Guanine nucleotide biosynthesis is regulated by the cellular p53 concentration

As an approach to defining the role of p53 in cellular proliferation, murine cell lines were derived which contain a stably transfected temperature-inducible p53 expression system. Cell lines derived with the system exhibited a 3-6-fold physiologic elevation in the cellular p53 concentration when grown at 32.5 degrees C. A p53 induction phenotype was defined by examination of the growth properties of these lines at 32.5 degrees C. The induction phenotype had three main features: 1) a 2-4-fold increase in doubling time and biphasic growth kinetics; 2) delayed early S phase transit; and 3) complete reversibility either by growth at 37 degrees C or by growth in the presence of added hypoxanthine or xanthosine. The reversal of the induction phenotype by these purine salvage precursors implicated the purine nucleotide biosynthetic pathway as the cellular target for the antiproliferative action of p53. Subsequent genetic and biochemical analyses identified a p53 induction-related purine pathway defect which was localized to the step of inosine 5'-monophosphate conversion to xanthosine 5'-monophosphate. This enzymatic step catalyzed by inosine 5'-monophosphate dehydrogenase (EC 1.2.1.14) is the rate-limiting step for GTP synthesis. Extracts from p53-inducible cells growing at the induction temperature show a specific reduction in inosine 5'-monophosphate dehydrogenase enzymatic activity. These findings define p53 as a cellular regulator of the synthesis of GTP, a key regulatory nucleotide for many important cellular processes. Moreover, observations of the growth behavior of p53-inducible cells suggest that by regulating the production of GTP, p53 can control cellular quiescence.