Pyridine nucleotide levels as a function of growth in normal and transformed 3T3 cells.

The levels of NAD (NAD⁺ + NADH) and NADP (NADP⁺ + NADPH) and their redox states were measured as a function of growth in 3T3 mouse fibroblasts which exhibit density-dependent inhibition of growth and SV40 (simian virus #40)-transformed 3T3 cells (SVT2) which have lost this property. The levels were related to cell numbers, protein content, and rates of DNA synthesis. At corresponding cell densities, 3T3 cells contain approximately twice as much total protein as SVT2 cells. The levels of NAD relative to total cellular protein are density dependent in both 3T3 and SVT2, increasing with increasing cell density. Over a 30-fold range of cell densities, the NAD levels in 3T3 increase 2.4-fold, while the levels in SVT2 increase 1.6-fold. The levels of NAD are very similar in dividing 3T3 and SVT2 cells at corresponding cell densities; however, a marked increase in the levels of NAD is observed in 3T3 cells, but not in SVT2 cells, at cell densities just prior to where 3T3 cells enter density-dependent inhibition of growth. This increase in NAD levels is correlated with the cessation of DNA synthesis. The NAD pools are 15–25% NADH for 3T3 and 5–15% NADH for SVT2. NADP levels relative to protein in 3T3 and SVT2 are less density dependent, with overall increases of 1.3- and 1.2-fold, respectively, observed over the range of cell densities examined. NADP levels relative to protein are nearly twice as high in SVT2 cells as in 3T3 cells of corresponding cell densities. The NADP pools are approximately 70–80% NADPH in both cell types.     

Binding of methylmercury(II) by HeLa S3 suspension-culture cells: Intracellular methylmercury levels and their effect on DNA replication and protein synthesis

HeLa S3 cells were exposed to varied concentrations of methylmercury over varied periods of time and its binding by the cells was studied using ²⁰³Hg-labeled methylmercuric chloride as radioactive marker. Also studied was the effect of cell-bound methylmercury on DNA replication and protein synthesis and on the growth rate of the cells. The results show that methylmercury binding is a rapid process, with much of the organomercurial bound within the the first 60 min of incubation, and that considerable quantities of organic mercury become affixed to the cells. The amounts of bound methylmercury, [CH₃Hg(II)]_bound, given in mol/cell, range from 2 × 10^?16 (at 1 h of incubation and at 1 μM CH₃Hg(II) in the medium) to almost 4 × 10^?14 (at 24 h of incubation and at 100 μM CH₃Hg(II) in the medium). A [CH₃Hg(II)]_bound value of about 30 × 10^?16 mol/cell appears to be the threshold below which cells display a normal growth pattern and below which metabolic events such as DNA replication or protein synthesis are affected only to a minor degree but above which major changes in cell metabolism and cell growth take place. Methylmercury binding by the cells is tight so that only 20% of the bound material is released from the cells over a 3-h incubation period when the cells are placed into fresh, methylmercury-free growth medium. Analysis of the binding data in terms of binding to identical and completely independent sites yields an association constant K of 7.92 × 10⁴ l/mol and for the maximum concentration of cellular binding sites the value 2.40 × 10^?14 mol/cell or 1.45 × 10¹⁰ sites/cell. Evidence is presented which shows that cellular sulfhydryl groups do not suffice to provide all the sites taken up by methylmercury and that binding, in all likelihood, involves basic nitrogen, too. The levels of cell-bound methylmercury are such that binding to HeLa DNA and HeLa chromatin, for instance, can readily take place. Methylmercury binding data obtained by using the technique of particle-induced X-ray emission (PIXE) are in good agreement with the data obtained via isotope dilution.     

Cell growth and ouabain-sensitive Rb uptake and (Na + K)-ATPase activity in 3T3 and SV40 transformed 3T3 fibroblasts

The uptake of ouabain-sensitive ⁸⁶Rb⁺ uptake measured at 5 min and the uptake measured at 60 min was 4.5- and 2.7-fold greater respectively for SV40 transformed 3T3 cells compared to 3T3 cells during the late log phase of growth. This uptake, however, varied markedly with cell growth. Ouabain-sensitive ⁸⁶Rb⁺ uptake was found to be a sensitive indicator of protein synthesis as measured by total protein content. Cessation of cell growth as measured by total protein content was associated with a decline in ouabain-sensitive ⁸⁶Rb⁺ uptake in both cell types. This increased ouabain-sensitive cation transport was reflected in increased levels of (Na⁺ + K⁺)-ATPase activity for SV40 3T3 cells, which showed a 2.5-fold increase V but the same K_rmm as 3T3 cells.These results are compared with the results of related work. Possible mechanisms for these effects are discussed and how changes in cation transport might be related to alterations in cell growth.     

Role of calcium in growth inhibition induced by a novel cell surface sialoglycopeptide

Our laboratory has purified an 18 kDa cell surface sialoglycopeptide growth inhibitor (CeReS-18) from intact bovine cerebral cortex cells. Evidence presented here demonstrates that sensitivity to CeReS-18-induced growth inhibition in BALB-c 3T3 cells is influenced by calcium, such that a decrease in the calcium concentration in the growth medium results in an increase in sensitivity to CeReS-18. Calcium did not alter CeReS-18 binding to its cell surface receptor and CeReS-18 does not bind calcium directly. Addition of calcium, but not magnesium, to CeReS-18-inhibited 3T3 cells resuts in reentry into the cell cycle. A greater than 3-hour exposure to increased calcium is required for escape from CeReS-18-induced growth inhibition. The calcium ionophore ionomycin could partially mimic the effect of increasing extracellular calcium, but thapsigargin was ineffective in inducing escape from growth inhibition. Increasing extracellular calcium 10-fold resulted in an approximately 7-fold increase in total cell-associated ⁴⁵Ca⁺², while free intracellular calcium only increased approximately 30%. However, addition of CeReS-18 did not affect total cell-associated calcium or the increase in total cell-associated calcium observed with an increase in extracellular calcium. Serum addition induced mobilization of intracellular calcium and influx across the plasma membrane in 3T3 cells, and pretreatment of 3T3 cells with CeReS-18 appeared to inhibit these calcium mobilization events. These results suggest that a calcium-sensitive step exists in the recovery from CeReS-18-induced growth inhibition. CeReS-18 may inhibit cell proliferation through a novel mechanism involving altering the intracellular calcium mobilization/regulation necessary for cell cycle progression. © 1995 Wiley-Liss, Inc.     

Turnover, change of composition with rate of cell growth and effect of phenylxyloside on synthesis and structure of cell surface sulfated glycosaminoglycans of normal and transformed cells

The synthesis of sulfated glycosaminoglycans was analysed in mouse fibroblasts during the transition from exponential growth to quiescent monolayers. 'Normal' Swiss 3T3 fibroblasts were compared with SV40 transformed 3T3, C6, ST1 and HeLa cells. p-Nitrophenyl-beta-D-xyloside, an artificial acceptor for glycosaminoglycans synthesis, was used as a probe. Exponentially growing 'normal' 3T3 cells synthesized both dermatan sulfate and chondroitin 4-sulfate, retaining the latter and releasing the former to the medium. Upon reaching quiescence these cells switched to retention of dermatan sulfate and release of chondroitin 4-sulfate. SV3T3 cells synthesized several fold less sulfated glycosaminoglycans than 'normal' 3T3. Even though SV3T3 cells are able to synthesize dermatan sulfate, they only retained chondroitin 4-sulfate, never switching to retention of dermatan sulfate. These results indicated that the transition from rapidly proliferating to resting G0 state in normal cells is accompanied by a switch from chondroitin-sulfate rich to dermatan-sulfate-rich cells. This switching was not observed with transformed cells, which are unable to enter the G0 state. Phenylxyloside caused a several fold increase in glycosaminoglycans released to the medium in both cell types, but it did not interfere with either growth rate or cell morphology. Particularly the phenylxyloside treatment led to an increase of more than 10-fold in production of dermatan and chondroitin sulfate by SV3T3, C6, ST1 and HeLa cells. This demonstrated that transformed cells have a high capacity for glycosaminoglycan synthesis. Analysis of enzymatic degradation products of glycosaminoglycans, synthesized in the presence of phenylxyloside, by normal and transformed cells, led to the finding of 4- and 6-sulfated iduronic and glucuronic acid-containing disaccharides. This result indicated that the xyloside causes the synthesis of a peculiar chondroitin sulfate/dermatan sulfate, in both normal and transformed cells.     

High chemotactic response to platelet-derived growth factor of a teratocarcinoma differentiated mesodermal cell line

Summary Evidence is presented that a differentiated mesodermal line (MES-1) from P19 EC cells express a high chemotactic response to platelet-derived growth factor (PDGF) as assayed in a blind-well modified Boyden chamber. Compared to the NIH 3T3 fibroblasts the chemotactic response of MES-1 is increased by 10-fold at 0.3 ng/ml of PDGF, 4-fold at 1.25 ng/ml of PDGF, 2-fold at 2.5 ng/ml of PDGF. In contrast, PDGF induces the same increase in [³H]thymidine incorporation in both cell lines, made quiescent under reduced serum concentration. This high chemotactic response to PDGF seems specific for these mesodermal cells. Among the different teratocarcinoma cells tested, including stem cells (F9, PC 13, PCC4) and endodermal derivatives (PYS, F9 with retinoic acid, PSA 5E), only the visceral endodermlike cells (PSA5E) are slightly attracted by PDGF. This chemotactic response to PDGF is not related to the presence or characteristics of the type B PDGF receptors, which are less numerous in MES-1 cells (10⁵ receptors/cell, KDa 1,2 mM) compared to NIH 3T3 cells (64×10⁴ receptors per cell, KDa 1,8 nM). The MES-1 cell line might be of interest for studying the chemotactic effect of PDGF. These results also suggest a role for this soluble factor in cell migration during early embryogenesis. This investigation was supported by a grant of La Fondation pour la Recherche Médicale.     

In vitro drug delivery mediated by ecto-NAD+-glycohydrolase ligand-targeted liposomes

We have studied the growth-inhibitory potency of methotrexate and methotrexate-γ-aspartate encapsulated in liposomes conjugated to ligands of ecto-NAD⁺-glycohydrolase (Salord, J. et al., Biochim. Biophys. Acta 886 (1986) 64–75). The ability of targeted liposomes to enhance growth inhibition, which amounted to a 4-fold reduction of the drug concentration required to inhibit cell growth by 50% as compared to nontargeted liposomes, was observed only with cells expressing this ecto-enzyme activity, i.e., Swiss 3T3 fibroblasts and RAJI, a Burkitt-type lymphoma cell line. Delivery of the encapsulated drugs was inhibited by NH₄Cl and varied with the endocytic capacity of the cells. Only small unilamellar vesicles affected the growth of the lymphoma cells, whereas the fibroblasts were more sensitive to large unilamellar vesicles. With vesicles of appropriate size, there was a good correlation between the specific binding of the targeted liposomes to cells and drug delivery. Our results suggest that ecto-NAD⁺-glycohydrolase can provide a recognition site on target cells and mediate the internalization of targeted liposomes by a mechanism most probably related to adsorptive endocytosis.     

Lys-tRNA4 levels and cell division in mouse 3T3 cells

tRNA₄^lys is an isoaccepting tRNA^lys which has been proposed as a necessary requirement for cell division in mammalian cells. We have measured the levels of this tRNA^lys during the growth cycle of mouse 3T3 fibroblasts. High levels of tRNA₄^lys were seen throughout exponential growth. However, a marked decrease in tRNA₄^lys occurred 24 h before the cells became confluent. This decrease was observed in three different 3T3 cell lines, but was not seen in a transformed 3T3 cell line. Trypsinization and replating of contact-inhibited cells returned tRNA₄^lys to the levels characteristic of exponential cells. Data from these and other cell lines show a direct relationship between the levels of tRNA₄^lys and the growth rate of cells in culture.     

PI3K pathway inhibitor LY294002 alters Jurkat T cell biobehaviours via ERK1/2-ICBP90 mediation

Little is known about whether there is a relationshipbetweenPI3K/AKT, ERK1/2 and an inverted CCAAT box binding protein (ICBP90) in biological behaviours of tumour cells. The aim of this study was to determine thisusing Jurkat T cells. Compared to PD98059 (an ERK1/2 signaling inhibitor), DAPT (a Notch signaling inhibitor) or adriamycin (a classical anti-tumour drug), the inhibition of Jurkat T cell growth by LY294002 (a PI3K/Akt signaling inhibitor) was more obvious. LY294002 combined with adriamycin appeared to have a synergistic effect. LY294002 strongly blocked Jurkat T cells at each phase of cell cycle with a decrease of DNA content, superior to adriamycin. Consistent with these changes, the expression of phosphorylated ERK1/2 was markedly decreased in the LY294002-treated Jurkat T cells, leading to the reduction of ICBP90 production, followed by moderate attenuation of TGF-β secretion. The results suggest that deactivation of PI3K/Akt signalling can surpress Jurkat T cell growth through inhibiting cell proliferation and blocking the cell cycle. ICBP90 may mediate the PI3K/AKT-ERK1/2 signalling to regulate leukemia cell growth.     

Stimulation of cell growth and proliferation in NIH-3T3 cells by onion and garlic oils

Onion and garlic oil were seen to shorten the cell-doubling time and stimulate the growth and proliferation of NIHY-3T3 cells. FNollowing treatment with either onion or garlic oil, an increase in the growth rate and almost a 2-fold increase in cell number over the control was observed within a 24-hour period. Phorbol myristate acetate when given simultaneously with either oil appeared to nullify both effects. FNollowing exposure to low doses (< 10 µm/ml) of either oil, an increase in cell survival, not seen with the oil control tricaprylin, was observed following a 5-day exposure period. At higher concentrations cell survival decreased proportionately in all cases. The appearance of multinucleated cells, which increased with dose and time, was also observed following treatment with both garlic and onion oil.Abbreviations EMEMM Eagles Minimal Essential Medium - MCN mean cell number - PMA phorbol myristateacetate - RCE relative cloning efficiency - SEM standard error of the mean     

Influences of clotting factors (thrombin, factor XIII) and of fibronectin on the growth of tumor cells and leukemic cells in vitro

Thrombin, factor XIII and fibronectin were incubated with cultures of mouse sarcoma cells, human cervix carcinoma cells (HeLa cells) and cells of an acute lymphoblastic leukemia. Thrombin induced a significant increase of 3H-thymidine uptake into cells with a 1,5- to 2-fold increase of cell count. The cells of an acute lymphoblastic leukemia showed a similar response to the influence of thrombin. Factor XIII in tumor cells merely induced an increase of 3H-thymidine uptake, the cell count remained constant. The cells of an acute lymphoblastic leukemia showed under the influence of factor XIII a significant increase of cell count and thymidine uptake. HeLa cell growth was optimal at low fibronectin concentrations. Fibronectin concentrations of 1 mg/ml to 3 mg/ml inhibited HeLa and mouse sarcoma cell growth.     

The biotin requirement of HeLa cells

We have examined the effect of alterations in the biotin content of the medium on the growth, viability, biotin content, and the activities of biotin-dependent and biotin-independent enzymes of the HeLa cells. The inclusion in the growth medium of avidin, which almost irreversibly binds with biotin (K_d, 10^?15 M), results in an increase in cellular biotin content and biotin enzyme activity over that seen when the cells are grown in a biotin-depleted medium. The addition of avidin-bound biotin to the growth medium led to a forty-fold increase in cellular biotin when compared to the inclusion of an equivalent amount of free biotin in the medium. HeLa cells are able to internalize avidin-bound biotin. Biotin is released from this complex to function as the prosthetic group of biotin enzymes. HeLa cells do have a nutritional requirement for biotin.     

Membrane effects of tumor promoters: stimulation of sugar uptake in mammalian cell cultures.

Phorbol esters stimulate 2-deoxy-D-glucose (DG) uptake in rodent and human cell cultures. The potent tumor promoting agent, 12-0-tetradecanoyl phorbol-13 acetate (TPA), induces a 12-fold stimulation in confluent 3T3 cells and 2.5-fold stimulation in HeLa cells. When a series of macrocyclic deterpenes are assayed, their relative potencies in stimulating DG uptake in 3T3 cells correlate with other known biologic effects of these compounds. On a molar basis, TPA is a much more potent stimulator of DG transport than insulin or epidermal growth factor. In HeLa cells, the ED50 value of the TPA effect is 0.2 nM. The increase in DG uptake occurs immediately after the addition of TPA, reaches a maximum at 90 minutes, persists for at least three hours after removal of TPA from the medium, and is temperature dependent. The stimulation is not inhibited by cycloheximide or actinomycin D. As in control cells, DG uptake in TPA treated cells is inhibited by p-hydroxymercuribenzoate, phyloridzin, cytochalasin B, and dexamethasone. Although the precise mechanism is not known, evidence is presented that the TPA stimulation of DG uptake is due to enhanced transport of the sugar rather than to effects on intracellular metabolism. The enhanced transport may be secondary to a more generalized change in membrane structure.     

Involvement of protein kinase C in phorbol ester-induced sensitization of HeLa cells to cis-diamminedichloroplatinum(II)

We have investigated the effect of tumor promoting phorbol esters on the antiproliferative actions of several antitumor agents. Pretreatment of HeLa cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) or phorbol 12,13-dibutyrate (PDBu) caused a significant (9-fold) increase in cellular sensitivity to cis-diamminedichloroplatinum(II) (CP). TPA also sensitized HeLa cells to melphalan (2.5-fold) but had no effect on the antiproliferative activity of bleomycin, doxorubicin, vincristine, or mitomycin C. The sensitization of HeLa cells by TPA was concentration-dependent up to 1 nM and paralleled the activation of protein kinase C by TPA measured in vitro. The maximum stimulation of protein kinase C (6-fold) was observed with 10 nM TPA. 4 alpha-Phorbol 12,13-didecanoate neither activated protein kinase C nor sensitized HeLa cells to CP. 4-O-Methyl-TPA, which does not affect cell cycle distribution of HeLa cells, also sensitized these cells to CP by 6-fold and activated protein kinase C by 3-fold. Inhibitors of protein kinase C, such as palmitoylcarnitine and sphingosine, antagonized PDBu-induced sensitization of HeLa cells to CP. The maximum sensitization of HeLa cells to CP required prolonged pretreatment (greater than or equal to 24 h) with phorbol esters but could not be explained by down-regulation of protein kinase C. For example, 4-O-methyl-TPA caused no down-regulation of protein kinase C. Moreover, TPA caused substantial down-regulation of protein kinase C (1% of control) in A-253 cells but failed to sensitize A-253 cells to CP. TPA (100 nM), however, activated protein kinase C in A-253 cells by 5.5-fold. Therefore, activation of protein kinase C by TPA appears to be necessary but not sufficient for cellular sensitization to CP. The sensitization of HeLa cells by TPA was associated with a concentration- and time-dependent increase in cellular platinum content. The protein synthesis inhibitor cycloheximide (10 micrograms/ml) blocked sensitization of HeLa cells to CP as well as the increase in platinum content caused by a 24-h pretreatment with PDBu.     

Increased alkaline phophatase activity in HeLa cells mediated by aliphatic monocarboxylates and inhibitors of DNA synthesis

Alkaline phosphatese activity of HeLa cells is increased from 3- to 8-fold during growth in medium with certain aliphatic monocarboxylates. The four-carbon fatty acid salt, sodium butyrate, is the most effective “inducer” with propionate (C₃), pentanoate (C₅) and hexanoate (C₆) having lesser effects. Other straight-chain aliphatic monocarboxylates, branched-chain analogues of inducers, hydroxylated derivatives, and metabolytes structurally related to butyrate are ineffective in mediating an increase in enzyme activity, indicating stringent structural requirements for inducers. The kinetics of increase in alkaline phosphatase activity in HeLa cells shows a 20–30 h lag period after adding the aliphatic acid followed by a rapid linear increase of enzyme activity. Protein synthesis is required for “induction”. The isozyme of HeLa alkaline phosphatase induced by monocarboxylates is the carcinoplacental form of the enzyme as determined by stereospecific inhibition by the l-enantiomorphs of phenylalanine and tryptophan, heat stability, and immunoreactivity with antibody against the human placental enzyme.Monocarboxylates that mediate increased alkaline phosphatase activity inhibit HeLa cell multiplication. Inhibition of HeLa cell growth may be necessary for induction and this hypothesis is supported by the findings that three different inhibitors of DNA synthesis, i.e. hydroxyurea, 1-β-d-arabinfuranosyl cytosine and methotrexate, also increase alkaline phosphatase activity. These inhibitors are synergistic with butyrate in causing HeLa cells to assume a more spindle-like shape and in producing an up-to 25-fold increase of enzyme activity. Studies on the modulation of carcinoplacental alkaline phosphatase by monocarboxylates commonly used as antimicrobial food additives and by anti-neoplastic agents may provide methods to evoke “tumor markers” of human occult malignancies. These drug-induced elevations of fetal isozyme activity may further our understanding of gene expression in human cells.     

A complement-resistant HeLa cell line (T638) is blocked at the step of C3 deposition

A complement-resistant line of HeLa cells (T638) was derived by serial passage of complement-susceptible HeLa cells in anti-beta 2-microglobulin (b2m) antiserum and complement. The T638 line maintained stable complement resistance when passed for an additional 1500 generations in the absence of antiserum and complement. T638 cells expressed equivalent levels of cell-associated b2m as did the parent HeLa cell line. Furthermore, T638 cells were resistant to killing by complement and anti-HeLa antiserum with specificity for molecules other than b2m. These results indicate that the resistance of T638 cells does not simply reflect loss of anti-b2m binding antigens. We next investigated the mechanism of resistance of T638 cells to complement-mediated killing. Antibody-sensitized HeLa and T638 cells both consumed CH50 activity completely from normal human serum; cytotoxicity was not mediated via the alternative complement pathway. HeLa and T638 cells caused equivalent utilization of C4 from normal human serum in the presence of antibody. Consumption of C2, greater with T638 than with HeLa cells during incubation in serum, was complete when cells bearing purified C1 and limited C4 were incubated with C2. T638 cells bound more 3H-C4 than HeLa cells during incubation in serum, but binding of 3H-C3 by T638 cells was fourfold to fivefold less than by HeLa cells. Finally, we investigated the rate of decay in the capacity of C142 on HeLa and T638 to cleave and deposit 3H-C3. The T1/2 for decay of C142-mediated binding of 3H-C3 on HeLa was 3.9 min, whereas minimal C3 deposition was detected on T638 cells at all time points. These results show that T638 cells evade complement-mediated lysis despite activating early components of the classical complement pathway. The mechanism of resistance is a failure to form an effective C3 convertase.     

Characterization of the cellular binding site for the urokinase-type plasminogen activator

Human urokinase-type plasminogen activator (uPA) binds rapidly and with high affinity to a number of human cell types; this localizes plasmin generation to the close environment of the cell surface. uPA binding to HeLa and U937 cells is mediated by a single class of sites with an affinity of 3.4 +/- 1.3 x 10(-10) M. Binding is abolished by treatment of the cells with trypsin. Chemical cross-linking of Mr 55,000 125I-uPA to the surface of HeLa and U937 cells with disuccinimidyl suberate or with formaldehyde results in the formation of a labeled complex of Mr 100,000, suggesting a Mr of 45,000 +/- 5,000 for the receptor or a subunit thereof. When cells solubilized in Triton X-114 are subjected to heat-induced phase separation, unoccupied receptor, receptor-bound 125I-uPA, and cross-linked 125I-uPA-receptor complex all partition in the detergent phase, whereas the unbound ligand remains in the aqueous phase; similar phase partitioning is observed with endogenous uPA-receptor complexes from cultured human and murine cells. Thus, uPA bound at the cell surface is tightly associated with an amphiphilic membrane protein. Interaction of uPA with this plasma membrane receptor is species-specific, since human uPA fails to bind to murine cells, and murine uPA does not bind to human cells. Finally, incubation of HeLa cells in the presence of epidermal growth factor or phorbol 12-myristate 13-acetate results, over a period of 24 h, in a progressive change in uPA binding: an approximately 10-fold increase in the number of sites is accompanied by a 10-fold decrease in their affinity. Cross-linking and phase partitioning of 125I-uPA bound to epidermal growth factor- or phorbol 12-myristate 13-acetate-treated cells indicate that, as in control conditions, it is associated with a Mr 45,000 cell surface amphiphilic polypeptide.     

Use of lanthanum to accurately quantify insulin-like growth factor binding to proteins on cell surfaces

Insulin-like growth factor binding proteins (IGFBPs) are found both associated with cells and in extracellular fluids. Cell-associated IGFBPs increase [¹²⁵I]-IGF binding to cell monolayers, whereas extracellular (soluble, released) IGFBPs decrease binding. In the current study, we show that either IGFBP-3 or IGFBP-5 are the major forms of IGFBP released from monolayers of human GM10 fibroblasts, T98G glioblastoma cells and forskolin-treated bovine MDBK cells. IGFBPs represent the most abundant [¹²⁵I]-IGF-I binding site on GM10 and T98G cell monolayers, but 4-17% of the total cell-associated IGFBPs are released from the cell monolayer at 8°C during their quantification. Most of the IGFBPs (> 70%) are released from MDBK cells. Quantitative estimates of [¹²⁵I]-IGF binding to the cell monolayers are altered because of the ability of the released IGFBPs to reduce the amount of radiolabeled ligand that is available to bind to the cell surface. Lanthanum (La³⁺) depresses IGFBP release from all three cell types (> 80% for GM10 and T98G cells and > 65% for MDBK cells). The effect was cation specific, noted with La³⁺ or Zn²⁺ but not with either Mn²⁺, Sr²⁺ or Se³⁺. The effect was also IGFBP specific; La³⁺ markedly depressed the release of IGFBP-3 and IGFBP-5, but had less of an effect on IGFBP-2 and IGFBP-4. Concomitant with a decrease in IGFBP-3 and IGFBP-5 release, La³⁺ caused an increase in [¹²⁵I]-IGF-I binding to cell-associated IGFBPs and type I IGF receptors. The released soluble IGFBPs have a three- to 20-fold greater affinity (K_a) for [¹²⁵I]-IGF-I compared to cell-associated IGFBPs. La³⁺ did not alter the affinity constants of cell-associated IGFBPs. In summary, we have identified a means to prevent loss of IGFBPs from cell monolayers during binding assays. This procedure will be useful in accurately quantifying the levels of IGFBPs on cell monolayers and in determining the role of cell-associated IGFBPs in controlling IGF activity. Retention of cell-associated low affinity IGFBPs may be important in controlling the size of the pericellular IGF pool and in regulating IGF-I access to the type I IGF receptor. J. Cell. Biochem. 66:256-267. © 1997 Wiley-Liss, Inc.     

Inhibition of cell growth by bafilomycin A1, a selective inhibitor of vacuolar H+-ATPase

Summary Bafilomycin A₁, a potent selective inhibitor of vacuolar H⁺-ATPase, inhibited the growth of a variety of cultured cells dose-dependently, including golden hamster embryo and NIH-3T3 fibroblasts, whether or not they were transformed, and PC12 and HeLa cells. The concentration of bafilomycin A₁ for 50% inhibition of cell growth ranged from 10 to 50 nM. The dose response was nearly parallel with that of the bafilomycin A₁-induced lysosomal pH increase. The degree of pH increase for growth inhibition produced by bafilomycin A₁ was similar to that produced by NH₄Cl in which little difference was recognized in effect among cell types.