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Metabolic diseases are closely linked to aberrant synthesis of endogenous fatty acids in the liver, called de novo lipogenesis (DNL), which is mediated by the enzyme fatty acid synthase (FASN). The composition of complex lipids consists of saturated or monosaturated fatty acids, which can be endogenously produced, and polyunsaturated fatty acids (PUFA), which are strictly dietary. Compositional differences between individuals are insufficiently understood and may influence the onset and progression of metabolic and cardiovascular diseases. Here we show that DNL critically determines the use of dietary PUFA. A patient with a hypofunctional heterozygous de novo Arg2177Cys variant in FASN exhibited an elevated composition of PUFA, which was phenocopied by pharmacological inhibition of FASN with TVB-2640 in patients with nonalcoholic steatohepatitis (NASH). In mice, the incorporation rate of supplemented omega-3 PUFA during an obesogenic diet was increased by genetic or pharmacologic reduction of DNL. Mechanistically, we show that the FASN variant exhibited a cysteine-dependent, non-enzymatic acetylation of FASN, which resulted in hyperubiquitinylation and decreased protein stability. The study reveals that PUFA storage is an active, enzymatic process controlled by FASN, DGAT2 and MFSD2A and that combining FASN inhibition and PUFA supplementation exerts additive beneficial metabolic effects. These findings provide evidence that the success of PUFA supplementation may depend on the rate of endogenous DNL and that combined PUFA supplementation and FASN inhibition may be a promising approach in metabolic disease.

Metabolic diseases such as obesity, type 2 diabetes mellitus and non-alcoholic steatohepatitis (NASH) are closely linked to aberrant synthesis of endogenous fatty acids in the liver, called de novo lipogenesis, which is mediated by the enzyme fatty acid synthase (FASN). Endogenously synthesized fatty acids are either saturated or monounsaturated and can adversely affect metabolic health through mechanisms that are not sufficiently understood. Here we show that lipogenesis critically determines the use of food-derived, metabolically beneficial polyunsaturated fatty acids (PUFA). We found a patient with a hypofunctional heterozygous de novo Arg2177Cys mutation in FASN, which resulted in a cysteine-dependent, non-enzymatic acetylation of FASN and decreased protein stability. Remarkably, the patient showed markedly elevated levels of polyunsaturated fatty acids, whereas patients with high FASN expression in the liver showed diminished levels. Moreover, pharmacological intervention with the FASN-inhibitor TVB-2640 in patients with NASH was sufficient to increase the rate of PUFA use in circulating lipids. In line, the incorporation rate of supplemented omega-3 PUFA in obesogenic diet could be dramatically increased by reducing the lipogenesis rate in mice. Mechanistically, we show that fatty acid storage preferences are determined by an active, enzymatic process dependent on FASN and DGAT2. Our results demonstrate that low lipogenesis rates increase the efficiency of PUFA incorporation into complex lipids, whereas high lipogenesis rates lead to low PUFA use. This has critical clinical implications for the prediction of low therapeutic success of omega-3 supplementation in patients with high lipogenesis and provides evidence for the urgent development of combined therapy options targeting lipogenesis and PUFA supplementation.

The small GTPase RAC1B has been shown to act as a powerful inhibitor of the transforming growth factor (TGF)&beta, type I receptor ALK5 and TGF&beta, 1/ALK5-induced epithelial&ndash, mesenchymal transition and cell motility. However, the precise mechanism has remained elusive. RNAi-mediated knockdown of RAC1B in the pancreatic ductal adenocarcinoma (PDAC)-derived cell line Panc1 failed to alter transcriptional activity from a transfected ALK5 promoter&ndash, reporter construct. In contrast, pharmacological inhibition of the proteasome decreased the abundance of ALK5 protein in cell lines of the mesenchymal subtype (Panc1, IMIM-PC-1, and breast cancer MDA-MB-231), but not in a PDAC cell line of the epithelial subtype (Colo357). Here, we focused on the inhibitory Smad protein, SMAD7, as a potential candidate for RAC1B-mediated inhibition of cell migration. In Panc1 cells devoid of RAC1B, SMAD7 protein was dramatically reduced and these cells were refractory to TGF&beta, 1-induced upregulation of SMAD7 protein but not mRNA expression. Intriguingly, RNAi-mediated knockdown or ectopic overexpression of SMAD7 in Panc1 cells up- or downregulated, respectively, ALK5 protein expression and mimicked the suppressive effect of RAC1B on TGF&beta, /SMAD3-dependent transcriptional activity, target gene expression and cell migration. Transfection of SMAD7 was further able to partially rescue cells from the RAC1B knockdown-mediated increase in migratory properties. Conversely, knockdown of SMAD7 was able to partially rescue Panc1 and MDA-MB-231 cells from the antimigratory effect of ectopically expressed RAC1B. Finally, we demonstrate that RAC1B upregulation of SMAD7 protein requires intermittent transcriptional induction of the deubiquitinating enzyme USP26. Our data suggest that RAC1B induces SMAD7 by promoting its deubiquitination and establishes this Smad as one of RAC1B&rsquo, s downstream effectors in negative regulation of ALK5 and TGF&beta, 1-induced cell migration in mesenchymal-type carcinoma cells.