Depletion of Liver Kupffer Cells Prevents the Development of Diet-Induced Hepatic Steatosis and Insulin Resistance

OBJECTIVE

Increased activity of the innate immune system has been implicated in the pathogenesis of the dyslipidemia and insulin resistance associated with obesity and type 2 diabetes. In this study, we addressed the potential role of Kupffer cells (liver-specific macrophages, KCs) in these metabolic abnormalities.

RESEARCH DESIGN AND METHODS

Rats were depleted of KCs by administration of gadolinium chloride, after which all animals were exposed to a 2-week high-fat or high-sucrose diet. Subsequently, the effects of these interventions on the development of hepatic insulin resistance and steatosis were assessed. In further studies, the effects of M1-polarized KCs on hepatocyte lipid metabolism and insulin sensitivity were addressed.

RESULTS

As expected, a high-fat or high-sucrose diet induced steatosis and hepatic insulin resistance. However, these metabolic abnormalities were prevented when liver was depleted of KCs. In vitro, KCs recapitulated the in vivo effects of diet by increasing hepatocyte triglyceride accumulation and fatty acid esterification, and decreasing fatty acid oxidation and insulin responsiveness. To address the mechanisms(s) of KC action, we inhibited a panel of cytokines using neutralizing antibodies. Only neutralizing antibodies against tumor necrosis factor-α (TNFα) attenuated KC-induced alterations in hepatocyte fatty acid oxidation, triglyceride accumulation, and insulin responsiveness. Importantly, KC TNFα levels were increased by diet in vivo and in isolated M1-polarized KCs in vitro.

CONCLUSIONS

These data demonstrate a role for liver macrophages in diet-induced alterations in hepatic lipid metabolism and insulin sensitivity, and suggest a role for these cells in the etiology of the metabolic abnormalities of obesity/type 2 diabetes.The physiological purpose of inflammation, which is an adaptive response to infection, injury, or exposure to toxic substances, is to reestablish a homeostatic state that entails removal of the source of infection, tissue repair, or resolution of toxin-induced stress. Upon the reestablishment of homeostasis, the necessity for the inflammatory response is removed, allowing immune system function to return to the basal state. However, under pathological conditions, a state of chronic inflammation is established, and the consequences of this inappropriate condition are the development of diseases of autoimmunity, sepsis, fibrosis, and cellular stress. Most recently, it has become apparent that the major metabolic diseases of this generation, namely obesity, type 2 diabetes, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, and atherosclerosis, are states of chronic inflammation, and a series of studies have demonstrated a role for inflammation in the pathophysiology of the metabolic abnormalities associated with a number of these conditions (1–5).Macrophages are a heterogeneous population of myeloid-derived mononuclear cells that are a critical component of the innate immune response (6,7). They are resident in practically all tissues of the body, are recruited to tissues in response to infection or tissue damage, and are particularly enriched in tissues that are frequently exposed to exogenous and endogenous antigens and toxins, such as the lungs and liver. In all tissues, they act as the first responders to pathogens, toxins, and tissue damage by producing a panel of M1 (Th-1) proinflammatory cytokines, the prototypical ones being tumor necrosis factor-α (TNFα), γ-interferon (IFN-γ), and interleukin (IL)-1β. In states of overnutrition such as obesity, the number and activity of macrophages in adipose tissue are increased in rodents (8,9) and humans (10,11). Furthermore, interventions that inhibit macrophage recruitment to adipose tissue (12,13) or decrease proinflammatory activity (14–16) of macrophages improve the insulin resistance associated with obesity, whereas interventions that induce macrophage recruitment exacerbate insulin resistance (12,17). However, although these studies demonstrate a pathophysiological role for adipose macrophages in the metabolic abnormalities of overnutrition, the role of macrophages in other tissues and the mechanisms of their effects are largely unknown. In this regard, the potential role of liver macrophages (Kupffer cells [KCs]) in the development of hepatic dyslipidemia (steatosis) and insulin resistance is largely unknown. Furthermore, recent studies (15,16) demonstrate that blocking the anti-inflammatory or alternative (M2 or Th-2) activation program of KCs exacerbates obesity-induced insulin resistance and decreases hepatocyte fatty acid oxidation. However, although suggestive, these studies do not directly address the contribution of KCs to the development of diet-induced steatosis and insulin resistance, or the mechanisms of these effects. The current study addressed these issues. The data demonstrate that the depletion of KCs protects against the development of diet-induced steatosis and insulin resistance, and that M1 activation of KCs induces changes in hepatocyte lipid metabolic pathways and insulin action that are consistent with the effects of diet in vivo. Finally, data are presented suggesting that KC–derived TNFα plays a role in mediating the detrimental effects of KCs on hepatocyte lipid metabolism and insulin action.