Biochemical heterogeneity governs functional disparities among lipoproteins. We examined charge-defined VLDL subfractions in metabolic syndrome (MetS) to determine whether their increased electronegativity is associated with increased cytotoxicity and whether high concentrations of highly electronegative subfractions render VLDL harmful to the vascular endothelium.

Metabolic syndrome (MetS) represents a cluster of metabolic derangements. Dyslipidemia is an important factor in MetS and is related to atrial fibrillation (AF). We hypothesized that very low density lipoproteins (VLDL) in MetS (MetS-VLDL) may induce atrial dilatation and vulnerability to AF. VLDL was therefore separated from normal (normal-VLDL) and MetS individuals. Wild type C57BL/6 male mice were divided into control, normal-VLDL (nVLDL), and MetS-VLDL (msVLDL) groups. VLDL (15 µg/g) and equivalent volumes of saline were injected via tail vein three times a week for six consecutive weeks. Cardiac chamber size and function were measured by echocardiography. MetS-VLDL significantly caused left atrial dilation (control, n = 10, 1.64 ± 0.23 mm; nVLDL, n = 7, 1.84 ± 0.13 mm; msVLDL, n = 10, 2.18 ± 0.24 mm; p < 0.0001) at week 6, associated with decreased ejection fraction (control, n = 10, 62.5% ± 7.7%, vs. msVLDL, n = 10, 52.9% ± 9.6%; p < 0.05). Isoproterenol-challenge experiment resulted in AF in young msVLDL mice. Unprovoked AF occurred only in elderly msVLDL mice. Immunohistochemistry showed excess lipid accumulation and apoptosis in msVLDL mice atria. These findings suggest a pivotal role of VLDL in AF pathogenesis for MetS individuals.

Individuals with metabolic syndrome (MetS) are at high risk for atrial myopathy and atrial fibrillation. Very low-density lipoproteins (VLDLs) of MetS (MetS-VLDLs) are cytotoxic to atrial myocytes in vivo and in vitro. The calcineurin-nuclear factor of activated T-cells (NFAT) pathway, which is regulated by stromal interaction molecule 1 (STIM1)/ calcium release-activated calcium channel protein 1 (Orai1)-mediated store-operated Ca2+ entry (SOCE), is a pivotal mediator of adaptive cardiac hypertrophy. We hypothesized that MetS-VLDLs could affect SOCE and the calcineurin-NFAT pathway. Normal-VLDL and MetS-VLDL samples were isolated from the peripheral blood of healthy volunteers and individuals with MetS. VLDLs were applied to HL-1 atrial myocytes for 18 h and were also injected into wild-type C57BL/6 male mouse tails three times per week for six weeks. After the sarcoplasmic reticulum (SR) Ca2+ store was depleted, SOCE was triggered upon reperfusion with 1.8 mM of Ca2+. SOCE was attenuated by MetS-VLDLs, along with reduced transcriptional and membranous expression of STIM1 (P = 0.025), and enhanced modification of O-GlcNAcylation on STIM1 protein, while Orai1 was unaltered. The nuclear translocation and activity of calcineurin were both reduced (P < 0.05), along with the alteration of myofilament proteins in atrial tissues. These changes were absent in normal-VLDL-treated cells. Our results demonstrated that MetS-VLDLs suppressed SOCE by modulating STIM1 at the transcriptional, translational, and post-translational levels, resulting in the inhibition of the calcineurin-NFAT pathway, which resulted in the alteration of myofilament protein expression and sarcomere derangement in atrial tissues. These findings may help explain atrial myopathy in MetS. We suggest a therapeutic target on VLDLs to prevent atrial fibrillation, especially for individuals with MetS.

Very-low-density lipoproteins (VLDL) is a hallmark of metabolic syndrome (MetS) and each manifestation of MetS is related to atrial fibrillation (AF) risks. Slowed atrial conduction is a mechanism of AF in MetS. We hypothesized that VLDL can modulate and reduce atrial gap junctions. VLDLs were separated from normal (Normal-VLDL) and MetS (MetS-VLDL) individuals. VLDLs (15 µg/g) and equivalent volumes of saline (CTL) were injected respectively to C57BL/6 mice for 6 weeks. Electrocardiograms demonstrated that MetS-VLDL induced prolongation of P wave (P = 0.041), PR intervals (P = 0.014), QRS duration and QTc interval (both P = 0.003), but Normal-VLDL did not. Optical mapping of perfused hearts confirmed slowed conduction on atria and ventricles of MetS-VLDL mice. Slowed cardiac conduction was associated with significant atrial and ventricular remodeling, along with systolic dysfunction and comparable intra-cardiac fibrosis. MetS-VLDL induced downregulation of Cx40 and Cx43 at transcriptional, translational and tissue levels, and it also enhanced O-GlcNAcylation of Cx40 and Cx43. Protein structure analyses predicted O-GlcNAcylation at serine 18 of Cx40 and Cx43 which may impair stability of gap junctions. In conclusion, MetS-VLDL modulates gap junctions and delays both atrial and ventricular conduction. VLDL may contribute to the pathophysiology of atrial fibrillation and ventricular arrhythmias in MetS.