The APC tumor suppressor controls entry into S-phase through its ability to regulate the cyclin D/RB pathway

Background & Aims: APC gene mutation is an early alteration in most colorectal tumors. In an attempt to determine its role in tumor development, we asked whether reintroducing wild-type APC into colorectal cancer cells with mutant APC affected cell cycle progression. Methods: Using transient transfection, a plasmid containing the APC complementary DNA and DNA encoding the green fluorescent protein was expressed in SW480 cells. In addition, several other constructs were co-expressed with APC to determine their combined effects. Results: We report that colorectal cancer cell lines transfected with wild-type APC arrest in the G₁phase of the cell cycle and that this arrest is abrogated by cotransfecting constitutively active β-catenin or cyclin D1 and cMYC together. This APC-induced cell cycle arrest involves the disruption of β-catenin-mediated transcription and depends on components of the G₁/S regulatory machinery, as overexpression of E1a or E2F-1, -2, or -3 overrides the G₁ arrest. Consistent with this, APC transfection inhibits RB phosphorylation and reduces levels of cyclin D1. Conclusions: Our results suggest that APC functions upstream of RB in the G₁/S regulatory pathway, cyclin D1 and cMYC affect APC-mediated arrest equivalently to oncogenic β-catenin, and most colon tumors disrupt control of G₁/S progression by APC mutation.