Immortalization of human lymphocytes by transfection with DNA from mouse L929 cytoplasts

Study of cytotoxic and therapeutic effects of stable and purified silver nanoparticles on tumor cells

We have synthesized and purified silver nanoparticles (Ag NPs) (11.3+/-2.3 nm) that are stable (non-aggregated) in cell culture medium and inside single living cells. We have developed new imaging methods to characterize sizes and number of single NPs in the medium and in single living cells in real-time and determine their stability (non-aggregation) in the medium and in single living cells at single NP resolution. These new approaches allow us to study toxic and therapeutic effects of single Ag NPs on tumor cells (L929, mouse fibroblast cells) with determined sizes and concentrations (doses) of NPs over time at single NP and single cell resolution. We found that Ag NPs inhibited the growth and division of tumor cells and their nuclei, in a dose and time dependent manner, showing significant inhibitory effects and abnormal cells with giant undivided nuclei or multiple nuclei beyond 12 h incubation. The results show that Ag NPs inhibited the segregation of chromosomes, but not their replication. Intracellular Ag NPs were well distributed in the cell population, and located in the nuclei and cytoplasm with higher numbers in the cytoplasm. This study demonstrates the possibility of using Ag NPs to inhibit the growth and division of tumor cells and using their cytotoxicity for potential therapeutic treatments. This study offers a new method to count the number of single NPs in the medium for characterization of their concentration and stability at single NP resolution over time.

Transportin mediates nuclear entry of DNA in vertebrate systems

Delivery of DNA to the cell nucleus is an essential step in many types of viral infection, transfection, gene transfer by the plant pathogen Agrobacterium tumefaciens and in strategies for gene therapy. Thus, the mechanism by which DNA crosses the nuclear pore complex (NPC) is of great interest. Using nuclei reconstituted in vitro in Xenopus egg extracts, we previously studied DNA passage through the nuclear pores using a single-molecule approach based on optical tweezers. Fluorescently labeled DNA molecules were also seen to accumulate within nuclei. Here we find that this import of DNA relies on a soluble protein receptor of the importin family. To identify this receptor, we used different pathway-specific cargoes in competition studies as well as pathway-specific dominant negative inhibitors derived from the nucleoporin Nup153. We found that inhibition of the receptor transportin suppresses DNA import. In contrast, inhibition of importin beta has little effect on the nuclear accumulation of DNA. The dependence on transportin was fully confirmed in assays using permeabilized HeLa cells and a mammalian cell extract. We conclude that the nuclear import of DNA observed in these different vertebrate systems is largely mediated by the receptor transportin. We further report that histones, a known cargo of transportin, can act as an adaptor for the binding of transportin to DNA.

Short DNA sequences from the cytoplasm of mouse tumor cells induce immortalization of human lymphocytes in vitro

Cytoplasts of mouse L929 and Ehrlich ascites tumor cells harbor DNA sequences that induce unlimited proliferation ("immortalization") of human lymphocytes after transfection in vitro. By equilibrium centrifugation of cytoplasmic lysates in a neutral CsCl gradient, the immortalizing activity was recovered together with extramitochondrial fractions at high salt densities (1.85-1.87 g/cm3). Unexpectedly, these fractions contain linear DNA molecules of 50-500 bp in length. In contrast, cytoplasts of primary, senescent cells (mouse embryo fibroblasts, human lymphocytes) do not harbor DNA in the corresponding fractions. Cytoplasmic DNA isolated from high-density fractions of mouse tumor cells was cloned in subset libraries, and of 45 DNA sequences we identified 2 clones--one from L929 cytoplasts (203 bp) and another one from the cytoplasm of Ehrlich ascites cells (372 bp)--that induce unlimited proliferation of human lymphocytes in vitro. Immortalized lymphoid cells harbor 1-5 copies of transfected DNA integrated into chromosomal DNA, whereas about 100 copies were found as episomal DNA in the cytoplasmic fraction. No immortalization could be induced by transfection of nuclear DNA randomly fragmented to 200-500 bp. Although the cloned DNA sedimented at 1.70 g/cm3, after transient transfection into lymphocytes, these DNA sequences form salt-stable complexes that sediment in fractions at the same high density (1.82-1.88 g/cm3) from which they were originally cloned. The high-density banding of these cytoplasmic DNA sequences may be due to association with RNA and/or with (metallo-) proteins in vivo. Since both cloned DNA sequences with immortalizing activity have stop codons for protein translation in all possible reading frames, immortalization may be induced by insertional inactivation or functional suppression of genes that are needed to be expressed during cellular senescence or programmed cell death.

Re-organization of the immunoglobulin kappa gene on both alleles is not an obligatory prerequisite for Ig lambda gene expression in human cells

Re-arrangement and expression of the immunoglobulin (Ig) genes in B cells occurs under an ordered developmental control. The sequential model of Ig light chain exclusion predicts that only after non-productive re-organization or deletion of both Ig kappa alleles would re-arrangement of Ig lambda gene segments occur. To prove this model, we asked whether expression of Ig lambda light chains is always associated with rearrangement and/or deletion of both Ig kappa alleles in human cells. Therefore, we established human diploid B-cell clones in vitro that produce Ig lambda light chains. Southern blot analysis of the Ig kappa alleles revealed that three Ig lambda expressing cell lines (out of six Ig lambda+ cell lines tested) harbour one Ig kappa allele in germline configuration. Furthermore, a 1.5 kb RNA derived from the germline Ig kappa locus was detected by Northern blot hybridizations. The results implicate that the mechanism of Ig light chain exclusion is not precisely sequential and that it does not necessarily need re-arrangement or deletion of both Ig kappa alleles as a prerequisite for Ig lambda light chain expression in human cells.

Factors influencing transient expression in cytotoxic T cells following DEAE dextran-mediated gene transfer

A number of transfection protocols have been tested for the introduction of exogenous DNA into cytotoxic T cells. These included electroporation, lipofection, calcium phosphate coprecipitation, polybrene-assisted gene transfer, and DEAE dextran-mediated transfer. Only the latter gave significant and reproducible transfection efficiencies coupled with low toxicity. The DEAE dextran protocol was optimized for the transfection of a transcription reporter construct pRSVcat into a cloned cytotoxic cell line. Among the parameters investigated were cell density, amount of input DNA, concentration of DEAE dextran, DNA adsorption time, temperature, use of permeabilization and expression facilitators, and recovery time. The optimized protocol was then used to demonstrate the presence of cis-acting regulatory regions in the 5'-flanking sequences of two cytotoxic cell-specific serine protease genes and, in addition, was shown to be applicable to other cloned T-cell lines.

The size of small polydisperse circular DNA (spcDNA) in angiofibroma-derived cell cultures from patients with tuberous sclerosis (TSC) differs from that in fibroblasts

Cell cultures were derived from angiofibromas of three patients with tuberous sclerosis (TSC), from the unaffected skin of these patients, and from the skin of five healthy donors. The length distributions of the small polydisperse circular DNA (spcDNA) fraction of these cell cultures were then analyzed. Nearly half the spcDNA molecules from the angiofibroma cultures were longer than 0.4 micron, whereas only about 7% exceeded this threshold in the spcDNA preparations from the skin fibroblast cultures. The percentage of the larger size class of spcDNA showed an increase at higher numbers of in vitro passages in all three types of cultures, but this effect was much more conspicuous in the angiofibroma-derived cultures than in those from the skin fibroblasts. An age-dependent increase in the overall amount of spcDNA was only seen in the angiofibroma-derived cultures. Our earlier finding of elevated amounts of spcDNA in angiofibroma cultures was confirmed in cultures from an additional TSC patient.