Hannelore Heemers

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.

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.

Androgens, lipogenesis and prostate cancer.

Both experimental and epidemiological data indicate that androgens are among the main factors controlling the development, maintenance and progression of prostate cancer. Identifying the genes that are regulated by androgens represents a major step towards the elucidation of the mechanisms underlying the impact of androgens on prostate cancer cell biology and is an attractive approach to find novel targets for prostate cancer therapy. Among the genes that have been identified thus far, several genes encode lipogenic enzymes. Studies aimed at the elucidation of the mechanisms underlying androgen regulation of lipogenic genes revealed that androgens coordinately stimulate the expression of these genes through interference with the molecular mechanism controlling activation of sterol regulatory element-binding proteins (SREBPs), lipogenic transcription factors governing cellular lipid homeostasis. The resulting increase in lipogenesis serves the synthesis of key membrane components (phospholipids, cholesterol) and is a major hallmark of cancer cells. Pharmacologic inhibition of lipogenesis or RNA-interference-mediated down-regulation of key lipogenic genes induces apoptosis in cancer cell lines and reduces tumor growth in xenograft models. While increased lipogenesis is already found in the earliest stages of cancer development (PIN) and initially is androgen-responsive it persists or re-emerges with the development of androgen-independent cancer, indicating that lipogenesis is a fundamental aspect of prostate cancer cell biology and is a potential target for chemoprevention and for antineoplastic therapy in advanced prostate cancer.

Androgens stimulate coordinated lipogenic gene expression in normal target tissues in vivo

In prostate cancer cell lines in culture androgens cause a marked and coordinated upregulation of the expression of several lipogenic genes. Here, using castrated male Wistar rats as an experimental paradigm, we investigated whether coordinated androgen stimulation of lipogenic gene expression represents a more general physiological regulation in non-cancerous androgen-responsive cells as well. In typical target tissues for androgen action such as the ventral prostate and the lacrimal gland, androgen deprivation resulted in a marked reduction in the mRNA and protein levels of genes involved in fatty acid (fatty acid synthase and acetyl-CoA-carboxylase) and cholesterol synthesis (HMG-CoA-reductase and farnesyl diphosphate synthase). Readministration of testosterone immediately following orchidectomy restored the expression of all four genes. Substitution of testosterone by the non-aromatizable androgen dihydrotestosterone gave rise to comparable changes in the mRNA and protein levels of the lipogenic genes under investigation, confirming the involvement of the androgen receptor in the observed effects. In support of the coordinate nature of this regulation, androgen-induced upregulation of lipogenic gene expression is accompanied by an increase in the nuclear content of SREBP, a key lipogenic transcription factor. Taken together, these findings provide evidence for a coordinate regulation of lipogenic gene expression not only in prostate cancer cell lines in culture but also in non-cancerous androgen-responsive tissues in vivo.

Androgen regulation of lipogenesis

In several tissues and cell lines, including epithelial cells of the prostate, breast and seminal vesicles, and in acinar cells of the meibomian and other sebaceous glands, androgens have a major stimulatory effect on the production, accumulation and/or secretion of lipids.1–11 In most of these tissues and cell lines, the composition of these lipids is complex. Accumulating lipids include cholesterol esters, diacylglycerides, triglycerides and wax esters. Depending on the tissue type, these lipids may have different physiological functions and dysregulation of their production is related to pathological conditions as diverse as acne, dry eye syndromes and cancer. In acne and dry eye syndromes local modulation of androgen action may provide an effective therapy.7,12

Androgens Stimulate the SREBP Pathway in Prostate Cancer Cells by Inducing a Shift in the SCAP-Retention Protein(s) Balance

In human prostate cancer (PCA) cells, androgens (As) coordinately stimulate the expression of genes involved in the synthesis of fatty acids and cholesterol. Interestingly, several of these genes are overexpressed in human cancers, particularly those with a poor prognosis, and are promising targets for anti-neoplastic therapy. Exploration of the mechanism underlying the lipogenic effects of As revealed that As induce activation of Sterol Regulatory Element-Binding Proteins (SREBPs), proteolytically activated transcription factors that play a central role in the control of cellular lipid homeostasis. The exact mechanism by which As interfere with the SREBP pathway remains to be investigated. Herein, we show that A-activation of the SREBP pathway depends on the presence of the structural elements required also for its sterol-regulated activation. Detailed studies revealed that As substantially increase the expression of the transporter protein SCAP in 2 of the 3 cell lines tested. Co-transfection of an expression vector encoding SCAP lead to a marked increase in the transcription of a key gene involved in cholesterol synthesis. In support of the involvement of activated SREBPs in the observed effects, SCAP-mediated transcriptional activation of this lipogenic gene was dependent on the presence of intact SREBP binding sites in its promoter region, and was counteracted by addition of dominant-negative SREBP forms. These data suggest that As activate the SREBP pathway by inducing a change in the normal cellular balance between SCAP and its retention protein complex.

Androgens and the Lipogenic Switch in Prostate Cancer

Androgens have a major impact on prostate cancer cell biology and modulate a variety of key cellular processes and functions. One of the processes that are most strikingly affected is lipid biosynthesis. Through a unique indirect mechanism that involves activation of the lipogenic transcription factor SREBP, androgens coordinately stimulate the expression of more than 20 enzymes involved in lipid synthesis, and in this way they affect the entire lipogenic program in prostate cancer cells. Through additional mechanisms, including the stimulation of an ubiquitin-specific protease that removes the degradation-tag ubiquitin from lipogenic enzymes such as fatty acid synthase, an even more complex network of regulatory control is created. Progressive deregulation of this network results in a marked overexpression of lipogenic enzymes, referred to as the lipogenic switch. This switch typically accompanies the development and progression of prostate cancer and is thought to play an active role in prostate cancer cell biology. In fact, interference with the lipogenic process impairs proper membrane formation and functioning, halts cell proliferation, and induces cell death selectively in cancer cells. These findings suggest that enhanced lipogenesis in cancer cells is an essential trait of prostate cancer progression and is a promising novel target for antineoplastic intervention.