Biochemistry, molecular biology, and pharmacology of fatty acid synthase, an emerging therapeutic target and diagnosis/prognosis marker

Abstract

Human fatty acid synthase (FASN) is a 270-kDa cytosolic dimeric enzyme that is responsible for palmitate synthesis. FASN is slowly emerging and rediscovered as a marker for diagnosis and prognosis of human cancers. Recent studies showed that FASN is an oncogene and inhibition of FASN effectively and selectively kill cancer cells. With recent publications of the FASN crystal structure and the new development of FASN inhibitors, targeting FASN opens a new window of opportunity for metabolically combating cancers. In this article, we will review critically the recent progresses in understanding the structure, function, and the role of FASN in cancers and pharmacologically targeting FASN for human cancer treatment.

Keywords: Fatty acid synthase; cancer; diagnosis; drug resistance; inhibitors; prognosis.

Figure 1.

De novo fatty acid synthesis. The de novo fatty acid synthesis pathway functions in both cancers and lipogenic tissues. In both cases, excess glucose goes through glycolysis and TCA cycle, and exits mitochondria as citrate which is then converted to acetyl-CoA by ATP citrate lyase. Carboxylation of acetyl-CoA to malonyl-CoA is catalyzed by acetyl-CoA carboxylase (ACC). FASN condenses one acetyl-CoA and seven malonyl-CoA into palmitate which can be then modified into various lipids such as phospholipids.

Figure 2.

Models of domain organization of FASN. (A) Conventional dimeric model of FASN. In this model, the two subunits in the homo-dimeric FASN are arranged in a fully extended head-to-tail organization. (B) Revised model of domain organization. In this revise model, FASN adopts an X-shaped dimeric form with each monomer in coiled structure to allow multiple intra- and inter-subunit interactions. KS = ketoacyl synthase; MAT = malonyl/acetyltransferase; DH = dehydrogenase; ER = enoyl reductase; KR=ketoacyl reductase, ACP = acyl carrier protein; TE = thio-esterase.

Figure 3.

Atomic structure of FASN. The overall structure of FASN dimer is X-shaped (viewed in perpendicular to its pseudo-2-fold axis). One subunit is colored by different shades of blue and green for different domains. The other subunit is in infrared colors ranging from magenta to orange. The two non-enzymatic domains, pseudo-ketoreductase (ΨKR) and pseudo-methyltransferase (ΨME), are colored in gray and black, respectively, for both subunits. This figure was created from FASN structure (PDF ID: 2VZ8)

Figure 4.

FASN-catalyzed palmitate synthesis. FASN-catalyzed palmitate synthesis involves three steps: initiation, elongation, and termination. The initiation step involves condensation of acetyl-CoA and malonyl-CoA catalyzed by the MAT domain. The elongation step of condensation of additional malonyl-CoA is catalyzed by KS, KR, DH, and ER domains. The final step of termination is catalyzed by the TE domain to release palmitate from FASN.

Figure 5.

FASN expression in MCF7 and the stepwise-selected drug resistant and revertant cell lines. (A). Western blot analyses. 20 mg proteins, each from MCF7, its stepwise-selected MCF7/AdVplO, MCF7/AdVplOO, and MCF7/AdVp3000 cells as well as the revertant cell line MCF7/Rev, were separated by SDS-PAGE followed by western blot analyses using FASN antibody. GAPDH was used as a loading control. (B). Real time RT-PCR analyses. RNAs isolated from MCF7, its stepwise-selected MCF7/AdVplO, MCF7/AdVplOO, and MCF7/AdVp3000 cells as well as the revertant cell line MCF7/Rev were subjected to real time RT-PCR analysis using SYBR green. The relative level of FASN mRNA calculated in the fold change (2^ΔΔCt) relative to that in MCF7 cells after normalization by internal control, GAPDH.

Figure 6.

Cell cycle analysis of FASN over-expressing MCF7 cells and vector transfected control MCF7 cells. 5x105 cells were harvested, labeled with propidium iodide and analyzed by flow cytometry analysis for stage of cell cycle, G0/G1, S, G2/M.

Figure 7.

Binding mode of Orlistat in the TE domain of FASN. Only the molecular surface of the TE domain where Orlistat binds is shown. The binding of Orlistat in the TE domain exists as a serine adduct intermediate (pink ball and stick, panel A) and hydrolyzed product (gray ball and stick, panel B). The serine intermediate is also superimposed to show the shifts and changes in the hydrolyzed product in panel B. This figure was generated from data in PDB (ID: 2PX6).