A Metabolic Basis for Endothelial-to-Mesenchymal Transition

Abstract

Endothelial-to-mesenchymal transition (EndoMT) is a cellular process often initiated by the transforming growth factor β (TGF-β) family of ligands. Although required for normal heart valve development, deregulated EndoMT is linked to a wide range of pathological conditions. Here, we demonstrate that endothelial fatty acid oxidation (FAO) is a critical in vitro and in vivo regulator of EndoMT. We further show that this FAO-dependent metabolic regulation of EndoMT occurs through alterations in intracellular acetyl-CoA levels. Disruption of FAO via conditional deletion of endothelial carnitine palmitoyltransferase II (Cpt2^E-KO) augments the magnitude of embryonic EndoMT, resulting in thickening of cardiac valves. Consistent with the known pathological effects of EndoMT, adult Cpt2^E-KO mice demonstrate increased permeability in multiple vascular beds. Taken together, these results demonstrate that endothelial FAO is required to maintain endothelial cell fate and that therapeutic manipulation of endothelial metabolism could provide the basis for treating a growing number of EndoMT-linked pathological conditions.

Keywords: endothelial-mesenchymal transition; fatty acid oxidation; metabolism; vascular permeability.

Figure 1. Induction of EndoMT is accompanied by an inhibition of FAO

(A) TGF-β1 and IL-1β induce morphological changes consistent with EndoMT. (B) Cytokine treatment induces EndoMT (technical triplicates per condition, data represents one of three similar, independent experiments). (C) Cytokine stimulation reciprocally alters short chain acylcarnitines and the organic acid profile (n=5 replicates per condition). (D) Level of endothelial CPT1A mRNA expression after cytokine treatment (n=3 independent experiments). (E) Cytokine and/or etomoxir treatment suppresses FAO as measured by the inability of palmitate conjugated bovine serum albumin (Palm-BSA) to stimulate the oxygen consumption rate (OCR) (n≥4 technical replicates per condition, data is one representative experiment of three similar independent experiments). (F) Rate of FAO after cytokine treatment, using [¹⁴C]-labeled palmitate (n=3 independent experiments). Data represents mean ± SEM, with significance determined by one-way ANOVA with a Bonferroni’s multiple comparison test (E). * p < 0.05; ** p < 0.01; ns, not significant.

Figure 2. FAO modulates in vitro EndoMT

(A) Western blot (WB) analysis demonstrating that heterologous expression of CPT1A inhibits cytokine-induced EndoMT markers. (B) qRT-PCR analysis of EndoMT markers with or without cytokine treatment, and with or without heterologous CPT1A expression (technical triplicates per condition, data represents one of two independent experiments). (C) Endothelial cell morphology following knockdown of CPT1A (CPT1A^KD). (D) Knockdown of CPT1A activates EndoMT (technical triplicates per condition, results are representative of one of three independent experiments). (E and F) Representative immunostaining of EndoMT markers in CPT1A^KD endothelial cells: mesenchymal marker PAI1 (E) or endothelial marker CD31 (F). Nuclei are stained with DAPI. (G) Quantification of average fluorescence intensity (AFI) of the indicated proteins following knockdown of CPT1A. Data represents mean ± SEM, with significance determined by one-way ANOVA with a Bonferroni’s multiple comparison test (B). * p < 0.05; ** p < 0.01; ns, not significant.

Figure 3. Acetyl-CoA levels modulate EndoMT

(A) Representative Western blot and quantification of phosphorylated SMAD2 (pSMAD2) in the presence or absence of the TGFβ signaling inhibitor SB431542 following control or CPT1A knockdown (n=3 independent experiments). (B) Levels of acetyl-CoA in control or cytokine-treated endothelial cells (n=3 independent experiments). (C) Cytosolic acetyl-CoA levels can be increased by ACSS2-mediated acetate conversion or reduced by the ACLY chemical inhibitor SB-204990. ACLY, ATP citrate lyase; ACSS2, acyl-coenzyme A synthetase short-chain family member 2. (D) Acetate treatment inhibits cytokine-induced pSMAD2 (n=3 independent experiments). (E) Acetate treatment inhibits EndoMT (technical triplicates per condition, data are representative of one of three independent experiments). (F) Acetate treatment inhibits cytokine-induced EndoMT protein expression. (G) SB-204990 induces EndoMT (technical triplicates per condition, data represents one of three independent experiments). (H and I) Western blot analysis of wildtype SMAD7 protein levels with or without acetate or SB204990 (H) or expression of the K64/70A acetylation-defective SMAD7 mutant under these same conditions (I) (n=3 independent experiments). Data represents mean ± SEM, with significance determined by one-way ANOVA with Bonferroni’s multiple comparison test (A, D, E, H, I). * p < 0.05; ** p < 0.01; ns, not significant.

Figure 4. FAO modulates in vivo EndoMT

(A) Western blot analysis of CPT2 expression in primary lung endothelial cells derived from Cpt2^E-WT or Cpt2^E-KO mice. (B) Basal OCR of indicated endothelial cells in the presence of BSA alone or Palm-BSA. (n=4 technical replicates per condition, data represent one of three independent experiments). (C) Level of acetyl-CoA in indicated endothelial cells with or without acetate supplementation (n=3 independent experiments). (D) Representative Western blot and quantification of pSMAD2 levels with or without acetate treatment as indicated (n=3 independent experiments). (E and F) Representative images (E) and quantification (F) of the thickness of the anterior mitral valve leaflet in Cpt2^E-WT and Cpt2^E-KO mice (n=9 mice per genotype). Data represent scatter plots with the median value indicated. (G and H) Representative images (G) and quantification (H) of EndoMT using lineage tracing within the anterior mitral valve leaflet in embryos at approximately E17.5 (n=5 mouse embryos per genotype). (I) Representative images from renal parenchyma identifying lineage-traced GFP-positive endothelial cells (white arrows). DAPI was used to stain nuclei. Yellow arrow indicates normal vimentin staining expressed in a non-endothelial cell. (J) Quantification of renal endothelial cells with histological evidence of EndoMT. Values were normalized to the GFP/vimentin co-localization levels observed in control animals (n=5 mice per genotype). (K) Quantification of Evans blue dye extravasation into the parenchyma of the kidney (n=10 mice per genotype), spleen (n=10 mice per genotype), and lung (n=9 mice per genotype) of Cpt2^E-WT and Cpt2^E-KO mice. Data represent scatter plots with the median value indicated. Data represents mean ± SEM, with significance determined by one-way ANOVA with Bonferroni’s multiple comparison test (B–D). * p < 0.05; ** p < 0.01; ns, not significant.