Because they are long-lived and cycle continuously, adult stem cells (ASCs) are predicted as the most common precursor for cancers in adult mammalian tissues. Two unique attributes have been proposed to restrict the carcinogenic potential of ASCs. These are asymmetric self-renewal that limits their number and immortal DNA strand cosegregation that limits their accumulation of mutations due to DNA replication errors. Until recently, the molecular basis and regulation of these important ASC-specific functions were unknown. We developed engineered cultured cells that exhibit asymmetric self-renewal and immortal DNA strand cosegregation. These model cells were used to show that both ASC-specific functions are regulated by the p53 cancer gene. Previously, we proposed that IMP dehydrogenase (IMPDH) was an essential factor for p53-dependent asymmetric self-renewal. We now confirm this proposal and provide quantitative evidence that asymmetric self-renewal is acutely sensitive to even modest changes in IMPDH expression. These analyses reveal that immortal DNA strand cosegregation is also regulated by IMPDH and confirm the original implicit precept that immortal DNA strand cosegregation is specific to cells undergoing asymmetric self-renewal (i.e., ASCs). With IMPDH being the rate-determining enzyme for guanine ribonucleotide (rGNP) biosynthesis, its requirement implicates rGNPs as important regulators of ASC asymmetric self-renewal and immortal DNA strand cosegregation. An in silico analysis of global gene expression data from human cancer cell lines underscored the importance of p53-IMPDH-rGNP regulation for normal tissue cell kinetics, providing further support for the concept that ASCs are key targets for adult tissue carcinogenesis.

Although senescence is a defining property of euploid mammalian cells, its physiologic basis remains obscure. Previously, cell kinetics properties of normal tissue cells have not been considered in models for senescence. We now provide evidence that senescence is in fact the natural consequence of normal in vivo somatic stem cell kinetics extended in culture. This concept of senescence is based on our discovery that cells engineered to conditionally express the well-recognized tumor suppressor protein and senescence factor, p53, exhibit asymmetric cell kinetics. In vivo, asymmetric cell kinetics are essential for maintenance of somatic stem cells; ex vivo, the same cell kinetics yield senescence as a simple kinetic endpoint. This new “asymmetric cell kinetics model” for senescence suggests novel strategies for the isolation and propagation of somatic tissue stem cells in culture.

The predominant type of cell division in adult mammals is renewal growth. Renewing stem cells in somatic tissues undergo continuous asymmetric divisions. One new daughter cell retains the division potential of the original stem cell, while the other differentiates into a functional constituent of the tissue. Disruptions of this process lead to the development of human cancers. We show that through a guanine nucleotide-dependent mechanism, the p53 antioncogene can induce exponentially dividing cells to switch to an asymmetric stem cell growth pattern. This finding suggests that the observed high frequency of p53 mutations in human cancers reflects a critical function in the regulation of somatic renewal growth.The p53 antioncogene has the distinction of being the most commonly altered genetic locus in examined human tumors (1,2). The high frequency of p53 gene alterations in diverse cancers suggests that the antioncogene plays a critical role in cellular processes that are fundamental to the development of human neoplasia. Recently, a number of hypotheses have been advanced for the cellular function of the p53 antioncogene (3-12). We have proposed a role for p53 in the regulation of guanine nucleotide biosynthesis. In our studies, regulation of the rate-limiting enzyme for guanine nucleotide biosynthesis, inosine 5'-monophosphate dehydrogenase (IMPD; IMP:NAD oxidoreductase, EC 1.1.1.205), by p53 expression can account for growth inhibition by the antioncogene (13).Our investigations of p53 function have been performed in cells that contain a stably integrated, conditional p53 expression system (13,14). C127, an immortal, nontumorigenic mouse mammary epithelial cell line, shown to express endogenous wild-type p53 protein (13), is the parent for the p53-inducible cells. When such cells are maintained at 37°C, the p53 transgene shows minimal expression. Culture at 32.5°C causes a 2-to 3-fold elevation in wild-type p53 protein concentration, a degree of induction within the physiologic range of p53 expression in C127 cells (13).In this report, we describe an investigation of changes in the growth properties of our experimental cell lines in response to elevated p53 expression. We find that increased p53 expression induces a switch from exponential growth kinetics to division kinetics similar to those of renewing stem cells in vivo. In addition, we found that primary human cells of both fibroblast and epithelial origin exhibit similar, though not quantitatively identical, properties. Guanine nucleotide precursors, which rescue cells from p53-induced growth inhibition (13), prevent the switch to renewal growth kinetics. The described studies functionally link stem cell renewal and guanine nucleotide metabolism to one another and to the expression of p53. This relationship can account for many features of p53 gene exThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate ...