Dynamic studies of signaling pathways reveal backwards propagation of information: implications in cancer

LAUREN VAN WASSENHOVE; PENG JIANG; ALEJANDRA C VENTURA; SOFIA D MERAJVER; ALEXANDER J NINFA

Conferencia; 2009 AACR Annual Meeting; 2009

Signaling pathways are the main structures whereby cells transmit information from the outside in and vice-versa.Derangements in key pathways are involved in malignant transformation and tumor progression. In spite of extensiveknowledge about signaling in cell growth, cell death, and invasion, for example, very little is known about their dynamicsand how they may be altered in disease. We hypothesized that many aspects of cancer biology, such as the metabolismof the cancer cell, are dependent on the dynamics of signaling pathways. As a paradigm, we have employed a wellcharacterized model to study pathway dynamics and response to stimulus. The nitrogen assimilation cycle in Escherichiacoli provides a good intermediate step in which to develop the theoretical and experimental frameworks needed to reacha better understanding of signaling in complex mammalian systems. Based on our previous theoretical work, wepredicted that information flows bidirectionally in signaling pathways, a phenomenon termed retroactivity.We used an experimental in vitro reconstituted system to measure the dynamic response to stimulus and a theoreticalapproach of ordinary differential equations and sensitivity analysis to characterize the dynamic behavior and makepredictions. PII is a homotrimeric signal transduction protein highly conserved in nature that can be modified by theaddition of up to three uridylyl groups. We used alpha-32P labeled UTP to quantitatively measure the covalentmodifications of PII over time. The addition and removal of uridylyl groups on PII is catalyzed by a single enzyme (UTase)which has uridylyl transferase (UT) and uridylyl removing (UR) activities. The modification of PII is regulated by glutamine,which activates the UR activity and inhibits the UT activity of UTase. By varying glutamine and the proportions of thedifferent system components, we generated dynamic data that we were able to model to understand the effects ofsignaling on downstream targets. These experiments were the first ever performed to test and quantify this novel,groundbreaking hypothesis of retroactivity in a fundamental cell signaling system.In order to study retroactivity in this pathway, the monocycle composed of PII/PII-UMP and UT/UR was allowed to reachsteady state with and without downstream protein NRII. The NRII dimer can bind to PII, thus sequestering it away fromthe PII/PII-UMP and UT/UR monocycle, causing a change in the ability of PII to signal. By studying this phenomenon, wewere able to quantify retroactive behavior and found that it indeed occurs.Since aberrant signaling drives cancer progression, these results, with their theoretical framework can be employed tomake predictions of outcomes for testing drug targets in a new light. Our team of biochemists, physicists,mathematicians, and physicians is an example of a new multidisciplinary team in translational cancer research.