Heat shock effects on cell cycle progression.
In mammalian cells, short-term (acute) exposure to a moderate heat shock leads to a transient arrest of cells at mainly two cell cycle checkpoints, the G1/S and G2/M transitions. This is documented by the more or less synchronous resumption of cell cycle progression from these checkpoints during recovery. The reason for the accumulation of cells at these checkpoints may be found in activity thresholds of cyclin-dependent kinases (Cdks) at both transitions which are determined by (i) the amounts of the responsible cyclins, (ii) regulatory phosphorylation of the Cdks and (iii) the inhibition of Cdks by associated regulatory proteins (Ckis). All three regulatory systems may be subject to heat-shock-dependent changes, the amounts of Ckis, in particular, being increased. Cdk-dependent phosphorylation of the retinoblastoma protein and the subsequent release of active S-phase-specific transcription factors E2F/DP are considered as major heat-sensitive steps in cell cycle progression. Furthermore, high acute heat shock and long-term (chronic) heat treatment may lead to cell-type-specific forms of cell death. All types of responses to heat treatment are subject to adaptation after a 'priming' treatment, probably due to higher levels of heat shock proteins.