigate the role of endogenous SOCS1 in regulating hepatocyte proliferation, we resorted to using Socs1−/−Ifng−/− mice because Socs1−/− mice die within 3 weeks of birth due to deregulated IFNγ signaling [13]. IFNγ-deficient mice were previously shown to display increased rate of liver regeneration [16]. Because IFNγ is a strong inducer of Socs1 gene expression in hepatocytes (Supplementary Fig. 1), it is possible that the IFNγ-mediated control of liver regeneration might be dependent on
Discussion
Among the SOCS family proteins, SOCS1 and SOCS3 are implicated in hepatocellular carcinoma [3], [5], [30]. Several lines of evidence suggest that SOCS1 and SOCS3 proteins may regulate proliferation of hepatocytes. Transcription of Socs1 and Socs3 genes in the liver is induced by PH and following systemic administration of IL-6 [20], [31], [32]. IL-6-induced SOCS3 expression occurs early and lasts longer, whereas SOCS1 expression begins later and occurs transiently, suggesting very tight
Conflict of interest
The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.
Acknowledgments
This work was supported by grants from CIHR (to SI, MOP-84234) and NSERC (to CS, NSERC-342061). SI is a CIHR new investigator. CS holds a FRSQ Junior 2 fellowship. MY is a recipient of FQRNT doctoral fellowship. Centre de Recherche Clinique Étienne-Le Bel is a FRSQ-funded research center.
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Cited by (45)
Essential role of suppressor of cytokine signaling 1 (SOCS1) in hepatocytes and macrophages in the regulation of liver fibrosis
2019, CytokineCitation Excerpt :SOCS1-deficient mice are highly susceptible to LF induced by dimethylnitorsamine or CCl4, and to experimental hepatocarcinogenesis elicited by diethylnitrosamine [18,19]. Investigations into the anti-tumor mechanisms of SOCS1 revealed that SOCS1 deficiency in hepatocytes increases their responsiveness to hepatocyte growth factor and their resistance to apoptosis [19,24,25]. However, the anti-fibrogenic mechanisms of SOCS1 remain to be elucidated.
Hepatocyte growth control by SOCS1 and SOCS3
2019, CytokineCitation Excerpt :TNFR2-deficient mice treated with TNFα strongly upregulate Socs1, Socs2 and Socs3 genes [79]. Even though the increased rate of DNA synthesis and the rapid restoration of functional liver mass in Socs1-null mice [43] could result from the loss of SOCS1-dependent regulation of NF-κB signaling (Fig. 3, discussed in the previous section), direct evidence linking SOCS1 to the regulation of TNFα-induced cell survival and apoptotic pathways in the liver is currently lacking. IL-6 deficient mice show impaired liver regeneration characterized by markedly reduced induction of the immediate early genes (c-Myc, c-Fos, c-Jun) and delayed DNA synthesis [80].
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These authors contributed equally to this work.