Synthesis and processing of human chorionic gonadotropin subunits in cultured choriocarcinoma cells

Substrate-dependent differences in production of extracellular matrix molecules by squamous carcinoma cells and diploid fibroblasts

Two human squamous carcinoma cell lines and human diploid fibroblasts were examined for the production of extracellular matrix (ECM) molecules including fibronectin (FN), laminin (LN), and thrombospondin (TSP) when grown on a number of different substrates. The substrates used included glass, plastic, collagen (gelatin), and DEAE-dextran. Levels of TSP as indicated by enzyme-linked immunosorbent assay did not vary significantly as a function of substrate. In contrast, LN levels in the culture medium were significantly decreased when the cells were grown on DEAE-dextran or collagen-linked dextran as compared to the other substrates. FN levels were slightly lower in the culture medium of the cells grown on DEAE-dextran. Biosynthetic labeling followed by immunoprecipitation indicated that the reduction in LN was due, in part, to decreased biosynthesis. Previous studies have indicated that LN influences the behavior of epithelial cells in culture and that the cells, themselves, are a major source of the LN. The differences in LN production noted here indicate that the production of this ECM component is influenced by the substratum on which the cells are grown. These differences could contribute to alterations in biological properties that are known to be influenced by the substratum.

Amphiregulin in lung branching morphogenesis: interaction with heparan sulfate proteoglycan modulates cell proliferation

Epithelial and mesenchymal cells isolated from mouse embryonic lungs synthesized and responded to amphiregulin (AR) in a different fashion. Mesenchymal cells produced and deposited 3- to 4-fold more AR than epithelial cells, proliferated in the presence of exogenous AR, and their spontaneous growth was blocked by up to 85% by anti-AR antibodies. In contrast, epithelial cells exhibited a broad response to this growth regulator factor depending on whether they were supplemented with extracellular matrix (ECM) and whether this ECM was of epithelial or mesenchymal origin. AR-treated epithelial cells proliferated by up to 3-fold in the presence of mesenchymal-deposited ECM, remained unchanged in the presence of epithelial-deposited ECM, and decreased in their proliferation rate below controls in the absence of ECM supplementation. This effect was abolished by treatment with the glycosaminoglycan-degrading enzymes heparinase and heparitinase suggesting the specific involvement of heparan sulfate proteoglycan (HSPG) in AR-mediated cell proliferation. In whole lung explants, branching morphogenesis was inhibited by antibodies against the AR heparan sulfate binding site and stimulated by exogenous AR. Since during development, epithelial cells are in contact with mesenchymal ECM at the tips of the growing buds and alongside the basement membrane, focal variations in the proportion of epithelial and mesenchymal HSPG will focally affect epithelial proliferation rates. Therefore, AR-HSPG interaction may underlie the process of branching morphogenesis by inducing differential cell proliferation.

Laminin expression in the mouse lung increases with development and stimulates spontaneous organotypic rearrangement of mixed lung cells

The recent establishment of a role for laminin in mouse lung organogenesis (Schuger et al. 1990a,b, 1991) prompted us to study its expression in the developing lung. Laminin A and B chains were detected in the murine lung from the first hours of development onward. In situ hybridization of mRNA as well as SDS-PAGE studies of lung cells in monoculture indicated that both epithelium and mesenchyme produce complete laminin molecules. Quantitative analysis of the in situ hybridization studies showed a gradual increase in laminin expression during development which was further supported by immunohistochemistry and ELISA. The overall pattern of expression suggested that the effects of laminin in morphogenesis were not restricted to a particular stage of development. Furthermore, the increase in expression during late development supported a role for the molecule in the fetal lung, which was not previously established. We next determined whether the increase in laminin production modulated the behavior of fetal lung cells as compared with their embryonic counterparts. We previously showed that organotypic pattern formation does not occur in cultures of mixed embryonic lung cells unless exogenous laminin is added (Schuger et al., 1990b). Organotypic pattern formation is the result of cell sorting into epithelial and mesenchymal compartments and further rearrangement in a pattern resembling the tissue of origin. In the present study, we demonstrated that organotypic pattern formation occurs spontaneously in cultures of mixed fetal lung cells, which express high laminin levels. Pattern formation was abolished by antibodies to laminin. These studies suggest a correlation between laminin expression and the ability of lung cells in culture to reproduce normal tissue patterns. We conclude that laminin is critical for epithelial-mesenchymal recognition and further morphogenic interaction during both the embryonic and fetal stages of lung development.

Properties of adenotin reconstituted into phospholipid vesicles

Adenotin is a low affinity adenosine binding protein that has amino terminal homology with mammalian and avian stress proteins. Human placental adenotin was solubilized and reconstituted into phospholipid vesicles with an overall yield of 30%. The properties of adenotin in vesicles were similar to the native membranes as follows: association has a Kobs of 0.61 +/- 0.03 minute-1; equilibrium is reached in approximately 15 minutes; and the first order dissociation constant is 5.0 +/- 0.3 minute-1. Displacement analysis reveals an agonist potency order and Ki values as follows: N-ethylcarboxamidoadenosine, 0.35 microM; 2-chloroadenosine, 1.5 microM; R-phenylisopropyladenosine, greater than 1000 microM. The addition of 100 microM 5'-guanylylimidodiphosphate did not decrease binding of 5'-N-ethylcarboxamidoadenosine (NECA) at 37 degrees C or 4 degrees C but did decrease the IC50 for PC12 and JAR cell membrane agonist binding from 9.9 to 3.3 microM and increase the binding to 150-211% of the control value at 37 degrees C. The latter studies at 37 degrees C showed high variability. Using binding sites reconstituted into vesicles and gel filtration chromatography and agonist related guanine nucleotide release, the authors investigated whether these changes were related to an interaction between adenotin and a guanine nucleotide regulatory protein. No evidence for such an interaction was found. These data suggest that adenotin retains its binding properties when reconstituted into phospholipid vesicles. The function of this low affinity adenosine binding site remains to be discovered. However, the reconstitution of adenotin into phospholipid vesicles provides a method to study its function.

Modulation of fibronectin synthesis and fibronectin binding during transformation and differentiation of mouse AKR fibroblasts

In previous studies it was shown that transformation of AKR fibroblasts with 3-methylcholanthrene was associated with a loss of surface fibronectin and that induction of differentiation of the transformed cells with N,N-dimethylformamide (DMF) was associated with reacquisition of surface fibronectin (Chakrabarty et al., J. Cell. Physiol. 133:415, 1987). It is shown in the present study that changes in surface fibronectin reflect altered fibronectin synthesis and altered fibronectin binding. Both the nontransformed cells (AKR-2B) and their transformed counterparts (AKR-MCA) bound 125I-fibronectin in a receptor-like fashion, but the AKR-MCA cells had only 20% of the receptors found on the AKR-2B cells. Whole cell extracts prepared from the AKR-2B cells and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions were examined for 125I-fibronectin binding. Under these conditions, the majority of binding occurred to moieties with molecular weights of 180 kD, 150 kD, and 97 kD. Binding to similar moieties on the AKR-MCA cells was virtually absent but occurred rapidly after treatment with DMF. The appearance of these moieties paralleled the acquisition of 125I-fibronectin binding activity by whole cells. Antibodies to the fibronectin receptor isolated from human placenta reacted with the DMF-sensitive moieties in immunoblot assays. Both the appearance of the fibronectin binding moieties and the acquisition of 125I-fibronectin binding activity by whole cells occurred within 6 hr of DMF treatment and increased over the subsequent 4 day period. The time course of these events paralleled closely the time course for induction of fibronectin biosynthesis by DMF. These changes in fibronectin binding and fibronectin production were associated with alterations in cell-substrate adhesion. The AKR-2B cells rapidly attached and spread on bovine serum albumin-coated dishes and on fibronectin-coated dishes, whereas the AKR-MCA cells were less adhesive on both substrates. Capacity to attach and spread was regained concomitantly with the induction of fibronectin binding and fibronectin production. Adhesion on both substrates was partially inhibited by antibodies to the fibronectin receptor and by RGDS. These studies suggest that fibronectin production and fibronectin binding are coregulated in AKR fibroblasts and that they function together to bring about changes in cell-substrate adhesion.

All-trans retinoic acid stimulates growth and extracellular matrix production in growth-inhibited cultured human skin fibroblasts

All-trans retinoic acid was examined for effects on human dermal fibroblast proliferation and for effects on fibroblast production and expression of non-collagenous and collagenous components of the extracellular matrix in vitro. Fibroblast proliferation was blocked when the cells were cultured in the presence of a serum-free culture medium containing epidermal growth factor, hydrocortisone, insulin, ethanolamine, phosphoethanolamine, and bovine pituitary extract as growth supplements and 0.15 mM Ca++. This level of extracellular Ca++ is lower than that needed to support fibroblast growth. Under these conditions, growth was stimulated by all-trans retinoic acid. Proliferation was also stimulated in the same basal medium without the growth supplements. Growth-promoting concentrations of all-trans retinoic acid ranged from 0.5-2.0 micrograms/ml (1.7-6.6 X 10(-6) M). Stimulation of proliferation was not seen at higher or lower concentrations. Concentrations of all-trans retinoic acid that stimulated proliferation also induced increased production of fibronectin as indicated by biosynthetic labeling/immunoprecipitation and by enzyme-linked immunosorbent assay. Increased production was associated with increased staining for fibronectin in the extracellular matrix. Increased production of two other non-collagenous extracellular matrix component, i.e., thrombospondin and laminin, also occurred in all-trans retinoic acid-treated cells. At 0.5 micrograms/ml, all-trans retinoic acid also stimulated production of type I collagen by the dermal fibroblasts, but at higher concentrations (2.5 micrograms/ml) production of type I collagen was inhibited. These data indicate that all-trans retinoic acid can induce changes in dermal fibroblasts in vitro (i.e., increased proliferation and extracellular matrix production) that mimic the major changes seen in the dermis after topical treatment with this agent.

Characterization of thrombospondin synthesis, secretion and cell surface expression by human tumor cells

Previous studies have shown that thrombospondin (TSP) is an adhesion factor for some human tumor cells. The previous studies have shown further that tumor cells which utilize TSP as an adhesion factor also synthesize it. This study continues the effort to understand how TSP production and expression are regulated in human tumor cells and the consequences of this for the cells. It is shown that differences among cell lines in their capacity to biosynthesize TSP are associated with differences in TSP specific mRNA levels. This indicates that biosynthesis is regulated at the transcriptional level. There is also a direct relationship between TSP biosynthesis and secretion into the culture medium and expression at the cell surface. The cells which are the most biosynthetically active secrete amounts of TSP into the culture medium that are sufficient to elicit a detectable response in the cell-substrate adhesion assay. The kinetics of TSP secretion by these cells are in accord with the kinetics of attachment and spreading of the same cells in the absence of exogenous adhesion factors. These data are consistent with the idea that endogenously produced TSP promotes the adhesion of the cells which synthesize it in an autocrine manner.

Modulation of fibronectin production in normal human melanocytes and malignant melanoma cells by interferon-gamma and tumor necrosis factor-alpha

Normal cultured human epidermal melanocytes and melanoma cells derived from three different malignant melanomas were examined for synthesis of extracellular matrix components before and after treatment for one day with interferon-gamma, tumor necrosis factor-alpha, or both. Treatment of the cells with either cytokine individually had minimal effects on fibronectin levels. Treatment of the cells with the two agents in combination greatly stimulated fibronectin production as indicated by biosynthetic labeling and immunoprecipitation and by enzyme-linked immunosorbent assay. Synthesis of laminin was decreased slightly by the same treatment whereas thrombospondin production was stimulated slightly. The same treatment reduced melanocyte viability slightly but significantly inhibited proliferation and altered the morphology of the melanocytes. The treated cells became flattened and polygonal in shape while the untreated cells exhibited a bipolar shape with one or more long dendritic processes. These morphologic changes were not seen in cultures treated with interferon-gamma or tumor necrosis factor-alpha individually. The effects of interferon-gamma and tumor necrosis factor-alpha on fibronectin production by epidermal melanocytes are in contrast to the effects of the same treatments on fibronectin production by epidermal keratinocytes where fibronectin production is decreased but are similar to the effects of transforming growth factor-beta on a number of other cell types in which increased synthesis of fibronectin occurs and is associated with decreased growth.

Phosphorylation of the secreted, free alpha subunit of human chorionic gonadotropin

Phosphorylation of secretory proteins is an uncommon event. In this manuscript, the phosphorylation of human chorionic gonadotropin, a glycoprotein hormone secreted by the JAR choriocarcinoma cell line, is described. Labeling of JAR cells with 32PO4 indicates that both the intracellular and the secreted forms of the free alpha subunit are phosphorylated. Although the secreted alpha beta dimer also incorporates 32PO4, there is little detectable phosphorylation of the intracellular precursors of alpha beta dimer, suggesting that dimer phosphorylation occurs as a late event in post-translational processing. In addition, phorbol 12-myristate 13-acetate markedly stimulates the phosphorylation of both intracellular and secreted forms of free alpha subunit and to a lesser extent of secreted alpha beta dimer. In vitro assays, using homogenates of JAR cells as a source of protein kinase activity, indicate that the uncombined alpha subunit is preferentially phosphorylated. The phosphorylation sites are on serine and threonine residues in the alpha subunit.

Laminin production by murine melanoma cells: possible involvement in cell motility

Three lines of B16 melanoma cells (B16-F1, B16-F10 and B16-BL6) were examined for motility in the micropore filter assay and for synthesis in culture of the basal lamina glycoprotein laminin. All three lines synthesized laminin as judged by the incorporation of [35S]methionine into immunoreactive laminin and secreted (or shed) laminin into the culture medium as indicated by biosynthetic labeling studies and enzyme-linked immunosorbent assays. Immunoreactive laminin was also seen on the surface of the cells as indicated by immunofluorescence staining and by complement-mediated killing. Analysis of [35S]methionine-labeled laminin immunoprecipitates by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) both with and without reduction of intersubunit disulfide bonds revealed that all three cell lines produced a similar array of laminin forms, and that the Mr = 950 kD laminin molecule (but not the uncombined subunits) was secreted into the culture medium. Laminin biosynthesis appeared to be limited by the availability of the Mr = 400 kD A subunit as shown by the intracellular accumulation of excess B subunit in the form of uncombined B subunit (Mr = 200 kD) and as a disulfide-linked B dimer (Mr = 400 kD). The motility of all three cell lines was stimulated four- to five-fold by the addition of either exogenous laminin from the EHS sarcoma or culture medium from the B16 cells containing the secreted laminin. The stimulated motility was inhibited by antilaminin serum. These observations suggest that the laminin synthesized by the B16 melanoma cells themselves may facilitate their motility.

Production of beta-human chorionic gonadotrophin by human squamous carcinoma cell lines

Eight out of 9 human squamous carcinoma cell lines ectopically secrete the beta subunit of human chorionic gonadotrophin (beta-HCG) in amounts detectable by radioimmunoassay. In contrast, both normal and SV40-transformed keratinocytes show no such evidence of beta-HCG secretion. Ectopic beta-HCG is identical in its gel exclusion and high-performance liquid chromatographic properties to placental beta-HCG and, in radioimmunoassay, yields a dilution curve parallel to that of placental beta-HCG. Dibutyryl cyclic AMP, but not sodium butyrate, stimulates beta-HCG secretion by the one human cell line examined, LICR-LON-HN-5, suggesting that ectopic production by these cells is similar to trophoblastic beta-HCG secretion, and differs from most other non-trophoblastic cell lines previously examined.

Intracellular transport and secretion of hepatitis B surface antigen in mammalian cells

The oligosaccharide processing and secretion of hepatitis B surface antigen (HBsAg) was studied in Chinese hamster ovary cells stably transfected with the gene coding HBsAg. HBsAg was secreted from cells with a relatively long half time (ca. 5 h). This appeared to be a characteristic of HBsAg itself, since HBsAg-producing cells infected with vesicular stomatitis virus transported the viral envelope glycoprotein to the cell surface with normal kinetics (half time of ca. 30 min). The secreted HBsAg was comprised of both the unglycosylated (P20) and the glycosylated (G25) polypeptides, characteristic of HBsAg isolated from human serum or secreted from other cell lines (C. W. Crowley, C.-C. Liu, and A. D. Levinson, Mol. Cell. Biol. 3:44-55, 1983; M. F. Dubois, C. Pourcel, S. Rousset, C. Chang, and P. Tiollais, Proc. Natl. Acad. Sci. U.S.A. 77:4549-4553, 1980; C.-C. Liu, D. Yansura, and A. D. Levinson, DNA, 1:213-221, 1982; G. M. Macnab, J. J. Alexander, G. Lecatsas, E. M. Bey, and J. M. Urbanocvicz, Br. J. Cancer, 24:509-515, 1976; A. M. Moriarity, B. H. Hoyer, J. W.-K. Shih, J. L. Gerin, and D. H. Hamer, Proc. Natl. Acad. Sci. U.S.A. 78:2606-2610, 1981; D. L. Peterson, J. Biol. Chem., 256:6975-6983, 1981). The glycosylated polypeptide (GP25) contained complex oligosaccharide chains. Cell-associated HBsAg also was comprised of both an unglycosylated and a glycosylated polypeptide; however, the glycosylated form (GP23) contained only high-mannose oligosaccharide chains. No oligosaccharide processing of the high-mannose chains could be detected within the cells. Thus, most of the time before secretion of HBsAg from cells must have been spent in a pre-Golgi or early Golgi compartment. Glycosylation was inhibited completely by tunicamycin, although unglycosylated particles were still secreted from cells and were antigenic. The secretion and oligosaccharide processing of HBsAg were inhibited with high concentrations of monensin, but at lower concentrations of monensin HBsAg was still secreted, although only half of the oligosaccharide chains were processed to the complex form.

Glycoprotein-hormone alpha-chain production by pancreatic endocrine tumors: a specific marker for malignancy. Immunocytochemical analysis of tumors of 155 patients

Human chorionic gonadotropin (hCG) or its alpha- and beta-subunits have been proposed as specific quantitative markers for malignant pancreatic endocrine tumors. Since proof of malignancy of pancreatic endocrine tumors is difficult early in the course of the illness, we tested retrospectively a series of 157 pancreatic endocrine tumors of 155 patients for alpha- or beta-subunits of hCG by immunocytochemistry. Human CG-alpha-immunoreactive cells were present in 42 of 56 (75%) functioning malignant pancreatic endocrine tumors but in only one, possibly benign, glucagonoma of 67 functioning benign tumors, in only one of 17 nonfunctioning malignant and in none of 17 nonfunctioning benign tumors. No beta-hCG-immunoreactivity was localized in the tumors. Human CG-alpha appears to be a reliable quantitative and qualitative marker for malignancy in functioning pancreatic endocrine tumors.

Combination of rat lutropin subunits occurs early in the secretory pathway

The combination of lutropin (LH; luteinizing hormone) alpha and beta subunits was examined in rat pituitaries incubated with [35S]methionine or [35S]sulfate. Combination was assessed by using antiserum directed against the beta subunit. The data show that combination of most of the subunits proceeds rapidly, well before the addition of sulfate and prior to the processing of asparagine-linked oligosaccharides to the complex form. Thus, combination appears to initiate in the endoplasmic reticulum and does not require those post-translational modifications. We observed that two forms of the LH-alpha subunit were processed--one that is secreted into the medium not associated with the LH beta subunit and another secreted as part of the alpha-beta dimer. Both forms of the alpha subunit are sulfated, and the data suggest that subsequent to sulfate addition, secretion of free alpha subunit and the dimer occur independently by separate pathways.

Cytological localization of chorionic gonadotropin alpha and placental lactogen mRNAs during development of the human placenta

Probes derived from clones bearing cDNAs corresponding to the alpha subunit of human chorionic gonadotropin (hCG) and human placental lactogen (hPL) were used to localize their respective mRNAs cytologically in sections of first trimester and term human placenta. hPL mRNA was exclusively localized to the syncytial layer, hCG alpha mRNA was found in the syncytial layer and also in some differentiating cytotrophoblasts. Hybridization was specific because no signal was observed when labeled pBR322 was hybridized to placental sections or when the placental probes were hybridized to sections of human tonsils. In addition, RNA in placental interstitial cells did not hybridize with hCG alpha and hPL probes. Hybridization with the hCG alpha probe was much greater in first trimester than in term sections, whereas hPL signals were comparable in both first trimester and term placentae. Syncytial formation proceeds through cellular intermediates of cytotrophoblastic origin, and the data suggest that transcription of the hCG alpha gene is initiated before the completion of syncytial formation. In contrast, hPL mRNA synthesis starts later in trophoblast differentiation, likely after the stage of syncytial formation. The data also suggested that hCG alpha mRNA synthesis becomes attenuated but that hPL is transcribed at a rather constant rate during placental development.

Studies on the biosynthesis of laminin by murine parietal endoderm cells

The biosynthesis and processing of the polypeptides A (Mr = 450 x 10(3)), B1 (Mr = 240 x 10(3)), B2 (Mr = 230 x 10(3)) and C (Mr = 150 x 10(3)) of the extracellular matrix protein, laminin, were studied in murine parietal endoderm cells labelled with [35S]methionine. Various lines of evidence suggest that the A chains are not precursors to the smaller B chains. Firstly, the pulse-chase experiments, radioactivity in cytoplasmic A and (B1 + B2) chains declines with the same half-life of about 70 min. Secondly, peptide maps generated by digestion of A and B (B1 + B2) chains with Staphylococcus aureus V8 protease are different. Finally, rabbit antibodies to isolated, denatured (B1 + B2) chains do not cross-react with reduced and alkylated A chains. A, B1, B2 and C polypeptides are all glycosylated by an intracellular process involving the addition of tunicamycin and endo-beta-N-acetylglucosaminidase-H-sensitive N-linked oligosaccharide side chains. Further glycosylation probably occurs around the time of secretion. Disulphide bonding of some A and B chains can be observed in the cytoplasm within 10 min of adding [35S]methionine. However, it appears that some free A and B2 chains are present in the cytoplasm and that free A chains exist in the medium. The relationship between the 150 x 10(3)-Mr C glycoprotein and the A and B components is discussed. Although B and C chains generate different peptide maps after digestion with S. aureus V8 protease, antibodies raised against isolated, denatured C chains cross-react with reduced and alkylated B (but not A) chains. This suggests that B and C chains may share some antigenic determinant(s).

Ectopic production in serum-free media of the common alpha subunit of the glycoprotein hormones

The HeLa-S3 cell strain grown in Ham's F12 medium supplemented with insulin, transferrin, cortisol, epidermal growth factor, fibroblast growth factor, and trace elements, but containing no serum, continued to produce the common alpha-subunit of the glycoprotein hormones for the 10 d study. The amounts of alpha-subunit secreted into the medium during the first 4 d were indistinguishable from those in F12 medium supplemented with 10% fetal bovine serum. During the remainder of the experiment the amounts of alpha-subunit reached 50 to 80% those in the serum-supplemented medium.