Human cytomegalovirus attenuates AKT activity by destabilizing insulin receptor substrate proteins

The phosphoinositide 3-kinase (PI3K)/AKT pathway plays crucial roles in cell viability and protein synthesis and is frequently co-opted by viruses to support their replication. Although many viruses maintain high levels of AKT activity during infection, other viruses, such as vesicular stomatitis virus and human cytomegalovirus (HCMV), cause AKT to accumulate in an inactive state. To efficiently replicate, HCMV requires FoxO transcription factors to localize to the infected cell nucleus (Zhang et. al. mBio 2022), a process directly antagonized by AKT. Therefore, we sought to investigate how HCMV inactivates AKT to achieve this. Subcellular fractionation and live cell imaging studies indicated that AKT failed to recruit to membranes upon serum-stimulation of infected cells. However, UV-inactivated virions were unable to render AKT non-responsive to serum, indicating a requirement for de novo viral gene expression. Interestingly, we were able to identify that UL38 (pUL38), a viral activator of mTORC1, is required to diminish AKT responsiveness to serum. mTORC1 contributes to insulin resistance by causing proteasomal degradation of insulin receptor substrate (IRS) proteins, such as IRS1, which are necessary for the recruitment of PI3K to growth factor receptors. In cells infected with a recombinant HCMV disrupted for UL38, AKT responsiveness to serum is retained and IRS1 is not degraded. Furthermore, ectopic expression of UL38 in uninfected cells induces IRS1 degradation, inactivating AKT. These effects of UL38 were reversed by the mTORC1 inhibitor, rapamycin. Collectively, our results demonstrate that HCMV relies upon a cell-intrinsic negative feedback loop to render AKT inactive during productive infection.