Fetuin-A is a major plasma glycoprotein which was discovered in 1944 
. For a long time, its biological function remained obscure. Using targeted gene disruption, fetuin-A was recently reported to inhibit ectopic calcification 
. In keeping with this, fetuin-A deficiency in humans was found to be associated with vascular calcification and mortality in patients on hemodialysis 
. However, fetuin-A might exert more functions: several studies demonstrated that fetuin-A can act as a natural inhibitor of the insulin receptor tyrosine kinase in liver and skeletal muscle 
, and fetuin-A knockout mice display improved insulin sensitivity and are resistant to weight gain upon a high-fat diet 
Besides the effects of fetuin-A on muscle and liver insulin signalling, there is increasing evidence that fetuin-A is important for insulin action in adipose tissue 
. Moreover, the variability of plasma adiponectin levels is largely explained by a locus on human chromosome 3q27 harbouring both ADIPOQ
and, in direct vicinity, AHSG
. It is of note that not all variability in plasma adiponectin is explained by genetic variation of the ADIPOQ
. This led us to investigate whether fetuin-A regulates adiponectin production and, thus, may explain the recently reported association between fetuin-A and the metabolic syndrome 
. In addition, the metabolic syndrome and hypoadiponectinemia are strongly associated with low-grade inflammation 
. Furthermore, high circulating fetuin-A was found to be associated with carotid arterial stiffness 
, a functional property of atherosclerosis that is accompanied by subclinical inflammation. Therefore, we further tested whether fetuin-A treatment affects expression of inflammatory cytokines, a critical step in the generation of low-grade inflammation.
With the present report, we provide novel data that highly purified fetuin-A exerts strong pro-inflammatory effects as it provoked cytokine expression in monocytes in vitro
. The latter are known to infiltrate hypertrophic adipose tissue and to essentially contribute to adipose tissue inflammation 
. In addition, we found that, in animals in vivo,
fetuin-A treatment increased adipose tissue RNA expression of Il6
174- and 27-fold, respectively.
Besides the effects of fetuin-A on the expression of these inflammatory cytokines, administration of fetuin-A into mice repressed Adipoq
mRNA expression in adipose tissue and decreased circulating adiponectin. Lep
mRNA expression were not affected by fetuin-A, and albumin did not impair Adipoq
mRNA expression. These findings provide evidence that the effect of fetuin-A on Adipoq
expression is not due to a general trophic effect 
. Furthermore, fetuin-A directly affected adipocyte gene expression and these findings were not restricted to rodents: it repressed ADIPOQ
expression of in vitro
differentiated human adipocytes.
Based on these findings in vitro
and in animals, we further investigated whether circulating fetuin-A was related to low-grade inflammation and circulating adiponectin in humans. Indeed, plasma fetuin-A levels correlated positively with hsCRP levels, as reported previously by Ix et al
These findings are somewhat unexpected because CRP is up-regulated in inflammatory states while acute inflammation down-regulates fetuin-A expression in the liver 
. Whether, the latter effect is transient and/or reflects compensational mechanisms, needs to be determined. Furthermore, plasma fetuin-A levels correlated negatively with plasma adiponectin levels. In addition, besides other determinants of adiponectin levels, plasma fetuin-A levels were identified as a contributor to the variability in circulating adiponectin. Of note, the observed relationships of fetuin-A with adiponectin in humans were not very strong, nevertheless, they remained statistically significant after adjustment for established determinants of plasma adiponectin levels. This finding supports the assumption of Ix et al. 
that the strong relationship between plasma fetuin-A levels and the metabolic syndrome may be a result of fetuin-A-induced suppression of adiponectin production.
In addition, fetuin-A was specifically associated with HMW and MMW forms of adiponectin, but not with the LMW form. HMW adiponectin is reported to be the most active adiponectin form 
and represents, among all multimeric forms of adiponectin, the major determinant of insulin resistance, an atherogenic lipoprotein profile, and the metabolic syndrome 
. How fetuin-A determines the assembly of trimeric adiponectin into hexameric MMW and higher-order HMW structures, a process that is supposed to occur during the entry of adiponectin into the bloodstream 
, is currently unknown and needs further investigation.
So far, no information is available on fetuin-A-specific cell surface receptors and intracellular signaling pathways making it difficult to assess the molecular mechanisms underlying fetuin-A-induced repression of ADIPOQ
expression in adipocytes as well as fetuin-A-mediated induction of cytokine expression in monocytes and adipocytes. Since the ADIPOQ
gene is under the control of the transcription factor peroxisome proliferator-activated receptor (PPAR) γ, we also assessed whether PPARγ expression was altered by fetuin-A treatment. Whereas fetuin-A reduced adipose tissue PPARγ expression in mice in vivo, PPARγ expression of adipocytes was not affected (data not shown). The down-regulation of adiponectin and possibly PPARγ expression, appears contradicting to the reported pro-adipogenic effect of fetuin-A 
. However, this observation may be due to differential effects of fetuin-A on mature adipocytes and pre-adipocytes, respectively.
Since we observed a strong effect of fetuin-A on cytokine expression, and IL-6 and TNF-α are well-described negative regulators of adiponectin expression and production 
, we suppose that fetuin-A-induced impairment of adiponectin synthesis may be at least partially mediated by effects of fetuin-A on IL-6 and TNF-α production. Whether this is an exclusive mechanism, or whether fetuin-A regulates adiponectin expression and secretion independently of cytokine production, needs to be determined in future studies.
Together, the results presented in this work advance our understanding of the role of fetuin-A in the pathophysiology of insulin resistance, atherosclerosis, and the metabolic syndrome. Hepatic steatosis that is strongly related to insulin resistance and type 2 diabetes 
, associates with enhanced hepatic fetuin-A production in rats 
and humans 
. Elevated circulating levels of fetuin-A negatively affect whole-body insulin sensitivity (i) by impairment of insulin signaling in muscle and liver 
and (ii) by triggering inflammation in adipose tissue and suppression of adiponectin production. Generation of an atherogenic lipoprotein profile 
, induction of inflammatory cytokines as well as suppression of the atheroprotective hormone adiponectin 
, therefore, represent plausible molecular pathways linking fatty liver and atherosclerosis 
. In addition, based on the findings from fetuin-A deficient mice that remain lean and insulin sensitive fed a high-fat diet 
, fetuin-A may have long-term effects on energy expenditure and lipid oxidation, independent of the aforementioned mechanisms.
In conclusion, we found that fetuin-A induces low-grade inflammation and represses adiponectin production in animals and in humans. These data provide novel evidence on the role of fatty liver in the pathophysiology of insulin resistance and atherosclerosis.