Thermosensitivity of Nuclear RNA Polymerase during the in vitro Senescence of Mouse Fibroblasts

Embryonic mouse fibroblasts divide approximately twelve times in vitro prior to cessation of mitotic activity. During this period of cellular senescence the thermosensitivity of the RNA polymerase activity of isolated nuclei has been examined as a means of detecting the possible accumulation of defective enzyme molecules, as has been found by other workers for several cytoplasmic enzymes during the ageing of human fibroblasts in vitro. The total RNA polymerase activity of nuclei isolated from old (10th generation) cells is more thermoresistant than that of young (2nd generation) cells. However, the net RNA polymerase activity of nuclei from non-dividing (confluent) cells is more thermoresistant than that of exponentially growing cells of the same age. When allowance is made for the state of growth of the cultures, little difference is seens in the thermosensitivity of the activities of nuclei from old and young cells. Neither is there any difference between the thermosensitivity of the net activity of an established line of murine fibroblasts (L-cells) and cells in primary culture. Preheating nuclei increases the inhibition of their total RNA polymerase activity by or-α-amanitin, indicating that RNA polymerase II is the most heat resistance species present. There appears to be no difference between the thermosensitivity of the α-amanitin sensitive and resistance species of the enzyme in the nuclei of old and young cells. It is concluded that old cells resemble non-dividing young cells in containing a higher proportion of RNA polymerase II in their nuclei, resulting in greater thermoresistance of the total RNA polymerase activity over that of exponentially growing cells. However, there appears to be no increase in thermosensitivity of the enzymes with age.