A study investigating the role of Arf1 shows promising results for new therapeutic strategies involving DAMP-mediated anti-tumour immunity to target cancer stem cells. This method of inducting an anti-tumour response could be another frontier in anti-tumour immune therapy.
“[This could be] the first demonstration that endogenous metabolic stress elicits anti-tumour immune responses by inducing DAMPs in rodent tumor models.”
Cancer stem cells (CSCs) are found in tumours and, like other stem cell types, have the ability to give rise to many different cell types. Their affinity for self-renewal, differentiation, tumourigenicity and their possible resistance to traditional treatments like chemotherapy suggests they could be responsible for treatment resistance, tumour metastasis, reoccurrence, evasion and patient death. CSCs, therefore, prove a good target for future tumour therapies.
CSCs have a unique metabolism. Leukaemia stem cells have been shown to rely on amino acids for oxidative phosphorylation and some CSC-enriched disseminated tumour cells obtain energy from fatty acids. The researchers’ previous work with drosophila found that the COPI/Arf1-mediated lipolysis pathway selectively sustains and transforms stem cells and that targeting this pathway by knocking out Arf1 kills stems cells via necrosis.
In this study, the researchers investigated the Arf1-mediated lipid metabolism pathway’s role in sustaining CSCs in mice and how disrupting this pathway would affect tumour growth and number. IMPC Arf1 embryonic stem cells were used in this study.
Does the Arf1 Lipolysis Pathway Sustain CSCs?
The researchers used the Lgr5-CreERT2/Apcf/f (Lgr5/Apc) mouse model, one known to be a good model for CSCs, and knocked out Arf1 to produce Lgr5-CreERT2/Arf1f/f/Apcf/f mice. They found a dramatic reduction in stem cell tumour numbers and a significant increase in lifespan for the Lgr4/Arf1/Apc mice. After examining intestinal cells, they found abnormal mitochondria, poor cristae, vacuoles, degeneration and necrosis.
They then took mouse colon cancer CT26 cells and human liver cancer Huh-7 cells and treated them with Arf1 inhibitors, BFA and GCA. The treatment dramatically increased lipid droplet accumulation in both cell types.
Deletion of Arf1 from the ventral foregut endoderm with Foxa3-Cre resulted in significantly underdeveloped livers and lethality at embryonic day 15.5. Further study found that it also caused a significant reduction in hepatoblast markers Hnf4α and Hnf6, lipid droplet accumulation and necrotic death of hepatoblasts.
“The Arf1-regulated lipolysis pathway selectively sustains stem cells, progenitors and cells enriched with CSCs in mice and that disrupting the pathway in these cells results in lipid droplet accumulation, mitochondrial defects and cell necrosis.”
Arf1-KO’s Effect on Intestinal CSCs
Arf1 depleted mice showed accumulation of CD3+, CD8+ and CD4+, evidence of a T-cell immune response, with dendritic cells (DCs) and inflammatory DCs also being significantly increased. Upon examining the population of lamina propria lymphocytes within the GI tract, they found that certain T-cells (CD8/4+ with and without IFN-γ+) were all significantly increased. These changes were not seen in other areas within the GI tract nor were these changes observed in other key immune cell sites, including the spleen, draining lymph nodes and inguinal lymph nodes.
T-cell-related chemokines (CCL5, CXCL10 and CCL22) were upregulated in the Lgr5/Arf1/Apc model. Real-time PCR showed elevated expressions of IFN-γ, perforin, granzyme A and B and IL-1β. The accumulated evidence suggested that:
“knocking out Arf1 in Lgr5+ stem cells triggers T cell infiltration and activation, leading to CSC death and prolonged survival of Lgr5/Arf1/Apc mice.”
Arf1-KO’s Effect on Liver CSCs
The researchers induced liver tumours using the Tet-o-MYC/LAP-tTA system before adding the Arf1 inhibitors GCA or BFA to mouse food, dramatically reducing the number of liver tumours and extending the lifespan of the mice. Selective depletion of Arf1 in Axin2+ liver cells or CSCs also produced this effect.
DCs, B cells and both CD4+ and CD8+ T-cells were significantly increased in the liver, as were many T-cell-related cytokines (CXCL10, CXCL11 and CCL22.) Real-time PCR showed elevated expression levels for the same molecules as in the intestine. Real-time PCR also, however, showed a significant decrease in PD-L1 expression in several mouse models and cell types.
“Arf1 inhibition may down-regulate PD-L1 through reducing the AP-1/C-Jun pathway…PD-L1 reduction could be partially responsible for the activation of infiltrated T cells that enhances the anti-tumour effect of Arf1 ablation”
Defining the Effect: T-cell Activation, DC Infiltration and DAMPs
In order to confirm that the effect relied on an adaptive immune response, they used anti-CD4 and anti-CD8 antibodies to deplete T-cells. In response, tumour numbers dramatically increased and lifespans were shortened. They then additionally knocked out either Rag1 or IFN-γ in the Lgr5/Arf1/Apc mice. Both caused survival times to significantly decrease. They concluded that the anti-tumour effects were due to an induction of a T-cell-dependant immune response.
Knocking out Arf1 had an effect on other aspects of the immune system. They found Arf1 inhibition may also induce inflammasome-mediated cell necrosis/pyroptosis. Arf1 inhibition or ablation also induced key DAMPS like Calr, HMGBI, ERp46 and LAMP1, as well as ER-stress markers. It was concluded that Arf1 inhibition triggers ER stress and, as a result, induces DAMPS and DC infiltration, enhancing the T-cell infiltration.
Through tests with a series of inhibitors, including the ATPase inhibitor ARL, it was concluded that the anti-tumour effect worked via ATP and HMGBI. Arf1 knockdown was not directly cytotoxic to tumour cells and, together with the above conclusions, the anti-tumour effect was DCs-ATP-IFN-γ-mediated T-cell immunity. Knockdown also prevented tumour metastasis.
Using Arf1-KO Cells for Vaccination
Vaccination with knockout Arf1 cells also proved successful in protecting animals from developing tumours. The vaccination was effective not only against melanoma but also against a variety of histologic types like colon cancer and breast carcinomas. Arf1 inhibition triggered a localised T-cell immune response that then attacked cancer throughout the body.
Simultaneous inhibition of T-cell checkpoint receptor PD-1 with Arf1 ablation had a synergistic effect and further reduced tumour numbers.
Upon analysing public datasets on human cancer, the researchers found Arf1 was amplified or overexpressed in the majority of cancer types examined. The cases with lower expression levels of Arf1 had significantly better survival probabilities for a number of cancer types, suggesting Arf1 is a negative prognostic factor.
How it All Happens
The researchers proposed a model:
“a small number of active anti-CSC T cells penetrate the tumor and attack…; as a result…the CTLs produce cytokines and re-stimulate the local environment to convert an immunosuppressive to an immunostimulatory environment;…[reawakening] the pre-existing inactive anti-tumor T cells [and recruiting] new anti-tumor T cells; these additional T cells are directed against tumour antigens other than the DAMPs to amplify the effects for destroying the bulk tumors; these active T cells may then migrate to other tumor sites and become memory T cells to produce widespread and durable antitumor effects.”
If results like the above are reproducible in humans, DAMP-mediated anti-tumour immune therapy could be a breakthrough for the treatment of reoccurring tumours, particularly those with high Arf1 expression. When used in conjunction with other already effective treatments or with the addition of successful checkpoint blockades, this method could prove even more effective.
Wang, G., Xu, J., Zhao, J. et al. Arf1-mediated lipid metabolism sustains cancer cells and its ablation induces anti-tumor immune responses in mice. Nat Commun11, 220 (2020). https://doi.org/10.1038/s41467-019-14046-9
Obesity and its related ailments like type 2 diabetes and fatty liver disease pose a major global health burden, but researchers report in Nature Communications that blocking an RNA-silencing protein in the livers of mice keeps the animals from getting fat and diabetic conditions.
Takahisa Nakamura, and colleagues at Cincinnati Children’s Hospital Medical Center genetically deleted a protein called Argonaute 2 (Ago2) from the livers of mice. Ago2 controls the silencing of RNA in cells, affecting energy metabolism in the body, according to the study. When Ago2 silences RNA in the liver, it slows metabolism and liver’s ability to process a high-fat diet, the scientists report.
When they deleted Ago2 from the livers of mice, it was not toxic to the animals but it did stabilize energy metabolism. This helped stave off obesity and prevented the mice from developing diabetes and fatty liver disease, which can severely damage the vital organ–which helps rid the body of toxic substances.
“Although this is still basic science, we propose that there may be important translational implications for our findings for chronic metabolic disorders like diabetes, fatty liver diseases, and other obesity associated illnesses,” said Nakamura, senior investigator and a member of the Division of Endocrinology. “This allows us to explore the potential of finding a novel therapeutic approach that alters energy balance in obesity and modulates the associated diseases.”
The scientists caution the research is early stage. Their findings still need additional study and verification in laboratory models and the development of a practical therapeutic to inhibit Ago2 in a clinical setting for patients. But the current paper provides a solid basis for subsequent work.
Ago2 was identified after the researchers conducted a thorough screen and analysis of the activity of genes and their molecular targets in the liver, such as critical proteins. They analyzed wild type and genetically modified mice with high-fat diets by deleting certain proteins that are critical to liver metabolism–such as one called AMPK (AMP-activated protein kinase).
Nakamura said that identifying Ago2’s role in the process “connects the dots” between how proteins are translated in the liver, how energy is produced and consumed and the activity of AMPK in these processes. He pointed out that disruption of these events is already a common feature in obesity and its related illnesses.
Seung-Hoi Koo discusses his recent article in Nature Communications: Hepatic Crtc2 controls whole body energy metabolism via a miR-34a-Fgf21 axis
More information can be found on the website of Professor Seung-Hoi Koo: https://koreauniv.pure.elsevier.com/en/persons/seung-hoi-koo
A team of researchers at the Harvard T. H. Chan School of Public Health has illuminated a critical player in cholesterol metabolism that acts as a molecular guardian in cells to help maintain cholesterol levels within a safe, narrow range. Known as Nrf1, it both senses and responds to excess cholesterol, and could represent a potential new therapeutic target in a multitude of diseases where cholesterol metabolism is disrupted.
“We’ve uncovered a key missing piece in our understanding of how cells can precisely control their cholesterol levels,” said senior author Gökhan S. Hotamisligil, J.S. Simmons Professor of Genetics and Metabolism and head of the Sabri Ülker Center for Nutrient, Genetic, and Metabolic Research at Harvard Chan School. “That piece forms part of a molecular yin-yang that is critical for keeping cholesterol levels in proper balance and ensuring proper cellular function.”
It has been accepted for decades that high cholesterol in the blood can set the stage for cardiovascular disease and other significant health problems. But elevated cholesterol is even more dangerous at the cellular level, leading to toxicity, inflammation, and eventually cell death. “The cell must guard against any rise in cholesterol — it cannot tolerate levels that are too high or too low,” said Hotamisligil. While there are well known cellular factors that send and receive signals when cholesterol is in short supply (orchestrated by a protein called SREBP2), it has been unclear how cells handle some crucial aspects of the converse problem of too much cholesterol.
— Dr Mandy Drake (@TeamDrakeUofE) November 17, 2017
To explore the mechanisms that defend cells against cholesterol, first author Scott Widenmaier, research fellow in the Department of Genetic and Complex Diseases and Sabri Ülker Center, and colleagues focused their attention on an area of the cell known as the endoplasmic reticulum or ER, which is surrounded by a membrane notoriously low in cholesterol — lower, in fact, than any other cellular membrane. “This would be a particularly vulnerable place in the cell, where a small increase in cholesterol would make a significant impact,” explained Hotamisligil. As an intracellular structure, the ER requires fluidity, and adding more cholesterol to its membrane makes it more brittle.
Based on their assumption, the scientists set out to find molecules that reside in the ER membrane and that might play a role in detecting or responding to cellular cholesterol levels; they homed in on a handful of likely suspects. In initial experiments, Nrf1 protein stood out because it responds to cholesterol — when cholesterol is added to cells, the levels of Nrf1 increase, indicating that the protein can react to high cholesterol. And when Nrf1 function is disrupted in mice, the liver becomes dramatically enlarged and overrun with excess cholesterol, suggesting that it normally acts to protect the liver from cholesterol accumulation.
To dig more deeply into Nrf1’s protective role in cholesterol metabolism, the researchers set out to determine how it works at the molecular level. They discovered that Nrf1 has the capacity to bind to cholesterol directly, and pinpointed specific regions of the protein that mediate this binding. Moreover, the binding of cholesterol triggers a cascade of molecular events that suppress inflammation and promote cholesterol removal from the cell.
Taken together, these findings highlight a novel program for responding to high cholesterol in the cell that operates alongside other molecular components that guard against low cholesterol.
“This discovery really teaches us a lot about how cells maintain cholesterol homeostasis,” said Hotamisligil. “Now, we demonstrate that there is a molecular yin-yang– formed by NRF1 and SREBP2 — that together keep cellular cholesterol within a safe, narrow range. That’s an exciting finding that could have broad, new therapeutic applications.”
Research article: NRF1 Is an ER Membrane Sensor that Is Central to Cholesterol Homeostasis