Conditionally Oncogenic Forms of the A-Raf and B-Raf Protein Kinases Display Different Biological and Biochemical Properties in NIH 3T3 Cells

Abstract

The protein kinase domains of mouse A-Raf and B-Raf were expressed as fusion proteins with the hormone binding domain of the human estrogen receptor in mammalian cells. In the absence of estradiol, 3T3 and rat1a cells expressing ΔA-Raf:ER and ΔB-Raf:ER were nontransformed, but upon the addition of estradiol the cells became oncogenically transformed. Morphological oncogenic transformation was more rapid and distinctive in cells expressing ΔB-Raf:ER compared with cells expressing ΔA-Raf:ER. Biochemical analysis of cells transformed by ΔA-Raf:ER and ΔB-Raf:ER revealed several interesting differences. The activation of ΔB-Raf:ER consistently led to the rapid and robust activation of both MEK and p42/p44 MAP kinases. By contrast, the activation of ΔA-Raf:ER led to a weak activation of MEK and the p42/p44 MAP kinases. The extent of activation of MEK in cells correlated with the ability of the different Raf kinases to phosphorylate and activate MEK1 in vitro. ΔB-Raf:ER phosphorylated MEK1 approximately 10 times more efficiently than ΔRaf-1:ER and at least 500 times more efficiently than ΔA-Raf:ER under the conditions of the immune-complex kinase assays. These results were confirmed with epitope-tagged versions of the Raf kinase domains expressed in insect cells. The activation of all three ΔRaf:ER proteins in 3T3 cells led to the hyperphosphorylation of the resident p74^raf-1 and mSOS1 proteins, suggesting the possibility of “cross-talk” between the different Raf kinases and feedback regulation of intracellular signaling pathways. The activation of either ΔB-Raf:ER or ΔRaf-1:ER in quiescent 3T3 cells was insufficient to promote the entry of the cells into DNA synthesis. By contrast, the activation of ΔA-Raf:ER in quiescent 3T3 cells was sufficient to promote the entry of the cells into S phase after prolonged exposure to β-estradiol. The ΔRaf:ER system has allowed us to reveal significant differences between the biological and biochemical properties of oncogenic forms of the Raf family of protein kinases. We anticipate that cells expressing these proteins and other estradiol-regulated protein kinases will be useful tools in future attempts to unravel the complex web of interactions involved in intracellular signal transduction pathways.