UCLA

Viruses are ancient pathogens that evolved to exploit cellular processes through sophisticated mechanisms that promote viral replication. Human adenovirus type 2 (Ad2) and the closely related adenovirus 5 (Ad5) are small DNA tumor viruses that infect the exposed, terminally differentiated epithelial cells that line the upper respiratory tract. Normally, these terminally differentiated cells are suboptimal for DNA virus replication since they have low rates of deoxynucleotide synthesis due to their cell cycle arrest in G0. Ad2 overcomes this obstacle through expression of early region genes immediately after infection that establish a cellular environment suitable for efficient viral replication. The first Ad2 gene expressed upon infection, early-region 1A (E1A). The N-terminal half of adenovirus E1A assembles multimeric complexes with host proteins that repress differentiated cell functions and force host cells into S-phase. In contrast, the functions of E1A’s C-terminal interactions with FOXK, DCAF7 and CtBP are unknown. We found that these interactions modulate RAS signaling, and that a single E1A molecule must bind all three of these host proteins to suppress activation of a subset of IFN-stimulated genes (ISGs). These ISGs were otherwise induced in primary respiratory epithelial cells at 12h p.i. This delayed activation of ISGs required IRF3 and coincided with an ~10-fold increase in IRF3 from protein stabilization. The induced IRF3 bound to chromatin and localized to the promoters of activated ISGs. While IRF3, STAT1/2 and IRF9 all greatly increased in concentration, there were no corresponding mRNA increases, suggesting that E1A regulates the stabilities of these key activators of innate immune responses, as shown directly for IRF3.

As a viral oncogene E1A promotes oncogenic transformation of primary mammalian cells. In cancer differentiated cells are reprogrammed leading to loss of their cell identity. Likewise, expression of E1A suppresses cellular differentiation. We find that eliminating E1A in Ad5-transformed human embryonic kidney cells induces their re-differentiation into cells with characteristics of mesenchymal stem cells (MSC). De-repression of ~1500 genes and a dramatic change in morphology requires chromatin association of Hippo pathway-regulated co-activators YAP and TAZ. E1A causes YAP/TAZ cytoplasmic sequestration. After eliminating E1A, YAP/TAZ are transported into the nucleus where they associate with poised enhancers with DNA-bound TEAD4 and H3K4me1. These complexes induce histone H3 acetylation, chromatin remodeling, and cohesin loading to establish enhancers and eventually super-enhancers. Activation of YAP/TAZ following E1A elimination requires signaling from the actin cytoskeleton. These results together with earlier studies suggest that YAP/TAZ are master regulators of MSC differentiation that function in a developmental check-point controlled by signaling from the actin cytoskeleton informing the cell if it is in the correct cellular and tissue environment before initiating terminal differentiation.