The regulation of de novo nucleotide synthesis is critical to the assembly of key intracellular building blocks such as the nucleic acid polymers DNA and RNA. Ultimately the rate of cell proliferation is determined by the rate of synthesis of these molecules; consequently these pathways have been targeted for therapeutic control of aberrant cell growth (cancer chemotherapy). However, the mechanisms by which normal growth signals control nucleotide synthesis are not well understood. In contrast, considerable progress has recently been achieved in the identification of growth factor-activated signaling pathways that contribute to cellular growth regulation. One such pathway is known as the mitogen-activated protein kinase (MAP kinase) cascade and is composed of protein kinases that regulate cellular signaling by reversible phosphorylation. The MAPK pathway is believed to play a central role in the determination of the rate of cellular proliferation and is activated by a multitude of growth promoting signals. Although the basic components of this pathway are known, many of the targets these regulations have yet to be elucidated. We recently found evidence for a link between MAP kinase signaling and the control of a key enzyme in uridine nucleotide biosynthesis known as CAD. CAD is composed of three enzymes carbamyl phosphate synthetase (CPS II), aspartate transcarbamylase and dihydrooratase. CAD catalyzes the rate- limiting step in uridine nucleotide synthesis in cells and the activity of this enzyme is increased in cells with increased growth rates (i.e. tumor cells). Therefore, the immediate objective of this proposal is to investigate at a biochemical level the involvement of MAP kinase in regulating CAD activity and the consequences of this regulation on de novo uridine nucleotide biosynthesis. The long-term goal of this study is to gain a greater understanding of the regulation of nucleotide biosynthesis by growth promoting signals. Eventually with the design of potent and selective inhibitors of growth factor-activated kinase signaling pathways, novel strategies for the regulation of nucleotide metabolism and cell growth may be achieved.
