Enhanced secretion of ApoB by transfected HepG2 cells overexpressing fibrinogen

Fibrinogen and coronary heart disease: test of causality by 'Mendelian randomization'

BACKGROUND

Blood concentrations of fibrinogen have been associated with coronary heart disease risk in epidemiological studies, but it is uncertain whether this association is causal or reflects residual confounding by other risk factors. We investigated the relationship between the single nucleotide polymorphism at position -148 in the beta-fibrinogen gene promoter (beta - 148C/T), blood fibrinogen levels, and risk of myocardial infarction (MI) in sufficiently large numbers of coronary disease cases to reliably address this question.

METHODS

Genotyping and measurement of blood fibrinogen concentration were carried out in 4,685 cases of confirmed MI and 3,460 controls with no history of coronary disease. A meta-analysis of ISIS and 19 other studies of beta-fibrinogen genotypes involving a total of 12,220 coronary disease cases and 18,716 controls was conducted.

RESULTS

Among the ISIS controls, mean plasma fibrinogen concentrations with the C/C, C/T and T/T genotypes were 3.34 (SE 0.015), 3.48 (0.022), and 3.60 (0.064) g/l, respectively, corresponding to an increase of 0.14 (0.024) g/l per T allele (trend P < 0.0001). In the case-control comparison, 0.14 g/l higher usual plasma fibrinogen concentration was associated with an age-adjusted and sex-adjusted risk ratio for MI of 1.17 [95% confidence interval (95% CI) 1.14-1.19; P < 0.0001]. But, after further adjustment for smoking, body mass index, and plasma apolipoprotein B/A(1) ratio, this risk ratio fell to 1.03 (95% CI 1.00-1.05; P = 0.05). Moreover, fibrinogen genotype was not significantly associated with MI incidence: risk ratio of 1.06 (95% CI 0.96-1.16) per higher-fibrinogen allele in ISIS alone and of 1.00 (95% CI 0.95-1.04) per allele in the meta-analysis.

CONCLUSIONS

Genotypes that produce lifelong differences in fibrinogen concentrations do not materially influence coronary disease incidence. As these genotype-dependent differences in fibrinogen were allocated randomly at conception (Mendelian randomization), this association is not likely to be confounded by other factors. Consequently, these genetic results provide strong evidence that long-term differences in fibrinogen concentrations are not a major determinant of coronary disease risk.

Regulation of gamma-fibrinogen chain expression by heterogeneous nuclear ribonucleoprotein A1

Earlier studies showed that HepG2 cells stably transfected with any one fibrinogen chain cDNA enhanced the expression of the other two fibrinogen chains. In this report, a regulatory element "TGCTCTC" in the gamma-fibrinogen promoter region, -322 to -316, is identified, which is involved in increased expression of gamma chain in HepG2 cells that are transfected with Bbeta fibrinogen cDNA. By electrophoretic mobility shift assay, three DNA-protein complexes were found to form with the regulatory element. The amount of the protein complexes that bind with the regulatory element was much reduced in HepG2 cells transfected with Bbeta cDNA. By DNA-affinity chromatography, mass spectrometry, and supershift assay, human heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) was identified as a component of the complexes. Overexpression of hnRNP A1 suppressed basal gamma-fibrinogen transcription. These results indicate that the basal expression of gamma-fibrinogen is regulated by a constitutive transcriptional repressor protein, hnRNP A1, and the decreased binding activity of hnRNP A1 leads to the overexpression of gamma chain in HepG2 cells that overexpress the Bbeta chain.

Ferritins can regulate the secretion of apolipoprotein B

Apolipoprotein B is secreted with atherogenic lipids as lipoprotein particles from hepatocytes. Regulation of the secretion of apolipoprotein B is largely post-translational and reflects the balance between processes that leads to particle assembly or to intracellular degradation. Previously, we conducted a proteomic screen to find proteins that bind apolipoprotein B in rat liver microsomes. We identified ferritin heavy and light chains in this screen among other proteins and showed that the two ferritins bind apolipoprotein B directly in vitro. In hepatocytes and other cells, ferritin heavy and light chains form cytosolic cages that store iron. We now show that ferritin heavy or light chains post-translationally inhibit the secretion of apolipoprotein B without altering the export of other hepatic proteins including albumin, factor XIII, and apolipoprotein A-I. This inhibition of apolipoprotein B secretion is not due to diminished lipid synthesis and can be partially overcome by stimulating triglyceride synthesis. The block in apolipoprotein B secretion by ferritins leads to an increase in endoplasmic reticulum-associated degradation of the apolipoprotein. Thus, despite being cytosolic proteins without known chaperone activity, ferritins can specifically regulate the secretion of apolipoprotein B post-translationally. The metabolic pathways for iron storage and intercellular cholesterol and triglyceride transport could intersect.