Correlation between sustained c-Jun N-terminal protein kinase activation and apoptosis induced by tumor necrosis factor-α in rat mesangial cells

Abstract

Rat mesangial cells are normally resistant to tumor necrosis factor-α (TNF-α)-induced apoptosis. In this report we show that the cells can be made susceptible to the apoptotic effect of TNF-α when pretreated with actinomycin D, cycloheximide, or vanadate. c-Jun N-terminal protein kinase (JNK) has been thought to mediate apoptotic processes elicited by some stimuli, but its involvement in TNF-α-induced apoptosis has been controversial. JNK activation was investigated under conditions where the mesangial cells were either resistant or susceptible to TNF-α-induced apoptosis. TNF-α alone stimulated a single transient JNK activity peak. However, when the cells were pretreated with actinomycin D or cycloheximide, TNF-α stimulated a second sustained JNK activity peak. When the cells were pretreated with the phosphatase inhibitor vanadate, TNF-α-induced JNK activation was greatly prolonged. In all three cases, a sustained JNK activation was associated with the initiation of apoptosis. Our data suggest that a sustained activation of JNX induced by these reagents may be associated with blocking the expression of a phosphatase that inactivates JNK. Further studies reveal that the expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) was induced by TNF-α, indicating that MKP-1 may be involved in protecting the cells from apoptosis by preventing a prolonged activation of JNK under normal conditions. Additional studies showed that extracellular signal-regulated protein kinase activation stimulated by TNF- α was unlikely to contribute to the resistance of mesangial cells to TNF-α cytotoxicity.