Bile acid metabolism mediates cholesterol homeostasis and promotes tumorigenesis in clear cell renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) incidence has risen steadily over the last decade. Elevated lipid uptake and storage is required for ccRCC cell viability. As stored cholesterol is the most abundant component in ccRCC intracellular lipid droplets, it may also play an important role in ccRCC cellular homeostasis. In support of this hypothesis, ccRCC cells acquire exogenous cholesterol through the HDL receptor SCARB1, inhibition or suppression of which induces apoptosis. Here, we showed that elevated expression of 3 beta-hydroxy steroid dehydrogenase type 7 (HSD3B7), which metabolizes cholesterol-derived oxysterols in the bile acid biosynthetic pathway, is also essential for ccRCC cell survival. Development of an HSD3B7 enzymatic assay and screening for small molecule inhibitors uncovered the compound celastrol as a potent HSD3B7 inhibitor with low micromolar activity. Repressing HSD3B7 expression genetically or treating ccRCC cells with celastrol resulted in toxic oxysterol accumulation, impaired proliferation, and increased apoptosis in vitro and in vivo. These data demonstrate that bile acid synthesis regulates cholesterol homeostasis in ccRCC and identifies HSD3B7 as a plausible therapeutic target.

Targeting the RNA-Binding Protein HuR in Cancer

The RNA-binding protein human antigen R (HuR) is a well-established regulator of gene expression at the posttranscriptional level. Its dysregulation has been implicated in various human diseases, particularly cancer. In cancer, HuR is considered "active" when it shows increased subcellular localization in the cytoplasm, in addition to its normal nuclear localization. Cytoplasmic HuR plays a crucial role in stabilizing and enhancing the translation of prosurvival mRNAs that are involved in stress responses relevant to cancer progression, such as hypoxia, radiotherapy, and chemotherapy. In general, due to HuR's abundance and function in cancer cells compared with normal cells, it is an appealing target for oncology research. Exploiting the principles underlying HuR's role in tumorigenesis and resistance to stressors, targeting HuR has the potential for synergy with existing and novel oncologic therapies. This review aims to explore HuR's role in homeostasis and cancer pathophysiology, as well as current targeting strategies, which include silencing HuR expression, preventing its translocation and dimerization from the nucleus to the cytoplasm, and inhibiting mRNA binding. Furthermore, this review will discuss recent studies investigating the potential synergy between HuR inhibition and traditional chemotherapeutics.

Deletion of the mRNA stability factor ELAVL1 (HuR) in pancreatic cancer cells disrupts the tumor microenvironment integrity

Stromal cells promote extensive fibrosis in pancreatic ductal adenocarcinoma (PDAC), which is associated with poor prognosis and therapeutic resistance. We report here for the first time that loss of the RNA-binding protein human antigen R (HuR, ELAVL1) in PDAC cells leads to reprogramming of the tumor microenvironment. In multiple in vivo models, CRISPR deletion of ELAVL1 in PDAC cells resulted in a decrease of collagen deposition, accompanied by a decrease of stromal markers (i.e. podoplanin, α-smooth muscle actin, desmin). RNA-sequencing data showed that HuR plays a role in cell-cell communication. Accordingly, cytokine arrays identified that HuR regulates the secretion of signaling molecules involved in stromal activation and extracellular matrix organization [i.e. platelet-derived growth factor AA (PDGFAA) and pentraxin 3]. Ribonucleoprotein immunoprecipitation analysis and transcription inhibition studies validated PDGFA mRNA as a novel HuR target. These data suggest that tumor-intrinsic HuR supports extrinsic activation of the stroma to produce collagen and desmoplasia through regulating signaling molecules (e.g. PDGFAA). HuR-deficient PDAC in vivo tumors with an altered tumor microenvironment are more sensitive to the standard of care gemcitabine, as compared to HuR-proficient tumors. Taken together, we identified a novel role of tumor-intrinsic HuR in its ability to modify the surrounding tumor microenvironment and regulate PDGFAA.

Abstract 3191: Tumor intrinsic HuR promotes stroma activation in pancreatic cancer

Background: One of the factors contributing to poor prognosis of pancreatic ductal adenocarcinoma (PDAC) is thought to be the characteristically dense stroma and heterogeneous tumor microenvironment. The PDAC stroma involves multiple players, including cancer associated fibroblasts (CAFs) and pancreatic stellate cells (PSCs). These stromal cells, when activated, promote extensive fibrosis in PDAC, which is associated with poor prognosis and chemoresistance.

We have previously established that the RNA-binding protein Human Antigen R (HuR, ELAVL1) facilitates PDAC progression. In a tumor microarray of matched normal and malignant patient tissues, we found high active (i.e. cytoplasmic) HuR staining in 80% of PDAC tumors, while low or absent in adjacent normal tissues (n=80). Moreover, in both the orthotopic and the genetically engineered mouse models of PDAC, active HuR staining in tumors was dramatically higher than in normal pancreas. Pancreas-specific overexpression of HuR in a transgenic mouse model did not initiate tumorigenesis, but did increase fibrosis compared to control mice. We hypothesize that tumor intrinsic HuR regulates transcripts that are needed for tumor cells to activate the surrounding stroma.

Methods: MiaPaCa-2 or PANC-1 human PDAC cells, either HuR-proficient or CRISPR-mediated knocked-out, were used in an orthotopic model where cells were injected into the tail of the pancreas. Both NRG and athymic mice were used. Resulting tumors underwent immunohistochemistry, immunofluorescence, or RNA-sequencing. Secreted proteins from cells in 2D culture were analyzed using a cytokine array.

Results: HuR knockout tumors showed a decrease in CAF and PSC markers (i.e. podoplanin, alpha-SMA, desmin; >55% decrease, p<0.05), as well as a decrease in collagen deposition (56% decrease, p=0.0004). Additionally, HuR expression in tumors positively correlated with collagen abundance (p= 0.0002).

RNA sequencing of PDAC tumors at various times during tumor development found that gene expression of stroma-activating ligands was significantly downregulated in HuR knockout tumors (e.g. TGFb, CCL2, PDGFA; -0.8<log2FC<-3.6, 0.0009<padj<2^-57). Moreover, we found a significant decrease of these stroma-activating ligands in conditioned media obtained from HuR knockout compared to wild type (e.g. PDGFAA, PTX3, CCL2; >50% decrease, p<0.0001).

Conclusions: We found inhibition of HuR led to a robust decrease in stroma activation (i.e. lack of collagen, CAFs, and PSCs). To our knowledge, this is the first report of PDAC tumor intrinsic HuR having an extrinsic effect on neighboring stromal cells. Specifically, we found loss of HuR results in a decrease of secreted stroma-activating ligands, likely to be the cause of the decreased fibrosis in HuR knockout tumors. With evidence that fibrosis is associated with poor survival rates and chemoresistance, inhibiting HuR may sensitize tumors to standard of care therapy.

Citation Format: Grace A. McCarthy, Jennifer M. Finan, Aditi Jain, Roberto DiNiro, Aaron Grossberg, Jonathan R. Brody. Tumor intrinsic HuR promotes stroma activation in pancreatic cancer [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 3191.

Abstract 3137: Elucidating the role of RNA-binding proteins in pancreatic cancer extracellular vesicle crosstalk

Many solid tumors, including pancreatic ductal adenocarcinoma (PDAC) tumors, rely on pro-tumorigenic intercellular paracrine signaling during tumor progression. Extensive work has been done to understand paracrine signaling via cytokines, chemokines, and metabolites secreted from tumor cells, but only recent studies have begun to investigate the role of extracellular vesicles (EVs) and their cargoes in the tumor microenvironment. Studies have illustrated the ability of PDAC-derived EVs to activate and recruit pancreatic stellate cells in the tumor microenvironment; however, no work has been done in vivo to determine whether this EV uptake occurs within tumors. Additionally, little mechanistic work has been done to understand the functional consequences of EV cargo within recipient cells.

Based on transcriptomic and proteomic analysis of PDAC patient-derived EVs, mRNAs and RNA-binding proteins (RBPs) are enriched within EVs. Our lab focuses on the role of RBPs in PDAC, and thus, we will utilize enhanced crosslinking immunoprecipitation to identify the RBPs within PDAC EVs and evaluate their impact on the EV transcriptome. We have successfully isolated and characterized EVs via size exclusion chromatography isolation paired with western blotting for classical EV markers, fluorescent nanoparticle tracking analysis, and transmission electron microscopy. We have also demonstrated that we can identify mRNA cargoes that are bound and unbound by RBPs. Additionally, we have optimized PKH67 labeling and detection of PDAC EVs to assess specific and preferential uptake in vitro. Further, we are establishing the validated PalmGRET bioluminescent reporter in our patient derived cell lines to track PDAC EVs in a pancreatic orthotopic mouse model. Utilizing this method, we intend to identify cell types in the microenvironment that are importing PDAC EVs and perform functional studies to understand how these cells are impacted by EV signaling.

These studies will elucidate which cells in the PDAC tumor microenvironment import PDAC EVs, as well as interrogate the mechanistic role of PDAC EV crosstalk. This work will further characterize the role of RBPs in PDAC and begin to evaluate their role in a cell extrinsic manner.

Citation Format: Jennifer M. Finan, Roberto Di Niro, Randall Armstrong, Jonathan R. Brody. Elucidating the role of RNA-binding proteins in pancreatic cancer extracellular vesicle crosstalk [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 3137.

Cholesterol Auxotrophy as a Targetable Vulnerability in Clear Cell Renal Cell Carcinoma

Clear cell renal cell carcinoma (ccRCC) is characterized by large intracellular lipid droplets containing free and esterified cholesterol; however, the functional significance of cholesterol accumulation in ccRCC cells is unknown. We demonstrate that, surprisingly, genes encoding cholesterol biosynthetic enzymes are repressed in ccRCC, suggesting a dependency on exogenous cholesterol. Mendelian randomization analyses based on 31,000 individuals indicate a causal link between elevated circulating high-density lipoprotein (HDL) cholesterol and ccRCC risk. Depriving ccRCC cells of either cholesterol or HDL compromises proliferation and survival in vitro and tumor growth in vivo; in contrast, elevated dietary cholesterol promotes tumor growth. Scavenger Receptor B1 (SCARB1) is uniquely required for cholesterol import, and inhibiting SCARB1 is sufficient to cause ccRCC cell-cycle arrest, apoptosis, elevated intracellular reactive oxygen species levels, and decreased PI3K/AKT signaling. Collectively, we reveal a cholesterol dependency in ccRCC and implicate SCARB1 as a novel therapeutic target for treating kidney cancer.

SIGNIFICANCE

We demonstrate that ccRCC cells are auxotrophic for exogenous cholesterol to maintain PI3K/AKT signaling pathway and ROS homeostasis. Blocking cholesterol import through the HDL transporter SCARB1 compromises ccRCC cell survival and tumor growth, suggesting a novel pharmacologic target for this disease. This article is highlighted in the In This Issue feature, p. 2945.

Mesenchymal Lineage Heterogeneity Underlies Non-Redundant Functions of Pancreatic Cancer-Associated Fibroblasts

Cancer-associated fibroblast (CAF) heterogeneity is increasingly appreciated, but the origins and functions of distinct CAF subtypes remain poorly understood. The abundant and transcriptionally diverse CAF population in pancreatic ductal adenocarcinoma (PDAC) is thought to arise from a common cell of origin, pancreatic stellate cells (PSCs), with diversification resulting from cytokine and growth factor gradients within the tumor microenvironment. Here we analyzed the differentiation and function of PSCs during tumor progression in vivo. Contrary to expectations, we found that PSCs give rise to a numerically minor subset of PDAC CAFs. Targeted ablation of PSC-derived CAFs within their host tissue revealed non-redundant functions for this defined CAF population in shaping the PDAC microenvironment, including production of specific extracellular matrix components and tissue stiffness regulation. Together, these findings link stromal evolution from distinct cells of origin to transcriptional heterogeneity among PDAC CAFs, and demonstrate unique functions for CAFs of a defined cellular origin.

Metabolic Enzyme DLST Promotes Tumor Aggression and Reveals a Vulnerability to OXPHOS Inhibition in High-Risk Neuroblastoma

High-risk neuroblastoma remains therapeutically challenging to treat, and the mechanisms promoting disease aggression are poorly understood. Here, we show that elevated expression of dihydrolipoamide S-succinyltransferase (DLST) predicts poor treatment outcome and aggressive disease in patients with neuroblastoma. DLST is an E2 component of the α-ketoglutarate (αKG) dehydrogenase complex, which governs the entry of glutamine into the tricarboxylic acid cycle (TCA) for oxidative decarboxylation. During this irreversible step, αKG is converted into succinyl-CoA, producing NADH for oxidative phosphorylation (OXPHOS). Utilizing a zebrafish model of MYCN-driven neuroblastoma, we demonstrate that even modest increases in DLST expression promote tumor aggression, while monoallelic dlst loss impedes disease initiation and progression. DLST depletion in human MYCN-amplified neuroblastoma cells minimally affected glutamine anaplerosis and did not alter TCA cycle metabolites other than αKG. However, DLST loss significantly suppressed NADH production and impaired OXPHOS, leading to growth arrest and apoptosis of neuroblastoma cells. In addition, multiple inhibitors targeting the electron transport chain, including the potent IACS-010759 that is currently in clinical testing for other cancers, efficiently reduced neuroblastoma proliferation in vitro. IACS-010759 also suppressed tumor growth in zebrafish and mouse xenograft models of high-risk neuroblastoma. Together, these results demonstrate that DLST promotes neuroblastoma aggression and unveils OXPHOS as an essential contributor to high-risk neuroblastoma. SIGNIFICANCE: These findings demonstrate a novel role for DLST in neuroblastoma aggression and identify the OXPHOS inhibitor IACS-010759 as a potential therapeutic strategy for this deadly disease.

Targeting glutamine metabolism slows soft tissue sarcoma growth

Tumour cells frequently utilize glutamine to meet bioenergetic and biosynthetic demands of rapid cell growth. However, glutamine dependence can be highly variable between in vitro and in vivo settings, based on surrounding microenvironments and complex adaptive responses to glutamine deprivation. Soft tissue sarcomas (STSs) are mesenchymal tumours where cytotoxic chemotherapy remains the primary approach for metastatic or unresectable disease. Therefore, it is critical to identify alternate therapies to improve patient outcomes. Using autochthonous STS murine models and unbiased metabolomics, we demonstrate that glutamine metabolism supports sarcomagenesis. STS subtypes expressing elevated glutaminase (GLS) levels are highly sensitive to glutamine starvation. In contrast to previous studies, treatment of autochthonous tumour-bearing animals with Telaglenastat (CB-839), an orally bioavailable GLS inhibitor, successfully inhibits undifferentiated pleomorphic sarcoma (UPS) tumour growth. We reveal glutamine metabolism as critical for sarcomagenesis, with CB-839 exhibiting potent therapeutic potential.

Glutamine is an energetic source required for the proliferation of cancer cells. Here, the authors show that soft tissue sarcomas expressing high levels of glutaminase (GLS) are particularly sensitive to glutamine starvation and GLS inhibition in tumour-bearing allograft and autochthonous mouse models.

Investigations into the DNA-binding mode of doxorubicinone

Cancer treatment is one of the major challenges facing the modern biomedical profession. Development of new small-molecule chemotherapeutics requires an understanding of the mechanism of action for these treatments, as well as the structure-activity relationship. Study of the well-known DNA-intercalating agent, doxorubicin, and its aglycone, doxorubicinone, was undertaken using a variety of spectroscopic and calorimetric techniques. It was found that, despite conservation of the planar, aromatic portion of doxorubicin, the agylcone does not intercalate; it instead likely binds to the DNA minor-groove.