PKR-Like Endoplasmic Reticulum Kinase Is Necessary for Lipogenic Activation during HCMV Infection

Abstract

PKR-like endoplasmic reticulum (ER) kinase (PERK) is an ER-associated stress sensor protein which phosphorylates eukaryotic initiation factor 2α (eIF2α) to induce translation attenuation in response to ER stress. PERK is also a regulator of lipogenesis during adipocyte differentiation through activation of the cleavage of sterol regulatory element binding protein 1 (SREBP1), resulting in the upregulation of lipogenic enzymes. Our recent studies have shown that human cytomegalovirus (HCMV) infection in human fibroblasts (HF) induces adipocyte-like lipogenesis through the activation of SREBP1. Here, we report that PERK expression is highly increased in HCMV-infected cells and is necessary for HCMV growth. Depletion of PERK, using short hairpin RNA (shRNA), resulted in attenuation of HCMV growth, inhibition of lipid synthesis and reduction of lipogenic gene expression. Examination of the cleavage of SREBP proteins showed PERK depletion inhibited the cleavage of SREBP1, but not SREBP2, in HCMV-infected cells, suggesting different cleavage regulatory mechanisms for SREBP1 and 2. Further studies showed that the depletion of SREBP1, but not SREBP2, reduced lipid synthesis in HCMV infection, suggesting that activation of SREBP1 is sufficient to induce lipogenesis in HCMV infection. The reduction of lipid synthesis by PERK depletion can be partially restored by expressing a Flag-tagged nuclear form of SREBP1a. Our studies also suggest that the induction of PERK in HCMV-infected cells stimulates SREBP1 cleavage by reducing levels of Insig1 (Insulin inducible gene 1) protein; this occurs independent of the phosphorylation of eIF2α. Introduction of an exogenous Insig1-Myc into HCMV infected cells significantly reduced HCMV growth and lipid synthesis. Our data demonstrate that the induction of PERK during HCMV infection is necessary for full activation of lipogenesis; this effect appears to be mediated by limiting the levels of Insig1 thus freeing SREBP1-SCAP complexes for SREBP1 processing.

ACSS2-mediated acetyl-CoA synthesis from acetate is necessary for human cytomegalovirus infection

“…Because ACLY is the primary enzyme producing cytosolic Ac-CoA for lipid synthesis, we tested de novo lipid synthesis from glucose carbon in HCMVinfected C9 cells. In agreement with our previous studies (6,9,26), serum-starved control HFs had a low level of lipid synthesis from glucose, which was increased significantly after HCMV infection (Fig. 1D).…”

“…In control HFs HCMV infection increased lipid droplet levels significantly (Fig. 1E), as we have reported previously (6,26). Interestingly, in HCMV-infected C9 cells lipid droplet levels were equivalent to the levels in controls (Fig.…”

Human Cytomegalovirus pTRS1 and pIRS1 Antagonize Protein Kinase R To Facilitate Virus Replication

“…Fibroblasts express three additional eIF2␣ kinases, GCN2, HRI, and PERK, which phosphorylate eIF2␣ in response to amino acid stress, reactive oxygen species (ROS), and activation of the unfolded protein response (UPR), respectively (30,31). Several studies show that HCMV induces ROS (32)(33)(34) and activates PERK as part of the virus-induced UPR (35,36). In addition, HCMV potentially induces amino acid stress due to high levels of ongoing host and viral protein synthesis.…”

Recent advances in CMV tropism, latency, and diagnosis during aging

“…Viral changes to fatty acid metabolism involve mTOR signaling (Spencer et al 2011;Purdy et al 2015). Protein kinase RNA-like endoplasmic reticulum kinase (PERK), an ER stress-responsive factor, contributes to HCMV-induced lipid synthesis (Yu et al 2013). Although these cellular kinases provide critical insight into the mechanisms HCMV is using to alter host metabolism, they only provide a partial picture.…”