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 (Ca-i), and dissociation of Na+-K+-ATPase from the cytoskeleton al ong with initiation of a stress response. We examined changes in Ca-i, 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 det ermine whether initiation of the stress response was related to any one of these perturbations alone. Ca-i increased first at 75% of control ATP. Trit on X-100 solubility of Na+-K+-ATPase increased below 70% control ATP. Reduc ing cellular ATP below 50% control consistently activated HSF. Stepped decr ements in cellular ATP below the respective thresholds caused incremental i ncreases in Ca-i, Na+-K+-ATPase solubility, and HSF activation. ATP depleti on 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 deple tion. Progressive accumulation of nonnative proteins may be the critical si gnal for the adaptive induction of the stress response in renal epithelia.