Cell Cycle Arrest by Transforming Growth Factor β1 near G₁/S Is Mediated by Acute Abrogation of Prereplication Complex Activation Involving an Rb-MCM Interaction

Abstract

Understanding inhibitory mechanisms of transforming growth factor beta1 (TGF-beta1) has provided insight into cell cycle regulation and how TGF-beta1 sensitivity is lost during tumorigenesis. We show here that TGF-beta1 utilizes a previously unknown mechanism targeting the function of prereplication complexes (pre-RCs) to acutely block S-phase entry when added to cells in late G(1), after most G(1) events have occurred. TGF-beta1 treatment in early G(1) suppresses Myc and CycE-Cdk2 and blocks pre-RC assembly. However, TGF-beta1 treatment in late G(1) acutely blocks S-phase entry by inhibiting activation of fully assembled pre-RCs, with arrest occurring prior to the helicase unwinding step at G(1)/S. This acute block by TGF-beta1 requires the function of Rb in late G(1) but does not involve Myc/CycE-Cdk2 suppression or transcriptional control. Instead, Rb mediates TGF-beta1 late-G(1) arrest by targeting the MCM helicase. Rb binds the MCM complex during late G(1) via a direct interaction with Mcm7, and TGF-beta1 blocks their dissociation at G(1)/S. Loss of Rb or overexpression of Mcm7 or its Rb-binding domain alone abrogates late-G(1) arrest by TGF-beta1. These results demonstrate that TGF-beta1 acutely blocks entry into S phase by inhibiting pre-RC activation and suggest a novel role for Rb in mediating this effect of TGF-beta1 through direct interaction with and control of the MCM helicase.