Hepatic lipid droplet-associated proteome changes distinguish dietary-induced fatty liver from glucose tolerance in male mice

Fatty liver is characterized by the expansion of lipid droplets (LDs) and is associated with the development of many metabolic diseases. We assessed the morphology of hepatic LDs and performed quantitative proteomics in lean, glucose-tolerant mice compared to high-fat diet (HFD) fed mice that displayed hepatic steatosis and glucose intolerance as well as high-starch diet (HStD) fed mice who exhibited similar levels of hepatic steatosis but remained glucose tolerant. Both HFD and HStD-fed mice had more and larger LDs than Chow-fed animals. We observed striking differences in liver LD proteomes of HFD and HStD-fed mice compared to Chow-fed mice, with fewer differences between HFD and HStD. Taking advantage of our diet strategy, we identified a fatty liver LD proteome consisting of proteins common in HFD- and HStD-fed mice, as well as a proteome associated with glucose tolerance that included proteins shared in Chow and HStD but not HFD-fed mice. Notably, glucose intolerance was associated with changes in the ratio of adipose triglyceride lipase to perilipin 5 in the LD proteome, suggesting dysregulation of neutral lipid homeostasis in glucose-intolerant fatty liver. We conclude that our novel dietary approach uncouples ectopic lipid burden from insulin resistance-associated changes in the hepatic lipid droplet proteome.

ACSM1 and ACSM3 regulate fatty acid metabolism to support prostate cancer growth and constrain ferroptosis

Solid tumors are highly reliant on lipids for energy, growth, and survival. In prostate cancer, the activity of the androgen receptor (AR) is associated with reprogramming of lipid metabolic processes. Here, we identified acyl-CoA synthetase medium chain family members 1 and 3 (ACSM1 and ACSM3) as AR-regulated mediators of prostate cancer metabolism and growth. ACSM1 and ACSM3 were upregulated in prostate tumors compared to non-malignant tissues and other cancer types. Both enzymes enhanced proliferation and protected prostate cancer cells from death in vitro, while silencing ACSM3 led to reduced tumor growth in an orthotopic xenograft model. ACSM1 and ACSM3 were major regulators of the prostate cancer lipidome and enhanced energy production via fatty acid oxidation. Metabolic dysregulation caused by loss of ACSM1/3 led to mitochondrial oxidative stress, lipid peroxidation and cell death by ferroptosis. Conversely, elevated ACSM1/3 activity enabled prostate cancer cells to survive toxic levels of medium chain fatty acids and promoted resistance to ferroptosis-inducing drugs and AR antagonists. Collectively, this study reveals a tumor-promoting function for medium chain acyl-CoA synthetases and positions ACSM1 and ACSM3 as key players in prostate cancer progression and therapy resistance.

Tumor Biomechanics Alters Metastatic Dissemination of Triple Negative Breast Cancer via Rewiring Fatty Acid Metabolism

In recent decades, the role of tumor biomechanics on cancer cell behavior at the primary site has been increasingly appreciated. However, the effect of primary tumor biomechanics on the latter stages of the metastatic cascade, such as metastatic seeding of secondary sites and outgrowth remains underappreciated. This work sought to address this in the context of triple negative breast cancer (TNBC), a cancer type known to aggressively disseminate at all stages of disease progression. Using mechanically tuneable model systems, mimicking the range of stiffness's typically found within breast tumors, it is found that, contrary to expectations, cancer cells exposed to softer microenvironments are more able to colonize secondary tissues. It is shown that heightened cell survival is driven by enhanced metabolism of fatty acids within TNBC cells exposed to softer microenvironments. It is demonstrated that uncoupling cellular mechanosensing through integrin β1 blocking antibody effectively causes stiff primed TNBC cells to behave like their soft counterparts, both in vitro and in vivo. This work is the first to show that softer tumor microenvironments may be contributing to changes in disease outcome by imprinting on TNBC cells a greater metabolic flexibility and conferring discrete cell survival advantages.

Peroxisomal β-oxidation enzyme, DECR2, regulates lipid metabolism and promotes treatment resistance in advanced prostate cancer

BACKGROUND

Peroxisomes are central metabolic organelles that have key roles in fatty acid homoeostasis. As prostate cancer (PCa) is particularly reliant on fatty acid metabolism, we explored the contribution of peroxisomal β-oxidation (perFAO) to PCa viability and therapy response.

METHODS

Bioinformatic analysis was performed on clinical transcriptomic datasets to identify the perFAO enzyme, 2,4-dienoyl CoA reductase 2 (DECR2) as a target gene of interest. Impact of DECR2 and perFAO inhibition via thioridazine was examined in vitro, in vivo, and in clinical prostate tumours cultured ex vivo. Transcriptomic and lipidomic profiling was used to determine the functional consequences of DECR2 inhibition in PCa.

RESULTS

DECR2 is upregulated in clinical PCa, most notably in metastatic castrate-resistant PCa (CRPC). Depletion of DECR2 significantly suppressed proliferation, migration, and 3D growth of a range of CRPC and therapy-resistant PCa cell lines, and inhibited LNCaP tumour growth and proliferation in vivo. DECR2 influences cell cycle progression and lipid metabolism to support tumour cell proliferation. Further, co-targeting of perFAO and standard-of-care androgen receptor inhibition enhanced suppression of PCa cell proliferation.

CONCLUSION

Our findings support a focus on perFAO, specifically DECR2, as a promising therapeutic target for CRPC and as a novel strategy to overcome lethal treatment resistance.

Fatty acid elongation regulates mitochondrial β-oxidation and cell viability in prostate cancer by controlling malonyl-CoA levels

Recently, the fatty acid elongation enzyme ELOVL5 was identified as a critical pro-metastatic factor in prostate cancer, required for cell growth and mitochondrial homeostasis. The fatty acid elongation reaction catalyzed by ELOVL5 utilizes malonyl-CoA as the carbon donor. Here, we demonstrate that ELOVL5 knockdown causes malonyl-CoA accumulation. Malonyl-CoA is a cellular substrate that can inhibit fatty acid β-oxidation in the mitochondria through allosteric inhibition of carnitine palmitoyltransferase 1A (CPT1A), the enzyme that controls the rate-limiting step of the long chain fatty acid β-oxidation cycle. We hypothesized that changes in malonyl-CoA abundance following ELOVL5 knockdown could influence mitochondrial β-oxidation rates in prostate cancer cells, and regulate cell viability. Accordingly, we find that ELOVL5 knockdown is associated with decreased mitochondrial β-oxidation in prostate cancer cells. Combining ELOVL5 knockdown with FASN inhibition to increase malonyl-CoA abundance endogenously enhances the effect of ELOVL5 knockdown on prostate cancer cell viability, while preventing malonyl-CoA production rescues the cells from the effect of ELOVL5 knockdown. Our findings indicate an additional role for fatty acid elongation, in the control of malonyl-CoA homeostasis, alongside its established role in the production of long-chain fatty acid species, to explain the importance of fatty acid elongation for cell viability.

Toward reconciling the roles of FGF21 in protein appetite, sweet preference, and energy expenditure

Fibroblast growth factor 21 (FGF21), an endocrine signal robustly increased by protein restriction independently of an animal's energy status, exerts profound effects on feeding behavior and metabolism. Here, we demonstrate that considering the nutritional contexts within which FGF21 is elevated can help reconcile current controversies over its roles in mediating macronutrient preference, food intake, and energy expenditure. We show that FGF21 is primarily a driver of increased protein intake in mice and that the effect of FGF21 on sweet preference depends on the carbohydrate balance of the animal. Under no-choice feeding, FGF21 infusion either increased or decreased energy expenditure depending on whether the animal was fed a high- or low-energy diet, respectively. We show that while the role of FGF21 in mediating feeding behavior is complex, its role in promoting protein appetite is robust and that the effects on sweet preference and energy expenditure are macronutrient-state-dependent effects of FGF21.

Atorvastatin improves cisplatin sensitivity through modulation of cholesteryl ester homeostasis in breast cancer cells

BACKGROUND

Acquired treatment resistance is a significant problem in breast cancer management, and alterations in lipid metabolism have been proposed to contribute to the development of drug resistance as well as other aspects of tumor progression. The present study aimed to identify the role of cholesterol metabolism in MCF-7 and MDA-MB-231 breast cancer cell response to cisplatin (CDDP) treatment in the acute setting and in a model of CDDP resistance.

METHODS

MCF-7 (luminal A), MDA-MB-231 (triple-negative) and CDDP-resistant MDA-MB-231 (MDACR) cell lines were grown in the presence or absence of CDDP in combination with atorvastatin (ATV), lipid depletion or low-density lipoprotein loading and were analyzed by a variety of biochemical and radiometric techniques.

RESULTS

Co-administration of CDDP and ATV strongly reduced cell proliferation and viability to a greater extent than CDDP alone, especially in MDA-MB-231 cells. These findings were associated with reduced cholesteryl ester synthesis and storage in MDA-MB-231 cells. In MDACR cells, acetyl-CoA acetyltransferase 1 (ACAT-1) was upregulated compared to naïve MDA-MB-231 cells and ATV treatment restored CDDP sensitivity, suggesting that aberrant ACAT-1 expression and associated changes in cholesterol metabolism contribute to CDDP resistance in MDA-MB-231 cells.

CONCLUSION

These findings indicate that the elevated susceptibility of MDA-MB-231 cells to co-administration of CDDP and ATV, is associated with an increased reliance on cholesteryl ester availability. Our data from these cell culture-based studies identifies altered cholesterol homeostasis as an adaptive response to CDDP treatment that contributes to aggressiveness and chemotherapy resistance.

Modulation of Plasma Lipidomic Profiles in Metastatic Castration-Resistant Prostate Cancer by Simvastatin

Elevated circulating sphingolipids are associated with shorter overall survival and therapeutic resistance in metastatic castration-resistant prostate cancer (mCRPC), suggesting that perturbations in sphingolipid metabolism promotes prostate cancer growth. This study assessed whether addition of simvastatin to standard treatment for mCRPC can modify a poor prognostic circulating lipidomic profile represented by a validated 3-lipid signature (3LS). Men with mCRPC (n = 27) who were not on a lipid-lowering agent, were given simvastatin for 12 weeks (40 mg orally, once daily) with commencement of standard treatment. Lipidomic profiling was performed on their plasma sampled at baseline and after 12 weeks of treatment. Only 11 men had the poor prognostic 3LS at baseline, of whom five (45%) did not retain the 3LS after simvastatin treatment (expected conversion rate with standard treatment = 19%). At baseline, the plasma profiles of men with the 3LS displayed higher levels (p < 0.05) of sphingolipids (ceramides, hexosylceramides and sphingomyelins) than those of men without the 3LS. These plasma sphingolipids were reduced after statin treatment in men who lost the 3LS (mean decrease: 23−52%, p < 0.05), but not in men with persistent 3LS, and were independent of changes to plasma cholesterol, LDL-C or triacylglycerol. In conclusion, simvastatin in addition to standard treatment can modify the poor prognostic circulating lipidomic profile in mCRPC into a more favourable profile at twice the expected conversion rate.

Glutamine addiction promotes glucose oxidation in triple-negative breast cancer

Glutamine is a conditionally essential nutrient for many cancer cells, but it remains unclear how consuming glutamine in excess of growth requirements confers greater fitness to glutamine-addicted cancers. By contrasting two breast cancer subtypes with distinct glutamine dependencies, we show that glutamine-indispensable triple-negative breast cancer (TNBC) cells rely on a non-canonical glutamine-to-glutamate overflow, with glutamine carbon routed once through the TCA cycle. Importantly, this single-pass glutaminolysis increases TCA cycle fluxes and replenishes TCA cycle intermediates in TNBC cells, a process that achieves net oxidation of glucose but not glutamine. The coupling of glucose and glutamine catabolism appears hard-wired via a distinct TNBC gene expression profile biased to strip and then sequester glutamine nitrogen, but hampers the ability of TNBC cells to oxidise glucose when glutamine is limiting. Our results provide a new understanding of how metabolically rigid TNBC cells are sensitive to glutamine deprivation and a way to select vulnerable TNBC subtypes that may be responsive to metabolic-targeted therapies.

Combined impact of lipidomic and genetic aberrations on clinical outcomes in metastatic castration-resistant prostate cancer

BACKGROUND

Both changes in circulating lipids represented by a validated poor prognostic 3-lipid signature (3LS) and somatic tumour genetic aberrations are individually associated with worse clinical outcomes in men with metastatic castration-resistant prostate cancer (mCRPC). A key question is how the lipid environment and the cancer genome are interrelated in order to exploit this therapeutically. We assessed the association between the poor prognostic 3-lipid signature (3LS), somatic genetic aberrations and clinical outcomes in mCRPC.

METHODS

We performed plasma lipidomic analysis and cell-free DNA (cfDNA) sequencing on 106 men with mCRPC commencing docetaxel, cabazitaxel, abiraterone or enzalutamide (discovery cohort) and 94 men with mCRPC commencing docetaxel (validation cohort). Differences in lipid levels between men ± somatic genetic aberrations were assessed with t-tests. Associations between the 3LS and genetic aberrations with overall survival (OS) were examined using Kaplan-Meier methods and Cox proportional hazard models.

RESULTS

The 3LS was associated with shorter OS in the discovery (hazard ratio [HR] 2.15, 95% confidence interval [CI] 1.4-3.3, p < 0.001) and validation cohorts (HR 2.32, 95% CI 1.59-3.38, p < 0.001). Elevated plasma sphingolipids were associated with AR, TP53, RB1 and PI3K aberrations (p < 0.05). Men with both the 3LS and aberrations in AR, TP53, RB1 or PI3K had shorter OS than men with neither in both cohorts (p ≤ 0.001). The presence of 3LS and/or genetic aberration was independently associated with shorter OS for men with AR, TP53, RB1 and PI3K aberrations (p < 0.02). Furthermore, aggressive-variant prostate cancer (AVPC), defined as 2 or more aberrations in TP53, RB1 and/or PTEN, was associated with elevated sphingolipids. The combination of AVPC and 3LS predicted for a median survival of ~12 months. The relatively small sample size of the cohorts limits clinical applicability and warrants future studies.

CONCLUSIONS

Elevated circulating sphingolipids were associated with AR, TP53, RB1, PI3K and AVPC aberrations in mCRPC, and the combination of lipid and genetic abnormalities conferred a worse prognosis. These findings suggest that certain genotypes in mCRPC may benefit from metabolic therapies.

Prostate cancer cell proliferation is influenced by LDL-cholesterol availability and cholesteryl ester turnover

BACKGROUND

Prostate cancer growth is driven by androgen receptor signaling, and advanced disease is initially treatable by depleting circulating androgens. However, prostate cancer cells inevitably adapt, resulting in disease relapse with incurable castrate-resistant prostate cancer. Androgen deprivation therapy has many side effects, including hypercholesterolemia, and more aggressive and castrate-resistant prostate cancers typically feature cellular accumulation of cholesterol stored in the form of cholesteryl esters. As cholesterol is a key substrate for de novo steroidogenesis in prostate cells, this study hypothesized that castrate-resistant/advanced prostate cancer cell growth is influenced by the availability of extracellular, low-density lipoprotein (LDL)-derived, cholesterol, which is coupled to intracellular cholesteryl ester homeostasis.

METHODS

C4-2B and PC3 prostate cancer cells were cultured in media supplemented with fetal calf serum (FCS), charcoal-stripped FCS (CS-FCS), lipoprotein-deficient FCS (LPDS), or charcoal-stripped LPDS (CS-LPDS) and analyzed by a variety of biochemical techniques. Cell viability and proliferation were measured by MTT assay and Incucyte, respectively.

RESULTS

Reducing lipoprotein availability led to a reduction in cholesteryl ester levels and cell growth in C4-2B and PC3 cells, with concomitant reductions in PI3K/mTOR and p38MAPK signaling. This reduced growth in LPDS-containing media was fully recovered by supplementation of exogenous low-density lipoprotein (LDL), but LDL only partially rescued growth of cells cultured with CS-LPDS. This growth pattern was not associated with changes in androgen receptor signaling but rather increased p38MAPK and MEK1/ERK/MSK1 activation. The ability of LDL supplementation to rescue cell growth required cholesterol esterification as well as cholesteryl ester hydrolysis activity. Further, growth of cells cultured in low androgen levels (CS-FCS) was suppressed when cholesteryl ester hydrolysis was inhibited.

CONCLUSIONS

Overall, these studies demonstrate that androgen-independent prostate cancer cell growth can be influenced by extracellular lipid levels and LDL-cholesterol availability and that uptake of extracellular cholesterol, through endocytosis of LDL-derived cholesterol and subsequent delivery and storage in the lipid droplet as cholesteryl esters, is required to support prostate cancer cell growth. This provides new insights into the relationship between extracellular cholesterol, intracellular cholesterol metabolism, and prostate cancer cell growth and the potential mechanisms linking hypercholesterolemia and more aggressive prostate cancer.

Annexin A6 and NPC1 regulate LDL-inducible cell migration and distribution of focal adhesions

Cholesterol is considered indispensable for cell motility, but how physiological cholesterol pools enable cells to move forward remains to be clarified. The majority of cells obtain cholesterol from the uptake of Low-Density lipoproteins (LDL) and here we demonstrate that LDL stimulates A431 squamous epithelial carcinoma and Chinese hamster ovary (CHO) cell migration and invasion. LDL also potentiated epidermal growth factor (EGF) -stimulated A431 cell migration as well as A431 invasion in 3-dimensional environments, using organotypic assays. Blocking cholesterol export from late endosomes (LE), using Niemann Pick Type C1 (NPC1) mutant cells, pharmacological NPC1 inhibition or overexpression of the annexin A6 (AnxA6) scaffold protein, compromised LDL-inducible migration and invasion. Nevertheless, NPC1 mutant cells established focal adhesions (FA) that contain activated focal adhesion kinase (pY397FAK, pY861FAK), vinculin and paxillin. Compared to controls, NPC1 mutants display increased FA numbers throughout the cell body, but lack LDL-inducible FA formation at cell edges. Strikingly, AnxA6 depletion in NPC1 mutant cells, which restores late endosomal cholesterol export in these cells, increases their cell motility and association of the cholesterol biosensor D4H with active FAK at cell edges, indicating that AnxA6-regulated transport routes contribute to cholesterol delivery to FA structures, thereby improving NPC1 mutant cell migratory behaviour.

Tumour fatty acid metabolism in the context of therapy resistance and obesity

Fatty acid metabolism is known to support tumorigenesis and disease progression as well as treatment resistance through enhanced lipid synthesis, storage and catabolism. More recently, the role of membrane fatty acid composition, for example, ratios of saturated, monounsaturated and polyunsaturated fatty acids, in promoting cell survival while limiting lipotoxicity and ferroptosis has been increasingly appreciated. Alongside these insights, it has become clear that tumour cells exhibit plasticity with respect to fatty acid metabolism, responding to extratumoural and systemic metabolic signals, such as obesity and cancer therapeutics, to promote the development of aggressive, treatment-resistant disease. Here, we describe cellular fatty acid metabolic changes that are connected to therapy resistance and contextualize obesity-associated changes in host fatty acid metabolism that likely influence the local tumour microenvironment to further modify cancer cell behaviour while simultaneously creating potential new vulnerabilities.

A feedback loop between the androgen receptor and 6-phosphogluoconate dehydrogenase (6PGD) drives prostate cancer growth

Alterations to the androgen receptor (AR) signalling axis and cellular metabolism are hallmarks of prostate cancer. This study provides insight into both hallmarks by uncovering a novel link between AR and the pentose phosphate pathway (PPP). Specifically, we identify 6-phosphogluoconate dehydrogenase (6PGD) as an androgen-regulated gene that is upregulated in prostate cancer. AR increased the expression of 6PGD indirectly via activation of sterol regulatory element binding protein 1 (SREBP1). Accordingly, loss of 6PGD, AR or SREBP1 resulted in suppression of PPP activity as revealed by 1,2-¹³C₂ glucose metabolic flux analysis. Knockdown of 6PGD also impaired growth and elicited death of prostate cancer cells, at least in part due to increased oxidative stress. We investigated the therapeutic potential of targeting 6PGD using two specific inhibitors, physcion and S3, and observed substantial anti-cancer activity in multiple models of prostate cancer, including aggressive, therapy-resistant models of castration-resistant disease as well as prospectively collected patient-derived tumour explants. Targeting of 6PGD was associated with two important tumour-suppressive mechanisms: first, increased activity of the AMP-activated protein kinase (AMPK), which repressed anabolic growth-promoting pathways regulated by acetyl-CoA carboxylase 1 (ACC1) and mammalian target of rapamycin complex 1 (mTORC1); and second, enhanced AR ubiquitylation, associated with a reduction in AR protein levels and activity. Supporting the biological relevance of positive feedback between AR and 6PGD, pharmacological co-targeting of both factors was more effective in suppressing the growth of prostate cancer cells than single-agent therapies. Collectively, this work provides new insight into the dysregulated metabolism of prostate cancer and provides impetus for further investigation of co-targeting AR and the PPP as a novel therapeutic strategy.

The diversity and breadth of cancer cell fatty acid metabolism

Tumor cellular metabolism exhibits distinguishing features that collectively enhance biomass synthesis while maintaining redox balance and cellular homeostasis. These attributes reflect the complex interactions between cell-intrinsic factors such as genomic-transcriptomic regulation and cell-extrinsic influences, including growth factor and nutrient availability. Alongside glucose and amino acid metabolism, fatty acid metabolism supports tumorigenesis and disease progression through a range of processes including membrane biosynthesis, energy storage and production, and generation of signaling intermediates. Here, we highlight the complexity of cellular fatty acid metabolism in cancer, the various inputs and outputs of the intracellular free fatty acid pool, and the numerous ways that these pathways influence disease behavior.

Abstract PO-036: ACSM1 and ACSM3 regulate fatty acid oxidation in prostate cancer to promote growth and protect against oxidative stress

Prostate tumours are highly reliant on lipids for growth and survival. Emerging evidence suggests that the androgen receptor (AR) is a major regulator of lipid metabolism in prostate cancer (PCa), but the molecular mechanisms underlying such regulation are largely unknown. By integrating cistromic and transcriptomic data, we identified Acyl-CoA Synthetase Medium Chain Family Members 1 and 3 (ACSM1 and ACSM3) as putative new AR-regulated genes in prostate cancer. These factors are thought to function to activate fatty acids for their utilisation in energy production via mitochondrial beta-oxidation. AR regulation was validated by demonstrating modulation of ACSM1 and ACSM3 by androgen and anti-androgen treatment and confirming direct binding of AR to proximal cis-regulatory elements by chromatin immunoprecipitation (ChIP). ACSM1 and ACSM3 are upregulated in prostate tumours compared to non-malignant prostate tissues and are expressed more highly in prostate cancer than other cancer types. We subsequently applied metabolomics, lipidomics and functional assays to decipher the roles of ACSM1 and ACSM3 in prostate cancer cells. Knockdown of ACSM1 and ACSM3 in PCa cells resulted in growth arrest and ATP depletion, supporting a key role for both factors in energy production from fatty acids. Furthermore, lipidomic analysis of cells showed that poly-unsaturated fatty acids accumulate in response to loss of ACSM1 and ACSM3. Metabolomics revealed that cells adapt to loss of ACSM1 and ACSM3 by switching to a glycolytic phenotype. The metabolic dysregulation induced by knockdown of ACSM1 and ACSM3 caused mitochondrial oxidative stress and subsequent lipid peroxidation, eventually resulting in cell death. Accumulation of mitochondrial reactive oxygen species was abrogated by ferrostatin-1 (an iron chelator), suggesting that cell death was due to an iron-dependent form of apoptosis termed ferroptosis. Supporting this concept, over-expression of ACSM1 and ACSM3 elicited resistance to the ferroptosis inducers Erastin and ML210. Our study has revealed a novel mechanism by which AR regulates lipid metabolism in prostate cancer cells. Importantly, the critical role of ACSM1 and ACSM3 as key regulators of growth and protectors against ferroptosis emphasises their potential as novel therapeutic targets

Citation Format: Raj Kumar Shrestha, Scott Townley, Adrienne Hanson, Marie Pickering, Zeyad D. Nassar, Chui Yan Mah, Mohammadreza A. Ghodsi, Andrew J. Hoy, Lake-Ee Quek, Wayne D. Tilley, Lisa M. Butler, Luke A. Selth. ACSM1 and ACSM3 regulate fatty acid oxidation in prostate cancer to promote growth and protect against oxidative stress [abstract]. In: Abstracts: AACR Special Virtual Conference on Epigenetics and Metabolism; October 15-16, 2020; 2020 Oct 15-16. Philadelphia (PA): AACR; Cancer Res 2020;80(23 Suppl):Abstract nr PO-036.

Synthesis and fluorine-18 radiolabeling of a phospholipid as a PET imaging agent for prostate cancer

INTRODUCTION

Altered lipid metabolism and subsequent changes in cellular lipid composition have been observed in prostate cancer cells, are associated with poor clinical outcome, and are promising targets for metabolic therapies. This study reports for the first time on the synthesis of a phospholipid radiotracer based on the phospholipid 1,2-didocosahexaenoyl-sn-glycero-3-phosphocholine (PC44:12) to allow tracking of polyunsaturated lipid tumor uptake via PET imaging. This tracer may aid in the development of strategies to modulate response to therapies targeting lipid metabolism in prostate cancer.

METHODS

Lipidomics analysis of prostate tumor explants and LNCaP tumor cells were used to identify PC44:12 as a potential phospholipid candidate for radiotracer development. Synthesis of phosphocholine precursor and non-radioactive standard were optimised using click chemistry. The biodistribution of a fluorine-18 labeled analogue, N-{[4-(2-[¹⁸F]fluoroethyl)-2,3,4-triazol-1-yl]methyl}-1,2-didocosahexaenoyl-sn-glycero-3-phosphocholine ([¹⁸F]2) was determined in LNCaP prostate tumor-bearing NOD SCID gamma mice by ex vivo biodistribution and PET imaging studies and compared to biodistribution of [¹⁸F]fluoromethylcholine.

RESULTS

[¹⁸F]2 was produced with a decay-corrected yield of 17.8 ± 3.7% and an average radiochemical purity of 97.00 ± 0.89% (n = 6). Molar activity was 85.1 ± 3.45 GBq/μmol (2300 ± 93 mCi/μmol) and the total synthesis time was 2 h. Ex vivo biodistribution data demonstrated high liver uptake (41.1 ± 9.2%ID/g) and high splenic uptake (10.9 ± 9.1%ID/g) 50 min post-injection. Ex vivo biodistribution showed low absolute tumor uptake of [¹⁸F]2 (0.8 ± 0.3%ID/g). However, dynamic PET imaging demonstrated an increase over time of the relative tumor-to-muscle ratio with a peak of 2.8 ± 0.5 reached 1 h post-injection. In contrast, dynamic PET of [¹⁸F]fluoromethylcholine demonstrated no increase in tumor-to-muscle ratios due to an increase in both tumor and muscle over time. Absolute uptake of [¹⁸F]fluoromethylcholine was higher and peaked at 60 min post injection (2.25 ± 0.29%ID/g) compared to [¹⁸F]2 (1.44 ± 0.06%ID/g) during the 1 h dynamic scan period.

CONCLUSIONS AND ADVANCES IN KNOWLEDGE

This study demonstrates the ability to radiolabel phospholipids and indicates the potential to monitor the in vivo distribution of phospholipids using fluorine-18 based PET.

Metabolic Remodeling Induced by Adipocytes: A New Achilles' Heel in Invasive Breast Cancer?

Metabolic reprogramming represents an important hallmark of cancer cells. Besides de novo fatty acid synthesis, it is now clear that cancer cells can acquire Fatty Acids (FA) from tumor-surrounding adipocytes to increase their invasive capacities. Indeed, adipocytes release FA in response to tumor secreted factors that are transferred to tumor cells to be either stored as triglycerides and other complex lipids or oxidized in mitochondria. Like all cells, FA can be released over time from triglyceride stores through lipolysis and then oxidized in mitochondria in cancer cells. This metabolic interaction results in specific metabolic remodeling in cancer cells, and underpins adipocyte stimulated tumor progression. Lipolysis and fatty acid oxidation therefore represent novel targets of interest in the treatment of cancer. In this review, we summarize the recent advances in our understanding of the metabolic reprogramming induced by adipocytes, with a focus on breast cancer. Then, we recapitulate recent reports studying the effect of lipolysis and fatty acid oxidation inhibitors on tumor cells and discuss the interest to target these metabolic pathways as new therapeutic approaches for cancer.

Abstract 237: DECR1: The rate limiting enzyme of polyunsaturated fatty acid metabolism and a novel therapeutic target in prostate cancer

Introduction: Fatty acid oxidation (FAO) is the dominant bioenergetic pathway in prostate cancer (PCa). Previous studies have demonstrated efficacy in targeting key FAO enzymes using in vitro and in vivo models of PCa. However, clinical evidence supporting FAO targeting in PCa is lacking. A recent study reported that FAO is upregulated in PCa cells and is essential for cell viability. Here, we demonstrated therapeutic efficacy of targeting FAO in clinical prostate tumors and identified DECR1 as a potential FAO-related survival factor.

Methods and Results: To examine the therapeutic efficacy of targeting FAO in a more clinically-relevant setting, patient-derived prostate explants (PDE) were treated with etomoxir (FAO inhibitor). Results showed significant inhibition of cell proliferation (evaluated using Ki67 immunohistochemistry staining) in drug-treated PDE by a mean of 48% (n=13, p&lt;0.05). To identify key targets of FAO in PCa, a meta-analysis was performed using four clinical RNA-sequencing datasets comprising of malignant and matched normal PCa patient tissues. DECR1 – the rate limiting enzyme of polyunsaturated fatty acid (PUFA) oxidation was identified as a robustly overexpressed FAO gene in PCa tissues; its expression is significantly associated with shorter biochemical recurrence-free survival and overall PCa patient survival. Consistent with the vital role of DECR1 in PUFA oxidation, knockdown of DECR1 impaired the cell's capacity to metabolize PUFAs; furthermore, FAO inhibition via DECR1 knockdown is specific to PUFA oxidation. Notably, DECR1 knockdown significantly inhibited PCa cell proliferation, migration, and invasion, and significantly reduced cellular proliferation in tumors. Mechanistically, DECR1 knockdown results in the induction of lipid peroxidation and cell death via ferroptosis in PCa cells. In contrast, overexpression of DECR1 significantly decreased mitochondrial oxidative stress and reduced levels of malondialdehyde. Treatment of PCa cells with ferrostatin successfully rescued PCa cells from death by DECR1 inhibition. Taken together, these data suggest that overexpression of DECR1 may offer PCa cells a survival advantage by protecting the cells from ferroptosis.

Conclusion: This study demonstrated for the first time clinically-relevant evidence that targeting FAO is efficacious and provides evidence for the importance of FAO, and specifically PUFA oxidation, in PCa progression. Importantly, this study identified DECR1 as a promising and novel therapeutic target for PCa.

Citation Format: Chui Yan Mah, Zeyad D. Nassar, Ingrid J. Burvenich, Swati Irani, Margaret M. Centenera, Max Moldovan, David J. Lynn, Andrew J. Hoy, Johannes V. Swinnen, Lisa M. Butler. DECR1: The rate limiting enzyme of polyunsaturated fatty acid metabolism and a novel therapeutic target in prostate cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 237.

Insulin signaling requires glucose to promote lipid anabolism in adipocytes

Adipose tissue is essential for metabolic homeostasis, balancing lipid storage and mobilization based on nutritional status. This is coordinated by insulin, which triggers kinase signaling cascades to modulate numerous metabolic proteins, leading to increased glucose uptake and anabolic processes like lipogenesis. Given recent evidence that glucose is dispensable for adipocyte respiration, we sought to test whether glucose is necessary for insulin-stimulated anabolism. Examining lipogenesis in cultured adipocytes, glucose was essential for insulin to stimulate the synthesis of fatty acids and glyceride-glycerol. Importantly, glucose was dispensable for lipogenesis in the absence of insulin, suggesting that distinct carbon sources are used with or without insulin. Metabolic tracing studies revealed that glucose was required for insulin to stimulate pathways providing carbon substrate, NADPH, and glycerol 3-phosphate for lipid synthesis and storage. Glucose also displaced leucine as a lipogenic substrate and was necessary to suppress fatty acid oxidation. Together, glucose provided substrates and metabolic control for insulin to promote lipogenesis in adipocytes. This contrasted with the suppression of lipolysis by insulin signaling, which occurred independently of glucose. Given previous observations that signal transduction acts primarily before glucose uptake in adipocytes, these data are consistent with a model whereby insulin initially utilizes protein phosphorylation to stimulate lipid anabolism, which is sustained by subsequent glucose metabolism. Consequently, lipid abundance was sensitive to glucose availability, both during adipogenesis and in Drosophila flies in vivo Together, these data highlight the importance of glucose metabolism to support insulin action, providing a complementary regulatory mechanism to signal transduction to stimulate adipose anabolism.

Human DECR1 is an androgen-repressed survival factor that regulates PUFA oxidation to protect prostate tumor cells from ferroptosis

Fatty acid β-oxidation (FAO) is the main bioenergetic pathway in human prostate cancer (PCa) and a promising novel therapeutic vulnerability. Here we demonstrate therapeutic efficacy of targeting FAO in clinical prostate tumors cultured ex vivo, and identify DECR1, encoding the rate-limiting enzyme for oxidation of polyunsaturated fatty acids (PUFAs), as robustly overexpressed in PCa tissues and associated with shorter relapse-free survival. DECR1 is a negatively-regulated androgen receptor (AR) target gene and, therefore, may promote PCa cell survival and resistance to AR targeting therapeutics. DECR1 knockdown selectively inhibited β-oxidation of PUFAs, inhibited proliferation and migration of PCa cells, including treatment resistant lines, and suppressed tumor cell proliferation and metastasis in mouse xenograft models. Mechanistically, targeting of DECR1 caused cellular accumulation of PUFAs, enhanced mitochondrial oxidative stress and lipid peroxidation, and induced ferroptosis. These findings implicate PUFA oxidation via DECR1 as an unexplored facet of FAO that promotes survival of PCa cells.

Assessment of Periprostatic and Subcutaneous Adipose Tissue Lipolysis and Adipocyte Size from Men with Localized Prostate Cancer

The prostate is surrounded by periprostatic adipose tissue (PPAT), the thickness of which has been associated with more aggressive prostate cancer (PCa). There are limited data regarding the functional characteristics of PPAT, how it compares to subcutaneous adipose tissue (SAT), and whether in a setting of localized PCa, these traits are altered by obesity or disease aggressiveness. PPAT and SAT were collected from 60 men (age: 42–78 years, BMI: 21.3–35.6 kg/m²) undergoing total prostatectomy for PCa. Compared to SAT, adipocytes in PPAT were smaller, had the same basal rates of fatty acid release (lipolysis) yet released less polyunsaturated fatty acid species, and were more sensitive to isoproterenol-stimulated lipolysis. Basal lipolysis of PPAT was increased in men diagnosed with less aggressive PCa (Gleason score (GS) ≤ 3 + 4) compared to men with more aggressive PCa (GS ≥ 4 + 3) but no other measured adipocyte parameters related to PCa aggressiveness. Likewise, there was no difference in PPAT lipid biology between lean and obese men. In conclusion, lipid biological features of PPAT do differ from SAT; however, we did not observe any meaningful difference in ex vivo PPAT biology that is associated with PCa aggressiveness or obesity. As such, our findings do not support a relationship between altered PCa behavior in obese men and the metabolic reprogramming of PPAT.

Lipid droplet-associated kinase STK25 regulates peroxisomal activity and metabolic stress response in steatotic liver

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are emerging as leading causes of liver disease worldwide and have been recognized as one of the major unmet medical needs of the 21st century. Our recent translational studies in mouse models, human cell lines, and well-characterized patient cohorts have identified serine/threonine kinase (STK)25 as a protein that coats intrahepatocellular lipid droplets (LDs) and critically regulates liver lipid homeostasis and progression of NAFLD/NASH. Here, we studied the mechanism-of-action of STK25 in steatotic liver by relative quantification of the hepatic LD-associated phosphoproteome from high-fat diet-fed Stk25 knockout mice compared with their wild-type littermates. We observed a total of 131 proteins and 60 phosphoproteins that were differentially represented in STK25-deficient livers. Most notably, a number of proteins involved in peroxisomal function, ubiquitination-mediated proteolysis, and antioxidant defense were coordinately regulated in Stk25 ^-/- versus wild-type livers. We confirmed attenuated peroxisomal biogenesis and protection against oxidative and ER stress in STK25-deficient human liver cells, demonstrating the hepatocyte-autonomous manner of STK25's action. In summary, our results suggest that regulation of peroxisomal function and metabolic stress response may be important molecular mechanisms by which STK25 controls the development and progression of NAFLD/NASH.

Lipid and glucose metabolism in hepatocyte cell lines and primary mouse hepatocytes: a comprehensive resource for in vitro studies of hepatic metabolism

The liver is a critical tissue for maintaining glucose, fatty acid, and cholesterol homeostasis. Primary hepatocytes represent the gold standard for studying the mechanisms controlling hepatic glucose, lipid, and cholesterol metabolism in vitro. However, access to primary hepatocytes can be limiting, and therefore, other immortalized hepatocyte models are commonly used. Here, we describe substrate metabolism of cultured AML12, IHH, and PH5CH8 cells, hepatocellular carcinoma-derived HepG2s, and primary mouse hepatocytes (PMH) to identify which of these cell lines most accurately phenocopy PMH basal and insulin-stimulated metabolism. Insulin-stimulated glucose metabolism in PH5CH8 cells, and to a lesser extent AML12 cells, responded most similarly to PMH. Notably, glucose incorporation in HepG2 cells were 14-fold greater than PMH. The differences in glucose metabolic activity were not explained by differential protein expression of key regulators of these pathways, for example glycogen synthase and glycogen content. In contrast, fatty acid metabolism in IHH cells was the closest to PMHs, yet insulin-responsive fatty acid metabolism in AML12 and HepG2 cells was most similar to PMH. Finally, incorporation of acetate into intracellular-free cholesterol was comparable for all cells to PMH; however, insulin-stimulated glucose conversion into lipids and the incorporation of acetate into intracellular cholesterol esters were strikingly different between PMHs and all tested cell lines. In general, AML12 cells most closely phenocopied PMH in vitro energy metabolism. However, the cell line most representative of PMHs differed depending on the mode of metabolism being investigated, and so careful consideration is needed in model selection.

Extracellular Fatty Acids Are the Major Contributor to Lipid Synthesis in Prostate Cancer

Prostate cancer cells exhibit altered cellular metabolism but, notably, not the hallmarks of Warburg metabolism. Prostate cancer cells exhibit increased de novo synthesis of fatty acids (FA); however, little is known about how extracellular FAs, such as those in the circulation, may support prostate cancer progression. Here, we show that increasing FA availability increased intracellular triacylglycerol content in cultured patient-derived tumor explants, LNCaP and C4-2B spheroids, a range of prostate cancer cells (LNCaP, C4-2B, 22Rv1, PC-3), and prostate epithelial cells (PNT1). Extracellular FAs are the major source (∼83%) of carbons to the total lipid pool in all cell lines, compared with glucose (∼13%) and glutamine (∼4%), and FA oxidation rates are greater in prostate cancer cells compared with PNT1 cells, which preferentially partitioned extracellular FAs into triacylglycerols. Because of the higher rates of FA oxidation in C4-2B cells, cells remained viable when challenged by the addition of palmitate to culture media and inhibition of mitochondrial FA oxidation sensitized C4-2B cells to palmitate-induced apoptosis. Whereas in PC-3 cells, palmitate induced apoptosis, which was prevented by pretreatment of PC-3 cells with FAs, and this protective effect required DGAT-1-mediated triacylglycerol synthesis. These outcomes highlight for the first-time heterogeneity of lipid metabolism in prostate cancer cells and the potential influence that obesity-associated dyslipidemia or host circulating has on prostate cancer progression. IMPLICATIONS: Extracellular-derived FAs are primary building blocks for complex lipids and heterogeneity in FA metabolism exists in prostate cancer that can influence tumor cell behavior.

The inverse relationship between prostate specific antigen (PSA) and obesity

Obese men have lower serum prostate-specific antigen (PSA) than comparably aged lean men, but the underlying mechanism remains unclear. The aim of this study was to determine the effect of obesity on PSA and the potential contributing mechanisms. A cohort of 1195 men aged 35 years and over at recruitment, with demographic, anthropometric (BMI, waist circumference (WC)) and serum hormone (serum testosterone, estradiol (E2)) PSA and hematology assessments obtained over two waves was assessed. Men with a history of prostate cancer or missing PSA were excluded, leaving 970 men for the final analysis. Mixed-effects regressions and mediation analyses adjusting for hormonal and volumetric factors explore the potential mechanisms relating obesity to PSA. After adjusting for age, PSA levels were lower in men with greater WC (P = 0.001). In a multivariable model including WC, age, E2/testosterone and PlasV as predictors, no statistically significant associations were observed between with PSA and either WC (P = 0.36) or PlasV (P = 0.49), while strong associations were observed with both E2/testosterone (P < 0.001) and age (P < 0.001). In the mediation analyses with PlasV as the mediator, the average causal mediation effect (ACME) explained roughly 20% of the total effect of WC on PSA (P = 0.31), while when E2/testosterone is a mediator, the ACME explained roughly 50% of the effect (P < 0.001). Our findings indicate that lower PSA levels in obese men, as compared to normal weight men, can be explained both by hormonal changes (elevated E2/testosterone ratio) and hemodilution. Hormonal factors therefore represent a substantial but underappreciated mediating pathway.

Heterogeneity of fatty acid metabolism in breast cancer cells underlies differential sensitivity to palmitate-induced apoptosis

Breast cancer (BrCa) metabolism is geared toward biomass synthesis and maintenance of reductive capacity. Changes in glucose and glutamine metabolism in BrCa have been widely reported, yet the contribution of fatty acids (FAs) in BrCa biology remains to be determined. We recently reported that adipocyte coculture alters MCF-7 and MDA-MB-231 cell metabolism and promotes proliferation and migration. Since adipocytes are FA-rich, and these FAs are transferred to BrCa cells, we sought to elucidate the FA metabolism of BrCa cells and their response to FA-rich environments. MCF-7 and MDA-MB-231 cells incubated in serum-containing media supplemented with FAs accumulate extracellular FAs as intracellular triacylglycerols (TAG) in a dose-dependent manner, with MDA-MB-231 cells accumulating more TAG. The differences in TAG levels were a consequence of distinct differences in intracellular partitioning of FAs, and not due to differences in the rate of FA uptake. Specifically, MCF-7 cells preferentially partition FAs into mitochondrial oxidation, whereas MDA-MB-231 cells partition FAs into TAG synthesis. These differences in intracellular FA handling underpin differences in the sensitivity to palmitate-induced lipotoxicity, with MDA-MB-231 cells being highly sensitive, whereas MCF-7 cells are partially protected. The attenuation of palmitate-induced lipotoxicity in MCF-7 cells was reversed by inhibition of FA oxidation. Pretreatment of MDA-MB-231 cells with FAs increased TAG synthesis and reduced palmitate-induced apoptosis. Our results provide novel insight into the potential influences of obesity on BrCa biology, highlighting distinct differences in FA metabolism in MCF-7 and MDA-MB-231 cells and how lipid-rich environments modulate these effects.

Altered hepatic glucose homeostasis in AnxA6-KO mice fed a high-fat diet

Annexin A6 (AnxA6) controls cholesterol and membrane transport in endo- and exocytosis, and modulates triglyceride accumulation and storage. In addition, AnxA6 acts as a scaffolding protein for negative regulators of growth factor receptors and their effector pathways in many different cell types. Here we investigated the role of AnxA6 in the regulation of whole body lipid metabolism and insulin-regulated glucose homeostasis. Therefore, wildtype (WT) and AnxA6-knockout (KO) mice were fed a high-fat diet (HFD) for 17 weeks. During the course of HFD feeding, AnxA6-KO mice gained less weight compared to controls, which correlated with reduced adiposity. Systemic triglyceride and cholesterol levels of HFD-fed control and AnxA6-KO mice were comparable, with slightly elevated high density lipoprotein (HDL) and reduced triglyceride-rich lipoprotein (TRL) levels in AnxA6-KO mice. AnxA6-KO mice displayed a trend towards improved insulin sensitivity in oral glucose and insulin tolerance tests (OGTT, ITT), which correlated with increased insulin-inducible phosphorylation of protein kinase B (Akt) and ribosomal protein S6 kinase (S6) in liver extracts. However, HFD-fed AnxA6-KO mice failed to downregulate hepatic gluconeogenesis, despite similar insulin levels and insulin signaling activity, as well as expression profiles of insulin-sensitive transcription factors to controls. In addition, increased glycogen storage in livers of HFD- and chow-fed AnxA6-KO animals was observed. Together with an inability to reduce glucose production upon insulin exposure in AnxA6-depleted HuH7 hepatocytes, this implicates AnxA6 contributing to the fine-tuning of hepatic glucose metabolism with potential consequences for the systemic control of glucose in health and disease.

A distinct plasma lipid signature associated with poor prognosis in castration-resistant prostate cancer

Lipids are known to influence tumour growth, inflammation and chemoresistance. However, the association of circulating lipids with the clinical outcome of metastatic castration-resistant prostate cancer (CRPC) is unknown. We investigated associations between the plasma lipidome and clinical outcome in CRPC. Lipidomic profiling by liquid chromatography-tandem mass spectrometry was performed on plasma samples from a Phase 1 discovery cohort of 96 CRPC patients. Results were validated in an independent Phase 2 cohort of 63 CRPC patients. Unsupervised analysis of lipidomic profiles (323 lipid species) classified the Phase 1 cohort into two patient subgroups with significant survival differences (HR 2.31, 95% CI 1.44-3.68, p = 0.0005). The levels of 46 lipids were individually prognostic and were predominantly sphingolipids with higher levels associated with poor prognosis. A prognostic three-lipid signature was derived (ceramide d18:1/24:1, sphingomyelin d18:2/16:0, phosphatidylcholine 16:0/16:0) and was also associated with shorter survival in the Phase 2 cohort (HR 4.8, 95% CI 2.06-11.1, p = 0.0003). The signature was an independent prognostic factor when modelled with clinicopathological factors or metabolic characteristics. The association of plasma lipids with CRPC prognosis suggests a possible role of these lipids in disease progression. Further research is required to determine if therapeutic modulation of the levels of these lipids by targeting their metabolic pathways may improve patient outcome.

Role of hepatic Annexin A6 in fatty acid-induced lipid droplet formation

Annexin A6 (AnxA6) has been implicated in the regulation of endo-/exocytic pathways, cholesterol transport, and the formation of multifactorial signaling complexes in many different cell types. More recently, AnxA6 has also been linked to triglyceride storage in adipocytes. Here we investigated the potential role of AnxA6 in fatty acid (FA) - induced lipid droplet (LD) formation in hepatocytes. AnxA6 was associated with LD from rat liver and HuH7 hepatocytes. In oleic acid (OA) -loaded HuH7 cells, substantial amounts of AnxA6 bound to LD in a Ca²⁺-independent manner. Remarkably, stable or transient AnxA6 overexpression in HuH7 cells led to elevated LD numbers/size and neutral lipid staining under control conditions as well as after OA loading compared to controls. In contrast, overexpression of AnxA1, AnxA2 and AnxA8 did not impact on OA-induced lipid accumulation. On the other hand, incubation of AnxA6-depleted HuH7 cells or primary hepatocytes from AnxA6 KO-mice with OA led to reduced FA accumulation and LD numbers. Furthermore, morphological analysis of liver sections from A6-KO mice revealed significantly lower LD numbers compared to wildtype animals. Interestingly, pharmacological inhibition of cytoplasmic phospholipase A2α (cPLA₂α)-dependent LD formation was ineffective in AnxA6-depleted HuH7 cells. We conclude that cPLA₂α-dependent pathways contribute to the novel regulatory role of hepatic AnxA6 in LD formation.

Insulin and diet-induced changes in the ubiquitin-modified proteome of rat liver

Ubiquitin is a crucial post-translational modification regulating numerous cellular processes, but its role in metabolic disease is not well characterized. In this study, we identified the in vivo ubiquitin-modified proteome in rat liver and determined changes in this ubiquitome under acute insulin stimulation and high-fat and sucrose diet-induced insulin resistance. We identified 1267 ubiquitinated proteins in rat liver across diet and insulin-stimulated conditions, with 882 proteins common to all conditions. KEGG pathway analysis of these proteins identified enrichment of metabolic pathways, TCA cycle, glycolysis/gluconeogenesis, fatty acid metabolism, and carbon metabolism, with similar pathways altered by diet and insulin resistance. Thus, the rat liver ubiquitome is sensitive to diet and insulin stimulation and this is perturbed in insulin resistance.

Adipocyte-Tumor Cell Metabolic Crosstalk in Breast Cancer

The tumor stroma is a heterogeneous ecosystem comprising matrix, fibroblasts, and immune cells and has an important role in cancer progression. Adipocytes constitute a major component of breast stroma, and significant emerging evidence demonstrates a reciprocal metabolic adaptation between stromal adipocytes and breast cancer (BC) cells. Recent observations promote a model where adipocytes respond to cancer cell-derived endocrine and paracrine signaling to provide metabolic substrates, which in turn drive enhanced cancer cell proliferation, invasion, and treatment resistance. Further defining the mechanisms that underpin this dynamic interaction between stromal adipocytes and BC cells, especially in the context of obesity, may identify novel therapeutic strategies. These will become increasingly important in addressing the clinical challenges presented by obesity and metabolic syndromes.

Mitochondrial mutations and metabolic adaptation in pancreatic cancer

BACKGROUND

Pancreatic cancer has a five-year survival rate of ~8%, with characteristic molecular heterogeneity and restricted treatment options. Targeting metabolism has emerged as a potentially effective therapeutic strategy for cancers such as pancreatic cancer, which are driven by genetic alterations that are not tractable drug targets. Although somatic mitochondrial genome (mtDNA) mutations have been observed in various tumors types, understanding of metabolic genotype-phenotype relationships is limited.

METHODS

We deployed an integrated approach combining genomics, metabolomics, and phenotypic analysis on a unique cohort of patient-derived pancreatic cancer cell lines (PDCLs). Genome analysis was performed via targeted sequencing of the mitochondrial genome (mtDNA) and nuclear genes encoding mitochondrial components and metabolic genes. Phenotypic characterization of PDCLs included measurement of cellular oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) using a Seahorse XF extracellular flux analyser, targeted metabolomics and pathway profiling, and radiolabelled glutamine tracing.

RESULTS

We identified 24 somatic mutations in the mtDNA of 12 patient-derived pancreatic cancer cell lines (PDCLs). A further 18 mutations were identified in a targeted study of ~1000 nuclear genes important for mitochondrial function and metabolism. Comparison with reference datasets indicated a strong selection bias for non-synonymous mutants with predicted functional effects. Phenotypic analysis showed metabolic changes consistent with mitochondrial dysfunction, including reduced oxygen consumption and increased glycolysis. Metabolomics and radiolabeled substrate tracing indicated the initiation of reductive glutamine metabolism and lipid synthesis in tumours.

CONCLUSIONS

The heterogeneous genomic landscape of pancreatic tumours may converge on a common metabolic phenotype, with individual tumours adapting to increased anabolic demands via different genetic mechanisms. Targeting resulting metabolic phenotypes may be a productive therapeutic strategy.

Adipocyte lipolysis links obesity to breast cancer growth: adipocyte-derived fatty acids drive breast cancer cell proliferation and migration

Background

Obesity is associated with increased recurrence and reduced survival of breast cancer. Adipocytes constitute a significant component of breast tissue, yet their role in provisioning metabolic substrates to support breast cancer progression is poorly understood.

Results

Here, we show that co-culture of breast cancer cells with adipocytes revealed cancer cell-stimulated depletion of adipocyte triacylglycerol. Adipocyte-derived free fatty acids were transferred to breast cancer cells, driving fatty acid metabolism via increased CPT1A and electron transport chain complex protein levels, resulting in increased proliferation and migration. Notably, fatty acid transfer to breast cancer cells was enhanced from “obese” adipocytes, concomitant with increased stimulation of cancer cell proliferation and migration. This adipocyte-stimulated breast cancer cell proliferation was dependent on lipolytic processes since HSL/ATGL knockdown attenuated cancer cell responses.

Conclusions

These findings highlight a novel and potentially important role for adipocyte lipolysis in the provision of metabolic substrates to breast cancer cells, thereby supporting cancer progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s40170-016-0163-7) contains supplementary material, which is available to authorized users.

Annexin A6 protein is downregulated in human hepatocellular carcinoma

Annexin A6 (AnxA6) is a lipid-binding protein highly expressed in the liver, regulating cholesterol homeostasis and signaling pathways with a role in liver physiology. Here, we analyzed whether hepatic AnxA6 levels are affected by pathological conditions that are associated with liver dysfunction and liver injury. AnxA6 levels in the fatty liver of mice fed a high-fat diet, in ob/ob and db/db animals and in human fatty liver are comparable to controls. Similarly, AnxA6 levels appear unaffected in murine nonalcoholic steatohepatitis and human liver fibrosis. Accordingly, adiponectin, lysophosphatidylcholine, palmitate, and TGFbeta, all of which have a role in liver injury, do not affect AnxA6 expression in human hepatocytes. Likewise, adiponectin and IL8 do not alter AnxA6 levels in primary human hepatic stellate cells. However, in hepatic tumors of 18 patients, AnxA6 protein levels are substantially reduced compared to nontumorous tissues. AnxA6 mRNA is even increased in the tumors suggesting that posttranscriptional mechanisms are involved herein. Lipidomic analysis shows trends toward elevated cholesteryl ester and sphingomyelin in the tumor samples, yet the ratio of tumor to nontumorous AnxA6 does not correlate with these lipids. The current study shows that AnxA6 is specifically reduced in human hepatocellular carcinoma suggesting a role of this protein in hepatocarcinogenesis.

Fetuin B Is a Secreted Hepatocyte Factor Linking Steatosis to Impaired Glucose Metabolism

Liver steatosis is associated with the development of insulin resistance and the pathogenesis of type 2 diabetes. We tested the hypothesis that protein signals originating from steatotic hepatocytes communicate with other cells to modulate metabolic phenotypes. We show that the secreted factors from steatotic hepatocytes induce pro-inflammatory signaling and insulin resistance in cultured cells. Next, we identified 168 hepatokines, of which 32 were differentially secreted in steatotic versus non-steatotic hepatocytes. Targeted analysis showed that fetuin B was increased in humans with liver steatosis and patients with type 2 diabetes. Fetuin B impaired insulin action in myotubes and hepatocytes and caused glucose intolerance in mice. Silencing of fetuin B in obese mice improved glucose tolerance. We conclude that the protein secretory profile of hepatocytes is altered with steatosis and is linked to inflammation and insulin resistance. Therefore, preventing steatosis may limit the development of dysregulated glucose metabolism in settings of overnutrition.

TPD52 expression increases neutral lipid storage within cultured cells

Tumor protein D52 (TPD52) is amplified and/or overexpressed in cancers of diverse cellular origins. Altered cellular metabolism (including lipogenesis) is a hallmark of cancer development, and protein-protein associations between TPD52 and known regulators of lipid storage, and differential TPD52 expression in obese versus non-obese adipose tissue, suggest that TPD52 might regulate cellular lipid metabolism. We found increased lipid droplet numbers in BALB/c 3T3 cell lines stably expressing TPD52, compared with control and TPD52L1-expressing cell lines. TPD52-expressing 3T3 cells showed increased fatty acid storage in triglyceride (from both de novo synthesis and uptake) and formed greater numbers of lipid droplets upon oleic acid supplementation than control cells. TPD52 colocalised with Golgi, but not endoplasmic reticulum (ER), markers and also showed partial colocalisation with lipid droplets coated with ADRP (also known as PLIN2), with a proportion of TPD52 being detected in the lipid droplet fraction. Direct interactions between ADRP and TPD52, but not TPD52L1, were demonstrated using the yeast two-hybrid system, with ADRP-TPD52 interactions confirmed using GST pulldown assays. Our findings uncover a new isoform-specific role for TPD52 in promoting intracellular lipid storage, which might be relevant to TPD52 overexpression in cancer.

ATGL-mediated triglyceride turnover and the regulation of mitochondrial capacity in skeletal muscle

Emerging evidence indicates that skeletal muscle lipid droplets are an important control point for intracellular lipid homeostasis and that regulating fatty acid fluxes from lipid droplets might influence mitochondrial capacity. We used pharmacological blockers of the major triglyceride lipases, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase, to show that a large proportion of the fatty acids that are transported into myotubes are trafficked through the intramyocellular triglyceride pool. We next tested whether increasing lipolysis from intramyocellular lipid droplets could activate transcriptional responses to enhance mitochondrial and fatty acid oxidative capacity. ATGL was overexpressed by adenoviral and adenoassociated viral infection in C2C12 myotubes and the tibialis anterior muscle of C57Bl/6 mice, respectively. ATGL overexpression in C2C12 myotubes increased lipolysis, which was associated with increased peroxisome proliferator-activated receptor (PPAR)-∂ activity, transcriptional upregulation of some PPAR∂ target genes, and enhanced mitochondrial capacity. The transcriptional responses were specific to ATGL actions and not a generalized increase in fatty acid flux in the myotubes. Marked ATGL overexpression (20-fold) induced modest molecular changes in the skeletal muscle of mice, but these effects were not sufficient to alter fatty acid oxidation. Together, these data demonstrate the importance of lipid droplets for myocellular fatty acid trafficking and the capacity to modulate mitochondrial capacity by enhancing lipid droplet lipolysis in vitro; however, this adaptive program is of minor importance when superimposing the normal metabolic stresses encountered in free-moving animals.

Targeting ASCT2-mediated glutamine uptake blocks prostate cancer growth and tumour development

Glutamine is conditionally essential in cancer cells, being utilized as a carbon and nitrogen source for macromolecule production, as well as for anaplerotic reactions fuelling the tricarboxylic acid (TCA) cycle. In this study, we demonstrated that the glutamine transporter ASCT2 (SLC1A5) is highly expressed in prostate cancer patient samples. Using LNCaP and PC‐3 prostate cancer cell lines, we showed that chemical or shRNA‐mediated inhibition of ASCT2 function in vitro decreases glutamine uptake, cell cycle progression through E2F transcription factors, mTORC1 pathway activation and cell growth. Chemical inhibition also reduces basal oxygen consumption and fatty acid synthesis, showing that downstream metabolic function is reliant on ASCT2‐mediated glutamine uptake. Furthermore, shRNA knockdown of ASCT2 in PC‐3 cell xenografts significantly inhibits tumour growth and metastasis in vivo, associated with the down‐regulation of E2F cell cycle pathway proteins. In conclusion, ASCT2‐mediated glutamine uptake is essential for multiple pathways regulating the cell cycle and cell growth, and is therefore a putative therapeutic target in prostate cancer. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Targeting ASCT2-mediated glutamine uptake blocks prostate cancer growth and tumour development

Glutamine is conditionally essential in cancer cells, being utilized as a carbon and nitrogen source for macromolecule production, as well as for anaplerotic reactions fuelling the tricarboxylic acid (TCA) cycle. In this study, we demonstrated that the glutamine transporter ASCT2 (SLC1A5) is highly expressed in prostate cancer patient samples. Using LNCaP and PC-3 prostate cancer cell lines, we showed that chemical or shRNA-mediated inhibition of ASCT2 function in vitro decreases glutamine uptake, cell cycle progression through E2F transcription factors, mTORC1 pathway activation and cell growth. Chemical inhibition also reduces basal oxygen consumption and fatty acid synthesis, showing that downstream metabolic function is reliant on ASCT2-mediated glutamine uptake. Furthermore, shRNA knockdown of ASCT2 in PC-3 cell xenografts significantly inhibits tumour growth and metastasis in vivo, associated with the down-regulation of E2F cell cycle pathway proteins. In conclusion, ASCT2-mediated glutamine uptake is essential for multiple pathways regulating the cell cycle and cell growth, and is therefore a putative therapeutic target in prostate cancer.

Uptake of 25-hydroxyvitamin D by muscle and fat cells

Vitamin D status, measured as serum 25-hydroxyvitamin D (25OHD) concentration, is determined by rates of input and of degradation. The half-life of 25OHD is surprisingly long for a steroid and much longer than its blood transporter, vitamin D binding protein. There is some evidence to suggest that vitamin D itself is stored in fat, whereas 25OHD concentrations are usually related to muscle-related parameters such as lean body mass and exercise. Both muscle and fat cells come from the mesenchymal cell lineage. We recently published evidence for net uptake of 25OHD into differentiated muscle cells, in a process that was megalin dependent, and speculated that this uptake might contribute to the extended half-life of 25OHD. Whether 25OHD is also taken up into cells of the adipocyte lineage is not clear. In the current study, we used the C2 muscle cell line as a source of myoblasts that were differentiated in culture to myotubes and 3T3-L1 pre-adipocytes that were differentiated into adipocytes in culture. We incubated the cells with trititated 25OHD and measured net uptake 4 and 16h afterwards. Differentiated myotubes took up labeled 25OHD in a time-dependent process to a far greater extent than myoblasts. In contrast, pre-adipocytes, but not differentiated adipocytes, accumulated labeled 25OHD in a time-dependent manner, though to a lesser extent than myotubes. Myotubes, but not myoblasts, showed megalin expression by immunohistochemistry. Pre-adipocytes, but not adipocytes, also showed expression of megalin. Since skeletal muscle consists mainly of differentiated muscle cells, while adipose tissue is mainly differentiated fat cells, it seems likely that muscle, but not fat tissue, provides a large extravascular pool through which 25OHD circulates and that this protects 25OHD from degradation. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

Evidence that diet-induced hyperleptinemia, but not hypothalamic gliosis, causes ghrelin resistance in NPY/AgRP neurons of male mice

High-fat diet (HFD) feeding causes ghrelin resistance in arcuate neuropeptide Y (NPY)/Agouti-related peptide neurons. In the current study, we investigated the time course over which this occurs and the mechanisms responsible for ghrelin resistance. After 3 weeks of HFD feeding, neither peripheral nor central ghrelin increased food intake and or activated NPY neurons as demonstrated by a lack of Fos immunoreactivity or whole-cell patch-clamp electrophysiology. Pair-feeding studies that matched HFD calorie intake with chow calorie intake show that HFD exposure does not cause ghrelin resistance independent of body weight gain. We observed increased plasma leptin in mice fed a HFD for 3 weeks and show that leptin-deficient obese ob/ob mice are still ghrelin sensitive but become ghrelin resistant when central leptin is coadministered. Moreover, ob/ob mice fed a HFD for 3 weeks remain ghrelin sensitive, and the ability of ghrelin to induce action potential firing in NPY neurons was blocked by leptin. We also examined hypothalamic gliosis in mice fed a chow diet or HFD, as well as in ob/ob mice fed a chow diet or HFD and lean controls. HFD-fed mice exhibited increased glial fibrillary acidic protein-positive cells compared with chow-fed mice, suggesting that hypothalamic gliosis may underlie ghrelin resistance. However, we also observed an increase in hypothalamic gliosis in ob/ob mice fed a HFD compared with chow-fed ob/ob and lean control mice. Because ob/ob mice fed a HFD remain ghrelin sensitive, our results suggest that hypothalamic gliosis does not underlie ghrelin resistance. Further, pair-feeding a HFD to match the calorie intake of chow-fed controls did not increase body weight gain or cause central ghrelin resistance; thus, our evidence suggests that diet-induced hyperleptinemia, rather than diet-induced hypothalamic gliosis or HFD exposure, causes ghrelin resistance.

Compound K modulates fatty acid-induced lipid droplet formation and expression of proteins involved in lipid metabolism in hepatocytes

BACKGROUND & AIMS

A key factor in the development of type 2 diabetes and non-alcoholic fatty liver disease (NAFLD) is hepatic steatosis. Incubation of human hepatic cells with free fatty acids (FFAs) causes accumulation of neutral lipids in lipid droplets (LDs) and serves as a model for hepatic steatosis. Ginsenosides, active constituents of ginsengs, have demonstrated beneficial effects in various pharmacological areas, including diabetes, however their effect on lipid accumulation in hepatocytes remains unclear. Here, we examine the effect of compound K (ComK), an active metabolite of ginsenosides, on the regulation of LD formation and on the expression of proteins involved in lipid homeostasis in hepatocytes.

METHODS

HuH7 cells were pretreated with ComK, followed by lipid loading with FFA. LDs were visualized using Oil Red O staining and immunohistochemistry for the LD-related protein PLIN2. Triglyceride levels were determined in isolated LDs. The expression of proteins involved in lipid homeostasis was examined by Western blotting.

RESULTS

Treatment with ComK significantly decreased LD formation in FFA-loaded HuH7 cells and increased phosphorylation levels of AMPK, and its substrate ACC. ComK also increased protein expression of peroxisome proliferator-activated receptor-α (PPAR-α) and acyl-CoA oxidase (ACOX1) together with elevated activity of a PPAR-α response element reporter construct. These effects were inhibited by the PPAR-α antagonist MK886.

CONCLUSIONS

ComK reduced LD formation and TG accumulation in FFA-loaded hepatocytes, in part by up-regulating AMPK activity and PPAR-α related pathways. These results suggest that ComK may have efficacy for the treatment of hepatic steatosis and associated diseases.

Loss of Krüppel-like factor 3 (KLF3/BKLF) leads to upregulation of the insulin-sensitizing factor adipolin (FAM132A/CTRP12/C1qdc2)

Krüppel-like factor 3 (KLF3) is a transcriptional regulator that we have shown to be involved in the regulation of adipogenesis in vitro. Here, we report that KLF3-null mice are lean and protected from diet-induced obesity and glucose intolerance. On a chow diet, plasma levels of leptin are decreased, and adiponectin is increased. Despite significant reductions in body weight and adiposity, wild-type and knockout animals show equivalent energy intake, expenditure, and excretion. To investigate the molecular events underlying these observations, we used microarray analysis to compare gene expression in Klf3(+/+) and Klf3(-/-) tissues. We found that mRNA expression of Fam132a, which encodes a newly identified insulin-sensitizing adipokine, adipolin, is significantly upregulated in the absence of KLF3. We confirmed that KLF3 binds the Fam132a promoter in vitro and in vivo and that this leads to repression of promoter activity. Further, plasma adipolin levels were significantly increased in Klf3(-/-) mice compared with wild-type littermates. Boosting levels of adipolin via targeting of KLF3 offers a novel potential therapeutic strategy for the treatment of insulin resistance.

Ceramides contained in LDL are elevated in type 2 diabetes and promote inflammation and skeletal muscle insulin resistance

Dysregulated lipid metabolism and inflammation are linked to the development of insulin resistance in obesity, and the intracellular accumulation of the sphingolipid ceramide has been implicated in these processes. Here, we explored the role of circulating ceramide on the pathogenesis of insulin resistance. Ceramide transported in LDL is elevated in the plasma of obese patients with type 2 diabetes and correlated with insulin resistance but not with the degree of obesity. Treating cultured myotubes with LDL containing ceramide promoted ceramide accrual in cells and was accompanied by reduced insulin-stimulated glucose uptake, Akt phosphorylation, and GLUT4 translocation compared with LDL deficient in ceramide. LDL-ceramide induced a proinflammatory response in cultured macrophages via toll-like receptor-dependent and -independent mechanisms. Finally, infusing LDL-ceramide into lean mice reduced insulin-stimulated glucose uptake, and this was due to impaired insulin action specifically in skeletal muscle. These newly identified roles of LDL-ceramide suggest that strategies aimed at reducing hepatic ceramide production or reducing ceramide packaging into lipoproteins may improve skeletal muscle insulin action.

Regulation of plasma ceramide levels with fatty acid oversupply: evidence that the liver detects and secretes de novo synthesised ceramide

AIMS/HYPOTHESIS

Plasma ceramide concentrations correlate with insulin sensitivity, inflammation and atherosclerotic risk. We hypothesised that plasma ceramide concentrations are increased in the presence of elevated fatty acid levels and are regulated by increased liver serine C-palmitoyltransferase (SPT) activity.

METHODS

Lean humans and rats underwent an acute lipid infusion and plasma ceramide levels were determined. One group of lipid-infused rats was administered myriocin to inhibit SPT activity. Liver SPT activity was determined in lipid-infused rats, and obese, insulin resistant mice. The time and palmitate dose-dependent synthesis of intracellular and secreted ceramide was determined in HepG2 liver cells.

RESULTS

Plasma ceramide levels were increased during lipid infusion in humans and rats, and in obese, insulin-resistant mice. The increase in plasma ceramide was not associated with changes in liver SPT activity, and inhibiting SPT activity by ~50% did not alter plasma ceramide levels in lipid-infused rats. In HepG2 liver cells, palmitate incorporation into extracellular ceramide was both dose- and time-dependent, suggesting the liver cells rapidly secreted the newly synthesised ceramide.

CONCLUSIONS/INTERPRETATION

Elevated systemic fatty acid availability increased plasma ceramide but this was not associated with changes in hepatic SPT activity, suggesting that liver ceramide synthesis is driven by substrate availability rather than increased SPT activity. This report also provides evidence that the liver is sensitive to the intracellular ceramide concentration, and an increase in liver ceramide secretion may help protect the liver from the deleterious effects of intracellular ceramide accumulation.