Chromosomal alterations of rat cell lines transformed by human adenovirus type-12 virion, whole DNA and left-end DNA fragments

c-myc induces autophagy in rat 3Y1 fibroblast cells

The proto-oncogene c-myc is a multifunctional gene that regulates cell division, cell growth, and apoptosis. Here we report a new function of c-myc: induction of autophagy. Autophagy is a bulk degradation system for intracellular proteins. Autophagy proceeds with characteristic morphologies, which begins with the formation of a double-membrane structure called the autophagosome surrounding a portion of the cytoplasm, after which its outer membrane then fuses with the lysosomal membrane to become an autolysosome. Autophagosomes and autolysosomes are generally called autophagic vacuoles. When c-Myc protein was overexpressed in rat 3Y1 fibroblasts or when the chimeric protein c-MycER was activated by estrogen, the number of autophagic vacuoles in cells increased significantly. The formation of autophagic vacuoles induced by c-Myc was completely blocked by a specific inhibitor of autophagosome formation, 3-methyladenine. A c-Myc mutant lacking Myc Box II induced neither apoptosis nor oncogenic transformation, but still stimulated autophagy. An inhibitor of caspases suppressed apoptosis but not autophagy. These results suggest that the autophagy caused by c-myc is not due to the apoptosis or tumorigenesis induced by c-myc. Taken together, our results suggest that the induction of autophagy is a novel function of c-myc.

Ras/MEK signaling suppresses Myc-dependent apoptosis in cells transformed by c-myc and activated ras

Cooperation of myc and activated ras has been suggested to cause malignant cell transformation but the mechanism is still unknown. Here we isolated a transformed cell line in which activation of c-Myc and Ras are independently controllable, and show that after establishment of the transformed state by c-myc and activated ras, removal of activated Ras initiates apoptosis that is dependent on c-Myc activity. Apoptosis is also initiated by an inhibitor of MEK (MAPK/ERK kinase), a kinase downstream of Ras, and apoptosis is blocked by activated Mek1. These results suggest that one of the conditions required for establishment of the transformed state is a block of apoptosis involving MEK activity. We tested the effect of MEK inhibition on cells transformed by various oncogenes. Suppression of apoptosis by MEK is not critical in general, but in cells transformed by c-myc plus a gene that activates the MAPK cascade it is necessary to avoid cell death. Activated Ras/MEK did not suppress c-myc-dependent apoptosis due to serum-limitation. Overexpression of chicken bcl-xL suppressed apoptosis under serum-limiting conditions, but not apoptosis initiated by Ras/MEK inhibition in cells transformed by myc and activated ras. Altogether, these results suggest the existence of a novel regulatory mechanism for myc-dependent apoptosis in certain transformed cells.

Both estradiol and tamoxifen decrease proliferation and invasiveness of cancer cells transfected with a mutated estrogen receptor

Previous studies have shown that, after wild-type estrogen receptor (ER) transfection in ER-negative breast cancer cells, estradiol but not tamoxifen prevents growth, invasiveness and metastasis of these cells in mice. Because an ER mutation at position 400 converts the triphenylethylene antiestrogen, OH-tamoxifen into a full estrogen agonist, we transfected this mutated form of human ER in an ER-negative rat cancer cell line. This was aimed at inducing an inhibitory, estrogen-like response of tamoxifen in these cells. In two stable ER-positive transfectants, OH-tamoxifen inhibited cell growth and invasiveness in vitro as efficiently as estradiol. The pure antiestrogen, ICI 164,384, was not agonistic alone and antagonized estrogen action. In contrast, the three compounds were ineffective in control mock-transfected cells. When injected into ovariectomized nude mice, ER-negative mock-transfected cells formed tumours which were significantly stimulated by estradiol and inhibited by tamoxifen treatment. This indicates that estradiol and tamoxifen altered the growth of ER-negative tumours via a general effect on the host response. Surprisingly, the hormone responsiveness of ER-positive tumours developed from ER-transfected cells did not significantly differ from that of ER-negative (mock-transfected) tumours. We conclude that transfection of a mutated human estrogen receptor inhibited, through an estrogenic activity of tamoxifen, the growth and invasiveness of these cancer cells in vitro. However, the low expression of ER did not allowed us to obtain the same effect of tamoxifen in vivo.

Presence of p53 mutations in 3Y1-B clone 1-6: a rat cell line widely used as a normal immortalized fibroblast

The 3Y1 cell line, established from a rat whole embryo, is widely used as a normal immortalized fibroblast. We analyzed p53 mutations in four clonal lines derived from the 3Y1 cell line; 3Y1-B clone 1-6, 3Y1-C and two clonal lines (3Y1 cl-3 and 3Y1 cl-6) which had been transformed by the human papilloma virus E6 gene. Polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) analysis and DNA sequencing showed that three clonal lines had a double mutation at codons 130 and 136 on the same allele and that the other clonal line, 3Y1 cl-3, had no mutations. 3Y1-B clone 1-6, which has been registered as the standard clonal line at the Japanese Cancer Research Resources Bank, demonstrated weak bands of the wild type allele, suggesting the existence of heterogeneous cell types in this "clonal" line. PCR-SSCP analysis of 25 subclones obtained by limiting dilution of 3Y1-B clone 1-6 cells revealed a mixture of two types of cells; 12 subclones showed only the bands of mutated allele, and 13 subclones showed both bands of the wild and mutated p53 alleles. These findings should be taken into consideration when using this cell line as a normal immortalized cell line.

Establishment and characterization of cell lines from human adenovirus type 12-induced murine tumors producing endogenous virus particles

Two cell lines designated IC-KMS and D-KMS were established from human adenovirus type 12-induced tumors of C3Hf/OK mouse. The cell lines retained the characteristics of the original tumor i.e., production of numerous C-type and intracisternal A-type particles, integration of Ad12 E1 region DNA and amplification of the myc gene family. Chromosomal analysis revealed chromosome aberrations in both IC-KMS and D-KMS cells. The modal chromosome number of IC-KMS cells was 54 and that of D-KMS cells was 48. Metacentric chromosomes and minichromosomes were found. Trisomy of chromosome 3, 7 and 12 was seen frequently in D-KMS cells. Although DNA aneuploidy was revealed by flow cytometry, the DNA indices of these cells showed no relation to the copy number of integrated Ad12 DNA. These cells have been propagated by serial culture during the past 17 months. Production of endogenous virus particles is a unique characteristic of IC-KMS and D-KMS cells. These cell lines would be useful materials for examining the contribution of Ad12 carcinogenesis to activation of endogenous virus particles, and also the correlation between Ad12 carcinogenesis and cancer-related genes.

A model of chromatin-dependent DNA replication sequences based on the decondensation units hypothesis

A model of chromatin-dependent DNA replication sequences was developed on the previously reported "decondensation units" hypothesis and its kinetic properties were examined by way of calculating various numerical indices using a Monte Carlo procedure. The model has much in common with the previous one but a fundamental difference is that the unit is assumed to consist of linearly arranged H-, D-, A- and S-zones each containing genes of different functional categories which are called H-, D-, A- and S-genes, respectively. The units are decondensed by the action of D-factors, i.e. decondensation factors, from H-zone to the end of S-zone and the genes in decondensed regions release signals to produce housekeeping enzymes, D-factors, A-factors and S-factors. These products are stored and at the same time degraded. A-factors activate replication origins in the decondensed regions and S-factors induce DNA synthesis at the activated origins. Replicated DNA is recondensed and gene activities are shut down in the recondensed chromatin. The factors are produced under the control of chromosome cycle and in turn affect chromosomes. Thus, dual control mechanism operates as Mazia and Prescott have argued. Biochemical and cytogenetic basis of this model was reviewed briefly and some results of simulation presented which include DNA synthesis rate vs. DNA content relationships. An outstanding characteristic of the model is the constancy of cellular state in A-subphase located in the late G1.

Influence of the deprivation of a single amino acid on cellular proliferation and survival in rat 3Y1 fibroblasts and their derivatives transformed by a wide variety of agents

We compared proliferation and survival of various syngeneic transformed cell lines under conditions of depletion of 15 amino acids in Dulbecco-Eagle's medium. We used a normal fibroblast line 3Y1 and 22 transformed sublines of 3Y1 which had been induced by one of seven transforming agents--simian virus 40, mouse polyomavirus, adenovirus type 12, E1A gene of adenovirus type 12, cDNA of Harvey murine sarcoma virus, Rous sarcoma virus, or N-methyl-N'-nitro-N-nitrosoguanidine. Unlike other untransformed cells examined (mouse BALB/c-3T3 line, mouse NIH-3T3 line, and primary Fischer rat embryo fibroblasts), 3Y1 ceased to proliferate and accumulated in a viable state with a G1-phase DNA content under 14 singular deprivations of amino acid. None of the transformed 3Y1 lines completely arrested in the G1 phase of the cell cycle and each showed different levels of survival, depending on each transforming agent. As for transformed 3Y1 cells induced by a given virus or a given transforming gene, any one of the three sublines shared the same trend with respect to proliferation and survival. Transformed derivatives induced by N-methyl-N'-nitro-N-nitrosoguanidine showed almost the same trend in proliferation, but the patterns of survival were not uniform. Our observations suggest that the unique responses of 3Y1 to amino acid depletion are differently modified by different transforming agents.

Commitment to ploidy conversion of 3Y1 cells during metaphase arrest by colcemid

Diploid rat 3Y1 fibroblasts proliferate to a saturation density, where they are arrested with a 2N DNA content. After treatment to induce ploidy conversion, the conversion rate can be estimated by determining the fraction of cells with a 4N DNA content in the confluent culture using flow cytometry. Using this method it was found that during mitotic inhibition with colcemid, 3Y1 cells were converted to tetraploids with a high efficiency (above 80%); the optimum colcemid concentration and exposure period were 40 ng/ml and 8 hr, respectively. When metaphase cells were reseeded with 40 ng/ml of colcemid, they delayed anchorage to a dish; 6 hr was required for complete adhesion (in the absence of colcemid only 1 hr was required). When reseeded metaphase cells were exposed to 40 ng/ml of colcemid for 5 hr followed by its removal, a greater fraction of the cells anchored to the substratum were converted to tetraploids, whereas most of the floating cells were not. A greater fraction of the anchored cells had formed nuclei, whereas most of the floating cells preserved condensed metaphase chromosomes. These results indicate that the cells which have formed nuclear structure without chromosome separation during mitotic inhibition are irreversibly committed to ploidy conversion, with restoration of anchorage.

Induction of endoreduplication in temperature-sensitive mutants of rat 3Y1 fibroblasts

Four temperature-sensitive mutants of rat 3Y1 fibroblasts belonging to separate complementation groups (3Y1tsD123, 3Y1tsF121, 3Y1tsG125, and 3Y1tsH203) are arrested mainly with a 2C DNA content, when cells proliferating at 33.8 degrees C are shifted up to 39.8 degrees C (Ohno et al., 1984). Zaitsu and Kimura (submitted for publication) showed that 3Y1tsF121 cells synchronized in the early S phase were arrested with a 4C DNA content at 39.8 degrees C. We studied the traverse through the S and G2 phases at 39.8 degrees C in the four ts mutants synchronized at the early S phase and found that 3Y1tsG125 and 3Y1tsH203 cells were arrested with a 4C DNA content as 3Y1tsF121, while 3Y1tsD123 cells went through S and G2 phases and underwent mitosis. When 3Y1tsF121 and 3Y1tsG125 mutants arrested at 39.8 degrees C were shifted down to 33.8 degrees C, a substantial fraction of the cells with a 4C DNA content started, with a certain lag period, DNA synthesis without intervening mitosis and underwent the first mitosis with a lag period similar to that in the cells arrested with a 2C DNA content. The tetraploid cells thus generated had a proliferating ability lower than that of diploid cells.

Formation of proliferative tetraploid cells after treatment of diploid cells with sodium butyrate in rat 3Y1 fibroblasts

When randomly proliferating rat 3Y1 fibroblasts were treated with sodium butyrate, more than 90% of their cells were arrested reversibly with a 2C DNA content at least 12 h before the G1/S boundary. When cells synchronized in the early S phase were treated with butyrate, approximately 70% of all cells were arrested with a 4C DNA content. The arrests in both G1 and G2 phases by the single inhibitor suggest that the two phases share a common mechanism. The ability of cells to undergo mitosis on time was quickly lost with time of arrest in the G2 phase. Upon removal of the inhibitor, the cells arrested with a 4C DNA content entered a new S phase without intervening mitosis. The tetraploid cells thus produced kept proliferating as fast as diploid cells. These results suggest that the inhibition of the normal G2 traverse is somehow responsible for the formation of the proliferative polyploid cells.

Genetic analysis of control of proliferation in fibroblastic cells in culture. I. Isolation and characterization of mutants temperature-sensitive for proliferation or survival of untransformed diploid rat cell line 3Y1

Mutants temperature sensitive for proliferation or survival were isolated from an untransformed diploid clone of fibroblastic rat cells (3Y1), according to an isolation protocol that selected for mutants defective at 38.5 degrees C (selection temperature) in undergoing the transition from quiescent to proliferating state while maintaining viability at 38.5 degrees C. Of the 108 temperature-sensitive clones isolated, 32 were examined for survival in sparse cultures at 39.8 degrees C (nonpermissive temperature) and classified into four classes. Results of temperature shift-up experiments suggest that functions defective in 11 of the 32 mutants are necessary not only for changing from the quiescent to proliferating state but also for maintenance of the proliferating state. Of the 32 mutants, 17 were assigned to eight complementation groups. Results of the physiological characterization of the representative mutants of each of the eight complementation groups are presented.

Difference in growth factor requirements of rat 3Y1 cells among growth in mass culture, clonal growth in low density culture, and stimulation to enter S phase in resting culture

A semiserum-free medium was developed for monolayer culture of rat 3Y1 fibroblastic cells. The main components of the developed medium added to Dulbecco's modified Eagle's medium (DMEM) were insulin, transferrin, epidermal growth factor, poly-D-lysine, bovine albumin, oleic acid, and bovine alpha-globulin. In this medium, 3Y1 cells grew in mass culture at much the same rate as in DMEM supplemented with 10% fetal bovine serum (FBS), and colonies, albeit of smaller sizes, did form. Virally transformed derivatives of 3Y1 (simian virus 40-3Y1, polyoma virus-3Y1 and adenovirus type 12-3Y1) also formed colonies in the semiserum-free medium. When trypsinized 3Y1 cells were seeded with the medium lacking alpha-globulin, neither growth in the mass culture nor clonal growth in the low density culture (clonal growth) occurred. In this case, cell spreading was inhibited by albumin, and this inhibition was overcome by adding alpha-globulin or treating dishes with serum. When albumin was excluded from the semiserum-free medium, clonal growth did not occur, whereas growth in mass culture and stimulation of DNA synthesis in the resting mass culture (stimulation of DNA synthesis) were not so drastically affected. When oleic acid was removed, growth in mass culture was inhibited considerably, but no considerable effect was seen on clonal growth or on stimulation of DNA synthesis. In the absence of insulin, stimulation of DNA synthesis was inhibited more markedly than when other components were removed, but such was not the case with growth in mass culture and clonal growth.

Chromosome changes in rat embryo cell lines transformed by temperature-sensitive mutants and sheared DNA of herpes simplex virus

The chromosomes of six rat embryo cell lines transformed with herpes simplex virus (HSV) temperature-sensitive (ts) mutants were examined at different passages of in vitro cultivation. Two cell lines were predominantly diploid, one cell line was hyperdiploid, one cell line was pseudodiploid, and two cell lines were hypotetraploid. In near-diploid cell lines chromosome No. 9 was most frequently involved in chromosome changes. All three cell lines derived from tumors obtained after one transplantation of HSV-transformed cells into baby rats were pseudodiploid, but each had different marker chromosomes. Chromosome No. 15 was involved in the formation of two out of four marker chromosomes. Four cell lines derived from tumors developing after two and three transplantations were hypodiploid and showed large chromosome variation. The occurrence of 25 marker chromosomes in three tumor-derived cell lines resulted in gains in parts from chromosomes No. 2, 6, and 7. One marker chromosome had a homogeneously faintly stained region. Chromosomes No. 2, 3, 7, and 12 were more frequently involved in the formation of marker chromosomes. No chromosome change was found to be specifically associated with HSV-induced transformation of rat cells, but chromosome changes in tumor-derived cell lines may provide selective advantage for survival and autonomous growth in the host animal.