Transcriptional activation of endoplasmic reticulum chaperone GRP78 by HCMV IE1-72 protein

Glucose-regulated protein 78 (GRP78), a key regulator of endoplasmic reticulum (ER) stress, facilitates cancer cell growth and viral replication. The mechanism leading to grp78 gene activation during viral infection is largely unknown. In this study, we show that the immediate-early 1 (IE1-72) protein of the human cytomegalovirus (HCMV) is essential for HCMV-mediated GRP78 activation. IE1-72 upregulated grp78 gene expression depending on the ATP-binding site, the zinc-finger domain and the putative leucine-zipper motif of IE1-72, as well as the ER stress response elements (ERSEs) on the grp78 promoter. The purified IE1-72 protein bound to the CCAAT box within ERSE in vitro, whereas deletion mutants of IE1-72 deficient in grp78 promoter stimulation failed to do so. Moreover, IE1-72 binding to the grp78 promoter in infected cells accompanied the recruitment of TATA box-binding protein-associated factor 1 (TAF1), a histone acetyltransferase, and the increased level of acetylated histone H4, an indicator of active-state chromatin. These results provide evidence that HCMV IE1-72 activates grp78 gene expression through direct promoter binding and modulation of the local chromatin structure, indicating an active viral mechanism of cellular chaperone induction for viral growth.     

Competitive inhibition of a set of endoplasmic reticulum protein genes (GRP78, GRP94, and ERp72) retards cell growth and lowers viability after ionophore treatment.

GRP78, a 78-kDa protein localized in the endoplasmic reticulum (ER), has been implicated in protein processing and stress protection. Its promoter contains a 36-bp region which is conserved among GRP genes across species and has the ability to compete for trans-acting factors mediating GRP gene expression. Integration of about 800 tandem copies of this sequence into the genome of a Chinese hamster ovary cell line (DG44) results in transfectants with the following phenotypes: (i) the induction level of GRP78 by the calcium ionophore A23187 and tunicamycin is reduced 4- and 2-fold, respectively, (ii) the induction levels of two other ER luminal protein genes, GRP94 and ERp72, are simultaneously down-regulated, (iii) the growth rate of these cells is half that of transfectants without the amplified sequence, and (iv) cell viability is decreased by 25-fold after A23187 treatment. These results provide new evidence that ERp72 shares common trans-acting regulatory factors with the GRP genes and that a reduction of this set of ER proteins correlates with lower viability after ionophore treatment.