P53 AND PRB PREVENT REREPLICATION IN RESPONSE TO MICROTUBULE INHIBITORS BY MEDIATING A REVERSIBLE G(1) ARREST

Cell cycle checkpoints are safeguards that ensure the initiation of do wnstream events only after completion of upstream processes. The tumor suppressors p53 and pRb prevent initiation of a second round of repli cation in response to spindle inhibitors, but it has yet to be proven that this is a mitotic checkpoint response, We show that asynchronous human fibroblasts arrest in G(1) with 4 N DNA content after nocodazole treatment, whereas isogenic p53- and pRb-deficient fibroblasts rerepl icate. Importantly, nocodazole elicits a reversible arrest in G(0)-G(1 ) synchronized normal human fibroblasts but not in isogenic p53-defici ent derivatives. Furthermore, the G(1) cyclin-dependent kinase inhibit ors p21 and p16 also play critical roles in limiting rereplication. He nce, p53 and pRb are required during G(1) to prevent entry into a repl icative cycle and appear to provide a connection between the structura l integrity of the microtubules and the cell cycle machinery in interp hase cells.