NRAS Mutant Dictates AHCYL1-Governed ER Calcium Homeostasis for Melanoma Tumor Growth

Calcium homeostasis is critical for cell proliferation, and emerging evidence shows that cancer cells exhibit altered calcium signals to fulfill their need for proliferation. However, it remains unclear whether there are oncogene-specific calcium homeostasis regulations that can expose novel therapeutic targets. Here, from RNAi screen, we report that adenosylhomocysteinase like protein 1 (AHCYL1), a suppressor of the endoplasmic reticulum (ER) calcium channel protein inositol trisphosphate receptor (IP3R), is selectively upregulated and critical for cell proliferation and tumor growth potential of human NRAS-mutated melanoma, but not for melanoma expressing BRAF V600E. Mechanistically, AHCYL1 deficiency results in decreased ER calcium levels, activates the unfolded protein response (UPR), and triggers downstream apoptosis. In addition, we show that AHCYL1 transcription is regulated by activating transcription factor 2 (ATF2) in NRAS-mutated melanoma. Our work provides evidence for oncogene-specific calcium regulations and suggests AHCYL1 as a novel therapeutic target for RAS mutant-expressing human cancers, including melanoma.

IMPLICATIONS

Our findings suggest that targeting the AHCYL1-IP3R axis presents a novel therapeutic approach for NRAS-mutated melanomas, with potential applicability to all cancers harboring RAS mutations, such as KRAS-mutated human colorectal cancers.

Trans-vaccenic acid reprograms CD8+ T cells and anti-tumour immunity

Diet-derived nutrients are inextricably linked to human physiology by providing energy and biosynthetic building blocks and by functioning as regulatory molecules. However, the mechanisms by which circulating nutrients in the human body influence specific physiological processes remain largely unknown. Here we use a blood nutrient compound library-based screening approach to demonstrate that dietary trans-vaccenic acid (TVA) directly promotes effector CD8+ T cell function and anti-tumour immunity in vivo. TVA is the predominant form of trans-fatty acids enriched in human milk, but the human body cannot produce TVA endogenously1. Circulating TVA in humans is mainly from ruminant-derived foods including beef, lamb and dairy products such as milk and butter2,3, but only around 19% or 12% of dietary TVA is converted to rumenic acid by humans or mice, respectively4,5. Mechanistically, TVA inactivates the cell-surface receptor GPR43, an immunomodulatory G protein-coupled receptor activated by its short-chain fatty acid ligands6-8. TVA thus antagonizes the short-chain fatty acid agonists of GPR43, leading to activation of the cAMP-PKA-CREB axis for enhanced CD8+ T cell function. These findings reveal that diet-derived TVA represents a mechanism for host-extrinsic reprogramming of CD8+ T cells as opposed to the intrahost gut microbiota-derived short-chain fatty acids. TVA thus has translational potential for the treatment of tumours.

Type I but Not Type II Calreticulin Mutations Activate the IRE1α/XBP1 Pathway of the Unfolded Protein Response to Drive Myeloproliferative Neoplasms

Approximately 20% of patients with myeloproliferative neoplasms (MPN) harbor mutations in the gene calreticulin (CALR), with 80% of those mutations classified as either type I or type II. While type II CALR-mutant proteins retain many of the Ca2+ binding sites present in the wild-type protein, type I CALR-mutant proteins lose these residues. The functional consequences of this differential loss of Ca2+ binding sites remain unexplored. Here, we show that the loss of Ca2+ binding residues in the type I mutant CALR protein directly impairs its Ca2+ binding ability, which in turn leads to depleted endoplasmic reticulum (ER) Ca2+ and subsequent activation of the IRE1α/XBP1 pathway of the unfolded protein response. Genetic or pharmacologic inhibition of IRE1α/XBP1 signaling induces cell death in type I mutant but not type II mutant or wild-type CALR-expressing cells, and abrogates type I mutant CALR-driven MPN disease progression in vivo.

SIGNIFICANCE

Current targeted therapies for CALR-mutated MPNs are not curative and fail to differentiate between type I- versus type II-driven disease. To improve treatment strategies, it is critical to identify CALR mutation type-specific vulnerabilities. Here we show that IRE1α/XBP1 represents a unique, targetable dependency specific to type I CALR-mutated MPNs. This article is highlighted in the In This Issue feature, p. 265.

Abstract LB134: Type 1 calreticulin mutations differentially activate the IRE1α-XBP1 pathway of the unfolded protein response to drive myeloproliferative neoplasms

Approximately 20% of patients with myeloproliferative neoplasms (MPN) harbor mutations in the gene calreticulin (CALR). 80% of CALR mutations are classified as either type 1 or type 2, exemplified by a 52 bp deletion (CALRdel52) and a 5 bp insertion (CALRins5), respectively. Despite their shared mutant C-termini and mutual ability to bind and activate MPL, patients with type 1 and type 2 CALR mutations display significant clinical and prognostic differences. Type 1 mutations are primarily associated with an MF phenotype and a higher risk of fibrotic transformation from ET, while type 2 mutations are more common in ET. Molecularly, type 2 CALR mutant proteins retain many of the calcium binding sites present in the wild type protein, while type 1 CALR mutant proteins lose these residues. The functional consequences of this differential loss of calcium binding sites remain yet unexplored. Current targeted therapies for CALR mutated MPN are not curative, and treatment does not differentiate between type 1 versus type 2 mutant CALR-driven disease, despite the different phenotypic and prognostic outcomes in these patients. In order to improve treatment strategies for CALR mutated MPN patients, it is critical to identify specific dependencies unique to each CALR mutation type that can be exploited for therapeutic gain. Here, we show that type 1 CALRdel52 but not type 2 CALRins5 mutations lead to activation of and dependency on the IRE1α-XBP1 pathway of the unfolded protein response (UPR). Mechanistically, we found that the loss of calcium binding residues in the type 1 mutant CALR protein directly impairs its calcium binding ability, which in turn leads to depleted ER calcium and subsequent activation of the IRE1α-XBP1 pathway. Using cell lines and primary MPN patient samples, we identified two novel transcriptional targets of XBP1 specific to CALRdel52-expressing cells - the anti-apoptotic protein BCL-2 and the calcium efflux channel IP3R. We show that BCL-2 acts downstream of XBP1 to promote survival in the face of depleted ER calcium, while IP3R is up-regulated downstream of XBP1 to promote continued ER calcium efflux in order to sustain IRE1α-XBP1 pathway activation and survival. We found that genetic or pharmacological inhibition of IRE1α-XBP1 signaling induced cell death only in type 1 mutant but not type 2 mutant or wild type CALR-expressing cells. Moreover, we show that in vivo inhibition of IRE1α significantly abrogates type 1 mutant CALR-driven disease in a bone marrow transplantation model. This work is the first to demonstrate that type 1 and type 2 mutant CALR-expressing cells display differential molecular dependencies that can be exploited for therapeutic gain. Moreover, this study answers an enduring question regarding the functional consequence of the loss of calcium binding sites on the type 1 mutant CALR protein, and demonstrates how type 1 CALR mutant-expressing cells rewire the UPR, downstream calcium signaling, and apoptotic pathways to drive MPN.

Citation Format: Juan Ibarra, Yassmin Elbanna, Katarzyna Kurylowicz, Michele Ciboddo, Harrison S. Greenbaum, Nicole S. Arellano, Deborah Rodriguez, Maria Evers, Dongbo Yang, Althea Bock-Hughes, Chenyu Liu, Quinn Smith, Julian Baumeister, Milena Kalmer, Kathrin Olschok, Benjamin Nicholson, Diane Silva, Jonathan Dowgielewicz3, Elisa Rumi, Daniela Pietra, Ilaria Carola Casetti, Steffen Koschmieder5, Sandeep Gurbuxani, Rebekka K. Schneider, Scott A. Oakes, Shannon E. Elf. Type 1 calreticulin mutations differentially activate the IRE1α-XBP1 pathway of the unfolded protein response to drive myeloproliferative neoplasms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB134.

Cellular signals converge at the NOX2-SHP-2 axis to induce reductive carboxylation in cancer cells

Environmental stresses, including hypoxia or detachment for anchorage independence, or attenuation of mitochondrial respiration through inhibition of electron transport chain induce reductive carboxylation in cells with an enhanced fraction of citrate arising through reductive metabolism of glutamine. This metabolic process contributes to redox homeostasis and sustains biosynthesis of lipids. Reductive carboxylation is often dependent on cytosolic isocitrate dehydrogenase 1 (IDH1). However, whether diverse cellular signals induce reductive carboxylation differentially or through a common signaling converging node remains unclear. We found that induction of reductive carboxylation commonly requires enhanced tyrosine phosphorylation and activation of IDH1, which, surprisingly, is achieved by attenuation of a cytosolic protein tyrosine phosphatase, Src homology region 2 domain-containing phosphatase-2 (SHP-2). Mechanistically, diverse signals induce reductive carboxylation by converging at upregulation of NADPH oxidase 2, leading to elevated cytosolic reactive oxygen species that consequently inhibit SHP-2. Together, our work elucidates the signaling basis underlying reductive carboxylation in cancer cells.

Lysine acetylation restricts mutant IDH2 activity to optimize transformation in AML cells

Mutant isocitrate dehydrogenase (IDH) 1 and 2 play a pathogenic role in cancers, including acute myeloid leukemia (AML), by producing oncometabolite 2-hydroxyglutarate (2-HG). We recently reported that tyrosine phosphorylation activates IDH1 R132H mutant in AML cells. Here, we show that mutant IDH2 (mIDH2) R140Q commonly has K413 acetylation, which negatively regulates mIDH2 activity in human AML cells by attenuating dimerization and blocking binding of substrate (α-ketoglutarate) and cofactor (NADPH). Mechanistically, K413 acetylation of mitochondrial mIDH2 is achieved through a series of hierarchical phosphorylation events mediated by tyrosine kinase FLT3, which phosphorylates mIDH2 to recruit upstream mitochondrial acetyltransferase ACAT1 and simultaneously activates ACAT1 and inhibits upstream mitochondrial deacetylase SIRT3 through tyrosine phosphorylation. Moreover, we found that the intrinsic enzyme activity of mIDH2 is much higher than mIDH1, thus the inhibitory K413 acetylation optimizes leukemogenic ability of mIDH2 in AML cells by both producing sufficient 2-HG for transformation and avoiding cytotoxic accumulation of intracellular 2-HG.

Tetrameric Acetyl-CoA Acetyltransferase 1 Is Important for Tumor Growth

Mitochondrial acetyl-CoA acetyltransferase 1 (ACAT1) regulates pyruvate dehydrogenase complex (PDC) by acetylating pyruvate dehydrogenase (PDH) and PDH phosphatase. How ACAT1 is "hijacked" to contribute to the Warburg effect in human cancer remains unclear. We found that active, tetrameric ACAT1 is commonly upregulated in cells stimulated by EGF and in diverse human cancer cells, where ACAT1 tetramers, but not monomers, are phosphorylated and stabilized by enhanced Y407 phosphorylation. Moreover, we identified arecoline hydrobromide (AH) as a covalent ACAT1 inhibitor that binds to and disrupts only ACAT1 tetramers. The resultant AH-bound ACAT1 monomers cannot reform tetramers. Inhibition of tetrameric ACAT1 by abolishing Y407 phosphorylation or AH treatment results in decreased ACAT1 activity, leading to increased PDC flux and oxidative phosphorylation with attenuated cancer cell proliferation and tumor growth. These findings provide a mechanistic understanding of how oncogenic events signal through distinct acetyltransferases to regulate cancer metabolism and suggest ACAT1 as an anti-cancer target.

Abstract 961: 6-phosphogluconate dehydrogenase links oxidative PPP, lipogenesis and tumor growth by inhibiting LKB1-AMPK signaling

6-phosphogluconate dehydrogenase (6PGD) in the oxidative pentose phosphate pathway (PPP) is upregulated in many cancers. However, how 6PGD is activated and provides a metabolic advantage to tumor growth is unknown. Here we show that 6PGD is commonly activated in human cancers by acetylation at K76 and K294,We found that 6PGD is commonly activated in human cancers by lysine acetylation. Acetylation at K76 and K294 promotes NADP+-binding to 6PGD and formation of active 6PGD dimers, respectively. Moreover, we found that lysine acetylation of 6PGD is important for cancer cell metabolism, proliferation and tumor growth. We also performed systematic RNAi screens and identified DLAT and ACAT2 as 6PGD acetyltransferases and HDAC4 as 6PGD deacetylase, using two novel “targeted” shRNA libraries that target the majority of acetyltransferases and deacetylases in the human genome, respectively. In addition, we found that 6PGD controls intracellular levels of its product ribulose-5-phosphate (Ru-5-P) to regulate lipogenesis. Ru-5-P inhibits LKB1 by disrupting active LKB1 complex, leading to inhibition of AMPK and subsequent activation of acetyl-CoA carboxylase 1 and lipogenesis. Furthermore, we screened out Physcion and its derivative S3 as novel 6PGD inhibitors, which are efficacious in treatment of xenograft nude mice and primary leukemia cells from human patients with minimal toxicity and no off-target effect.

Our findings for the first time decipher the molecular mechanisms underlying 6PGD upregulation in cancer cells and suggest that 6PGD provides an additional and novel link between oxidative PPP and lipogenesis through Ru-5-P-dependent inhibition of LKB1-AMPK signaling. Moreover, identification and characterization of 6PGD inhibitors in translational/pre-clinical studies for the first time provide “proof of principle” to suggest 6PGD as an attractive anti-cancer target.

Citation Format: Changliang Shan, Shannon E. Elf, Ting-Lei Gu, Hanna J. Khoury, Titus Boggon, Sumin Kang, Jing Chen. 6-phosphogluconate dehydrogenase links oxidative PPP, lipogenesis and tumor growth by inhibiting LKB1-AMPK signaling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 961. doi:10.1158/1538-7445.AM2014-961

Targeting glucose metabolism in patients with cancer

Nearly a century ago, Otto Warburg made the astute observation that the metabolic properties of cancer cells differ markedly from those of normal cells. Several decades passed before the concept of exploiting cancer cell metabolism came into clinical practice with the advent of chemotherapy, the underlying principle of which is to target rapidly dividing cells by interfering with critical processes that are all, on some level, driven by cell metabolism. Although chemotherapy can be quite effective, success rates are highly variable and the adverse effects associated with treatment often outweigh the benefits due to the fact that chemotherapy is indiscriminately cytotoxic against all rapidly dividing cells, cancerous or healthy. During the past several years, a more intricate understanding of cancer cell metabolism has permitted the development of targeted therapies that aim to specifically target cancer cells and spare healthy tissue by exploiting the altered metabolism of cancer cells. The identification of new metabolic targets and the subsequent development of small-molecule inhibitors of metabolic enzymes have demonstrated the utility and promise of targeting cancer cell metabolism as an anticancer strategy. This review summarizes recent advances in the identification and characterization of several metabolic enzymes as emerging anticancer targets.

Akt phosphorylates the transcriptional repressor bmi1 to block its effects on the tumor-suppressing ink4a-arf locus

The Polycomb group protein Bmi1 is a transcriptional silencer of the Ink4a-Arf locus, which encodes the cell cycle regulator p16(Ink4a) and the tumor suppressor p19(Arf). Bmi1 plays a key role in oncogenesis and stem cell self-renewal. We report that phosphorylation of human Bmi1 at Ser³¹⁶ by Akt impaired its function by triggering its dissociation from the Ink4a-Arf locus, which resulted in decreased ubiquitylation of histone H2A and the inability of Bmi1 to promote cellular proliferation and tumor growth. Moreover, Akt-mediated phosphorylation of Bmi1 also inhibited its ability to promote self-renewal of hematopoietic stem and progenitor cells. Our study provides a mechanism for the increased abundance of p16(Ink4a) and p19(Arf) seen in cancer cells with an activated phosphoinositide 3-kinase to Akt signaling pathway and identifies crosstalk between phosphorylation events and chromatin structure.

The p53 tumor suppressor protein is a critical regulator of hematopoietic stem cell behavior

In response to diverse stresses, the tumor suppressor p53 differentially regulates its target genes, variably inducing cell-cycle arrest, apoptosis or senescence. Emerging evidence indicates that p53 plays an important role in regulating hematopoietic stem cell (HSC) quiescence, self-renewal, apoptosis and aging. The p53 pathway is activated by DNA damage, defects in ribosome biogenesis, oxidative stress and oncogene induced p19 ARF upregulation. We present an overview of the current state of knowledge about p53 (and its target genes) in regulating HSC behavior, with the hope that understanding the molecular mechanisms that control p53 activity in HSCs and how p53 mutations affect its role in these events may facilitate the development of therapeutic strategies for eliminating leukemia (and cancer) propagating cells.

p53 regulates hematopoietic stem cell quiescence

The importance of the p53 protein in the cellular response to DNA damage is well known, but its function during steady-state hematopoiesis has not been established. We have defined a critical role of p53 in regulating hematopoietic stem cell quiescence, especially in promoting the enhanced quiescence seen in HSCs that lack the MEF/ELF4 transcription factor. Transcription profiling of HSCs isolated from wild-type and p53 null mice identified Gfi-1 and Necdin as p53 target genes, and using lentiviral vectors to upregulate or knockdown the expression of these genes, we show their importance in regulating HSC quiescence. Establishing the role of p53 (and its target genes) in controlling the cell-cycle entry of HSCs may lead to therapeutic strategies capable of eliminating quiescent cancer (stem) cells.