Decoding the obesity-cancer connection: lessons from preclinical models of pancreatic adenocarcinoma

Obesity is a metabolic state of energy excess and a risk factor for over a dozen cancer types. Because of the rising worldwide prevalence of obesity, decoding the mechanisms by which obesity promotes tumor initiation and early progression is a societal imperative and could broadly impact human health. Here, we review results from preclinical models that link obesity to cancer, using pancreatic adenocarcinoma as a paradigmatic example. We discuss how obesity drives cancer development by reprogramming the pretumor or tumor cell and its micro- and macro-environments. Specifically, we describe evidence for (1) altered cellular metabolism, (2) hormone dysregulation, (3) inflammation, and (4) microbial dysbiosis in obesity-driven pancreatic tumorigenesis, denoting variables that confound interpretation of these studies, and highlight remaining gaps in knowledge. Recent advances in preclinical modeling and emerging unbiased analytic approaches will aid in further unraveling the complex link between obesity and cancer, informing novel strategies for prevention, interception, and therapy in pancreatic adenocarcinoma and other obesity-associated cancers.

A genome-wide single-cell 3D genome atlas of lung cancer progression

Alterations in three-dimensional (3D) genome structures are associated with cancer1-5. However, how genome folding evolves and diversifies during subclonal cancer progression in the native tissue environment remains unknown. Here, we leveraged a genome-wide chromatin tracing technology to directly visualize 3D genome folding in situ in a faithful Kras-driven mouse model of lung adenocarcinoma (LUAD)6, generating the first single-cell 3D genome atlas of any cancer. We discovered stereotypical 3D genome alterations during cancer development, including a striking structural bottleneck in preinvasive adenomas prior to progression to LUAD, indicating a stringent selection on the 3D genome early in cancer progression. We further showed that the 3D genome precisely encodes cancer states in single cells, despite considerable cell-to-cell heterogeneity. Finally, evolutionary changes in 3D genome compartmentalization - partially regulated by polycomb group protein Rnf2 through its ubiquitin ligase-independent activity - reveal novel genetic drivers and suppressors of LUAD progression. Our results demonstrate the importance of mapping the single-cell cancer 3D genome and the potential to identify new diagnostic and therapeutic biomarkers from 3D genomic architectures.

Abstract PR015: Excess dietary oleic acid primes the pancreas for cancer

Human epidemiologic studies support a strong link between increased pancreatic adenocarcinoma (PDAC) risk and high fat diet (HFD) consumption, obesity, and overall energy imbalance. Given the rapid rise in worldwide obesity rates and the prevalence of western diets rich in fat, deciphering these mechanisms is not only a societal imperative but also represents a key untapped target to develop novel strategies for prevention and therapy. The translational relevance of diet research, however, has been limited by inconsistencies in fat source and consumption across human populations and mouse studies. Therefore, whether and how specific dietary fatty acids drive cancer development is poorly understood. To identify commonly consumed dietary fats capable of promoting pancreatic tumorigenesis, we fed a novel panel of 12 isocaloric HFDs – differing solely in fat source and representing the diversity of modern human fat consumption (as per statistics from the USDA) – to an oncogenic Kras-driven mouse model (KC: Pdx1-Cre; KrasLSL-G12D/+) that closely mimics the genetic and histologic features of human PDAC progression. Surprisingly, we found that diets rich in oleic acid (OA), a monounsaturated fat typically associated with good health, were strongly correlated with enhanced precancerous pancreatic intraepithelial neoplasia (PanIN) formation, arguing that OA enhances Kras-induced cellular transformation and early progression. High-oleic diets (HODs) and OA treatment of primary acinar cells induced loss of acinar cell identity and acquisition of ductal markers, consistent with a direct role for OA in acinar-to-ductal metaplasia (ADM), a prerequisite step in early PDAC development. Lipidomic analyses of plasma, liver, and muscle of mice fed HODs revealed greater circulating OA levels and increased tissue incorporation of OA into the acyl chains of phospholipids and sphingolipids, which make up the plasma membrane of cells and mediate intracellular and extracellular signaling. Furthermore, plasma and tissue OA levels more strongly correlated with tumor development, suggesting that direct OA pancreatic tissue incorporation could drive tumorigenesis. Indeed, molecular and biochemical analyses confirmed upregulation in the expression of de novo lipogenesis genes, alterations in lipid metabolism, and enhanced mTOR signaling in pancreata of mice fed HODs. Overall, these results directly link dietary OA to pancreatic lipid metabolism and transformation during PDAC development and highlight the complex pleiotropic effects of dietary fatty acids on health and disease: OA, while beneficial for heart health, may promote the development of certain cancers, such as PDAC. Uncovering the links between specific dietary fats and tumorigenesis are critical to enable precision nutritional guidance for the prevention and treatment of PDAC and potentially other obesity-associated cancers.

Citation Format: Christian F. Ruiz, Rylee McDonnell, Jennifer Kaplan, Jasper de Jong, Christine Shugrue, Michael C. Rudolph, Fred S. Gorelick, John Wysolmerski, Matthew Rodeheffer, Mandar D. Muzumdar. Excess dietary oleic acid primes the pancreas for cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr PR015.

Abstract B056: Wild-type RAS/MAPK signaling complexes mediate adaptive resistance to KRAS inhibition in PDAC

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy and predicted to be the second most common cause of death in the US within the next decade. A striking hallmark of PDAC is the presence of activating KRAS mutations in >90% of all cases. Studies in animal models and cell lines have suggested that oncogenic KRAS is important for both the initiation and progression of PDAC, making KRAS an attractive target for therapy. Recent FDA approvals of KRAS mutant-specific inhibitors has advanced the clinical treatment of patients with cancers harboring mutant KRAS, yet resistance inevitably occurs through both genetic (second-site mutations, bypass pathways) and non-genetic (adaptive) mechanisms. Elucidating the adaptive mechanisms by which cancers evade KRAS inhibition in PDAC will enable the development of more efficacious and durable combinatorial therapeutic strategies with emerging KRAS inhibitors. To address this gap in knowledge, we used CRISPR/Cas-mediated gene knockout technology to model KRAS inhibition in PDAC and found that KRAS is dispensable in a subset of human and mouse PDAC cells, arguing that KRAS is not absolutely required for tumor maintenance. KRAS-deficient cells showed enhanced sensitivity to phosphoinositide 3-kinase (PI3K) inhibitors (PI3Ki), which functioned not only to block canonical AKT signaling but also, unexpectedly, to suppress mitogen-activated protein kinase (MAPK) signaling through rapid attenuation of wild-type RAS/MAPK activity in single cells. Importantly, we confirmed this novel regulatory role of PI3K in mediating RAS/MAPK activity across multiple wild-type KRAS-expressing malignant and non-malignant cell lines, supporting the generalizability of this signaling axis. Furthermore, unbiased proximity protein labeling (BioID) revealed that PI3K inhibition reduces the interaction of KRAS with canonical regulators, effectors, and scaffolds in RAS/MAPK signaling. Parallel genome-wide CRISPR screens comparing KRAS intact and isogenic KRAS-deficient PDAC cells revealed specific RAS/MAPK scaffold/docking proteins as essential for PDAC cell fitness following KRAS ablation. Together, our work sheds new light into the regulatory role of PI3K in wild-type RAS/MAPK signaling, highlights the importance of functional wild-type RAS/MAPK signaling complexes in adaptive resistance to KRAS inhibition, and nominates potential therapeutic targets to overcome resistance to KRAS inhibition in PDAC and possibly other KRAS mutant cancers.

Citation Format: Xiangyu Ge, Christian F. Ruiz, Wenxue Li, Yanixa Quiñones-Avilés, Mandar Deepak Muzumdar. Wild-type RAS/MAPK signaling complexes mediate adaptive resistance to KRAS inhibition in PDAC [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B056.

1-Deoxysphinganine initiates adaptive responses to serine and glycine starvation in cancer cells via proteolysis of sphingosine kinase

Cancer cells may depend on exogenous serine, depletion of which results in slower growth and activation of adaptive metabolic changes. We previously demonstrated that serine and glycine (SG) deprivation causes loss of sphingosine kinase 1 (SK1) in cancer cells, thereby increasing the levels of its lipid substrate, sphingosine (Sph), which mediates several adaptive biological responses. However, the signaling molecules regulating SK1 and Sph levels in response to SG deprivation have yet to be defined. Here, we identify 1-deoxysphinganine (dSA), a noncanonical sphingoid base generated in the absence of serine from the alternative condensation of alanine and palmitoyl CoA by serine palmitoyl transferase, as a proximal mediator of SG deprivation in SK1 loss and Sph level elevation upon SG deprivation in cancer cells. SG starvation increased dSA levels in vitro and in vivo and in turn induced SK1 degradation through a serine palmitoyl transferase-dependent mechanism, thereby increasing Sph levels. Addition of exogenous dSA caused a moderate increase in intracellular reactive oxygen species, which in turn decreased pyruvate kinase PKM2 activity while increasing phosphoglycerate dehydrogenase levels, and thereby promoted serine synthesis. We further showed that increased dSA induces the adaptive cellular and metabolic functions in the response of cells to decreased availability of serine likely by increasing Sph levels. Thus, we conclude that dSA functions as an initial sensor of serine loss, SK1 functions as its direct target, and Sph functions as a downstream effector of cellular and metabolic adaptations. These studies define a previously unrecognized "physiological" nontoxic function for dSA.

Sphingosine kinase 1 downregulation is required for adaptation to serine deprivation

It has been well-established that cancer cells often display altered metabolic profiles, and recent work has concentrated on how cancer cells adapt to serine removal. Serine can be either taken exogenously or synthesized from glucose, and its regulation forms an important mechanism for nutrient integration. One of the several important metabolic roles for serine is in the generation of bioactive sphingolipids since it is the main substrate for serine palmitoyltransferase, the initial and rate-limiting enzyme in the synthesis of sphingolipids. Previously, serine deprivation has been connected to the action of the tumor suppressor p53, and we have previously published on a role for p53 regulating sphingosine kinase 1 (SK1), an enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P). SK1 is a key enzyme in sphingolipid synthesis that functions in pro-survival and tumor-promoting pathways and whose expression is also often elevated in cancers. Here we show that SK1 was degraded during serine starvation in a time and dose-dependent manner, which led to sphingosine accumulation. This was independent of effects on p53 but required the action of the proteasome. Furthermore, we show that overexpression of SK1, to compensate for SK1 loss, was detrimental to cell growth under conditions of serine starvation, demonstrating that the suppression of SK1 under these conditions is adaptive. Mitochondrial oxygen consumption decreased in response to SK1 degradation, and this was accompanied by an increase in intracellular reactive oxygen species (ROS). Suppression of ROS with N-acteylcysteine resulted in suppression of the metabolic adaptations and in decreased cell growth under serine deprivation. The effects of SK1 suppression on ROS were mimicked by D-erythro-sphingosine, whereas S1P was ineffective, suggesting that the effects of loss of SK1 were due to the accumulation of its substrate sphingosine. This study reveals a new mechanism for regulating SK1 levels and a link of SK1 to serine starvation as well as mitochondrial function.

SREBP1 regulates mitochondrial metabolism in oncogenic KRAS expressing NSCLC

Cancer cells require extensive metabolic reprograming in order to provide the bioenergetics and macromolecular precursors needed to sustain a malignant phenotype. Mutant KRAS is a driver oncogene that is well-known for its ability to regulate the ERK and PI3K signaling pathways. However, it is now appreciated that KRAS can promote the tumor growth via upregulation of anabolic metabolism. We recently reported that oncogenic KRAS promotes a gene expression program of de novo lipogenesis in non-small cell lung cancer (NSCLC). To define the mechanism(s) responsible, we focused on the lipogenic transcription factor SREBP1. We observed that KRAS increases SREBP1 expression and genetic knockdown of SREBP1 significantly inhibited the cell proliferation of mutant KRAS-expressing cells. Unexpectedly, lipogenesis was not significantly altered in cells subject to SREBP1 knockdown. Carbon tracing metabolic studies showed a significant decrease in oxidative phosphorylation and RNA-seq data revealed a significant decrease in mitochondrial encoded subunits of the electron transport chain (ETC). Taken together, these data support a novel role, distinct from lipogenesis, of SREBP1 on mitochondrial function in mutant KRAS NSCLC.

Decoupling of Nrf2 Expression Promotes Mesenchymal State Maintenance in Non-Small Cell Lung Cancer

Epithelial mesenchymal transition is a common mechanism leading to metastatic dissemination and cancer progression. In an effort to better understand this process we found an intersection of Nrf2/NLE2F2 (Nrf2), epithelial mesenchymal transition (EMT), and metabolic alterations using multiple in vitro and in vivo approaches. Nrf2 is a key transcription factor controlling the expression of redox regulators to establish cellular redox homeostasis. Nrf2 has been shown to exert both cancer inhibitory and stimulatory activities. Using multiple isogenic non-small cell lung cancer (NSCLC) cell lines, we observed a reduction of Nrf2 protein and activity in a prometastatic mesenchymal cell state and increased reactive oxygen species. Knockdown of Nrf2 promoted a mesenchymal phenotype and reduced glycolytic, TCA cycle and lipogenic output from both glucose and glutamine in the isogenic cell models; while overexpression of Nrf2 promoted a more epithelial phenotype and metabolic reactivation. In both Nrf2 knockout mice and in NSCLC patient samples, Nrf2^low was co-correlated with markedly decreased expression of glycolytic, lipogenic, and mesenchymal RNAs. Conversely, Nrf2^high was associated with partial mesenchymal epithelial transition and increased expression of metabolic RNAs. The impact of Nrf2 on epithelial and mesenchymal cancer cell states and metabolic output provide an additional context to Nrf2 function in cancer initiation and progression, with implications for therapeutic inhibition of Nrf2 in cancer treatment.

Inhibition of phosphoenolpyruvate carboxykinase blocks lactate utilization and impairs tumor growth in colorectal cancer

Background

Metabolic reprogramming is a key feature of malignant cells. While glucose is one of the primary substrates for malignant cells, cancer cells also display a remarkable metabolic flexibility. Depending on nutrient availability and requirements, cancer cells will utilize alternative fuel sources to maintain the TCA cycle for bioenergetic and biosynthetic requirements. Lactate was typically viewed as a passive byproduct of cancer cells. However, studies now show that lactate is an important substrate for the TCA cycle in breast, lung, and pancreatic cancer.

Methods

Metabolic analysis of colorectal cancer (CRC) cells was performed using a combination of bioenergetic analysis and ¹³C stable isotope tracing.

Results

We show here that CRC cells use lactate to fuel the TCA cycle and promote growth especially under nutrient-deprived conditions. This was mediated in part by maintaining cellular bioenergetics. Therefore targeting the ability of cancer cells to utilize lactate via the TCA cycle would have a significant therapeutic benefit. Phosphoenolpyruvate carboxykinase (PEPCK) is an important cataplerotic enzyme that promotes TCA cycle activity in CRC cells. Treatment of CRC cells with low micromolar doses of a PEPCK inhibitor (PEPCKi) developed for diabetes decreased cell proliferation and utilization of lactate by the TCA cycle in vitro and in vivo. Mechanistically, we observed that the PEPCKi increased nutrient stress as determined by decreased cellular bioenergetics including decreased respiration, ATP levels, and increased AMPK activation. ¹³C stable isotope tracing showed that the PEPCKi decreased the incorporation of lactate into the TCA cycle.

Conclusions

These studies highlight lactate as an important substrate for CRC and the use of PEPCKi as a therapeutic approach to target lactate utilization in CRC cells.

Use of chuck muscles and their acceptability in restructured beef/surimi steaks

Ten major muscles along with any unidentifiable lean, were carefully excised from 16 Choice square-cut chucks Yield Grade 2, and placed according to previously determined tenderness rankings, into one of three muscle groups. Group 1 was composed of the most tender muscles, and contained the infraspinatus, longissimus, and triceps brachii. Group 2 contained intermediate tenderness muscles and was composed of the serratus ventralis, deep pectoral, and complexus. Group 3 contained the least tender muscles and was composed of the biceps brachii, supraspinatus, rhomboideus, trapezius, deltoids, and neck muscles. Each group was restructured into beef/surimi steaks and was evaluated. Total muscle yield before trimming accounted for 66.2% of the chuck. Careful fat trimming, desinewing, and internal seam cutting on individual muscles resulted in 34.7% lean available for the restructuring of steaks. The triceps brachii, longissimus, supraspinatus, and infraspinatus required the least trimming and were easiest to excise. These muscles made up 49% of the trimmed meat and 13.7% of the total chuck. Steaks were evaluated by a consumer sensory panel for tenderness, flavor, overall preference, and intent to purchase. There were no differences detected by consumers among the muscle groups for the sensory traits studied. Tenderness and flavor were rated equal to intact steaks for all muscle groups studied. The consumer sensory panel indicated that Groups 1 and 2 would be purchased twice a month and Group 3 once a month.

Relationship between final temperature, thaw rate, and quality of bovine semen

Relationships between thaw rate, thaw bath time, and initial bath and final seminal temperature with coefficients of determination .99 and .97 were: bath time = -.01 + 220.25(1/thaw rate); initial bath temperature = final seminal temperature - 7.29 + 390.05 (1/bath time). Ejaculates from 10 bulls were split and processed in egg yolk-citrate-glycerol, egg yolk-Tris-glycerol, and whole milk-glycerol. All semen was packaged and frozen in .5-ml French straws at -196 degrees C. Sixteen thaw treatments consisted of factorial combinations of four final seminal temperatures and four thaw rates. Treatments were assessed by post-thaw acrosomal integrity after 3-h 37 degrees C incubation. Seminal quality improved with increasing final seminal temperature up to 31 degrees C and did not differ between 31 and 44 degrees C for any of the extenders. A slow thaw rate (3 degrees C/s) resulted in inferior quality for all extenders, and rates 11, 19, and 27 degrees C/s resulted in similar quality for citrate and milk extended semen. Acrosomal integrity was most for 19 degrees C/s in Tris extended semen. A significant factorial interaction existed for Tris and milk extended semen. Predicted acrosomal response of 57.7% across all extenders was at optimum final seminal temperature and thaw rate 37 degrees C and 18 degrees C/s. Bath temperature and bath time determine optimum thaw rate and final temperature of semen packaged in French straws and thus maximize seminal quality.