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 casca de comprising the consecutive activation of Mek and extracellular-sign a-regulated kinases (Erks). In v-abl-transformed cells the endogenous Raf-1 protein was phosphorylated on tyrosine and displayed high consti tutive kinase activity. The activities of the Erks were constitutively elevated in both v-raf- and v-abl-transformed cells. In both cell typ es 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 sub stantially diminished the activities of Erks in v-raf-transformed cell s but not in v-abl-transformed cells, indicating that v-abl can activa te Erks by a Raf-1-independent pathway. PKA activation induced apoptos is in v-abl-transformed cells while reverting v-raf transformation wit hout severe cytopathic effects. Overexpression of Raf-1 in v-abl-trans formed cells partially protected the cells from apoptosis induced by P KA 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 hig h constitutive levels of expression of c-myc, which were not reduced f ollowing PKA activation. Myc activation has been previously shown to b e essential for transformation by oncogenic Abl proteins. Using estrog en-regulated c-myc and temperature-sensitive Raf-1 mutants, we found t hat 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 surv ival signal which complements c-myc in transformation, and (ii) that c AMP agonists might become useful for the treatment of malignancies whe re abl oncogenes are involved, such as chronic myeloid leukemias.