Repositioning of human interphase chromosomes by nucleolar dynamics in the reverse transformation of HT1080 fibrosarcoma cells.

An experimental system which should be valuable for studying the role of spatial positioning of the nuclear genome in human cell function has been developed. Reverse transformation of the malignant HT1080 fibrosarcoma cell line upon treatment with 8-chloro-cAMP results in growth inhibition, cytoskeletal reorganization, changes in nuclear shape and chromatin accessibility, and formation of prominent nucleoli. Fluorescent in situ hybridization was used to study DNA positioning during nuclear remodeling. Morphometric analysis of the hybridization sites for both repetitive sequences and "painting probes" for whole chromosomes indicated dispersal of acrocentric chromosomes in untreated cells and a highly organized central location of these ribosome gene-containing chromosomes in association with one or a few large nucleoli in nondividing treated cells. The results suggest that there was a directed movement of interphase chromosomes during a response which normalized a malignant cell line. These large-scale repositionings may serve two functions in restoring a normal transcriptional setup to the nucleus. First, ribosome genes are placed in the nucleolus, their transcriptional suborganelle. Second, nucleolar anchorings together with additional perinucleolar centromeric associations orient the domain shapes of entire chromosomes, installing gene-rich chromosomal regions into pockets of (accessible) DNAse I-sensitive chromatin populated by spliceosomes.