Anchorage-independent colony formation of SV40 transformed BALB/c-3T3 cells in serum-free medium: role of cell- and serum-derived factors

Egg yolk lipoprotein, a new supplement for the growth of mammalian cells in serum-free medium : Egg yolk lipoprotein for serum-free growth of cells

Egg yolk lipoprotein promoted growth of a wide variety of mammalian cell lines, including plasma-cytomas and epithelial cell lines, in serum-free medium. The lipoprotein was active for cell growth when used with insulin, transferrin, ethanolamine and selenite. The most active lipoprotein fraction (YLP-pI7.5) was purified to give a single peak by chromatofocusing and gel filtration, and was homogeneous on a 0.35% agarose gel electrophoretogram. The lipoprotein was characterised as a very low density lipoprotein with a protein content of only 1.3%. This lipoprotein had an optimal concentration of 300 μg/ml (4 μg protein/ml). It was easily separable from proteinous molecules secreted into the serum-free medium by the cells, since it floated on the surface of the medium after addition of ammonium sulfate, to precipitate protein, and centrifugation. An associated structure of lipid and protein seemed to be still necessary for the lipoprotein to exhibit a growth promoting activity.

Adhesion to fibronectin stimulates proliferation of wild-type and bcr/abl-transfected murine hematopoietic cells

Cells of most tissues require adhesion to a surface to grow. However, for hematopoietic cells, both stimulation and inhibition of proliferation by adhesion to extracellular matrix components have been described. Furthermore, it has been suggested that progenitor cells from chronic myelogenous leukemia show decreased beta1 integrin-mediated adhesion to fibronectin, resulting in increased proliferation and abnormal trafficking. However, we show here that the chronic myelogenous leukemia-specific fusion protein p210bcr/abl stimulates the expression of alpha5beta1 integrins and induces adhesion to fibronectin when expressed in the myeloid cell line 32D. Moreover, proliferation of both p210bcr/abl-transfected 32D (32Dp210) cells and untransfected 32D cells is stimulated by immobilized fibronectin. Cell cycle analysis revealed that nonadherent 32D and 32Dp210 cells are arrested in late G1 or early S phase, whereas the adherent fractions continue cycling. Although both adherent and nonadherent p210bcr/abl-transfected and parental 32D cells express equal amounts of cyclin A, a protein necessary for cell cycle progression at the G1/S boundary, cyclin A complexes immunoprecipitated from 32D cells cultured on immobilized fibronectin were found to be catalytically inactive in nonadherent but not in adherent cells. In addition, as compared with untransfected 32D cells, cyclin A immunoprecipitates from 32Dp210 cells exhibited a greatly elevated kinase activity and remained partially active irrespective of the adhesion status. The lack of cyclin A/cyclin-dependent kinase (CDK) 2 activity in nonadherent 32D cells appeared to result from increased expression and cyclin A complex formation of the CDK inhibitor p27(Kip1). Taken together, our results indicate that adhesion stimulates cell cycle progression of hematopoietic cells by down-regulation of p27(Kip1), resulting in activation of cyclin A/CDK2 complexes and subsequent transition through the G1/S adhesion checkpoint.

A novel CCAAT-binding protein necessary for adhesion-dependent cyclin A transcription at the G1/S boundary is sequestered by a retinoblastoma-like protein in G0

Loss of adhesion leads to cell cycle arrest at the G1/S boundary in normal, adhesion-dependent, mesenchymal cells. This arrest is accompanied by the inability to produce cyclin A. Using deletional and mutational analysis of the cyclin A promoter, we have identified a CCAAT element that mediates the adhesion-dependent transcriptional activation of cyclin A in late G1 phase of the cell cycle. Specific binding of a novel 40/115-kDa heterodimeric protein complex, which we have named CBP/cycA, to this CCAAT element was detectable in growing but not in G0-arrested or nonadherent normal rat kidney fibroblasts. During G0 CBP/cycA appears to be present but sequestered by a retinoblastoma family member. These results suggest that expression of cyclin A, which controls cell cycle progression by adhesion at the G1/S boundary, is regulated by CBP/cycA and the phosphorylation status of the retinoblastoma protein or a retinoblastoma-related protein.

Fibroblasts isolated from human pterygia exhibit transformed cell characteristics

Pterygium is a degenerative corneal limbal process and UV irradiation has been suggested as being a major environmental predisposing factor. The invasive nature of the fibroblasts associated with pterygia raises the question as to whether these cells are transformed. To test this hypothesis, we established fibroblast strains from autologous and heterologous pterygial and conjunctival specimens, respectively, from subjects between 40 to 50 yr of age, and compared their growth characteristics in culture. All pterygial fibroblast strains exhibited a reduced dependence on serum and exogenous growth factors for growth and reached a saturation population density that was threefold higher than conjunctival fibroblasts cultured under the same conditions. In addition, all pterygial fibroblast strains were able to form colonies in soft agar in 5% fetal bovine serum at a 6.0 to 7.5% efficiency. Under the same experimental conditions, none of the conjunctival fibroblast strains were able to grow. The results presented support the conclusion that pterygial fibroblasts have acquired many of the properties of the transformed phenotype.

Nonresponsiveness of an SV40-transformed Balb/c-3T3 cell line to PDGF mitogenicity is correlated with an irreversible reduction in PDGF receptor number

The effect of SV40 transformation on the mitogenic responsiveness of Balb/c-3T3 cells to platelet-derived growth factor was examined. The SV40 transformed cell line (SV40-3T3) did not require exogenous PDGF for growth in a serum-free medium containing insulin, transferrin and high density lipoprotein as macromolecular supplements. The SV40-3T3 cells were found to have undetectable PDGF-stimulated PDGF receptor autophosphorylation and markedly reduced PDGF binding. PDGF-induced PDGF receptor autophosphorylation and PDGF binding of the normal 3T3 cells were not reduced by incubation with conditioned medium from SV40 transformed cells; nor could PDGF binding to SV40-3T3 be increased by an 8-hour incubation with 0.5 mM suramin, a compound known to prevent the binding of PDGF to its receptor. These data suggest that SV40-3T3 cells do not produce PDGF in quantities that are sufficient to down regulate their PDGF receptor and suggest that the reduced PDGF responsiveness and PDGF receptor number in SV40-3T3 cells may be a reflection of a more permanent, perhaps genetic, alteration.

Growth in serum-free medium of NIH3T3 cells transformed by the EJ-H-ras oncogene: evidence for multiple autocrine growth factors

Rodent fibroblastic cells transformed by ras oncogenes can grow in serum-free (S-) medium. We have studied clonal lines of mouse NIH3T3 fibroblasts transfected with the EJ-H-ras oncogene, and observed that practically all become independent of exogenous growth and attachment factors shortly after transfection. Moreover, all the clones tested soon form anchorage-independent (AI) colonies in S- medium, and most give rise to spheroids able to grow in suspension. The cell-conditioned S- medium of the transformed (TR) cells stimulates autocrinally the AI and anchored growth of these cells, in the absence of serum, and it contains growth factors related to TGF-alpha (or EGF), PDGF and bFGF, and other uncharacterized factors. Some of these factors are not found, or are found only in very small amounts, in the S- medium of non-transformed NIH3T3 cells, which also stimulates the growth of the TR cells, in the absence of serum. In addition, the TR cells contain 4-6 times more cell-associated bFGF than the non-transformed cells and release more latent TGF-beta activatable by acid treatments. However, no active TGF-beta is secreted by either cell type. Activated TGF-beta and pure TGF-beta 1 stimulate the growth of the anchored TR and NIH3T3 cells, but inhibit the AI growth of the TR cells. Another inhibitor of this growth is also found in the concentrated medium of the NIH3T3 cells.

Transforming growth factor beta stimulates mitogenically mouse NIH3T3 fibroblasts and those cells transformed by the EJ-H ras oncogene

TGF-beta 1 stimulates thymidine incorporation and the growth rate of mouse NIH3T3 fibroblasts and of those cells transformed by the EJ-H-ras oncogene (TR15 cells), in the presence and the absence of serum. Thymidine incorporation, in serum-deprived cells, is stimulated to a higher degree by 0.1-1 ng/ml of TGF-beta in NIH3T3 than in TR15 cells, which have a 10-fold higher basal level of incorporation. In both cell types TGF-beta 1 is as active, or more active than other mitogens (TGF-alpha, PDGF-AB, bFGF) at the same concentration. The growth rate of NIH3T3 cells, in low serum or serum-free (S-) medium, is stimulated by only 10 picograms/ml of TGF-beta 1, and that of TR15 cells, in S- medium, by only 1 picogram/ml. In contrast, TGF-beta 1 inhibits mitogenically unestablished mouse embryo fibroblasts and these fibroblasts immortalized spontaneously and able to grow in S- medium. It also inhibits the anchorage-independent growth of TR15 cells. NIH3T3 and TR15 cells respond, similarly, to TGF-beta activated by acification of their culture medium. The kinetics of thymidine incorporation and of activation of the c-myc proto-oncogene, observed already after 1 hr, in treated NIH3T3 and TR15 cells, suggests a direct mitogenic stimulation. The level of activated c-myc RNA is 2-fold higher at 2 hr, and subsequently decreases relatively less in the TR15 cells.

Serum protease inhibitors promote anchorage-independent growth of transformed cells in serum-free medium

The effect of three serum serine protease inhibitors on the serum-free agar growth of an SV40-transformed 3T3 cell line was investigated. Antithrombin III, alpha-2-macroglobulin and alpha-1-antitrypsin were found to potently stimulate colony growth in a semisolid medium because of their anti-proteolytic properties. These results indicate that protease inhibitors can facilitate tumor cell growth in serum-free agar cultures and suggest that the stimulatory effect of serum on the growth of certain transformed cells in agar may at least partially be due to the high levels of protease inhibitors found in serum.

Correlation of the inability to sustain growth in defined serum-free medium with the suppression of tumorigenicity in Wilms' nephroblastoma

We report the investigation of the growth properties of tumorigenic and reverted nontumorigenic Wilms' nephroblastoma cells when cultured in serum-free medium. Wilms' tumor, a pediatric nephroblastoma, has been associated with deletions encompassing the p13 band of chromosome 11 and an independent loss of heterozygosity at 11p15. Weissman et al. (Science 236:175-180, 1987) transferred a human der(11) chromosome into the G401.6TG.6 Wilms' tumor cell line via the microcell-mediated chromosome transfer technique. The resulting microcell hybrids were nontumorigenic when assayed in nude mice; however these cells retained all of the in vitro growth and morphological characteristics of the tumorigenic parental cells in 10% fetal calf serum (FCS). Segregation of the der(11) chromosome from the nontumorigenic microcell hybrid cells resulted in the reappearance of the tumorigenic phenotype in vivo. In vitro culture of these cell lines in serum-free medium supplemented with 0.1% bovine serum albumin (BSA) and 10 ng/ml Na2O3Se resulted in sustained growth of both the tumorigenic parent and the tumorigenic segregant while the nontumorigenic microcell hybrids were unable to divide. The separate addition of either 10 ng/ml of epidermal growth factor (EGF) or 5 micrograms/ml of insulin did not alter this effect. However, the addition of 5 micrograms/ml of transferrin stimulated the nontumorigenic microcell hybrid cells to grow at a rate comparable to the tumorigenic cells. In addition, conditioned serum-free medium from the tumorigenic parental or tumorigenic segregant cell lines was able to stimulate the growth of the nontumorigenic microcell hybrid cells, whereas the reciprocal experiment had no effect on the growth of the tumorigenic cells. These data suggest that the inability of the microcell hybrid cells to grow in serum-free conditions is correlated with their genetic nontumorigenic phenotype and that a specific growth factor, transferrin, can bypass or alter this negative growth regulatory pathway(s) in vitro.

Characterization of a serum-free culture system comparing growth factor requirements of transformed and untransformed cells

We describe the first completely serum-free model culture system for comparing growth control in transformed and untransformed cells. Continuous maintenance of untransformed AKR-2B fibroblasts and chemically transformed AKR-MCA cells in the presence of serum-free medium containing epidermal growth factor (E), insulin (I), and transferrin (T) resulted in cell lines which proliferated with similar doubling times (14 h), comparable to parental lines maintained in 10% serum (16 h). The transformed MCA-SF cells and untransformed AKR-SF cells did not differ in their saturation densities in medium containing E + I + T. However, the monolayer proliferation of MCA-SF cells was significantly greater than that of the AKR-SF cells in the presence of E + T, I + T, or T alone. Both cell lines required T to proliferate in monolayer culture. [3H]-Thymidine incorporation experiments and autoradiographic analysis indicated that quiescent MCA-SF cells could reenter the cell cycle by addition of nutrients alone. The combination of E + I + T produced no additional stimulation of DNA synthesis. In contrast, individual polypeptide growth factors (E, I, IGF-I, PDGF, FGF a or b, or TGF-beta 1) were required to elicit a mitogenic response in the untransformed AKR-SF cells. Peak mitogenesis occurred from 18-20 h for all growth factors except TGF-beta 1 (32 h). Neither AKR-SF nor MCA-SF cells could grow with anchorage independence in serum-free medium, unless both TGF-beta 1 and FGF a or b were simultaneously present. The results indicate that this well-defined, serum-free model system can be utilized to detect growth factor-related alterations associated with the transformed state.

Growth factor interactions between mouse mammary cell lines cocultured in collagen gels

Three related mouse mammary cell lines were cultured in collagen gels and assayed for growth factor responsiveness and interaction via soluble factors. The CL-S1 cell line is nontumorigenic and grows poorly in collagen gel culture. The +SA and -SA cell lines exhibit different degrees of malignant behavior in vivo and have different growth properties in vitro. In collagen gel culture, +SA growth was stimulated by serum but not by epidermal growth factor (EGF), whereas both serum and EGF were required for optimal growth of -SA cells of early passage number as well as CL-S1 cells. -SA cells of later passage repeatedly exhibited a change so as to no longer require serum while retaining EGF responsiveness. [125I]EGF binding analyses indicated that CL-S1 cells bound EGF with less affinity than did -SA cells whereas +SA cells bound almost no ligand. When cell lines were maintained in separate collagen gels but shared the same culture medium, growth of +SA or -SA cells was slightly enhanced in the presence of CL-S1 cells and -SA cell growth was enhanced by the presence of +SA cells. Using the normal rat kidney fibroblast line NRK (clone 49F) as an indicator, serum-containing conditioned media from each cell line and from each pair of cell lines cultured in collagen gels were tested for transforming growth factor (TGF) activity. Both the -SA and CL-S1 lines tested positive for TGF-alpha production and possibly released a TGF-beta activity. These results suggest mechanisms by which cell populations in and around tumors can modify one another's growth characteristics.

Effect of retinoids on growth factor-induced anchorage independent growth of human fibroblasts

We have studied the effects of all-trans retinol, all-trans retinoic acid, and anhydroretinol, a biologically inactive retinoid, on anchorage-independent growth of human fibroblasts induced by purified growth factors. The anchorage-independence assay was conducted in medium supplemented with serum that had had its peptide growth factors inactivated by treatment with dithiothreitol and iodoacetamide. Physiologic concentrations of either all-trans retinol (0.5 microM) or all-trans retinoic acid (1.0 nM) but not anhydroretinol (0.5 microM) reduced the frequency of anchorage-independent growth of normal human fibroblasts induced by platelet-derived growth factor (PDGF). All-trans retinol was also tested for its effect on the frequency of anchorage-independent growth induced by basic fibroblast growth factor (bFGF) and was found to decrease this growth. All-trans retinol also reduced the frequency of anchorage-independent growth of the human fibrosarcoma-derived cell line, HT1080, which grew in semisolid medium without added growth factors. Inasmuch as these retinoids reduced the frequency of anchorage-independent growth induced by either PDGF or bFGF and because PDGF and bFGF bind to independent cell membrane receptors and are known to stimulate different pathways leading to DNA synthesis, the data suggest that physiologically active retinoids have an effect on a step that is common to both signal pathways.

Growth factor requirements of normal and polyomavirus middle T gene transformed REF52 cells in serum-free medium: indications of a reduced vasopressin requirement and its relationship to the control of phosphatidylinositol metabolism

The growth factor requirement of normal and polyomavirus middle T gene transformed REF52 cells was studied in serum-free medium in an attempt to elucidate the possible linkage between an altered growth factor requirement and one or more altered physiological properties of the transformed cells. For optimal growth, REF52 cells required vasopressin, epidermal growth factor (EGF), high-density lipoprotein (HDL), hydrocortisone, insulin, transferrin, and fibronectin. Deletion of vasopressin or hydrocortisone from the medium resulted in a 50 to 60% reduction in cell growth and the deletion of HDL, transferrin, or the combination of EGF and vasopressin led to an 80 to 90% growth retardation. The same medium supported the growth of the transformed variant (PyMLV-REF52) at a rate comparable to that of 10% serum, and deletion of hydrocortisone, vasopressin, or the combination of EGF and vasopressin had virtually no effect on PyMLV-REF52 cell growth. In vasopressin-deleted medium, vasopressin elicited a rapid increase of intracellular inositol phosphate levels in REF52 cells and the control of phosphoinositide turnover was strictly regulated. In contrast, both cell proliferation and intracellular inositol phosphate levels of PyMLV-REF52 cells were not affected by vasopressin treatment under identical culture conditions, and control of phosphoinositide metabolism was lost. Thus, a correlation may exist between the trigger of a mitogenic signal and the stimulation of the phosphoinositol pathway by vasopressin in REF52 cells and this relationship was disrupted in PyMLV-REF52 cells.

Transformation by viral and cellular oncogenes of a mouse BALB/3T3 cell mutant resistant to transformation by chemical carcinogens

The mouse cell line MO-5 is resistant to transformation by various chemical carcinogens and also by UV irradiation (C. Yasutake, Y. Kuratomi, M. Ono, S. Masumi, and M. Kuwano, Cancer Res. 47:4894-4899, 1987). Northern (RNA) blot analysis showed active expression of ras and myc genes in MO-5 and BALB/3T3 cells. The effect of transfection of various oncogenes on transformation was compared in MO-5 cells and parental BALB/3T3 cells. Activated c-H-ras, c-N-ras, and v-mos gene induced transformation foci of MO-5 and BALB/3T3. Introduction of the polyomavirus middle T-antigen (mTag) or the Rous sarcoma virus-related oncogene v-src, however, efficiently transformed BALB/3T3 but not MO-5 cells. Expression and phosphorylation of mTag and the associated c-src proteins were observed in mTag-transfected clones of MO-5 as in BALB/3T3 and phosphorylation of the src protein was observed in v-src-transfected BALB/3T3 and MO-5 clones. Hybrids between mTag- or v-src-induced transformants of BALB/3T3 and untransformed MO-5 maintained the transformation phenotype, suggesting that no dominant suppressor of transformation exists in MO-5. A hybrid clone between BALB/3T3 and MO-5 induced efficient transformation foci after transfection with the mTag gene, suggesting that the deficient transformation phenotype of MO-5 was recessive. Instead, some other alteration of MO-5, plausibly membrane function, might lead to abortive transformation by chemical carcinogens and also by mTag and the v-src gene product.

Platelet-derived growth factor or basic fibroblast growth factor induce anchorage-independent growth of human fibroblasts

Anchorage-independent growth, i.e., growth in semi-solid medium is considered a marker of cellular transformation of fibroblast cells. Diploid human fibroblasts ordinarily do not exhibit such growth but can grow transiently when medium contains high concentrations of fetal bovine serum. This suggests that some growth factor(s) in serum is responsible for anchorage-independent growth. Much work has been done to characterize the peptide growth factor requirements of various rodent fibroblast cells for anchorage-independent growth; however, the requirements of human fibroblasts are not known. To determine the peptide growth factor requirements of human fibroblasts for anchorage-independent growth, we used medium containing serum that had had its peptide growth factors inactivated. We found that either platelet-derived growth factor (PDGF) or the basic form of fibroblast growth factor (bFGF) induced anchorage-independent growth. Epidermal growth factor (EGF) did not enhance the growth induced by PDGF, or did so only slightly. Transforming growth factor beta (TGF-beta) decreased the growth induced by PDGF. EGF combined with TGF-beta induced colony formation in semi-solid medium at concentrations at which neither growth factor by itself was effective, but the combination was much less effective in stimulating anchorage-independent growth than PDGF or bFGF. This work showed that PDGF, or bFGF, or EGF combined with TGF-beta can stimulate anchorage-independent growth of nontransformed human fibroblasts. The results support the idea that cellular transformation may reduce or eliminate the need for exogenous PDGF or bFGF.

Transformation by viral and cellular oncogenes of a mouse BALB/3T3 cell mutant resistant to transformation by chemical carcinogens

The mouse cell line MO-5 is resistant to transformation by various chemical carcinogens and also by UV irradiation (C. Yasutake, Y. Kuratomi, M. Ono, S. Masumi, and M. Kuwano, Cancer Res. 47:4894-4899, 1987). Northern (RNA) blot analysis showed active expression of ras and myc genes in MO-5 and BALB/3T3 cells. The effect of transfection of various oncogenes on transformation was compared in MO-5 cells and parental BALB/3T3 cells. Activated c-H-ras, c-N-ras, and v-mos gene induced transformation foci of MO-5 and BALB/3T3. Introduction of the polyomavirus middle T-antigen (mTag) or the Rous sarcoma virus-related oncogene v-src, however, efficiently transformed BALB/3T3 but not MO-5 cells. Expression and phosphorylation of mTag and the associated c-src proteins were observed in mTag-transfected clones of MO-5 as in BALB/3T3 and phosphorylation of the src protein was observed in v-src-transfected BALB/3T3 and MO-5 clones. Hybrids between mTag- or v-src-induced transformants of BALB/3T3 and untransformed MO-5 maintained the transformation phenotype, suggesting that no dominant suppressor of transformation exists in MO-5. A hybrid clone between BALB/3T3 and MO-5 induced efficient transformation foci after transfection with the mTag gene, suggesting that the deficient transformation phenotype of MO-5 was recessive. Instead, some other alteration of MO-5, plausibly membrane function, might lead to abortive transformation by chemical carcinogens and also by mTag and the v-src gene product.

Enhancement of transformed cell growth in agar by serine protease inhibitors

We investigated the effects of three serine protease inhibitors (leupeptin, soybean trypsin inhibitor, and aprotinin) on the serum-free growth of two transformed cell lines in soft agar. Aprotinin markedly enhanced the growth of rat embryo fibroblasts that had been transformed by polyoma middle T antigen (PyMLV-REF52), while having only a slight effect on the colonial growth of SV40 transformed Balb/c 3T3 cells (SV3T3-Aga). Leupeptin and soybean trypsin inhibitor, on the other hand, significantly enhanced the growth of SV3T3-Aga cells while having little effect on PyMLV-REF52 growth. We observed no stimulatory effect of any of the protease inhibitors on serum-free monolayer growth. Under conditions of excess aprotinin, PyMLV-REF52 cells were found to be unresponsive to epidermal growth factor (EGF) at a concentration that would normally stimulate agar colony growth. However, aprotinin was not capable of supporting colony formation with transforming growth factor-beta. These results indicate that aprotinin acts primarily as a protease inhibitor in spite of its structural homology to EGF and that EGF may promote the soft agar growth of these cell lines either by inhibiting proteolysis directly or by enhancing the synthesis of a serine protease inhibitor.

Altered low density lipoprotein receptor regulation is associated with cholesteryl ester accumulation in Simian virus 40 transformed rodent fibroblast cell lines

Several SV40 transformed REF52 cell lines were found to accumulate 5 to 10 times more cholesteryl esters compared to their parent line REF52 when cultured in 10% serum. Under this culture condition, the cholesteryl ester to phospholipid ratio was 0.4:1 and 2.0:1 to 3.8:1 for normal and SV40 transformed cells, respectively. The mechanism underlying cholesteryl ester accumulation in SV40 transformed lines was investigated. We found that 1) the rate of the de novo cholesterol and cholesteryl ester synthesis was roughly equal in normal and transformed derivatives; 2) the accumulation of cholesteryl esters in the transformants would not occur when cultured in lipoprotein-deficient medium and reappeared upon their return to low density lipoprotein-containing medium; 3) the transformants expressed twice as many low density lipoprotein receptors and were less sensitive to LDL-induced receptor down regulation compared to their nontransformed counterparts. The results indicate that SV40 transformed lines exhibited an accelerated lipid uptake from the culture medium due to an altered regulation of low density lipoprotein receptor activity.

Chemically and virally transformed cells able to grow without anchorage in serum-free medium: evidence for an autocrine growth factor

BA10-IR transformed cells, obtained by treating Syrian hamster embryo fibroblasts (HEF) with 7-methylbenz(a)anthracene and cultivated for a long period, are highly tumorigenic and grow in suspension as aggregates (spheroids) (Levy et al., 1976). They also grow in attached form or as spheroids in serum-free (S-) synthetic medium, without insulin and transferrin, and form anchorage-independent (AI) colonies in this same, but semi-solid, medium. This exceptional phenotype was acquired stepwise, after other transformation parameters, and appears to be related to the capacity of the transformed cells to respond to a mitogenic growth factor which they secrete. The response to this autocrine factor is amplified by insulin and transferrin. Untransformed HEF, at late and early passages, and also mouse and rat embryo fibroblasts, secrete factors equally active on BA10-IR cells; but HEF do not respond, in S- medium, to their factor, or that of BA10-IR cells. Rat FR3T3 fibroblasts transformed by Kirsten murine sarcoma virus (FR3T3-Ki cells) also form AI colonies in semi-solid S- medium, secrete an autocrine factor potentiated by insulin and transferrin, and respond to the factors active on BA10-IR cells. However, they form far fewer colonies without additives, and respond as well to the mitogenic factors only in the presence of insulin and transferrin. BA10-IR cells and FR3T3-Ki cells also release beta-TGF, or a related factor, in an active and a latent form, activable by acidification, and HEF latent, activable beta-TGF. However, the factors shed by BA10-IR cells or HEF which stimulate AI growth of BA10-IR and FR3T3-Ki cells are proteins which seem unrelated to known transforming growth factors. Two major cellular alterations characteristic of the transformed phenotype in vitro are the ability to grow in the absence of anchorage, in semi-solid medium, and reduced dependence on serum growth factors (Hanafusa, 1977; Tooze, 1980). These alterations are often expressed together, and anchorage independence also appears to be the in vitro transformation parameter which correlates best with the tumorigenicity of the transformed cells (Pollack et al., 1975; Shin et al., 1975; Cifone and Fidler, 1980). However, this correlation is not constant (cf., Tooze, 1980). The cellular changes which confer anchorage independence remain unknown, but the culture conditions which allow anchorage-independent (AI) growth are better known. This growth occurs in the same media which permit the growth of attached cells, but generally requires serum.(ABSTRACT TRUNCATED AT 400 WORDS)

Fibronectin-associated transforming growth factor

We have studied the ability of fibronectins to induce anchorage-independent growth of NRK-49F cells in serum-free medium. Cells were seeded in soft agar in the presence of various concentrations of plasma fibronectins, and colonies were counted after 10 days. It was found that, with some exceptions, human plasma fibronectins induced anchorage-independent growth at concentrations in 20-100 micrograms/ml range. The ability of exogenously supplied fibronectins to promote anchorage-independent growth of NRK cells is attributed to a transforming growth factor (TGF) activity associated with gelatin-agarose affinity purified plasma fibronectins. This TGF activity required epidermal growth factor (EGF) in our serum-free assay system. The TGF-like activity appears to either co-purify or to be associated with fibronectin at neutral pH during molecular sieve chromatography and during ultracentrifugation through sucrose density gradients. The TGF activity "dissociates" from fibronectin at extremes of pH, however, and can be separated from fibronectin by molecular sieve chromatography in 1 M acetic acid. Under these conditions, the TGF-like activity chromatographed as a single peak with an apparent molecular weight of approximately 30 kDa. The physical-chemical properties, chromatographic behavior, and biological activity of this TGF suggest that it is type-beta transforming growth factor/growth inhibitor (beta-TGF/GI). The TGF activity has been observed in fibronectin isolated from fresh human plasma as well as in fibronectins from several other species obtained from commercial suppliers. Our results would suggest that caution be applied in the interpretation of experiments in which gelatin affinity purified fibronectins are used at micrograms/ml concentrations.

Growth factor requirements of oncogene-transformed NIH 3T3 and BALB/c 3T3 cells cultured in defined media

We report conditions for the efficient growth of NIH 3T3 and BALB/c 3T3 cells cultured in a defined medium supplemented with either platelet-derived growth factor (PDGF) or pituitary-derived fibroblast growth factor (FGF). The oncogenes v-mos, v-src, v-sis, and c-H-ras Val 12 can induce morphological transformation of these cells and can release them from the mitogen requirement for growth, while the oncogene v-fos cannot abrogate the PDGF-FGF requirement. The radically different behavior of normal and transformed NIH 3T3 cells in PDGF-FGF-free defined medium can form the basis of a sensitive new fibroblast transformation assay.

Mouse Balb/c3T3 cell mutant with low epidermal growth factor receptor activity: induction of stable anchorage-independent growth by transforming growth factor beta

A mutant clone (MO-5) was originally isolated as a clone resistant to Na+/K+ ionophoric antibiotic monensin from mouse Balb/c3T3 cells. MO-5 was found to show low receptor-endocytosis activity for epidermal growth factor (EGF): binding activity for EGF in MO-5 was less than one tenth of that in Balb/c3T3. Anchorage-independent growth of MO-5 was compared to that of Balb/c3T3 when assayed by colony formation capacity in soft agar. Coadministration of EGF and TGF-beta efficiently enhanced anchorage-independent growth of normal rat kidney (NRK) cells, but neither factor alone was competent to promote the anchorage-independent growth. The frequency of colonies appearing in soft agar of MO-5 or Balb/c3T3 was significantly enhanced by TGF-beta while EGF did not further enhance that of MO-5 or Balb/c3T3. Colonies of Balb/c3T3 formed in soft agar in the presence of TGF-beta showed low colony formation capacity in soft agar in the absence of TGF-beta. Colonies of MO-5 formed by TGF-beta in soft agar, however, showed high colony formation capacity in soft agar in the absence of TGF-beta. Pretreatment of MO-5 with TGF-beta induced secretion of TGF-beta-like activity from the cells, while the treatment of Balb/c3T3 did not induce the secretion of a significant amount of TGF-beta-like activity. The loss of EGF-receptor activity in the stable expression and maintenance of the "transformed" phenotype in MO-5 is discussed.

Growth factor requirements of oncogene-transformed NIH 3T3 and BALB/c 3T3 cells cultured in defined media

We report conditions for the efficient growth of NIH 3T3 and BALB/c 3T3 cells cultured in a defined medium supplemented with either platelet-derived growth factor (PDGF) or pituitary-derived fibroblast growth factor (FGF). The oncogenes v-mos, v-src, v-sis, and c-H-ras Val 12 can induce morphological transformation of these cells and can release them from the mitogen requirement for growth, while the oncogene v-fos cannot abrogate the PDGF-FGF requirement. The radically different behavior of normal and transformed NIH 3T3 cells in PDGF-FGF-free defined medium can form the basis of a sensitive new fibroblast transformation assay.

Purified HDL-apolipoproteins, A-I and C-III, substitute for HDL in promoting the growth of SV40-transformed REF52 cells in serum-free medium

The lipid-free apolipoproteins of human high density lipoprotein (HDL) have been assayed for their ability to substitute for native HDL in promoting the growth of a SV40-transformed REF52 cell line in serum-free medium. Total HDL-apolipoproteins (apoHDL) were found to mimic almost exactly the growth promoting effects of whole HDL. The apoHDL-associated growth promoting activity eluted from a Sephacryl S-200 column in two separate fractions coinciding with the protein peaks of apolipoprotein A-I and the C group of apolipoproteins. These two fractions, designated S-II and S-IV, respectively, acted additively in promoting WT1A cell growth when tested at saturating concentrations. The active component in the S-II fraction maximally stimulated WT1A cell growth at 40-60 micrograms/ml and was identified as apolipoprotein A-1 by NaDodSO4 polyacrylamide gel electrophoresis and affinity chromatography on anti-(apoA-I). The active component in the S-IV fraction was maximally active at 1-2 micrograms/ml and was identified as apolipoprotein C-III by DEAE ion exchange high pressure liquid chromatography and polyacrylamide gel electrophoresis (at pH 8.3) in 6 M urea. These results indicate that the growth promoting effect of HDL on WT1A cells is mediated via the HDL-apolipoproteins, A-I and C-III, and that the mechanism responsible does not necessarily involve their participation in the uptake (or utilization) of HDL-associated lipids.

Biochemical characterization of the cholesterol-dependent growth of the NS-1 mouse myeloma cell line

The biochemical basis for the cholesterol-dependent growth phenotype of the NS-1 myeloma cell line has been investigated. In one series of experiments, the growth response of NS-1 cells to several of the intermediates of cholesterol biosynthesis was studied in serum-free medium. The cholesterol precursors, squalene and lanosterol, were totally ineffective in promoting NS-1 cell growth. In contrast, cholesterol precursors downstream from lanosterol, i.e., desmosterol and 7-dehydrocholesterol, completely replaced cholesterol in supporting NS-1 cell growth. In a second series of experiments, NS-1 cells and NS-1-503 cells (a cholesterol growth-independent variant of NS-1 cells) were labelled with [2-14C]acetate and the distributions of radioactivity between cholesterol and its precursors were determined by thin-layer chromatography using two different solvent systems. The major labelled sterol product (greater than 80%) in NS-1 cells after a 24-h exposure to [2-14C]acetate was lanosterol. In contrast, the major labelled sterol product (greater than 95%) in NS-1-503 cells after a 24-h exposure to [2-14C]acetate was cholesterol. Taken together, these results indicate that NS-1 cells are defective in cholesterol biosynthesis and identify the site of lesion as the demethylation of lanosterol to C-29 sterol intermediates.

Anchorage-independent growth of primary rat embryo cells is induced by platelet-derived growth factor and inhibited by type-beta transforming growth factor

Several growth factors implicated in the process of cellular transformation were tested for their ability to induce anchorage-independent (AI) growth of primary rat embryo (RE) cells. Our results show that in the presence of 10% calf serum, platelet-derived growth factor (PDGF), 1-30 ng/ml, has the strongest effect of all growth factors tested on AI growth. Type-beta transforming growth factor (TGF-beta), by itself, does not stimulate AI growth, and it inhibits the PDGF-induced colony formation in a dose-dependent manner (ED50 approximately 0.03 ng/ml). Qualitatively similar responses are obtained by using an established line of fibroblasts, NIH 3T3 cells; the principal difference between the response of the primary cells and the established cell line is in colony-forming efficiency in soft agar culture (15% and 90%, respectively, for growth of colonies greater than 1,500 micron2 diameter in the presence of 10 ng/ml PDGF). Since AI growth has been shown to correlate well with tumorigenicity in vivo, our results suggest that the transforming potential of PDGF in an appropriate responsive cell can be controlled not only through its interaction with its own receptor, but also by the presence of inhibitory factors such as TGF-beta.

Selection of transformed cells in serum-free media

NIH3T3 cells grow in a serum-free basal nutrient medium supplemented with fibronectin, transferrin, insulin, epidermal growth factor (EGF) and high density lipoprotein (HDL). The individual omission from the serum-free medium of insulin, EGF, or HDL results in greatly reduced cell growth. These growth-restrictive conditions can be used to select for cells transformed with SV40, the polyomavirus middle T antigen gene, the activated human ras gene, and the mouse c-myc gene.

Autocrine growth factors for human tumor clonogenic cells

A human epithelial-derived cell line, SW-13, releases a soluble substance that functions as an autocrine growth factor. SW-13 cells, derived from a human adenocarcinoma of the adrenal cortex, form a few small colonies when suspended in soft agar at low densities. The number of colonies increased significantly when either viable SW-13 cells or serum-free medium conditioned by SW-13 cells (CM) was added to agar underlayers. CM increased colony formation in a dose-dependent fashion. Clonal growth at low cell densities was dependent on the presence of both horse serum and SW-13 CM. Neither activity alone was capable of sustaining growth. Even when cells were plated at high densities CM could not substitute for serum, but could reduce the threshold serum concentration. The results suggest that autocrine and serum-derived factors act in concert to maintain clonal growth of epithelial tumor cells in soft agar.

Analysis of a transgenic mouse containing simian virus 40 and v-myc sequences

Two plasmids, one containing the simian virus 40 (SV40) genome and the mouse metallothionein I gene and one containing the v-myc gene of avian myelocytomatosis virus MC29, were coinjected into mouse embryos. Of the 13 surviving mice, one, designated M13, contained both myc and SV40 sequences. This mouse developed a cranial bulge identified as a choroid plexus papilloma at 13 weeks and was subsequently sacrificed; tissue samples were taken for further analysis. Primary cell lines derived from these tissues contained both myc and SV40 DNA. No v-myc mRNA could be detected, although SV40 mRNA was present in all of the cell lines tested. T antigen also was expressed in all of the cell lines analyzed. These data suggest that SV40 expression was involved in the abnormalities of mouse M13 and was responsible for the transformed phenotype of the primary cell lines. Primary cell lines from this mouse were atypical in that the population rapidly became progressively more transformed with time in culture based on the following criteria: morphology, growth rate, and the ability to grow in soft agar and in serum-free medium. The data also suggest that factors present in the mouse regulated the ability of SV40 to oncogenically transform most cells and that in vitro culture of cells allowed them to escape those factors.

Phenotypic transformation of normal rat kidney cells in a growth-factor-defined medium: induction by a neuroblastoma-derived transforming growth factor independently of the EGF receptor

Polypeptide growth factor activity in serum can be destroyed by treatment with dithiothreitol. When such growth-factor-inactivated serum is used as a supplement of culture media instead of regular serum, normal rat kidney (NRK) cells become quiescent unless defined polypeptide growth factors like insulin and epidermal growth factor (EGF) are added. On this basis a growth-factor-defined medium has been developed for NRK cells, which permits cell proliferation as rapidly as in media supplemented with serum, even at low cell densities. Moreover, cells can be serially passaged in this medium. NRK cells can be induced to grow in semisolid media when incubated with transforming growth factors. The growth-factor-defined medium permits soft agar growth experiments of NRK cells, without interference from polypeptide growth factors in serum. Using this assay system we have shown that EGF alone is unable to induce any degree of anchorage-independent growth in NRK cells. However, a recently identified transforming growth factor from mouse neuroblastoma cells which does not compete with EGF for receptor binding is able to induce progressively growing colonies of NRK cells in soft agar, even without additional EGF.

Analysis of a transgenic mouse containing simian virus 40 and v-myc sequences

Two plasmids, one containing the simian virus 40 (SV40) genome and the mouse metallothionein I gene and one containing the v-myc gene of avian myelocytomatosis virus MC29, were coinjected into mouse embryos. Of the 13 surviving mice, one, designated M13, contained both myc and SV40 sequences. This mouse developed a cranial bulge identified as a choroid plexus papilloma at 13 weeks and was subsequently sacrificed; tissue samples were taken for further analysis. Primary cell lines derived from these tissues contained both myc and SV40 DNA. No v-myc mRNA could be detected, although SV40 mRNA was present in all of the cell lines tested. T antigen also was expressed in all of the cell lines analyzed. These data suggest that SV40 expression was involved in the abnormalities of mouse M13 and was responsible for the transformed phenotype of the primary cell lines. Primary cell lines from this mouse were atypical in that the population rapidly became progressively more transformed with time in culture based on the following criteria: morphology, growth rate, and the ability to grow in soft agar and in serum-free medium. The data also suggest that factors present in the mouse regulated the ability of SV40 to oncogenically transform most cells and that in vitro culture of cells allowed them to escape those factors.

Secretion of transforming growth factors by primary human tumour cells

We examined the ability of primary human tumour cells to secrete diffusible factors capable of stimulating anchorage independent growth of normal rat kidney fibroblast (NRK) cells. Conditioned media (CM) prepared from cells derived from 31/43 patients with adenocarcinoma of the breast, colon, ovary or lung were found to induce growth of NRK cells in soft agar. The ability of the CM to induce anchorage independent growth was enhanced in 25/35 cases by the presence of epidermal growth factor (EGF). The CM did not compete with EGF for binding to the EGF receptor site. CM from cells derived from nonmalignant effusions also supported the growth of NRK cells in soft agar. There was no significant difference in the ability of the CM derived from malignant or normal cells to support NRK colony growth. The ability of primary human tumour cells to clone in soft agar was compared to the ability of these cells to produce diffusible colony stimulating factors for NRK cells. No correlation was observed between the ability of the primary human tumour cells to clone in soft agar and their ability to induce anchorage independent growth of NRK cells. The secretion of substances with TGF like activity may be a property of many types of primary human cells.

Analysis of the reduced growth factor dependency of simian virus 40-transformed 3T3 cells

We have measured in a defined serum-free medium the platelet-derived growth factor (PDGF) and insulin requirements of normal Swiss 3T3 cells, simian virus 40-transformed 3T3 cells, and partial revertants of simian virus 40-transformed 3T3 cells. Swiss 3T3 cells displayed strong requirements for both PDGF and insulin. Both of these requirements were significantly diminished in simian virus 40-transformed 3T3 cells. Analysis of the PDGF and insulin requirements of the revertants indicated that the loss of either of these two growth factor requirements was not necessarily linked to the other; rather, the growth factor requirements were specifically associated with other parameters of transformation. The reacquisition of a PDGF requirement cosegregated with reversion to density-dependent growth inhibition, whereas reacquisition of a normal insulin requirement cosegregated with reversion to a normal growth dependence on calf serum. Anchorage dependence was dissociable from both growth factor requirements. The relationship between the PDGF requirement and density-dependent growth inhibition was further analyzed in normal 3T3 cells by measuring the PDGF requirement at different cell densities. At high cell densities, the requirement for PDGF became significantly greater. We suggest that at least in part the ability of transformed cells to grow to high saturation densities results from their loss of a requirement for PDGF.

Growth control variant cell line having increased serum requirement and decreased response to platelet-derived growth factor: reversion by 5-azacytidine

Variants of the mouse embryo fibroblast X melanoma hybrid clone 100A have been isolated by a procedure that selects against cells that are able to grow in medium containing low concentrations of serum plus insulin. Three variant clones derived from this selection were found to have a much higher serum requirement than the parental clone 100A cells, as evidenced by a very low rate of DNA synthesis and growth in medium containing low concentrations of serum. Two of the variants had approximately double the number of chromosomes as the parental cell line, while one had approximately the same number of chromosomes as the parental cells. One of the variants was very strongly reverted by 5-azacytidine but not by ethyl methanesulfonate, suggesting that it reverted by a nonmutational mechanism such as a stable change in DNA methylation. Analysis of the growth requirements in hormone-supplemented serum-free media of the 100A parent, the INS 471 variant, and revertants of the variant indicated that the variant had a specific deficiency in its growth response to platelet-derived growth factor (PDGF). PDGF dose-response curves obtained with the variant cells were shifted approximately an order of magnitude toward higher PDGF concentrations relative to PDGF dose-response curves obtained with the parental 100A cells. This quantitative increase in PDGF requirement of the INS 471 variant appears to explain the increased serum requirement of this variant. Equilibrium binding experiments performed with 125I-PDGF suggest that the variant does not have a decreased number of PDGF receptors.

Flat revertants derived from Kirsten murine sarcoma virus-transformed cells produce transforming growth factors

Two flat cellular revertant cell lines, F-2 and C-11, which were originally selected from the DT line of Kirsten murine sarcoma virus (Ki-MuSV)-transformed NIH/3T3 cells, were examined for the production of transforming growth factors (TGFs). The revertant cells fail to grow in semisolid medium as colonies and exhibit a markedly reduced level of tumorigenicity in nude mice, although they are known to express high levels of p21ras, the product of the Kirsten sarcoma virus oncogene, ras, and they contain a rescuable transforming virus. TGF activity associated with the transformed, revertant, and non-transformed cell lines was measured by the ability of concentrated conditioned medium (CM) from these cells to induce normal rat kidney (NRK) and NIH/3T3 cells to form colonies in semisolid agar suspension cultures and to inhibit the binding of 125I epidermal growth factor (EGF) to specific cell surface receptors. CM from the transformed DT cells and from both the F-2 and C-11 revertants contains TGF activity, in contrast to CM obtained from normal NIH/3T3 cells. Furthermore, unlike NIH/3T3 cells, neither the DT nor the revertant cells were able to bind 125I EGF. All four cell lines were able to proliferate in serum-free medium supplemented with transferrin, insulin, EGF, and Pedersen fetuin. However, in basal medium lacking these growth factors, only DT cells and, to a lesser extent, the revertant cells were able to grow. These results suggest that the F-2 and C-11 revertants fail to exhibit all of the properties associated with transformation because the series of events leading to the transformed phenotype is blocked at a point(s) distal both to the expression of the p21 ras gene product and also to the production of TGFs and that the production of TGFs may be necessary but not sufficient for maintaining the transformed state.

Behavior of transforming growth factors in serum-free media: an improved assay for transforming growth factors

Transforming growth factors (TGFs) are a relatively new category of factors that induce the anchorage-independent growth of non-transformed cells. These factors are usually detected by their ability to induce normal rat kidney (NRK) fibroblasts to grow in soft agar. Until now, this assay has been performed in serum-containing medium (SCM). Unfortunately, the background activity of this assay is variable and dependent on several factors, including passage number of the cells and the serum lot used. Furthermore, the addition of either EGF or TGF-beta alone results in the appearance of additional colonies, which decreases the sensitivity of the assay. To circumvent these problems, serum-free media have been developed that support the growth of the NRK cells at low density in both monolayer culture and soft agar. Long-term growth in monolayer cultures occurs in serum-free medium supplemented with laminin, insulin, transferrin, epidermal growth factor (EGF), fibroblast growth factor (FGF) and high density lipoprotein (HDL). Growth in soft agar occurs when TGFs are added to a serum-free medium, AIG medium, that contains insulin, transferrin, FGF and HDL. In contrast to the background activity observed when the assay is performed in SCM, no colonies form in the AIG medium unless TGFs are added and few, if any, colonies form if EGF or TGF-beta are added alone. Thus, the AIG medium provides an improved assay for TGFs. In addition, the AIG medium should prove useful for examining other factors, including serum factors, for TGF activity.

Morphological transformation, autonomous proliferation and colony formation by chicken heart mesenchymal cells infected with avian sarcoma, erythroblastosis and myelocytomatosis viruses

Normal chicken heart mesenchymal cells at low density in monolayer culture in plasma-containing medium have a polygonal shape and are proliferatively quiescent. The combination of epidermal growth factor and insulin at hyperphysiological concentration, an insulin-like growth factor surrogate, causes these cells to assume a fusiform shape and to increase 40-fold in number during four days of incubation. These mitogenic hormones do not, however, induce normal chicken heart mesenchymal cells to form colonies in agarose suspension culture. Chicken heart mesenchymal cells infected with the Schmidt-Ruppin or Prague-A strains of Rous sarcoma virus or with the Fujinami or Y73 avian sarcoma viruses assume spindle and round shapes, increase 50-100 fold in number during four days of monolayer culture in the absence of mitogenic hormones and form macroscopic colonies during 3-4 days of agarose suspension culture. The autonomous (mitogenic hormone-independent) proliferation, in monolayer culture, of cells infected with temperature-sensitive transformation mutants of Rous sarcoma virus (tsNY68, tsNY72, tsLA24, tsLA29) is temperature-sensitive. Chicken heart mesenchymal cells infected with avian erythroblastosis virus assume spindle shapes and proliferate in monolayer culture at a rate comparable to that of sarcoma virus-infected cells but do not, however, form colonies in agarose suspension culture. Cells infected with the myelocytomatosis virus MC29 assume stellate shapes and increase 18-fold in number during four days of monolayer culture. Cells infected with the myelocytomatosis virus MH2 assume fusiform shapes and increase fourfold in number during four days of monolayer culture. Neither MC29 nor MH2 renders chicken heart mesenchymal cells capable of colony formation in agarose suspension culture. Infection with avian leukosis viruses (RAV-1, RAV-2, RPL-42) or with transformation-defective mutants of Rous sarcoma virus (tdNY105, 107, 109) does not affect the morphology or proliferative behavior of chicken heart mesenchymal cells. Monolayer culture of chicken heart mesenchymal cells in plasma-containing medium appears, therefore, to define the ability of onc genes of acute transforming avian retroviruses to induce autonomous (mitogenic hormone-independent) cell proliferation, the essential characteristic of neoplasia. The differences in transformed morphology and rates of autonomous proliferation between cells infected with different acute transforming retroviruses probably reflects differences in the modes of action of the transforming proteins encoded by the onc genes of the respective viruses.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the reduced growth factor dependency of simian virus 40-transformed 3T3 cells

We have measured in a defined serum-free medium the platelet-derived growth factor (PDGF) and insulin requirements of normal Swiss 3T3 cells, simian virus 40-transformed 3T3 cells, and partial revertants of simian virus 40-transformed 3T3 cells. Swiss 3T3 cells displayed strong requirements for both PDGF and insulin. Both of these requirements were significantly diminished in simian virus 40-transformed 3T3 cells. Analysis of the PDGF and insulin requirements of the revertants indicated that the loss of either of these two growth factor requirements was not necessarily linked to the other; rather, the growth factor requirements were specifically associated with other parameters of transformation. The reacquisition of a PDGF requirement cosegregated with reversion to density-dependent growth inhibition, whereas reacquisition of a normal insulin requirement cosegregated with reversion to a normal growth dependence on calf serum. Anchorage dependence was dissociable from both growth factor requirements. The relationship between the PDGF requirement and density-dependent growth inhibition was further analyzed in normal 3T3 cells by measuring the PDGF requirement at different cell densities. At high cell densities, the requirement for PDGF became significantly greater. We suggest that at least in part the ability of transformed cells to grow to high saturation densities results from their loss of a requirement for PDGF.

Spreading of non-transformed and transformed cells

Mechanisms of cellular reactions responsible for the spreading of non-transformed cultured tissue cells on the surface of various substrata and relationships of these reactions to the control of cell proliferation are reviewed; the special role of the membrane-cytoskeleton interactions leading to extension and attachment of pseudopods is stressed. Transition of cells from non-transformed to transformed phenotype is characterized by decreased spreading and by decreased dependence of proliferation on spreading. Manifestations of both of these spreading-associated changes are reviewed and their possible mechanisms are discussed. It is suggested that cell transition to transformed phenotype involves shift of an equilibrium between the reactions induced by the two groups of membrane-bound ligands: those attached and those not attached to the substratum.