Mechanism of Tetinoblastoma Protein-Mediated Terminal Cell Cycle Arrest

摘要

A prognostic feature of many human cancers is a high mitotic index and the inability to maintain a terminal cell cycle arrest (TCCA). The Rb gene product retinoblastoma protein has been implicated in the maintenance of a terminal cell cycle arrest. Likewise the inactivation of retinoblastoma gene (Rb) is observed in several human cancers including those of the breast. However, in contrast to our knowledge of how pRb regulates proliferation in a cycling population, little is known how it maintains a permanent cell cycle arrest. The proposed study was aimed at elucidating the molecular mechanism by which pRb accomplishes this task and plays the role of tumor suppressor of tumor formation. Our working hypothesis was that pRb in cooperation with basic helix loop helix (bHLH) protein MyoD participates in the transcriptional repression of one or more immediate early genes required for the induction of cyclin Dl. And this event ultimately prevents the re%entry into the cell cycle, thus maintaining a terminal cell cycle arrest. To test this hypothesis myogenic differentiation has been used as model, because it represents a differentiation system in which pRb has been implicated in a terminal cell cycle arrest both in vitro and in vivo. In this study I have established that, among various immediate early genes only the induction of Fra-1 is blocked in pRb and myoD dependent manner leading to cyclin Dl control and is independent of the cell cycle inhibitory protein pl 6. Whereas an ectopic expression of Fra-1 by adenovirus-vector which leads to induction of cyclin Dl in identical settings confirmed a causal relationship between Fra-1 expression and cyclin Dl. Further a biochemical analysis of the Fra-1 gene regulation using wild type promoter reporter (LUC) and a mutant for MyoD binding constructs confirmed the loss of MyoD binding to the promoter renders its ability to inhibit the Fra-1 induction.