Diacylglycerol kinase eta augments C-Raf activity and B-Raf/C-Raf heterodimerization

The Ras/B-Raf/c-Raf/MEK/ERK signaling cascade is critical for the control of many fundamental cellular processes, including proliferation, survival, and differentiation. This study demonstrated that small interfering RNA-dependent knockdown of diacylglycerol kinase eta (DGKeta) impaired the Ras/B-Raf/c-Raf/MEK/ERK pathway activated by epidermal growth factor (EGF) in HeLa cells. Conversely, the overexpression of DGKeta1 could activate the Ras/B-Raf/c-Raf/MEK/ERK pathway in a DGK activity-independent manner, suggesting that DGKeta serves as a scaffold/adaptor protein. By determining the activity of all the components of the pathway in DGKeta-silenced HeLa cells, this study revealed that DGKeta activated c-Raf but not B-Raf. Moreover, this study demonstrated that DGKeta enhanced EGF-induced heterodimerization of c-Raf with B-Raf, which transmits the signal to c-Raf. DGKeta physically interacted with B-Raf and c-Raf, regulating EGF-induced recruitment of B-Raf and c-Raf from the cytosol to membranes. The DGKeta-dependent activation of c-Raf occurred downstream or independently of the already known c-Raf modifications, such as dephosphorylation at Ser-259, phosphorylation at Ser-338, and interaction with 14-3-3 protein. Taken together, the results obtained strongly support that DGKeta acts as a novel critical regulatory component of the Ras/B-Raf/c-Raf/MEK/ERK signaling cascade via a previously unidentified mechanism.