Kenneth R. Feingold

TNF-α is not the cause of fatty liver disease in obese diabetic mice

To the editor—The article by Lin et al. in the September 2000 issue of Nature Medicine proposed that metformin treatment reverses fatty liver disease (FLD) in obese, leptin-deficient (ob/ob) mice by decreasing hepatic TNF-α mRNA expression. In that study, metformin reversed hepatomegaly, lowered elevated serum transaminase levels, and decreased TNFα, uncoupling protein 2 (UCP2) mRNA and fatty acid synthase protein in the liver in ob/ob mice. Because earlier studies have shown that TNF-α is elevated in ob/ob mice, stimulates hepatic fatty acid synthesis and induces UCP2 expression in the liver, the authors postulated that TNF-α and TNF-inducible factors that promote hepatic lipid accumulation contribute to FLD in obese mice. While the hypothesis that increased hepatic TNF-α expression caused FLD is attractive, it is also conceivable that the increase in hepatic TNF-α expression is a consequence of lipid accumulation rather than the cause. One approach to obtain definitive evidence for or against the role of TNF-α as a mechanism of FLD is to use knockout mice. Previous studies have shown that ob/ob mice that lack both TNF receptors (ob/ob–p55/p75-null) and therefore lack functional TNF-α activity, have improved insulin sensitivity, but no change in body weight or fat mass per lean body mass. We have reported that ob/ob–p55/p75-null mice have twice as much UCP2 expression in the liver and adipose tissue compared with ob/ob mice suggesting that endogenous TNFα has an inhibitory effect on UCP2 and therefore a lack of TNF-α further upregulates UCP2 expression in obese mice. Since these data were in contrast to the proposed hypothesis of Lin et al. that TNF-α may be the cause of FLD and subsequent susceptibility to liver toxicity in ob/ob mice, we have measured hepatic lipid content and serum transaminase levels in control, ob/ob, p55/p75-null and ob/ob–p55/p75-null mice. We show that hepatic lipid content is similar in ob/ob and ob/ob–p55/p75null mice (Table 1). Moreover, serum transaminase levels are also not significantly different between ob/ob mice with or without TNF receptors. Thus, these data demonstrate that lack of TNFα action neither reverses hepatic lipid accumulation nor lowers elevated serum transaminase levels in obese mice, indicating that TNF-α is not the mechanism of FLD in obese diabetic mice. Our data indicate that FLD occurs in ob/ob mice, even in the absence of TNF action and TNF-induced insulin resistance.

Epidermal injury stimulates prenylation in the epidermis of hairless mice

Isoprenylation is the covalent attachment of isoprenyl groups, intermediates of the cholesterol biosynthesis pathway, to carboxy terminal cysteine residues of proteins. Numerous proteins are isoprenylated including small GTP binding proteins, trimeric G proteins, and nuclear lamins, and these prenylated proteins regulate a variety of cell functions, including cell growth, cytokinesis, and differentiation. Here, we quantitated protein prenylation and determined which proteins are prenylated in the epidermis of hairless mice by radiolabeling with 3H-mevalonolactone following acute or chronic epidermal injury. In normal epidermis, four major radiolabeled bands, with molecular weights of 17–26, 48, 54, and 68 kDa, were observed. The levels of each of these bands increased by 24–63% 16 h following acute epidermal injury induced by topical acetone treatment or tape stripping, returning to normal by 24 h. On 2D gel electrophoresis, there were no major differences between the patterns of labeling following barrier disruption. Subacute epidermal injury induced by either acetone or tape stripping twice a day for 7 days and chronic injury induced by feeding an essential fatty acid-deficient (EFAD) diet, also resulted in a significant increase in protein prenylation. As with acute injury, SDS-PAGE and 2D gel electrophoresis did not reveal marked differences in the pattern of protein prenylation. These results demonstrate that the prenylation of proteins in the epidermis is stimulated by injury, suggesting that one or more of these prenylated species may be important in epidermal proliferation or differentiation.