Evidence for an alternative fatty acid desaturation pathway increasing cancer plasticity.

Most tumours have an aberrantly activated lipid metabolism1,2 that enables them to synthesize, elongate and desaturate fatty acids to support proliferation. However, only particular subsets of cancer cells are sensitive to approaches that target fatty acid metabolism and, in particular, fatty acid desaturation3. This suggests that many cancer cells contain an unexplored plasticity in their fatty acid metabolism. Here we show that some cancer cells can exploit an alternative fatty acid desaturation pathway. We identify various cancer cell lines, mouse hepatocellular carcinomas, and primary human liver and lung carcinomas that desaturate palmitate to the unusual fatty acid sapienate to support membrane biosynthesis during proliferation. Accordingly, we found that sapienate biosynthesis enables cancer cells to bypass the known fatty acid desaturation pathway that is dependent on stearoyl-CoA desaturase. Thus, only by targeting both desaturation pathways is the in vitro and in vivo proliferation of cancer cells that synthesize sapienate impaired. Our discovery explains metabolic plasticity in fatty acid desaturation and constitutes an unexplored metabolic rewiring in cancers.

Targeted Determination of Tissue Energy Status by LC-MS/MS.

Intracellular nucleotides and acyl-CoAs are metabolites that are central to the regulation of energy metabolism. They set the cellular energy charge and redox state, act as allosteric regulators, modulate signaling and transcription factors, and thermodynamically activate substrates for oxidation or biosynthesis. Unfortunately, no method exists to simultaneously quantify these biomolecules in tissue extracts. A simple method was developed using ion-pairing reversed-phase high-performance liquid chromatography-electrospray-ionization tandem mass spectrometry (HPLC-ESI-MS/MS) to simultaneously quantify adenine nucleotides (AMP, ADP, and ATP), pyridine dinucleotides (NAD+ and NADH), and short-chain acyl-CoAs (acetyl, malonyl, succinyl, and propionyl). Quantitative analysis of these molecules in mouse liver was achieved using stable-isotope-labeled internal standards. The method was extensively validated by determining the linearity, accuracy, repeatability, and assay stability. Biological responsiveness was confirmed in assays of liver tissue with variable durations of ischemia, which had substantial effects on tissue energy charge and redox state. We conclude that the method provides a simple, fast, and reliable approach to the simultaneous analysis of nucleotides and short-chain acyl-CoAs.

A high-fat diet suppresses de novo lipogenesis and desaturation but not elongation and triglyceride synthesis in mice.

Intracellular lipids and their synthesis contribute to the mechanisms and complications of obesity-associated diseases. We describe an NMR approach that provides an abbreviated lipidomic analysis with concurrent lipid biosynthetic fluxes. Following deuterated water administration, positional isotopomer analysis by deuterium NMR of specific lipid species was used to examine flux through de novo lipogenesis (DNL), FA elongation, desaturation, and TG-glycerol synthesis. The NMR method obviated certain assumptions regarding sites of enrichment and exchangeable hydrogens required by mass isotope methods. The approach was responsive to genetic and pharmacological gain or loss of function of DNL, elongation, desaturation, and glyceride synthesis. BDF1 mice consuming a high-fat diet (HFD) or matched low-fat diet for 35 weeks were examined across feeding periods to determine how flux through these pathways contributes to diet induced fatty liver and obesity. HFD mice had increased rates of FA elongation and glyceride synthesis. However DNL was markedly suppressed despite insulin resistance and obesity. We conclude that most hepatic TGs in the liver of HFD mice were formed from the reesterification of existing or ingested lipids, not DNL.

Colesevelam suppresses hepatic glycogenolysis by TGR5-mediated induction of GLP-1 action in DIO mice.

Bile acid sequestrants are nonabsorbable resins designed to treat hypercholesterolemia by preventing ileal uptake of bile acids, thus increasing catabolism of cholesterol into bile acids. However, sequestrants also improve hyperglycemia and hyperinsulinemia through less characterized metabolic and molecular mechanisms. Here, we demonstrate that the bile acid sequestrant, colesevelam, significantly reduced hepatic glucose production by suppressing hepatic glycogenolysis in diet-induced obese mice and that this was partially mediated by activation of the G protein-coupled bile acid receptor TGR5 and glucagon-like peptide-1 (GLP-1) release. A GLP-1 receptor antagonist blocked suppression of hepatic glycogenolysis and blunted but did not eliminate the effect of colesevelam on glycemia. The ability of colesevelam to induce GLP-1, lower glycemia, and spare hepatic glycogen content was compromised in mice lacking TGR5. In vitro assays revealed that bile acid activation of TGR5 initiates a prolonged cAMP signaling cascade and that this signaling was maintained even when the bile acid was complexed to colesevelam. Intestinal TGR5 was most abundantly expressed in the colon, and rectal administration of a colesevelam/bile acid complex was sufficient to induce portal GLP-1 concentration but did not activate the nuclear bile acid receptor farnesoid X receptor (FXR). The beneficial effects of colesevelam on cholesterol metabolism were mediated by FXR and were independent of TGR5/GLP-1. We conclude that colesevelam administration functions through a dual mechanism, which includes TGR5/GLP-1-dependent suppression of hepatic glycogenolysis and FXR-dependent cholesterol reduction.

Hyperpolarized 13C-Pyruvate Metabolism as a Surrogate for Tumor Grade and Poor Outcome in Renal Cell Carcinoma-A Proof of Principle Study.

Differentiating aggressive clear cell renal cell carcinoma (ccRCC) from indolent lesions is challenging using conventional imaging. This work prospectively compared the metabolic imaging phenotype of renal tumors using carbon-13 MRI following injection of hyperpolarized [1-13C]pyruvate (HP-13C-MRI) and validated these findings with histopathology. Nine patients with treatment-naïve renal tumors (6 ccRCCs, 1 liposarcoma, 1 pheochromocytoma, 1 oncocytoma) underwent pre-operative HP-13C-MRI and conventional proton (1H) MRI. Multi-regional tissue samples were collected using patient-specific 3D-printed tumor molds for spatial registration between imaging and molecular analysis. The apparent exchange rate constant (kPL) between 13C-pyruvate and 13C-lactate was calculated. Immunohistochemistry for the pyruvate transporter (MCT1) from 44 multi-regional samples, as well as associations between MCT1 expression and outcome in the TCGA-KIRC dataset, were investigated. Increasing kPL in ccRCC was correlated with increasing overall tumor grade (ρ = 0.92, p = 0.009) and MCT1 expression (r = 0.89, p = 0.016), with similar results acquired from the multi-regional analysis. Conventional 1H-MRI parameters did not discriminate tumor grades. The correlation between MCT1 and ccRCC grade was confirmed within a TCGA dataset (p < 0.001), where MCT1 expression was a predictor of overall and disease-free survival. In conclusion, metabolic imaging using HP-13C-MRI differentiates tumor aggressiveness in ccRCC and correlates with the expression of MCT1, a predictor of survival. HP-13C-MRI may non-invasively characterize metabolic phenotypes within renal cancer.

Serine metabolism remodeling after platinum-based chemotherapy identifies vulnerabilities in a subgroup of resistant ovarian cancers.

Resistance to platinum-based chemotherapy represents a major clinical challenge for many tumors, including epithelial ovarian cancer. Patients often experience several response-relapse events, until tumors become resistant and life expectancy drops to 12-15 months. Despite improved knowledge of the molecular determinants of platinum resistance, the lack of clinical applicability limits exploitation of many potential targets, leaving patients with limited options. Serine biosynthesis has been linked to cancer growth and poor prognosis in various cancer types, however its role in platinum-resistant ovarian cancer is not known. Here, we show that a subgroup of resistant tumors decreases phosphoglycerate dehydrogenase (PHGDH) expression at relapse after platinum-based chemotherapy. Mechanistically, we observe that this phenomenon is accompanied by a specific oxidized nicotinamide adenine dinucleotide (NAD+) regenerating phenotype, which helps tumor cells in sustaining Poly (ADP-ribose) polymerase (PARP) activity under platinum treatment. Our findings reveal metabolic vulnerabilities with clinical implications for a subset of platinum resistant ovarian cancers.

Analyzing the Metabolism of Metastases in Mice.

Metastasis formation is the leading cause of death in cancer patients. It has recently emerged that cancer cells adapt their metabolism to successfully transition through the metastatic cascade. Consequently, measuring and analyzing the in vivo metabolism of metastases has the potential to reveal novel treatment strategies to prevent metastasis formation. Here, we describe two different metastasis mouse models and how their metabolism can be analyzed with metabolomics and 13C tracer analysis.

Pyruvate-Carboxylase-Mediated Anaplerosis Promotes Antioxidant Capacity by Sustaining TCA Cycle and Redox Metabolism in Liver.

The hepatic TCA cycle supports oxidative and biosynthetic metabolism. This dual responsibility requires anaplerotic pathways, such as pyruvate carboxylase (PC), to generate TCA cycle intermediates necessary for biosynthesis without disrupting oxidative metabolism. Liver-specific PC knockout (LPCKO) mice were created to test the role of anaplerotic flux in liver metabolism. LPCKO mice have impaired hepatic anaplerosis, diminution of TCA cycle intermediates, suppressed gluconeogenesis, reduced TCA cycle flux, and a compensatory increase in ketogenesis and renal gluconeogenesis. Loss of PC depleted aspartate and compromised urea cycle function, causing elevated urea cycle intermediates and hyperammonemia. Loss of PC prevented diet-induced hyperglycemia and insulin resistance but depleted NADPH and glutathione, which exacerbated oxidative stress and correlated with elevated liver inflammation. Thus, despite catalyzing the synthesis of intermediates also produced by other anaplerotic pathways, PC is specifically necessary for maintaining oxidation, biosynthesis, and pathways distal to the TCA cycle, such as antioxidant defenses.

Sustained SREBP-1-dependent lipogenesis as a key mediator of resistance to BRAF-targeted therapy.

Whereas significant anti-tumor responses are observed in most BRAFV600E-mutant melanoma patients exposed to MAPK-targeting agents, resistance almost invariably develops. Here, we show that in therapy-responsive cells BRAF inhibition induces downregulation of the processing of Sterol Regulator Element Binding (SREBP-1) and thereby lipogenesis. Irrespective of the escape mechanism, therapy-resistant cells invariably restore this process to promote lipid saturation and protect melanoma from ROS-induced damage and lipid peroxidation. Importantly, pharmacological SREBP-1 inhibition sensitizes BRAFV600E-mutant therapy-resistant melanoma to BRAFV600E inhibitors both in vitro and in a pre-clinical PDX in vivo model. Together, these data indicate that targeting SREBP-1-induced lipogenesis may offer a new avenue to overcome acquisition of resistance to BRAF-targeted therapy. This work also provides evidence that targeting vulnerabilities downstream of oncogenic signaling offers new possibilities in overcoming resistance to targeted therapies.

Mitochondrial metabolism mediates oxidative stress and inflammation in fatty liver.

Mitochondria are critical for respiration in all tissues; however, in liver, these organelles also accommodate high-capacity anaplerotic/cataplerotic pathways that are essential to gluconeogenesis and other biosynthetic activities. During nonalcoholic fatty liver disease (NAFLD), mitochondria also produce ROS that damage hepatocytes, trigger inflammation, and contribute to insulin resistance. Here, we provide several lines of evidence indicating that induction of biosynthesis through hepatic anaplerotic/cataplerotic pathways is energetically backed by elevated oxidative metabolism and hence contributes to oxidative stress and inflammation during NAFLD. First, in murine livers, elevation of fatty acid delivery not only induced oxidative metabolism, but also amplified anaplerosis/cataplerosis and caused a proportional rise in oxidative stress and inflammation. Second, loss of anaplerosis/cataplerosis via genetic knockdown of phosphoenolpyruvate carboxykinase 1 (Pck1) prevented fatty acid-induced rise in oxidative flux, oxidative stress, and inflammation. Flux appeared to be regulated by redox state, energy charge, and metabolite concentration, which may also amplify antioxidant pathways. Third, preventing elevated oxidative metabolism with metformin also normalized hepatic anaplerosis/cataplerosis and reduced markers of inflammation. Finally, independent histological grades in human NAFLD biopsies were proportional to oxidative flux. Thus, hepatic oxidative stress and inflammation are associated with elevated oxidative metabolism during an obesogenic diet, and this link may be provoked by increased work through anabolic pathways.