Thresholds for cellular disruption and activation of the stress response in renal epithelia.

Renal ischemia causes a rapid fall in cellular ATP, increased intracellular calcium (Cai), and dissociation of Na+-K+-ATPase from the cytoskeleton along with initiation of a stress response. We examined changes in Cai, Na+-K+-ATPase detergent solubility, and activation of heat-shock transcription factor (HSF) in relation to graded reduction of ATP in LLC-PK1 cells to determine whether initiation of the stress response was related to any one of these perturbations alone. Cai increased first at 75% of control ATP. Triton X-100 solubility of Na+-K+-ATPase increased below 70% control ATP. Reducing cellular ATP below 50% control consistently activated HSF. Stepped decrements in cellular ATP below the respective thresholds caused incremental increases in Cai, Na+-K+-ATPase solubility, and HSF activation. ATP depletion activated both HSF1 and HSF2. Proteasome inhibition caused activation of HSF1 and HSF2 in a pattern similar to ATP depletion. Lactate dehydrogenase release remained at control levels irrespective of the degree of ATP depletion. Progressive accumulation of nonnative proteins may be the critical signal for the adaptive induction of the stress response in renal epithelia.