The role of FOXK1/K2 transcription factors in adenovirus 5 E1A functions

Adenovirus (Adv) is a model DNA tumor virus that has been widely used to decipher critical pathways of oncogenesis. The adenovirus early gene E1A is an intensely investigated viral oncogene and has been instrumental in uncovering common cell cycle-regulatory pathways shared by other DNA tumor virus oncogenes. E1A promotes cellular entry into S phase by deregulating the cell cycle and activates other early viral genes to facilitate viral replication. As a consequence of nonproductive infection, E1A can oncogenically transform rodent cells in cooperation with other viral or cellular oncogenes. E1A stimulates cell proliferation and inhibits differentiation. These activities are primarily linked to the N-terminal region (exon 1) of E1A, which interacts with multiple cellular protein complexes. The C terminus (exon 2) of E1A antagonizes these processes, mediated in part through interaction with C-terminal binding proteins 1 and 2 (CtBP1/2). The aim of this study was to identify additional cellular E1A targets that are involved in the modulation of E1A C-terminus activities.;Tandem affinity purification of E1A-associated proteins was performed and mass spectrometric analysis identified several known E1A-interacting proteins as well as novel E1A targets, such as the forkhead transcription factors, FOXK1/K2. Interaction with FOXK1/K2 was mapped to a Ser/Thr-containing sequence motif in a C-terminal region of E1A. Furthermore, E1A mutants deficient in interaction with FOXK1/K2 or the dual-specificity kinase DYRK1A and its cofactor HAN11 induced enhanced cell proliferation, cooperative transformation and tumorigenesis. These results suggest that the E1A C-terminal region may suppress oncogenic transformation through interaction with three different cellular protein complexes: FOXK1/K2, DYRK(1A/1B)/HAN11, and CtBP1/2. To determine the molecular consequences of E1A interaction with FOXK1/K2 and DYRK1A/HAN11, gene expression changes in cells infected with Adv C-terminal deletion mutants was evaluated. E1A interaction with FOXK1/K2 suppressed expression of mitosis-related genes including Plk1 and Cyclin B1. FOXK1 occupation at Plk1 and Cyclin B1 promoter regions was detected in Adv infected cells. In addition, E1A promotes FOXK1 association with the Sin3B corepressor. These results suggest that C-terminal regions of E1A may suppress cell proliferation through modulating FOXK1 transcriptional regulation of mitotic genes.