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Human adenovirus‐36 and non‐alcoholic fatty liver disease

In this issue, Trovato et al. (1) report the first evaluation of the association of non-alcoholic fatty liver disease (NAFLD) and human adenovirus-36 (Adv36). This evaluation is relevant because Adv36 infection has been associated with obesity in humans and animals (2–9). Obesity is known to be associated with an increased prevalence of NAFLD (10). Chickens, mice, rats and monkeys that were experimentally infected with Adv36 were shown to increase body fat, gain weight and have paradoxically reduced serum cholesterol and triglycerides (2–4). Adult humans in the US and Italy and children in Korea who have naturally contracted Adv36 infection are heavier than uninfected individuals (5–9). The pro-inflammatory cytokine, interleukin-6 (IL-6), has been reported to be increased in the liver of people with NAFLD and is thought to play a role in fat accumulation and insulin resistance (11). Bouwman et al. (12) reported increased IL-6 in human adipocytes in vitro, but there are no data in humans on presence of Adv36 in the liver or its effects in hepatocytes. Kapila et al. (13) found Adv36 DNA in the livers of experimentally infected hamsters along with a significantly altered lipoprotein metabolism. Based on these prior publications it might have been expected that individuals infected with Adv36 would have an increased prevalence of NAFLD. Surprisingly, Trovato et al. (1) found an inverse relationship between the two. The authors studied 115 patients with a ‘bright liver score’ (BLS) on ultrasound indicative of NAFLD and 114 control subjects without liver disease. Fifty NAFLD patients were excluded because of factors including history of diabetes, hepatitis B or C, and liver enzyme assays above normal, leaving a total of 65 NAFLD patients in the analysis. Thus, this population is highly selected and represents those subjects with pure NAFLD in its mildest form. Adv36 infection was documented by detecting antibodies to Adv36 in the serum by serum neutralization assay. The prevalence of Adv36 antibodies in this population of NAFLD patients was 32% compared with an infection rate of 47% in those without NAFLD (1). Of the 179 total patients, 41% had been infected with Adv36 based on the presence of antibodies in serum. This paper is only the second study reporting the prevalence of Adv36 in adult humans and the authors found a much higher prevalence in Italy than has been reported in the US. Atkinson et al. (5) found that 30% of USA obese adults and 11% of non-obese adults had Adv36 antibodies. Insulin resistance is known to be strongly associated with NAFLD, and obesity, particularly visceral obesity, is associated with insulin resistance (10). Trovato et al. (1) confirmed that in their total population, NAFLD patients had much higher insulin levels and insulin resistance than non-NAFLD patients. Because the liver appears protected by Adv36 infection in this study, the question becomes what are the mechanisms of the Adv36 protection from NAFLD? The analyses in this paper reveal that there is a complex relationship between Adv36 infection and insulin/insulin resistance and their associations with liver disease. Individuals, all of whom had NAFLD and also were positive for Adv36, had a higher body mass index (BMI) and fat mass than those who were Adv36 negative. However, serum glucose, total and HDL cholesterol, insulin, and insulin resistance by homoeostasis model-insulin resistance index (HOMA) were all similar between Adv36-positive and -negative patients with NAFLD. Thus, Adv36-infected NAFLD patients were heavier and fatter than non-Adv36 patients, but there were no differences in biochemical measures. This is in marked contrast to the situation in individuals all of whom were positive for Adv36, but did or did not have NAFLD. If NAFLD was present in Adv36 positive individuals, the serum insulin, HOMA insulin resistance and HDL cholesterol were highly significantly different than in those without NAFLD. This is similar to the situation in uninfected individuals: those with NAFLD also had much higher levels of insulin and insulin resistance and lower levels of HDL cholesterol than those without NAFLD. Although the lower prevalence of NALFD in patients who were Adv36 positive might appear to be a paradoxical response based on prior reports of Adv36 induced obesity and increased IL-6 (12), there are data in the literature that may shed light on the mechanisms of this response. Pasarica et al. (14) showed that mice experimentally infected with Adv36 had significantly lower serum levels of several cytokines. Rogers et al. (15) found that experimental infection of human adipose tissue explants and human adipose-derived stem cells with Adv36 caused a ‘metabolically favourable’ response. Adv36 increased glucose transport and upregulated expressions of several proadipogenic genes, adiponectin and fatty acid synthase. It also reduced the expression of inflammatory cytokine macrophage chemoattractant protein-1 in a phosphotidylinositol 3-kinase (PI3K)dependent manner. Fasting glucose and A1C levels were