Synthesis of insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor ternary binding protein complex in primary cultures of human hepatocytes

Involvement of insulin-like growth factor-binding protein-3 in the effects of histone deacetylase inhibitor MS-275 in hepatoma cells

Insulin-like growth factor-binding protein-3 (IGFBP-3) expression is frequently suppressed in liver cancers and can be reactivated by histone deacetylase (HDAC) inhibition. This study examined the role of IGFBP-3 in mediating the effects of the HDAC inhibitor MS-275 in liver cancer cells and identified IGFBP-3-dependent proteins that regulate proliferation and migration. In HepG2 cells, MS-275 inhibited DNA synthesis, cell cycle activity, and cell viability concomitantly with increased binding of acetylated histone H3 to IGFBP-3 promoter sequences and induction of IGFBP-3 expression. IGFBP-3 down-regulation by siRNA significantly reversed the inhibition of cell viability and DNA synthesis by MS-275, indicating an intermediary role for IGFBP-3. Induction of the cyclin-dependent kinase inhibitor p21 by MS-275 was attenuated by IGFBP-3 down-regulation, providing an explanation for IGFBP-3-dependent effects of MS-275 on cell cycle activity. In contrast, MS-275 stimulated HepG2 cell migration, an effect also inhibited by IGFBP-3 down-regulation. Among genes whose induction by MS-275 was attenuated by IGFBP-3 down-regulation, LYVE1 and THBS2 (thrombospondin-2) were identified as mediators of IGFBP-3-dependent effects of MS-275. Silencing of either protein had no effect on the inhibition of HepG2 viability by MS-275 but reversed its stimulatory effect on cell migration. We conclude that among genes up-regulated by MS-275, IGFBP-3 is a key mediator of effects on hepatoma cell growth and migration, involving IGFBP-3-dependent proteins p21 (proliferation) and LYVE1 and THBS2 (migration). The enhanced cell motility that accompanies reactivation of IGFBP-3 expression in liver cancer by HDAC inhibition suggests the possibility of increased metastatic spread despite inhibited cell proliferation.

Increased degradation of insulin-like growth factor-I in serum from feed-deprived steers

Severe feed restriction decreases serum insulin-like growth factor I (IGF-I) concentration in animals, and this decrease is thought to be due to reduced IGF-I production in the liver. The objective of this study was to determine whether feed deprivation also increases degradation of serum IGF-I and serum levels of IGF binding protein 3 (IGFBP-3) and acid-labile subunit (ALS), which inhibit IGF-I degradation and increase IGF-I retention in the blood by forming a ternary complex with IGF-I, in cattle. Five steers had free access to pasture, and another five were deprived of feed for 60 h. Serum concentration of IGF-I and liver abundance of IGF-I mRNA at the end of the 60-h period were 50% and 80% lower, respectively, in feed-deprived steers than in fed steers. Less (125)I-labeled IGF-I remained intact after a 45-h incubation in sera of feed-deprived steers than in sera of fed steers, suggesting that serum IGF-I is more quickly degraded in feed-deprived animals. Serum levels of IGFBP-3 and ALS were decreased by 40% and 30%, respectively, in feed-deprived steers compared with fed steers. These decreases were associated with more than 50% reductions in IGFBP-3 and ALS mRNA in the liver, the major source of serum IGFBP-3 and ALS. Taken together, these results suggest that feed deprivation reduces serum concentration of IGF-I in cattle not only by decreasing IGF-I gene expression in the liver, but also by increasing IGF-I degradation and reducing IGF-I retention in the blood through decreasing IGFBP-3 and ALS production in the liver.

Estrogen regulation of growth hormone action

GH plays a pivotal role in regulating body growth and development, which is modulated by sex steroids. A close interplay between estrogen and GH leads to attainment of gender-specific body composition during puberty. The physiological basis of the interaction is not well understood. Most previous studies have focused on the effects of estrogen on GH secretion. There is also strong evidence that estrogen modulates GH action independent of secretion. Oral but not transdermal administration of estrogen impairs the metabolic action of GH in the liver, causing a fall in IGF-I production and fat oxidation. This results in a loss of lean tissue and a gain of body fat in postmenopausal women and an impairment of GH effect in hypopituitary women on GH replacement. The negative metabolic sequelae are potentially important because of the widespread use of oral estrogen and estrogen-related compounds. Estrogen affects GH action at the level of receptor expression and signaling. More recently, estrogen has been shown to inhibit Janus kinase/signal transducer and activator of transcription signaling by GH via the induction of suppressor of cytokine signaling-2, a protein inhibitor for cytokine signaling. This represents a novel paradigm of steroid regulation of cytokine receptors and is likely to have significance for a diverse range of cytokine function.

Expression and regulation of the insulin-like growth factor axis components in rat liver myofibroblasts

Apart from hepatic stellate cells (HSC), liver myofibroblasts (MF) represent a second mesenchymal cell population involved in hepatic fibrogenesis. The IGF system including the insulin-like growth factors I and II (IGF-I, -II), their receptors (IGF-I receptor, IGF-IR; IGF-II/mannose 6-phosphate receptor, IGF-II/M6-PR), and six high affinity IGF binding proteins (IGFBPs) participate in the regulation of growth and differentiation of cells of the fibroblast lineage, possibly contributing to the fibrogenic process. The aim of this work was to study the expression and regulation of the IGF axis components in rat liver MF.

METHODS

Cultures of MF from passages 1 to 4 (P1-4) were studied. IGFBP secretion was analyzed by [(125)I]-IGF-I ligand and immunoblotting. IGF-I, IGF-IR, IGF-II/M6-PR, and IGFBP messenger RNA (mRNA) expression was assessed by Northern blot hybridization. DNA synthesis was evaluated by 5-bromo-2'-deoxyuridine (BrdU) incorporation assay.

RESULTS

MF from P1 to 4 constitutively expressed mRNA transcripts specific for IGF-I, IGF-IR, and IGF-II/M6-PR. In MF, biosynthesis of IGFBP-3 and -2 was observed that was stimulated by IGF-I, insulin, and transforming growth factor beta (TGF-beta), whereas platelet-derived growth factor (PDGF-BB) revealed inhibitory effects. IGF-I and to a lesser extent insulin increased DNA synthesis of MF. Simultaneous addition of recombinant human IGFBP-2 or -3 with IGF-I diminished the mitogenic effect of IGF-I on MF whereas preincubation of MF with IGFBP-2 or -3 further potentiated the IGF-I stimulated DNA synthesis. In conclusion, the present study demonstrates that the IGF axis may play a role in the regulation of MF proliferation in vitro which might be relevant in vivo for the process of fibrogenesis during acute and chronic liver injury.

MDCK cells secrete neutral proteases cleaving insulin-like growth factor-binding protein-2 to -6

Proteolysis of insulin-like growth factor-binding proteins (IGFBPs) may be an important mechanism to regulate IGF availability and IGF-independent functions of IGFBPs. We analyzed the secretion of IGFBP proteases in Madin-Darby canine kidney (MDCK) cells. The results showed that several specific proteases were secreted, cleaving IGFBP-2 to -6 at neutral pH. The proteolytic activity against IGFBP-6 differed at least from IGFBP-5 protease activity in its sensitivity both to IGF-II and to the hydroxamic acid-based disintegrin metalloprotease inhibitor, as well as serine protease inhibitors. During partial purification steps, the serine protease inhibitor-sensitive fraction with IGFBP-6 protease activity was separated from fractions characterized by the presence of a 30-kDa disintegrin immunoreactive band. Whereas the IGFBP-4 and -6 proteases are predominantly secreted across the basolateral membrane, the majority of IGFBPs are sorted to the apical medium from filter-grown cells. These studies indicate that the side-specific secretion of several distinct IGFBP proteases with partially overlapping IGFBP specificities may be another level in the regulation of IGF-dependent epithelial functions.

Changes in the IGF-IGFBP axis in critical illness

In critical illness, serum concentrations of the growth hormone-dependent complexes containing insulin-like growth factors I and II are decreased. This is initially due to a transient growth hormone resistance, but in the longer term, the less pulsatile pattern of growth hormone secretion may be a major factor since only pulsatile growth hormone increases the levels of insulin-like growth factor-I and the acid-labile subunit. Other factors contributing to a low insulin-like growth factor level are nutritional deficiency and the direct effects of inflammatory cytokines. The growth hormone-independent proteins IGFBP-2, IGFBP-4 and IGFBP-6 increase in critical illness, suggesting a redistribution of insulin-like growth factors from growth hormone-dependent ternary complexes with IGFBP-3 and IGFBP-5 to binary complexes with these binding proteins, which might facilitate transport to the tissues. IGFBP-1, which is acutely regulated by metabolic status, is elevated on admission to intensive care but may fall in response to nutritional support. Initial evidence suggests that the level of IGFBP-1 may be predictive of outcome in critically ill patients, suggesting a possible prognostic role for this protein.

Regulation of the components of the 150 kDa IGF binding protein complex in cocultures of rat hepatocytes and Kupffer cells by 3',5'-cyclic adenosine monophosphate

In the circulation, most of IGFs are bound to a high molecular mass complex of 150 kDa that consists of IGF-I (or IGF-II), IGFBP-3 and the acid-labile subunit (ALS). Within rat liver, biosynthesis of these components has been localized to different cell populations with hepatocytes as source of ALS and nonparenchymal cells (endothelial and Kupffer cells (KC)) as source of IGFBP-3. In the present study, the regulatory effects of the cAMP analogs dibutyryl-cAMP (db-cAMP) and 8-bromo-cAMP (8-br-cAMP) on IGF-I, ALS, and IGFBP expression were evaluated in primary cultures of rat hepatocytes, KC as well as in cocultures of hepatocytes and KC. In cocultures, biosynthesis of IGFBP-3 and ALS was inhibited dose-dependently by db-cAMP and 8-br-cAMP while that of IGF-I, IGFBP-1, and -4 was stimulated as demonstrated by ligand and Northern blotting. IGFBP-3 expression in primary cultures of pure KC did not respond to cAMP treatment indicating the importance of a cellular interaction between KC and hepatocytes for the decreased IGFBP-3 synthesis. The inhibition of IGFBP-3 in db-cAMP-treated cocultures was due to a decrease of IGFBP-3 mRNA level accompanied by a reduced cellular degradation of IGFBP-3. We conclude that cAMP stimulate the biosynthesis of IGF-I, IGFBP-1, and -4 in cocultures of hepatocytes and KC thereby enabling the formation of binary IGF/IGFBP complexes while the formation of the 150 kDa complex is impaired through downregulation of IGFBP-3 and ALS. This complex regulation may be a prerequisite for the effects of cAMP-dependent hormones on the transfer of IGFs from circulation to peripheral tissues.

Estrogens exert route- and dose-dependent effects on insulin-like growth factor (IGF)-binding protein-3 and the acid-labile subunit of the IGF ternary complex

We have previously shown that exogenous estrogens exert route-dependent effects on serum GH and insulin-like growth factor I (IGF-I) levels. IGF-I circulates as a ternary complex with IGF-binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). It is not known whether IGFBP-3 and ALS in blood are regulated by estrogen and, if so, whether this is also route dependent. In the present study we investigate the effects on IGFBP-3 and ALS of oral and transdermal estrogens (study 1), of different oral estrogen formulations (ethinyl estradiol, conjugated estrogen, and estradiol valerate; study 2), of different estrogen dosages (study 3) in normal postmenopausal women, and of oral estrogen in hypogonadal GH-deficient women (study 4). Administration of oral, but not transdermal, estrogen in normal postmenopausal women significantly decreased serum levels of IGFBP-3 and ALS (P < or = 0.005). The suppressive effects were similar with different oral estrogen formulations, and the degree of suppression increased with estrogen dosage. In hypogonadal GH-deficient women, oral estrogen treatment also significantly reduced IGFBP-3 and ALS (P = 0.02). The changes in IGF-I in each of the four studies paralleled the changes in both IGFBP-3 and ALS. In conclusion, exogenous estrogens suppress serum IGFBP-3 and ALS in a route- and dose-dependent manner, which are in parallel with the effects on serum IGF-I. These actions of oral estrogen are independent of endogenous GH status.

The acid-labile subunit of the ternary insulin-like growth factor complex in cirrhosis: relation to liver dysfunction

BACKGROUND/AIMS

In the circulation, insulin-like growth factor-I (IGF-I) is bound in a trimeric complex of 150 kDa with IGF binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). Whereas circulating IGF-I and IGFBP-3 are reported to be low in patients with chronic liver failure, the level of ALS has not been described in relation to hepatic dysfunction. The aim of the present study was therefore to measure circulating and hepatic venous concentrations of ALS in relation to hepatic function and the IGF axis.

METHODS

Twenty-five patients with cirrhosis (Child class A/B/C:5/10/10) and 30 controls with normal liver function were studied. During a haemodynamic investigation, blood samples were collected from the hepatic vein and femoral artery, and the plasma concentrations of ALS, IGF-I and IGFBP-3 were determined.

RESULTS

Hepatic venous and arterial concentrations of ALS were significantly decreased in the cirrhotic patients compared with the controls (-69% and -68%, respectively, both p<0.001). IGF-I and IGFBP-3 were similarly decreased in the cirrhotic patients (-51%,p<0.001). A significant hepatic extraction of ALS was found in the controls (6%, p<0.01) and in the cirrhotic patients (8%, p=0.08). ALS correlated significantly with indicators of liver dysfunction, including the Child-Turcotte score (r=-0.69, p<0.0001), IGF-I (r=0.82, p<0.0001) and IGFBP-3 (r=0.74, p<0.0001).

CONCLUSIONS

Circulating and hepatic venous ALS are decreased in patients with cirrhosis with significant relations to liver dysfunction and other components of the IGF complex. A small hepatic extraction was found in controls, which suggests extrahepatic production of ALS. Future studies should focus on organ-specific removal of ALS.

Liver-specific and proliferation-induced deoxyribonuclease I hypersensitive sites in the mouse insulin-like growth factor binding protein-1 gene

The insulin-like growth factor binding protein-1 (IGFBP-1) gene is highly expressed in fetal, perinatal, and regenerating liver. Up-regulation is transcriptionally mediated in regenerating liver and occurs in the first few minutes to hours after partial hepatectomy. In transgenic mice a 970-bp region from -776 to +151 of the IGFBP-1 promoter was sufficient for tissue-specific and induced expression of the gene in fetal and hepatectomized livers. However weak and/or poorly regulated expression in some transgenic lines suggested the existence of other regulatory regions. Here, genomic clones containing large regions 5' of the mouse IGFBP-1 gene sequence were isolated, subcloned, and sequenced. Deoxyribonuclease I (DNaseI) hypersensitivity analyses identified clusters of tissue-specific nuclease-sensitive sites in the promoter region, -100 to -300, -2,300, -3,100, and -5,000 along with other weak sites. After partial hepatectomy, enhanced sensitivity and/or novel sites were detected in the -100/-300, -5,000, and -3,100 regions, the promoter region remaining the most hypersensitive. A subset of these sites was present in fetal and perinatal livers. Novel tissue-specific sites that interacted with C/EBP and hepatic nuclear factor 3 (HNF3) transcription factors were identified in the -3,100 region. A hepatectomy-induced DNA binding complex containing the transcription factor USF1 was identified within the -100 to -300 region of the promoter. These results suggested that a complex array of tissue-specific and hepatic proliferation-induced transcription factors combine to regulate both the proximal promoter and more distal regulatory elements of the IGFBP-1 gene.

Molecular regulation of insulin-like growth factor-I and its principal binding protein, IGFBP-3

The insulin-like growth factors (IGFs) have diverse anabolic cellular functions, and structure similar to that of proinsulin. The distribution of IGFs and their receptors in a wide variety of organs and tissues enables the IGFs to exert endocrine, paracrine, and autocrine effects on cell proliferation and differentiation, caloric storage, and skeletal elongation. IGF-I exhibits particular metabolic responsiveness, and circulating IGF-I originates predominantly in the liver. Hepatic IGF-I production is controlled at the level of gene transcription, and transcripts are initiated largely in exon 1. Hepatic IGF-I gene transcription is reduced in conditions of protein malnutrition and diabetes mellitus, and our laboratory has used in vitro transcription to study mechanisms related to diabetes. We find that the presence of sequences downstream from the major transcription initiation sites in exon 1 is necessary for the diabetes-induced decrease in IGF-I transcription. Six nuclear factor binding sites have been identified within the exon 1 downstream region, and footprint sites III and V appear to be necessary for metabolic regulation; region V probes exhibit a decrease in nuclear factor binding with hepatic nuclear extracts from diabetic animals. IGFs in biological fluids are associated with IGF binding proteins, and IGFs circulate as a 150-kDa complex that consists of an IGF, an IGFBP-3, and an acid-labile subunit. Circulating IGFBP-3 originates mainly in hepatic nonparenchymal cells, where IGF-I increases IGFBP-3 mRNA stability, but insulin increases IGFBP-3 gene transcription. Regulation of IGFBP-3 gene transcription by insulin appears to be mediated by an insulin-responsive element, which recognizes insulin-responsive nuclear factors in both gel mobility shift assays and southwestern blots. Studies of mechanisms underlying the modulation of IGF-I and IGFBP-3 gene transcription, and identification of critical nuclear proteins involved, should lead to new understanding of the role and regulation of these important growth factors in diabetes mellitus and other metabolic disorders.

Characterization of the IGF axis components in isolated rat hepatic stellate cells

The insulin-like growth factors I and II (IGF-I, -II) are circulating peptides known to participate in the regulation of metabolism, growth, and cellular differentiation. In the present study, "early cultured" (days 2-3 of culture) and "culture-activated" (days 6-7 of culture) rat hepatic stellate cells (HSCs) were analyzed for expression of individual components of the IGF axis. Northern blot analysis of IGF-I messenger RNA (mRNA) revealed transcripts of 7.5, 4, 2, and 1.0 to 1.5 kb in culture-activated HSCs, while early cultured HSCs did not express IGF-I mRNA. In culture-activated HSCs, an IGF-I secretion of 8.3+/-2.5 ng/10(6) cells per 24 hours was determined radioimmunologically. In media from early cultured HSCs, IGF-I was not detectable. The IGF-I receptor (IGF-I-R) mRNA expression was three-fold higher in early cultured HSCs than in culture-activated HSCs. By immunohistochemistry, a decrease of IGF-I-R expression of HSCs in vivo following CCl4-induced liver damage was noted as well. IGF binding proteins (IGFBPs) were detected in conditioned media from HSCs by 125I-IGF-I ligand blotting at apparent molecular masses of 24 and 41 to 45 kd that were immunologically identified as IGFBP-4 and -3, respectively. Synthesis of these IGFBPs increased with time of culture. At neutral pH, no IGFBP proteolysis was observed in conditioned media of early cultured and culture-activated HSCs, whereas at acidic pH, protease activities against IGFBP-3 and -4 were detectable. IGFBP protease activities were completely abolished by inhibitors of aspartyl and cysteine proteases. Addition of 100 nmol/L IGF-I stimulated cell proliferation of early cultured HSCs 5.6+/-1.1- and 4.6+/-0.2-fold as measured by [3H]thymidine and 5-bromo-2'-deoxyuridine incorporation, respectively. In culture-activated HSCs, proliferation was increased 1.2+/-0.1-fold in the presence of 100 nmol/L IGF-I in both proliferation assays. It can be concluded that due to a higher expression of the IGF-I-R and lower levels of IGFBPs, early cultured HSCs are more susceptible to the mitogenic actions of IGFs than the culture-activated HSCs. The present data suggest a role for the IGF axis components in the initiation rather than the perpetuation of HSC proliferation during hepatic fibrogenesis.

Characterization and regulation of insulin-like growth factor binding proteins in human hepatic stellate cells

Cultured hepatic stellate cells (HSCs), the cell type primarily involved in the progression of liver fibrosis, secrete insulin-like growth factor-I (IGF-I) and IGF binding protein (IGFBP) activity. IGF-I exerts a mitogenic effect on HSCs, thus potentially contributing to the fibrogenic process in an autocrine fashion. However, IGF-I action is modulated by the presence of specific IGFBPs that may inhibit and/or enhance its biologic effects. Therefore, we examined IGFBP-1 through IGFBP-6 mRNA and protein expression in HSCs isolated from human liver and activated in culture. Regulation of IGFBPs in response to IGF-I and other polypeptide growth factors involved in the hepatic fibrogenic process was also assessed. RNase protection assays and ligand blot analysis demonstrated that HSCs express IGFBP-2 through IGFBP-6 mRNAs and release detectable levels of IGFBP-2 through IGFBP-5. Because IGF-I, platelet-derived growth factor-BB (PDGF-BB), and transforming growth factor-beta (TGF-beta) stimulate HSC proliferation and/or matrix production, we tested their effect on IGFBPs released by HSCs. IGF-I induced IGFBP-3 and IGFBP-5 proteins in a time-dependent manner without an increase in the corresponding mRNAs. IGFBP-4 protein levels decreased in response to IGF-I. TGF-beta stimulated IGFBP-3 mRNA and protein but decreased IGFBP-5 mRNA and protein. In contrast, PDGF-BB failed to regulate IGFBPs compared with controls. Recombinant human IGFBP-3 (rhIGFBP-3) was then tested for its effect on IGF-I-induced mitogenesis in HSCs. rhIGFBP-3 inhibited IGF-I-stimulated DNA synthesis in a dose-dependent manner, with a peak effect observed at 25 nM IGFBP-3. Because TGF-beta is highly expressed in cirrhotic liver tissue, we determined whether IGFBP-3 mRNA expression is increased in liver biopsies obtained from patients with an active fibroproliferative response due to viral-induced chronic active hepatitis. In the majority of these samples, IGFBP-3 mRNA was increased compared with normal controls. These findings indicate that human HSCs, in their activated phenotype, constitutively produce IGFBPs. IGF-I and TGF-beta differentially regulate IGFBP-3, IGFBP-4, and IGFBP-5 expression, which, in turn, may modulate the in vitro and in vivo action of IGF-I.

Growth hormone-stimulated insulin-like growth factor (IGF) I and IGF-binding protein-3 in liver cirrhosis

BACKGROUND/AIMS

The aim of this study was to evaluate the liver's potential to generate insulin-like growth factor (IGF) I and IGF-binding protein-3 (IGFBP-3), following stimulation by human recombinant growth hormone, as a possible marker for liver functional reserve in patients with liver cirrhosis.

METHODS

In a pilot study, 15 patients (mean age 56 years) with postnecrotic liver cirrhosis were divided into two groups according to disease severity (Child-Pugh score): Group 1 (n=8) with scores of 5-8 and Group 2 (n=7) with scores of 9-12. Five age-matched healthy subjects served as controls. Human recombinant growth hormone (0.06 mg/kg) was administered subcutaneously on 2 consecutive days. Serum levels of IGF-I and IGFBP-3 were measured before and up to 48 h after human recombinant growth hormone injection. Nutritional status was assessed by the creatinine-height index and was compared to lymphocyte count, body mass index, and muscle arm circumference.

RESULTS

Baseline IGF-I levels were significantly lower in patients with cirrhosis than in controls, while no differences were noted between the two patient groups. IGF-I levels increased significantly after rhGH administration to the healthy controls, to a lower degree in Group 1, while no change occurred in Group 2. IGF-I levels at 24 h and beyond correlated significantly with the nutritional status, the Child-Pugh score, and the basal levels of GH-binding protein and IGFBP-3. IGFBP-3 serum levels did not change after rhGH stimulation.

CONCLUSIONS

IGF-I generation after GH stimulation may provide a new dimension in the assessment of liver function and nutritional status in patients with liver cirrhosis.