Induction of receptors for tumor necrosis factor-alpha by interferons is not a major mechanism for their synergistic cytotoxic response

We have investigated the effects of various interferons on the receptors for recombinant tumor necrosis factor-alpha (rTNF-alpha) and also their effects on rTNF-alpha-mediated cytotoxicity on human cervical carcinoma cell line ME-180. Preincubation of cells with interferon (IFN)-gamma causes a concentration- and time-dependent increase in rTNF-alpha receptor number without any change in the affinity constant of the receptors. The increase in receptor number is caused only by IFN-gamma and not by IFN-alpha or IFN-beta. Approximately 4-6 h of preincubation with IFN-gamma are required for maximum increase in rTNF-alpha binding to the cells, and this increase can be abolished by inhibitors of protein synthesis, suggesting de novo synthesis of rTNF-alpha receptors. The half-life of both uninduced and induced receptors of rTNF-alpha is approximately 2 h, indicating a rapid turnover. The binding of rTNF-alpha to the cells can also be eliminated by pretreatment of cells with trypsin. Following the removal of trypsin, binding of rTNF-alpha gradually increases, and this requires the synthesis of new proteins. The cytotoxic effect of rTNF-alpha on ME-180 cells is potentiated severalfold by the addition of either IFN-alpha, -beta, or -gamma. However, at similar concentrations, relatively higher potentiation of rTNF-alpha cytotoxicity is observed with IFN-gamma as compared to IFN-alpha and IFN-beta. The pre-exposure of cells to IFNs is as effective as co-exposure in enhancing cytotoxic effects of TNF-alpha. The induction of TNF-alpha receptors by IFNs is observed in different cell types regardless of their sensitivity to TNF-alpha, suggesting that increase in receptor number alone is not sufficient for the enhanced cytotoxic response. Because the enhancement of cytotoxic effects of TNF-alpha is observed by all IFNs but receptor induction in ME-180 cells occurs only with INF-gamma and because metabolic inhibitors which down-regulate TNF-alpha receptors also enhance cytotoxic response, we suggest that the induction of TNF-alpha receptor by IFNs is not a major mechanism of synergism between these cytokines.     

Effect of phorbol esters on down-regulation and redistribution of cell surface receptors for tumor necrosis factor-alpha

The effects of various phorbol esters on the interaction of human cells with recombinant human tumor necrosis factor-alpha (rTNF-alpha) was investigated. Preexposure of several different types of cells with only biologically active tumor promoter, i.e. 4 beta-phorbol 12-myristate 13-acetate (PMA), inhibited the specific binding of rTNF-alpha to its receptor. The reduction in specific binding of TNF-alpha was observed only by PMA but not with several other phorbol esters tested. 1-oleoyl-2-acetylglycerol, which is an analogue of the natural protein kinase C activator, diacylglycerol, was active in down-regulating TNF-alpha receptors but only at 1000 times concentration than PMA. Scatchard analysis of the binding data on U-937 cells revealed that PMA caused a decrease in high affinity cell surface receptor number (approximately 8300 versus approximately 2500 binding sites/cell) without any significant change in the dissociation constant (0.38 nM versus 0.32 nM). This decrease in receptor number is dependent on temperature, the time of exposure, and dose of PMA. Greater than 95% of the specific binding of 125I-TNF-alpha could be abolished within 10 min by preexposure of cells to 10 nM PMA at 37 degrees C. The down-regulation of receptors by PMA occurred only at 37 degrees C but not at 4 degrees C, suggesting a probable internalization of the receptors. The specific binding of TNF-alpha to detergent-solubilized cell extracts remained unchanged after exposure of cells to PMA. The rates of dissociation of TNF-alpha from the cell surface and the rate of internalization was not significantly affected by PMA, but the rate of disappearance from cell interior and its appearance into the medium was slightly enhanced by PMA. PMA did not alter the rate of degradation of the TNF-alpha nor cause the shedding of receptors into the medium. Approximately 70% of TNF-alpha cell surface receptors could be regenerated within 16 h after PMA removal. These results suggest the involvement of PMA-activated protein kinase C in down-regulation and redistribution of TNF-alpha receptors.     

Immunoregulatory activity of recombinant human tumor necrosis factor (TNF)-alpha: induction of TNF receptors on human T cells and TNF-alpha-mediated enhancement of T cell responses

The expression of specific tumor necrosis factor (TNF) membrane receptors and biological effects of recombinant TNF (rTNF)-alpha on normal human T lymphocytes were studied. Although resting T cells lacked specific binding capacity for rTNF-alpha, high affinity (Kd 70 pM) TNF receptors were de novo induced upon primary activation of T cells. Comparison of TNF receptor expression with that of high affinity interleukin 2 (IL-2) and interferon-gamma (IFN-gamma) receptors, respectively, revealed similarities to IL 2-receptor expression with respect to kinetics of induction. However, maximum expression of TNF receptors (approximately equal to 5000/cell at day 6) and subsequent decline occurred approximately 3 days after the peak of IL 2-receptor expression. In contrast, no change in the expression of IFN-gamma receptors (Kd 10 pM, 300 to 400 receptors/cell) was found in the course of T cell activation. On activated TNF receptor positive T cells, TNF-alpha exerted multiple stimulatory activities. Thus TNF increased the expression of HLA-DR antigens and high affinity IL 2 receptors. As a consequence, TNF-treated T cells showed an enhanced proliferative response to IL 2. Moreover, TNF-alpha was effective as a co-stimulator of IL 2-dependent IFN-gamma production. These data indicate that TNF-alpha may regulate growth and functional activities of normal T cells.     

Expression and characterization of erythropoietin receptors on normal human bone marrow cells

We studied the specific binding of 125I-labeled bioactive recombinant human erythropoietin (Epo) to human bone marrow mononuclear cells (BMNC) obtained from normal subjects. The 125I-labeled Epo bound specifically to the BMNC. Scatchard analysis of the data showed two classes of binding sites; one high affinity (Kd 0.07 nM) and the other low affinity (Kd 0.38 nM). The number of Epo binding sites per BMNC was 46 +/- 16 high-affinity receptors and 91 +/- 51 low-affinity receptors. The specific binding was displaced by unlabeled Epo, but not by other growth factors. Receptor internalization was observed significantly at 37 degrees C, but was prevented by the presence of 0.2% sodium azide. These findings indicate that human BMNC possess two classes of specific Epo receptors with characteristics of a hormone-receptor association.     

Distinct subsets of somatostatin receptors on cultured human lymphocytes

Somatostatin (SOM) is a neuroendocrine tetradecapeptide that suppresses specific functions of differentiated T-cells and antibody-producing cells. The Jurkat line of human leukemic T-cells and U266 IgE-producing human myeloma cells bound [I-Tyr11]SOM specifically. The maximal level of specific binding was attained by 1-2 h at 22 degrees C for both types of cells and reversed by 70-85% within 2-3 h after the addition of excess nonradioactive SOM. Computer-assisted Scatchard analysis of the competition curves revealed two classes of binding sites for both cells. An average of 144 and 1295 high affinity receptors per Jurkat and U266 cells had a Kd value of 3 pM and 5 pM, respectively, whereas a large number of low affinity sites had Kd values of 66 nM and 100 nM. The affinity of the analogs somatostatin 28, [I-Tyr11]SOM, and [D-Trp8, D-Cys14]SOM for Jurkat and U266 cell lines, relative to SOM, suggested a degree of specificity similar to receptors on neuroendocrine cells.     

Atrial natriuretic peptide binding, cross-linking, and stimulation of cyclic GMP accumulation and particulate guanylate cyclase activity in cultured cells

The stimulation of cyclic GMP accumulation and particulate guanylate cyclase activity by atrial natriuretic peptide (ANP) was compared to the affinity and number of ANP receptors in eight cultured cell types. At 100 nM, ANP increased cyclic GMP by 13-fold in bovine adrenal cortical, 35-fold in human lung fibroblast, 58-fold in canine kidney epithelial, 60-fold in bovine aortic smooth muscle, 120-fold in rat mammary epithelial, 260-fold in rat Leydig, 300-fold in bovine kidney epithelial, and 475-fold in bovine aortic endothelial cells. ANP (1 microM) increased particulate guanylate cyclase activity by 1.5-, 2.5-, 3.1-, 3.2-, 5.0-, 7.0-, 7.8-, and 8.0-fold in bovine adrenal cortical, bovine aortic smooth muscle, human lung fibroblast, canine kidney epithelial, rat mammary epithelial, rat Leydig, bovine kidney epithelial, and bovine aortic endothelial cells, respectively. Specific 125I-ANP binding to intact rat Leydig (3,000 sites/cell; Kd = 0.11 nM), bovine aortic endothelial (14,000 sites/cell; Kd = 0.09 nM), bovine adrenal cortical (50,000 sites/cell; Kd = 0.12 nM), human lung fibroblast (80,000 sites/cell; Kd = 0.32 nM), and bovine aortic smooth muscle (310,000 sites/cell; Kd = 0.82 nM) cells was saturable and high affinity. No specific and saturable ANP binding was detected in bovine and canine kidney epithelial and rat mammary epithelial cells. Two ANP-binding sites of 66,000 and 130,000 daltons were specifically labeled by 125I-ANP after cross-linking with disuccinimidyl suberate. The 130,000-dalton ANP-binding sites bound to a GTP-agarose affinity column, and the specific activity of guanylate cyclase was increased by 90-fold in this fraction. Our results demonstrate that the increase in cyclic GMP accumulation and particulate guanylate cyclase activity by ANP does not correlate with the affinity and number of ANP-binding sites. These results suggest that multiple populations of ANP receptors exist in these cells and that only one receptor subtype (130,000 daltons) is associated with particulate guanylate cyclase activity.     

Characterization of the cell surface receptors for human B cell growth factor of 12,000 molecular weight

Quiescent normal human B cells have been shown to require an activation step before proliferating in response to B cell growth factor (BCGF) of 12,000 m.w. (12 kd). One effect of cell activation has been the putative acquisition of specific cell surface growth factor receptors. In this report, the existence of such receptors has been confirmed by using purified radioiodinated BCGF-12 kd. BCGF-12 kd receptors on activated B cells have been shown to be distinct form those interacting with IL 2. Scatchard analysis revealed both high affinity receptor sites with an apparent Kd of 28.6 pM and low affinity receptor sites with Kd of 1.2 nM on freshly prepared, anti-IgM activated peripheral blood B cells. Human B cells grown in culture for extended periods of time in the presence of BCGF-12 kd also displayed high affinity receptor sites (Kd, 41.4 pM) and low affinity receptor sites (Kd, 0.9 nM). The action of BCGF-12 kd therefore appears to be mediated by binding to its lineage-specific receptors on the cell surface.     

Analysis of interleukin 5 receptors on murine eosinophils: a comparison with receptors on B13 cells

The interleukin 5 receptor (IL-5R) on murine eosinophils and a mouse B cell line (B13) was investigated using iodinated murine IL-5 produced in the baculovirus system. Electrophoretic analysis of this recombinant protein identified a range of bands Mr 26,000 to 32,000 resulting from differential glycosylation. The specific activity and binding kinetics of the iodinated IL-5 (125I-IL-5) were essentially identical to unlabeled material. Both high-affinity (Kd approximately 50 pM) and low-affinity (Kd approximately 1 nM) receptor populations were identified on murine eosinophils. Approximately 50 high-affinity receptors and 10,000 low-affinity receptors were present. This was compared with approximately 2,000 high-affinity (Kd approximately 80 pM) and about 8,000 low-affinity (Kd approximately 3 nM) sites on B13 cells. An antibody that inhibits IL-5 binding to, and proliferation of, B13 cells (R52.120) was also shown to inhibit eosinophil proliferation, suggesting that eosinophils and B cells bear the equivalent IL-5 binding proteins.     

Use of a sensitive receptor binding assay to discriminate between full-length and truncated human recombinant tumor necrosis factor proteins

A radioreceptor assay (RRA) capable of detecting picomolar concentrations of human recombinant tumor necrosis factor (TNF) was used to compare the relative binding affinities of genetically engineered full-length and truncated TNF proteins. The specific cell-surface receptors for TNF present on the human cervical carcinoma cell line ME-180 were characterized as having a Kd of 0.2 nM and a density of 2700 sites/cell. Conditions were then defined for an RRA that maximized the specific binding of 125I-TNF to this adherent cell line. Incubation of ME-180 cells with 125I-TNF at 37 degrees C in the presence of 0.02% sodium azide resulted in a 4-fold increase in assay sensitivity and a doubling of specific counts bound, as compared to binding done at 4 degrees C with or without sodium azide. Inhibition of receptor-ligand internalization under these conditions was a likely reason for the increases. This system was utilized to compare low concentrations of the full-length TNF protein and a genetically altered TNF protein (mutein) which lacks the 10 N-terminal amino acids and contains an N-terminal methionine. Previous studies showing the truncated TNF to be 2- to 3-fold lower in cytotoxic activity on a variety of tumor cell lines were corroborated by our findings that the mutein was also three and one-half times lower in relative affinity for the TNF receptor on ME-180 cells. These results suggest a possible role for these residues in receptor binding and illustrate the use of a highly sensitive RRA for the evaluation of TNF molecules altered by recombinant DNA technology.     

Tumor necrosis factor up-regulates gamma-interferon binding in a human carcinoma cell line

WiDR colorectal carcinoma cells are highly sensitive to the synergistic cytotoxic effects of tumor necrosis factor (TNF) and gamma-interferon (IFN-gamma). In the present study, we have investigated the effects of recombinant human (rh) TNF and IFN-gamma on the binding of both ligands in this cell line. WiDR cells exhibited high affinity binding sites for both 125I-rhTNF (Kd = 1.66 x 10(-10) M, 920 sites/cell) and 125I-rhIFN-gamma (Kd = 4.15 x 10(-10) M, 18,960 sites/cell). Preincubation of the cells with rhTNF (24 h) increased cell-associated 125I-rhIFN-gamma radioactivity by 129% when binding was carried out at 37 degrees C, as a result of an increase in both surface bound and internalized 125I-rhIFN-gamma. However, rhTNF did not alter the degradation profile of released 125I-rhIFN-gamma radioactivity. Scatchard analysis of 125I-rhIFN-gamama binding data (4 degrees C) revealed that rhTNF induced a 245% increase in 125I-rhIFN-gamma binding sites. Conversely, rhIFN-gamma caused a 68% increase in 125I-rhTNF binding sites and a 58% increase in receptor affinity. rhIFN-gamma also increased the subsequent binding of 125I-rhIFN-gamma, whereas rhTNF increased the subsequent binding of 125I-rhTNF. Furthermore, preincubation of the cells with both rhTNF and rhIFN-gamma also resulted in an increase in the binding of both ligands. Actinomycin D and cycloheximide blocked all the effects of rhTNF and rhIFN-gamma on ligand binding. However, the basal level of 125I-rhIFN-gamma binding was insensitive to either inhibitor, whereas the basal level of 125I-rhTNF binding was decreased by both inhibitors. These data indicate that in some cell types TNF and IFN-gamma may induce an increase in their own receptors (homologous up-regulation) and concomitantly increase each other's receptors (heterologous up-regulation) and that these actions are due, in part, to enhanced receptor synthesis.     

Biochemical properties of the 75-kDa tumor necrosis factor receptor. Characterization of ligand binding, internalization, and receptor phosphorylation.

An expression plasmid encoding the human 75-kDa tumor necrosis factor (TNF) type 2 receptor (TNF-R2) was constructed and used to generate a stable human cell line (293/TNF-R2) overexpressing TNF-R2. Ligand binding analysis revealed high affinity binding (Kd = 0.2 nM) with approximately 94,000 +/- 7,500 sites/cell for 125I-TNF-alpha and approximately 5-fold lower affinity for TNF-beta (Kd = 1.1 nM) with 264,000 +/- 2,000 sites/cell. Cross-linking of 125I-TNF-alpha and 125I-TNF-beta to 293/TNF-R2 cells yielded predominant complexes with apparent molecular weights of 211,000 for TNF-alpha and 205,000 and 244,000 for TNF-beta, suggesting these complexes contain two or three TNF-R2 molecules. Immunoprecipitation of TNF-R2 from 32P-labeled 293/TNF-R2 cells demonstrated that the receptor is phosphorylated. The majority (97%) of 32Pi incorporation was found in serine residues with a very low level of incorporation (3%) in threonine residues. TNF-alpha treatment of 293/TNF-R2 cells did not significantly affect the degree or pattern of phosphorylation. Cell surface-bound 125I-TNF-alpha was slowly internalized by the 293/TNF-R2 cell line with a t1/2 = 25 min. Shedding of the extracellular domain of TNF-R2 was induced by 4 beta-phorbol 12-myristate 13-acetate but not by TNF-alpha or TNF-beta.     

Identification of cell surface receptors for murine macrophage inflammatory protein-1 alpha.

We have produced recombinant proteins for a cytokine, L2G25BP (macrophage inflammatory protein-1 alpha) (MIP-1 alpha). By using the recombinant protein (rMIP-1 alpha), receptors for MIP-1 alpha were identified on Con A-stimulated and unstimulated CTLL-R8, a T cell line, and LPS-stimulated RAW 264.7, a macrophage cell line. The 125I-rMIP-1 alpha binds to the receptor in a specific and saturable manner. Scatchard analysis indicated a single class of high affinity receptor, with a Kd of approximately 1.5 x 10(-9) M and approximately 1200 binding sites/Con A-stimulated CTLL-R8 cell and a Kd of 0.9 x 10(-9) M and approximately 380 binding sites/RAW 264.7 cell. 125I-rMIP-1 alpha binding was inhibited by unlabeled rMIP-1 alpha in a dose-dependent manner, but not by IL-1 alpha or IL-2. rMIP-1 alpha inhibited the proliferation of unstimulated CTLL-R8 cells. Rabbit anti-rMIP-1 alpha antibodies blocked the growth-inhibitory effect of the rMIP-1 alpha on CTLL-R8 cells.     

Human macrophage maturation in vitro: expression of functional transferrin binding sites of high affinity

Human blood monocytes (mo) when cultured in suspension on hydrophobic teflon membranes undergo terminal maturation to macrophages (MO). Together with the appearance of new lineage-restricted differentiation antigens, mature MO but not blood mo, express transferrin (TF) receptor molecules as detected by immunostaining methods. Here we report that radio- and fluorescein-labelled TF binds to a single class of high-affinity binding sites on MO but not on mo. As mo mature in vitro in the presence of human serum, their receptor numbers increase to about 10(6) per cell, showing an apparent Kd for Fe2TF of approximately 5 nM. These receptor numbers were comparable with our estimates for cultured K 562 human tumor cells, and about 20x greater than reported for human MO cultured in the presence of fetal calf serum. Our MO showed 58Fe uptake comparable with uptake by tumor cells and also exhibited TF-promoted uptakes of 61Ga. The possibility that MO might recycle stored iron through receptor-bound apoTF was not supported by experiments which showed that their Fe2TF receptors had much lower affinity for apoTF (Kd greater than 1 microM) and which could not detect separate high-affinity receptors specific for apoTF. Expression of TF receptors was not substantially altered by treatment with human recombinant interferon-gamma.     

Intracellular [3H]dopamine binding sites in normal and malignant cells: relationships to cell proliferation

Dopamine interaction with target cells undoubtably involves binding to plasma membrane receptors. However, the well documented cell growth inhibitory activity of this catecholamine suggests nuclear regulation. To evaluate this possibility, we determined the intracellular localization and binding of [3H]dopamine in human retinoblastoma (Y-79 cells), normal mouse fibroblasts (LM-cells), and in the rat uterus. Cytosol and purified nuclear preparations devoid of plasma membrane components contained specific, saturable, high affinity (Kd approximately 20 nM) binding sites for [3H]dopamine. The nuclear binding affinity for dopamine, L-dopa, and L-dopa methyl ester correlated with the inhibitory effects of these compounds on cell proliferation, suggesting that intracellular dopamine binding sites may also be involved in cellular response to catecholamines.     

Differential expression of functional adrenocorticotropic hormone receptors by subpopulations of lymphocytes

In an effort to investigate the presence of adrenocorticotropic hormone (ACTH) receptors on rat lymphocytes, cells were separated by a panning procedure into T and B cell populations. By using the radiolabeled ACTH agonist, (125I-Tyr23) phenylalanine2-norleucine4-ACTH1-24, substantial numbers of ACTH binding sites were detected on T and B lymphocytes, but not on thymocytes. Scatchard analysis revealed two types of binding sites on each cell population, one with Kd1 = 0.088 +/- 0.025 nM and one with Kd2 = 4.2 +/- 0.6 nM; however, the absolute number of binding sites per cell was different. B lymphocytes expressed approximately three times the number of Kd1 binding sites per cell when compared with T lymphocytes. However, ACTH receptor expression by these cell populations was not static as suggested by the ability to induce receptor expression via mitogens. B or T cells and thymocytes stimulated with the mitogens LPS or Con A, respectively, substantially increased their number of Kd1 binding sites per cell (approximately three-fold). Even more dramatic increases in Kd1 receptor expression (approximately 100-fold) were observed when comparing "normal" and stimulated thymocytes. To demonstrate that these ACTH binding sites were in fact functional, cAMP levels were measured in lymphocytes 10 min after exposure to varying concentrations of ACTH. Dose-dependent increases in cAMP levels were observed, with significant stimulation occurring with as little as 0.1 nM ACTH added. Taken together, these studies demonstrate the presence of functional ACTH receptors on normal, rat T and B lymphocytes.     

Specific, high-affinity binding sites for angiotensin II on Mycoplasma hyorhinis.

Mycoplasmataceae are known to express various proteins that are similar to those present in mammals. We report a strain of Mycoplasma hyorhinis isolated from opossum kidney cells with specific, high-affinity binding sites for human angiotensin II (Kd = 5.1 +/- 1.9 nM). In contrast, two strains of M. hominis revealed no specific binding. These binding sites resembled mammalian angiotensin II receptors by their high affinity and by their sensitivity to dithiothreitol. However, they are different from mammalian angiotensin II receptors in that they bind angiotensin I with high affinity (Kd = 1.6 +/- 0.29 nM) but not angiotensin III (Kd approximately 330,000 nM). [125I]-angiotensin II binding was not inhibited by angiotensin receptor subtype antagonists DuP 753 and CGP 42112A but it was sensitive to bacitracin and aprotinin. Positions Asp1, Ile5, His6 and Pro7 were essential for binding to M. hyorhinis as deletion of these residues led to a more than 10,000-fold decrease in affinity.     

High and low affinity receptors for human interleukin for DA cells/leukemia inhibitory factor on human cells. Molecular characterization and cellular distribution.

Radioiodinated recombinant human interleukin DA (HILDA)/leukemia inhibitory factor (LIF) purified from conditioned medium of Chinese hamster ovary transfected cells enabled the identification of specific receptor sites on a variety of human cell types. Using low concentrations (up to 500 pM) of the ligand iodinated at a high specific radioactivity, high affinity receptors (equilibrium dissociation constant Kd in the range of 30-100 pM) were first demonstrated. They were expressed at low levels by human peripheral blood monocytes but not by lymphocytes, NK cells, granulocytes, and platelets. The myelomonocytic cell line THP1 as well as the T lymphoma cell line HSB2 and the lymphoblastoid B cell line DAB were also receptor-negative. In contrast, most of the non-lymphoid tumoral cell lines tested, including melanomas, neuroblastomas, and carcinomas, expressed high affinity HILDA/LIF receptors at variable levels (Bmax from 20 to 600 sites/cell). The kinetics of HILDA/LIF high affinity binding to the choriocarcinoma JAR cell line were characterized at 4 degrees C with association and dissociation rate constants of k1 = 2.2 10(9) M-1 min-1 and k-1 = 0.0084 min-1, respectively, corresponding to a steady-state dissociation constant k1/k-1 = 3.8 pM. The subsequent use of higher concentrations of HILDA/LIF labeled at a lower specific radioactivity enabled the identification of a low affinity component on several cell lines (Kd in the range of 1-4 nM; Bmax from 1,000 to 5,000 sites/cell). On JAR cells, this low affinity component was characterized by association and dissociation rate constants at 4 degrees C of k1 = 7.3 10(7) M-1 min-1 and k-1 = 0.19 min-1, respectively (k-1/k1 = 2.6 nM). Affinity cross-linking of HILDA/LIF to JAR cells showed two cross-linked species under both reducing and nonreducing conditions corresponding to receptor species of 120 and 250 kDa, respectively. Whereas both bands had similar intensities under high affinity conditions, the higher band predominated under low affinity conditions. Our data suggest that the 250-kDa chain could constitute the low affinity binding component whereas the association of both 250- and 120-Da subunits would form the high affinity structure.     

Characterization of hepatocyte-growth-factor receptors on Meth A cells.

Hepatocyte growth factor (HGF) is a heparin-binding polypeptide mitogen for a variety of cell types including hepatocytes. HGF also has cytotoxic activity on some tumor cell lines as well as scattering activity on epithelial cells. In this study, recombinant human HGF was used to identify HGF-binding cell surface receptors on Meth A cells, whose growth is inhibited by HGF. Scatchard analysis of binding data indicated that there were two classes of binding sites with high affinity (Kd = 17 pM) and low affinity (Kd = 6.7 nM) and the average numbers were 6600 and 2,600,000 per cell, respectively. Affinity cross-linking of 125I-HGF to Meth A cells resulted in a major and a minor specifically labeled complex. Competition analysis followed by cross-linking indicated that the HGF-binding proteins were involved in the formation of the high-affinity binding. The existence of the two HGF-binding surface proteins was confirmed by HGF-dependent immunoprecipitation of the binding proteins with an anti-HGF polyclonal antibody. The molecular masses of the major and the minor surface proteins were 160 kDa and 130 kDa, respectively. The 160-kDa protein was autophosphorylated in vitro on tyrosine residue and was immunoprecipitated with an antiserum against the c-met proto-oncogene product. These results indicate that the 160-kDa HGF-binding surface protein on Meth A cells is the c-met protein. Furthermore, tyrosine phosphorylation of the c-met protein was stimulated by HGF treatment of Meth A cells, suggesting that it may be involved in the signal transduction of the growth inhibition of Meth A cells by HGF.     

High affinity binding of VIP to human lung cancer cell lines

The binding of ¹²⁵I-VIP to human lung cancer cell lines was investigated. Radiolabeled VIP bound to adenocarcinoma, squamous cell carcinoma, large cell carcinoma and small cell lung cancer (SCLC) cell lines. As SCLC cell line NCI-N592 bound radiolabeled VIP well, its binding was further characterized. ¹²⁵I-VIP bound to membranes in a specific and time dependent manner. ¹²⁵I-VIP bound with high (Kd=0.8 nM) and moderate affinity (Kd=66 nM) to two classes of sites. Pharmacology studies indicated that the order of peptide potency was VIP PHI > secretin > VIP^10–28. Because VIP receptors are present on human lung cancer cells, VIP may function as a regulatory peptide in lung cancer.