Effects of Growth State and 3 H Labeling Level on RNA Turnover in WI-38 Fibroblasts and HeLa Cells

Abstract

The rate of turnover of prelabeled RNA in WI-38 human diploid fibroblasts varied with the level of /sup 3/H incorporated, the cell density of cultures, and the arrest of growth by senescence. The half-life of RNA in sparse cultures of growing WI-38 diploid fibroblasts depended on the level of (/sup 3/H)uridine incorporated; extrapolated to zero levels of incorporation, the half-life was 15 to 20 days. At any level of incorporated (/sup 3/H)uridine, however, RNA half-life decreased to 4 to 5 days in superconfluent cultures as the culture growth slowed. A similar shortening of half-life was observed when growth was stopped by /sup 3/H irradiation or clonal senescence. However, the rate of turnover was not simply dependent on whether cells were growing; for example, turnover did not increase when growth was arrested by incubating cells in conditioned medium. HeLa and L cells also showed an RNA half-life of about 14 to 20 days with an increase in turnover rate of crowded cultures. However, this increase occurred at higher cell densities than with the diploid fibroblasts. Also, the growth rate and rate of RNA turnover of HeLa and L cells were much less affected by incorporated /sup 3/H. The differential responses tomore » confluence and /sup 3/H label can explain the higher turnover rate of RNA in normal human fibroblasts compared to SV40-transformed cells (S.A. Liebhaber, S. Wolf, and D. Schlessinger, Cell 13, 121-127 (1978)).« less