Increase of thermoresistance after growth stimulation of resting Reuber H35 hepatoma cells : Alteration of nuclear characteristics, non-histone chromosomal protein phosphorylation and basal heat shock protein synthesis

In this paper we demonstrate an increase in thermoresistance of resting Reuber H35 cells upon growth stimulation by serum-containing medium: late G1/early S-phase cells were thermoresistant as compared with G0 phase cells. Increase of thermoresistance during early cell cycle runs parallel with increased tolerance of structural and molecular properties of the cell nucleus. Nuclear shape and chromatin structuring became thermotolerant as determined by geometric and densitometric analysis of Feulgen-stained nuclei. Moreover, increased tolerance was demonstrated by means of the capability for endogenous phosphorylation of isolated non-histone chromosomal proteins (NHCPs). We discuss the molecular basis for this increased thermoresistance after growth stimulation and make a comparison with induction of 'acquired thermotolerance' such as has been observed in studies on fractionated hyperthermia. Both after growth stimulation and after heat shock, an increase of endogenous phosphorylation capacity of isolated NHCPs was observed, while a main enhancement of phosphorylation was found for a NHCP of Mr 95,000. Moreover, the basal synthesis of proteins inducible by heat shock (heat shock proteins) and indicated as HSP65, HSP68 and HSP84 was enhanced in thermoresistant late G1/early S phase cells as compared with thermo-sensitive G0 phase cells. A role for chromatin structuring, NHCP phosphorylation and HSPs in the regulation of thermosensitivity and cell cycling is discussed.