The Unfolded Protein Response Induces the Angiogenic Switch in Human Tumor Cells through the PERK/ATF4 Pathway

Abstract

Neovascularization is a limiting factor in tumor growth and progression. It is well known that changes in the tumor microenvironment, such as hypoxia and glucose deprivation (GD), can induce VEGF production. However, the mechanism linking GD to tumor growth and angiogenesis is unclear. We hypothesize that GD induces the angiogenic switch in tumors through activation of the unfolded protein response (UPR). We report that UPR activation in human tumors results in elevated expression of proangiogenic mediators and a concomitant decrease in angiogenesis inhibitors. cDNA microarray results showed that GD-induced UPR activation promoted upregulation of a number of proangiogenic mediators (VEGF, FGF2, IL6, etc.) and downregulation of several angiogenic inhibitors (THBS1, CXCL14 and CXCL10). In vitro studies revealed that partially blocking UPR signaling by silencing PERK or ATF4 significantly reduced the production of angiogenesis mediators induced by GD. However, suppressing the alpha subunit of hypoxia-inducible factors had no effect on this process. Chromatin immunoprecipitation confirmed binding of ATF4 to a regulatory site in the VEGF gene. In vivo results confirmed that knockdown of PERK in tumor cells slows down tumor growth and decreases tumor blood vessel density. Collectively, these results demonstrate that the PERK/ATF4 arm of UPR mediates the angiogenic switch and is a potential target for antiangiogenic cancer therapy.

Endoplasmic reticulum proteostasis in glioblastoma—From molecular mechanisms to therapeutic perspectives

“…Activation of the PERK/eIF2a/ATF4 axis may promote metastasis through the up-regulation of the metastasis-associated LAMP3 gene (41) or induction of matrix metalloproteinases MMP2 and MMP7 (42). Through the direct binding to the VEGFA gene promoter, ATF4 also increases angiogenesis in tumors (43). Moreover, either pharmacological inhibition or engineered knockdown of PERK attenuates tumor growth and significantly reduces blood vessel density in mouse models (43,44).…”

“…Through the direct binding to the VEGFA gene promoter, ATF4 also increases angiogenesis in tumors (43). Moreover, either pharmacological inhibition or engineered knockdown of PERK attenuates tumor growth and significantly reduces blood vessel density in mouse models (43,44). PERK-mediated inhibition of protein translation also reduces the inhibitor of kB (IkB) to nuclear factor kB (NF-kB) ratio, enabling the translocation of NF-kB to the nucleus and transcription of its downstream inflammatory genes (45).…”

Identification of G-protein-coupled receptor 120 as a tumor-promoting receptor that induces angiogenesis and migration in human colorectal carcinoma

“…Angiogenesis is regulated by a balance of proangiogenic and angiostatic factors. 21,22 Therefore, identification of proangiogenic genes and their products that lead to angiogenesis is critical for providing new potential therapeutic targets. In current oncological practice, the VEGF-targeted agent bevacizumab has been established as a first-line treatment for metastatic CRC, [23][24][25] suggesting especially important roles for angiogenic factors in CRC progression.…”

Protein Folding and Tumour Angiogenesis-Do We Know Enough?

“…and a concomitant decrease in angiogenesis inhibitors (CXCL14, and CXCL10) in tumors. 34 Also, partially blocking UPR signaling by activating transcription factor 4 (ATF4) or silencing protein kinase RNA-like ER kinase (PERK) reduced the production of VEGF. In vivo the knockdown of PERK in tumor cells slows down tumor growth and decreases blood vessel density.…”

“…In vivo the knockdown of PERK in tumor cells slows down tumor growth and decreases blood vessel density. 34 Collectively, knowledge about unfolded protein response in context of angiogenesis, and upstream downstream pathways could bring specific new targets, and therefore inhibiting those mediators of ER stress may lead for the development of new therapeutics to inhibit tumour angiogenesis. Although computational approaches have greatly helped and increased our knowledge about protein folding and regulation of various diseases, still a lot needs to be done.…”