Multiple forms of growth inhibitors secreted from cultured rat liver cells: purification and characterization

It was found that a non-tumorigenic epithelial cell line from the liver of a Buffalo-strain rat (BRL) secreted into the culture medium various inhibitors of the growth of BRL and RSV-BRL (tumorigenic BRL transformed by infection of Rous sarcoma virus). The secreted inhibitors were classified into two types: one inhibited the growth of BRL to a greater extent than that of RSV-BRL (non-tumorigenic BRL growth inhibitor, NGI), and the other, vice versa (tumorigenic BRL growth inhibitor, TGI). Two NGI (NGI-I and NGI-II) and two TGI (TGI-I and TGI-II) were highly purified from the serum-free conditioned medium. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis without 2-mercaptoethanol, NGI-I and II gave protein bands with molecular weights (Mr) of 56,000 and 21,000, respectively. TGI-I and II gave a band that migrated faster than bromophenol blue marker dye, but they did not pass through an ultrafiltration membrane with an Mr cutoff of 5,000. In the presence of a reducing reagent, only NGI-II showed a decrease of Mr, from 21,000 to 11,000. NGI and TGI showed 50% growth inhibition with BRL and RSV-BRL, respectively, at 5-15 ng/ml in the medium containing 10% fetal calf serum. NGI and TGI all were stable to 1 M acetic acid (pH 2.3) and 6 M urea, but labile to 5 mM dithiothreitol or trypsin. Of the eight cell lines tested, NGI-I was most effective on BRL, NGI-II on BRL and HSC-3 (human tongue squamous carcinoma), and both TGI-I and II on RSV-BRL.     

Growth-inhibitory protein present in rabbit serum, which is more effective on tumorigenic rat liver epithelial cells than on non-tumorigenic ones: its species, and mode of existence

We have previously reported that in culture, rabbit serum inhibits the growth of the epithelial cell line from Buffalo rat liver (BRL) lower than that of the tumorigenic one transformed by Rous sarcoma virus (RSV-BRL). Here, the serum was fractionated by several different methods. The findings are: 1) the growth inhibitor present (GI) existed as large complexes with non-inhibitory proteins; 2) the complexes were dissociated by 1 M NaCl plus 6 M urea; 3) the dissociated GI did not pass through membrane filter with Mr cutoff 10k; 4) it was stable in 8.5 M urea and 1 M acetic acid (pH 2.5), but labile against either dithiothreitol and trypsin; 5) it was separable into two species with pI 7.5 and 9.5; 6) both species were more effective on RSV-BRL than on BRL.     

Purification and properties of growth inhibitor from normal rabbit serum.

It was previously found that rabbit serum contains a growth-inhibitory substance for a tumorigenic rat liver cell line RSV-BRL. In the present study, the growth inhibitor was purified from normal rabbit serum to show a homogeneous protein band with a molecular weight (Mr) of 56 k on SDS-polyacrylamide gel electrophoresis under non-reducing conditions. The purified growth inhibitor, tentatively named rabbit serum-derived growth inhibitor (RSGI), potently inhibited the growth of RSV-BRL and nine kinds of other cell lines including three human tumor cell lines at a concentration of 20 ng/ml or higher. The growth-inhibitory effect of RSGI was reversible and appeared to be cytostatic rather than cytotoxic. RSGI was stable to heating at 56 degrees C for 30 min or treatment with 0.1 M 2-mercaptoethanol, but labile to heating at 100 degrees C for 3 min or treatment with 1 M acetic acid (pH 2.3), 6 M urea, 50% (v/v) 1-propanol, or 0.1% (w/v) trypsin. These properties of RSGI suggested that it was different from type beta transforming growth factors, tumor necrosis factor-alpha, and other known growth-regulatory factors.     

Purification and partial characterization of two types of growth-inhibitory protein latently present in rabbit serum.

Normal rabbit serum contained two kinds of growth-inhibitory protein, GI-I and GI-II, in latent forms. These latent inhibitors were activated by incubation at 37 degrees C for 12 h, and their activation was lowered by inhibitors for serine, cysteine and metalloproteinases. Both growth inhibitors were highly purified in active forms by successive column chromatographies. GI-I showed a major protein band with an Mr of 18,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, while GI-II showed a major protein band with an Mr of 36,000. GI-I and GI-II half-inhibited the growth of rat tumorigenic cell line (RSV-BRL) at concentrations of 0.5 ng/ml and 10 ng/ml, excess concentrations. Of the 15 cell lines tested, GI-I specifically inhibited the growth of rodent and lagomorph cells, whereas GI-II nonspecifically inhibited the growth of all cell lines tested. Specificities for cell type and malignancy were not observed with either inhibitor. These growth inhibitors were stable to a reducing reagent and proteinase inhibitors, but labile to urea, acid, organic solvents, trypsin, plasmin and heating at 95 degrees C for 5 min. These properties suggested that both growth inhibitors might be distinct from known growth-inhibitory factors.     

Transformation of rat liver cell line by Rous sarcoma virus causes loss of cell surface fibronectin, accompanied with secretion of metallo-proteinase that preferentially digests the fibronectin

An epithelial cell line derived from the liver of a normal Buffalo rat (BRL) was transformed by Rous sarcoma virus (RSV). The RSV-transformed cells were separated into five clones (RSV-BRL1 through 5), which were morphologically different. RSV-BRL cells exhibited the following characteristics distinct from those of BRL cells: tumorigenicity, irregular cell arrangement, loose intercellular junction, growth in soft agar (anchorage-independent growth) except for RSV-BRL3 and 5, and loss of cell surface fibronectin. When BRL cells were cultured in the standard medium supplemented with the serum-free conditioned medium of RSV-BRL cells, the amount of the cell surface fibronectin decreased significantly. It was found that RSV-BRL cells secreted a proteinase capable of hydrolyzing the fibronectin, whereas BRL cells secreted hardly any of this proteinase. The fibronectin-hydrolyzing proteinase (FNase) could also hydrolyze plasma fibronectin added as an exogenous substrate. The hydrolysis of plasma fibronectin was inhibited by ethylenediamine tetraacetate, but stimulated by rho-chloromercuribenzoate and calcium ion. This indicates that FNase is a metallo-enzyme, but not a serine or thiol enzyme. In addition to the proteinase, RSV-BRL cells secreted plasminogen activator and a proteinase inhibitor which inhibited the activity of plasmin but not FNase.     

Characterization of a growth-inhibiting protein present in rat serum that exerts a differential effect onin vitro growth of nonmalignant rat liver cells when compared with Rous sarcoma virus-transformed rat liver cells

Summary We have previously reported the transformation by Rous sarcoma virus of a cloned epithelial cell line (BRL) established from Buffalo rat liver by H. Coon. The nontransformed (BRL) and transformed (RSV-BRL) cells grew at comparable rates in culture, whereas only the transformed cells were tumorigenic in vivo. We report here on the existence in rat and mouse sera of a growth inhibitor for the nontransformed BRL cells. The transformed BRL cells (RSV-BRL) were insensitive to this inhibitor. The inhibitory activity was not prominent in sera from other species of animals tested except for rabbit; this serum inhibited the growth of RSV-BRL cells more strongly than that of BRL cells. The growth inhibitor was partially purified from rat serum. It is a protein free of lipid and has a molecular weight of about 220 000. The inhibitor could be separated into three components of pI 4.6, 5.2 (major) and 5.6 by isoelectric electrophoresis. EDITOR'S STATEMENT Although compelling theoretical arguments sometimes can be made for the likely existence of growth-inhibitory substances of physical relevance in the control of cell proliferation, experiments aimed at identifying and studying such factors often are difficult to design and interpret, and little strong data exists to suggest that growth-inhibitory substances are important regulatorsin vivo. The information presented in this paper represents a start toward developing a useful system for studying growth-inhibitory factor. David W. Barnes     

Efficient purification of TIMP-2 from culture medium conditioned by human hepatoma cell line, and its inhibitory effects on metalloproteinases and in vitro tumor invasion.

Metalloproteinase inhibitors were surveyed with the culture media of 19 kinds of human tumor cell lines, using transin (rat stromelysin) as the target enzyme. This survey showed that most of the cell lines more or less secreted inhibitor activity, and that a human hepatoma cell line, HLE, secreted an extremely high inhibitor activity into the culture medium. Two kinds of metalloproteinase inhibitors were purified from the serum-free conditioned medium of HLE cells. The major inhibitor, which showed a single protein band with a molecular weight (Mr) of 21,000 (21k) (nonreduced) or 24k (reduced) on SDS-polyacrylamide gel electrophoresis, was identified as TIMP-2 (tissue inhibitor of metalloproteinases-2) by the analysis of its N-terminal amino acid sequence. The other was immunologically identified as TIMP. Purified TIMP-2 inhibited the activities of transin, matrin (pump-1), Mr 72k gelatinase, and interstitial collagenase with 1:1 stoichiometry. When the latent precursor form (Mr 57k) of transin was incubated with p-aminophenylmercuric acetate as an activating reagent, TIMP-2 inhibited the conversion of the intermediate form (Mr 45k) into the mature enzyme (Mr 42k). This indicated that TIMP-2 regulates not only the activity of the mature enzyme but also the autolytic processing of the proenzyme. TIMP-2 also inhibited in vitro tumor invasion through reconstituted basement membrane (matrigel) in chemotaxis chambers, showing that the metalloproteinase inhibitors as well as the extracellular matrix metalloproteinases are involved in tumor invasion through basement membrane and other extracellular matrices.     

Purification and properties of epithelial growth inhibitor (EGI) from human platelets: its separation from type beta transforming growth factor (TGF-beta)

We previously reported that sera from various kinds of animals contain a protein(s) capable of inhibiting the growth of the non-malignant epithelial cell line derived from Buffalo rat liver (BRL). In the present study, a similar epithelial cell-specific growth inhibitor (EGI) was purified to homogeneity from an acid-ethanol extract of human platelets. During purification, EGI was separated from the major component of type beta transforming growth factor (TGF-beta), which can stimulate the colony formation of the non-malignant fibroblastic cell line derived from rat kidney (NRK) in soft agar in the presence of epidermal growth factor (EGF). The purified EGI had an Mr of 27,000, and was composed of two subunits identical in Mr. It significantly inhibited the growth in monolayer cultures of three non-malignant epithelial cell lines, BRL, MDCK (from Madin-Darby canine kidney) and BSC-1 (from African green monkey kidney), at doses lower than 40 pg/ml in medium containing 10% fetal calf serum. Its inhibitory activity was stable against heating at 90 degrees C for 3 min, but not against treatment with 50 mM dithiothreitol. In addition, TGF-beta was also partially purified from the same extract. The purified TGF-beta did not show any inhibitory activity toward the growth of BRL, MDCK, BSC-1, or NRK.     

Purification and properties of extracellular matrix-degrading metallo-proteinase overproduced by Rous sarcoma virus-transformed rat liver cell line, and its identification as transin

Rous sarcoma virus-transformed rat liver cell line RSV-BRL secreted a neutral proteinase in a latent precursor form with a molecular weight (Mr) of 57,000 (57k) as a major secreted protein. This enzyme was a calcium-dependent metallo-proteinase. The proenzyme was purified from the serum-free conditioned medium of the transformed cells by affinity chromatographies on a zinc chelate Sepharose column and a reactive red agarose column. When activated by treatment with trypsin or p-aminophenylmercuric acetate (APMA) in the presence of Ca2+, the purified enzyme effectively hydrolyzed casein, fibronectin, and laminin. Type IV collagen was hydrolyzed at 37 degrees C but not at 30 degrees C by the enzyme, whereas type I and type III collagens were hardly hydrolyzed even at 37 degrees C. The treatment with trypsin or AMPA in the presence of Ca2+ converted this 57k proenzyme to an active and stable enzyme with Mr 42k. In the absence of Ca2+, however, APMA converted the proenzyme to an intermediate form with Mr 45k, while trypsin digested it to an inactive peptide with Mr 30k. These results demonstrate that calcium ion is essential for the activation, activity expression, and stabilization of this metallo-proteinase. Analysis of its partial amino acid sequence and amino acid composition showed that the 57k proenzyme was identical or closely related to the putative protein transin, a rat homologue of stromelysin.     

Demonstration of two subtypes of insulin-like growth factor receptors by affinity cross-linking

The structure of receptors for insulin-like growth factors in rat liver plasma membranes and the BRL 3A2 rat liver cell line has been examined by chemical cross-linking with disuccinimidyl suberate and sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and nonreducing conditions. Two receptor subtypes have been identified: (i) 125I multiplication-stimulating activity cross-linked to liver membranes or intact cells appeared in a complex of Mr = 260,000 (reduced) and 220,000 (nonreduced) and (ii) 125I-insulin-like growth factor I cross-linked to BRL 3A2 cells appeared predominantly in two bands of Mr greater than 300,000 without disulfide reduction and in a Mr = 130,000 complex following reduction. The two subtypes of insulin-like growth factor receptors identified by structural analysis correspond to previously observed differences in their specificity for insulin and insulin-like growth factors.     

Influence of feeder layer type on the efficiency of isolation of porcine embryo-derived cell lines

Experiments were conducted to determine the effects of feeder layers composed of different cell types on the efficiency of isolation and the behavior of porcine embryo-derived cell lines. Inner cell masses (ICM) isolated from 7- to 8-d-old embryos were plated on feeder layers composed of Buffalo rat liver cells (BRL), a continuous cell line of murine embryonic fibroblasts (STO), STO combined with BRL at a 9:1 and 1:1 ratio, STO with BRL-conditioned medium (STO + CM), porcine embryonic fibroblasts (PEF), PEF combined with BRL at a 9:1 and 1:1 ratio, porcine uterine epithelial cells (PUE), murine embryonic fibroblasts (MEF), or an epithelial-like porcine embryo-derived cell line (PH3A). It was found that embryo-derived cell lines could be isolated only from the STO and the STO with BRL-conditioned medium treatments. The isolated cell lines were of epithelial-like and embryonic stem cell-like (ES-like) morphology. The feeders tested had an effect on the behavior of plated ICM. Some feeders, represented by PUE, BRL, STO:BRL (1:1), PEF:BRL (1:1), and PH3A, did not promote attachment of the ICM to the feeder layer; others, represented by STO and MEF, allowed attachment, differentiation and proliferation. On PEF feeders the ICM spread onto the feeder layer after attachment without apparent signs of proliferation or differentiation. None of the feeders tested increased the efficiency of isolation or the growth characteristics of embryo-derived (both ES-like and epithelial-like) cell lines over that of STO feeders.     

Normal and Ha-ras-1 oncogene transformed Buffalo rat liver (BRL) cells show differential resistance to cytoskeletal protein inhibitors.

In the present study, using immunofluorescence microscopy, we have demonstrated that normal and Ha-ras-1 transformed Buffalo rat liver (BRL) cells which were exposed to cytoskeletal protein inhibitors, showed a differential resistance of their microfilament and microtubule networks. One hour exposure of normal BRL cells to 10(-5) M cytochalasin B provoked a clear and already total breakdown of actin filaments. However, at this concentration of cytochalasin B, the microfilaments of transformed BRLHO6T1-1 cells were not seriously affected; a higher cytochalasin B concentration (> or = 2 x 10(-5) M) was required to induce a significant breakdown of microfilaments in these transformed cells. The two cell lines also demonstrated differential microtubule stability when they were treated with either colchicine or triethyllead. Three hours exposure to 10(-6) M of either antimicrotubule agents was sufficient to disrupt the microtubules of normal BRL cells, without affecting their counterparts in the transformed BRLHO6T1-1 cells. A 10-fold higher drug concentration (10(-5) M) was required to induce microtubular breakdown in the transformed BRL cells. The differential stability of microfilaments and microtubules in normal and transformed BRL cells that was observed could not be attributed to a differential internalization of the agents, as shown by experiments on the uptake of [3H]-cytochalasin B and triethyllead. In addition, the transformed BRLHO6T1-1 cells did not express altered actin and tubulin isoforms, as demonstrated by isoelectric focusing followed by immunoblotting analysis. We conclude that the transformation of BRL cells with the Ha-ras-1 oncogene results in a greater stability of microfilaments and microtubules, leading to a structurally firmer cell shape.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that viral transforming gene products and epidermal growth factor stimulate phosphorylation of the same cellular protein with similar specificity

Treatment of A-431 human epidermoid carcinoma cells with epidermal growth factor (EGF) was shown to enhance the phosphorylation of a Mr = 34,000 protein. Because the phosphorylation of an analogous protein is enhanced in various cell lines transformed by Rous sarcoma virus (RSV) (Erikson, E., and Erikson, R. L. (1980) Cell 21, 829-836), we characterized the phosphorylation of the A-431 Mr = 34,000 protein under these two conditions in order to determine whether there are common pathways between viral transformation and EGF stimulation. The results of tryptic phosphopeptide mapping and phosphoamino acid analysis showed that the Mr = 34,000 protein was phosphorylated in an identical manner by the EGF-stimulated protein kinase activity and by the protein kinase activity of the RSV transformation-specific protein or of its normal cell homolog. Although the specific protein kinase that phosphorylates the Mr = 34,000 protein under conditions of EGF-stimulation is not yet identified, these studies demonstrate that at least one consequence of EGF stimulation is identical with one of the consequences of viral transformation.     

Studies of cAMP metabolism in cultured hepatoma cells: presence of functional adenylate cyclase despite low cAMP content and lack of hormonal responsiveness.

The ability of isoproterenol, glucagon, PGE1 and cholera toxin to stimulate the synthesis of cAMP and protein kinase activity in line of liver cells (BRL) and a line of rat hepatoma cells (H35) has been determined. The concentration of cAMP in BRL cells (approximately 10 pmoles/mg protein) is in the range reported for other cultured cell lines but H35 cells contain extraordinarily low amounts of this cyclic nucleotide (approximately 0.05 pmoles/mg protein). Isoproterenol and PGE1 caused an increase in cAMP content, and protein kinase activation in BRL cells, although glucagon was ineffective. H35 cells, in contrast, were completely insensitive to all hormonal agonists. Despite this fact, cholera toxin was able to produce a marked increase in cAMP content, adenylate cyclase activity and protein kinase activation in H35 cells. binding studies with [125 I]-iodohydroxybenzylpindolol, a specific beta-adrenergic receptor antagonist, revealed that each H35 cell possesses fewer than 10 beta-adrenergic receptors whereas BRL cells contain 2-5,000 receptors per cell. The low level of cAMP in H35 cells appears to result from a combination of totally unstimulated adenylate cyclase and apparently elevated phosphodiesterase activities.     

Characterization of insulin-like growth factor binding proteins from human breast cancer cells

The insulin-like growth factor binding proteins (IGF-BPs) are structurally and immunologically distinct from the IGF type 1 or type 2 receptors and are characterized by two major forms: a large, GH-dependent BP found in human plasma (Mr = 150 k) and a small GH-independent BP (Mr = 28-42 k) present in human plasma, amniotic fluid, and HEP G2 cells. Using affinity cross-linking techniques, we have identified several binding proteins secreted by human breast cancer cell lines (Hs578T, MDA-231, T-47D, and MCF-7). Under nonreducing conditions these proteins migrated at an apparent Mr = 35, 28, 27, and 24 k, while reducing conditions revealed bands of apparent Mr = 35, 32, 27, and 24 k. Competitive binding studies in T-47D-conditioned media demonstrated that these BPs bound more IGF-II than IGF-I, and that IGF-II potently inhibited binding of either IGF-I or -II. Immunological studies using a polyclonal antibody against the HEP G2 small BP revealed no immunoreactive BP in conditioned media from MCF-7 and T-47D and only slight immunoreactivity in conditioned media from Hs578T and MDA 231. Analysis by Northern blot, using a probe from the cDNA sequence of the HEP G2 BP, demonstrated that Hs578T and MDA-231 cell lines contained small amounts of the 1.65 kilobase mRNA characteristic of the HEP G2 BP, while MCF-7 and T-47D tested negative.(ABSTRACT TRUNCATED AT 250 WORDS)     

Soluble insulin-like growth factor II/mannose 6-phosphate receptor inhibits DNA synthesis in insulin-like growth factor II sensitive cells

The soluble form of the insulin-like growth factor II (IGF-II)/mannose 6-P (IGF-II/M6P) receptor is released by cells in culture and circulates in the serum. It retains its ability to bind IGF-II and blocks IGF-II-stimulated DNA synthesis in isolated rat hepatocytes. Because these cells are not normally stimulated to divide by IGF-II in vivo, the effect of soluble IGF-II/M6P receptor on DNA synthesis has been further investigated in two cell lines sensitive to IGF-II; mouse 3T3(A31) fibroblasts, stimulated by low levels of IGF-II following priming by epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) and Buffalo rat liver (BRL) cells, which secrete IGF-II and proliferate in the absence of exogenous growth factors. Soluble IGF-II/M6P receptor (0.2-2.0 microgram/ml) purified from a rat hepatoma cell line inhibited DNA synthesis (determined by dThd incorporation) in both cell lines. Basal DNA synthesis was very low in serum-free 3T3 cells, but high in serum-free BRL cells, possibly as a result of autocrine IGF-II production. The inhibitory effect was reversible in cells preincubated with soluble receptor prior to incubation with growth factors and could also be overcome by excess IGF-II. Soluble receptor was more potent in IGF-II-stimulated 3T3 cells and serum-free BRL cells than in BRL cells incubated with serum. Mean inhibition by four preparations of soluble receptor (1 microgram/ml) was 34.7% +/- 4.4% in BRL cells stimulated with fetal calf serum (FCS) (5%) compared to 54.8% +/- 4.2% in serum-free BRL cells (P = 0.05) and 60.6% +/- 6.5% (P = 0.02) in 3T3 cells stimulated by PDGF, EGF, and IGF-II. Soluble receptor had no effect on DNA synthesis in 3T3 cells stimulated with IGF-I. These results demonstrate that soluble receptor, at physiological concentrations, can block proliferation of cells by IGF-II and could therefore play a role in blocking tumor growth mediated by IGF-II.     

Identification of the types of insulin-like growth factor-binding proteins that are secreted by muscle cells in vitro

We have previously shown that muscle cells secrete insulin-like growth factor-binding proteins. In the present study, BC3H-1 cells were shown to secrete one binding protein of Mr 32,000, whereas L6 cells secreted two binding proteins of Mr 31,000 and 24,000, as determined by ligand blotting. Subconfluent proliferating L6 cells secrete more of the Mr 24,000 binding protein, relative to the Mr 31,000 form. In contrast, differentiated L6 myotubes secreted similar quantities of the two forms. Insulin-like growth factor I preferentially stimulated secretion of the Mr 31,000 versus the Mr 24,000 binding protein from L6 cells and caused an increase in the secretion of the Mr 32,000 binding protein from BC3H-1 cells. The Mr 31,000 binding protein from L6 cells had a greater affinity for insulin-like growth factor II compared with insulin-like growth factor I, as did the Mr 32,000 binding protein of BC3H-1 cells. In contrast, the Mr 24,000 binding protein of L6 cells preferred insulin-like growth factor I. Neither porcine insulin nor relaxin competed for 125I-IGF-I binding. In conclusion, these muscle cell lines secrete only one or two forms of insulin-like growth factor-binding proteins. L6 cell differentiation is associated with a relative increase in the secretion of the Mr 31,000 binding protein compared with the Mr 24,000 form. Insulin-like growth factor I stimulates the secretion of its own binding proteins from muscle cells, and this may be an important mechanism for modulating cellular responsiveness to this growth factor.     

Different susceptibility of lung cell lines to inhibitors of tumor promotion and inducers of differentiation

Histologically distinct lung tumor and normal cell lines were treated with a variety of potential inhibitors of cell growth such as inducers of cell differentiation, inhibitors of protein kinase C and inhibitors of tumor promotion. The response was assessed by 3H thymidine incorporation and cloning efficiency. Both phorbol retinoate acetate and mezerein stimulated growth in lung normal cell lines (human fibroblastic PEH cells and rat epithelial TP9 cells) while inhibiting growth in lung tumor cell lines (human small-cell cancer-derived cell line IRSC-10M and adenocarcinoma-derived cell line A549). Likewise, the hydrophobic peptide melittin did not inhibit growth and cloning efficiency of normal cells at 1 microM, a concentration which prevented proliferation in tumor cells. Protein kinase C inhibitors, chlorpromazine, trifluoperazine and 1-(5 isoquinolinylsulfonyl) 2-methylpiperazine, were much more effective on proliferation of IRSC-1OM than of A549 cells. In contrast, the latter cells were more susceptible to anti-promoters such as glycyrrhetic acid, an anti-inflammatory agent, and 3,4',2', 4'-tetrahydroxychalcone or 2,3,5-trimethyl-6 (12-hydroxy-5,10-dodecadiynyl)-1,4-benzoquinone, two inhibitors of lipoxygenase, a key enzyme in arachidonate metabolism. Our results provide evidence that small-cell carcinoma-derived cells, in contrast with adenocarcinoma-derived cells, are growth-inhibited by protein kinase C inhibitors and poorly dependent on the arachidonate metabolism. This difference in responsiveness suggests that different growth signalling pathways are preferentially triggered in these histologically distinct lung tumor cell lines. As a consequence, the proper susceptibility of tumor cells to phenotype modifiers has to be taken into account in cancer therapy.