Ciliary neurotrophic factor induces acute-phase protein expression in hepatocytes.

During inflammatory states, hepatocytes are induced to synthesize and secrete a group of proteins called acute-phase proteins. It has recently been shown that besides interleukin-6 (IL-6), related cytokines such as leukemia inhibitory factor, oncostation M and interleukin-11 are also mediators of the hepatic acute-phase response. All these mediators belong to the hematopoietic family of alpha-helical cytokines. Here we show that an additional member of this cytokine family, ciliary neurotrophic factor (CNTF), induces the hepatic acute-phase protein genes haptoglobin, alpha 1-antichymotrypsin, alpha 2-macroglobulin and beta-fibrinogen in human hepatoma cells (HepG2) and in primary rat hepatocytes with a time course and dose-response comparable with that of IL-6. Our next aim was to define the receptor components used by CNTF on hepatic cells. Using a cell-free binding assay we exclude that CNTF binds to the 80 kDa IL-6 receptor, a protein with significant homology to the CNTF receptor which has recently been cloned from neuroblastoma cells. In human hepatoma cells (Hep3B) which lack the leukemia inhibitory factor receptor, CNTF was not able to induce acute-phase protein synthesis, indicating that this receptor protein may be part of the functional CNTF receptor on hepatic cells.     

Induction of phospholipase A2 gene expression in human hepatoma cells by mediators of the acute phase response.

Serum levels of phospholipase A2 (PLA2) activity have been shown to be elevated in cases of septic shock and rheumatoid arthritis. The cellular origin of serum PLA2, however, is not known. In this report, we demonstrate that human group II PLA2 expression and secretion are induced in hepatoma cells (HepG2) following treatment with interleukin-6 (IL-6), tumor necrosis factor (TNF), and interleukin-1 (IL-1). Of the three cytokines, IL-6 is the most potent. Significant synergy is observed between IL-6 and IL-1 and between IL-6 and TNF, but not between IL-1 and TNF. PLA2 induction does not occur in human YT cells, which are known to have receptors for both IL-1 and IL-6, indicating that the regulatory mechanism involved is cell type-specific. The results of RNA blot analysis indicate that the PLA2 gene is regulated in HepG2 cells at the pretranslational level. Induction of PLA2 synthesis in HepG2 cells in response to these cytokines resembles the induction of the acute phase plasma proteins which are synthesized in cultured hepatocytes and hepatoma cells following exposure to the same cytokines and in liver in response to inflammation and infection. In addition, a putative IL-6-responsive element, which is homologous to a similar element found in several acute phase genes, is present in the 5'-promoter-proximal region of the PLA2 gene. These results suggest that serum PLA2 is synthesized in and secreted from liver cells in response to inflammatory stimuli, mediated primarily by IL-6, and therefore should be classified as an acute phase protein.     

CD14 is an acute-phase protein

The origin of soluble CD14 (sCD14) in the circulation is uncertain. To examine whether CD14 could be an acute-phase protein (APP), the levels of sCD14, IL-6, and C-reactive protein were determined by ELISA in serum and synovial fluid (SF) of patients with various arthropathies, and the regulation of CD14 synthesis was examined in liver cells. In patients with crystal-mediated or immunologically mediated arthritis (rheumatoid arthritis), serum levels of sCD14 were higher than or similar to those found in infection-mediated arthritis (reactive arthritis), precluding a relation with bacteria exposure. Levels of sCD14 were similar in SF and serum, and did not correlate with the number of SF leukocytes, excluding an important source from leukocyte membrane-bound CD14, by protease-mediated shedding. In contrast, serum levels of sCD14 in patients correlated with those of C-reactive protein, a classical APP, and IL-6, a cytokine known to regulate the synthesis of APP in the liver. Serum levels of sCD14 also correlated with disease activity in rheumatoid arthritis and reactive arthritis patients. IL-6 stimulated the production of CD14 by HepG2 hepatoma cells. By real-time PCR, the inducibility of CD14 by IL-6 was also observed at the mRNA level both in HepG2 cells and human primary hepatocytes. These in vitro results were confirmed by in vivo studies in IL-6(-/-) mice injected with turpentine, an experimental model of acute-phase response. Liver levels of CD14 mRNA increased in IL-6(+/+), but not in IL-6(-/-) mice. These results indicate that sCD14 can be considered as a type 2 APP.     

Differential responses of hematopoietic and non-hematopoietic cells to anti-inflammatory cytokines: IL-4, IL-13 and IL-10.

Recombinant preparations of human anti-inflammatory cytokines: IL-4, IL-13 and IL-10, inhibited LPS-induced synthesis of TNFalpha and IL-6 in the whole human blood tested in vitro. These cytokines also inhibited LPS-induced IL-6 and TNF mRNA accumulation in isolated human blood monocytes/macrophages. On the other hand, similar concentrations of IL-4 and IL-13 (but not IL-10) enhanced synthesis of IL-6 in cultured human umbilical vein endothelial cells (HUVEC). In human hepatoma HepG2 cells IL-4 and IL-13 (but not IL-10) inhibited IL-6-induced synthesis of haptoglobin. These differential responses to the tested anti-inflammatory cytokines were observed at mRNA and protein levels and may reflect cell specificities in signalling pathways and gene expression. When HUVEC and HepG2 cells were cultured together and stimulated with LPS the addition of IL-4 or IL-13 resulted in the reduction of LPS-induced and IL-6-mediated haptoglobin synthesis. Thus in co-culture the inhibitory effects of IL-4 or IL-13 on HepG2 cells prevail over stimulation of IL-6 synthesis in HUVEC.     

Complex of soluble human IL-6-receptor/IL-6 up-regulates expression of acute-phase proteins.

IL-6 is a major regulator of acute phase protein synthesis in the liver. It exerts its action via a plasma membrane receptor consisting of two subunits, a ligand binding 80-kDa glycoprotein and a 130-kDa glycoprotein involved in signal transduction. We genetically generated a soluble form of the 80-kDa subunit of the human IL-6R (shIL-6R) in mouse fibroblasts (NIH/3T3 cells). The shIL-6R added to human hepatoma cells (HepG2) amplified the induction of alpha 1-antichymotrypsin and haptoglobin by IL-6 at the mRNA and protein level. Moreover, a model for a liver permanently exposed to high IL-6 concentrations has been developed; HepG2 cells were stably transfected with human IL-6-cDNA; 10(6) of the transfected cells (HepG2-IL-6) synthesized and secreted 2 micrograms of IL-6 within 24 h. Incubation of these cells with endogenous or exogenous IL-6 did not result in acute-phase protein induction. However, these IL-6-desensitized cells responded to other cytokines such as leukemia inhibitory factor, transforming growth factor beta 1, and IFN-gamma, known to modulate acute phase protein synthesis in the liver. Incubation of HepG2-IL-6 cells with shIL-6R reconstituted their responsiveness to IL-6 in a dose- and time-dependent manner. The possible biologic role that might be played by the shIL-6R in disease is discussed.     

Cross-talk between alpha(1B)-adrenergic receptor (alpha(1B)AR) and interleukin-6 (IL-6) signaling pathways. Activation of alpha(1b)AR inhibits il-6-activated STAT3 in hepatic cells by a p42/44 mitogen-activated protein kinase-dependent mechanism

Treatment of primary rat hepatocytes or tranfected HepG2 cells with the alpha(1B)-adrenergic receptor (alpha(1B)AR) agonist phenylephrine (PE) significantly inhibited interleukin 6 (IL-6)-induced STAT3 binding, tyrosine phosphorylation, and IL-6-induced serum amyloid A mRNA expression. Western analyses and in vitro kinase assays indicate that this inhibition is not due to either down-regulation of STAT3 protein expression nor inactivation of upstream-located JAK1 and JAK2. Blocking the new RNA and protein syntheses antagonized the inhibitory effect of PE on IL-6-activated STAT3, suggesting synthesis of an inhibitory factor(s) is involved. The inhibitory effect of PE on IL-6 activation of STAT3 was also abolished by the tyrosine phosphatase inhibitor sodium vanadate, indicating involvement of protein tyrosine phosphatases. Furthermore, preincubation of the cells with the specific MEK1 inhibitor PD98059 or a dominant negative MEK1 reversed the inhibitory effect of PE, and expression of constitutively activated MEK1 alone abolished IL-6-activated STAT3. Taken together, these data indicate that PE inhibits IL-6 activation of STAT3 in hepatic cells by a p42/44 mitogen-activated protein kinase-dependent mechanism, and tyrosine phosphatases are involved. This inhibitory cross-talk between the alpha(1B)AR and IL-6 signaling pathways implicates the alpha(1B)AR involvement in regulating the IL-6-mediated inflammatory responses.     

A new role for epidermal cell-derived thymocyte activating factor/IL-1 as an antagonist for distinct epidermal cell function

It is known that many immunologic responses to IL-1 are antagonized by the neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH). This led us to investigate the possible reciprocal effects of IL-1 and the functionally related epidermal cytokines, epidermal cell-derived thymocyte activating factor (ETAF) and IL-6, on the melanogenic effect of alpha-MSH on murine Cloudman melanoma cells. When these cells were treated with ETAF in combination with alpha-MSH or its potent analog [Nle4,D-Phe7]-alpha-MSH, the melanotropin induced increase in tyrosinase activity, and thus melanin synthesis, was abrogated. This inhibitory effect of ETAF was not mediated by competitive binding to the melanotropin receptor, because ETAF also blocked the melanogenic response of melanoma cells to isobutyl methylxanthine (IBMX) and to PGE1 and PGE2. ETAF had no effect on cellular proliferation. Inhibition of the stimulated tyrosinase activity by ETAF was not due to diminished cAMP synthesis or increased cAMP degradation. Cells treated concomitantly with ETAF and alpha-MSH, IBMX, or PGE1 had the same cAMP levels as cells treated with alpha-MSH, IBMX, or PGE1 alone. In contrast to ETAF, human rIL-1 alpha or IL-1 beta alone or in combination did not have an inhibitory effect on melanogenesis. IL-6 significantly inhibited the basal level of tyrosinase and partially abrogated the alpha-MSH-induced tyrosinase activity. IL-6 also stimulated cellular proliferation when added alone or in combination with alpha-MSH. Granulocyte-macrophage colony stimulating factor (GM-CSF) did not alter either the tyrosinase activity or cellular replication at the concentrations tested. IL-1 alpha, GM-CSF, and IL-6 or IL-1 alpha and GM-CSF added together did not significantly affect the MSH-induced tyrosinase activity. These results ascribe a new potential function for ETAF and IL-6 as modulators of the melanogenic response of pigment cells.     

Translation of the alzheimer amyloid precursor protein mRNA is up-regulated by interleukin-1 through 5'-untranslated region sequences

The amyloid precursor protein (APP) has been associated with Alzheimer's disease (AD) because APP is processed into the beta-peptide that accumulates in amyloid plaques, and APP gene mutations can cause early onset AD. Inflammation is also associated with AD as exemplified by increased expression of interleukin-1 (IL-1) in microglia in affected areas of the AD brain. Here we demonstrate that IL-1alpha and IL-1beta increase APP synthesis by up to 6-fold in primary human astrocytes and by 15-fold in human astrocytoma cells without changing the steady-state levels of APP mRNA. A 90-nucleotide sequence in the APP gene 5'-untranslated region (5'-UTR) conferred translational regulation by IL-1alpha and IL-1beta to a chloramphenicol acetyltransferase (CAT) reporter gene. Steady-state levels of transfected APP(5'-UTR)/CAT mRNAs were unchanged, whereas both base-line and IL-1-dependent CAT protein synthesis were increased. This APP mRNA translational enhancer maps from +55 to +144 nucleotides from the 5'-cap site and is homologous to related translational control elements in the 5'-UTR of the light and and heavy ferritin genes. Enhanced translation of APP mRNA provides a mechanism by which IL-1 influences the pathogenesis of AD.     

Interleukin-1 beta is a potent growth inhibitor of adult rat hepatocytes in primary culture

Interleukin-1 beta (IL-1 beta) strongly inhibited DNA synthesis of adult rat hepatocytes in primary culture stimulated by insulin and epidermal growth factor (EGF). Its effect was dose-dependent and was maximal at 2 ng/ml. IL-1 beta had no cytotoxic effect but changed the cells from a flat to a spindle shape as shown by phase-contrast microscopy. The inhibition of DNA synthesis by IL-1 beta was closely correlated with a decrease in the labeling index. This inhibitory effect was observed only when IL-1 beta was added for 10 h to cultured hepatocytes in the G1 phase within 12 h after addition of insulin and EGF: it was not observed in the S phase, which starts about 24 h after addition of the mitogens. Exposure of the hepatocytes to IL-1 beta for two 1-h periods, one at an early stage (0-6 h) and one at a late stage (6-12 h) of the G1 phase, resulted in the same marked inhibition of DNA synthesis as exposure to IL-1 beta for 10 h in the G1 phase. This requirement of IL-1 beta at two stages in the G1 phase for inhibition of DNA synthesis of hepatocytes is different from that with transforming growth factor-beta, which is required for only 1 h in the early G1 phase for a similar inhibition. These findings suggest that IL-1 beta acts at two distinct stages in the G1 phase and that its cooperative actions are necessary to inhibit growth of adult rat hepatocytes in primary culture. Other cytokines, such as IL-6/B-cell stimulating factor-2, were less potent, but caused significant inhibition of DNA synthesis of adult rat hepatocytes at 2 ng/ml, whereas IL-2 and tumor necrosis factor did not affect hepatocyte growth. From these results it is suggested that Kupffer cells in liver lobules and macrophages in the blood may play important roles, mainly via IL-1, in repair of liver damage and regeneration.     

Phorbol ester modulates interleukin 6- and interleukin 1-regulated expression of acute phase plasma proteins in hepatoma cells

Interleukin 6 (IL 6) and interleukin 1 (IL-1) regulate the expression of acute phase plasma proteins in rat and human hepatoma cells. Phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), partially mimics the stimulatory effect of IL-6 but reduces that effect of IL-1. TPA and IL-6 act synergistically. These regulatory properties of TPA are also manifested in HepG2 cells transiently transfected with an indicator gene construct carrying the IL-1/IL-6 regulatory enhancer element of the rat alpha 1-acid glycoprotein gene. IL-6 and IL-1 act independently of TPA-inducible kinase C, and of changes in intracellular Ca2+ concentrations. However, prolonged pretreatment of HepG2 cells with TPA results in a drastically reduced cytokine response that is proportional to the loss of cell surface binding activity for the cytokine. These data suggest that hormones activating protein kinase C probably play a contributing role in stimulating the expression of acute phase plasma protein genes but they may be crucial in controlling the responsiveness of liver cells to inflammatory cytokines during subsequent stages of the hepatic acute phase reaction.     

A cytokine-selective defect in interleukin-1 beta-mediated acute phase gene expression in a subclone of the human hepatoma cell line (HEPG2)

Several well-differentiated human hepatoma cell lines (HepG2, Hep3B) have been used to identify factors which regulate hepatic gene expression during the host response to inflammation/tissue injury (acute phase response). Studies in these cell lines, as well as in primary cultures of rat, rabbit, and mouse hepatocytes, have demonstrated that interleukin-1 beta (IL-1 beta), tumor necrosis factor (TNF-alpha), and interferon-beta 2 (IFN-beta 2) each mediate changes in expression of several hepatic acute phase genes. In this study we identify a subclone of the HepG2 cell line in which there is a selective defect in IL-1 beta-mediated acute phase gene expression. Recombinant human IL-1 beta mediates an increase in synthesis of the positive acute phase complement protein factor B and a decrease in synthesis of negative acute phase protein albumin in the parent uncloned HepG2 cell line (HG2Y), but not in the subclone HG2N. Recombinant human IFN-beta 2 and TNF-alpha, however, regulate acute phase protein synthesis in the subclone HG2N; i.e. IFN-beta 2 and TNF-alpha increase synthesis of factor B and decrease synthesis of albumin in both HG2Y and HG2N cells. Equilibrium binding analysis with 125I-rIL-1 beta at 4 degrees C showed that both HG2N and HG2Y cells bind IL-1 beta specifically and saturably. HG2N and HG2Y possess 3.8 and 4.0 x 10(3) plasma membrane receptors/cell with affinities of 0.96 and 1.07 x 10(-9) M, respectively. Thus, the defect in this subclone of the HepG2 cell line is likely to involve the signal transduction pathway for the biological activity of IL-1 beta and will be useful in elucidation of this signal transduction pathway.     

Differential gene regulation in human versus rodent hepatocytes by peroxisome proliferator-activated receptor (PPAR) alpha. PPAR alpha fails to induce peroxisome proliferation-associated genes in human cells independently of the level of receptor expresson.

We compared the ability of rat and human hepatocytes to respond to fenofibric acid and a novel potent phenylacetic acid peroxisome proliferator-activated receptor (PPAR) alpha agonist (compound 1). Fatty acyl-CoA oxidase (FACO) activity and mRNA were increased after treatment with either fenofibric acid or compound 1 in rat hepatocytes. In addition, apolipoprotein CIII mRNA was decreased by both fenofibric acid and compound 1 in rat hepatocytes. Both agonists decreased apolipoprotein CIII mRNA in human hepatocytes; however, very little change in FACO activity or mRNA was observed. Furthermore, other peroxisome proliferation (PP)-associated genes including peroxisomal 3-oxoacyl-CoA thiolase (THIO), peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (HD), peroxisomal membrane protein-70 (PMP-70) were not regulated by PPAR alpha agonists in human hepatocytes. Moreover, other genes that are regulated by PPAR alpha ligands in human hepatocytes such as mitochondrial HMG-CoA synthase and carnitine palmitoyl transferase-1 (CPT-1) were also regulated in HepG2 cells by PPAR alpha agonists. Several stably transfected HepG2 cell lines were established that overexpressed human PPAR alpha to levels between 6- and 26-fold over normal human hepatocytes. These PPAR alpha-overexpressing cells had higher basal mRNA levels of mitochondrial HMG-CoA synthase and CPT-1; however, basal FACO mRNA levels and other PP-associated genes including THIO, HD, or PMP-70 mRNA were not substantially affected. In addition, FACO, THIO, HD, and PMP-70 mRNA levels did not increase in response to PPAR alpha agonist treatment in the PPAR alpha-overexpressing cells, although mitochondrial HMG-CoA synthase and CPT-1 mRNAs were both induced. These results suggest that other factors besides PPAR alpha levels determine the species-specific response of human and rat hepatocytes to the induction of PP.