A reassessment of methylcholanthrene transformation in the C3H10T1/2 cell culture system

We previously demonstrated that four tumorigenic methylcholanthrene (MCA) transformed cell lines derived from C3H10T1/2 cells each contain a common G34----T nucleotide alteration in the c-Ki-ras gene. In contrast, a non-tumorigenic MCA transformant does not contain this mutation. We have now examined 75 newly isolated MCA transformants of C3H10T1/2 cells for their degree of morphological transformation, the presence of the c-Ki-ras G34----T mutation, colony formation in soft agar, and tumorigenicity in nude mice. Although many of these new MCA transformants exhibit morphological characteristics indistinguishable from previously isolated tumorigenic MCA transformants, none contain the G34----T mutation in the c-Ki-ras gene. Only one newly isolated MCA transformant can grow in soft agar. Of 14 tested, none of the new MCA C3H10T1/2 transformants are tumorigenic in nude mice.     

Tumorigenic conversion resulting from inhibition of apoptosis in a nontumorigenic HeLa-derived hybrid cell line

Although tumorigenicity in nude mice is one of the most important transformed phenotypes, its mechanism has been little analyzed. To understand the molecular basis of tumorigenicity, we characterized nontumorigenic CGL1 and tumorigenic CGL4 cell lines, both of which were originated from a common ancestral HeLa-human diploid fibroblast hybrid cell clone and retained a malignant state except tumorigenicity. When injected into nude mice, nontumorigenic CGL1 cells underwent apoptosis, but tumorigenic CGL4 cells did not. In vitro, CGL1 was also less resistant to various apoptotic stimuli than CGL4. These results suggested that inhibition of apoptosis may lead to tumorigenicity. To examine this hypothesis, we introduced antiapoptotic genes into the CGL1 cell line and injected the resulting clones into nude mice. The results showed that the ectopic expression of Bcl-2 or E1B19k, but not of crmA, converted CGL1 cells to tumorigenicity, suggesting strongly that this phenotype may be conferred by evasion of apoptosis.     

Growth control of heterologous tissue culture cells in the congenitally athymic nude mouse.

A variety of heterologous mammalian cells were inoculated into nude mice and scored for tumorigenicity. The cells tested were from primary cell cultures, established cell lines of neoplastic origin, established cell lines of nontumor origin, and primary cell cultures transformed by oncogenic viruses. Regardless of the animal species of origin, every cell line that was tumorigenic in some other animal host and every cell line of neoplastic origin was tumorigenic in nude mice. Several tissue culture cells lines capable of indefinite growth in vitro failed to form tumors in nude mice, and the basis of this growth suppression was investigated. The findings suggest that the failure of an established cell line to form tumors in nude mice is an authentic response to host-mediated growth-regulatory signals.     

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 sub-cutaneously 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.     

Establishment and characterization of human renal cancer and normal kidney cell lines

We have reviewed our laboratory's efforts to establish continuous human renal cancer cell lines. During the 16-year period of 1972 through 1987, 498 successive attempts resulted in establishment of 63 renal cancer cell lines. Of these lines, 46 were derived from primary kidney tumors and 17 from metastatic sites (lung, brain, bone, and lymph node). Forty-three of these lines have been characterized with regard to morphology, growth kinetics, anchorage-independent growth, tumorigenicity in athymic nude mice, and expression of kidney cell surface antigens. These results were compared with data from primary short term cultures of normal kidney epithelium. The overall success rate of establishing continuous renal cancer cell lines was 12.7%. In general, no significant difference in success was noted based on whether the specimen was derived from a primary or a metastatic lesion. However, all successfully established lines were derived from tumors exhibiting clinically "aggressive" behavior. All cell lines expressed proximal tubular cell differentiation antigens. Significant morphological heterogeneity was observed among normal kidney as well as kidney cancer cell lines in vitro. No significant difference in doubling time was found between cell lines of renal cancer and passage 1 cultures of normal kidney epithelium. Twenty-one of 30 (70%) lines assayed formed clones on soft agar and 26 of 33 (79%) lines grew in athymic mice. Among the 25 lines which were assayed for both soft agar growth and tumorigenicity in nude mice, this pair of phenotypic traits were concordant in 17 lines (60%). Four lines (16%) grew on agar but not in mice, while four other lines (16%) failed to grow in agar but were tumorigenic in mice.     

Tumorigenic conversion of xeroderma pigmentosum lymphoblastoid cells without karyotypic alteration

In order to examine the process of malignant transformation of human somatic cells, we studied the tumorigenic conversion of an Epstein-Barr-virus-immortalized lymphoblastoid cell line (LCL) derived from a patient with xeroderma pigmentosum (XP) complementation group A. Repeated irradiation of the XP cells, XP7NI, with UV-light and subsequent treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in the acquisition of tumorigenicity in athymic nude mice. The tumorigenicity of XP7NI cells was also induced by TPA treatment alone. The tumors formed in athymic mice were of B-cell lymphoma with characteristic histology, cell surface immunoglobulins and an antigen as detected by a B-cell-specific monoclonal antibody (MAb), CD20. The surface immunoglobulins and the HLA type of these tumor cells were identical with those of the parental cells. These malignantly transformed cells retained the same UV sensitivity, serum requirement, colony-forming ability in soft agar, and normal human karyotype as the parental cells. Unlike other tumorigenic lymphoblastoid cell lines, this XP lymphoblastoid cell line provides a unique case in that process(es) leading to tumorigenicity may be induced by UV and TPA without apparent karyotypic changes.     

Intracranial heterotransplantation of human hematopoietic cells in nude mice.

A panel of established cell lines and many primary cell specimens from lymphomas and leukemias as well as from normal lymphatic tissues were tested for tumorigenicity by intracranial heterotransplantation in nude mice. Not only lymphoma and leukemia cell lines, but also lymphoblastoid cell lines, lacking markers of malignancy, were tumorigenic in the brains of nude mice. These findings indicate that tumorigenicity following intracranial heterotransplantation in nude mice cannot be used as proof for the malignant nature of established cell lines. Heterotraplantation of primary cell specimens yielded only a few tumor takes. When primary cells were infected with exogenous Epstein-Barr virus prior to the transplantation procedure, tumorigenicity could be significantly increased. Cytogenetic evaluation of tumors growing after intracranial transplantation of human hematopoietic cells showed, in some cases, a selection of cytogenetically aberrant cell clones.     

Immune response to somatic cell hybrids between ultraviolet radiation-induced regressor and spontaneous progressor C3H mouse tumor cells

We used somatic cell hybridization to determine whether the regressor phenotype exhibited by UV-induced murine tumors was dominant or recessive and whether this technique could confer immunogenic properties on nonimmunogenic syngeneic tumors. We transfected a highly antigenic UV-induced C3H mouse tumor cell line (UV-2240) with the plasmid pSV2-neo and selected G418-resistant clones. The resulting cell line was fused with a spontaneously transformed nonimmunogenic C3H progressor tumor cell line (SF-2T) that had been selected previously for resistance to 3.0 mM ouabain. These two cell lines were fused by a brief exposure to polyethylene glycol and heterokaryons isolated by growth in medium containing both G418 and ouabain. Hybrid cell lines established from individual colonies and from pools of colonies were tested for tumorigenicity in normal C3H and athymic nude mice. The results indicated that all the hybrid cell lines tested were highly antigenic in that they were completely rejected when transplanted into normal syngeneic mice but grew progressively in nude mice. Furthermore, immunization of C3H mice with the hybrid cell lines induced protective immunity against challenge with the immunizing tumor and generated cross-protective immunity against challenge with the regressor parental cell line but not against challenge with the progressor parental cell line. These results demonstrate that the regressor phenotype of the UV-2240 tumor is dominant in nature and that the immune response induced by somatic cell hybrids is uniquely directed against the dominant tumor-specific transplantation antigens expressed on the regressor tumor. This implies that introduction of tumor-specific transplantation antigens from an immunogenic tumor into a nonimmunogenic tumor, although sufficient to confer immunogenic properties to the hybrid, is insufficient to induce cross-protective transplantation immunity against the nonimmunogenic tumor.     

Growth potential of human colorectal carcinomas in nude mice: association with the preoperative serum concentration of carcinoembryonic antigen in patients

A preoperative serum carcinoembryonic antigen (CEA) concentration greater than 5 ng/ml portends a poor prognosis for patients with colorectal carcinoma. The purpose of this study was to determine if the tumorigenicity of colorectal carcinomas in nude mice was associated with the preoperative serum CEA concentration. Neoplasms from 53 patients were either implanted as fragments or dissociated with collagenase and DNase, and 3 x 10(6) viable cells were injected into the flanks of BALB/c nude mice. The growth potential of tumors resected from patients with CEA levels exceeding 5 ng/ml was greater than that of tumors from patients with normal serum CEA: 26 of 33 carcinomas from patients with CEA greater than or equal to 5 ng/ml were tumorigenic in nude mice, whereas only 8 of 22 neoplasms from patients with normal serum CEA were tumorigenic in nude mice (P less than 0.001). Primary colorectal cancers, not metastases, were the basis for the association between tumorigenicity and preoperative CEA. Tumorigenicity was also associated with stage of disease, since Dukes' D primary tumors and metastases were more tumorigenic than Dukes' A to C primary tumors. Growth in nude mice was not associated with other prognostic factors such as tumor site, mucin production, local invasion, or stage of histological differentiation. The tumorigenic capability of human colorectal carcinomas may be associated with the preoperative serum CEA concentration and may reflect an increased potential to develop clinical metastases.     

Immune control of SV40-induced tumors in mice

The ability of mice to mount a cytotoxic T-lymphocyte (CTL) immune response to SV40 T-antigen is determined by the H-2 haplotype of the host; H-2b and k mice are high responders and H-2d mice are low responders. Mice of these 3 H-2 haplotypes were challenged with SV40 and their ability to generate and sustain an antibody response to SV40 T-antigen was found to be equivalent. To investigate the role of the different components of the host immune response in controlling growth of SV40-induced tumors, the tumorigenic potential of freshly established cell lines, obtained by SV40 transformation of cells from normal tissues of inbred strains of mice of 6 H-2 haplotypes, was assessed. Each cell line was tumorigenic in athymic and newborn mice but not in adult syngeneic immunocompetent mice. Cells from these initial SV40-transformed lines were then passaged in athymic (nu/nu) mice, re-established in vitro and again transferred into syngeneic animals. Transfer of H-2d SV40 transformants to low or non-responder mice of the H-2d haplotype resulted in tumor formation in some animals. Cells derived from these tumors expressed both the viral encoded T-antigen and the H-2Dd restriction element. Furthermore, the proportion of animals with tumors varied with the strength of their CTL-responsiveness to SV40 T-antigen in association with H-2Dd. Therefore, in H-2d animals, tumor cell growth appears to result from escape of cells from inefficient CTL surveillance. No tumors were formed by transfer of the in vivo selected H-2b or H-2k SV40 transformants to syngeneic high-responder mice. We therefore investigated the role of CTL in the selection of SV40-transformed cells able to escape immune surveillance. Under conditions of stringent immune selection by CTLs, tumorigenic cells that no longer expressed the relevant H-2 class-I restriction element were obtained. Although interaction between the various immune effector mechanisms may play a role in the recognition and elimination of SV40 transformants, our results were consistent with the hypothesis that the SV40-specific CTL response is the predominant control of SV40 tumor growth.     

Antisense expression for amphiregulin suppresses tumorigenicity of a transformed human breast epithelial cell line

The epidermal growth factor (EFG) family of receptors and their respective ligands play a major role in breast cancer progression and are the targets of new therapeutic approaches. Following immortalization with SV40 T antigen of normal human breast epithelial cells, a transformed variant cell line (NS2T2A1) was selected for its increased tumorigenicity in nude mice. This cell line was shown to have a higher expression of EGF receptors (EGFR) and amphiregulin (AR) when compared to their normal counterparts or less aggressive transformed cells. Dual staining of EGFR and AR was observed in 50-60% of NS2T2A1 cells, while 30-40% cells expressed AR only. To explore the potential tumorigenic role of AR, a 1.1 kb AR cDNA in an antisense orientation was transfected in NS2T2A1 cells. Three clones, selected by hygromycin B, expressed AR antisense RNA (AR AS1, AR AS2 and AR AS3 cell lines) in which AR protein expression was reduced (ranging from about 50 to < 5%). The anchorage-independent growth of AR AS cell lines was reduced to levels ranging from 32.4-6.8% relative to the control cell line transfected with the vector alone. The clones expressing AR antisense RNA showed a reversion of the malignant phenotype when injected in nude mice, since a significant reduction of tumor intake was observed coincident with a significant tumor mass reduction (> 96%). Moreover, intra-tumoral vascularization decreased significantly in tumors derived from AR AS cells (26.7, 70.7 and 50.4% of control). These in vitro and in vivo data reveal the oncogenic nature of AR in transformed breast epithelial cells and imply a role for AR in tumor angiogenesis.     

Analysis of immune surveillance of sequentially derived cell lines that differ in their tumorigenic potential

The immune surveillance hypothesis suggests that cancer evolves as a multistage process. Further, it predicts that cells intermediate on the pathway to cancer are susceptible to host protective mechanisms, and only those variants that are able to escape the protective mechanisms are able to grow as tumors. We have isolated, as lineages, fibroblast lines that express phenotypes predicted by the surveillance hypothesis. The lineages were derived by treating nontransformed cells (N-cells) with chemical carcinogens and by isolating transformed variants in vitro. From the transformants that are tumorigenic in immune-depressed ATXFL mice but rejected by normal mice (I-cells), variants were selected in vivo that had escaped the rejection mechanism(s) and had grown as tumors in normal mice (C-cells). Thus lineages were established comprised of sequentially derived cell lines with the following phenotypes: nontransformed, transformed but susceptible to host protective mechanisms, transformed and resistant to host protective mechanisms (i.e., N----I----C). With the use of in vivo cross-protection experiments, two independently derived I-cell lines were shown to express non-cross-reactive antigens that are not expressed by the parental nontransformed N-cells (i.e., transformation-associated antigens). The transformation-associated antigens are expressed at an equivalent level on the cells that are susceptible to rejection (i.e., I-lines) and those that have escaped rejection (i.e., C-lines). In addition, although the transformation-associated antigens expressed by I-cells induce an effective immune response capable of rejecting both the I-line and C-line, the expression of these antigens on C-cells does not induce an effective immune response. The role of host defense mechanisms in the rejection of these chemically transformed I-cells and the possible mechanisms by which C-cells escape rejection are discussed.     

Cell aggregation assay: a rapid means of evaluating and selecting in vitro transformed cells

A cell aggregation assay for evaluating in vitro transformation has recently been reported. Transformed cells formed larger cell aggregates than counterpart normal cells when suspended in liquid media above an agar base, a property which correlates with growth in soft agar and tumorigenicity. Using a human osteosarcoma cell line transformed by viruses and chemicals, we found increased cell growth of the transformed aggregates over untransformed cells. Moreover, certain aggregation properties (size/survival of aggregates) of transformed rat, mouse, hamster, human, dog, cat, chimpanzee, and sheep cell lines were found to be correlated with tumor potential, regardless of the method of transformation (spontaneous, chemical, or virus induced). Certain lines derived from cell aggregates growing in liquid medium above an agar layer were tumorigenic in nude mice, whereas the parent transformed line was not. Therefore, this assay can be utilized not only to evaluate tumorigenic potential, but also for selection of cells, which have undergone malignant transformation.     

Primary transfection as a mechanism for transformation of host cells by human tumor cells implanted in nude mice

Establishment of cell lines in vitro from a human lung cancer xenograft in nude mice resulted in transformed mouse cell lines. The transformed mouse cell lines expressed both mouse-specific and human-specific histocompatibility antigens. Of 3 cell lines, 2 were tumorigenic in BALB/c nude mice but not in normal mice. Tumors formed by the transformed mouse cell lines were fibroblastoid and epithelioid by histology. In addition, tumors exhibited neuroepithelial differentiation by ultrastructural and immunohistochemical analysis. Phenotypically they were similar to the original patient and human xenograft tumor. These data suggest that previous reports of host cell transformation and induction of fibrosarcomas may not be true fibrosarcomas. Human DNA sequences were present in the tumorigenic cell lines, indicating that spontaneous transfection of human tumor DNA into host cells had occurred. The implication of these findings is that human genetic information has been transferred to primary mouse host fibroblasts, which resulted in a transformed as well as a differentiated phenotype.     

Human tumor spontaneous metastasis in immunosuppressed newborn rats. I. Characterization of the bioassay

We characterize a new model of spontaneous metastasis of human tumor cells using anti-thymocyte serum (ATS) immunosuppressed newborn rats. We analyzed the intrinsic value of the bioassay of measurement of tumorigenicity and metastatic capacity using 17 human tumor cell lines, of which were 9 human malignant melanomas. Most of these cell lines revealed as tumorigenic and metastatic in lungs and/or lymph nodes 3 weeks after s.c. injection in the ventral flank of newborn rats, irrespective of their origin. All the melanoma cell lines that we injected were tumorigenic and 77% were metastatic, whereas the same cell lines grafted in nude mice showed no evidence of metastases after 6 weeks' examination. We were not able to show any relationship between tumorigenicity in nude mice and the malignant behavior of these cells in ATS-treated newborn rats. Likewise, neither chromosome abnormalities, nor antigenic marker expression were found to be related to tumor growth in nude mice or newborn rats. Two intrinsic parameters of the model were studied: number of cells injected vs. dose of ATS injected for one melanoma cell line; and role of the 3rd and 4th injections of ATS in the establishment and development of pulmonary metastases. Moreover, we show that s.c. injection in the ATS-treated newborn rat may represent a suitable method for studying melanoma cell tumor growth and spontaneous dissemination.     

Decreased expression of CD80 is a marker for increased tumorigenicity in a new murine model of oral squamous-cell carcinoma.

We established a new syngeneic murine model of oral squamous-cell carcinoma (SCC) to analyze the potential role of immune recognition determinants in the early development of oral cancer. In this study, we examined whether SCC that undergo transformation and development in the absence of specific immunity exhibit differences in tumorigenicity that relate to differences in expression of CD80, CD86 or MHC class I. Mucosal keratinocytes from BALB/c mice were transformed in vitro with 4-nitroquinolone-1-oxide (4-NQO) and inoculated into SCID mice to obtain tumorigenic cell lines. Five SCC cell lines were re-isolated from tumors, and 4 retained cytokeratin and beta4-integrin markers of epithelial origin. Their growth was compared in BALB/c and in congenic SCID mice to establish whether the cell lines exhibit differences in immunogenicity. Three lines that showed slower growth or completely regressed when implanted in immune competent hosts retained or developed increased expression of CD80 during development in SCID mice. Conversely, 2 SCC lines that lost expression of CD80 after passage in vivo grew progressively in immune-competent hosts. MHC-class-I and CD86 expression did not correlate with tumorigenicity. These observations provide evidence that decreased expression of CD80 may serve as a marker for increased tumorigenicity during early development of oral SCC. The development of this new murine oral SCC model should prove useful in determining the potential effects of CD80 expression on the immune pathogenesis and therapy of SCC.     

Tumorigenicity and in vitro characteristics of rat liver epithelial cells and their adenovirus-transformed derivatives

Cloned and uncloned epithelial cultures were established from the liver of a 3-week-old AS rat. These epithelial cultures were neither tumorigenic nor did they display anchorage-independent growth. One of the clones was cytogenetically normal after 53 in vitro passages (approximately 200 population doublings after cloning). Eight transformed lines were isolated from the liver epithelial cells after infection with adenovirus type 12 (Ad-12). Five of these produced typical Ad-12 T-antigen, whereas three appeared to be T-antigen-negative. All were tumorigenic in newborn syngeneic rats. The T-antigen-positive transformed lines produced anaplastic-epithelial tumors, whereas the T-antigen-negative transformed lines produced adenocarcinomas. Although all the transformed lines were tumorigenic, some were fibronectin-positive while others produced no detectable fibronectin. The normal (untransformed) epithelial cells produced fibronectin. These results are interesting for two reasons: (1) there are relatively few reports of fibronectin on epithelial cells and (2) they emphasize the view that there is no absolute correlation between reduced fibronectin and tumorigenicity in transformed cells. The transformed lines displayed in vitro characteristics similar to those of transformants derived from embryonic and fibroblastic cell strains, notably, increased saturation density and changes in cellular morphology. Some of the transformed cell lines, but not all, displayed anchorage-independent growth. All the transformed cell lines were picked from multi-layered foci so that morphological criteria (i.e. piling-up focus) for isolating transformants from the epithelial cultures were similar as in embryonic and fibroblastic transforming cell systems. With the new cell system we have developed we can, using the same epithelial cell line (clone C3), study both virus transformation and virus mutagenesis.     

Burkitt's lymphoma cell lines reveal different degrees of tumorigenicity in nude mice

A quantitative in vivo assay for evaluating the tumorigenicity of Burkitt's lymphoma (BL) cell lines in nude mice is described. It is based on the dose-response kinetics of BL cell lines in pre-irradiated (480 rad) nude mice following the s.c. injection of 4 different cell doses. This model system was used to estimate the xenografting potential of 26 BL cell lines derived from BL patients of different geographic and ethnic origins, as well as lymphoblastoid cell lines (LCLs) established by Epstein-Barr virus (EBV) immortalization of normal B lymphocytes. The results indicate that most BL cell lines are tumorigenic, but LCLs fail to produce progressively growing tumours in nude mice. However, BL cell lines revealed individual degrees of tumorigenicity and accordingly could be divided into 4 groups with high, moderate, low or no tumorigenicity. Preliminary attempts to correlate the xenografting phenotype of BL lines with other characteristics indicate that: (1) the aberrations of chromosome 1 are more often encountered in cell lines with high and moderate tumorigenicity; (2) EBV-positive BL lines do not reveal a higher tumorigenic phenotype in comparison with EBV-negative ones; (3) cell lines carrying translocations t(8;22) and t(2;8) might fall more frequently in the group of lines with high and moderate tumorigenicity; and (4) when LCLs grow in nu/nu mice, rejection always occurs and is associated with massive tumour necrosis. These findings suggest that the tumorigenicity of BL cell lines in immunosuppressed animals is not related with EBV, but with certain chromosomal abnormalities (BL-specific and non-specific) indicating that this in vivo model system can be instrumental for the identification of other factors or stages involved in BL development.     

Tumorigenicity in the nude mouse of cocultures derived from two nontumorigenic cell types, human pituitary adenomas and mouse C3H 10T1/2 fibroblasts

Human pituitary adenoma tissues were not tumorigenic in the hormonally manipulated nude mouse. Mouse fibroblast cells (C3H 10T1/2) also did not form tumors when inoculated alone into nude mice. When these two tissues were cocultured and coinoculated into nude mice however, the majority of inocula developed progressively enlarging tumors which could be established in tissue culture and passaged through the nude mouse. These tumors were sarcomatous histologically and thus did not resemble any human pituitary adenoma tissue injected. In order to detect any human cells in these tumors, tumor genomic DNA was subjected to Southern analysis using human repetitive Alu and HGH DNA sequence probes. Southern blot analysis of the nude mouse derived tumor genomic DNA revealed no sizeable human DNA in the mouse tumor cell genome indicating the absence of significant numbers of human cells in the tumors or the transfer of human DNA to the mouse cells. The tumors therefore arose from transformed C3H 10T1/2 cells after coculture with the human pituitary adenoma cells. These results implied that the tumorigenic transformation of susceptible C3H 10T1/2 cells in the cocultures occurred as a result of the secretion by the adenoma cells of transforming substances in the culture media or the induction of tumorigenicity through direct cell-cell contact between the two cell types.