Increased tissue iron levels are associated with diabetes, both in human hereditary hemochromatosis (HH) and in dietary iron overload (1
). Although this is at least partially the result of decreased insulin secretion (15
), tissue iron overload also results in significant changes in glucose metabolism in skeletal muscle (18
). In that case, however, the effect of iron is to increase glucose uptake, a change that would be predicted to be protective of diabetes. To better understand the changes in overall fuel economy caused by iron that might contribute to diabetes, we have investigated muscle and hepatic carbohydrate and fatty acid metabolism in a mouse model with targeted deletion of the gene most commonly mutated in human HH, Hfe
. We show that Hfe−/−
mice exhibit a shift in fuel preference from glucose to fatty acid oxidation in muscle. The decreased glucose oxidation in muscle is associated with increased pyruvate/lactate recycling to liver, as demonstrated in our previous metabolic flux studies (18
), in , and by the increased hepatic glucose output. These combined effects may contribute to the increased prevalence of diabetes in individuals with HH.
Our results are consistent with the close coupling of metabolism to iron availability such as has been demonstrated in lower eukaryotes (31
). The decreased glucose oxidation in skeletal muscle is due, at least in part, to decreased PDH activity. The mechanism is likely the observed increase in Pdk4
mRNA, PDK4 activity being largely regulated at the transcriptional level (28
). A candidate mediator for the regulation of Pdk4
may be increased AMPK activity (34
) that we previously reported in Hfe−/−
mouse muscle (18
). Other factors contributing to the decreased glucose oxidation include a modest degree of mitochondrial dysfunction and increased fatty acid oxidation (Randle effect) (35
). The mitochondrial dysfunction in Hfe−/−
muscle is likely, however, to be a relatively minor contributor given the increased capacity for fatty acid oxidation apparent in mice on high-fat diets.
The mechanism for the increased fatty acid oxidation is also likely multifactorial. We previously reported higher serum adiponectin levels in Hfe−/−
) that would contribute to higher rates of fatty acid oxidation through increased AMPK activation and decreased ACC activity (36
). Increased Cpt1b
expression may also play a role, as might changes in malonyl-CoA, which were not assessed. Perhaps related to the current findings, mice overexpressing erythropoietin in muscle exhibit a similar phenotype to iron-overloaded mice, including increased fat oxidation, decreased glucose oxidation, and protection from diet-induced obesity, although tissue iron levels were not measured in that model (38
). Insofar as iron sufficiency should facilitate unimpaired erythropoiesis, it would seem advantageous for an iron-sufficient mouse to shift to the more energy-efficient but oxygen-inefficient fuel source of fatty acids to make use of that full capacity for oxygen transport. Indeed, under the opposite condition of hypoxia, fatty acid oxidation decreases, enzymes for fat metabolism downregulate, and glucose oxidation increases (39
). Consistent with this proposed adaptive relationship between iron and fuel choice, Hfe−/−
mice consume more oxygen and produce more heat when fat is available in the diet. A decreased ability to transition between utilization of carbohydrate and lipid fuel sources, so-called “metabolic inflexibility,” is a characteristic of the metabolic syndrome and type 2 diabetes (19
). Chronically increased fat oxidation, especially in the setting of decreased mitochondrial function, might also contribute to the accumulation of lipid products that have been implicated in the pathogenesis of diabetes (41
). Whether these pathways contribute to increased diabetes risk in human hemochromatosis, however, is unknown.
The data are most consistent with the source of the increased hepatic glucose production being increased recycling of lactate and pyruvate, as was also previously documented by isotopomer analysis of the fate of 1,2[13
). It appears that this increase in hepatic glucose production is mainly substrate driven as the levels of mRNA for gluconeogenic enzymes were not increased (B
) and glucose production from exogenous pyruvate was not enhanced (C
). The data also do not support changes in insulin signaling as the underlying cause of the phenotype. We previously demonstrated no change in basal or insulin-stimulated (in vivo) pAkt in skeletal muscle (18
), and herein we shown that the increased hepatic glucose production occurs in the face of paradoxically increased insulin signaling in liver (D
and E). Why insulin signaling is upregulated is not known. Levels of the insulin-sensitizing adipokine have been shown to be elevated in Hfe−/−
), but other unknown factors may also contribute, including compensation for decreased insulin levels (17
) or decreased inflammatory signaling in macrophages based on their lower iron content (42
). Why this increased insulin signaling does not translate to decreased levels of gluconeogenic enzymes is unclear and is under investigation.
In sum, we have shown here and in previous work that the diabetes risk engendered by iron operates not only through its toxic effects on β cells mediated by increased oxidative stress and mitochondrial dysfunction. Rather, iron also exerts regulatory effects on metabolism and fuel choice. Iron overloaded skeletal muscle demonstrates decreased glucose oxidation and increased fatty acid oxidation. Liver of Hfe−/− mice exhibits increased glucose output, with the patterns of hepatic gene regulation and metabolite levels suggesting that the gluconeogenesis is largely substrate-driven and results from the altered fuel choice in muscle. These findings should help elucidate the association between iron and diabetes, not only in hemochromatosis, but also in nonhemochromatotic individuals with the metabolic syndrome or type 2 diabetes associated with dietary iron excess.