Studies on the growth of endocrine cells have identified two major pathways of regulating proliferation. These pathways are initiated by growth factor stimulation of receptor/tyrosine kinases and hormonal activation of G protein-coupled receptors. Recent data in our laboratory demonstrate that these pathways intersect at multiple levels. For example, hormones that elevate intracellular levels of cyclic adenosine monophosphate (cAMP) have a wide range of cell type-specific effects on cell growth and differentiation. MAP kinase activation mediates the mitogenic response to growth factors in a number of cell types including endocrine cells. We propose that cAMP's mitogenic actions require MAP kinase activation. We have identified a novel pathway mediating cAMP signals in neuroendocrine cells. We show data that one substrate of PKA, the GTPase Rap1, is an activator of MAP kinase in neuroendocrine cells and is activated by cAMP in these cells, and we propose that Rap1 dictates cAMP's activation of MAP kinase in these cells. We have identified the proximal target of Rap activation: the MAP kinase B-Raf. B-Raf is expressed in a subset of cells including neuronal and endocrine cells. We propose that its expression dictates cAMP's ability to activate MAP kinase in neuroendocrine cells. We will examine the mechanism of the activation of B-Raf by Rap and determine whether this pathway mediates cAMP's cell type-specific activation of MAP kinase. Specifically, we will test the hypothesis that Rap1 is a selective activator of B-Raf (Specific Aim 1), and will determine whether Rap1 mediates the actions of cAMP on MAP kinase in B-Raf-expressing cells (Specific Aim 2). Lastly, we will determine whether other physiological signals can activate this novel pathway (Specific Aim 3).
