Induction of supermelanin synthesis and morphological changes in interspecific reconstituted cells and its reversal by tumor promoter

Pleiotropic phenotypic expression in cybrids derived from mouse teratocarcinoma cells fused with rat myoblast cytoplasts

Cybrid clones were isolated by fusing mouse embryonal carcinoma (PCC4) cells with cytoplasts of rat myoblastic cells (L6TG X CAPr). Although some clones were similar to PCC4 (Type II), a high proportion (88%) were differentiated; the differentiated cells had a mesh-like arrangement (Type I) or were flat with many projections (Type III). Protein patterns of both Type I and Type III cells changed markedly from that of PCC4 cells. Type III cells lacked alkaline phosphatase and expressed endo A and B proteins predominantly. One Type III clone produced alpha-fetoprotein and plasminogen activator (visceral endoderm-like), while another clone consisted of trophectodermal cell-like giant cells. Therefore it was shown that introduction of the somatic cell cytoplasm induces differentiation of teratocarcinoma stem cells, suggesting a cytoplasmic element (or elements) regulating gene expression.

Histone expressions in mouse-rat somatic reconstituted cells

A critical analysis of histone expression was performed on the four interspecific and the two intraspecific reconstituted cells formed between karyoplast from mouse B16 cells and the cytoplast from rat cells (L6TG.CAPr) or mouse cells (B82.CAPr). All the reconstituted cells had the same pattern of mouse histones and the same amount of mouse-specific H2B. 2 histone as that of mouse nuclear donor cells. A hybrid between B16 and L6TG.CAPr contained both mouse and rat-specific H1b subtypes, whereas no rat-specific H1b was detected in the interspecific reconstituted cells. In both intra- and interspecific reconstituted cells, the proportion of H1b content was lower than that of B16 cells but that of H1 degree was higher, indicating that the mouse H1 patterns from these cells slightly resembled the pattern of slower growing and differentiated cytoplast donor cells. As an effect of the tumor promoter, the H1 pattern tended to revert to that of the nuclear donor cells in agreement with the phenotypic reversion, without any significant change in cell growth.

Induction of erythroid differentiation by cytoplast fusion in mouse erythroleukemia (Friend) cells

An intracellular activity, which is induced by dimethyl sulfoxide (DMSO) or hexamethylenebisacetamide (HMBA) and leads to erythroid differentiation in mouse Friend cells, was characterized by cell fusion between genetically marked intact cells and cytoplasts. For this, a procedure for rapid selection of cybrids was devised by sensitizing non-fused cells with oligomycin. We were able to demonstrate that cytoplasts derived from DMSO- (or HMBA)-treated cells trigger erythroid differentiation upon fusion with UV-irradiated cells. The activity in the cytoplasts remained only transiently and its induction was inhibited by biologically active phorbol esters or cycloheximide. The activity, however, was not induced in cytoplasts by directly treating them with DMSO (or HMBA). These results indicate that (1) the intracellular erythroid-inducing activity is located in cytoplasts, (2) it acts in trans and induces erythroid differentiation as a dominant factor and (3) its production requires de novo nuclear protein synthesis. The mechanisms of the induction of the intracellular activity and of how it triggers erythroid differentiation are discussed.

Effects of actinomycin D and cycloheximide on the increase in tyrosinase activity of hamster amelanotic melanoma cells in vitro

Tyrosinase activity in the Ab hamster amelanotic melanoma cells cultured in serum-free Eagle's MEM increased 3 times after 6 h of primary cell culture. This increase was inhibited completely by cycloheximide, while actinomycin D had no effect on this process. After 24 h of culture in MEM with calf serum, further increase of the tyrosinase activity was inhibited by both cycloheximide and actinomycin D. The data presented may indicate that the increase of tyrosinase activity in the primary cell culture of the Ab melanoma is due initially to the unblocking of translation and later to the activation of transcription of the gene controlling the enzyme.

Correction of sister chromatid exchanges in Bloom's syndrome fibroblasts after fusion with cytoplasts from a Chinese hamster cell line

Cytoplasmic hybrids (cybrids) were prepared by fusion of Bloom's syndrome (BS) fibroblasts with cytoplasts derived from a chloramphenicol (CAP)-resistant cell line of hypoxanthine phosphoribosyl transferase (HPRT)-deficient Chinese hamster wg3h cells to study the role of cytoplasmic and nuclear factors which may involve corrective factor(s) of sister chromatid exchanges (SCEs) of the BS cells. The high rate of SCEs characteristic of the cultured BS fibroblasts was found to be fully corrected in cybrids. This result suggests the existence of a transmissible corrective factor(s) in the cytoplasm of the Chinese hamster cells. Long-term cultures of such cybrids after 3 and 5 weeks of fusion, however, showed a reversion to a high SCE level similar to the parental BS cells, indicating that the corrective factors have not been stable in the cybrid cells.

Supermelanotic hybrids between mouse teratocarcinoma and mouse melanoma cells

Each of four kinds of teratocarcinoma cells, OTT6050P, PCC4, PSA1 and LT, derived from 129 or LT mouse strain, was fused with B16-CAPr melanoma cells derived from C57BL/6J by using Sendai virus. The resultant hybrids were morphologically melanotic melanoma cells which were larger and more heavily pigmented than the parental B16-CAPr melanoma cells. The chromosome analysis and GPI electrophoresis demonstrated that all hybrids were products of fusion between a single teratocarcinoma cell and a single melanoma cell. The pigmentation in the hybrids between a 129 teratocarcinoma cell and a melanoma cell was much stronger than that in hybrids between an LT teratocarcinoma cell and a melanoma cell. This phenomenon was consistent with the difference of coat color between 129 and LT mouse strain. From these results, it was suggested that the genes of teratocarcinoma cells involved in the pigmentation are activated in the hybrids with B16-CAPr melanoma cells.

Isolation and characterization of intraspecific cybrids. Effect of mitochondrial DNA on their cellular properties

Cybrid clones were obtained by fusing whole cells of rat glioma C6BU-1, resistant to 5-bromodeoxyuridine (BrdU), with cytoplasts of embryonic rat 3Y1CAP cells, resistant to chloramphenicol (CAP), in selective medium with BrdU and CAP. The clones resistant to BrdU and CAP were confirmed to be cybrids by chromosome and mtDNA analyses. More than half the mtDNA of all the cybrid clones was from the 3Y1CAP cells. After cultivation of a cybrid clone Y22 for 3 months in the absence of CAP, subclones were isolated. One subclone Y22-22 contained predominantly mitochondrial DNA (mtDNA) from the 3Y1CAP cells. Using this subclone, the effects of the mitochondrial genome on cellular properties were examined. The growth patterns, expression of glioma-specific beta-adrenergic receptor, and composition of the major proteins of C6BU-1 cells were not affected by transmitted mtDNA from the 3Y1CAP cells. This procedure for isolating cells containing predominantly foreign mtDNA will be useful in studies on the interaction between genomes of the mitochondria and nucleus.

Intracellular location of small circular DNA complexes in mammalian cell lines

For determination of the cellular location of small polydisperse circular DNA complexes, rat myoblastic L6 cells, HeLa cells, and mouse L cells were enucleated and processed by the micapress-adsorption method for electron microscopy (H. Yamagishi, T. Kunisada, and T. Tsuda, 1982, Plasmid 8, 299-306). Small circular DNA complexes from intact cells showed a heterogeneous size distribution of from 0.1 to more than 2 micron with a mean contour length of 0.6 to 0.8 micron, like that of covalently closed circular DNAs. Cells contained 400 to 1200 copies. The size distribution in the cytoplasts was narrow and the number-average length was 0.3 to 0.4 micron, whereas that in L6 karyoplasts was wide and the average length was 0.9 micron. The longer circular complexes appeared to be absent from the cytoplasts. The origin and biological functions of these complexes are discussed in relation to the cellular locations of the complexes.

Effect of mitochondrial DNA composition on the cellular properties of interspecific hybrid cells

Four subclones with single species of mitochondria and three subclones with both parental mitochondria were isolated from a mouse-rat hybrid cell line H2. The effects of the coexistence of different species of mitochondria on cellular properties were examined in these clones. Growth properties were studied by comparing the plating efficiencies and doubling times. The numbers of growing colonies and the doubling times of all the subclones were found to be almost the same, indicating that these growth properties were not affected by the presence of both mouse and rat mitochondria within the cells. The correlation between the expression of chloramphenicol (CAP)-resistance and the relative contents of mtDNA of CAP-resistant (CAPr) rat and CAP-sensitive (CAPs) mouse parent cells in the subclones were also examined. The expression of CAP resistance was measured as the relative plating efficiency. Subclones with a high content of mtDNA from CAPr rat parent cells showed high relative plating efficiency.

Identification of key regulated events early in the life of hybrid animal cells constructed by nuclear transplantation

Reconstituted cells were constructed by fusion of cytoplasts from the human diploid fibroblast cell strain Detroit 532 and karyoplasts from the mouse fibroblast cell line A9. Several cellular properties were examined during the first 48 hr after nuclear transplantation. (i) The overall morphology of the cells originally resembled that of the cytoplasmic donor, Detroit 532, but rapidly changed to approximate that of the nuclear donor, A9. However, definitive changes in the microfilament structure of the reconstituted cells were not seen until 24-48 hr after fusion. These observations support the idea that the presence or absence of an ordered array of microfilament bundles is not the sole determinant of cell shape. (ii) Although cytoplasts and karyoplasts were prepared from cultures of randomly growing cells, the first division of reconstituted cells occurred in a synchronous manner. However, the initiation of DNA synthesis was not synchronized. It thus appeared that, in their first cell cycle, the cells had a G2 period of variable length. The results further suggest that the cytoplasm of interphase fibroblasts contains the material necessary to initiate or support DNA synthesis in a transplanted nucleus but not entry into mitosis. (iii) A two-dimensional gel electrophoretic analysis of polypeptide synthesis in reconstituted cell cultures showed that synthesis directed by transplanted mouse nuclei could be detected as early as 3-6 hr after fusion. Some of the mouse polypeptides detected at the earliest time points studied were not among the major polypeptides synthesized by the parental A9 cells. By about 48 hr after fusion, the pattern of polypeptides produced by reconstituted cells was almost indistinguishable from that of the nuclear donor parent cells.