INDUCTION OF STRESS-ACTIVATED PROTEIN-KINASES C-JUN N-TERMINAL KINASES BY THE P55 TUMOR-NECROSIS-FACTOR RECEPTOR DOES NOT REQUIRE SPHINGOMYELINASES

Ceramide has been implicated in the activation of stress-activated pro tein kinases/c-Jun N-terminal kinases (SAPK/JNK). Binding of tumour ne crosis factor (TNF) to its 55 kDa receptor (TR55) leads to the generat ion of ceramide through activation of either acid or neutral sphingomy elinase (A/N-SMase) as well as to potent activation of SAPK/JNK, We ha ve examined a putative role of both N- and A-SMase in the TR55-depende nt activation of SAPK/JNK. The analysis of TR55 deletion mutants expre ssed in 70Z/3 pre-B cells revealed that activation of SAPK/JNK occurs independently of N-SMase. Although both SAPK/JNK and A-SMase are activ ated by the death domain of TR55, pharmacological prevention of the TR 55-dependent activation of A-SMase, or proteolytic degradation of A-SM ase in 70Z/3 cells, did not impair SAPK/JNK activation, indicating tha t SAPK/JNK are not secondary to A-SMase. In addition, proteolytic degr adation of A-SMase also did not affect SAPK/JNK activation by ultravio let (UV-C) irradiation, arguing against a general role of A-SMase in s tress-mediated responses. Furthermore, fibroblasts from Niemann-Pick A patients deficient in A-SMase did not show altered activation of SAPK /JNK in response to either TNF or W-C. These results suggest that TR55 can activate SAPK/JNK without direct participation of sphingomyelinas es or ceramide.