An early event in the response of cells to a wide variety of growth factors is the initiation of a protein kinase cascade referred to as the mitogen activated protein kinase (MAP kinase) pathway that has been evolutionarily conserved from yeast to mammals and is thought to play a critical role in the transduction of growth and differentiative signals from the membrane to the nucleus. The MAP kinase cascade involves the sequential activation of a mitogen activated protein kinase kinase kinase (MAPKKK), a mitogen activated protein kinase kinase (MAPKK) and MAP kinases (ERK1 and ERK2). Despite the significant progress that has been made in the molecular characterization of the MAP kinase pathway, its precise role in cellular responses remains unclear. Stimulation of lymphoid cells through a variety of surface receptors can result in clonal unresponsiveness, proliferation, functional differentiation or programmed cell death depending on which receptors are stimulated and differentiative state of the cell.
The aim of this proposal is to investigate the role of the MAP kinase pathway in signaling by lymphoid receptors that regulate these diverse cellular responses and thereby gain insight into the role of this pathway in the regulation of cellular activation, growth, and apoptosis. Since the guanine nucleotide binding proteins encoded by the c-ras proto-oncogenes function upstream of the MAP kinase pathway, we will study regulation of p21/ras, MAPKKK, MAPKK and MAP kinase activities using techniques already established or under active development in this laboratory. Regulation of the MAP kinase pathway in response to receptors that control lymphocyte activation and proliferation will be investigated by studying the effect of ligation of CD28, a structure expressed on the majority of T lymphocytes that delivers a co-stimulatory signal during activation via the T cell receptor (TCR) and the T cell growth factor interleukin 2 (IL- 2). We will also investigate regulation of the MAP kinase pathway by receptor stimuli that induce apoptosis in lymphoid cells. The role of the MAP kinase pathway in these signaling pathways will be addressed by overexpressing both wild type and mutant MAPKK (either inactive or constitutively active) in cell lines that are responsive to the appropriate stimuli. The role of the MAP kinase pathway in T lymphocyte development will be investigated by expression of wild-type and mutant MAPKKK as transgenes under the control of a T lymphocyte-specific promoter. It is hoped that the results of these studies will increase our understanding of the mechanisms by which receptors regulate the MAP kinase pathway and the role of this pathway in the diverse responses of lymphoid and non-lymphoid cells to receptor stimuli.
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