It is currently considered that during metastasic progression of human malignancies the intracellular FASN is up-regulated and over-expressed and the excess is finally released from the cytosol to the circulation in a stage-related manner [11–14]. Using a proteomics approach, it has been demonstrated that FASN is significantly up-regulated during both HIV infection and HCV infection [4, 5, 7]. Our data confirm and extend further these in vitro findings in a clinical setting revealing for the first time that HIV-infected patients show significant elevations in circulating serum FASN concentration with respect to healthy uninfected donors. Moreover, the increase in serum FASN concentration is even greater in HIV-patients co-infected with HCV. FASN, an intracellular protein, appears to exit cells during HIV infection and this release into the circulation is exacerbated in the presence of HCV co-infection, a condition that is frequently observed in HIV-infected patients. Based on the gender dependencies that we observed (i.e. women with undetectable HIV viral load or in those undergoing NNRTI-based antiretroviral regimen), and considering that women infected with HIV have a higher rate of gonadal dysfunction , it could be argued that enhanced levels of serum FASN might arise, at least in part, from stimulatory effects of estrogen on FASN gene expression which, in turn, will result in enhanced release of FASN protein [20–22]. Conversely, and perhaps surprisingly, we did not observe any significant relationship between circulating levels of the extracellular form of FASN and HIV-related dyslipidemia and lipodystrophy, two conditions in which enhanced endogenous fatty acid synthesis is closely linked to the accumulation of lipids and disproportionate distribution of tissue-associated fats. Although cytosolic FASN, by the coordinated action of its seven active sites, catalyzes all of the necessary reactions in the synthesis of palmitate [23–25], we failed also to identify statistically significant changes in serum NEFAs concentrations in both groups of HIV-infected patients, with and without HCV co-infection. Therefore, our findings support a model in which, upon infection with HIV and/or HCV, changes in circulating levels of extracellular FASN take place through molecular mechanisms likely unrelated to established pathways that regulate the intracellular FASN expression [9, 10, 23–25]. Notably, integration of HIV into the host cell chromosome occurs preferentially within genes. Particularly, insulin receptor (IRS), 19p13.3-13.2, is one of the identified integration sites  and the knockdown of this gene results in a significant up-regulation of FASN . It is plausible, therefore, that increased levels of serum FASN are not a mere epiphenomenon related to cell destruction and leakage but rather to a pathophysiologic response occurring during the viral infection.
In multivariate analysis, serum MCP-1, but not other inflammatory biomarkers, significantly contributed to explain serum FASN concentration in HCV co-infected patients despite the strong association with serum ALT activity and treatment regimen. This is not unexpected as we have already shown that serum MCP-1 concentration may be a reliable marker of inflammation in hepatic derangements in patients with chronic liver disease, a characteristic that is not shared by serum CRP concentration . As previously described , HCV co-infection increased serum IL-8 levels with respect to healthy individuals. Surprisingly, our comparison did not reach statistically significance, suggesting that IL-8 liver expression could be partially affected by antiretroviral HIV therapy as shown in Figure 2D. Moreover, the correlation between serum insulin and serum FASN was maintained irrespective of HCV co-infection or altered ALT values in HIV-infected patients.
Abnormalities of glucose regulation, including impaired glucose tolerance and insulin resistance, are often developed among HIV-infected patients, and co-infection with HCV appears to exacerbate insulin resistance in these patients [30–33]. Insulin resistance in this population may result from antiviral medication, effects of HIV and/or HCV per se, or from other indirect effects, such as fat redistribution. Considering that increased MCP-1 forms also a vicious adipokine network causing insulin resistance and metabolic syndrome , both the chronic viral state itself and the host immune response can give rise to glucose and lipid metabolic disorders which, in turn, are risk factors for hepatic damage. Therefore, it is tempting to suggest that molecular determinants of HIV/HCV action on FASN release are closely linked to the overall metabolic regulation. In this scenario, increased concentrations of serum FASN might universally occur in metabolic disorders in which insulin resistance could be prominent. Supporting this notion, we and others have found that serum FASN concentration is increased in patients with non-alcoholic steatohepatitis or chronic liver impairment [35, 36]. We have also found higher concentrations of circulating FASN in patients with type 2 diabetes . Administration of insulin sensitizers significantly prevented FASN release from cultured human adipose tissue explants whereas treatment with inflammatory stimuli increased the amount of extracellular FASN . We now add chronic infection with HIV/HCV as a novel pro-insulin resistance setting in which serum FASN concentration is significantly increased.
We have also provided experimental evidence to suggest that FASN release is an active and controlled process through the activation of AMP-activated protein kinase (AMPK) , a key enzyme that regulates the body energy balance [37–39]. We acknowledge that the ultimate mechanisms through which HIV/HCV infections influence the FASN release cannot by addressed by our study but HIV/HCV-induced changes on AMPK function might play a pivotal role. HIV and HCV are slow-growing viruses that should maintain beneficial host cell functions for an extended period as signs of disease do not usually show up until months or years after initial exposure. This requires a strict control of cellular AMPK because intense response and energy utilization could be deleterious to the viral infection . We hypothesize that upon AMPK activation, the extracellular release of excess FASN may provide a rapid mechanism to prevent further energy consumption. Conversely, if we assume that FASN up-regulation is important in the pathogenesis of HIV and HCV infection, as it has been already demonstrated in infections by the hepatitis B virus and the coxsackievirus B3 , it is conceivable that FASN might be a potential therapeutic target for an antiviral therapy.