ACTIVATION OF THE RAS SIGNALING PATHWAY BY THE CD40 RECEPTOR

The CD40 receptor is an important molecule regulating B lymphocyte pro liferation, maturation, Ab class switching, and cell survival, In the present study, we identified signal transduction events triggered by c ross-linking the CD40 receptor, Stimulation of Daudi B cells with anti -CD40 resulted in activation of p21(ras), an important switch point in the regulation of cell growth and differentiation. Ras activation cor related with a stimulation of Rad and MEK-1 as well as tyrosine phosph orylation of phosphatidylinositol 3-kinase, Inhibition of endogenous R as by transfection of transdominant inhibitory Ras prevented tyrosine phosphorylation or stimulation of phosphatidylinositol 3-kinase, Rad, or MEK-1 upon CD40 receptor triggering, proving an activation of the R as pathway by CD40, Ras activation was partially inhibited by either h erbimycin A or calphostin pretreatment and completely inhibited by pre incubation with a combination of both inhibitors, indicating a synergi stic role for protein tyrosine kinases and diglycerides in Ras activat ion after CD40 stimulation, In support of a role for diglycerides, we detected a 30 +/- 5% decrease of cellular phosphatidylcholine content, correlating with a threefold increase of diacylglycerol synthesis ind uced by CD40, Supporting a role for protein tyrosine kinase, we measur ed a five- to eightfold stimulation of p56(lyn) and p58(blk) kinase ac tivity, These results suggest the activation of the Ras pathway via an additive function of src kinases and phospholipases that may be impor tant in the mediation of biologic effects after CD40 receptor engageme nt.