Arginase 1: a potential marker of a common pattern of liver steatosis in HCV and NAFLD children.

We thank Alisi and colleagues for their comments [1] on our recent report characterizing the effects of the hepatitis C virus (HCV) proteins on metabolic liver zonation and steatosis [2]. In a transgenic mouse model with hepato-specific expression of the full complement of HCV proteins we found that fatty acid synthase, a major lipogenic enzyme, was redistributed to the midzone of the lobule, coinciding with zonated accumulation of lipids. Based on these and additional results, we hypothesized that low levels of viral proteins are sufficient to drive striking alterations of hepatic metabolic zonation. However, not all genes that display zonated expression in the liver were altered in the animal model studied. Notably, we found no change in the pattern of expression of E-cadherin or arginase 1. In contrast to the results reported by Alisi et al. we did not study the global level of expression of the latter. Alisi et al. compared arginase 1 expression in liver tissues from children with no hepatic pathologies or suffering from non-alcoholic fatty liver disease (NAFLD) or hepatitis C [1]. The quantification of the immunhistochemical staining showed identical mean intensity of arginase 1 in healthy livers and in HCVpositive patients without steatosis. This is consistent with our results showing no difference of arginase 1 expression in hepatocytes from control and HCV-transgenic mice [2]. In our work, appearance of steatosis did not alter arginase distribution. In contrast, Alisi et al. observed increased arginase 1 expression in HCV patients with steatosis and in NAFLD. Their results highlight the link between lipid accumulation and arginase expression, as well as the effect of HCV infection on arginase 1 accumulation. Noteworthy, Alisi et al. give no information on the HCV genotype, while our study focused solely on genotype 1. It would be of interest to determine whether the reported observations are genotype specific or more generalizable. As Alisi and colleagues correctly pointed out, the role of arginase 1 in steatosis is not well understood and possibly underestimated. Interestingly, its increased expression has been described in HCV infection and it was suggested that it might participate in promoting cell growth and proliferation [3]. Indeed, arginase 1 converts arginine to ornithine, which is further metabolized to polyamines that promote cellular proliferation. Thus, elevated arginase 1 expression may be hepatoprotective during chronic infection by promoting survival and proliferation of HCV-infected hepatocytes. In consequence, the inflammatory immune response that contributes to liver damage would be inefficient in eliminating virus-infected cells. Elevated arginase 1 expression is associated with many tumor types [4] and therapeutic strategies aiming at inhibiting this metabolic pathway are being tested in the clinic. Interestingly, in HCC, the predominant peri-tumoral distribution of arginase 1 expression is in agreement with proteomic analyses of samples from HCV infected HCC and non-tumoral liver [3,5]. Recent evidence indicates that interactions between tumor cells and the host microenvironment have a major role in driving cancer progression and metastasis [6,7]. The abundance of macrophages, pivotal members of tumor stroma, strongly correlates with poor prognosis in different types of solid tumors, including HCC [8]. It has been proposed that tumor-associated macrophages (M2-polarized) play an important role in generating overall immunosuppressive milieu within the tumor microenvironment that suppresses anti-tumor immunity and promotes tumor progression [6]. Interestingly, the differential metabolism of L-arginine provides a means of distinguishing the two macrophage activation states. M1, or classically activated macrophages, upregulate iNOS to catabolize L-arginine to nitric oxide and citrulline, while M2, or alternatively activated macrophages, induce arginase 1, the enzyme upstream of polyamines production, and thus increase collagen synthesis and cellular proliferation [9]. In HCC, high numbers of peri-tumoral M2-polarized TAMs are associated with poor patient prognosis, and while no analyses of arginase 1 expression have yet been conducted in this context, it is likely that they express high levels of this enzyme along with the other M2 genes described. In this context, TAM-derived arginase 1 might promote tumorigenesis in a non-cell autonomous manner. Furthermore, several TAM-derived factors, such as