Protection of renal inner medullary epithelial cells from apoptosis by hypertonic stress-induced p53 activation

Acute hypertonicity causes cell cycle delay and apoptosis in mouse renal in ner medullary collecting duct cells (mIMCD3) and increases GADD45 expressio n. Be cause the tumor suppressor protein p53 may be involved in these effec ts, we have investigated the role of p53 in mIMCD3 response to hyperosmotic stress. Acute elevation of osmolality with NaCl addition from the control level of 320 mosmol/kg to 500-600 mosmol/kg greatly increased the levels of total and Ser(15)-phosphorylated p53 within 15 min. However, similar eleva tion of osmolality with urea did not increase p53 levels. Our studies indic ate that induced p53 is transcriptionally active because NaCl addition to 5 00-600 mosmol/kg stimulated transcription of a luciferase reporter containi ng a p53 consensus element and appropriately altered mRNA levels of known t ranscriptional targets of p53, i.e. increased MDM-2 and decreased BCL-2 lev els. Elevating NaCl further to 700-800 mosmol/kg rapidly killed most of the cells by apoptosis, At these higher NaCl concentrations, p53 levels were f urther increased although Ser(15) phosphorylation and transcriptional activ ity were significantly lower than levels at 500-600 mosmol/kg. At NaCl-indu ced 500 mosmol/kg, apoptosis was rare in the presence of control, nonspecif ic oligonucleotide but highly prevalent upon addition of p53 antisense olig onucleotide that substantially reduced p53 levels. We conclude that inducti on of active p53 in mIMCD3 cells by hypertonic stress contributes to cell s urvival.