Effects of normal XXXX fibroblast mitochondrial DNA on segregation of HeLaTG Mitochondrial DNA and on tumorigenicity of HeLaTG cells.

@entity1 @entity19 @entity5
We isolated hybrids and cybrids using HeLaTG cells and @entity1 normal primary fibroblasts to examine the functional differences between the mitochondrial genomes of @entity5 and normal cells with respect to their possible involvement in the regulation of tumorigenicity. Hybrids contained mitochondrial DNA (mtDNA) predominantly from the fibroblast parent and their tumorigenicity was suppressed completely. Then, cytoplasmic transmission of primary fibroblast mtDNA to HeLaTG cells was carried out using toxin-antitoxin selection. Two cybrid clones containing a HeLaTG nucleus only and more than 60% of transmitted fibroblast mtDNA were isolated and injected into @entity19 to test their tumorigenicity. They formed @entity5 when 2 X 10(6) cells were injected, whereas no @entity5 were formed after injection of 5 X 10(5) cells (a concentration at which HeLaTG subclones formed @entity5 ). These cybrids were cultivated in normal medium for two additional months and the content of fibroblast mtDNA increased gradually, resulting in HeLaTG mtDNA eventually being lost from both cybrid clones. We again examined their tumorigenicity and found that they recovered tumorigenicity completely. These results indicate that tumorigenicity of HeLaTG cells could not be suppressed by replacing their mitochondrial genomes with those of normal primary fibroblasts. Further, the partial suppression of tumorigenicity observed in the cybrid clones was temporary and may be due to cytoplasmic factors other than the mitochondrial genomes. Although we can find no difference between the mitochondrial genomes of normal and @entity5 cells regarding the regulation of tumorigenicity, the segregation pattern of the mtDNA in the cybrids was of interest: in the absence of any mitochondrial selection, HeLaTG mtDNA was lost while fibroblast mtDNA was retained, even though the nuclear component of these cybrids was from the HeLaTG cells. Thus, there should be some functional differences between the mitochondrial genomes of HeLaTG cells and primary fibroblasts that are responsible for the preferential segregation of HeLaTG mtDNA from the cybrids.
