Cirrhotic livers reveal genetic changes in the MDM2-P14ARF system of cell cycle regulators

The genesis of hepatocellular carcinoma is promoted by changes in the regulatory MDM2-P14ARF system. The incidence of such changes has to date not been analysed in non-tumourous livers showing regenerative proliferation. In the present study, 24 cirrhotic livers of alcohol-, autoimmue disorder- or HCV-caused genesis were screened for MDM2-P14ARF alterations at the level of protein, DNA and mRNA. Using confocal laser scanning microscopy, the absence of MDM2 and P14ARF expression was detected in all samples except three HCV-infected livers (four livers) which contained hepatocytes overexpressing MDM2 (P14ARF) protein. In two of the samples lacking P14ARF expression, laser microdissection and PCR demonstrated deletion of the P14ARF gene. The P14ARF gene amplified from other specimens did not carry mutations. MDM2 splicing variants were present in tissues from alcohol- and autoimmune disorder-induced cirrhoses. Sequencing of full-size mRNA revealed a MDM2 mis-sense mutation in an alcohol-induced cirrhosis. One sample contained regenerative nodules with genetic instability occurring at MDM2 locus D12S83 according to the data of automatic PCR fragment analysis. In summary, this study gives first evidence for different types of MDM2 and P14ARF alterations in cirrhotic livers. We suggest that the changes impair the regulatory MDM2-P14ARF system, thus possibly favouring regenerative proliferation and transformation.

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.

Regulation of insulin-like growth factor-I and of insulin-like growth factor binding protein-1, -3 and -4 in cocultures of rat hepatocytes and Kupffer cells by interleukin-6

BACKGROUND/AIMS

Catabolism is associated with decreased serum concentrations of insulin-like growth factor (IGF)-I and insulin-like growth factor binding protein (IGFBP)-3 associated with elevated IGFBP-3 protease activity and increased concentrations of IGFBP-1 and -4. The effects of the acute phase mediators interleukin (IL)-6, IL-1beta and tumor necrosis factor alpha (TNFalpha) on the biosynthesis of IGF-I and IGFBPs were studied in primary rat liver cells.

METHODS

mRNA levels of IGF-I and of IGFBPs were analyzed by Northern blotting, secretion of IGFBPs by [(125)I]IGF-I ligand blotting. Proteolytic activity was measured using iodinated recombinant IGFBP-3 as the substrate.

RESULTS

In hepatocytes, Kupffer cells (KC) and cocultures of hepatocytes with KC, IL-6 reduced IGF-I biosynthesis dose-dependently. IL-6 stimulated mRNA expression and protein secretion of IGFBP-1 and -4 in hepatocytes and that of IGFBP-3 in KC, respectively. In cocultures, biosynthesis of IGFBP-1, -3 and -4 was increased dose-dependently by IL-6, while the effects of IL-1beta or TNFalpha were less prominent. At neutral pH, proteolytic activity against IGFBP-3 was not detected in media of cocultures treated with IL-6.

CONCLUSIONS

The alterations of IGF-I, IGFBP-1 and -4 observed in catabolism correlate with the effects of IL-6 on the biosynthesis of these components in primary rat liver cells, while a neutral IGFBP-3 protease was not detectable.

The IGF axis and hepatocarcinogenesis

Deregulation of the insulin-like growth factor (IGF) axis, including the autocrine production of IGFs, IGF binding proteins (IGFBPs), IGFBP proteases, and the expression of the IGF receptors, has been identified in the development of hepatocellular carcinoma (HCC). Characteristic alterations detected in HCC and hepatoma cell lines comprise the increased expression of IGF-II and the IGF-I receptor (IGF-IR), which have emerged as crucial events in malignant transformation and the growth of tumours. Alterations of IGFBP production and the proteolytic degradation of IGFBPs resulting in an excess of bioactive IGFs, as well as the defective function of the IGF degrading IGF-II/mannose 6-phosphate receptor (IGF-II/M6PR), may further potentiate the mitogenic effects of IGFs in the development of HCC.

Fragments of human oncoprotein MDM2 reveal variable distribution within and on cultivated human hepatoma cells

Human oncoprotein MDM2 reveals a MHC class I binding motif HMDM441 characterizing MDM2 as a potential tumor antigen. To analyze the distribution of MDM2 proteins containing this motif in liver cancer cells we produced rabbit anti-HMDM441 serum. The novel antibodies bound to an MDM2 fragment of approximately 55 kDa which lacked the N-terminal region and was present in lysate and supernatant of a human hepatoma cell line overexpressing normal 90-kDa MDM2. The 55-kDa fragment was detected in the cytoplasm and nucleoli and at the nuclear envelope of hepatoma cells, whereas normal hepatocytes were negative. Double-fluorescence labeling indicated that the MDM2 fragments and MHC class I molecules were coexpressed on the surface of the hepatoma cells. Further studies must clarify whether MDM2 fragments containing motif HMDM441 are novel targets of immunotherapy and immunochemical tumor diagnosis.

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.

Analysis of the IGF axis in preneoplastic hepatic foci and hepatocellular neoplasms developing after low-number pancreatic islet transplantation into the livers of streptozotocin diabetic rats

Preneoplastic hepatic foci have been demonstrated in liver acini, which drain the blood from intraportally transplanted pancreatic islets in streptozotocin-induced diabetic rats with mild persisting diabetes. In long-term studies of this animal model, hepatocellular adenomas and carcinomas (HCC) developed after a sequence of characteristic preneoplastic hepatic foci. In this experimental model, the local hyperinsulinism is thought to have a causative role. Because insulin and the insulin-like growth factor (IGF) axis are closely linked, an altered gene expression of the IGF axis components is likely. Therefore, preneoplastic hepatic foci and HCC were studied for the expression of IGF axis components. Glycogen-storing "early" preneoplastic hepatic foci were detectable several days after pancreatic islet transplantation. Northern blot analysis, in-situ hybridization, and immunohistochemical studies of these "early" lesions demonstrated increased expressions of IGF-I and IGF binding protein-4 (IGFBP-4) in altered parenchymal cells, and a decreased expression of IGFBP-1. IGF-II was not detected in these preneoplastic foci. HCC arising in this model had decreased expressions of IGF-I and IGFBP-4 but IGFBP-1 expression was not significantly altered. Some HCC showed a more than 100-fold overexpression of IGF-II, whereas other tumors were completely negative for IGF-II expression. Low IGF-I receptor expression was detected in preneoplastic foci and adjacent nonaltered liver tissue. However, HCC tissue consistently showed an increased IGF-I receptor expression, rendering these tissues susceptible to the mitogenic effects of IGF. The altered gene expression in glycogen-storing preneoplastic hepatic foci, especially the up-regulation of IGF-I and IGFBP-4 with the down-regulation of IGFBP-1, resemble the insulin-dependent regulation of these components in normal rat hepatocytes. These data agree with previous studies demonstrating a correspondence of the focal character, morphology, and enzyme pattern of preneoplastic hepatic foci with insulin effects on hepatocytes. The development from preneoplastic foci to HCC may be driven by insulin itself and/or an altered IGF axis component or yet unidentified factors.

Expression of matrix metalloproteinases and their inhibitors during hepatic tissue repair in the rat

Matrix metalloproteinases (MMPs) and their specific inhibitors (TIMPs) are thought to play an essential role in liver injury associated with tissue remodeling. However, their distinct expression profile in different liver repair models still remains to be established. Hepatic expression of collagenase (MMP-13), gelatinases A and B (MMP-2, -9), stromelysin-1 and -2 (MMP-3, -10), membrane-type MMP-1 (MMP-14), and TIMP-1 and -2 was studied following single and repeated CCl4-mediated injury and after partial hepatectomy. Expression was analyzed by reverse transcription-PCR (RT-PCR), northern blot analysis, zymography, and immunohistochemistry. Following a single toxic liver injury, MMPs and TIMPs were induced in a distinct time frame in that expression of most MMPs was induced during the early phase of liver injury, was maximal during the inflammatory reaction, and was diminished in the recovery phase. In contrast, TIMP and MMP-2 steady state mRNA levels remained constant in the early phase, were strongly induced during tissue inflammation, and remained increased until the recovery phase. Interestingly, hepatic TNF-alpha expression paralleled the MMP induction profile, while the increase of TGF-beta1 expression mapped to the increase of TIMPs. Chronic liver injury was accompanied by an increase in the steady state mRNA levels of MMP-2 and TIMPs, while other MMPs remained more or less unchanged or were diminished. Partial hepatectomy was followed by a dramatic increase of MMP-14 and to a lesser extent also of TIMP-1 expression; other MMPs and TIMPs were not significantly induced. Liver injury is accompanied by profound changes in hepatic MMP/TIMP expression, the latter being critically dependent on the type of injury. Single toxic injury resulting in complete restoration was characterized by a sequential induction of MMPs and TIMPs suggesting initial matrix breakdown and matrix restoration thereafter. Chronic liver injury leading to fibrosis displays overall diminished matrix degradation mainly through TIMP induction, while liver regeneration induced by partial hepatectomy caused an induction of MMP-14 and TIMP-1 only, which might be unrelated to matrix turnover but connected to pericellular fibrinolysis or fibrolysis required for hepatocellular replication.

Ultrastructural characterization of isolated rat Kupffer cells by transmission X-ray microscopy

The ultrastructure of primary cultured rat Kupffer cells was studied using transmission X-ray microscopy as well as transmission electron microscopy. X-ray microscopical images of intact, hydrated Kupffer cells demonstrated structures such as cell nucleus separated by a nuclear membrane and filaments concentrated in the perinuclear area. Within the cytoplasm, a number of vacuoles were visible; some of these were crescent-shaped vacuoles that were half X-ray lucent, half X-ray dense; others were uniformly dense. The number of crescent-shaped vacuoles was predominant. After phagocytosis of haematite particles, enlarged vacuoles containing the ingested material were visible within the cytoplasm of Kupffer cells while crescent-shaped vacuoles were no longer detectable. Densitometric analysis of the two types of vacuole revealed that the X-ray absorption of the uniform vacuole was approximately half that of the dense part of the crescent-shaped vacuoles. This observation led to speculation on the existence of only one type of vacuole in the cytoplasm of Kupffer cells. The different morphological aspects--crescent-shaped versus uniform vacuoles--might be due to different three-dimensional orientation with respect to the image plane. Using transmission electron microscopy, the morphology of vacuoles differed more widely in diameter, density and shape. Two main types of vacuole were identified: electron-lucent and electron-dense. Based on the observation of only one type of vacuole by transmission X-ray microscopy, the different morphological aspects of vacuoles obtained by transmission electron microscopy could be explained by imaging several different sections of a crescent-shaped vacuole. From the present data it can be concluded that transmission X-ray microscopy is a versatile technique that reveals the ultrastructure of intact, unsectioned biological specimens in their aqueous environment, thereby allowing a more comprehensive interpretation of data obtained by transmission electron microscopy.

Characterization of the insulin-like growth factor axis in a human hepatoma cell line (PLC)

The insulin-like growth factors (IGF-I and -II) are structurally related peptides participating in the regulation of metabolism, growth and cellular differentiation. In the present study, the human hepatoma cell line PLC was studied for the expression of individual components of the IGF axis. Northern blot analysis using IGF-I and -II coding cDNAs failed to detect IGF-I- or -II-specific transcripts in total RNA from PLC cells. Biosynthesis of type I and II IGF receptors was demonstrated by northern blotting and binding studies as well as cross-linking of the respective radiolabeled ligand. Both IGF-I and -II stimulated [3H]-thymidine incorporation dose-dependently. The mitogenic activity of exogenously added IGFs was reduced by the presence of IGF-binding proteins of 24, 30, 34, 41 and 45 kDa in supernatants of PLC cells identified as IGFBP-4, -1, -2 and -3, respectively, by [125I]IGF-I ligand-, immuno- and northern blotting. Biosynthesis of IGFBP-3 was stimulated dose-dependently by IGF-I and -II, while IGFBP-1, -2 and -4 were not affected. The increase of IGFBP-3 in response to IGF-I and -II was due to a stimulation of IGFBP-3 specific mRNA as well as to an inhibition of IGFBP-3 endocytosis. Proteolytic activity for rhIGFBP-3 was detected in media from PLC cells at acidic pH that was inhibited by the aspartyl protease inhibitor pepstatin A as well as after immunodepletion of cathepsin D from media of PLC cells. Thus, a role of cathepsin D for the regulation of IGFBP-3 bioavailability via endocytosis in acidic prelysosomal compartments was suggested. The susceptibility of PLC for IGF-I and -II was restricted by their ability to increase the abundance of inhibitory IGFBPs and to decrease the level of IGF-I receptor expression. The present data point to the IGF axis as a complex regulatory system that self limits the mitogenic activity of exogenous IGFs.

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.

Mouse insulin-like growth factor binding protein-6: expression, purification, characterization and histochemical localization

The mitogenic and metabolic activities of insulin-like growth factors (IGF) are modulated by a family of six high affinity IGF binding proteins (IGFBPs). This study describes the expression of the mouse IGFBP-6 which is unique among IGFBPs in its preferential binding of IGF II, in insect cells using the baculovirus system. The purified, O-glycosylated IGFBP-6 was functional as shown by IGF binding and by inhibition of IGF II-stimulated DNA synthesis in human fibroblasts. Specific antibodies generated in chicken against the recombinant IGFBP-6 were used for Western blotting analysis and immunohistochemistry. Strong immunoreactivity was found in ossifying bones of the cranial base, in cell clusters of the pancreas anlage, in the trigeminal ganglion, on myoblasts, on motoneurons of the spinal cord of embryonic mice. In tissues of adult mouse, strong IGFBP-6 immunostaining was present in epidermal and peridermal layers of the skin, in meningeal layers, in long-striated skeletal muscle, and in the Langerhans' islets of the pancreas. No immunopositive staining was observed in lung and liver indicating that sites of synthesis and IGFBP action are different.

Structural and functional analysis of the promoter of the hepatic lipase gene

Hepatic lipase (HL) gene transcription is almost exclusively limited to hepatocytes. Here we have studied sequences and transcription factors regulating basal and hepatocyte-restricted HL promoter activity. Sequencing of a cloned 3.4-kb HL promoter fragment revealed three Alu repeat sequences and a consensus hepatocyte-enriched nuclear transcription factor 1 (HNF1) binding site located upstream of one major and one minor transcription initiation site. By transfection of cell lines of hepatic and non-hepatic origin and of primary hepatocyte cultures, sequences controlling basic HL promoter activity and negative elements located downstream and upstream thereof which extinguish or enhance this activity were defined. Some HL-promoter fragments with internal deletions were active only in primary hepatocyte cultures. Human HNF1 protein was shown to bind to the HL-specific HNF1 response element and the activity of a heterologous promoter was enhanced by HL-HNF1 in rat primary hepatocyte cultures but not in the context of the authentic 3.4-kb HL promoter sequences. In cell lines the presence of HNF4 but not of HNF1 and vHNF1 mRNA was found to correlate with HL gene expression although no perfect consensus HNF4 binding motif was detected in the promoter region tested. Taken together, these data indicate that hepatocyte-specific HL gene transcription is controlled by positive and negative transcription regulatory proteins which bind to sequence motifs within and outside of the proximal 3.4-kb promoter fragment studied. For the elucidation of the control of HL promoter activity in vivo the use of primary hepatocyte cultures is essential.

Tri-iodothyronine increases insulin-like growth factor binding protein-4 expression in rat hepatocytes

Previous in vivo studies demonstrated significant variations in insulin-like growth factor binding protein-1 (IGFBP-1), IGFBP-2 and IGFBP-4 hepatic mRNAs and/or serum levels depending on the rat thyroid status. In this study we employed cultured hepatocytes from adult rats to demonstrate a possible direct regulation of these genes by tri-iodothyronine (T3). Northern blot analysis revealed that IGFBP-1 and -4 messages were clearly expressed, whereas IGFBP-2 signal was barely detectable. No significant effects on IGFBP-1 mRNA level or on peptide secretion were detected in T3-cultured hepatocytes. In contrast, significant increases in IGFBP-4 mRNA steady-state levels as well as in IGFBP-4 secretion were observed in hepatocytes cultured for 12-24 h in the presence of T3. The T3 effect on IGFBP-4 transcript levels appears to consist of enhanced gene transcription and is independent of ongoing protein synthesis. The T3-increased IGFBP-4 expression in cultured hepatocytes is consistent with our in vivo experiments demonstrating an increase in hepatic IGFBP-4 mRNA and serum IGFBP-4 levels in T3-treated rats. Furthermore, significant decreases in hepatic IGFBP-4 message and serum IGFBP-4 levels were observed in hypothyroid rats compared with euthyroid controls. Our data establish an important direct role for thyroid hormone in regulating IGFBP-4 expression and consequently IGF activity.

Insulin-like growth factor-I serum concentrations and patterns of insulin-like growth factor binding proteins in patients with chronic liver disease

BACKGROUND/AIMS

Serum concentrations of insulin-like growth factor-I are decreased in liver cirrhosis. However, this growth factor is bound for the most part to specific binding proteins that are known to modulate biological actions. Plasma insulin-like growth factor binding proteins are predominantly synthesized in the liver.

METHODS

The effect of liver disease on basal and on growth hormone-stimulated serum concentrations of total and "free" insulin-like growth factor-I and on insulin-like growth factor binding protein patterns is reported. Sera were obtained from 20 patients with non-cirrhotic chronic liver diseases and from 20 patients with cirrhosis before and 24 h after a single subcutaneous dose of growth hormone. Samples were analyzed using radioimmunoassays, gel chromatography, ligand blotting and immunoblotting.

RESULTS

In cirrhosis, serum concentrations of total and "free" insulin-like growth factor-I were decreased, the binding protein pattern was changed profoundly showing a reduction in the 150 kD complex and an increase in the 30-40 kD complexes. Concentrations of binding protein-1 and -2 were increased, while that of binding protein-3 was decreased in cirrhosis. The response to growth hormone was blunted. These changes were related to the degree of liver dysfunction as assessed by the Child-Pugh classification.

CONCLUSIONS

A pathogenetic link of altered bio-availability of insulin-like growth factor-I to clinical characteristics of advanced liver disease, e.g. insulin resistance or skeletal muscle wasting, may be suggested by the present data.

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

BACKGROUND/AIMS

The liver is the main source of circulating insulin-like growth factor binding proteins. In man, the cellular origin of insulin-like growth factor binding proteins has remained obscure.

METHODS

Human hepatocytes isolated from surgical specimens were purified and cultured using a collagen gel immobilization technique. Gene expression of individual insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor binding proteins by Western ligand blotting and immunoblot analysis. Neutral size chromatography of medium samples was used to detect insulin-like growth factors binding protein complexes.

RESULTS

In cultured hepatocytes transcripts for insulin-like growth factor binding protein-1, -2, -3, -4 and for acid labile subunit could be demonstrated. Ligand blotting revealed the secretion of insulin-like growth factor binding proteins of molecular weights of 24 kD, 30 kD, 34 kD, 43 kD and 46 kD, respectively. Using polyclonal antisera, these proteins were identified as insulin-like growth factor binding protein-1, -2 and the insulin-like growth factor binding protein-3 doublet. Neural size chromatography of culture supernatants showed the presence of an insulin-like growth factor binding protein complex of approximately 40 kD, but absence of the high molecular weight ternary complex of 150 kD.

CONCLUSIONS

It is concluded that in man parenchymal liver cells have to be regarded as a source of acid-labile subunit and of circulating insulin-like growth factor binding proteins including insulin-like growth factor binding protein-3.

Cellular localization and hormonal regulation of biosynthesis of insulin-like growth factor binding proteins and of the acid-labile subunit within rat liver

In the circulation, most of the IGFs are bound to a high molecular weight binding protein complex of 150 kDa that consists of IGF-I (or IGF-II), IGFBP-3 and the acid-labile subunit (ALS). Within rat liver, individual components of the 150 kDa complex are synthesized in different cellular compartments. ALS expression is localized in hepatocytes, but not in non-parenchymal cells. IGFBP-3 mRNA, however, is exclusively expressed in non-parenchymal and among them in endothelial and Kupffer cells. Co-cultures of hepatocytes and Kupffer cells were used as a model to study the hormonal regulation of biosynthesis of the components of the 150 kDa complex. Although expressed in different liver cell populations IGFBP-3 and ALS were regulated synergistically. Insulin stimulated both the expression of ALS and IGFBP-3 in co-cultures in a dose-dependent manner, while expression of IGFBP-I was decreased. Regulation of IGFBP-3 synthesis of Kupffer cells required a mediator that is secreted by hepatocytes, since IGFBP-3 expression in cultures of pure Kupffer cells did not respond to the stimulating effect of insulin.

Biochemical and immunological characterization of micellar complexes of the envelope glycoprotein of a simian immunodeficiency virus isolated from an African green monkey

The external envelope glycoprotein gp130 of a simian immunodeficiency virus isolated from an African green monkey (SIVagmTYO-7) was purified as micellar complexes. The molecular weight of the gp130 micelles was about 700 K. On electron microscopy, the micelles appeared as spherical particles with a diameter of 15 to 20 nm. Such aggregates consisted of about 4 to 5 gp130 monomers. Hyperimmune sera raised in rabbits and rhesus monkeys against these gp130 micelles exhibited titers between 10(5) and 10(6). Such sera inhibit the CD4 binding of gp130 and neutralize SIVagmTYO-7 and SIVmac251 but not HIV-2ben.

CD4-binding of gp130 micelles isolated from SIVagmTYO-7

The binding and binding inhibition of the SIVagmTYO-7 external glycoprotein gp130 in micellar form to the CD4 molecule on human Molt-4 clone 8 cells was investigated. The best binding of gp130 to Molt-4 clone 8 cells occurred at pH 5.5 to 6.5 at 37 degrees C after 4 h or at room temperature after 10 h. The dissociation constant of this reaction was 0.2-0.4 nM, with both soluble CD4 or CD4 on Molt-4 clone 8 cells. This value is close to 0.15 nM determined for the antihuman CD4 monoclonal antibody 30F16H5. After partial deglycosylation of gp130, a 90 kD product arose which still bound to CD4. Fully deglycosylated gp130 of 60 kD was still immunoprecipitable, but had lost the CD4 binding activity. Lens culinaris agglutinin was able to inhibit the gp130-CD4 interaction very efficiently, while the agglutinin of Phaseolus vulgaris was half as efficient and Canavalia ensiformis was inefficient. CD4 binding of gp130 micelles was also inhibited with several anti CD4 monoclonal antibodies directed against the OKT4a epitope as well as with soluble recombinant CD4.

Expression and frameshifting but extremely inefficient proteolytic processing of the HIV-1 gag and pol gene products in stably transfected rodent cell lines

Expression, ribosomal frameshifting, and proteolytic processing of HIV-1 GAG and POL proteins were investigated in heterologous mammalian cells in order to elucidate the influence of the cellular background on these events. DNA fragments encoded by the gag and pol region were expressed in two rodent cell lines, LTK- and BHK. Both stably transfected cell lines continuously produce recombinant proteins which react with HIV-specific antisera. The GAG precursor and a 39-kDa proteolytic fragment thereof were the major recombinant proteins detected. Expression of the gag-pol region leads to the production of the GAG-POL precursor. Ribosomal frameshifting at the HIV-1 shifty sequence to a typical extent could be positively demonstrated by an enzyme assay. Despite the presence of the viral protease within the GAG-POL precursors, proteolytic processing of the HIV-derived polyproteins was extremely inefficient. The efficiency could not be enhanced by overexpression of the HIV-1 protease encoding region.