For years, researchers have been trying to target a gene called MYC that is known to drive tumor growth in multiple cancer types when it is mutated or over-expressed, but hitting that target successfully has proven difficult.
Now researchers in the Perelman School of Medicine of the University of Pennsylvania have identified a new pathway that works as a partner to MYC and may be its Achilles’ Heel.
The pathway involves a protein called ATF4, and when it’s blocked, it can cause cancer cells to produce too much protein and die.
These findings in cell lines and mouse models could point the way toward a new therapeutic approach as inhibitors that can block synthesis of ATF4 already exist.
The journal Nature Cell Biology published the findings today.
MYC is a gene that controls normal cell growth, but when it is mutated or amplified in cancer, it sets off a chain reaction that helps tumors grow uncontrollably.
While there is currently no specific way to target it, previous research has focused on blocking other steps in the chain as a workaround to impede tumor growth.
The team, led by Constantinos Koumenis, Ph.D., the Richard Chamberlain Professor of Radiation Oncology and vice chair and research division director of Radiation Oncology, have previously shown that in certain tumors, one of these steps is regulated by a kinase called PERK, which activates ATF4.
However, in this new study, they’ve shown that blocking PERK does not always stop tumor growth because MYC actually controls a second process that can work in parallel as a redundancy in the system.
This study identified this second kinase, which is called GCN2.
“What we’ve learned is that we need to go further downstream to block tumor growth in a way that cancer cells can’t easily escape, and our study identifies the target to do just that,” said Koumenis, who is the co-senior author on this study along with Davide Ruggero, Ph.D., a professor of Urology in the Helen Diller Family Comprehensive Cancer Center at the University of California, San Francisco (UCSF).
This study shows the alternative approach is to target ATF4 itself, since it’s the point where both signal pathways converge, meaning there’s less redundancy built in to allow cancer to survive.
The findings also show that ATF4 turns on the genes MYC needs for growth and also controls the rate at which cells make specific proteins called 4E-BP.
When the researchers knocked out ATF4 in cells or mice, they found tumor cells continued to build up those proteins and eventually died as a result of stress.
This blocked tumor growth in mice with lymphomas and colorectal cancer.
This study also found that when tumors in humans are driven by MYC, ATF4 and its protein partner 4E-BP are also overly expressed, which is further evidence that these findings may point to an approach that could work for humans.
“This shows us the potential impacts of targeting ATF4 in MYC-dependent tumors, something we’re already studying.
We’re also working to confirm this approach will not cause any serious off-target effects,” said lead author Feven Tameire, Ph.D., who conducted this research while she was a doctoral candidate at Penn.
Researchers say future studies will also focus on continuing to investigate why ATF4 works the way it does, which may help their understanding of whether there are other potential targets in the chain.
Activating transcription factor 4 (ATF4), a member of the ATF/CREB family, plays a major role in regulating genes that are involved in the integrated stress response (ISR) [1, 2], amino acid metabolism, redox homeostasis and endoplasmic reticulum (ER) stress responses.
Previous studies showed that ATF4 is over-expressed in many human solid tumors, such as lung cancer, prostate cancer and human hepatocellular carcinoma, suggesting that it may play important roles in tumor progression [3, 4].
ATF4 is induced by numerous stress signals, including anoxia/hypoxia, ER stress, amino acid deprivation, and oxidative stress. ATF4 expression is regulated transcriptionally and translationally by the PERK pathway, which involves the phosphorylation of eIF2α, and post-translationally by phosphorylation, which targets ATF4 for proteasomal degradation .
Previous studies have demonstrated that ATF4 is involved in oncogenic process. For example, Horiguchi et al. reported that ATF4 promoted oncogene-induced neoplastic transformation by suppressing the expression of the cellular senescence-associated genes INK4a and ARF [6, 7].
ATF4 over-expressing cell lines were used to show that ATF4 increased drug resistance to cisplatin, doxorubicin, etoposide, SN-38 and vincristine . Li et al. reported that the knockdown of PERK and ATF4 attenuated LPS-induced autophagy and promoted cell survival .
ATF4 is the master coordinator of the integrated stress response (ISR), which is an adaptive pathway that is triggered by multiple stressors. Magne et al. observed that treating liver cells with ethanol up-regulated ATF4 as well as the expression of ISR target genes (i.e., HMOX-1, GCLC, AsnS, IGFBP-1, GADD34, CHOP, ATF3, and CHAC1) . Increased CHOP, GADD34, ATF3, and CHAC1 expression has been associated with growth arrest and apoptosis . CHOP controls Puma expression in islet cells that are experiencing ER stress .
It has been suggested that CHOP induces cell death via a variety of mechanisms that include depleting cellular glutathione, sensitizing the cell to oxidative stress, up-regulating pro-apoptotic genes  and down-regulating the pro-survival molecule Bcl-2 .
ER stress, which is induced by the activation of CHOP, has been shown to activate the pro-apoptotic molecules Bim and Puma in several cell types and tissues.
Ionizing radiation (IR) activates the unfolded protein response (UPR) and modulates important radiosensitivity-associated factors. Kim et al. observed enhanced UPR signaling in cells exposed to IR, and IR induced eIF2α phosphorylation and increased ATF4 levels in both HUVECs and HCAECs .
Irradiation (IR) can directly or indirectly cause damage to biological molecules to affect cell viability. IR-induced DNA damage activated NF-κB in Glioblastoma cells which promoted expression of IL-6, IL-8 and Bcl-xL, thereby contributing to cell survival and invasion . Recent investigations have suggested that in addition to DNA, proteins can be important targets of IR that can also cause damage [17, 18].
It has also been shown that IR increases the mRNA levels of ATF4 target genes in a dose-dependent manner .
However, the precise function of ATF4 in response to 60Coγradiation has never been reported.
In the present study, we found that 60Coγradiation up-regulated ATF4 expression in different types of cells. We also investigated the function of ATF4 in radiation using knockdown and overexpression systems.
Thus, the aim of this study was to investigate the role of ATF4 in 60Coγradiation and assess whether ATF4 promotes sensitivity to IR.
More information: ATF4 couples MYC-dependent translational activity to bioenergetic demands during tumour progression, Nature Cell Biology(2019). DOI: 10.1038/s41556-019-0347-9 , https://nature.com/articles/s41556-019-0347-9
ournal information: Nature Cell Biology
Provided by Perelman School of Medicine at the University of Pennsylvania