Autoradiographic studies of nicotinic acid utilization in human-mouse heterokaryons and inhibition of utilization in newly-formed hybrid cells

Formation and fate of heterokaryons involving embryonal carcinoma cells

We have shown that murine embryonal carcinoma (EC) cells will readily fuse to non-EC cells following treatment with polyethylene glycol (PEG). Heterokaryons were detected by autoradiography and by the induction of premature chromosome condensation (PCC). Those heterokaryons formed between EC cells and certain of the non-EC cell types did not proliferate to form viable hybrids. For example, hybrids were not recovered from EC/HeLa heterokaryons in spite of the fact that both EC/3T3 and HeLa/3T3 heterokaryons gave rise to hybrids at high frequency. No hybrids were recovered from fusion mixtures involving Chinese hamster (CHO) cells and EC; however, if the differentiated progeny of the EC cells were used, hybrids were recovered. These results suggest that both developmental and phylogenetic differences between parental cells may contribute to the ability of heterokaryons to proliferate indefinitely.

Kinetics of human chromosome loss from 3T3-human hybrid cells

Human chromosomes were lost from populations of 3T3-HeLa and 3T3-diploid human fibroblast (HF) hybrids with similar first-order kinetics. Whereas loss began immediately in 3T3-HF hybrids, there was a lag of 5-10 cell divisions before chromosome loss began in 3T3-HeLa hybrids. Human chromosome loss was not affected by aminopterin selection, the use of polyethylene glycol rather than Sendai virus as fusagen, or by the presence of one or two 3T3 genomes. However, when cell division was retarded by growing 3T3-HF hybrids in low serum or at low temperatures, fewer human chromosomes were lost. This suggests that cell cycle traverse is important in chromosome loss. The distribution of human chromosomes among hybrid metaphases indicated that gradual chromosome loss occurred in all hybrids rather than extensive loss from a portion of the hybrids. During the period of chromosome loss, increased numbers of individual asynchronously condensed human chromosomes were randomly distributed among hybrid metaphases.

Studies on the intranuclear distribution of human and mouse genomes and formation of human-mouse hybrid cells

Autoradiographic examination of early hybrid cells formed by the fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells revealed that human and mouse chromosomes are often separated within metaphase and interphase nuclei. Although the marked separation of human and mouse chromosomes progressively disappeared with succeeding cell division, the occurrence of sectored nuclei in 16-cell hybrid colonies and the labeling pattern of human chromosomes within separated metaphases indicate that separation of human and mouse chromosomes may persist through several mitoses. Fusion of 3H-thymidine labeled D98/AH2 cells and 3T3-4E cells was coupled with aminopterin selection to study other aspects of hybrid cell formation. Hybrid cells arose from heterokaryons by cell division. Nuclear fusion may have occurred in two out of several thousand heterokaryons examined. However, these could reflect the close apposition of adjacent nuclei. A large fraction (greater than 0.5) of D98/3T3 heterokaryons underwent at least one cell division. However, the number of hybrid colonies containing more than eight cells at ten days following fusion was about 0.03 of the total number of heterokaryons. Many hybrid colonies arrested growth before the 8-cell stage, and the cells in such colonies exhibited nuclear abnormalities.