Adenoviral proteins mimic nutrient/growth signals to activate the mTOR pathway for viral replication

Abstract

Like tumor cells, DNA viruses have had to evolve mechanisms that uncouple cellular replication from the many intra-and extracellular factors that normally control it. Here we show that adenovirus encodes two proteins that activate the mammalian target of rapamycin (mTOR) for viral replication, even under nutrient/growth factor-limiting conditions. E4-ORF1 mimics growth factor signaling by activating PI3-kinase, resulting in increased Rheb.GTP loading and mTOR activation. E4-ORF4 is redundant with glucose in stimulating mTOR, does not affect Rheb.GTP levels and is the major mechanism whereby adenovirus activates mTOR in quiescent primary cells. We demonstrate that mTOR is activated through a mechanism that is dependent on the E4-ORF4 protein phosphatase 2A-binding domain. We also show that mTOR activation is required for efficient S-phase entry, independently of E2F activation, in adenovirus-infected quiescent primary cells. These data reveal that adenovirus has evolved proteins that activate the mTOR pathway, irrespective of the cellular microenvironment, and which play a requisite role in viral replication.

Stress and mTORture signaling

“…The importance of mTOR as key regulator of translation can also be inferred from studies showing that some viruses co-opt insulin/insulin-like growth factor signaling (IIS)/TOR signaling to support translation of viral mRNAs (Frese et al, 2003;Mannova and Beretta, 2005;Moody et al, 2005;O'Shea et al, 2005).…”

Adenovirus Overrides Cellular Checkpoints for Protein Translation

“…Moreover, E4-ORF4 is the critical viral protein necessary for activation of the mTOR pathway in infected primary quiescent cells, while E4-ORF1/PI3-kinase plays a minor role. 5 A trivial explanation for the minor contribution of E4-ORF1/PI3-kinase, compared to E4-ORF4, in activating mTOR in infected SAECs may be that E4-ORF1 is not highly expressed in quiescent SAECs. However, a more intriguing explanation is that the hard-wiring of mTOR signaling pathways is different in specific tissue types or cellular states (such as G 0 versus G 1 ), with those activated by E4-ORF4 playing a particularly important role in quiescent SAECs.…”

“…Using adenoviral mutants, we identified two early viral proteins, E4-ORF1 and E4-ORF4, which stimulate the activity of mTOR in distinct but complementary ways. 5 One of these viral proteins, E4-ORF1, uses a C terminal PDZ binding motif to activate PI3-kinase, a well known regulator of mTOR activity. However, an E4-ORF1 mutant virus that fails to activate PI3-kinase in infected primary cells is only partially compromised in its ability to activate p70 S6K .…”

“…As such, DNA viruses are a powerful genetic system with which to identify and examine the integration of the pivotal cellular pathways that are necessary to drive aberrant cellular replication. In a recent study, 5 we used adenovirus to gain new insights into the regulation of protein translation and nutrient/ growth factor signaling pathways to mTOR, an important tumor target.…”

Generation of a functional and durable vascular niche by the adenoviral E4ORF1 gene

“…Among the known E4 ORFs, E4ORF1 primarily affects survival but not cell proliferation (9,10). Therefore, we hypothesized that E4ORF1 may modulate survival in ECs without promoting oncogenic transformation.…”