Aberrant regulation of the Akt signaling network by human cytomegalovirus allows for targeting of infected monocytes

Abstract

Primary peripheral blood monocytes are responsible for the hematogenous dissemination of human cytomegalovirus (HCMV) following a primary infection. In order to facilitate viral spread, HCMV extends the naturally short 48-h lifespan of monocytes by stimulating a non-canonical activation of Akt during viral entry, which leads to the increased expression of a specific subset of antiapoptotic proteins. In this study, global analysis of the Akt signaling network showed HCMV induced a more robust activation of the entire network when compared to normal myeloid growth factors. Furthermore, we found a unique interplay between HCMV-activated Akt and the stress response transcription heat shock factor 1 (HSF1) that allowed for the synthesis of both cap- and internal ribosome entry site (IRES)-containing antiapoptotic mRNAs such as myeloid cell leukemia-1 (Mcl-1) and X-linked inhibitor of apoptosis (XIAP), respectively. As generally a switch from cap-dependent to IRES-mediated translation occurs during cellular stress, the ability of HCMV to concurrently drive both types of translation produces a distinct milieu of prosurvival proteins needed for the viability of infected monocytes. Indeed, we found inhibition of XIAP led to death of ∼99% of HCMV-infected monocytes while having minimal effect on the viability of uninfected cells. Taken together, these data indicate that the aberrant activation of the Akt network by HCMV induces the upregulation of a unique subset of antiapoptotic proteins specifically required for the survival of infected monocytes. Consequently, our study highlights the possibility of exploiting these virus-induced changes to prevent viral spread in immunocompromised patients at high-risk for HCMV exposure.

HCMV modulation of cellular PI3K/AKT/mTOR signaling: New opportunities for therapeutic intervention?

“…Targeting these pathways may be key to not only suppressing virus replication, but also eliminating latent and/or quiescent viral reservoirs, which are highly dependent on cellular kinases to effect cellular change due to the limited expression of viral gene products. In particular, a number of prosurvival pathways have been recently identified to be specifically upregulated in HCMV-infected monocytes but not uninfected cells (Chan et al, 2010;Cojohari et al, 2016;Collins-McMillen et al, 2015;Peppenelli et al, 2016;Peppenelli et al, 2018;Stevenson et al, 2014). These pathways represent a unique opportunity for the design of new antivirals that target the virally infected cell rather than the virus itself.…”

“…In contrast, GM-CSF and M-CSF treatments stimulate both S473 and T308 phosphorylation (Baran et al, 2003;Goyal et al, 2002), indicating that the growth factor and HCMV-initiated PI signaling have distinct functional outputs. Indeed, global analysis revealed differential phosphorylation of downstream targets between HCMV-and growth factor-activated Akt (Peppenelli et al, 2018). One major downstream target differentially regulated was mTOR (Peppenelli et al, 2018), which is responsible for controlling protein translation.…”

“…Indeed, global analysis revealed differential phosphorylation of downstream targets between HCMV-and growth factor-activated Akt (Peppenelli et al, 2018). One major downstream target differentially regulated was mTOR (Peppenelli et al, 2018), which is responsible for controlling protein translation. HCMV rapidly phosphorylated mTOR in an Akt-dependent manner following HCMV infection, while GM-CSF and M-CSF treatment had no effect on mTOR activity despite also activating Akt, suggesting that downstream targets may provide increased selectivity in terms of antivirals (Peppenelli et al, 2016).…”

“…Other results suggest that mTOR does not play a role in reactivation from latency (Glover et al, 2014). During a quiescent infection of monocytes, HCMV stimulates mTOR activity in a PI3K/Akt dependent manner (Peppenelli et al, 2018). Despite increasing mTOR activity, lytic replication is not initiated indicating either a threshold level is needed to drive replication or additional factors are required in combination with mTOR to drive replication.…”

“…Activated during times of cellular stress, heat shock factor 1 (HSF1) transcription factor responsible for the expression of stress-associated proteins, which are generally independent of cap-mediated translation (Calderwood et al, 2010;Wu, 1995). HCMV rapidly phosphorylates HSF1 in an Akt-dependent fashion while myeloid growth factors have little effect on HSF1 activity (Peppenelli et al, 2018). A positive feedback loop from HSF1 to mTOR was found to exist in HCMV-infected monocytes where inhibition of HSF1 activity decreased mTOR activity ( Figure 2B).…”