Urea induces the heat shock response in human neuroblastoma cells

Uremic encephalopathy is a complication of renal failure that reflects stresses exerted by as yet poorly defined uremic toxins. All cells respond to stresses by undergoing the "heat shock" response. Although urea kinetics and creatinine concentration are routinely used to assess dialysis adequacy, the roles of urea and creatinine as uremic toxins remain controversial. To investigate their potential roles in uremic encephalopathy, cultured human neuroblastoma cells (SK-N-SH) were exposed to 0.5 to 14 mg/dL creatinine, or to 20 to 200 mg/dL urea, or to mannitol, NaCl, or glycerol at equivalent osmolalities for 30 min to 48 h, and the induction of Hsp72 (heat shock) protein was used as a marker of cell stress. Although creatinine failed to elicit a heat shock response, urea in clinically relevant concentrations (40 to 200 mg/dL) induced it at 30 min. The response peaked at 10 h and returned to zero by 48 h. Cells exposed to equivalent osmolalities of mannitol, NaCl, or glycerol failed to exhibit this response. Protein extracts from cells exposed to urea showed significant carbamylation that increased as a function of time. These results demonstrate: (1) that urea is neurotoxic in vitro and that creatinine is not: (2) that the insult urea causes is not simply the result of hypertonicity; but rather (3) that urea, via breakdown to cyanate and ammonium ions, may cause cell stress because of its ability to cause carbamylation of cellular proteins. The cells attenuation of the heat shock response after 10 h of exposure to urea suggests that they can adapt to the presence of urea or carbamylation. This may explain, in part, why the same degree of azotemia causes fewer neurological symptoms in patients with chronic as opposed to acute renal failure.