Factors Affecting the Heat-Induced Increase in Protein Content of Chromatin

Abstract

When HeLa cells are heated (40 to 48/sup 0/C), a time- and temperature-dependent increase in the chromatin protein content is observed. If isolated nuclei are heated (45/sup 0/C, 30 min) in medium with 7% calf serum, in 100% calf serum, or in a suspension of cell sonicates, no increase in the chromatin protein content is observed. When cells are permeabilized using Tween 80, a 70% increase in the protein content of chromatin occurs without hyperthermia. These results suggest that cellular structure is required for the heat-induced increase in chromatin protein content to occur and that the increase can be caused by membrane damage. When cells are heated (15 min at 45/sup 0/C) and returned to medium at 37/sup 0/C, they can restore the protein content of chromatin to control levels. Thus restitution of the chromatin protein content requires 3 h. Restitution is not inhibited by hydroxyurea (2 mM), excess thymidine (3 mM), actinomycin-D (5 ..mu..g/ml), lucanthone (55 ..mu..g/ml), 2,4-dimitrophenol (10 mM), ouabain (1 mM), or cyclohexamide (50 ..mu..g/m.). Thus major metabolic processes are not required for restitution. However, high (42/sup 0/C) and low (0/sup 0/C) temperatures inhibit the restitution process.