Johannes V. Swinnen

Lipogenesis and lipolysis: the pathways exploited by the cancer cells to acquire fatty acids

One of the most important metabolic hallmarks of cancer cells is enhanced lipogenesis. Depending on the tumor type, tumor cells synthesize up to 95% of saturated and mono-unsaturated fatty acids (FA) de novo in spite of sufficient dietary lipid supply. This lipogenic conversion starts early when cells become cancerous and further expands as the tumor cells become more malignant. It is suggested that activation of FA synthesis is required for carcinogenesis and for tumor cell survival. These observations suggest that the enzymes involved in FA synthesis would be rational therapeutic targets for cancer treatment. However, several recent reports have shown that the anti-tumor effects, following inhibition of endogenous FA synthesis in cancer cell lines may be obviated by adding exogenous FAs. Additionally, high intake of dietary fat is reported to be a potential risk factor for development and poor prognosis for certain cancers. Recently it was reported that breast and liposarcoma tumors are equipped for both de novo fatty acid synthesis pathway as well as LPL-mediated extracellular lipolysis. These observations indicate that lipolytically acquired FAs may provide an additional source of FAs for cancer. This review focuses on our current understanding of lipogenic and lipolytic pathways in cancer cell progression.

Increased lipogenesis in cancer cells: new players, novel targets.

Purpose of reviewThis review evaluates recent findings on the mechanisms by which lipogenic enzymes are upregulated or activated in cancer cells, the implications of increased lipogenesis for cancer cell biology and the feasibility of exploiting this pathway and its regulators as targets for antineoplastic intervention. Recent findingsThe list of cancer types showing increased lipogenic enzyme expression keeps growing and further evidence is accumulating that growth factor signaling and particularly activation of the phosphatidylinositol 3′-kinase/protein kinase B pathway plays a role in this process. This signaling pathway stimulates lipogenic gene transcription through activation of the lipogenic transcription factor sterol regulatory element-binding protein-1 and directly activates lipogenic enzymes such as ATP-citrate lyase, linking the upregulation of lipogenesis in cancer cells to the well known tumor-associated increase in glycolysis. Steroid hormones, overexpression of the ubiquitin-specific protease-2a and mutations in breast cancer susceptibility gene 1 may further enhance lipid synthesis. While fatty acid synthase is further established as a target for antineoplastic intervention, recent findings show that interference with acetyl-CoA carboxylase-α, ATP citrate lyase or the AMP-activated protein kinase limits cancer cell proliferation and survival. SummaryThe same disturbances in signaling pathways responsible for oncogenic transformation may also contribute to the increased lipogenesis observed in tumor cells. Increased lipogenesis involves modulation of multiple lipogenic enzymes at both transcriptional and posttranscriptional level and is linked to other cancer-associated metabolic changes. Not only fatty acid synthase, but in fact all key enzymes involved in fatty acid synthesis as well as key metabolic regulators are potential targets for antineoplastic intervention.

Overexpression of fatty acid synthase is an early and common event in the development of prostate cancer

The expression of fatty acid synthase (FAS), a key lipogenic enzyme and potential target for antineoplastic therapy, was analyzed in 87 frozen needle biopsies of prostate cancer using a highly sensitive immunohistochemical detection technique (Envision). In comparison to normal or benign, hyperplastic glandular structures, which were all negative for FAS staining, immunohistochemical signal was evident in 24/25 low grade prostatic epithelial neoplasia (PIN) lesions, in 26/26 high grade PIN lesions and in 82/87 invasive carcinomas. Staining intensity tended to increase from low grade to high grade PIN to invasive carcinoma. Cancers with a high FAS expression had an overall high proliferative index. No correlation was found between FAS expression and lipid accumulation. These findings indicate that increased FAS expression is one of the earliest and most common events in the development of prostate cancer, suggesting that FAS may be used as a general prostate cancer marker and that antineoplastic therapy based on FAS inhibition may be an option for chemoprevention or curative treatment for nearly all prostate cancers.

De novo Lipogenesis Protects Cancer Cells from Free Radicals and Chemotherapeutics by Promoting Membrane Lipid Saturation

Activation of de novo lipogenesis in cancer cells is increasingly recognized as a hallmark of aggressive cancers and has been implicated in the production of membranes for rapid cell proliferation. In the current report, we provide evidence that this activation has a more profound role. Using a mass spectrometry-based phospholipid analysis approach, we show that clinical tumor tissues that display the lipogenic phenotype show an increase in the degree of lipid saturation compared with nonlipogenic tumors. Reversal of the lipogenic switch in cancer cells by treatment with the lipogenesis inhibitor soraphen A or by targeting lipogenic enzymes with small interfering RNA leads to a marked decrease in saturated and mono-unsaturated phospholipid species and increases the relative degree of polyunsaturation. Because polyunsaturated acyl chains are more susceptible to peroxidation, inhibition of lipogenesis increases the levels of peroxidation end products and renders cells more susceptible to oxidative stress-induced cell death. As saturated lipids pack more densely, modulation of lipogenesis also alters lateral and transversal membrane dynamics as revealed by diffusion of membrane-targeted green fluorescent protein and by the uptake and response to doxorubicin. These data show that shifting lipid acquisition from lipid uptake toward de novo lipogenesis dramatically changes membrane properties and protects cells from both endogenous and exogenous insults. These findings provide important new insights into the role of de novo lipogenesis in cancer cells, and they provide a rationale for the use of lipogenesis inhibitors as antineoplastic agents and as chemotherapeutic sensitizers.

RNA Interference–Mediated Silencing of the Acetyl-CoA-Carboxylase-α Gene Induces Growth Inhibition and Apoptosis of Prostate Cancer Cells

Overexpression of lipogenic enzymes is a common characteristic of many cancers. Thus far, studies aimed at the exploration of lipogenic enzymes as targets for cancer intervention have focused on fatty acid synthase (FAS), the enzyme catalyzing the terminal steps in fatty acid synthesis. Chemical inhibition or RNA interference (RNAi)–mediated knockdown of FAS consistently inhibits the growth and induces death of cancer cells. Accumulation of the FAS substrate malonyl-CoA has been implicated in the mechanism of cytotoxicity of FAS inhibition. Here, using RNAi technology, we have knocked down the expression of acetyl-CoA carboxylase-α (ACC-α), the enzyme providing the malonyl-CoA substrate. Silencing of the ACC-α gene resulted in a similar inhibition of cell proliferation and induction of caspase-mediated apoptosis of highly lipogenic LNCaP prostate cancer cells as observed after FAS RNAi. In nonmalignant cells with low lipogenic activity, no cytotoxic effects of knockdown of ACC-α or FAS were observed. These findings indicate that accumulation of malonyl-CoA is not a prerequisite for cytotoxicity induced by inhibition of tumor-associated lipogenesis and suggest that in addition to FAS, ACC-α is a potential target for cancer intervention.

Fatty acid synthase drives the synthesis of phospholipids partitioning into detergent-resistant membrane microdomains

Fatty acid synthase (FAS) is a key metabolic enzyme catalyzing the synthesis of long-chain saturated fatty acids. It plays a central role in the production of surfactant in fetal lungs, in the supply of fatty components of milk, and in the conversion and storage of energy in liver and adipose tissue. Remarkably high levels of FAS expression are found in the majority of human epithelial cancers. As the role of FAS in cancer cells remains largely unknown, we have initiated studies to assess the fate of newly synthesized lipids in cancer cells and have estimated the contribution of FAS to the synthesis of specific lipid classes by treating the cells with small interfering RNAs targeting FAS. Here, we show that in cancer cells FAS plays a major role in the synthesis of phospholipids partitioning into detergent-resistant membrane microdomains. These are raft-aggregates implicated in key cellular processes including signal transduction, intracellular trafficking, cell polarization, and cell migration. These findings reveal a novel role for FAS, provide important new insights into the otherwise poorly understood mechanisms underlying the control of lipid composition of membrane microdomains, and point to a link between FAS overexpression and dysregulation of membrane composition and functioning in tumor cells.

Chemical Inhibition of Acetyl-CoA Carboxylase Induces Growth Arrest and Cytotoxicity Selectively in Cancer Cells

Development and progression of cancer is accompanied by marked changes in the expression and activity of enzymes involved in the cellular homeostasis of fatty acids. One class of enzymes that play a particularly important role in this process are the acetyl-CoA carboxylases (ACC). ACCs produce malonyl-CoA, an intermediate metabolite that functions as substrate for fatty acid synthesis and as negative regulator of fatty acid oxidation. Here, using the potent ACC inhibitor soraphen A, a macrocyclic polyketide from myxobacteria, we show that ACC activity in cancer cells is essential for proliferation and survival. Even at nanomolar concentrations, soraphen A can block fatty acid synthesis and stimulate fatty acid oxidation in LNCaP and PC-3M prostate cancer cells. As a result, the phospholipid content of cancer cells decreased, and cells stopped proliferating and ultimately died. LNCaP cells predominantly died through apoptosis, whereas PC-3M cells showed signs of autophagy. Supplementation of the culture medium with exogenous palmitic acid completely abolished the effects of soraphen A and rescued the cells from cell death. Interestingly, when added to cultures of premalignant BPH-1 cells, soraphen A only slightly affected cell proliferation and did not induce cell death. Together, these findings indicate that cancer cells have become dependent on ACC activity to provide the cell with a sufficient supply of fatty acids to permit proliferation and survival, introducing the concept of using small-molecule ACC inhibitors as therapeutic agents for cancer.

Selective activation of the fatty acid synthesis pathway in human prostate cancer

A substantial subset of breast, colorectal, ovarian, endometrial and prostatic cancers displays markedly elevated expression of immunohistochemically detectable fatty acid synthase, a feature that has been associated with poor prognosis and that may be exploited in anti‐neoplastic therapy. Here, using an RNA array hybridisation technique complemented by in situ hybridisation, we report that in prostate cancer fatty acid synthase expression is up‐regulated at the mRNA level together with other enzymes of the same metabolic pathway. Contrary to the observations that in many cell systems (including androgen‐stimulated LNCaP prostate cancer cells) fatty acid and cholesterol metabolism are co‐ordinately regulated so as to supply balanced amounts of lipids for membrane biosynthesis, storage or secretion, no changes in the expression of genes involved in cholesterol synthesis were found. These findings point to selective activation of the fatty acid synthesis pathway and suggest a shift in the balance of lipogenic gene expression in a subgroup of prostate cancers. Int. J. Cancer 88:176–179, 2000.

ATP-Citrate Lyase: A Key Player in Cancer Metabolism

ATP-citrate lyase (ACLY) is a cytosolic enzyme that catalyzes the generation of acetyl CoA from citrate. Acetyl CoA is a vital building block for the endogenous biosynthesis of fatty acids and cholesterol and is involved in isoprenoid-based protein modifications. Acetyl CoA is also required for acetylation reactions that modify proteins, such as histone acetylation. ACLY is upregulated or activated in several types of cancers, and its inhibition is known to induce proliferation arrest in cancer cells both in vitro and in vivo. The present review highlights current knowledge about the role of ACLY in cancer cells, with special reference to the different pathways that are linked by ACLY.

Stimulation of tumor-associated fatty acid synthase expression by growth factor activation of the sterol regulatory element-binding protein pathway

Increased expression of fatty acid synthase (FAS) is observed in a clinically aggressive subset of various common cancers and interference with FAS offers promising opportunities for selective chemotherapeutic intervention. The mechanisms by which FAS expression is (up)-regulated in these tumors remain, however, largely unknown. Recently we demonstrated that in LNCaP prostate cancer cells FAS expression is markedly elevated by androgens via an indirect pathway involving sterol regulatory element-binding proteins (SREBPs). Here, we also show that growth factors such as EGF are able to stimulate FAS mRNA, protein and activity. Several observations also indicate that the effects of EGF on FAS expression are ultimately mediated by SREBPs. EGF stimulates SREBP-1c mRNA expression and induces an increase in mature nuclear SREBP-1. Moreover, in transient transfection studies EGF stimulates the transcriptional activity of a 178 bp FAS promoter fragment harboring a complex SREBP-binding site. Deletion or mutation of this binding site abolishes these effects and ectopic expression of dominant negative SREBP-1 inhibits FAS expression and induction in intact LNCaP cells. Given the frequent dysregulation of growth factor signaling in cancer and the key role of SREBP-1 in lipid homeostasis, growth factor-induced activation of the SREBP pathway is proposed as one of the mechanisms responsible for up-regulation of lipogenic gene expression in a subset of cancer cells.