Fetuin-A induces cytokine expression and suppresses adiponectin production

Abstract

Background: The secreted liver protein fetuin-A (AHSG) is up-regulated in hepatic steatosis and the metabolic syndrome. These states are strongly associated with low-grade inflammation and hypoadiponectinemia. We, therefore, hypothesized that fetuin-A may play a role in the regulation of cytokine expression, the modulation of adipose tissue expression and plasma concentration of the insulin-sensitizing and atheroprotective adipokine adiponectin.

Methodology and principal findings: Human monocytic THP1 cells and human in vitro differenttiated adipocytes as well as C57BL/6 mice were treated with fetuin-A. mRNA expression of the genes encoding inflammatory cytokines and the adipokine adiponectin (ADIPOQ) was assessed by real-time RT-PCR. In 122 subjects, plasma levels of fetuin-A, adiponectin and, in a subgroup, the multimeric forms of adiponectin were determined. Fetuin-A treatment induced TNF and IL1B mRNA expression in THP1 cells (p<0.05). Treatment of mice with fetuin-A, analogously, resulted in a marked increase in adipose tissue Tnf mRNA as well as Il6 expression (27- and 174-fold, respectively). These effects were accompanied by a decrease in adipose tissue Adipoq mRNA expression and lower circulating adiponectin levels (p<0.05, both). Furthermore, fetuin-A repressed ADIPOQ mRNA expression of human in vitro differentiated adipocytes (p<0.02) and induced inflammatory cytokine expression. In humans in plasma, fetuin-A correlated positively with high-sensitivity C-reactive protein, a marker of subclinical inflammation (r = 0.26, p = 0.01), and negatively with total- (r = -0.28, p = 0.02) and, particularly, high molecular weight adiponectin (r = -0.36, p = 0.01).

Conclusions and significance: We provide novel evidence that the secreted liver protein fetuin-A induces low-grade inflammation and represses adiponectin production in animals and in humans. These data suggest an important role of fatty liver in the pathophysiology of insulin resistance and atherosclerosis.

Figure 1. mRNA expression in human THP1 monocytes following fetuin-A treatment.

Expression of TNF (A), IL1B (B), PBEF1 (C), and RETN (D) mRNA in human THP1 monocytes before (control) and after treatment with human albumin or human fetuin-A for 8 h. Cellular mRNA contents were corrected for 28S-rRNA (RAU = relative arbitrary units). Data are given as means±SEM (n = 5). Data were analyzed by ANOVA, followed by Dunnett's test.

Figure 2. Cytokine and adipokine mRNA expression in adipose tissue of mice.

Adipose tissue expression of Il6 (A), Tnf (B), Adipoq (C), and Lep (D) mRNA in mice after bolus treatment with human fetuin-A (0.5 mg/g body weight), human albumin (0.5 mg/g body weight), or diluent (control) for 8 h. Cellular mRNA contents were corrected for 28S-rRNA (RAU = relative arbitrary units). Data are given as means±SEM. Data were analyzed by ANOVA, followed by Dunnett's test.

Figure 3. Adiponectin mRNA expression in cultured human adipocytes.

ADIPOQ mRNA expression of human in vitro differentiated adipocytes after treatment with bovine (A) and human (B) fetuin-A for 24 h. Cellular mRNA contents were corrected for 28S-rRNA. Data from adipocyte cultures of four donors are presented. Data were analyzed by Student's t-test.

Figure 4. Relationships between circulating fetuin-A and circulating adiponectin in humans.

Relationship between plasma levels of fetuin-A with total adiponectin and adiponectin's multimeric forms (A–D) in 49 healthy human subjects after adjustment of log-transformed data for age, sex, and percentage of body fat by multivariate linear regression analysis. The regression coefficients as well as the p-value are indicated (HMW–high molecular weight; LMW–low molecular weight; MMW–middle molecular weight).