Inhibition of the Raf-1 kinase by cyclic AMP agonists causes apoptosis of v-abl-transformed cells.
Here we investigate the role of the Raf-1 kinase in transformation by the v-abl oncogene. Raf-1 can activate a transforming signalling cascade comprising the consecutive activation of Mek and extracellular-signal-regulated kinases (Erks). In v-abl-transformed cells the endogenous Raf-1 protein was phosphorylated on tyrosine and displayed high constitutive kinase activity. The activities of the Erks were constitutively elevated in both v-raf- and v-abl-transformed cells. In both cell types the activities of Raf-1 and v-raf were almost completely suppressed after activation of the cyclic AMP-dependent kinase (protein kinase A [PKA]), whereas the v-abl kinase was not affected. Raf inhibition substantially diminished the activities of Erks in v-raf-transformed cells but not in v-abl-transformed cells, indicating that v-abl can activate Erks by a Raf-1-independent pathway. PKA activation induced apoptosis in v-abl-transformed cells while reverting v-raf transformation without severe cytopathic effects. Overexpression of Raf-1 in v-abl-transformed cells partially protected the cells from apoptosis induced by PKA activation. In contrast to PKA activators, a Mek inhibitor did not induce apoptosis. The diverse biological responses correlated with the status of c-myc gene expression. v-abl-transformed cells featured high constitutive levels of expression of c-myc, which were not reduced following PKA activation. Myc activation has been previously shown to be essential for transformation by oncogenic Abl proteins. Using estrogen-regulated c-myc and temperature-sensitive Raf-1 mutants, we found that Raf-1 activation could protect cells from c-myc-induced apoptosis. In conclusion, these results suggest (i) that Raf-1 participates in v-abl transformation via an Erk-independent pathway by providing a survival signal which complements c-myc in transformation, and (ii) that cAMP agonists might become useful for the treatment of malignancies where abl oncogenes are involved, such as chronic myeloid leukemias.
Weissinger
E M
EM
Institut für Klinische Hämatologie, GSF, Hamatologikum, Munich, Germany.
Eissner
G
G
Grammer
C
C
Fackler
S
S
Haefner
B
B
Yoon
L S
LS
Lu
K S
KS
Bazarov
A
A
Sedivy
J M
JM
Mischak
H
H
Kolch
W
W
D016475:3T3 Cells; D000483:Alleles; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002472:Cell Transformation, Viral / Q000187:drug effects; D000242:Cyclic AMP / Q000819:agonists* / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D051379:Mice; D008954:Models, Biological; D016374:Oncogene Proteins v-abl / Q000494:pharmacology*; D027384:Oncogene Proteins v-raf; D010641:Phenotype; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf; D015682:Retroviridae Proteins, Oncogenic / Q000378:metabolism
D016374:Oncogene Proteins v-abl; D011518:Proto-Oncogene Proteins; D016271:Proto-Oncogene Proteins c-myc; D015682:Retroviridae Proteins, Oncogenic; D000242:Cyclic AMP; D011505:Protein-Tyrosine Kinases; D027384:Oncogene Proteins v-raf; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D017868:Cyclic AMP-Dependent Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases