Complement Factor B Is a Determinant of Both Metabolic and Cardiovascular Features of Metabolic Syndrome

Abstract

CFB (complement factor B) is elevated in adipose tissue and serum from patients with type 2 diabetes mellitus and cardiovascular disease, but the causal relationship to disease pathogenesis is unclear. Cfb is also elevated in adipose tissue and serum of the spontaneously hypertensive rat, a well-characterized model of metabolic syndrome. To establish the role of CFB in metabolic syndrome, we knocked out the Cfb gene in the spontaneously hypertensive rat. Cfb^-/- rats showed improved glucose tolerance and insulin sensitivity, redistribution of visceral to subcutaneous fat, increased adipocyte mitochondrial respiration, and marked changes in gene expression. Cfb^-/- rats also had lower blood pressure, increased ejection fraction and fractional shortening, and reduced left ventricular mass. These changes in metabolism and gene expression, in adipose tissue and left ventricle, suggest new adipose tissue-intrinsic and blood pressure-independent mechanisms for insulin resistance and cardiac hypertrophy in the spontaneously hypertensive rat. In silico analysis of the human CFB locus revealed 2 cis-regulated expression quantitative trait loci for CFB expression significantly associated with visceral fat, circulating triglycerides and hypertension in genome-wide association studies. Together, these data demonstrate a key role for CFB in the development of spontaneously hypertensive rat metabolic syndrome phenotypes and of related traits in humans and indicate the potential for CFB as a novel target for treatment of cardiometabolic disease.

Keywords: adipose tissue; blood pressure; complement system proteins; glucose; hypertension.

Figure 1.

Glucose homeostasis. A, Glucose concentration curve during oral glucose tolerance (OGTT; inset, area under the curve, area under the curve (AUC), glucose. B, Plasma insulin concentration curve of OGTT (inset; area under the curve insulin). C, G:I ratio, (AUC_glucose:AUC_insulin). D, Insulin-stimulated glucose clearance (K_ITT). Spontaneously hypertensive rat (SHR), filled bars/circles, Cfb^−/−, open bars/circles. *P<0.05, **P<0.01, ***P<0.005. G:I indicates ratio of area under the curve of plasma glucose concentration to area under the curve of plasma insulin concentration.

Figure 2.

Adipose tissue and adipocyte morphometry, gene expression, and respiratory capacity. A, Adipose tissue wet masses, including subcutaneous (SAT), epididymal (EAT), retroperitoneal (RAT), mesenteric (MAT), and brown (BAT; n=6 per group). B, Epididymal mean cell number (n=6 per group). C, EAT, (D) SAT, (E) BAT gene expression levels in Cfb^−/−, normalized to Actb (n=5 per group). F, Maximal respiratory rates in primary epididymal adipocytes. Spontaneously hypertensive rat (SHR), filled bars, Cfb^-/-, and open bars. Aco1 indicates aconitase 1; Adipoq, adiponectin; Cpt1, carnitine palmitoyltransferase I; Fasn, fatty acid synthetase; normalized expression, gene of interest normalized to β-actin; Lep, leptin; Pgc1a, peroxisome proliferator-activated receptor gamma coactivator 1 alpha Slc2a4, solute carrier family 2 member 4; and Ucp1, uncoupling protein 1. *P<0.05, **P<0.01, ***P<0.005.

Figure 3.

Left ventricle morphometry, blood pressure, and rate pressure product before and after 72-h infusion of isoproterenol or saline. A, Left ventricle wet mass and (B) mean left ventricular cardiomyocyte diameter. C, baseline mean systolic blood pressure and (D) rate pressure product recorded telemetrically. E, Mean systolic blood pressure and (F) rate pressure product recorded telemetrically during infusion of isoproterenol or saline. Black-filled bars, spontaneously hypertensive rat (SHR), saline-treated; stripe-filled bars, SHR, isoproterenol-treated; white-filled bars, Cfb^-/-, saline-treated; hatch-filled bars, Cfb^-/-, isoproterenol-treated. Differences in genotype *P<0.05, ***P<0.0005 or treatment †P<0.05, ††P<0.005, †††P<0.0005.

Figure 4.

Gene expression levels in left ventricles after 72-h isoproterenol or saline treatment. A, Nppa, natriuretic peptide a, (B) Nppb, brain natriuretic peptide, (C) Camk2d, calcium/calmodulin dependent protein kinase II delta Myh6, (D) Myh7, myosin heavy polypeptide 7, (E) Actc1, α-cardiac actin, (F) Acta1, α-skeletal actin. Black-filled bars, SHR, saline-treated; stripe-filled bars, spontaneously hypertensive rat (SHR), isoproterenol-treated; white-filled bars, Cfb^-/-, saline-treated; hatch-filled bars, Cfb^-/-, isoproterenol-treated. Differences in genotype *P<0.05, **P<0.005, ***P<0.0005 or treatment †P<0.05, ††P<0.005, †††P<0.0005.

Figure 5.

Cardiometabolic genome-wide association study (GWAS) hits and cis-eQTLs (quantitative trait loci) located in the human the complement factor B (CFB) locus. Eighteen relevant cardiometabolic single-nucleotide polymorphisms (SNPs) located <1 Mb from the boundaries of the human CFB gene (upper; red). Twenty-six SNPs were retrieved from the GTEx Portal that were found to be significantly associated with CFB expression (P<0.05), blue SNPs are associated with a significant negative effect, whereas red SNPs are associated with a significant positive effect. Four SNPs (with 1 overlapping) were determined to be correlated to both CFB expression, as well as being GWAS hits for relevant cardiometabolic traits (lower; red/blue). See Table S8 for a list of genes located in the CFB locus.