Reply:

Abstract

We appreciate the interest of Dr. Leclercq and colleagues in our article.1 In their correspondence,2 Leclercq and colleagues take issue with respect to the design and some of the outcomes of our experiments, which revealed important cross-talk between Kupffer cells and hepatocytes in the regulation of hepatic triglyceride storage. Here, we would like to take the opportunity to respond to the specific issues raised and continue to make a strong case for a relation between Kupffer cells and liver triglycerides. It should be emphasized that the chronic high-fat feeding protocol as employed in our study represents the most widely used model for diet-induced obesity and metabolic syndrome in mice. Leclercq et al. raise the point that Kupffer cells are not activated in this model, which is based on the lack of change in hepatic expression of macrophage markers Cd68 and F4/80, as well as interleukin-1beta (IL-1β). Although this may be true, it is unclear to us why Kupffer cells have to be activated to be able to study the effect of Kupffer cell deletion on hepatic lipid storage. In our study, the purpose of chronic high-fat feeding was to induce obesity-related hepatic steatosis, not to create a state of steatohepatitis. Second, Leclercq et al. assert that intraperitoneal injection of clodronate liposomes not only depletes hepatic Kupffer cells but also adipose tissue macrophages. Unfortunately, the experimental backing of this claim is weak. As revealed by the gene expression data, the effects of clodronate liposomes on expression of macrophage markers in epididymal adipose tissue are small and dwarfed by the dramatic changes observed in liver. Nevertheless, we acknowledge in the discussion that we cannot completely rule out a role of nonhepatic macrophages in the observed reduction in hepatic lipids and increase in peroxisome proliferator-activated receptor α activity but consider this explanation very unlikely. Third, it is argued that, in conflict with existing literature, the mice fed a high-fat diet do not exhibit adipose tissue inflammation. However, our expression data perfectly copy the established literature by showing marked induction of macrophage markers, including all those that were reported in the original publication by Xu et al.3 Histological data that were not included in our manuscript support enhanced macrophage infiltration in the mice fed a high-fat diet. The alleged absence of adipose tissue inflammation is based on perceived lack of induction of tumor necrosis factor-α, IL-1β, and IL-6 expression in adipose tissue of the mice. However, induction of these cytokines is rarely observed in diet-induced obesity models and does not define adipose tissue inflammation. Perhaps more importantly, it is unclear to us why induction of adipose tissue inflammation by the dietary treatment has any relevance to our findings that entirely pertain to liver. Finally, we would like to stress that a number of very recent studies reported findings similar to ours and corroborate a role for Kupffer cells in hepatic fatty acid metabolism and hepatic steatosis.4-7 Thus, we feel that a role of Kupffer cells in ameliorating steatosis has been convincingly demonstrated. Sander Kersten*, Rinke Stienstra*, Michael Müller*, * Nutrigenomics Consortium, TI Food and Nutrition, Wageningen, the Netherlands, and Nutrition, Metabolism and Genomics Group, Division of Human Nutrition, Wageningen University, Wageningen, the Netherlands.