Correlation of cellular aggregation of transformed cells with their growth in soft agar and tumorigenic potential

Rigidity sensing at the leading edge through alphavbeta3 integrins and RPTPalpha

Cells require optimal substrate stiffness for normal function and differentiation. The mechanisms for sensing matrix rigidity and durotaxis, however, are not clear. Here we showed that control, Shp2-/-, integrin beta1-/-, and talin1-/- cell lines all spread to a threefold greater area on fibronectin (FN)-coated rigid polyacrylamide surfaces than soft. In contrast, RPTPalpha-/- cells spread to the same area irrespective of rigidity on FN surfaces but spread 3x greater on rigid collagen IV-coated surfaces than soft. RPTPalpha and alphavbeta3 integrins were shown previously to be colocalized at leading edges and antibodies to alphavbeta3 blocked FN rigidity sensing. When FN beads were held with a rigid laser trap at the leading edge, stronger bonds to the cytoskeleton formed than when held with a soft trap; whereas back from the leading edge and in RPTPalpha-/- cells, weaker bonds were formed with both rigid and soft laser traps. From the rigidity of the trap, we calculate that a force of 10 pN generated in 1 s is sufficient to activate the rigidity response. We suggest that RPTPalpha and alphavbeta3 at the leading edge are critical elements for sensing FN matrix rigidity possibly through SFK activation at the edge and downstream signaling.

Collateral expression of proangiogenic and tumorigenic properties in intestinal epithelial cell variants selected for resistance to anoikis

Although in vitro anchorage-independent growth is widely used as a marker of cell transformation, the biological implications of this trait are poorly understood. We previously demonstrated that enforced anchorage-independent growth of a nontumorigenic, immortalized epithelial cell line (IEC-18) in multicellular spheroid culture results in massive apoptotic cell death. This death process, termed anoikis, is prevented by expression of transforming oncogenes, which also confer tumorigenic competence. This study examines whether acquisition of an anoikis-resistant phenotype is causally related to the tumorigenic capacity of transformed epithelial cells. Parental IEC-18 cells were subjected to 10 cycles of selection for survival in speroid culture. Unlike parental cells, the resulting anoikis-resistant variants (AR1.10 and AR2.10) formed relatively large tumors in nude mice. Both anoikis-resistant sublines displayed upregulated expression of vascular endothelial growth factor (VEGF), a potent angiogenesis stimulator. VEGF121 overexpression alone did not induce tumorigenic conversion of parental IEC-18 cells, which remained highly susceptible to anoikis. We postulate that both anoikis-resistance and angiogenic-competence contribute to tumor formation. Development of anoikis-resistance can be then viewed as a precondition for expression of the tumorigenic phenotype. Our results suggest that even when angiogenesis is not a rate limiting factor (e.g. in vitro) the selective pressures of solid tumor-like, 3-dimensional growth conditions favoring anoikis resistance result in collateral induction of a proangiogenic phenotype.

Development of intracytoplasmic lumens in colon cancer cells cultured on non-adhesive surface

Cell-matrix interactions have important effects on phenotypic features, such as morphology, differentiation and cell growth. Several papers have suggested that when cell-matrix interaction is interrupted, cells grow as multicellular spheroids and eventually undergo apoptosis. We found that when ET(-), a laminin non-adherent colon cancer cell line, was cultured on poly-2-hydroxyethyl methacrylate (HEMA) coated plastic, the cells floated as cellular aggregates of spheroids or as single cells. Some of the single cells contained large intracytoplasmic lumens (ICL) and appeared similar to signet ring cells. These ICL were lined by a layer of short microvilli. The number of the cell did not increase when cultured on poly-HEMA. Another type of single cells, usually without ICL, demonstrated the characteristics of apoptotic cells by histologic examination. Acridine orange staining, flow cytometry and electron microscopy confirmed the apoptotic nature of those cells. On immunohistochemical staining for proliferating cell nuclear antigen, spheroids of cells and single cells with ICL were immunoreactive, while most of the single cells without ICL were negative. These results suggest that multicellular aggregates and formation of ICL were induced by the adaptation of ET(-) colon cancer cells in a harmful environment caused by reduced adhesiveness, and these changes might be related to cell survival.

Vitamin E and coenzyme Q concentrations in the thyroid tissues of patients with various thyroid disorders

To clarify the different roles of free radical scavenging systems in various thyroid disorders, we measured the levels of alpha-, beta-, and gamma-tocopherols and coenzyme Q in the thyroid tissues of patients with thyroid tumors and Graves' disease using high-performance liquid chromatography. The levels of alpha-tocopherols and gamma-tocopherols in the thyroid tissue of patients with papillary carcinoma and the level of gamma-tocopherol in the thyroid tissue of patients with malignant lymphoma were elevated compared with those in normal thyroid tissues. The level of coenzyme Q was reduced in the thyroid tissue of patients with Graves' disease and follicular and papillary thyroid carcinomas. These findings imply that vitamin E and coenzyme Q as scavengers play some role in thyroid follicular cell hyperfunction or dysfunction.

Massive programmed cell death in intestinal epithelial cells induced by three-dimensional growth conditions: suppression by mutant c-H-ras oncogene expression

Deregulation of molecular pathways controlling cell survival and death, including programmed cell death, are thought to be important factors in tumor formation, disease progression, and response to therapy. Studies devoted to analyzing the role of programmed cell death in cancer have been carried out primarily using conventional monolayer cell culture systems. However the majority of cancers grow as three-dimensional solid tumors. Because gene expression, and possibly function, can be significantly altered under such conditions, we decided to analyze the control and characteristics of cell death using a compatible three-dimensional tissue culture system (multicellular spheroids) and compare the results obtained to those using two-dimensional monolayer cell culture. To do so we selected for study an immortalized, but nontumorigenic line of rat intestinal epithelial cells, called IEC-18, and several tumorigenic variants of IEC-18 obtained by transfection with a mutant (activated) c-H-ras oncogene. The rationale for choosing these cell lines was based in part on the fact that intestinal epithelial cells grow in vivo in a monolayer-like manner and form solid tumors only after sustaining certain genetic mutations, including those involving the ras gene family. We found that the IEC-18 cells, which grow readily and survive in monolayer cell culture, undergo massive cell death within 48-72 h when cultured as multicellular spheroids on a nonadhesive surface. This process was accompanied by a number of features associated with programmed cell death including chromatin condensation (Hoechst 33258 staining) apoptotic morphology, DNA degradation, and a virtual complete loss of colony forming (clonogenic) ability in the absence of apparent membrane damage as well as accumulation of lipid containing vacuoles in the cytoplasm. Moreover, enforced over-expression of a transfected bcl-2 gene could prevent this cell death process from taking place. In marked contrast, three different stably transfected ras clones of IEC-18 survived when grown as multicellular spheroids. In addition, an IEC cell line (called clone 25) carrying its mutant transfected ras under a glucocorticoid inducible promoter survived in three-dimensional culture only when the cells were exposed to dexamethasone. If exposure to dexamethasone was delayed for as long as 48 h the cells nevertheless survived, whereas the cells became irreversibly committed to programmed cell death (PCD) if exposed to dexamethasone after 72 h. These results suggest that intestinal epithelial cells may be programmed to activate a PCD pathway upon detachment from a physiologic two-dimensional monolayer configuration, and that this process of adhesion regulated programmed cell death (ARPCD) can be substantially suppressed by expression of a mutant ras oncogene.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

A method for accelerating progression to a malignant phenotype in rodent cells

This report describes a system of suspension culturing that enhances the expression of transformed cells in carcinogen-treated rodent cells and decreases the time required to observe clear evidence of the neoplastic or malignant phenotype by 2-8 wk or more. Retrovirus-infected Fischer rat embryo cells, uninfected Balb/c 3T3 mouse cells and Syrian hamster embryo cells in monolayer culture were treated with the chemical carcinogens, dimethylbenzanthracene, benzo[a]pyrene or N-methyl-N'-nitro-N-nitrosoguanidine, following a protocol appropriate to each cell type. The cultures were divided into two groups, one seeded directly onto a plastic surface, and the other suspended in liquid medium over agar before seeding onto a plastic surface. In all three cell types incluson of the suspension phase accelerated chemically induced transformation as indicated by clonogenicity in soft agarose (rat, mouse and hamster cells) and by morphological transformation and formation of tumours in athymic nude mice (hamster cells).

A selection procedure for the rapid expression of carcinogen-induced malignant transformation of retrovirus-infected Fischer rat embryo cells

This study evaluates a procedure that accelerates the expression of the transformation of retrovirus-infected Fischer rat embryo cells. The endpoints used were anchorage-independent growth (formation of colonies in soft agarose) and formation of foci on a contact-inhibited monolayer. The cells were treated in monolayer with chemical, solvent (negative control) or medium alone for 3 days; then the chemical was removed and the cultures re-fed with medium alone for an additional 3 days. Cells in monolayer were disaggregated, suspended in liquid medium over solid agar for 4 days, disaggregated again and seeded into monolayer. After 2-4 wk without additional subculturing, cells from monolayer were seeded in soft agarose. Suspension of retrovirus-infected rat embryo cells above agar after chemical treatment resulted in rapid expression of neoplastic phenotypes. Cells treated in monolayer with the carcinogens, benzidine dichloride, dimethyl benzanthracene, 4,4-oxydianiline, 4-nitroquinoline-N-oxide and N-methyl-N'-nitro-N-nitrosoguanidine demonstrated colony formation in soft agarose or morphological transformation within 2-4 wk after being held in suspension for 4 days. In addition two carcinogens, benzo[a]pyrene and N-2-acetylaminofluorene and two noncarcinogens, benzo[e]pyrene and N-4-acetylaminofluorene did not induce neoplastic changes in this time period. The suspension technique may be a useful modification of this assay because it selectively amplifies neoplastic transformation after treatment with a number of chemicals.

Potential carcinogenicity of aza-aromatic hydrocarbons: azabenz(a)anthracenes

The potential carcinogenicity of 3 azabenz(a)anthracenes was determined in vitro. The 3 compounds tested were 1-, 2-, and 9-azabenz(a)anthracene. The initial assay was chemical carcinogen-induced enhancement of anchorage-independent survival of Rauscher leukemia virus-infected Fischer rat embryo cells, 2FR(4)50 (2FR4). Cells treated with 2- and 9-azabenz(a)anthracene showed dose-dependent increased survival. After continued subculturing, the surviving cells from 2- and 9-azabenz(a)anthracene-treated cultures displayed morphological transformation and ability to grow in semi-solid medium. Mock-treated controls and I-azabenz(a)anthracene-treated cultures did not show either of these properties. These data suggest that certain azabenz(a)anthracenes are potential carcinogens.

Chemical enhancement of survival in aggregation of retrovirus-infected rat cells: an interlaboratory comparison

This study provides a comparative evaluation among three independent laboratories of the responses to 16 chemicals in the retrovirus (Rauscher leukemia) infected Fischer rat embryo (RIFRE) cell-survival-in-aggregation assay. When suspended in liquid media above an agar base, control cells showed a rapid decline in cell survival, whereas cells that had previously been treated with chemical carcinogen survive in suspension longer than control cells. The endpoint, survival in aggregation, is measured by counting viable cells dissociated from aggregates in suspension for 4 days. By modifying previously reported procedures, we have improved the system so that a clear differential (positive or negative) response is achieved by cells treated with either a known carcinogen or known noncarcinogen. Using procedures designed to minimize assay variability, replicate assays were performed and the data analyzed for inter- and intralaboratory concordance. The RIFRE cell-survival-in-aggregation assay demonstrated a high degree of interlaboratory reproducibility in assessing the overall positive or negative responses of known carcinogens and noncarcinogens, and good qualitative reproducibility in assessing compounds tested under code. The assay could discriminate between known carcinogens and noncarcinogens. All chemicals were tested without the addition of a metabolic activation system. Cells exhibiting carcinogen-induced enhancement of survival in aggregation, when plated back onto a solid substrate and carried in culture, subsequently expressed transformation-associated changes in their cellular morphology, growth in semisolid media, and tumorigenicity in nude mice. These results indicate that retrovirus-infected Fischer rat embryo cells detect a carcinogen-mediated early event that progresses to neoplastic phenotypes. Survival in aggregation appears to require the presence of the exogenous retrovirus, since uninfected cells did not show a differential survival response when carcinogen-treated, noncarcinogen-treated, or control cells were compared. This system provides a reproducible method of detecting carcinogenic chemicals based on their ability to induce enhanced survival in aggregation of treated cells.

Increased carcinogen metabolism and survival of retrovirus-infected Fischer rat embryo cells following repetitive carcinogen treatment

The carcinogens N-2-acetylaminofluorene (AAF) and diethylnitrosamine (DEN) often give negative results when tested in the Fischer rat embryo cell survival assay with the standard single 72-hour regimen, whereas another carcinogen, benzo(a)pyrene [B(a)P], may yield varied results between different laboratories and may require relatively high concentrations (compared with other polycyclic aromatic hydrocarbons) for a positive result to occur. Enhanced survivals (compared with controls) were 56% or less with these carcinogens. In place of the standard single 72-hour treatment with test chemical, the cells were exposed to three consecutive 24-hour treatments. The amount of B(a)P metabolized during the last of the three 24-hour treatment periods was 3.2 times greater than that during the first 24-hour period, indicating that an induction effect occurred. Furthermore, the total amount of metabolites of B(a)P formed with repetitive treatments was 2.1 times greater than with a single 72-hour treatment. The total amount of AAF metabolites formed with repeated treatments was 1.6 times greater than with the single treatment regimen, although no induction effect was observed between treatment periods. Survival enhancement with the repetitive regimen increased to 181% with B(a)P, 172% with AAF, and 188% with DEN. With benzo(e)pyrene, anthracene, and pyrene, enhanced survival was 14% or less following the single treatment regimen and did not increase following repetitive treatments. When the carcinogen cinnamyl anthranilate was tested using repetitive treatments, survival enhancement was more than 100% at three of six doses, versus less than 0% when the standard single treatment regimen was used.

Neoplastic transformation of fetal lamb kidney cells by bovine leukemia virus

Neoplastic transformation of fetal lamb kidney (FLK) cells by bovine leukemia virus (BLV) is reported. Morphological transformation was observed in FLK cells within a few weeks of BLV inoculation. BLV-transformed FLK cells had the following properties generally associated with viral transformation: (1) altered morphology; (2) increased growth rate; (3) colony formation in soft agar medium; (4) formation of large cell aggregates and growth in this aggregate form above an agar base; and (5) tumorigenicity in nude mice. The transformed cells contained BLV-specific p24 and gp51 antigens and were virus producers. BLV was detected in transformed cells by syncytial assay. The transformed cells yielded high-titered viral reverse transcriptase activity and viral particles. The transformed cells contained marker chromosomes (M1, M2, M3, M4 and M7) and a large submetacentric chromosome, but control cells did not. Thus, we demonstrated the neoplastic transformation of FLK cells by BLV.

Establishment and characterization of a new cell line from a hereditary renal rat tumor

A new cloned tissue culture cell line, HRRT, has been established from a hereditary renal rat tumor. Electron microscopic studies indicate that the cells are fibroblasts. The cell line has been maintained in monolayer cultures for more than 3 yr and multiplies with a population doubling time of 21 h. The HRRT cells were found to have a plating efficiency of 50% and were not able to grow at low serum concentrations; they did not multiply in suspension culture and were unable to form colonies in soft agar or to proliferate in the aggregate form. The HRRT cells did not form tumors in nude mice. The new cell line was adapted to grow in serum-free medium by decreasing the serum concentration gradually over a period of several months and by addition of sodium pyruvate, insulin, alanine, serine, ribose, thymidine, and uridine to the medium. In serum-free medium HRRT cells multiplied with a population doubling time of 42 h and were not able to form colonies in soft agar.

Chemical carcinogenesis in vitro: an improved method for chemical transformation in Rauscher leukemia virus-infected rat embryo cells

In vitro assay for carcinogenesis, using mammalian cells, provide opportunity for rapid and inexpensive means, compared to in vivo assays, for studying carcinogenesis and for identifying potential carcinogens. These assays must, however, be shown to be reproducible, reliable and able to detect a variety of known carcinogens before they can be recommended for general use. We have, independently, reproduced a transformation assay which utilizes murine leukemia virus-infected rat embryo cells as targets. In the process a new culture, designated 2FR4(50), was generated to replace the F1706 line, of Freeman, which is no longer available. Through careful control of the assay parameters a readily reproducible test has been developed. In 2-4 culture passages after chemical treatment, morphologically transformed foci of cells are observed while no such foci are found in noncarcinogen treated or control cultures. Over 75 compounds have been tested in this assay; 20 of these are detailed here as representative of the chemically diverse types of carcinogens detected.

Phenotypic diversification of a cultured tumor line as a function of substratum

We have found that a murine hepatoma displays a considerable phenotypic diversification in culture, which depends upon the substratum utilized, and is manifested by the formation of multicellular structures of differing geometry: Monolayer on glass and plastic, thick multilayer pads on Gelfilm, and spheroids on agar and agarose. These multicellular morphological phenotypes were assayed without disruption to ascertain their antigenicity in vitro and their tumorigenicity in vivo and to obtain quantitative information on the effect of the spatial arrangement of the hepatoma cells upon the ability of each multicellular structure to interact, as a whole, with molecules and cells in its surroundings. The antigenicity of the multicellular structures was determined with calibrated probes and a methodology that measures the total antigenicity, as well as antigenicity per unit of surface area. Antigenicity was found to differ in the following decreasing order: Monolayer on plastic greater than spheroids on agarose greater than spheroids on agar greater than multilayer on Gelfilm. At least part of these antigenic variants arise from different degrees of masking of the structures' surface determinants by a trypsin-sensitive material. The multicellular phenotypes also differed in tumorigenicity. When assayed in syngeneic hosts under comparable conditions, agar-grown spheroids produced the fewest tumors, whereas Gelfilm-grown multilayers produced the most. These two independent sets of data show that the various geometries that a tumor tissue is induced to acquire by the culture substratum are accompanied by a distinctive combination of surface and biological properties.

Transformation of feline embryo cells in culture by a chemical carcinogen

A feline embryo cell line was treated in vitro with various levels of 7,12-dimethylbenz[a]anthracene (DMBA) or dimethyl sulfoxide (control). Repeat treatment of DMBA only induced in vitro transformation of feline embryo cells following clonal growth and selection. The morphologically altered cells formed large cell aggregates and grew in this aggregate form when suspended in liquid growth medium above an agar base, formed colonies in soft agar, and grew to high saturation densities. However, no progressively growing tumors were produced when cells were inoculated into nude athymic mice. The transformed lines were negative for feline oncornavirus-associated cell membrane antigen (FOCMA).

Neoplastic transformation of canine embryo cells in vitro by N-methyl-N'-nitro-N-nitrosoguanidine

A cell line derived from a normal beagle embryo was treated in vitro with various levels of N-methyl-N'-nitro-N-nitrosoguanidine or dimethyl sulfoxide (control). Cells treated only with the carcinogen underwent morphologic alteration in vitro, and one of these altered cell lines produced tumors subcutaneously when injected into NIH nude mice. The tumorigenic transformed line formed larger cell aggregates and grew in this aggregate form when suspended in liquid growth medium above an agar base.

Chemical transformation of human revertant cells induced by murine sarcoma virus

A human revertant cell line, derived from non-producer human osteosarcoma cells (NP/KHOS) induced by Kirsten murine sarcoma virus, was treated in vitro with various levels of polycyclic aromatic hydrocarbons (3-methylcholanthrene, 7,12-dimethylbenz(alpha)anthracene, and benzo(alpha)pyrene [BP] or dimethyl sulfoxide (control). Cells treated only with 3-methylcholanthrene and 7,12-dimethylbenz(alpha)anthracene underwent morphological alteration in vitro, and produced tumors when injected into NIH nude mice. These human revertant cells may be a useful in vitro tool in screening for the potential chemical carcinogens in human cell systems.