Regulation of TNF-alpha : implications for health and disease

Abstract

Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine, which is implicated in some metabolic disorders and may play a role in the development of cardiovascular disease.

When healthy 50-year-old men were studied, the plasma TNF-alpha concentration was associated with systolic and diastolic blood pressure, degree of alimentary lipaemia, plasma VLDL triglyceride and LDL cholesterol concentrations, peak LDL particle size and all measured cell adhesion molecules. Two indices of insulin resistance also correlated positively with TNF-alpha. The plasma TNF-alpha concentration was associated with common carotid IMT in univariate but not in multivariate analysis where soluble E-selectin and postprandial triglycerides were independent predictors of IMT.

The regulation of TNF-alpha expression in man is indicated to be partly genetically determined. Therefore, a 1263 base pair section of the proximal promoter of the TNF-alpha gene was screened for common functional polymorphisms. A C to A substitution at position -863 was found to exhibit distinct differences between the alleles in the binding of monocytic and hepatic nuclear factors. The rare -863A-allele was associated with 31% lower basal transcriptional activity (p<0.05) in-CAT reporter gene studies in human HepG2 cells. Allele frequency for the rare A- allele was 17% amongst 254 apparently healthy men of Swedish origin, aged 35-50 years. In 156 men, carriers of the rare allele had significantly lower TNF-alpha serum levels (p<0.05).

Further, the molecular mechanism underlying the allele-specific regulation of TNF-alpha gene expression under both basal and LPS-stimulated conditions was studied. Several transcription factors, in particular the NF-kappaB p50/p65 heterodimer, were shown to be involved in LPS-induced allele-specific regulation of TNF-alpha gene expression. Despite a strong LPS-dependent activation of the endogenous TNF-alpha gene expression, LPS did not activate the TNF-alpha promoter in transient transfections. Accordingly, it was hypothesized that the lack of LPS- mediated transcriptional activation in transient transfection assays could be explained by the missing chromatin structure, normally acting as a repressor of the transcriptional machinery. It was demonstrated with DNase I hypersensitivity assays that the chromatin structure is remodeled upon LPS stimulation and that the DNA-chromatin configuration is changed over time.

The association of different TNF-alpha promoter polymorphisms with TNF-alpha secretion from human subcutaneous adipose tissue was studied in 24 non-obese (BMI<30) healthy control subjects and 59 obese subjects (BMI>30). The secretion rate of TNF-alpha was about 3 times greater in non-obese subjects homozygous for the -863C allele than in carriers of the A-allele (p<0.05). Of note, the levels of TNF-alpha did not differ between carriers and non-carriers of the A- allele in obese individuals.

In conclusion, our results show that the plasma TNF-alpha concentration is associated with degree of early atherosclerosis and correlates with metabolic and cellular perturbations that are considered important for the progression of atherosclerosis. The common -863C/A polymorphism in the promoter region of the TNF-alpha gene is functional in vitro in monocytic and hepatic cells, and influences the serum TNF-alpha concentration in vivo in healthy middle-aged men and the secretion of TNF-alpha from adipose tissue from non-obese individuals ex vivo.